Acta Geophysica vol. 58, no. 5, pp DOI: /s x

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1 Acta Geophysica vol. 58, no. 5, pp DOI: /s x Toward a Unified Solid State Theory for Pre-Earthquake Signals Friedemann FREUND NASA Ames Research Center, Moffett Field, CA, USA friedemann.t.freund@nasa.gov Department of Physics, San Jose State University, San Jose, CA, USA Carl Sagan Center, SETI Institute, Mountain View, CA, USA Abstract Many different non-seismic pre-earthquake signals have been reported but there is great uncertainty about their origin, their correlation to each other and to the impending seismic event. The discovery of stressactivated electric currents in rocks provides a possible explanation. Stresses activate electronic charge carriers, namely defect electrons in the oxygen anion sublattice, equivalent to O in a matrix of O 2, also known as positive holes. These charge carriers pre-exist in unstressed rocks in a dormant, electrically inactive state as peroxy links, O 3 Si-OO-SiO 3, where two O are tightly bound together. Under stress dislocations sweep through the mineral grains causing the peroxy links to break. Positive holes, thus generated, flow down stress gradients, constituting an electric current with attendant magnetic field variations and EM emissions. The positive holes accumulate at the surface, creating electric fields, strong enough to field-ionize air molecules. They also recombine leading to a spectroscopically distinct IR emission seen in laboratory experiments and night-time infrared satellite images. In addition positive holes interact with radon in the soil, affecting the radon emanation. Key words: pre-earthquake signals, peroxy, positive holes, EM emissions, earthquake lights, thermal infrared anomalies, radon emanation Institute of Geophysics, Polish Academy of Sciences

2 720 F. FREUND 1. INTRODUCTION: NON-SEISMIC PRE-EARTHQUAKE SIGNALS Seismology is, by definition, earthquake science. For over a hundred years seismology has provided invaluable insight into the hidden structures of the Earth, into the distribution and orientation of faults in the Earth s crust, and into the dynamics of earthquakes. However, there is one area in which seismology has not done well: Elucidating the transient signals, which the Earth sends out prior to major earthquakes, sometimes strong, more often subtle and fleeting. Some pre-earthquake (pre-eq) signals are seismic such as the increase in microseismicity recently reported by Sobolev and his coworkers (Sobolev 2004, Sobolev et al. 2005, Sobolev and Lyubushin 2006, 2007) and by Korneer (2010), but most of them are non-seismic. If those non-seismic pre-eq signals are studied solely with the tools of seismology, they are and will remain inexplicable. At the same time most seismologists consider earthquake prediction or, more appropriately, earthquake forecasting as the holy grail of their discipline. However, the conventional approach in seismology is based on the analysis of historical earthquakes, large and small, and on the attempt to find patterns in the spatial and temporal distribution of earthquakes along given faults. Such information is fed into computer models, often very sophisticated and complex, to estimate the probability of future events. This approach can only give statistical estimates, burdened by wide uncertainty margins (Console et al. 2002, Grunewald and Stein 2006, Stewart 2000). In this context it is understandable why prominent seismologists have made categorical statements such as earthquakes cannot be predicted (Geller et al. 1997, Mulargia and Geller 2003). However, the literature is replete with reports of non-seismic signals prior to major earthquakes. They fall into very different categories: magnetic field variations, electromagnetic (EM) emissions over a wide range of frequencies from visible (VIS) through the infrared (IR) to radiofrequency (RF) and ultralow/extremely low frequency (ULF/ELF), radon emanation from the ground, atmospheric and ionospheric phenomena, unusual animal behavior, and others. Faced with such a bewildering multitude of reported pre-eq signals, the questions must be raised: (i) How are these signals generated, and (ii) is there an underlying physical process? Here I present evidence that there is indeed such a basic physical process, long ignored in the geoscience community, which allows us to approach non-seismic pre-eq signals from a unifying solid state physics perspective. The work on which I report here would not have been possible without the help of dedicated collaborators, whose names are listed in the Acknowledgments.

3 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 721 As part of this presentation I need to introduce some terms that are familiar to members from other disciplines but probably not to seismologists. Electronic charge carriers in rocks exist in two types: electrons and defect electrons. In semiconductor physics they are symbolized by e and h, where superscript prime and superscript dot stand for a negative and positive charge, respectively, with reference to the ideal charge that should be at a given lattice site. For instance, if an oxygen anion O 2 changes its valence to 1, it chemically becomes O but physically it becomes a defect electron in the oxygen anion sublattice. Defect electrons are also called holes or positive holes h. When two O pair up, they form what chemists call a peroxy bond, for instance, O 3 Si-OO-SiO 3, and what we sometimes call a positive hole pair, PHP. The tight peroxy bond strictly localizes the two hole states. For this reason the peroxy bond is electrically inactive. 2. SOLID STATE BACKGROUND: POSITIVE HOLE CHARGE CARRIERS Processes related to seismic and pre-seismic activity are generally studied and discussed using the tools of rock mechanics, applying them to largescale systems like faults and large volumes of adjacent rocks. For the approach chosen here it is necessary to descend to the atomic level. Peroxy links are well-known point defects in fused silica (Ricci et al. 2001), when one of the usual O 3 Si-O-SiO 3 bonds is replaced by an O 3 Si-OO- SiO 3 link. The same type of peroxy defects exist in the structures of rockforming minerals, O 3 X-OO-YO 3 with X,Y = Si 4+, Al 3+ etc. They are introduced through the incorporation of H 2 O into nominally anhydrous minerals that crystallize in H 2 O laden magmas or recrystallize in high-temperature H 2 O laden environments (Freund 1985). The incorporation of H 2 O leads to hydroxyls, O 3 (X,Y)-OH, most commonly hydroxyl pairs associated with other defect sites such as cation vacancies. During cooling through the C window, the hydroxyl pairs electronically rearrange in such a way that their protons, H +, steal an electron from their respective oxygens, O 2, turning into an H 2 molecule. while the two oxygens are oxidized from the valence 2 to the valence 1. The two O then form a peroxy link: Water splits Si-O-Si bond Hydroxyl pair equal dissolved H 2 O Redox conversion to peroxy bond plus H 2 Self-trapped positive hole pair (1) e e H 2 O + O 3 Si-O-SiO 3 O 3 Si-OH HO-SiO 3 O 3 Si/ OO \SiO 3 O 3 Si/ OO \SiO 3 + H 2 H 2

4 722 F. FREUND Peroxy defects are ubiquitous in the mineral world. Every igneous and high-grade metamorphic rock in the Earth s crust carries a non-zero complement of peroxy. The same is true for every sedimentary rock that contains detridal mineral grains from igneous and high-grade metamorphic rocks. This has far-reaching consequences for the capability of crystalline and most sedimentary rocks to (i) generate electronic charge carriers when subjected to tectonic stresses, and (ii) to allow those electronic charge carriers to flow through rocks and carry a current. The structure of every silicate mineral is made up of large O 2 anions filling most of the space and the usually much smaller metal cations occupying interstitial sites. An O represents a missing electron in the O 2 sublattice. Thus an O is a defect electron in the O 2 sublattice, also known as a positive hole (Griscom 1990) or hole for short, symbolized by h. Two O bonded together in a peroxy link represent a positive hole pair, PHP, where the two positive holes are self-trapped and strictly localized. PHPs are inconspicuous and electrically inactive. They are practically undetectable. For this reason the ubiquity of peroxy in minerals and rocks has escaped the attention of researchers for a long time. There are several ways to break the PHPs, most significantly through the application of uniaxial stresses (Freund et al. 2006). Such stresses cause mineral grains to plastically deform. Even at low stress levels deformation can take place at grain-to-grain contact points, which act as stress concentrators. Plastic deformation is achieved through the movement of dislocations, e.g., zipper-like displacements of rows of atoms in the mineral structure. Each time a dislocation intersects a peroxy link, it causes the O O bond to break. A neighboring O 2 then sends in an electron as outlined in eq. (2), where an [SiO 4 ] 4 structural unit is taken as the electron donor: (2) The broken peroxy link acts as electron receptor: it takes the electron and holds on to it, while the [SiO 4 ] 4, which had donated the electron, thereby turns into a positive hole, symbolized by h. The situation, which then develops, requires some more semiconductor terminology. As stated above the h is an electronic state associated with a

5 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 723 missing electron in the O 2 sublattice. It has no physical reality except that it has a wavefunction that is associated with the O 2 where an electron is missing. At the same time h are electronic charge carriers and quite mobile too. They reside in the valence band, which forms an energetic continuum across all grain boundaries. If all energy levels in the valence band are fully occupied, the h have no way to move. A suitable analogy is that of a movie theater, where all seats are occupied. Most silicate minerals are good insulators, meaning that they disallow the flow of electrons. However, if their valence bands contain some h, these h are like empty seats in a movie theater. The empty seats can move by people on occupied seats exchanging places. In the case of h electrons are moving from O 2 onto O. For this reason, if minerals contain an ever so low concentration of h, charges can move as depicted in eq. (3). There is evidence that, indeed, all rock-forming minerals contain positive holes at least at some level, providing energy levels at the upper edge of their valence bands along which the h can propagate. The h propagate not only through individual mineral grains. They can also cross grain boundaries, e.g., they can propagate from grain to grain in solid rocks and even from grain to grain in sand and soil wherever mineral grains are in physical contact. e e e O 2 O 2 O O 2 O 2 O 2 O 2 O 2 O 2 O 2 O O 2 O 2 O 2 O 2 O 2 O 2 O 2 O O 2 O 2 h h h (3) Positive holes are thought to propagate by electrons hopping from O 2 to O in the opposite direction as indicated by eq. (3). Ideally, each time two oxygens vibrate against each other, an electron can hop. If this process occurs at the frequency of thermal phonons, about Hz, with the h advancing by 2.8 Å (the average O 2 O 2 distance, m) at every step, then the maximum speed with which an h pulse can propagate will be on the order of 280 m s 1. The measured phase velocity of h charge spreading through various rocks, m s 1 (Freund 2002, Hollerman et al. 2006), is consistent with this phonon-assisted electron hopping mechanism (Shluger et al. 1992). The drift velocity of an actual h concentration wave traveling along its self-generated electric field gradient will be significantly lower and its activation energy, e.g., the height of the energy barrier over which the electron has to hop is on the order of 1 ev. Figure 1a shows the electrical conductivity σ = σ 0 e E/kT of a high purity MgO single crystal over the temperature range from C, where σ 0 is the pre-exponential factor, E the activation energy, k the Boltzmann con-

6 724 F. FREUND Fig. 1a. Electrical conductivity of single crystal MgO during heating dominated by the thermal activation of h charge carriers with a characteristic 1 ev activation energy (after Kathrein and Freund 1983). Fig. 1b. Selection of log σ versus 1/T [K] plots of different mafic and ultramafic rocks to illustrate the common 1 ev activation energy for their electrical conductivity response (after Parkhomenko and Bondarenko 1986). stant, and T the absolute temperature (Kathrein and Freund 1983). In the C window h charge carriers become thermally activated, causing the electrical conductivity to increase steeply. A special heating program with intermittent cooling demonstrates how the number of h charge carriers increases over the C window and how they conduct the current with

7 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 725 a characteristic 1 ev activation energy. The apparent activation energy of 3.4 ev associated with the steeply rising portion of the log σ versus 1/T curve arises from the combined activation of the h charge carriers, 2.4 ev, and their 1 ev conduction mechanism (Freund et al. 1993). Figure 1b shows the electrical conductivity σ of representative igneous rocks over the temperature range C typical of the Earth s continental crust and into the upper mantle (Parkhomenko and Bondarenko 1986). Despite their chemical and mineralogical complexity all rocks in this plot conduct electric current from below 200 C to approximately 600 C with the same 1 ev activation energy as MgO single crystals. The C temperature window corresponds to the depth range, 7-35 km, where no open porosity persists and where no electrical conductivity contribution from interconnected water films is possible (Walder and Nur 1984, Wendebourg and Ulmer 1992). The range 7-35 km is also the depth window in which the majority of earthquakes occur. The constancy of the 1 ev activation energy mechanism points to the presence of h charge carriers in all igneous rocks and, hence, to their presence in almost all crustal rocks. It also points to the fact that the electrical conductivity response of these rocks is entirely dominated by these previously overlooked or ignored electronic charge carriers. Using temperature to activate h charge carriers has provided important insight into the processes that take place in oxides and silicates, including rocks, which can lead to changes in the electrical conductivity (Freund et al. 1993, Freund 2003). As exemplified by Fig. 1a the conductivity of nominally high purity MgO single crystals increases by as much as 5-6 orders of magnitude over the temperature range where h charge carriers are activated. In typical igneous rocks the increase reaches 3-5 orders of magnitude as shown by Fig. 1b. However, heating is only one way to activate h charge carriers in rocks. As indicated above, the application of uniaxial stresses provides an alternative way. During the lead-up to any earthquake, tectonic forces will subject rocks at depths to increasingly strong stresses. Deformation will happen on all scales, from microscopic volumes at contact points between mineral grains, acting as stress concentrators, to large volumes comprising hundreds to thousands, sometimes tens to hundreds of thousand cubic kilometer of rocks. Long before the stresses reach values causing catastrophic rupture, h charge carriers will be activated in large, potentially in very large numbers. Therefore the questions to be addressed are: What are the consequences of h activation? How large an effect can we expect to see in the electrical conductivity? What other recognizable effects will arise from h activation?

8 726 F. FREUND 3. EXPECTED OBSERVABLES 3.1 Flow of positive hole charge carriers through the bulk During uniaxial loading, which simulates tectonic stresses, existing dislocations move and new dislocations are generated in response to shear forces acting on mineral grains. The dislocations activate h charge carriers alongside electrons e. The h generated are mobile and capable of spreading out into the unstressed rock volume, while the e remain trapped at the sites of the broken peroxy bonds (Freund 2002, Freund et al. 2006). Stressing thus creates elevated concentrations of h and e inside the stressed subvolume as outlined at the bottom of Fig. 2. Figure 2 describes a situation similar to that in a battery. In the case of an electrochemical battery metal cations are the positive charges in the anode. They spread into the electrolyte. The electrons, which cannot enter or flow through the electrolyte, can pass from the anode to the cathode, but a metal wire is needed to close the battery circuit. In a rock the stressed subvolume turns into the anode from where positive charges can flow out. If we attach a metal contact to the unstressed rock, we create a cathode. The difference to an electrochemical battery is that, in the rock battery, the outflowing charges are positive holes, h. Also, in sharp contrast to an electrochemical battery, anode and cathode of the rock battery Fig. 2. Set-up to demonstrate the activation of positive holes, which flow out of the stressed rock volume. Bottom: Before application of stress the h concentration is low and uniform throughout the rock (dashed line). After application of stress the concentration of e and h increases in the stressed subvolume (dot-dashed). The h flow out causing the h concentration in the stressed rock decrease and in the unstressed rock to increase (dotted). The unstressed rock becomes positively charged relative to the stressed rock.

9 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 727 are not chemically different but only physically different, e.g., they are the stressed and unstressed subvolume, which contain different concentrations of mobile electronic charge carriers. The h outflow from the stressed rock subvolume has two immediate consequences: it sets up a potential difference, which causes the unstressed rock to become positively charged relative to the stressed rock. This is equivalent to a battery voltage. It creates an electric field, which counteracts the h outflow and will eventually stop it. since the h charge carriers are the only mobile charge carriers in the unstressed rock, they repel each other electrostatically and, hence, will be pushed toward the surface. Accumulating at the surface they produce a positive surface/subsurface charge. The build-up of the surface/subsurface charge can be followed experimentally with a non-contact capacitive sensor as depicted in Figs. 3a and 3b measuring the potential difference between the unstressed rock and the pistons. The pistons are in electrical contact with the stressed rock. Figure 3a shows how to obtain the surface potential off the bare rock surface. Figure 3b shows the configuration to obtain the surface potential off a sand or soil surface on top of the rock. Fig. 3a. Set-up to measure the positive surface potential with a capacitive sensor. Fig. 3b. Demonstration that the h propagate through sand/soil building up a surface charge.

10 728 F. FREUND Fig. 3c. Set-up to measure the battery current with a Cu contact placed directly on the rock. Fig. 3d. Demonstration that the h current also flows through sand and/or soil. The battery circuit is completed by placing a Cu contact on the surface of the unstressed rock as shown in Fig. 3c. This becomes the cathode. Replacing the voltmeter with an ammeter allows us to measure the battery current. Upon pressing a Cu contact on a layer of sand or soil as in Fig. 3d the battery current flows first through the rock, then through sand and soil, even moist sand and soil. This response is consistent with the information provided above that the h charge carriers propagate by using energy levels at the upper edge of the valence band. Since the valence bands of all mineral grains are electrically connected, the h charge carriers are able to spread from grain to grain through solid rock as well as through loosely consolidated sand and soil, and even through moist sand and soil. 3.2 Expected bulk conductivity effects Basically, taking a rock as a semiconductor, its electrical conductivity σ will be given by σ = σ 0 [n µ ] [n µ ], where σ 0 is a frequency factor, n and n are number densities (concentrations) of e and h charge carriers, and µ and µ the mobilities of e and h charge carriers. Figure 2 illustrates that, upon stressing a rock, n and n increase in the stressed subvolume with initially n = n. Because the h charge carriers have the capability to flow out of the stressed rock into unstressed rock, their number density n in the stressed

11 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 729 rock must decrease but increases in the unstressed rock. Hence, with µ and µ constant and µ» µ, we can conduct standard dc conductivity measurements through stressed and unstressed rocks. We expect to see a significant increase in the electrical conductivity through the stressed rock and a lesser, but noticeable conductivity increase through the unstressed rock. 3.3 Expected reactions at the rock air interface The accumulation of positive hole charge carriers at the surface leads to a surface charge layer and an associated electric field. In fact two electric fields are to be considered: the field exterior to the rock surface and the surface/subsurface field caused by the accumulation of the charge carriers in the top atomic layers of the solid, e.g., in the top nm of the surface. The exterior field is amenable to measurement by a capacitive sensor as depicted in Fig. 3. The subsurface charge distribution, electric potential and associated electric field can be calculated using Schottky barrier theory. Figure 4 shows an example of the potential and electric field calculated for the flat surface of a dielectric medium with a dielectric constant ε = 10 Fig. 4. Surface potential and subsurface electric fields calculated for a flat surface of a dielectric medium with a dielectric constant of 10 and two charge carrier concentrations, 10 and 100 ppm (after King and Freund 1984).

12 730 F. FREUND assumed to contain and mobile positive charges per cm 3, equivalent to 10 and 100 ppm, respectively, but no mobile negative charges (King and Freund 1984). The potential is +400 mv and independent of the charge carrier concentration. However, because the charge carriers repel each other electrostatically, they accumulate in the subsurface. The more charge carriers in the bulk, the thinner the subsurface layer and the steeper the associated electric field. Under the conditions chosen here, flat surface and 100 ppm charge carrier concentration, E reaches 400,000 V cm 1. At corners, edges and any sharp point the E will be significantly higher. The electric field can therefore be expected to reach and exceed the ionization threshold of air, which is on the order of V cm 1 (Manna and Chakrabarti 1987). To test this prediction we modify the set-up in Fig. 3c. Using the capacitor plate as an ion collector we apply a bias voltage and replace the voltmeter with an ammeter. Figure 5a depicts the field ionization of air molecules: as expected molecules loose an electron to the rock surface and turn into airborne positive ions. When such positive ions are forming, a current flows through the air gap from the rock surface to the negatively bias ion collector plate. Fig. 5a. Set-up to measure the field ionization of air molecules, here assumed to be O 2, leading to airborne positive ions. Fig. 5b. Set-up to measure free electrons and negative airborne ions during corona discharges symbolized by small flashes of the surface.

13 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 731 If the electric fields at the rock surface increase further, they can accelerate electrons to energies sufficiently large to impact-ionize neutral gas molecules. This would lead to ionization avalanches, triggering corona discharges as reported earlier (Freund 2002). Corona discharges produce free electrons plus positive and negative airborne ions. To measure these processes we apply a positive bias to the ion collector plate as depicted in Fig. 5b. 4. LABORATORY VALIDATION 4.1 DC conductivity through the bulk of the rocks Measuring the dc conductivity of rocks has been reported many times in the literature. It is done in by applying voltages to a pair of electrodes on opposite sides of a rock and measuring the current that flows through the bulk, either at ambient pressure or under various stress levels (Bai et al. 2002, Duba and Constable 1993, Glover and Vine 1994, Kazatchenko et al. 2004, Shankland et al. 1997). To prevent currents flowing along the surface from contaminating the measurements it is common practice to apply a grounded guard electrode. Figure 6 shows the conductivity of dry granite increasing as a function of uniaxial stress. It was measured across a circular area, 50 mm diameter, in the center of a large 9 mm thick granite plate. The I-V characteristic was ohmic over the range up to 1000 V, except for the 0 to 3 V interval, where a non-linear behavior was observed. As demonstrated by Fig. 6 the conductivity Fig. 6. Change in electrical conductivity (solid circles) through a rock volume subjected to uniaxial stress. The conductivity increases sharply at low stress levels (open squares) and then reaches a plateau, increasing only slightly with further loading.

14 732 F. FREUND increases with increasing stress by a factor of about 3.5, rapidly at first, then more slowly, approaching a factor of 5. Any increase in the conductivity as a function of stress under conditions such as depicted in Fig. 6 is normally explained by improved grain-to-grain contacts (Glover and Vine 1992). However, as stated in Section 3.2, we have to consider the possibility that the conductivity increase may be due to the stress activation of e and h charge carriers in the stressed rock volume through break-up of peroxy links. In other words, the increase in the conductivity would arise from an increase in the number density of e and h charge carriers in the stressed rock volume. The insets in Fig. 7a/b depict a different type of conductivity measurement, one that has probably never been performed before: attaching two pairs of contact electrodes to a rock, one underneath the pistons, which apply stress, and another one on an unstressed portion of the rock. Figure 7a shows the quasi-linear increase of the stress from 0 MPa to failure in little more than 20 min. The voltage applied to both sets of contact electrodes was 100 V, well within the range of ohmic behavior. Two major crack events, marked by dotted lines, occurred close to failure. Figure 7b shows the current flowing through the stressed rock volume between the pistons (thick line) and the current flowing through the unstressed rock (thin line). The current through the rock between the piston starts out at 2.1 µa and rises continuously, but not smoothly, to around 40 µa Fig. 7a. Uniaxial stress versus time for current measurement through stressed and unstressed rock under externally applied d.c. voltage. Fig. 7b. Current flowing through stressed rock subvolume (bold line) and unstressed rock subvolume under externally applied d.c. voltage.

15 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 733 close to failure (factor ~20), followed by a sharp maximum up to 63.3 µa (factor ~30) within the last minutes before failure. Remarkably, the current through the unstressed portion of the rock also increases as stress is applied to the stressed portion. Initially the current through the unstressed portion was barely 0.1 µa but rose very rapidly to above 0.8 µa (factor ~8). It strongly fluctuated and dropped back to about 0.3 µa and then precipitously to around or below 0.1 µa, while the current through the stressed rock increased in a stepwise fashion. At the failure the sign of the current flowing through the unstressed rock even briefly reversed. The fact that the current flowing through the unstressed rock increased confirms the prediction implicit in Fig. 2 that charge carriers activated in the stressed rock volume were spreading into the unstressed portion and affected its electrical conductivity. However, given the complexity of the response, the situation is clearly more complicated. In fact, at the time when these experiments were conducted, it appeared inexplicable why the current through the unstressed portion of the rock behaved so strangely, increasing very rapidly at the beginning of loading and then suddenly collapsing about half way through the run. A number of experiments with the same set-up confirmed this overall behavior, though every run produced slightly different results. Follow-up work described further below has since provided some insight into the processes that are mostly likely responsible for the complicated current flow through the unstressed portion of the rock as a function of the stress. 4.2 Battery currents flowing through the bulk of the rocks For electrical conductivity measurements external voltages are applied, often artificially high, to pull charge carriers through the sample. In nature, however, there are no artificially high voltages, at least not under near dc conditions. If voltages develop, they are generally low. This applies to the battery voltages that develop between different portions of a volume of rock subjected to different stress levels (Takeuchi et al. 2006). To demonstrate the rock battery we apply Cu contacts to both ends of a slab of a rock, connect them through a wire and an ammeter, and simply load one end (Freund et al. 2006). Figure 8a shows the current generated when a slab of granite, 4 m long, about cm 2 cross section is loaded between two pistons, 20 cm diameter, insulated from the press. The load was increased within 5 min from 0 to 60,000 lbs, equivalent to about 20 MPa, 10% of the fracture strength. The load was held constant for 5 min and taken back to 0 over 5 min. A current began to flow immediately upon loading and increased rapidly. It saturated at less than 5% of the fracture strength reaching a maximum around 1 na, then went through a brief maximum-minimum

16 734 F. FREUND Fig. 8a. Current versus time flowing through a 4 m long granite loaded/unloaded at one end. Fig. 8b. Outflow current a stressed gabbro tile over 10 hrs at constant load (48 MPa). sequence. Under constant load the current stayed at the elevated level, decreasing slowly. Upon unloading the current stayed high at first then decayed rapidly when the stress level dropped below 5% of the fracture strength and returned to the slowly decreasing background after some time. The current that flows through the rock is limited by the internal resistance R, which is constant at constant temperature T. The self-generated battery V is a function of the rate of loading and is found to be higher at high stress rates (Takeuchi et al. 2006). The reason is that, during stressing, different types of h charge carriers are activated, characterized by different decay times. Short-lived h appear and disappear during rapid loading. Some h remain active only for seconds or minutes, others for longer. This behavior is illustrated in Fig. 8b, showing the outflow of current from a 10 cm 3 rock volume at the center of a cm 3 gabbro tile, loaded within 5 s from 0 to a constant stress level of 48 MPa, approximately 20% the fracture strength of this rock. On the scale of this plot the stress-activated current starts around 250 pa and decays over 10 hrs, here shown in a log I versus time t plot. The three straight sections suggest stressing activates three types of h charge carriers with different lifetimes increasing from a few hours to several days. Long-term experiments have indicates that some h charge carriers remain active for more than 2½ months provided the stress level on the rock is maintained or increased. Results from fast loading experiments, up to 20 MPa s 1 from 0 to 48 MPa, indicate that even shorter-lived h charge carriers are activated under these conditions, which give rise to initial voltage pulses and brief current pulses up to 100 stronger as the currents shown in Fig. 8b.

17 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 735 Understanding whether and how currents can flow through rocks at depth is intimately linked to the question of EM emissions, in particular ELF/ULF emissions coming from deep below. The crucial point is battery circuit closure which is a prerequisite for allowing sustained h currents to flow. The reason is that, if h charge carriers only flow out of a stressed rock and spread into the surrounding unstressed rocks without circuit closure, the build-up of the battery voltage will soon put an end to such the h outflow. Therefore, as long as the battery circuit is not closed, allowing for a return current of equal magnitude, the h outflow current can only be transient and weak. Any persisting stress-activated h current requires a process by which electrons that are co-activated in the stressed rock volume follow suit and also flow out. Somewhere along their path the electrons must recombine with the h to close the current loop. In the laboratory it is easy to close the battery circuit. As depicted in Fig. 3c/d, we do it by running a metal wire from the pistons or a Cu contact on the stressed rock volume (acting as anode) to a Cu contact on the unstressed portion of the rock (acting as cathode). The h charge carriers traveling through the rock will then recombine with the electrons traveling through the metal wire. In the field, in the Earth s crust, closure of the battery circuit between stressed and unstressed, or less stressed rock subvolumes is more difficult to achieve. Three scenarios have been identified: Circuit closure is achieved by electrons in the stressed rock volume flowing downward in the crust to reach the postulated boundary where, at a temperature of about 500 C, equivalent to about km depth, the rocks turn from a p-type semiconducting state to an n-type state, e.g., where rocks are hot enough to be able to conduct electrons. Circuit closure is achieved by h charge carriers encountering a rockwater interface where they convert H 2 O to H 2 O 2, while the actual current continues to flow due to the electrolytical conductivity of water, in particular of saline water. If the local conditions are such that an electrolytically conductive path exists, which connect back to the stressed rock volume, circuit closure can conceivably be achieved via this mechanism. Circuit closure is achieved by currents flowing through the air, when massive air ionization takes place at the rock-to-air or ground-to-air interface as will be discussed in Section 5.2. The first possibility of closing the battery circuit by extending the stressed subvolume downward into the hot and ductile mid-crust has been discussed elsewhere (Freund 2007a, b). Space is too limited to address this scenario. The same is true for the second possibility, closure of the battery circuit through the electrolytical conductivity of brines, which has been sug-

18 736 F. FREUND gested recently on the basis of laboratory experiments aimed at understanding what happens at the rock-water interface (Balk et al. 2009, Freund 2009). Here the third possibility will be discussed, which involves processes triggered by the arrival of h charge carriers at the Earth s surface. 4.3 Charge carriers arriving at the surface Many pre-earthquake signals point to processes taking place at the Earth surface or the ground-to-air interface. The reported phenomena include Various atmospheric effects, Perturbations in the ionosphere, Luminous phenomena, often called earthquake lights, Enhanced infrared emission from around the epicentral region seen in satellite images, Radon emanation from the ground. As indicated in Fig. 3a-c and Figs. 4a,b the positive hole charge carriers, which flow out of the stressed subvolume, repel each other electrostatically. They are expected to do the same in the semi-infinite half-space the Earth surface. Hence, the positive holes have a strong tendency to be pushed to the surface, where they form thin surface/subsurface charge layers. The electric fields associated with such surface/subsurface charge layers are very steep. Theoretical methods used to address boundary layers in Schottky diodes allow us to calculate the electric fields associated with surface/subsurface charge layers (King and Freund 1984). These electric fields are microscopic on the nanometer scale. The calculated example given in Fig. 5 shows that they can reach high values even for an atomically flat surface and for moderate charge carrier concentrations in the bulk. At corners and edges, e.g., at any topographic high points, the h will accumulate in still higher concentrations and produce electric fields that are expected to exceed the ionization threshold of air, typically V cm 1 (Manna and Chakrabarti 1987) Surface potential Though the surface/subsurface electric fields are not amenable to direct measurements, the surface potential can be obtained with a capacitive sensor as sketched in Fig. 3a,b (Takeuchi et al. 2006). Figure 9 shows how the surface potential evolves during loading of a block of gabbro up to failure. The measurement was conducted inside a Faraday cage, cm 3, with bellows to transmit the load to one end of a cm 3 slab of gabbro. In order to achieve sufficiently high stress levels we used steel pistons with hardened teeth that act as stress concentrators allowing us to bring the rock to the breaking point with relatively modest loads on the order of 45,000 lbs, about 20 tons (Freund et al. 2009). The capacitive sensor consisted of a

19 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 737 Fig. 9. Evolution of the surface potential of a gabbro loaded at one end at a constant rate in a hydraulic press using pistons with teeth causing massive deformation on a local scale and eventual failure of the rock. Because of the use of pistons with stress concentrators the applied load is given, not the stress in MPa. Immediately upon loading the surface potential becomes distinctly positive, then fluctuates and collapses to reverse to weakly negative, still with continuing fluctuations. (Dotted line: unstable initial surface charges caused by ever to slight stresses acting on the rock). 200 cm 2 Al plate about 5 mm above the rock surface, extending slightly over the edges of the block. The surface potential is extremely sensitive to slight mechanical stresses acting on the rock by, for instance, the platen of the hydraulic press inching upward at the beginning of the run (dashed section of the curve). As the hydraulic press bears down on the rock, the surface potential over the unstressed rock shoots up immediately to about +3 V, significantly higher than the +0.4 V in Fig. 4 calculated for an atomically flat surface. The full surface potential is reached at relatively modest load, 5-10% of the load needed to achieve failure. Figure 9 also shows that, while the load continues to increase, the surface potential fluctuates. These fluctuations suggest some form of instability affecting the build-up of a steady positive surface/subsurface charge beyond +3 V. Around the same time as the surface potential fluctuates, the initially smooth loading curve is interrupted by steps marking episodes where the teeth of the pistons sink into the rock. Further on, close ½ to the load necessary to achieve failure, the positive surface potential suddenly collapses, turns negative, but continues to fluctuate. Experimentally every run produces slightly different results, but the voltage versus load and load versus time curves shown in Fig. 9 are typical.

20 738 F. FREUND To build up a surface potential of +3 V or higher a large number of h charge carriers has to flow out of the stressed rock at the beginning of the loading. According to Fig. 4a,b (King and Freund 1984) the surface/subsurface charge layer, which the h generate, may be only a few nm thick but it is associated with a high field, probably exceeding the field strength > 10 6 V cm 1 needed to ionize gas molecules in contact with the rock surface (Manna and Chakrabarti 1987). Hence, the surface potential fluctuations seem to point to a process involving the ionization of air molecules at the rock surface. Evidence for such a process will be presented in Section Why the positive surface potential suddenly collapses and turns negative, while continuing to fluctuate, points at follow-on processes to be discussed in Section Airborne positive ions The formation of positive airborne ions at the rock surface can be studied with the same set-up as used before for the surface potential measurements, by replacing the voltmeter with an ammeter and applying a negative bias to the capacitive sensor, thus turning it into a collector plate for positive airborne ions. No positive airborne ions form during the early stages of the loading as shown by Fig. 10a. When the load reaches values within the range where we can reasonably assume that the surface potential has increased to around +3 V, an ion current starts to flow between the rock surface and the collector plate. The current flows in pulses, which appear to be related to the surface potential fluctuations around +3 V seen in Fig. 9. The current pulses continue into the load range where we can reasonably assume that the surface potential has reversed to negative values and continues to fluctuate. Occasionally the current pulses are correlated with periods when the teeth of the pistons sink into the rock causing massive local deformation, but the correlation is weak. Positive ion current pulses may also occur without the episodes of massive local deformation of the rock. The currents measured with the metal ion collector plate can reach values around 20 na as shown in Fig. 10a. This is equivalent to an airborne ion production rate of about 10 9 s 1 cm -2. At the end of run No. 37, when the rock failed, the ion concentration in our Faraday cage, hence the ion current rose sharply to more than 425 na as shown in Fig. 10b and decayed exponentially over the next s. During other experiments such as depicted in Fig. 10c less intense positive ion currents were recorded, while the load increased smoothly. This run was marked by a single sharp ion emission pulse in the latter part of the loading program, rising to about 185 na. Occasionally, as shown in Fig. 10d, the positive ion emission remains low over the course of most of the loading cycle.

21 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 739 Fig. 10a. Generation of positive airborne ions upon loading a gabbro at the far end. Run #37. Open circles: Load. Bold line: Ion current. Inset: Set-up for measuring air ions. Fig. 10b. Whole run #37. Open circles: Load. Bold line: Ion current. Inset: Setup for measuring ± ions. Very large positive airborne ion pulse generated during rock failure. Fig. 10c. Load curve (thin dotted line) and positive airborne ions produced by a gabbro. Left inset: weak early ion current pulse; right inset: exponential decay after rock failure. Fig. 10d. Load curve (open circles) and positive airborne ions produced by a gabbro loaded as shown in inset. Note the exponential decay after rock failure. Immediately after failure of the rocks the positive ion current measured by the collector plate always increased sharply, followed by an exponential decay. The most likely cause is that, when the rock fails at the stressed end, opposite to the location of the ion collector plate, the fracture surface releases a large number of airborne ions, which fill the Faraday cage. These ions will drift to the grounded walls and discharge or recombine with their counter ions. Some are being captured by the ion collector plate causing the exponential decay seen in Figs. 10b-d.

22 740 F. FREUND Fig. 11a. Schematic representation of the ion collector plate and field ionization of molecules, here presumed to be O 2 turning into O 2 +. Fig. 11b. Field ionization of air molecules delivers electrons to the rock surface, which cause more h influx from the bulk. The formation of positive airborne ions suggests that air molecules are being ionized at the rock surface. This is an indication that, early in the loading cycle, the surface/subsurface electric field reaches values sufficiently high to field-ionize air molecules in contact with the rock surface. The molecules loose an electron as illustrated in Fig. 11a. They become airborne as positive ions capable of forming an ion current between the rock surface and the negatively biased ion collector plate. Air molecules, which have a low ionization potential such as O 2, are candidates for this process. Topographic heights, corners and edges are the likely places where the critical electric field is reached as illustrated in Fig. 11b Corona discharges Figure 9 provides an example of the positive surface potential, which builds up first upon stressing the rock, suddenly collapsing at higher stress levels and changing to a negative surface potential. This recurring observation indicates that, during loading, the conditions leading to field-ionization of air molecules and formation of positive airborne ions, is followed by conditions leading to the generation of free electrons or negative airborne ions, or both.

23 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 741 Fig. 12a. Generation of negative airborne ions and electrons upon loading a gabbro at the far end. Inset: Photodiode signal of light flash. Fig. 12b. Generation of an RF pulse during loading a gabbro at the far end. The transition from positive to negative surface potential is often very sudden. Figure 12a shows an example where the ion collector plate was positively biased. For most of the early loading cycle no current flows between the rock surface and the collector plate, indicating no negative airborne ions and/or electrons are formed. As the load reaches values closer to failure, the sudden onset of the current across the air gap is accompanied by flashes of visible light. Flashes of light coming off the edge of the rock and lasting about 1 ms has been reported earlier (Freund 2002). Figure 12b, based on data from a different run, shows a radiofrequency (RF) pulse, also similar to what had been reported earlier (Freund 2002). The flashes of visible light and RF pulses are clear indications of corona discharges and, hence, indications that the electric fields at corners and edges reach such high values that they can trigger ionization avalanches in the air. The process probably involves run-away acceleration of electrons that are always available at low numbers in ambient air, due to cosmic rays or radon decay (Sugawara and Sakai 2003, Trakhtengerts et al. 2003). When these electrons reach kinetic energies high enough to impact-ionize neutral gas molecules, they produce positive ions and free electrons, which become available for acceleration and additional impact ionization. During the ensuing formation of small air volumes of highly ionized plasma, enough free electrons are generated to shower onto the rock surface and wipe out its positive charge. The collapse of the positive surface potential leads to the collapse of the electric field, which had accelerated electrons and triggered the

24 742 F. FREUND corona discharges. At the same time, however, as indicated by the arrows in Fig. 11b, h charge carriers continue to flow from the bulk to the surface to replenish the h. The dynamic interplay between h influx from below and the electron showers from above gives rise to rapid-fire sequences of corona discharges and to the continuing fluctuations of the surface potential, even in the realm of weakly negative values, as evidenced by Fig. 9. The sequence of ionization events at the rock surface, in particular the switch from the generation of exclusively positive airborne ions to corona discharges and their profuse production of free electrons, helps us understand the evolution of the surface potential as a function of stress as depicted in Fig. 9. It may also shed light on the observation depicted in Fig. 7b regarding the changes in electrical conductivity (under externally applied voltage) across a slab of unstressed granite subjected to uniaxial stress. More work will be needed to further characterize the complex interactions between the activation and flow of h charge carriers in the bulk and the various ionization processes at the surface Infrared emission There is one other process among the various processes at the rock surface, which deserves special attention. It derives from the observation that it costs energy to break the peroxy bond according to eq. (2). As the h charge carriers flow out of the stressed rock volume and accumulate in a thin subsurface layer, they locally reach high number densities. Under these conditions the h will have a finite probability to recombine. During recombination the h will recover at least part of the energy that had be required to break the peroxy bond. To estimate how much energy is needed to break the peroxy bond we turn to experiments such as depicted in Fig. 1a, where the electrical conductivity σ of a high purity MgO single crystal is plotted as log σ versus 1/T (T in K). Between about 400 C and 600 C the peroxy bonds break, releasing h charge carriers, which increase the conductivity along the approximately straight section with an apparent activation energy of 3.4 ev. The straight log σ versus 1/T sections, measured during intermittent cooling, show that the activation energy for h to conduct the current is 1.0 ev. Hence, the energy needed to thermally activate h by breaking the peroxy bond is ev. This value will be conserved regardless of whether the peroxy bonds are broken through the action of temperature or through the action of dislocations during application of stress. If the energy required to break the peroxy bond is around 2.4 ev, then the energy recovered during recombination of two h will be of the same order or somewhat less, say 2.1 ev. When this energy is deposited into the

25 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 743 newly forming O O bond, the two oxygens, which participate in the recombination, will enter into a vibrationally excited state. The degree of vibrational excitation is very high, equivalent to T K. There are only two ways for this excess energy to dissipate: (i) by disproportionation of the peroxy bond and emission of an O atom, possibly in an electronically excited state, or (ii) by radiative de-excitation through emission of mid-ir photons at energies corresponding to the transition energies of the vibrational manifold that describes the peroxy bond (Ricci et al. 2001). The emission of IR photons was experimentally confirmed by recording the IR spectrum off the surface of a cm 3 block of anorthosite, a nearly mono-mineralic feldspar rock, stressed about 50 cm away from the emitting surface. As soon as load was applied to the rock, the rock surface at the unstressed end began to emit a characteristic IR spectrum overlying the broad graybody 300 K emission. The spectrum consisted of a series of narrow emission bands due to (i) the radiative de-excitation of highly excited O 3 Si-OO-SiO 3 bonds, e.g., tumbling down the quantum stairs of their vibrational manifold, and (ii) secondary excitations of Si-O and Al-O stretching modes, e.g., kicking neighboring bonds and causing them to also emit narrow IR bands due to transitions between higher vibrational quantum levels (Freund et al. 2007). The demonstration of the excess IR emission from the surface of a rock stressed at the far end provides a ground to reassess the thermal anomalies seen in night-time infrared satellite images from areas around epicenters of impending earthquakes. Presently these anomalies are explained by assuming an emanation of CO 2 and other greenhouse gases from the ground due to regional stress-induced microfracturing of rocks, or due to emanation of radon, which would ionize the air, lead to condensation of water and release of latent heat (Pulinets et al. 2006, Saraf et al. 2008, Singh 2008, Tramutoli et al. 2005, Tronin 1999). The laboratory observation of excess IR intensity emitted from a rock surface due to a quantum mechanically controlled emission process provides an explanation, which fits the larger picture of stress-activated h charge carriers in rocks, their capability to spread over long distances through unstressed rock, sand and soil, and their propensity to recombine at the surface (Freund et al. 2007). 5. CORRELATION WITH FIELD OBSERVATIONS The recognition that peroxy defects are ubiquitous in rock-forming minerals and that, upon stress activation, they release positive hole charge carriers provides an entry to a better understanding of many processes linked to the build-up of tectonic stresses in the Earth crust prior to earthquakes.

26 744 F. FREUND As pointed out in the introductory section there are different types preearthquake signals that have been reported. One of their distinctive features is that there seemed to be no single cause for the multitude of physical phenomena. The discovery of h charge carriers in rocks and their interesting properties can change this assessment. 5.1 Air ionization at the ground-to-air interface Enhanced air ionization at the ground-to-air interface has long been discussed in the context of pre-earthquake phenomena (Omori et al. 2009, Pulinets 2007). A rise in air conductivity is thought to influence in characteristic ways processes such as cloud formation (Guo and Wang 2008, Pulinets et al. 2006), ionospheric perturbations (Chen et al. 1999, Hayakawa 2007, Hayakawa et al. 2005, Liperovsky et al. 2000, Pulinets 2007, Sorokin et al. 2006b, Zakharenkova et al. 2007), thermal infrared anomalies (Ouzounov et al. 2006, Ouzounov and Freund 2004, Qiang et al. 1999, Saraf et al. 2008, Singh 2008, Tramutoli et al. 2005, Tronin 2000, Tronin et al. 2004), and other phenomena that might be indicators for impending seismic activity. The unanswered question has always been: if the transient maxima in air conductivity are real and linked to the build-up of tectonic stresses below, what are the underlying physical processes? Two main explanations have been offered, both based on the concept that the build-up of stress in the hypocentral region will lead to stresses in a halo surrounding the hypocenter and cause microfracturing. The first explanation invokes the stresses necessary to cause microfractures and assumes that these stresses would lead to locally high voltages due to the piezoelectricity of quartz crystals, hence, to the ionization of air. The second explanation considers that the opening of cracks during microfracturing would be a mechanism allowing radon to escape from the ground and to ionize the air. 5.2 Piezoelectric effect The concept that microfracturing affects a large region around the future hypocenter is a variant of the dilatancy model proposed in the 1960s (Brace et al. 1966). Dilatancy has been used for some time to explain data from laboratory and field studies (Brace 1975, Dobrovolsky et al. 1979, Hadley 1975, Nur 1974) but has been effectively abandoned in recent years in the light of the fact that there is no evidence for the build-up of sufficiently high stresses to cause microfracturing far from the hypocenter (Johnston 1997, Scholz 2002). The concept that stressed quartz crystals in rocks might produce piezoelectric voltages of sufficient field strength to cause air ionization (Bishop 1981, Tuck et al. 1977) is also not physically supported despite extensive modeling efforts (Ogawa and Utada 2000).

27 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS Radon emanation The idea that radon gas would emanate from the ground in large quantities to significantly ionize the air has been widely used to explain pre-earthquake ionospheric perturbations (Ondoh 2003, Oyama et al. 2008, Pulinets 2007, 2009) and thermal IR anomalies (Ouzounov et al. 2006, Qiang et al. 1991, Tronin 2002, Tronin et al. 2004, Xu et al. 1991). The scenario generally envisioned can be described as follows: As stresses in the hypocentral region build up, a halo of pervasive microfracturing spreads outward through the crust, allowing radon gas trapped in uranium- and radium-bearing rocks to escape and percolate upward through rocks and soil. The radioactive decay of 222 Ra atoms through high energy alpha particle emission causes the formation of about 10 5 ion pairs cm 3 and, hence, an increase in air conductivity. The radius r of the so-called earthquake preparation zone, where microfracturing will occur is thought to scale with the magnitude M of the impending earthquake as r = M km (Dobrovolsky et al. 1979). This implies that the diameter of the preparation zone for an M = 5 seismic event would extend over km and be significantly larger for more powerful earthquakes. The ionospheric perturbations are explained by the effect that increased air conductivity in the lower atmosphere over such a wide region would have on the ionosphere and on the vertical distribution of electrons and ions in the ionospheric plasma. A follow-on idea is that radon emanation from the ground and ionization of the air would lead to condensation of water vapor on the airborne ions and to the release of latent heat of condensation. This would heat up the air volume and cause the thermal anomalies reported from night-time infrared satellite images (Ouzounov et al. 2006, Qiang et al. 1991, Tronin 2002, Tronin et al. 2004, Xu et al. 1991). Figure 13 shows the rate of airborne ionizations under fair weather conditions in the lower 10 km of the atmosphere (Gringel et al. 1986, Hoppel et al. 1986). The contribution from cosmic rays is near-constant below 1000 m, amounting to ~2 ions cm 3 s 1. The contribution from backscattered beta and gamma rays and ionizing radiation from radioactive decays in the Earth, except for radon, is on the order of 20 ion cm 3 s 1 at the surface, decreasing to nil in the first m. The contribution from radon varies from ~30 ions cm 3 s 1 to barely 300 ions cm 3 s 1 at the Earth s surface and decreases to less than 5 ions cm 3 s 1 within the first 1000 m. Field data near the Earth surface or in shallow holes indicate that, prior to seismic activity within a radius up to about 100 km, the radon concentrations can increase by a factor up to 10 over periods of days to months and decrease again after the seismic activity has subsided (Chyi et al. 2002, İnan et al. 2008, Nagarajaa et al. 2003, Tsvetkova et al. 2001). Occasionally the

28 746 F. FREUND Fig. 13. The rate of air ionization in the first 10 km of the atmosphere under fair weather conditions due to cosmic rays, back-scattered beta and gamma radiation from the ground and radon emanation (after Gringel et al. 1986). radon concentrations increase by only 20-30% above longtime background values (Yasuoka et al. 2009). In some cases radon variations are found to be narrowly constrained to active sections of a fault on the scale of tens of meters (King 1980). 5.4 Massive air ionization At the same time, there are reports from the field of episodes of massive air ionization exceeding 10 5 ions cm 3, mostly lasting from a few hours to up to 2 days (Hattori et al. 2008, Wasa and Wadatsumi 2003). The airborne ions are positive, sometimes exclusively positive, at other times both positive and negative. The data suggests that the airborne ion concentrations are often 2-3 orders of magnitudes higher than those, which can reasonably be associated with changes in the radon emanation from the ground and they tend to spread over tens, possibly hundreds of kilometers. An episode of strongly enhanced air conductivity was also observed in California during the lead-up to the M = 5.4 Alum Rock earthquake of Oct. 30, In the weeks prior to this event an air conductivity sensor at one of the CalMagNet stations situated only 2 km from the epicenter recorded short pulses. About 20 hrs before the earthquake, a sustained period of massive air ionization saturated the sensor for over 13 hrs (Bleier et al. 2009). Simultaneously, an identical air conductivity sensor at another Cal- MagNet station at Portola Valley, about 35 km from the Alum Rock epicenter, recorded elevated air conductivity as well, supporting the regional character of this phenomenon.

29 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 747 In addition to the 13 hrs long episode of air ionization before the Alum Rock earthquake, the same CalMagNet station also recorded at 50 min long episode of intense ULF emission (Bleier et al. 2009). Such an ULF signal requires a strong electric current, e.g. closure of the battery circuit. It is worth noting that the ULF emission occurred around the midpoint of the 13 hrs long episode of air ionization, possibly marking the time of most intense ionization. In this case, current closure might have been achieved by an ion current flowing through the air, allowing an equivalent h current to flow in the crust. Conceivably, the air ion current would flow from the periphery, where positive airborne ions were generated at the Earth s surface, to a central area above the hypocenter, where the electric fields at the ground-to-air interface become high enough to trigger corona discharges and change the surface potential to negative. The positive surface potential around the periphery and the negative surface potential closer to the epicenter would create a lateral potential difference, e.g. an electric field along which positive airborne ions can drift. As mentioned in Section the rate of positive air ion production measured in the laboratory can reach values as high as 10 9 s 1 cm 2, occasionally ~10 10 s 1 cm -2. Provided the conditions in the field are the same, this translates to air ion currents on the order of A km 2. Depending on how many km 2 of the area around the periphery produce positive airborne ions and how large the area is close to the epicenter, where the surface potential turns negative, the air ion currents available to close the battery circuit can conceivably be quite large. 5.5 Cloud formation Injection of ions into the air at ground level in the field will have other predictable consequences. The airborne ions can act as condensation nuclei for water droplets, leading to fog and haze. The latent heat released during condensation will warm the air and cause it to rise (Dunajecka and Pulinets 2005). Under the right relative humidity conditions clouds can form. If the ionization the ground-to-air interface persists for some length of time, such a pre-earthquake cloud can be expected to remain stationary relative to the source of airborne ions, for instance over a fault zone, and relative to other clouds, which may drift with the prevailing winds. Cloud formation as potential indicator of impending seismic activity has been mentioned repeatedly in the literature (Lu 1988, Ondoh 2003, Tramutoli 1998). A few days before M = 6.7 Bam earthquake of Dec. 26, 2003 in Iran a distinct cloud formed above the future epicenter, as documented by MeteoSat images (Guo and Wang 2008). It persisted for about 24 hrs, apparently reached high altitudes and drifted ESE for over 2000 km

30 748 F. FREUND to NW India. There is no confirmation of increased air ionization in the area near Bam from this time, due to the lack of ground stations. 5.6 Ionospheric perturbations Numerous examples of ionospheric perturbations have been reported (Chen et al. 2004, Depuev and Zelenova 1996, Hayakawa et al. 2006, Liu et al. 2004, 2006, Maekawa et al. 2006, Oyama et al. 2008, Pulinets et al. 2005, Singh 2008, Trigunait et al. 2004, Zakharenkova et al. 2007). They occur in the form of changes in the F-region, at altitudes greater than 160 km, and are typically reported as increased Total Electron Content (TEC). They include changes in the subionospheric VLF/LF propagation and shifts in the terminator times (Hayakawa 2007). Though pre-earthquake ionospheric perturbations are statistically well documented (Chen et al. 2004) and distinct from the model of the International Reference Ionosphere (Bilitza 2001), their cause is still not fully understood. Air ionization due to radon released at the ground level has been forcefully called into question (Rishbeth 2007, 2006). However, the injection of massive amounts of positive ions at the ground-to-air interface due to field-ionization of air molecules as described in this report is expected to lead to an upward expansion of highly ionized air and to changes in the vertical electric field between the ground and the lower edge of the ionosphere. The upward drift of airborne ions will form a vertical current, which may reach values on the order of A km 2. Such currents, even if they occur only episodically, may play an important role in the global electric circuit (Rycroft et al. 2008). In addition perturbations are expected to occur in the uppermost atmosphere and lower ionosphere (Sorokin et al. 2006a, b). Sky radiation measurements have indicated anomalous values several days before a strong earthquake, both in the red and the green parts of the spectrum. Before and after the M = 7.4 Petatlán, Mexico earthquake of March 14, 1979, for instance, large fluctuations in sky luminosity were recorded, at times exceeding the standard value for a transparent atmosphere (Araiza-Quijano and Hernández-del-Valle 1996). A possible explanation is that massive injection of positive ions at the ground level and their upward expansion led the electrons at the lower edge of the ionosphere to be pulled down and to interact with the uppermost atmosphere, causing the excitation of the 630 and 557 nm emission lines of atomic O. 5.7 Geo-electric anomalies The electrical resistance of the ground has been monitored for many years, in particular in China, using ground electrode arrays derived from the Schlumberger geoelectric sounding techniques (Patra and Nath 1999). Distinct

31 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 749 changes in the resistivity of the soil have been noted, apparently linked to impending earthquake activity. Figure 14a shows the principle of the measurement where the distance AB is typically 1 km and the distance MN on the order of 200 m (Qian et al. 1983, Zhao and Qian 1994). The changes in resistivity are usually explained as the result of changes in the pore volume of basement rocks due to regional stresses. It is assumed that the stresses squeeze out pore water, thereby affecting the moisture level in the soils. However, no detailed understanding seems to be available as to how this process works, in particular in view of the fact that rainfall appears not to totally mask the changes of the pore water level assumed to come from below. Often a tidal component can also be seen in the soil resistivity data. An example of small but seemingly reliable ground resistivity changes observed in the week before the M = 7.9 Wenchuan earthquake of May 12, 2008 in Sichuan Province, China, is depicted in Fig. 14b (Qian et al. 2009). Small magnitude excursions occurred on a daily basis, followed by a relatively large drop in the ground resistivity a few hours before the earthquake. More ground resistivity data will be needed to validate the link to impending seismic activity and to understand the underlying mechanism. Though the most favored explanation invokes microfracturing within the assumed earthquake preparation zone (Dobrovolsky et al. 1979), the work presented here points to h charge carriers, which become activated at depth and spread to the Earth surface. Thus the observed small changes in soil resistivity exemplified in Fig. 14b may be due to the arrival of waves of h charge carriers from below. This as yet tentative explanation provides a more Fig. 14a. Schematic of geoelectric sounding and soil conductivity measurements by means of a Schlumberger array.

32 750 F. FREUND Fig. 14b. Changes in the electric resistivity of the soil prior to the M = 7.9 Wenchuan earthquake (after Qian et al. 2009). robust physical basis for the observed effects than microfracturing of the rocks below the soil as a result of stresses transmitted over large distances (Qian et al. 1983, Zhao and Qian 1994). 5.8 Radon emanation again Understanding and validating radon emanation in relation to impending seismic activity has been an elusive goal as indicated in Section 5.3. If stress-activated h charge carriers are the mostly likely agents for the range of pre-earthquake phenomena discussed in Sections , they may also play a role in the release of radon from the soil. Figure 15 shows the 2002 record measured at Armutlu, one of a soil radon station network along the North Anatolian Fault in Turkey with the vertical arrows marking M > 4 and lesser earthquakes within about 100 km (İnan et al. 2003, 2008). While there are apparent correlations between the radon count rates and the regional seismicity, details are not clear. Sometimes, as in January February 2002, there seems to be no regional seismicity inspite of a broad maximum of radon release. Conversely, as in July 2002, some smaller seismic events occur without recognizable increase in radon emission. At other times M > 4 seismic events occur either soon after a radon peak, just before a radon peak, at a radon peak, or seemingly at random. During the last quarter of 2002 the regional seismicity around the Armutlu station died down and the radon dropped to background levels. Observations like these indicate that the relationship between radon release and local or regional seismicity is far from simple.

33 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 751 Fig. 15. Continuous record for 2002 of the radon soil sensor at the ARMR station along the North Anatolian Fault in Turkey. The bold arrows 1, 2 and 3 refer to M > 4 earthquakes, the thin unlabeled arrows refer to smaller magnitude earthquakes within 100 km (İnan et al. 2003). Figure 16 reports an observation that may add to a better understanding of the interplay between stress and radon release. In the case shown here sudden stress pulses were applied through large explosions in a quarry about 1.5 km from the Gemlik station, one of the above-mentioned soil radon stations along the North Anatolian Fault in Turkey. After the first quarry explosion at 14:51 LT on February 19 the radon count rate at Gemlik started to increase sharply about 3 hrs 25 min later, at 18:15 LT. After the second explosion at 18:40 LT on February 20 a similar increase was recorded with a time delay of 6 hrs, at 22:30 LT (İnan 2009). Data from more such cases will have to be collected to establish whether or not there is a causal link between quarry explosions and the radon release at a near-by soil radon station. If such a causal link exists, the observations depicted in Fig. 16 would rule out that the stress pulses of the propagating P or S waves from the quarry explosions would trigger the increased radon emanation at this particular distance, 1.5 km. However, from the laboratory experiments described here it is certain that any large quarry explosion will activate h charge carriers in a finite volume of rock. This raises the following questions: Will these h charge carriers spread out from the activation volume, e.g., the quarry? How fast will they travel? Can they trigger the release of radon from the soil about 1.5 km from the explosion site? There is little doubt that, once activated, the h charge carriers will flow out of the stressed rock volume and spread into the surrounding unstressed

34 752 F. FREUND Fig. 16. Abrupt changes in the radon release at the Gemlik station along the North Anatolian Fault, Turkey, about 1.5 km from a quarry where explosions were set off to produce gravel for road construction (courtesy of Sedat İnan). or less stressed rocks. On the basis of impact experiments we know that the phase velocity is on the order of 200 m/s (Freund 2002, Hollerman et al. 2006), consistent with the speed, with which a charge pulse can propagate by way of a phonon-assisted electron hopping mechanism (Shluger et al. 1992). However the drift velocity, e.g., the speed with which h charge carriers can actually travel in an electric field and/or concentration gradient, will be much less. To address the question how h charge carriers may interact with radon, we need to consider the chemical reactivity of radon. Radon is a noble gas. As such it is widely believed to be chemically inert and unreactive. However, with the possible exception of helium, all noble gases exhibit chemical reactivity (Li et al. 2008). As the heaviest of the noble gases, radon is particularly prone to chemical bonding. Likely bonding partners are fluorine and triple-bonded carbon, C=C, the latter giving rise to acetylene-derived compounds, which can be stable at ambient surface temperatures (Gerber 2004). Thus there is the possibility that, when 222 Rn is generated through radioactive decay of 226 Ra (radium) in igneous rocks and/or detrital igneous mineral grains in sedimentary rocks, it will not diffuse or percolate freely through the overlying soil. Instead, the radon atoms can be expected to interact with the soil in ways similar to the interaction of many other gases in chromatography columns (Willett 1987). Soils often contain organic compounds with triple-bonded carbon, which will be the most efficient in retain-

35 UNIFIED SOLID STATE THEORY FOR PRE-EARTHQUAKE SIGNALS 753 ing radon (Conklin 2006). If triple-bonded carbon compounds which retain Rn are present, the rate at which 222 Rn can emanate from soil will be determined by (i) its average retention time in the soil, (ii) the interaction of positive hole charge carriers with the 222 Rn atoms chemisorbed in the soil, and (ii) its intrinsic halflife of ~3.8 days. To model the observations presented in Fig. 16 we may assume that, after the explosions at the quarry, h charge carriers spread outward from the stress-activated volume, traveling along their concentration gradients. If their average drift velocity was on the order of 6-12 cm s 1, they would have arrived 3-6 hrs later at the measuring station 1.5 km away. To understand what may happen at the measuring station we note that the h are not only mobile electronic charge carriers but also highly oxidizing radicals (Balk et al. 2009). They can be expected to interact with organic compounds in the soil, preferentially with those that are most highly reduced. Compounds with triple-bonded carbon fall into this category. The h can be expected to oxidize carbon triple bonds to double bonds as depicted in Fig. 17. Since doublebond carbon compounds cannot retain radon (Gerber 2004), the 222 Rn atoms would then become free to percolate through the soil and eventually escape through the surface. This as yet tentative explanation proposes a connection between radon release from the soil and regional tectonic stresses that is not predicated on the assumed microfracturing of rocks in the underground. As outlined in Section 5.2 microfracturing over a wide area is part of the dilatancy model (Brace et al. 1966), which has been used extensively for some time to explain data from laboratory and field studies (Brace 1975, Dobrovolsky et al. 1979, Hadley 1975, Nur 1974), but has been effectively abandoned in view of the fact that little or no evidence exists for pervasive microfracturing far from the hypocenter (Johnston 1997, Scholz 2002). Fig. 17. Schematic representation how radon may be retained by triple-bond organic soil compounds, but released when the carbon triple bonds are oxidized to double bonds through h capture.