A hypothesis for the seismogenesis of a double seismic zone

Transcription

1 Geophys. J. Znt. (1995) l23,71-84 A hypothesis for the seismogenesis of a double seismic zone Honn Kao and Lin-gun Liu Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan 115, ROC Accepted 1995 April 3. Received 1995 April 3; in original form 1994 August 11 INTRODUCTION SUMMARY The seismogenesis of a double seismic zone, in particular the lower layer of a double seismic zone, has not been adequately explained in the literature. On the basis of seismic data and geothermal structures along three well-studied cross-sections in the Kuril-Kamchatka and Japan subduction zones, we investigate the temperature/pressure conditions associated with seismogenic structures of the double seismic zones. The corresponding TIP loci seem to suggest that earthquakes observed in the lower layer and in the lower part (below approximately km) of the top layer of a double seismic zone were caused by metastable phase transition-a mechanism similar to that responsible for deep-focus earthquakes only at lower temperature/pressure conditions. Under this hypothesis, the wedge-shaped configuration of a double seismic zone is interpreted to represent the loci of the kinetic boundary of the phase transition. According to theoretical/experimental studies and the constraints imposed by our observations, a likely candidate for such a phase transition is the metastable Al-rich enstatite decomposing into the assemblage of Al-poor enstatite plus garnet. Earthquakes in the upper part of the top layer were most probably due to conventional mechanisms such as dehydration of subducted materials and/or facies change from basalt to eclogite. That the top layer involves more than one seismogenic mechanism is also implied by the distinct behaviour of seismicity in the vicinity of 130 f 20 km. Because the presence of deviatoric stress is critical to the reaction rate of a metastable phase transition, it is inferred that single seismic zones are also caused by the same mechanisms, except that the implicit layer of a supposed double seismic zone is missing, due to the insufficient amount of appropriate metastable minerals or to the lack of appropriate deviatoric stresses in the source region. Key words: double seismic genesis. Seismogenesis, the occurrence of earthquakes, has been one of the central platforms in geosciences for decades. Owing to the interdisciplinary nature of the subject, researchers have approached it in various ways, including studies in the fields of seismology, geodynamics, rock mechanics, mineral physics, and so on. Most shallow earthquakes are now understood as a manifestation of the frictional instability of geological materials, or faulting (e.g. Stuart & Mavko 1979; Ruina 1983; Shimamoto 1985; Tse & Rice 1986; Ord & Hobbs 1989; Scholz 1990; Rice 1993). On the other hand, the same seismogenic mechanism cannot account for earthquakes that occurred within subduction zones at depths greater than approximately 50 km, because the corresponding pressure and temperature conditions in the mantle zone, enstatite, metastable phase transition, seismo- would not allow any brittle or frictional behaviour to take place (e.g. Jeffreys 1929; Griggs & Hardin 1960; Scholz 1990). Consequently, seismogenesis of intermediate- and deep-focus earthquakes must invoke other mechanisms which are appropriate for the temperature and pressure conditions in the deep mantle. From a seismological point of view, seismic patterns exhibited by intermediate-depth earthquakes are by no means uniform and have been the focus of intensive studies since the discovery of Wadati-Benioff zones. With the establishment of the World-Wide Standardized Seismographic Network (WWSSN) in the early 1960s and the rapid advent of seismic source studies, earthquakes with focal depths greater than -50 km are now believed to reflect the state of internal deformation within a subducted lithosphere (e.g. Isacks & Molnar 1969, 1971), shown by consistent RAS 71

2 72 H, Kao and L. Liu patterns of downdip compressional or extensional focal mechanisms. However, the simultaneous presence of both downdip compression and extension at intermediate depths (the so-called double seismic zone) was reported for many, if not all, subduction zones around the world [see Kao & Chen (1994) for a brief update]. The downdip compression is usually observed along the top layer of seismicity, and the downdip extension along the bottom layer. The two layers are separated by km at a depth of km and gradually merge into one single Wadati-Benioff zone at depths greater than approximately km. The state of strain shown by a single seismic zone at intermediate depths is interpreted conventionally as a result of the interaction between the subducting slab and its surrounding mantle (i.e. the stress-guide model; Isacks & Molnar 1969, 1971). On the other hand, the origin of the contrasting state of strain for a double seismic zone has been the centre of debate for years. Possible models include unbending of the subducting slab (e.g. Engdahl & Scholz 1977; Isacks & Barazangi 1977; Samowitz & Forsyth 1981; Tsukahara 1980; Kawakatsu 1986a), thermoelastic stresses (e.g. Fujita & Kanamori 1981; House & Jacob 1982; Hamaguchi, Goto & Suzuki 1983; Goto, Hamaguchi & Suzuki 1985) and sagging of the subducted slab (e.g. Sleep 1979). Based on precisely determined source parameters from the inversion of body waveforms, Kao & Chen (1994) found that the double seismic zone along the Kuril- Kamchatka arc exists only in the central portion of the arc and is, in fact, an overlap of two single seismic zones: a downdip compressional layer of seismicity existing along the northern two-thirds of the arc and another downdip extensional layer along the southern two-thirds. They interpreted the lateral changeover from a double seismic zone in the central portion to single seismic zones towards the two ends as a consequence of the changing state of stress transmitted from the deep slab (i.e. a combination of the,traditional stress-guide model and a double seismic zone model), based on the strain segmentation found at depths of km by Glennon & Chen (1993). Despite all the recent progress in delineating the configuration of seismogenic structures at intermediate depths and the possible relationships among them, seismological studies alone cannot unravel what seismogenic processes are responsible for the occurrence of intermediate-depth earthquakes; they can, however, provide crucial observational constraints. Meanwhile, significant insight into the seismogenesis for deep-focus earthquakes (>300 km) has been gained, particularly from results of laboratory experiments under high-temperature and high-pressure conditions similar to those found in the mantle. For example, Bridgman (1945) proposed that polymorphic phase changes could result in sudden volumetric contraction and the release of elastic waves. Liu (1983) investigated the various types of energy dissipated from metastable phase transformations for a number of silicates and suggested that they were capable of generating seismic waves, a hypothesis later proved by experimental studies (e.g. Green et al. 1992; Meade & Jeanloz 1989). It has generally been inferred that the shear instability due to the development of anti-cracks from metastable olivine to spinel (or modified spinel) phase transformation in the deep mantle is responsible for the seismogenesis of deep-focus earthquakes (e.g. Vaisnys & Pilbeam 1976; Kirby 1987; Green & Burnley 1989; Green et al. 1990; Burnley, Green & Prior 1991; Kirby, Durham & Stern 1991). The same phase transformation, however, cannot account for intermediate-depth earthquakes, including the double seismic zone, simply because olivine is not under a metastable state between 50 and 300km. Many studies advocated that fluid released from dehydration of hydrous silicates (e.g. serpentine) can induce brittle deformation (e.g. Raleigh & Peterson 1965; Raleigh 1967) and/or amorphization of crystal structures (e.g. Meade & Jeanloz 1989, 1991)-mechanisms that could account for the seismogenesis of intermediate-depth earthquakes. However, as pointed out by Kao & Chen (1994), the limited thickness of the oceanic crust, where sepentine is present, fails to explain the wide separation ( km) between the top and bottom layers of a double seismic zone at an intermediate depth. In the present study, we investigate the temperature/ pressure conditions corresponding to a double seismic zone, based on precisely delineated seismogenic structures and geothermal models. Specifically, we consider the double seismic zones between depths of 50 and 200 km along three profiles in the Kuril-Kamchatka and Japan arcs, which are two of the best-established examples in the world. Combining recent theoretical and experimental progress in mineral physics, we propose the hypothesis that seismogenesis of a double seismic zone is most probably due to metastable phase transition-a mechanism similar to that proposed for deep-focus events. Under this hypothesis, the wedge-shaped double seismic zone is interpreted as corresponding to the kinetic boundary of the metastable phase transition. We also discuss a number of criteria that such phase transition(s) must satisfy. It is suggested that metastable Al-rich pyroxene (the second most abundant mineral in the mantle, next to olivine) decomposing into Al-poor pyroxene plus garnet (or majorite) could be a likely candidate. Our model predicts a kink in seismicity at depths between 100 and 150km, a phenomenon already observed in many subduction zones. Finally, we will extend our model to explain a variety of observed seismic patterns, including single Wadati-Benioff zones found in many other regions. SEISMOGENIC STRUCTURES AT INTERMEDIATE DEPTHS The occurrence of earthquakes along subduction zones generally follows systematic patterns, although seismogenic structures might vary in detail from one to another. As demonstrated by Kao & Chen (1991), erroneously determined focal depths and mechanisms can lead to complicated seismogenic patterns which are difficult to interpret. Therefore, precisely determined earthquake source parameters are critical to the success of an integrated seismogenic model that incorporates observations from various disciplines. In this section, we will review the characteristics of seismogenic and geothermal patterns in the subducted lithosphere, using two of the best-studied subduction zones in the world as examples (the Kuril- Kamchatka and Japan arcs; Fig. 1). Both subduction zones are known to have double seismic zones at depths between 50 and 200 km and high seismicity along plate interfaces RAS, GJI 123,71-84

3 Seismogenesis of a double seismic zone " 145" 150" 155" 160" 50" 50' 45' 40" 140" 145" 150" 155' 160' Figure 1. A map showing the Kamchatka-Kuril-Japan subduction system. Boxes labelled KP, KS and JH are the locations of seismic profiles shown in Fig. 2. The patterns are considered to be representative of a tectonic setting, where an old oceanic lithosphere subducts beneath a continental overriding plate. Double seismic zones The double seismic zone was first discovered in the Kuril-Kamchatka (e.g. Veith 1974) and the Japan subduction zones (e.g. Umino & Hasegawa 1975; Hasegawa, Umino & Takagi 1978a, b) two decades ago. The existence of double seismic zones was later reported for many other subduction zones, including those of the eastern Aleutian arc (Engdahl & Scholz 1977; Reyners & Coles 1982; House & Jacob 1983), the Mariana arc (Samowitz & Forsyth 1981), the Tonga arc (Kawakatsu l985,1986b), central Peru (Isacks & Barazangi 1977), and northern Chile (Malgrange, Deschamps & Madariaga 1981; Malgrange & Madariaga 1983; Kono, Takahashi & Fukao 1985). As far as detailed seismogenic structures are concerned, the Kuril- Kamchatka and the Japan arcs remain two of the most studied regions in the world. 45' 40' The precise configuration of double seismic zones is one of the most important constraints on the understanding of the seismogenesis of intermediate-depth earthquakes. Figs 2(a) and (b) show two cross-sections (profiles KS and KP) along the central Kuril-Kamchatka arc where the detailed seismogenic structures between 0 and 200 km were delineated from high-resolution teleseismic studies using body waveform inversion (Kao & Chen 1994, 1995). Fig. 2(c) displays an E-W cross-section between 39" and 40"N along Honshu, Japan [profile JH; modified from Fig. 4(b) of Hasegawa et al. (1978a)], where a detailed distribution of seismogenic structures was mapped out using microearthquakes recorded by a densely distributed local seismograph network (e.g. Hasegawa et al. 1978a,b, 1979; Nakanishi 1980; Nakanishi, Suyehiro & Yokota 1981; Hori et al. 1985; Matsuzawa et al. 1986; Obara & Sat0 1988). In general, the most prominent feature of a double seismic zone is the simultaneous presence of two layers of seismicity: earthquakes along the top layer are consistent with a pattern of downdip compressional mechanisms and RAS, CJI 123,Il-84

4 74 H. Kao and L. Liu NW Volcanic Arc Kuril Trench v I I I I I I SE -_ KSll k KS KS F KS L NW KSS A Volcanic Arc A KS18 A KS12 I I I I I m I 3 Kurill 'rench - SE -I 100 km 200 km 100 km 200 km Km Figure 2. (a) A NW-SE cross-section (without vertical exaggeration) of the region KS marked in Fig. 1. Earthquake hypocentres (small symbols) and mechanisms (large symbols) are adapted from Kao & Chen (1995). Only focal mechanisms of earthquakes that occurred in the double seismic zone are shown. The back-hemispheres of focal spheres were projected onto a vertical plane perpendicular to the trench axis. A double seismic zone is delineated in the range km, as indicated by broken shaded lines. Downdip compressional (solid triangles) and downdip extensional (open triangles) events are found along the upper and lower layers of the double seismic zone, respectively. A shaded line from the trench axis to -30 km marks the location of the interplate thrust zbne. The 'I-beam' symbols mark the position of a low-velocity zone, which is presumably the oceanic crust of the subducted Pacific plate (Kao & Chen 1995). (b) A NW-SE cross-section (without vertical exaggeration) of the region KP marked in Fig. 1. Earthquake hypocentres and mechanisms are adapted from Kao & Chen (1995). The layout and symbols are the same as in (a). The solid square indicates an outer-rise compressional event. (c) An E-W cross-section (without vertical exaggeration) of the region JH marked in Fig. 1. Seismicity is adapted from Hasegawa e? al. (1978a). A wedge-shaped double seismic zone is delineated in the range km. The layout is the same as in (a) RAS, GJZ 123,71-84

5 Seismogenesis of a double seismic zone 75 krn Figure 2. (Continued.) those along the lower layer are characterized by downdip extension. The distinct focal mechanism patterns are conspicuous in Figs 2(a) and (b). Besides the opposite patterns in focal mechanisms, the geometry of a double seismic zone is also distinguished by the wedge shape that extends down to km. This geometry is most visible in Fig. 2(c) for the cross-section through the Japan arc. Furthermore, a recent study using seismic data locally recorded in the Kamchatka peninsula has revealed a similar geometry for the double seismic zones observed beneath the Kuril-Kamchatka arc (Gorbatov et al. 1994). The top layer of seismicity has an average dip angle of 30"-40", whereas the dip angle of the bottom layer is approximately 5"-10" less. The onset of the bottom layer is at a depth of km, which corresponds to a pressure of kbar. The onset of the top layer, on the other hand, is difficult to determine by seismicity alone, because structures representing interplate slip and those of intraplate deformation seem to abut on each other. By identifying the distinguished downdip compressional focal mechanisms (Figs 2a and b), Kao & Chen (1995) identified that the onset of the top layer of the double seismic zone along the Kuril-Kamchatka arc is at approximately km. The separation between the top and bottom layers at this depth is 30-40km. Similar conclusions were also reached for the profile JH (Fig. 2c) along the Japan arc (e.g. Hasegawa et al. 1978a,b, 1979; Yoshii 1979). On the basis of the observed patterns presented in Fig. 2, a schematic summary of these double seismic zones is shown in Fig. 3, in which the solid circles represent earthquakes related to processes along the plate interface, the solid triangles represent earthquakes having downdip compressional focal mechanisms and the open triangles represent those having downdip extensional mechanisms. The top layer of a double seismic zone has customarily been regarded as the top boundary of a subducting slab, with a slight increase in the dip angle at a depth of around 50 km. The same interpretation has also been applied to all the observable single seismic zones at this depth range RAS, GJI 123, krn analysing seismograms recorded at local distances, several studies have further pointed out that earthquakes within the top layer of the double seismic zone along the Japan arc took place in a thin low-velocity zone (40 km), which presumably represents the subducted oceanic crust of the descending slab (e.g. Nakanishi 1980; Nakanishi et al. 1981; Hori et al. 1985; Matsuzawa et al. 1986; Obara & Sat0 1988). Teleseismic studies along the Kuril-Kamchatka arc also reported the existence of a subducted oceanic crust in the vicinity of the onset of the top layer (Kao & Chen 1994, 1995). Based on waveform analysis, Kao & Chen (1995) suggested that events along the top layer at depths greater than 100 km probably begin to occur beneath the subducted crust. In order to show these features in Fig. 3, we placed the top boundary of the subducting slab (the heavy solid line) slightly above the top layer of the double seismic zone (the solid triangles) at depths below about 50 km. Thus, the dip angle along the top boundary of the subducting slab beneath 50km is slightly less than that deduced from seismicity data. Geothermal structures within the subducted lithosphere There are extensive studies concerning the geothermal structures within the subducted lithosphere (e.g. Toksoz, Sleep & Smith 1973; Hsui & Toksoz 1979; Hamaguchi ef al. 1983; Honda 1985; Helffrich, Stein & Wood 1989). In general, the subducted slab is heated from both the top and bottom boundaries due to its subduction into the hot surrounding mantle. The heating rate for regions near the initially colder top boundary is relatively higher than that near the lower boundary, resulting in a larger thermal gradient in the upper portion of the subducted slab. The exact temperature profile for the region where a double seismic zone exists is model-dependent. Controlling factors include the age of the subducted slab, the relative slip rate between the subducting and overriding plates, the mode of induced mantle flow, the amount of shear heating along the plate interface, and the possible reaction heat

6 76 H. Kao and L. Liu dissipated from dehydration or phase transformation of subducted materials. We adapted two of the most recent models (i.e. Honda 1985 and Helffrich et al. 1989) in our later calculations. Both models were calculated specifically for the subduction zone in the north-eastern Japan region, near to the double seismic zones used in the present study. It will be shown, however, that the choice of a particular geothermal model is not critical in our interpretation. In Fig. 3, we have superimposed two of the isotherms (250" and 500 "C, dashed lines) calculated by Honda (1985) with projected earthquake hypocentres (circles and triangles). The positions of A, C, and B shown in Fig. 3 mark three subduction paths at different depths inside the subducting slab. Path A is initially at 10 km depth beneath the surface (Al) and corresponds to the onset of the top layer of the double seismic zone at a depth of 60 km (A2). Path B is initially at -40 km (Bl) and corresponds to the location of the bottom layer of the double seismic zone directly below point A2 at 95 km (B2). The deepest position of the double seismic zone (C3) is reached by a path that is initially at 20km beneath the surface (Cl). The narrow shaded band in Fig. 3 connects the loci of the double seismic zone, except for the upper half of the top layer. This band will be explained in detail in the next section. Based on a normal oceanic geotherm (e.g. Turcotte & Schubert 1982) and the geothermal model proposed by Honda (1985), the temperatures at points Al, C1 and B1 are 120", 260" and 550"C, respectively (solid circles, Fig. 4). After the initiation of subduction, path A experiences a relatively faster heating rate than do paths B and C, as indicated by the steeper slope of the TIP path (dotted line) Volcanic Arc shown in Fig. 4. It reaches a temperature of approximately 200 "C near the onset of the top layer of the double seismic zone at a depth of 6Okm (solid circle, A2), whereas the temperatures for points C2 at -75 km and B2 at -95 km are 300 "C and 560 "C, respectively. The temperature reaches approximately 400 "C at C3 (solid circle, -185 km), where the double seismic zone terminates. At the same depth, the temperatures at A3 and B3 are approximately 850 C and 600 "C, respectively. If, on the other hand, the geothermal model of Helffrich et al. (1989) is used, the temperatures corresponding to the three paths become somewhat lower (open circles in Fig. 4). The differences between the two models increase with depth, up to -150 C at point A3. Despite the variation in temperature, however, it is evident from Fig. 4 that the general features of the TIP loci for the three paths remain unchanged. SEISMOGENIC MODEL OF DOUBLE SEISMIC ZONES: A KINETIC BOUNDARY The most interesting feature in Fig. 4 is probably the TIP loci that correspond to the double seismic zone. By following the shaded band in Fig. 3 along the lower layer of the double seismic zone from point B2 to point C3, the corresponding TIP path in Fig. 4 is shown by the dot-dashed line connecting solid or open circles of B2 and C3, depending upon which thermal model is used. The TIP loci of path A intercept this line at point AA, which corresponds to a depth of between 100 and 150 km along the top layer of the double seismic zone in Fig. 3. On the basis of the data presented in Figs 2-4, it is proposed that Trench w 0.0 kb 14.6 kb 31.2 kb kb 64.5 kb Figure 3. A schematic diagram showing seismogenic and geothermal structures along a subduction zone. The interface between the subducting and overriding plates is indicated by the thick solid line. The bottom of the subducting lithosphere is assumed to be at 100 km and is marked by the broken line. Solid circles represent earthquakes related to the interplate thrust zone. Solid and open triangles show earthquakes with downdip compressional and extensional focal mechanisms, respectively. Together, they form a double seismic zone. AI-A2-A3, Cl-C2-C3 and Bl-B2-B3 represent the subduction paths for materials originally at 10, 20 and 40 km depth before subduction. Isotherms of 250 C and 500 C (the dotted lines) are from Honda (1985). Based on the temperature/pressure conditions associated with the double seismic zones, it is proposed that earthquakes in a double seismic zone are due to the metastable phase transition (probably from Al-rich enstatite to Al-poor enstatite plus garnet). The wedge-shaped double seismic zone represents the kinetic boundary of the phase transition and is shown by a shaded band. The extrapolated kinetic boundary (broken shaded lines) does not exist outside the double seismic zone region RAS, GJI 123,71-84

7 Seismogenesis of a double seismic zone 77 I l l I [ I I I I \ I I l / l 1 I I I / / / - / / / / I I I I I I Pressure, kbar Figure 4. A temperature/pressure (TIP) diagram for regions in the vicinity of a subduction zone. The three subduction paths (A, B and C) in Fig. 3 are plotted as dotted lines. Open and solid circles represent the temperatures derived from Helffrich et at. (1989) and Honda (1985), respectively. A normal oceanic geotherm is shown by a thin curve which passes through the points Al, C1 and B1. The TIP conditions of the lower layer of the double seismic zone are shown by a dot-dashed line connecting B2 and C3. The extrapolation of this line to depths shallower than -70 km is not observable and is labelled by question marks. The phase diagrams for MgSiO,.n per cent A1,0, are overlaid for reference. The equilibrium boundary between Al-rich enstatite and Al-poor enstatite plus garnet (or majorite) was determined by Boyd & England (1964). According to our hypothesis, the TIP loci associated with the double seismic zone (the dot-dashed lines) represent the kinetic boundary of the metastable phase transition. Notice that a kinetic boundary defined in this way coincides well with the equilibrium boundary. Path A intersects the kinetic boundary at AA, which defines the onset of the same phase transformation along the top layer of the double seismic zone. earthquakes that occurred in the lower layer and in the lower half of the top layer (i.e. below point AA in Fig. 3) of a double seismic zone are most probably due to a metastable phase transition of the same geological material-a mechanism similar to that proposed for the seismogenesis of deep-focus earthquakes, except that it takes place at lower TIP conditions. In the following discussion, we shall explain the hypothesis and its justification in detail. In addition, the most likely candidates for such a phase transition will be examined. Equilibrium boundary, kinetic boundary and earthquakes Thermodynamically speaking, there exists an equilibrium boundary between two different phases of the same composition, and the TIP loci of that boundary can be calculated theoretically, if appropriate thermodynamic data are available, and verified by laboratory experiments. In reality, however, the equilibrium phase transition may not be able to proceed if the temperature is not high enough, despite the fact that the temperature/pressure condition has RAS, GJI 123,71-84

8 78 H. Kao and L. Liu already reached the other side of the equilibrium boundary. In this case, the material is said to be in a metastable state. When the temperature rises, a metastable material must eventually undergo a metastable phase transition to reach its stable state by passing through the so-called kinetic boundary. The exact location of a kinetic boundary depends not only on the reaction components but also on several other factors, such as the grain size and the presence of shear stress and catalysts (Sung & Burns 1976a, b). It was proposed as long as a half century ago that metastable phase transitions might emit seismic energy in regions where brittle behavior is no longer valid (e.g. Bridgman 1945; Dennis & Walker 1965; Sung & Burns 1976a, b; McGarr 1977). Liu (1983) discussed the theoretical aspects of metastable phase transitions from the energy point of view and concluded that there are several necessary conditions for such a phase transition to release energy in the form of seismic waves. These conditions are as follows: (1) the equilibrium boundary must have a positive Clapeyron slope: (2) the temperature must reach and/or be higher than the characteristic temperature of a kinetic boundary; and (3) the pressure must be less than the characteristic pressure of a kinetic boundary. In addition, Liu (1983) pointed out that there are at least three forms of energy involved in a metastable phase transition, namely, the kinetic energy due to the sudden volume contraction, the latent-heat energy originating from the entropy decrease, and the metastability energy caused by overpressure from the equilibrium boundary. For most silicates in the lithosphere (olivine, pyroxene, basalt), the energy density of metastable phase transition was estimated to be over 2.5 X erg kmp3 (Liu 1983). Therefore, if the reaction of a metastable phase transition is sufficiently fast that the released energy is unable to dissipate completely by thermal conduction or radiation, it is inevitable that a fraction of the involved energy must be released in the form 0.f seismic waves. In fact, recent high-temperaturelhigh-pressure experiments have provided direct access to the explanation of how metastable phase transitions from olivine to spinel (or modified spinel) are related to shear failures or earthquakes (e.g. Green & Burnley 1989; Green et al. 1990; Burnley et al. 1991; Green ef al. 1992). The general scenario is that lens-shaped anti-cracks filled with very fine-grained spinel are first created nearly perpendicular to the direction of maximum compression. The high shear stress creates stress concentrations that favour the growth of the transformed region (Kirby 1987). The lenses of spinel then link together to produce shear failure and emit acoustic waves (Green & Burnley 1989; Green ef al. 1990; Burnley et al. 1991; Green et al. 1992). In essence, the shear faulting caused by a metastable phase transition is analogous to a normal brittle fracture in which tensile (mode I) microcracks link up to form shear fractures. Sources produced in this way are seismically equivalent to double couples. Because experiments on other metastable phase transitions are relatively few at the moment, there is no a przori information to exclude all reactions but one from producing double couples. On the contrary, the success in generating double couples from metastable phase transitions between olivine and spinel might suggest the opposite. Continuing this line of thought, we now speculate that double couple mechanisms might be produced by metastable phase transitions other than olivine-spinel. It should be pointed out, however, that this speculation is still not experimentally confirmed at the moment. Consequently, modifications to our proposed model might be required, should our assumption be proved wrong by future experiments. A likely candidate: Al-rich enstatite decomposing into Alpoor enstatite plus garnet In searching for possible candidates of phase transitions that might be responsible for the double seismic zone, there are several obvious criteria, in addition to those discussed in the previous section. First, the TIP loci of the equilibrium boundaries of possible phase transitions must agree with the kinetic boundaries that we assumed in Fig. 4 (the dot-dashed lines). Secondly, the specific mineral(s) must be of sufficient quantity in the subducting lithosphere. Thirdly, the mineral(s) must remain in a metastable state until reaching intermediate depths. Finally, the rate of reaction must be fast enough to emit seismic signals. The mineralogical assemblage of an oceanic lithosphere is complex. Generally speaking, a typical oceanic lithosphere has a thin basaltic top layer (-5-10 km), which results from partial melting along the mid-ocean ridge. The immediately underlying mantle is presumed to be composed of peridotite, in which olivine and pyroxene are the most predominant minerals. The olivine content has been estimated to be at least per cent of the subducting lithosphere, while aluminous pyroxene is likely to make up approximately one-third of it (Liu 1983). The amounts of other minerals are not considered to be very great and therefore do not meet the second requirement listed above. In Fig. 4, the phase diagram for aluminous enstatite (MgSiO,.5 per cent A1203 and MgSiO,. 10 per cent A1203) that was derived from the work of Boyd & England (1964) is shown. The appropriate equation of the phase transition, according to Liu (1980), is described as x MgSiO,. Mg,Al,-,,,(Mg, Si)ySi30,2 Al-rich enstatite garnet (1) + (x-3-y)mgsi03. yal,o, Al-poor enstatite where x > (3 + y)/(l - y) and 1 > y 2 0. The equilibrium boundary of reaction (1) has been determined in the range C, and is shown by thick solid lines, with their extrapolations shown by dashed lines (Boyd & England 1964; MacGregor & Ringwood 1964). The question marks at the left end of the dot-dashed lines in Fig. 4 indicate the extrapolation of the assumed kinetic boundaries. It is clear that the Clapeyron slope for this particular phase transition is positive and that the equilibrium and kinetic boundaries are in good agreement. According to Fig. 4, Al-rich enstatite is stable under conditions of low pressure and high temperature-an environment which is normally found in the mid-ocean ridge. A normal oceanic geotherm does not intersect the RAS, GJI 123,71-84

9 equilibrium boundary of reaction (1) until depths of at least 70 km, if 5 per cent of A1,0, is assumed. In fact, there is no intersection at all if the aluminous content reaches 10 per cent. Therefore, Al-rich enstatite is unstable inside most of the Earth except near the mid-ocean ridge. This is essentially the same situation as for basalt. In other words, the equilibrium boundary between Al-rich enstatite and the assemblage of Al-poor enstatite plus garnet does not exist in a normal oceanic upper mantle. By analogy to basalt, Al-rich enstatite is most likely formed at the shallow part (probably less than km) of a mid-ocean ridge, where new lithosphere is generated. When the plate moves away from the mid-ocean ridge, Al-rich enstatite becomes unstable due to the decrease in temperature. Thermodynamically speaking, all Al-rich enstatite should have undergone reaction (1) when the plate reaches its normal geothermal state. On the other hand, just like basalt, whether the kinetics of the transition is fast enough to do so is in doubt, particularly for the shallow portion of the lithosphere where the temperature is relatively low. Unlike basalt, which is the melting part of a partial melting process, Al-rich enstatite is probably the residual of a partial melting. A sketch displaying this situation near the subducting region is shown in Fig. 5. While basalt forms the oceanic crust and covers the very shallow part (5-10km) of a subducting lithosphere, the various types of rock immediately beneath basalt contain metastable Al-rich enstatite. The exact thickness of these rocks and the percentage of A1,0, content are probably region-dependent and are not completely known at present. We speculate that the thickness is probably less than km as indicated by the onset the bottom layer of the double seismic zone (Fig. 2). The aluminous content has been estimated to be slightly less than 10 per cent, according to the analysis of mantle nodules by MacGregor & Ringwood (1964). Seismogenesis of the lower layer of the double seismic zone Seismogenesis of a double seismic zone, in particular the lower layer, has not been adequately explained since its discovery nearly two decades ago. Mechanisms such as the dehydration of hydrous silicates (e.g. serpentine: Meade & Jeanloz 1991) and the facies change from basalt to eclogite (Pennington 1983) were proposed to account for the occurrence of intermediate-depth earthquakes. Unfortunately, none of them is applicable to both the top and lower layers of a double seismic zone. The depth range of the lower layer is much too shallow for olivine to become metastable, and there are no hydrous minerals or basalt at these locations either. In our model, the occurrence of earthquakes along the lower layer of a double seismic zone is interpreted as being due to a metastable phase transition of subducted materials, probably the Al-rich enstatite decomposing to Al-poor enstatite plus garnet. In Fig. 5, the uppermost 5-10 km of the top boundary of the subducting lithosphere (the dark shaded band) is a thin oceanic crust made up of metastable basalt. Immediately beneath the basaltic crust, there are rocks containing metastable Al-rich enstatite (represented by the white wedge-shaped region) on top of those rocks containing the stable assemblage of Al-poor enstatite plus RAS, GJI 123,7144 Seismogenesis of a double seismic zone 79 garnet (the hatched region). The physical interface between these two types of rocks is indicated by a thin solid line. As the metastable Al-rich enstatite is subducted towards greater depths, the deepest parts will first pass the kinetic boundary (shaded band) at approximately 70 km depth, marking the onset of the lower layer of the double seismic zone. The kinetic boundary will then move progressively inwards with increasing depth as the Al-rich enstatite that was originally at shallower depths moves towards greater depths. The coldest part of the subducted slab, which contains metastable Al-rich enstatite (the white region in Fig. 5), reaches the greatest depth where the lower layer joins the top layer. The original boundary that separates the metastable Al-rich enstatite from the stable assemblage of Al-poor enstatite plus garnet before subduction is no longer meaningful and is now represented by the thin broken line in the hatched region. Seismogenesis of the upper layer of the double seismic zone On the other hand, along path A in Fig. 3 (i.e. the upper seismic zone), it is not so obvious where the equivalent metastable phase transformation should take place. However, this information can easily be found from Fig. 4. Given that path A has a higher thermal gradient than those of other paths, it will intersect the kinetic boundary defined by B2 and C3 at the point AA. Thus, according to our model, earthquakes observed along the upper seismic zone with focal depths greater than approximately 130 f 20 km (i.e. events between points AA and C3 in Fig. 3) could also be interpreted by the same phase transformation that accounts for events in the lower layer of the double seismic zone. What seismogenic processes, then, are responsible for events occurring in the upper seismic zone at depths between approximately 50 and 100 km? We attribute their occurrence to the various dehydration reactions (e.g. Raleigh & Paterson 1965; Raleigh 1967; Evison 1970; Meade & Jeanloz 1991) and to the facies change from metastable basalt to eclogite (e.g. Ringwood & Green 1966; Fukao, Hori & Ukawa 1983; Pennington 1983). The transition boundary between basalt and eclogite is very similar to that shown in Fig. 4, except that it is at lower temperatures (Liu 1983). Therefore, our interpretation is qualitatively compatible with previously proposed models regarding the seismogenesis of intermediate-depth earthquakes. In our interpretation, seismogenesis in the upper seismic zone probably consists of at least two different mechanisms. Earthquakes are probably caused by dehydration-related processes or by metastable basalt transforming to eclogite within the subducted oceanic crust. Below a depth of approximately 130 f 20 km, metastable Al-rich enstatite near the top boundary of the subducting lithosphere starts to transform into Al-poor enstatite plus garnet-similar to what happens in the lower seismic zone. Moving from AA to C3 in Fig. 4 would require the kinetic boundary of the same transition in the top seismic layer to move progressively downwards with increasing depth and finally to join the kinetic boundary associated with the lower seismic zone near the coldest core of the subducted slab at a depth of approximately km (Figs 3 and 5).

10 00 H. Kao and L. Liu Volcanic Arc O h A Trench v le Al-rich Enaatite 50km - A Figure 5. A sketch showing the subducting metastable basalt and rocks containing metastable Al-rich enstatite. Before subduction, the oceanic crust is made up of a thin layer (<lokm) of metastable basalt (shown by the dark shaded band). In the top km of the lithosphere, where the temperature is not high enough to promote equilibrium transition, Al-rich enstatite remains metastably just as basalt. Below this depth, Al-rich enstatite has already decomposed into Al-poor enstatite plust garnet (shown by hatched region). The physical (not equilibrium) boundary separating these two types of rocks is shown by a thin solid line. When these rocks are subducted to a depth of about 70 km, the lowermost AI-rich enstatite passes the kinetic boundary shown in Figs 3 and 4 and transforms into Al-poor enstatite plus garnet. This point corresponds to the onset of the lower layer of the double seismic zone. The original physical boundary is no longer meaningful and is now represented by a thin broken line. Earthquakes that occurred in the upper seismic zone are probably caused by various reactions within the subducted oceanic crust. Upon reaching a depth between 100 and 150 km, the same metastable transition that is responsible for the lower layer becomes effective. As is required by the TIP conditions of the kinetic boundary shown in Fig. 4, both the upper (corresponding to AA-C3) and lower (corresponding to B2-C3) kinetic boundaries move progressively inwards with increasing depth, and join together at the coldest core (C3) of the subducted slab. The earthquake foci of the Japan arc shown in Fig. 2(c) also seem to hint that different seismogeneses may be invoked in the vicinity of 100 km depth along the top layer. First, the density of earthquake occurrence above this depth seems much greater than that below. The spread of the earthquake foci is also much wider for the shallow portion. Second, the dip angle along the upper layer of the double seismic zone seems to increase slightly at depths below about 100 km, as would be expected from our model (Fig. 3). Seismic data along the two profiles of the Kuril- Kamchatka arc are too sparse to confirm such detailed changes. None the less, these features may serve as testable measures if further high-resolution observations in other subduction zones become available. DISCUSSION Despite the fact that double seismic zones were reported for a number of subduction zones, we cannot ignore that there are quite a few places where only single seismic zones exist (e.g. Fujita & Kanamori 1981; Kao & Chen 1991). Conventionally, a single seismic zone is referred as a Wadati-Benioff zone in a single state of strain, i.e. mechanisms of all events are consistent with one seismic pattern: mostly either in downdip compression or in downdip extension, but not in both. It is important to point out that the present model does not concern itself with any particular earthquake focal mechanism. The orientation of earthquake fault planes is primarily determined by the local state of stress, not by the seismogenic process itself. In other words, the occurrence of an intermediate-depth earthquake depends on at least two critical factors: the existence of deviatoric stress and seismogenic mechanisms (e.g. the metastable phase transition in our model) at the source region. Following this argument, a double seismic zone with both the top and lower layers in a uniform state of strain is compatible with the present model, as long as the wedge-shaped configuration is maintained and the strain field at the source region can be adequately justified. On the other hand, a real single seismic zone (i.e. only one seismogenic structure exists) requires additional explanation, because the wedge shape of seismicity is no longer there. One possible explanation is the lack of an appropriate metastable phase transition in the source region. For example, if the subducted lithosphere has completed reaction (1) and contains little metastable Al-rich enstatite when it reaches the intermediate depth, then the lower layer may not exist. It could also be that the thickness of the rocks containing the Al-rich enstatite is very thin, so that the separation between the top and lower layers can hardly be distinguished. Similarly, if the TIP loci of the subducting paths (e.g. A, B and C paths in Fig. 4) do not intersect the kinetic boundary due to abnormal thermal structure of the subducted slab, then no double seismic zone would be expected. In addition to the lack of an appropriate metastable phase transition in the source region, single seismic zones may also be the result of a lack of sufficient deviatoric stresses along either of the two layers of the double seismic zone. In the case of the Kuril-Kamchatka arc, for example, Kao & Chen (1994) reported that the double seismic zone found in the RAS, GJI 1W,71-84

11 central portion of the arc is actually an overlap of two real single seismic zones: a compressional seismic zone near the top boundary of the subducted slab extending along the northern two-thirds of the arc, and an extensional seismic zone located in the interior of the subducted lithosphere along the southern two-thirds. Based on the strain segmentation observed for deep-focus earthquakes of the same arc, they interpreted the changeover from a double to a single seismic zone as being a consequence of the lateral variation of deviatoric stresses transmitted from the deep slab. Towards the northern end of the arc, the transmitted deviatoric stress is stronger in compression, so that it neutralizes the deviatoric extension that originally existed in the lower seismic zone. Consequently, the lack of deviatoric stress in the lower layer fails to generate seismogenic deformation, causing the disappearance of earthquakes. Similar reasoning can be applied to the southern portion of the arc, where the double seismic zone changes to a single seismic zone in extension. Although their results were derived from events with rn,>5.1 and the exact lateral extent of the double seismic zone may be wider if a smaller magnitude threshold is adopted (Gorbatov et al. 1994), the conclusion that stresses within the subducted slab at intermediate depths and at greater depths are interacting with each other seems to be unchanged. That the presence of deviatoric stress may promote the reaction rate of metastable transformation has been proved by laboratory experiments (e.g. Rubie & Thompson 1985). Kirby (1987) further pointed out that the presence of deviatoric stress would create stress concentrations which favour growth of the transformation region. Loss of cohesion within the transformed zones would then evolve into a fault, causing an earthquake. Based on these contentions, we argue that the lack of deviatoric stress may result in a slower reaction rate for metastable phase transformation, so that no seismic energy can be generated. Therefore, the present model is also applicable to single seismic zones, under the assumption that not enough deviatoric stress exists along either the top or the lower kinetic boundary depicted in Fig. 5. It is important to realize that reactions involving X+ Y or X+ Y + Z are fundamentally different from those involving two or more reactants. This is particularly true for a metastable phase transition, which usually occurs at relatively low temperatures with a rapid reaction rate in a narrow temperature interval (Green & Burnley 1989). The reaction rate involving two or more solid reactants is usually slower, due to the necessity of a solid-solid diffusion process. As it is always emphasized that a sufficiently rapid reaction rate is crucial to the generation of seismic energy, this constraint presumably rules out the possibility that intermediate-depth earthquakes could be caused by chemical reactions involving two (or more) solid geological materials. Given the present knowledge on phase transitions of minerals and the TIP structures within the subducted lithosphere, Al-rich enstatite decomposing into Al-poor enstatite plus garnet seems to be the most appropriate candidate for the seismogenesis of earthquakes in a double seismic zone. In the present model, all metastable Al-rich enstatite in the upper portion of the subducted lithosphere should have undergone reaction (1) before reaching a depth of RAS, GJI 123,71-84 Seismogenesis of a double seismic zone 81 approximately km. Thus, one may wonder which seismogenic mechanisms are responsible for earthquake occurrences in the depth range between -180 and -300 km, where the metastable phase transformation of olivine to spinel (or modified spinel) is still not possible. It is suggested that their occurrences might be linked to metastable phase transformations (or maybe some other processes) of minerals other than olivine, Al-rich enstatite or basalt. In fact, the distribution of the number of earthquakes versus focal depth indicates that seismicity decreases steadily from -70 km to a minimum at depths between 300 and 400 km, and then resurges to another maximum at a depth of approximately 600 km (Frohlich 1989). Such a trend seems to suggest that whatever is responsible for events between -180 and -300 km, it must be secondary to the metastable phase transformation proposed in this study. Given that olivine and pyroxene are the most and the second most abundant minerals in the subducted lithosphere, this inference is probably reasonable. Meade & Jeanloz (1991) reported no acoustic emission associated with solid-state transformations in olivine and pyroxene, and concluded that high-pressure phase transitions of anhydrous mantle phases release no seismic energy, even in metastable conditions. The conclusion was contradicted by Green et al. (1992), who presented experimental evidence of acoustic emission associated with the metastable phase transformation from olivine to spinel. Since Meade & Jeanloz (1991) did not specifically mention the metastable phase transformation from Al-rich enstatite to Al-poor enstatite plus garnet, the question whether such a reaction can radiate seismic energy is still unanswered. It is possible that previous experiments were not operated under appropriate conditions to detect the acoustic emission associated with reaction (1) (similar to what happened in the case of the olivine to spinel phase transformation). None the less, confirmation of the release of seismic energy associated with the metastable reaction (1) in future hightemperature/high-pressure experiments is anticipated. The present model incorporates results of theoretical and observational studies from various disciplines. The location of double seismic zones is constrained by precisely determined earthquake source parameters (Hasegawa et al. 1978a, b, 1979; Kao & Chen 1994, 1995). The corresponding temperature profiles in the subducting lithosphere come from geodynamical modelling that takes geothermal measurements as its constraints (Honda 1985; Helffrich et al. 1989). The phase diagram was determined by hightemperature/high-pressure experiments (Boyd & England 1964), while mineralogical studies provided an insight into the relationship between the seismogenesis of intermediatedepth earthquakes and metastable phase transformation (e.g. Liu 1983). The consistency among these data is probably too remarkable to be fortuitous. CONCLUSIONS The well-determined seismogenic structures along three profiles in the Kuril-Kamchatka and Japan subduction zones clearly show that double seismic zones exist in the depth range from -60 to -185 km. The separation between the top and bottom layers is more than 30 km at its onset

12 82 H. Kao and L. Liu and gradually decreases with increasing depth. Such a wide separation cannot be accommodated by conventional interpretations, which attribute seismogenesis of intermediate-depth earthquakes to dehydration of subducted materials and/or facies change of basalt to eclogite. On the basis of seismic data and the geothermal structures calculated for subduction zones, we investigated the temperature/pressure conditions associated with seismogenic structures of the double seismic zones. The corresponding TIP loci seem to suggest that earthquakes observed in the lower seismic zone were caused by a metastable phase transition-a mechanism similar to that responsible for deep-focus earthquakes, except at lower TIP conditions. Under this hypothesis, the wedge-shaped configuration of a double seismic zone is interpreted as representing the loci of the kinetic boundary of the phase transition. The successful candidates for such a phase transition, according to theoretical/experimental studies and the constraints imposed by our observations, must have a positive Clapeyron slope for its equilibrium boundary and a relatively rapid reaction rate. In addition, such a metastable mineral must be abundant in the oceanic lithosphere, and remains in a metastable state until reaching intermediate depths. It is proposed that Al-rich enstatite (aluminous pyroxene) decomposing into the assemblage of Al-poor enstatite plus garnet (or majorite) is one of the major contributors to the seismogenesis of earthquakes in a double seismic zone. According to the phase diagram of MgSiO,.n per cent A1,0,, Al-rich enstatite is stable under conditions of high temperature and low pressure (e.g. in the vicinity of the mid-ocean ridge). As the oceanic lithosphere moves towards subduction zones, the Al-rich enstatite near the top becomes metastable due to low temperatures. Once it has been subducted, the deepest Al-rich enstatite will first pass the kinetic boundary at a depth of approximately 70 km, marking the onset of the lower layer of the double seismic zone. The Al-rich enstatite originally at a shallower portion will then progressively pass the kinetic boundary as it moves towards greater depths, until about 185km, where the coldest core of the subducted slab is reached. The uppermost Al-rich enstatite would undergo the same metastable phase transformation at depths greater than approximately 130 f 20 km. Therefore, seismogenesis along the top layer of a double seismic zone involves at least two different seismogenic mechanisms. Earthquakes in the upper portion of the top layer are probably caused by dehydration of subducted materials and/or facies change from basalt to eclogite, whereas the same metastable phase transition responsible for the lower layer accounts for events in the lower portion. Single seismic zones (i.e. Wadati-Benioff zones with only one seismogenic layer) can be explained by insufficient amounts of metastable Al-rich enstatite in the subducted lithosphere. Alternatively, a lack of deviatoric stresses in the source region may also result in the disappearance of either layer of the double seismic zone. This is based on the argument that the presence of sufficient deviatoric stress is critical to a rapid reaction rate of metastable phase transition. Therefore, the present model is applicable to single seismic zones, as long as the lack of Al-rich enstatite and/or deviatoric stresses can be adequately justified. ACKNOWLEDGMENTS We thank Ban-Yuan Kuo, Wang-Ping Chen, Ho-kwang Mao and Francis T. Wu for constructive discussions. K. C. Hsiang assisted us with some of the graphics. HK was supported by grants NSC M and NSC M from the National Science Council, Taiwan. REFERENCES Boyd, F.R. & England, J.L., The system enstatite-pyrope, Carnegie Inst. 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