The practical application of seismology is to prevent

Transcription

1 7 2 What determines the spatial extent of earthquakes? C. BERGE-THIERRY IRSN) B. HERNANDEZ IRSN) F. COTTON IRSN) O. SCOTTI IRSN) The practical application of seismology is to prevent or reduce the damage caused by earthquakes. Our current knowledge of earthquakes is such that it is not possible to predict the instant an earthquake will occur in a given place. On the other hand, it is possible to identify zones where the seismic hazard is significant and accordingly to construct buildings there that are capable of withstanding shocks. In order to be able to build in a seismic zone, it is first necessary to define the characteristics of potential earthquakes in that zone. The vibrations that the buildings or installations are liable to experience in the event of such an earthquake are then evaluated. Locating active faults and determining their geometry requires a multidisciplinary approach, but the exercise remains difficult in some areas, especially those characterized by small deformations of the earth s crust. Faults along which earthquakes occur have a complicated geometry and consist of several segments. Observations from recent earthquakes show that a rupture can propagate from one fault segment to another, and that a variety of phenomena may influence earthquake rupture initiations and arrests. Elastic rebound: apparent simplicity From the least vibration up to the greatest upheaval, earthquakes are caused by two segments of the earth s crust sliding along a preexisting fracture called a fault. In 1912, after the San Francisco earthquake of 1906, Harry Fielding Reid was the first to suggest the mechanism of elastic rebound. Fielding Reid revealed the elastic properties of rocks. Under the accumulated stresses caused by tectonic movements, the rocks in the vicinity of the locked fault are gradually deformed, at first elastically without flowing or rupturing, like a spring being stretched. For tens, or even hundreds or thousands of years, the forces of friction keep the rocks in place on each side of the fault, the stresses gradually building up between the two blocks due to slow movement of the order of a millimeter a year). When these stresses exceed the strength of the material, sudden slip occurs between the two sides of the fault, releasing the accumulated energy, partly in the form of seismic waves. After this seismic episode, stresses continue to build up until the next earthquake. The elastic rebound model suggests that earthquakes of similar magnitudes regularly recur on a given fault. Experience shows that this model is too simplistic given that in areas of high seismicity, the periodic recurrence of an earthquake of constant magnitude is rarely observed. The fact that earthquakes do not recur identically on a given fault makes it hard to assess the magnitude of the next earthquake on this fault based on the observation of past quakes. This information would, however, be essential in preventing and reducing any damage that could be caused in a region by future earthquakes. In order to meet this need, seismologists are now trying to understand in more detail the physics of rupture along faults causing earthquakes, and in particular the various phenomena involved in initiating, propagating and arresting rupture, so as to deduce practical applications from them in terms of assessing the seismic hazard. The case study of the Landers earthquake: earthquake jogs In 1992, an earthquake of magnitude 7.2 Landers earthquake) shook the Mojave Desert in California. This desert region, situated between Los Angeles and Las Vegas, is sparsely populated and the damage was therefore limited. Nevertheless, this earthquake provided the scientific community with an unprecedented set of data. First of all, the slip zone reached the ground surface. The displacement between the two sides of the fault created by the earthquake was traced over 220 INSTITUT DE RADIOPROTECTION ET DE SÛRETÉ NUCLÉAIRE

2 7 more than 70 km in the desert. In addition, the Californian seismological networks were able to detect and locate the aftershocks, minor quakes that occur after the main shock. These aftershocks occurred predominantly along the fault plane of the main earthquake. In the case of the Landers earthquake, they were localized along a vertical strip coincident with the earthquake s traces at the surface. This observation showed that the faults where the earthquake took place were vertical: the geometry of the rupture was determined at the surface and at depth figure 1). The displacements observed along the surface trace were variable in size, generally over 2 m, with maxima of up to 6 m; these displacements were observed on a series of fault segments with an overall SSW/NNE orientation. The regional networks located the epicenter at the southern end of the rupture zone and showed that the slip propagated from south to north over several kilometers. Thus, the Johnson Valley, Landers, Homestead Valley, Emerson and Camp Rock faults were successively affected by the earthquake. The earthquake did not therefore take place along a single plane: it occurred in a complex manner, jumping from one fault segment to another along a staggered fault system figure 2). Figure 1 Landers earthquake 1992). The green, blue and red lines are the surface traces of fault segments activated during the Landers earthquake; the epicenter of this earthquake is located in the southern part of the red segment. The other two stars are the epicenters of the two largest aftershocks. The green dots are the epicenters of small quakes that occurred after the main Landers shock. Figure 2 Geometry of the Landers earthquake 1992). Safety of radioactive waste Ineffective geometric barriers When the earth s crust is subjected to the effect of plate movement, it can result in an earthquake or, on the contrary, in an aseismic deformation, in other words creep. Several factors govern this release of energy, including the rate of deformation, the thermal gradient and the mechanical state of the material. This deformation takes place through the reactivation of old faults and very rarely through the creation of new ones. This property of seismic faults is due to the fact that they are not plane structures of zero thickness. Faults are zones consisting of crushed rock due to previous movements that occurred along them. Since the fault forms an ideal path for the movement of fluids, these rocks are often weathered. This crushed region is thus a zone of least resistance when compared to the neighboring intact rocks. Faults where earthquakes occur have therefore been in action for a very long time. For example, the Middle Durance fault, where major earthquakes have periodically occurred in 1509, 1708, The earthquake propagated from south to north along several faults. Fault jumps are located at points 1, 2 and 3. When the earth s crust is subjected to the effect of plate movement, it can result in an earthquake or, on the contrary, in an aseismic deformation. SCIENTIFIC AND TECHNICAL REPORT

3 Fracture in the highway caused by the Landers earthquake California). cross them and propagate along nearby faults; it may also stop at points in the middle of a fault. Observing such complexity in seismic rupture was not completely new: it had already been suggested in the case of several past earthquakes. But, for the first time during the Landers earthquake, seismologists had a multiplicity of strongmotion station data, enabling them to analyze this complexity in a new way. Seismological tools with new capabilities Reference 1 - P. Volant, C. Berge, P. Dervin, M. Cushing, G. Mohammadioun and F. Mathieu, The South Eastern Durance Fault Permanent Network : Preliminary Results. Journal of Seismology, April ) : , 1913), have been in operation for more than 250 million years 1. The Landers earthquake itself occurred on pre-existing faults, whose trace was already known from geological maps. Since the earthquake took place on pre-existing faults, it might be assumed that its size is determined by the fault s dimensions. The terminations of a fault would then act as geometric barriers and delimit the size of the earthquake. But this is not necessarily the case: we noted earlier that the Landers earthquake had affected various faults. Yet the earthquake repeatedly encountered fault terminations. To the south, the end of the Johnson Valley fault did form a sturdy geometric barrier which halted the spread of the earthquake southward. On the other hand, in spreading northward, the rupture repeatedly encountered fault terminations. But these terminations did not halt the spread, which continued over other faults. Conversely, the spread of the rupture may end in the middle of a fault. This is, for example, the case of the rupture termination in the middle of the Camp Rock fault. Thus, fault terminations may act as geometric barriers, but these barriers are not absolute: an earthquake rupture may The quality and quantity of data that can be used in studying earthquakes have greatly improved over the last few years. Before the 1960s, the study of an earthquake relied predominantly on macroseismic data. Macroseismic effects effects felt by eyewitnesses, damage to buildings, landslides) can first of all be used to approximately locate the zone where the earthquake has taken place and to estimate its size. These observations were often the only ones the seismologist had for studying former earthquakes. In the case of some seismic events, like at the time of the Landers earthquake, the rupture reached the ground surface. These observations are important, since they can be used to measure the displacement created at the surface between the two sides of the fault: the fault geometry thus defined at the surface cannot, however, be extrapolated at depth. Earthquakes have also been studied in the past thanks to traditional geodetic techniques. With the advent of GPS Global Positioning System) it is now possible to measure from space a position on the Earth s surface with an error of less than one centimeter. The deformation caused by the earthquake close to the fault is deduced by comparing the location of a point on the ground surface before and after the earthquake. These increasingly accurate and numerous GPS measurements, together with the progress in space geodesy, especially in It is now possible thanks to GPS to measure, from space, a position on the Earth s surface with an error of less than one centimeter. 222 INSTITUT DE RADIOPROTECTION ET DE SÛRETÉ NUCLÉAIRE

4 7Safety of radioactive waste Slipping first commences at a particular point, called the hypocenter, then a rupture front develops at a speed of approximately 3 km per second along the surface of the fault. radar interferometry, can be used to define the geometry of a fault, its location and the slip between the two sides of the fault that takes place at depth between two series of measurements. The time interval between these two measurements is, however, too great to know whether the slip is pre-seismic before the quake), co-seismic during the few seconds of the quake) or post-seismic in the days or hours following the quake). Studying the slip distribution produced during the few seconds of an earthquake is nonetheless possible thanks to seismometers, which measure ground movement several times a second during an earth tremor. The shorter the distance between the station and the fault, the richer the ground movement record is in terms of information on the slip. Conventional seismological stations, dedicated to the detection and location of distant earthquakes or to local microseismicity, are very sensitive and can record very weak ground movements. On the other hand, these conventional stations contribute very little information when they are situated near major earthquakes since, as they are limited by the dynamics of digital acquisition, they cannot be used to record all ground movement. So-called strongmotion stations are accelerometers designed with a great dynamic range that does not saturate and can thus record ground movement during major earthquakes. Strong-motion networks have become highly advanced over the last ten years or so. This equipment is now present near many faults in high-seismicity zones California, Japan, Mexico, India, Iran, etc.). More than ten strong-motion stations located within a radius of 60 km from the earthquake epicenter, recorded the Landers earthquake. The nearest station, Lucerne, was located less than 2 km from the fault. Film of the rupture: illustration of its complex nature Knowing the slip distribution, the geometry of the fault plane and the laws of wave propagation in the medium surrounding the fault, IRSN has developed a method 1,2 to define rupture scenarios, i.e. the history of slip along the fault. This method consists in reconstructing the seismic rupture propagation along the fault, over time and space, so as to best reproduce the recorded signals by modeling. The major features of the phenomenon can be summarized as follows. When faults slip during an earthquake, rupturing will cause the two blocks on each side of the fault to slide. Slipping first commences at a particular point, called the hypocenter, then a rupture front develops at a speed of approximately 3 km per second along the surface of the fault. Displacement between the two sides of the fault starts at a point when this rupture front reaches it. Behind this rupture front, slipping between the two blocks lasts a few seconds or a few tenths of a second depending on the earthquake. The history of the slip at each point of the fault can thus be analyzed by determining the following three parameters: the position of the rupture front at each instant, the duration of the rupture, and the final slip between the two sides of the fault at each point figure 3, page 224). The results of these calculations can be used to obtain a film of the rupture, showing the displacement of both sides of the fault at depth, a few moments after the beginning of the earthquake. It took just 20 seconds to rupture 60 km from south to north of the fault and cause displacements of several meters. The rupture took place down to a depth of some fifteen kilometers. At each point of the fault, this displacement occurred in a few seconds the duration of the References 1 - F. Cotton and M. Campillo, Stability of the Rake during the 1992 Landers Earthquake. An Indication for a Small Stress Release. Geophysical Research Letters, , B. Hernandez, F. Cotton and M. Campillo, Contribution of Radar Interferometry to a Two Step Inversion of the Kinematic Process : Variability of the Rupture Front Velocity during the 1992 Landers Earthquake. Journal of Geophysical Research , SCIENTIFIC AND TECHNICAL REPORT

5 Figure 3 The data from strong-motion stations enable the history of the rupture to be deduced using an inversion method. The northward progress of the rupture is shown for every 1.5 seconds interval in this figure. Figure 4 Geometry of a fault at the surface and at depth: nucleation point of the rupture focus) and its projection at the surface epicenter). Fault trace at the surface Epicenter Fault plane Focus Seismic wave propagation along wave fronts Rupture propagation across fault jogs rupture at each point varied between 2 and 3 seconds). While the rupture front had a velocity of the order of 3 km per second, this velocity was not constant. The variability in displacement between the two sides of the fault at the surface is confirmed at depth. The slip distribution is heterogeneous. It should be noted that there are three zones where the slip is considerable, much more than 1 m. Two regions where the slip is less separate these zones. The earthquake could therefore be broken down into three smaller quakes that took place successively we then speak of three sub-events). This spatio-temporal history of the rupture can be used to explain the movements recorded at the surface both by the seismological stations and by the geodetic stations GPS). The slip in the broken part of the fault expresses itself as a dynamic increase in stresses in the vicinity of the rupture figure 4). But this variation in stresses diminishes with distance from the rupture zone. If there is another fault in the vicinity of the fault where the rupture occurs and if the variation in stress on this fault exceeds the stress threshold necessary for its rupture, the earthquake will be able to continue onto this other fault. It is clear then that an earthquake may be propagated over several faults without necessarily rupturing the part situated between the faults. Geometrical discontinuities of faults are therefore not effective barriers if the distance between the two faults is too small. The earthquake then consists of several sub-events separated by zones where the slip is small, or even zero. Imaging of the rupture shows that this is the scenario that probably occurred in the case of the Landers earthquake. Indeed, our results indicate the presence of three zones of strong slip separated by two zones where the slip is less, and which are located at two points where the fault system s geometry becomes complex, and where the earthquake jumped from the Johnson Valley fault to the Homestead fault and, farther north, from the Homestead fault to the Emerson fault. 224 INSTITUT DE RADIOPROTECTION ET DE SÛRETÉ NUCLÉAIRE

6 7Safety of radioactive waste Earthquake arrest: numerous hypotheses When fault segments are not responsible for earthquake arrest, why do earthquakes stop propagating? Several hypotheses may be put forward. RUPTURE ARREST CONTROLLED BY SPATIAL VARIATIONS IN FRICTION LAWS The arrest of seismic rupture below a depth of 15 km is probably due to the change in characteristics of the rock with depth. Beyond about fifteen kilometers in California, the materials slip aseismically and continuously. Is there reason to believe that local material properties along faults between 0 and 15 km depth portray similar characteristics? Might the north of the Camp Rock fault or the north of the Johnson Valley fault also act aseismically, as does the deeper part of the faults? This hypothesis is unlikely, as observation of the slow motion of the faults, made possible by geodesy and radar interferometry, does not show any aseismic response of these faults to the stress variations caused by the earthquake s dynamics 1. Other hypotheses have to be formulated for these arrest points. RUPTURE ARREST CONTROLLED BY THE HISTORY OF PREVIOUS EARTHQUAKES In order to understand why the Landers earthquake stopped at points where the faults are rather smooth and intrinsically favorable to the propagation of the rupture, American seismologists trenched across the faults responsible for the Landers earthquake. These studies led to the discovery and dating of fault displacements caused by previous earthquakes. Paleoseismicity studies thus contribute vital information for a better understanding of earthquake initiation and arresting processes. In actual fact, in the zone where a recent earthquake has taken place, the stresses on the fault have been relaxed and are weak. It is therefore to be expected that a considerable time must elapse before the stress level again reaches the necessary level for a new slip to occur. Trench studies show that along the Camp Rock fault and to the south of the Emerson fault, earthquakes have taken place in the last few thousand years. Along the other fault segments of the Landers earthquake, the previous events are much older. Thus, the north of the Camp Rock fault and the south of the Emerson fault may be regarded as possible relaxation barriers for the Landers earthquake. The existence of earthquakes in the recent past relative to the average period of earthquake recurrence along this fault system hampers the propagation of a present-day earthquake in these zones. The relaxation barrier hypothesis cannot, however, adequately explain the physical mechanism that arrested the earthquake propagation to the north of the Johnson Valley fault, since no recent rupture event could be detected along this fault. The influence of former earthquakes is not limited to rupture arrest. Each event contributes to the redistribution of stress along the fault plane, and the observed complexity of the slip is therefore linked to the heterogeneity of the release of stresses occurring during previous earthquakes 2. RUPTURE ARREST DUE TO A CHANGE IN FAULT ORIENTATION WITH RESPECT TO THE STRESS FIELD Another hypothesis has been put forward to explain the rupture arrest on the Camp Rock and Johnson Valley faults. When the geometry of the Landers earthquake faults is analyzed, it can also be seen that the direction of the faults where the References 1 - B. Hernandez, F. Cotton and M. Campillo, A Comparison between Short Term Co-seismic) and Long Term one Year) Slip for the Landers Earthquake : Measurements from Strong Motion and SAR Interferometry. Geophysical Research Letters, , M. Bouchon, M. Campillo and F. Cotton, Stress Field Associated with the Rupture of the 1992 Landers, California, Earthquake and its Implication Concerning the Fault Strength at the Onset of the Earthquake. Journal of Geophysical Research, The observed complexity of the slip is linked to the heterogeneity of the release of stresses occurring during previous earthquakes. SCIENTIFIC AND TECHNICAL REPORT

7 The orientation of the fault plane with respect to the directions of the tectonic forces caused by plate movement is significant. Reference 1 - E. Calais, R. Bayer, J. Chéry, F. Cotton, E. Doerflinger, M.Flouzat, F. Jouanne, M. Kasser M. Laplanche, D. Maillard, J. Martinod, F. Mathieu, P. Nicolon, J. M. Nocquet, O. Scotti, L. Serrurier, M. Tardy and C. Vigny, REGAL : réseau GPS permanent dans les Alpes occidentales. Configuration et premiers résultats. Bulletin Soc. Géol., France, , rupture develops varies from north-south southern part of the Johnson Valley) to northwest/south-east in the Camp Rock and Emerson faults and the northern part of the Johnson Valley. The orientation of the fault plane with respect to the directions of the tectonic forces caused by plate movement is, in fact, significant. The forces perpendicular to the fault increase the contact between its two sides, increasing the friction, and raising the rupture threshold. The analogy of a block pulled by a spring can also be applied here: if the block is pressed down, it is harder for it to slide. In the Landers region, regional seismicity studies have shown that the main direction of the stress field is north-east/south-west. North-west/south-east oriented faults thus have an alignment that is not very favorable to slipping in this stress field strong normal stress and weak tangential stress). The faults in the south, with a north-south alignment, on the other hand, are favorably aligned for slipping weak normal stress and strong tangential stress). As the slip propagated northward, the fault orientation became progressively less favorable with respect to the stress field, and the rupture threshold increased until it eventually arrested the rupture. The Durance at Saint-Paul-lez-Durance Bouches-du-Rhône). In the background: the Mirabeau fold. Application to seismic risk The imaging studies that we have carried out reveal the difficulties in estimating the maximum magnitude of an earthquake, since geometric barriers alone may not stop a rupture from propagating. The variations in the geometry of faults and their segmentation affect the progress of rupture, but this can occur over several faults and thus jump over geometrical jogs that are intrinsically solid. In spite of these limitations, the multidisciplinary studies currently in progress are on the way to improving the assessment of seismic hazard. The geodetic networks 1 will thus soon enable the accurate determination of tectonic movements and therefore the fault loading rate. They will also help to determine whether these faults are locked or whether they slip aseismically. Seismological and geological studies are therefore complementary and can be used to determine whether locked faults release accumulated elastic energy via minor tremors or via rare, destructive earthquakes which, owing to long intervals between events, are not yet visible on our seismicity maps with their limited period of recording, but whose traces are still seen in the landscape or in catalogs of historical seismicity. Finally, rupture-imaging methods are opening up new prospects. They can additionally be used to locate zones that did not slip during the earthquake and that remain potentially dangerous in the near future. In conjunction with laboratory friction studies and dynamic theoretical modeling, they might be able to better characterize the constitutive friction laws at depth and to distinguish the geometrical jogs that can be jumped by an earthquake rupture from those that remain insurmountable, irrespective of the earthquake scenario considered. 226 INSTITUT DE RADIOPROTECTION ET DE SÛRETÉ NUCLÉAIRE