Low Stress Drop Research Articles

Do Strike-Slip Faults of Molise, Central-Southern Italy, Really Release a High Stress?

The 31 October and 1 November 2002 Molise earthquakes (both M w 5.7) were caused by right-lateral slip between 12 and 20 km depth. These earthquakes are the result of large-scale reactivation of preexisting, left-lateral, regionally extended east–west structures of Mesozoic age. Although recorded ground motions were generally smaller than expected for typical Italian earthquakes, a recent paper attributes a stress drop as high as 180 bars to the Molise earthquakes. We remark that a high stress drop is in contrast both with the relatively long source duration inferred in previous investigations and with geodetic evidence for a significantly smaller fault slip compared with other Apennine earthquakes having similarly large rupture area (e.g., 1997 Umbria–Marche earthquakes). We analyzed both ground acceleration spectra of the mainshocks and single-station spectral ratios of broadband seismograms in an extended magnitude range (2.7≤ M w≤5.7). Our results show that neither the spectral amplitudes of recorded ground motions nor the spectral ratios can be fit by a high stress-drop source. Instead we find that the observations are consistent with a low stress drop, our best estimates ranging between 6 and 25 bars, in agreement with the relatively long source duration and small coseismic slip. We interpret the low stress of the 2002 Molise earthquakes in terms of lower energy release mechanisms due to the reutilization of faults reactivated opposite to their original sense of slip.

Dynamic modeling of slow earthquakes based on thermoporoelastic effects and inelastic generation of pores

We numerically simulate features of slow earthquakes to understand their generation mechanism in the framework of dynamic modeling. Their typical features will be that the fault rupture velocity and stress drop are markedly lower than those of ordinary earthquakes. We assume a fault in a thermoporoelastic medium taking account of fluid flow and inelastic creation of pores on the fault. This paper is an extension of our studies published in the work of Suzuki and Yamashita (2006, 2007, 2008), in which a nondimensional parameter Su was shown to play a critical role in dynamic fault rupture. The parameter Su represents the dominance of the effect of inelastic pore creation over that of frictional heating under the condition of no fluid flow. However, it can be shown that Su plays an important role even if the fluid flow is considered. In the present study we successfully simulate slow fault rupture growth and low stress drop, which characterize the slow earthquakes. Critical ingredients of our modeling are assumptions of (1) Su considerably larger than assumed for the simulation of ordinary earthquakes, (2) fluid flow into the inelastically created pores, and (3) initial shear stress significantly smaller than assumed for the simulation of ordinary earthquakes. The assumption of a large value of Su corresponds to that of a slip‐resistant and low‐stress‐drop zone. The fluid inflow can promote the fault rupture in the slip‐resistant zone.

Statistics of Earthquake Stress Drops on a Heterogeneous Fault in an Elastic Half-Space

We investigate properties of earthquake stress drops in simulations of evolving seismicity and stress field on a heterogeneous fault. The model consists of an inherently discrete strike-slip fault surrounded by a 3D elastic half-space. We consider various spatial distributions of frictional properties and analyze results generated by 150–300 model years. In all cases, the self-organized heterogeneous initial stress distributions at the times of earthquake failure lead to stress drops that are systematically lower than those predicted for a homogeneous process. In particular, the large system-sized events have stress drops that are consistently ∼25% of predictions based on the average fault strength. The type and amount of assumed spatial heterogeneity on the fault affect the stress-drop statistics of small earthquakes ( M L <5) more than those of the larger events. This produces a decrease in the range of stress drops as the earthquake magnitudes increase. The results can resolve the discrepancy between traditional estimates of stress drops and seismological observations. The general tendency for low stress drops of large events provides a rationale for reducing the statistical estimates of potential ground motion associated with large earthquakes.

Apparent Slow Oceanic Transform Earthquakes Due to Source Mechanism Bias

Slow earthquakes, characterized as ones whose magnitude increases with period, are often observed occurring on oceanic transform faults (Kanamori and Stewart 1976; Okal and Stewart 1982; Stein and Pelayo 1991; Choy and Boatwright 1995; Newman and Okal 1998; Perez-Campos et al. 2003). Previous work has found that oceanic transform earthquakes often have lower body wave magnitudes compared to ridge and oceanic intraplate earthquakes of comparable surface wave magnitude and seismic moment. This phenomenon might result from the slow events having lower stress drop. Alternatively, the effect could be due to a source mechanism bias resulting from the fact that transform earthquakes are primarily strike-slip events on steeply dipping faults. In this geometry, body waves arriving at teleseismic distances left the source near nodal planes, resulting in smaller amplitudes than for dip-slip events of the same moment. Slow earthquakes are ones that release energy more “slowly” than typical earthquakes and therefore radiate more energy at longer periods. Such earthquakes have been identified in various ways. One is from an anomalous ratio of the body wave magnitude mb , which reflects the energy release at a period of 1 s, to the surface wave magnitude MS , which reflects the energy release at a period of 20 s. Although slow earthquakes have been observed in various tectonic environments, they are especially common for oceanic transform earthquakes that have strike-slip mechanisms, as shown by analysis of a global catalog of oceanic earthquakes (Stein and Pelayo 1991). The transform earthquakes generally had lower body wave magnitudes compared to ridge and oceanic intraplate earthquakes of comparable surface wave magnitude and seismic moment. This effect is illustrated in Figure 1A, where a transform event with a moment larger than the ridge event shown has lower mb than …

Maximum entropy production and earthquake dynamics

We examine the consistency of natural and model seismicity with the maximum entropy production hypothesis for open, slowly‐driven, steady‐state, dissipative systems. Assuming the commonly‐observed power‐law feedback between remote boundary stress and strain rate at steady state, several natural observations are explained by the system organizing to maximize entropy production in a near but strictly sub‐critical state. These include the low but finite seismic efficiency and stress drop, an upper magnitude cut‐off that is large but finite, and the universally‐ observed Gutenberg‐Richter b‐value of 1 in frequency‐magnitude data. In this state the model stress field organizes into coherent domains, providing a physical mechanism for retaining a finite memory of past events. This implies a finite degree of predictability, strongly limited theoretically by the proximity to criticality and practically by the difficulty of directly observing Earth's stress field at an equivalent resolution.

Spectral P-wave magnitudes, magnitude spectra and other source parameters for the 1990 southern Sudan and the 2005 Lake Tanganyika earthquakes

Teleseismic Broadband seismograms of P-waves from the May 1990 southern Sudan and the December, 2005 Lake Tanganyika earthquakes; the western branch of the East African Rift System at different azimuths have been investigated on the basis of magnitude spectra. The two earthquakes are the largest shocks in the East African Rift System and its extension in southern Sudan. Focal mechanism solutions along with geological evidences suggest that the first event represents a complex style of the deformation at the intersection of the northern branch of the western branch of the East African Rift and Aswa Shear Zone while the second one represents the current tensional stress on the East African Rift. The maximum average spectral magnitude for the first event is determined to be 6.79 at 4 s period compared to 6.33 at 4 s period for the second event. The other source parameters for the two earthquakes were also estimated. The first event had a seismic moment over fourth that of the second one. The two events are radiated from patches of faults having radii of 13.05 and 7.85 km, respectively. The average displacement and stress drop are estimated to be 0.56 m and 1.65 MPa for the first event and 0.43 m and 2.20 MPa for the second one. The source parameters that describe inhomogeneity of the fault are also determined from the magnitude spectra. These additional parameters are complexity, asperity radius, displacements across the asperity and ambient stress drop. Both events produce moderate rupture complexity. Compared to the second event, the first event is characterized by relatively higher complexity, a low average stress drop and a high ambient stress. A reasonable explanation for the variations in these parameters may suggest variation in the strength of the seismogenic fault which provides the relations between the different source parameters. The values of stress drops and the ambient stresses estimated for both events indicate that these earthquakes are of interplate type.

Coseismic source model of the 2003Mw6.8 Chengkung earthquake, Taiwan, determined from GPS measurements

A coseismic source model of the 2003Mw6.8 Chengkung, Taiwan, earthquake was well determined with 213 GPS stations, providing a unique opportunity to study the characteristics of coseismic displacements of a high‐angle buried reverse fault. Horizontal coseismic displacements show fault‐normal shortening across the fault trace. Displacements on the hanging wall reveal fault‐parallel and fault‐normal lengthening. The largest horizontal and vertical GPS displacements reached 153 and 302 mm, respectively, in the middle part of the network. Fault geometry and slip distribution were determined by inverting GPS data using a three‐dimensional (3‐D) layered‐elastic dislocation model. The slip is mainly concentrated within a 44 × 14 km slip patch centered at 15 km depth with peak amplitude of 126.6 cm. Results from 3‐D forward‐elastic model tests indicate that the dome‐shaped folding on the hanging wall is reproduced with fault dips greater than 40°. Compared with the rupture area and average slip from slow slip earthquakes and a compilation of finite source models of 18 earthquakes, the Chengkung earthquake generated a larger rupture area and a lower stress drop, suggesting lower than average friction. Hence the Chengkung earthquake seems to be a transitional example between regular and slow slip earthquakes. The coseismic source model of this event indicates that the Chihshang fault is divided into a creeping segment in the north and the locked segment in the south. An average recurrence interval of 50 years for a magnitude 6.8 earthquake was estimated for the southern fault segment.

SPECTRAL P-WAVE MAGNITUDES, MAGNITUDE SPECTRA AND OTHER SOURCE PARAMETERS FOR THE 1990 SOUTHERN SUDAN AND THE 2005 LAKE TANGANYIKA EARTHQUAKES

Abstract Teleseismic Broadband seismograms of P-waves from the May 1990 southern Sudan and the December, 2005 Lake Tanganyika earthquakes; the western branch of the East African Rift System at different azimuths have been investigated on the basis of magnitude spectra. The two earthquakes are the largest shocks in the East African Rift System and its extension in southern Sudan. Focal mechanism solutions along with geological evidences suggest that the first event represents a complex style of the deformation at the intersection of the northern branch of the western branch of the East African Rift and Aswa Shear Zone while the second one represents the current tensional stress on the East African Rift. The maximum average spectral magnitude for the first event is determined to be 6.79 at 4 s period compared to 6.33 at 4 s period for the second event. The other source parameters for the two earthquakes were also estimated. The first event had a seismic moment over fourth that of the second one. The two events are radiated from patches of faults having radii of 13.05 and 7.85 km, respectively. The average displacement and stress drop are estimated to be 0.56 m and 1.65 MPa for the first event and 0.43 m and 2.20 MPa for the second one. The source parameters that describe inhomogeneity of the fault are also determined from the magnitude spectra. These additional parameters are complexity, asperity radius, displacements across the asperity and ambient stress drop. Both events produce moderate rupture complexity. Compared to the second event, the first event is characterized by relatively higher complexity, a low average stress drop and a high ambient stress. A reasonable explanation for the variations in these parameters may suggest variation in the strength of the seismogenic fault which provides the relations between the different source parameters. The values of stress drops and the ambient stresses estimated for both events indicate that these earthquakes are of interplate type.

A scaling law for slow earthquakes

Recently, a series of unusual earthquake phenomena have been discovered, including deep episodic tremor, low-frequency earthquakes, very-low-frequency earthquakes, slow slip events and silent earthquakes. Each of these has been demonstrated to arise from shear slip, just as do regular earthquakes, but with longer characteristic durations and radiating much less seismic energy. Here we show that these slow events follow a simple, unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes. We find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant, with a spectral high-frequency decay of f(-1). This scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category. The observed scale dependence of rupture velocity for these events can be explained by either a constant low-stress drop model or a diffusional constant-slip model. This new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation.

Spatial and temporal stress drop variations in small earthquakes near Parkfield, California

We estimate source parameters from spectra of 42367 earthquakes between 1984 and 2005 that occurred in the Parkfield segment of the San Andreas Fault in central California. We use a method that isolates the source term of the displacement spectra based on a convolutional model and correct the observed P wave source spectra with a spatially varying empirical Green's function (EGF). Our Brune‐type stress drop estimates vary from 0.1 to over 100 MPa with a median value of 6.75 MPa, which is nearly constant with moment, implying self‐similarity over the ML = 0.5 to 3.0 range of our data. The corner frequency decreases for earthquakes at shallower depths, consistent with slower rupture velocities and reduced shear wave velocities in local velocity models. The estimated median stress drops show significant lateral variations: we find lower stress drops in the Middle Mountain asperity and along the creeping fault section, and higher stress drops in the hypocentral region of the 2004 M6.0 Parkfield earthquake. The main shock did not alter the overall pattern of high and low stress drop regions. However, a statistical test reveals areas with significant changes in computed stress drops after the main shock, which we compare to estimated absolute shear stress changes from a main shock slip model. By calculating Δt* from the spectral EGF ratio, we also identify areas with increased attenuation after the main shock, and we are able to distinguish source effects and near‐source attenuation effects in the spectral analysis. These results are confirmed independently from spectral ratios of repeating microearthquake clusters.

Highland Boundary Fault Zone: Tectonic implications of the Aberfoyle earthquake sequence of 2003

The Highland Boundary Fault Zone (HBFZ) is one of the major faulted tectonic boundaries in Great Britain. Historically, seismicity has occurred in this zone around the town of Comrie. But an earthquake sequence that occurred in 2003 near the village of Aberfoyle ( M L 1.3–3.2) was the first significant activity to be recorded in the HBFZ since the installation of modern seismograph networks in the 1970s. This study describes detailed analysis of these data. The waveform signals of the events were almost identical and by applying a cross-correlation technique combined with multiple event location, the alignment of the events was found to be WSW–ENE. This alignment matches one of the nodal planes determined by joint focal mechanism analysis. The fault plane dips to the northwest, and shows oblique sinistral strike–slip with normal movement. The orientation of the event alignment matches the direction and orientation of observed features in the HBFZ. Hence, it is concluded that the WSW–ENE striking nodal plane was the causative fault that is associated with the HBFZ. The orientation of maximum compressional stress is rotated from the regional average expected due to the Mid-Atlantic ridge-push force. This rotation is possibly explained by stresses due to postglacial rebound. Smaller events in the sequence were used as empirical Green's functions and deconvolved from the larger events to determine source time functions. The corresponding corner frequencies matched results from spectral fitting, showing that the events were of relatively low stress drop.

Earthquake scaling, fault segmentation, and structural maturity

Slip and length measurements on earthquakes suggest large stress drop variability. We analyze an extended set of slip-length measurements for large earthquakes ( M ≥ 6) to seek for the possible origin(s) of this apparent variability. We propose that such variability arises from earthquakes breaking a variable number of major fault segments. That number depends on the strength of the inter-segment zones, which itself depends on the structural maturity of the faults. We propose new D max– L parameterizations based on that idea of multiple segment-ruptures. In such parameterizations, each broken segment roughly scales as a crack, while the total multi-segment rupture does not. Stress drop on individual segments is roughly constant, only varying between 3.5 to 9 MPa. The slight variation that is still observed depends on fault structural maturity; more mature faults have lower stress drops than immature ones. The new D max– L functions that we propose reduce uncertainties with respect to available relationships. They thus provide a more solid basis to estimate seismic hazard by integrating fault properties revealed by geological studies.

Testing the Validity of Simulated Strong Ground Motion from the Dynamic Rupture of a Finite Fault, by Using Empirical Equations

This paper is concerned with testing the validity of the ground motions estimated by combining a boundary integral equation method to simulate dynamic rupture along finite faults with a finite difference method to compute the subsequent wave propagation. The validation exercise is conducted by comparing the calculated ground motions at about 100 hypothetical stations surrounding the pure strike-slip and pure reverse faults with those estimated by recent ground motion estimation equations derived by regression analysis of observed strong-motion data. The validity of the ground motions with respect to their amplitude, frequency content and duration is examined. It is found that the numerical simulation method adopted leads to ground motions that are mainly compatible with the magnitude and distance dependence modelled by empirical equations but that the choice of a low stress drop leads to ground motions that are smaller than generally observed. In addition, the scatter in the simulated ground motions, for which a laterally homogeneous crust and standard rock site were used, is of the same order as the scatter in observed motions therefore, close to the fault, variations in source propagation likely contribute a significant proportion of the scatter in observed motions in comparison with travel-path and site effects.

A seismological reassessment of the source of the 1946 Aleutian ‘tsunami’ earthquake

SUMMARY We present a re-evaluation of the seismological properties of the Aleutian ‘tsunami earthquake’ of 1946 April 1, characterized by a deceptively low conventional magnitude (7.4) in view of its catastrophic tsunami, both in the near and far fields. Relocation of 40 aftershocks show that the fault zone extends a minimum of 181 km along the Aleutian trench, in a geometry requiring a bilateral rupture from the original nucleation at the epicentre. Their spatial and temporal distribution are typical of the aftershock patterns of a large earthquake, and rule out the model of a landslide source exclusive of a dislocation. The analysis of the spectra of mantle waves favours the model of a large seismic source, with a static moment of 8.5 × 10 28 dyn-cm, making the event one of the ten largest earthquakes ever recorded (hence the destructive tsunami in the far field), and of a slow bilateral rupture, at an average velocity of only 1.12 km s −1 , hence the destructive interference in all azimuths for all but the longest mantle waves. The exceptionally slow character of the earthquake is confirmed by a deficiency in radiated seismic energy expressed by the lowest value measured to date of the energy-tomoment ratio. The earthquake appears as an end member in the family of ‘tsunami earthquakes’, resulting from the combination of anomalous, but not unprecedented, parameters, such as low stress drop and rupture velocity.

Very low frequency earthquakes within accretionary prisms are very low stress‐drop earthquakes

We report characteristic P wave spectra and stress drops of very low frequency (VLF) earthquakes that occurred within the accretionary prism along the Nankai Trough, southwestern Japan. Although many VLF earthquakes showed no distinct phases on a raw seismogram, a few showed slightly distinct phases. The arrival times of some phases were consistent with the calculated P wave arrival times. The corner frequencies of the VLF earthquakes were calculated using the stacked P wave spectra assuming an omega‐square spectrum. Stress drops were very low, i.e., in the range of 0.1–10 kPa, which corresponds to 0.1%–1% of those of ordinary earthquakes. Such extremely low stress drops of the VLF earthquakes suggest that the fault strength within the accretionary prism may have weakened due to the existence of a fluid within the thrust fault system of the accretionary prism.

Nucleation process with dilatant hardening on a fluid‐infiltrated strike‐slip fault model using a rate‐ and state‐dependent friction law

Dilatancy and fluid movements in a fault zone significantly affect the mode of the nucleation process and are one of the key factors for understanding the physical mechanisms of some short‐term precursory phenomena. We investigated a model of the earthquake nucleation processes on a vertical fluid‐infiltrated strike‐slip fault in a three‐dimensional elastic half‐space using a Dieterich/Ruina rate‐ and state‐dependent friction law and an evolution equation for plastic porosity. Because of dilatancy, porosity is thought to increase with slip velocity. We assume that plastic porosity depends logarithmically on the state variable in the friction law. The results of numerical simulations show that porosity in the fault zone increases with increasing slip velocity in the nucleation zone. Then pore fluid pressure in the fault zone drops significantly, resulting in dilatant hardening. With increasing dilatancy coefficient ɛ or with decreasing pore compressibility βφ, larger pore fluid pressure drop is observed during the nucleation process, which in turn increases the size of the nucleation zone. We observe that when a dilatancy coefficient ɛ is large, a quasi‐static nucleation zone extends widely with a low stress drop. Finally, slip velocity is accelerated in a localized area in the quasi‐static nucleation zone, which is a place where a dynamic rupture originates. The size of the accelerated slip zone is significantly smaller than that of the quasi‐static nucleation zone. Numerical results using a larger dilatancy coefficient can explain the extension of foreshock activity, which is occasionally observed in large earthquakes.

The nature of volcanic earthquake swarm preceding the 2000 flank eruption at Usu volcano, Hokkaido, Japan

The 2000 eruption of Usu volcano, Hokkaido, occurred on the northwestern flank of volcano on March 31 at 13:07 (Local time), after about 4 days of intensive seismic activity. We study the seismic process prior to the eruption using the routine earthquake catalogue of the Japan Meteorological Agency (JMA) and the digital seismograms of the short-period station NBBT (JMA). Earthquakes began beneath the summit and southern slope of the volcano. Then seismic foci clustered along two linear zones intersecting to the south of the initial epicentral zone. The majority of earthquakes occurred at depth within the range between 4 and 7 km. The magnitude range of located earthquakes varied between 0.8 and 4.4. The earthquake epicentres did not reach the sites of the forthcoming eruption; they stopped at a distance of about 1–3 km. The eruption began during the ceasing of seismic activity. A study of the nature of the swarm was performed for a sample of 303 earthquakes with a magnitude interval from 2.5 to 3.4 that were representative of the swarm. Visual analysis allowed separating the earthquakes into two groups: 286 volcano-tectonic (VT) earthquakes and 17 long-period (LP) earthquakes. The spectral analysis of S waves of 274 VT earthquakes in the frame of Brune's model showed that the VT earthquakes may be separated into two groups as characterized by high stress drops (HSD) and low stress drops (LSD). The swarm began with HSD earthquakes. The LSD and LP earthquakes were recorded about 30–40 hr later, simultaneously with the beginning of deformation of volcanic edifice, and accompanied the HSD earthquakes up to the beginning of eruption. We attribute the HSD earthquakes to the probable product of magmatic thermal fracturing of host rocks. The relatively long LSD ruptures may be attributed to the slips along the pre-existing fractures during magma movement.

Apparent stress, fault maturity and seismic hazard for normal-fault earthquakes at subduction zones

SUMMARY The behavior of apparent stress for normal-fault earthquakes at subduction zones is derived by examining the apparent stress (τa = µES/M 0 ,w here ES is radiated energy and M 0 is seismic moment) of all globally distributed shallow (depth, h 1M Pa )are also generally intraslab, but occur where the lithosphere has just begun subduction beneath the overriding plate. They usually occur in cold slabs near trenches where the direction of plate motion across the trench is oblique to the trench axis, or where there are local contortions or geometrical complexities of the plate boundary. Lower τa (< 1M Pa )i sassociated with events occurring at the outer rise (OR) complex (between the OR and the trench axis), as well as with intracrustal events occurring just landward of the trench. The average apparent stress of intraslab-normal-fault earthquakes is considerably higher than the average apparent stress of interplate-thrust-fault earthquakes. In turn, the average τa of strike-slip earthquakes in intraoceanic environments is considerably higher than that of intraslab-normal-fault earthquakes. The variation of average τa with focal mechanism and tectonic regime suggests that the level of τa is related to fault maturity. Lower stress drops are needed to rupture mature faults such as those found at plate interfaces that have been smoothed by large cumulative displacements (from hundreds to thousands of kilometres). In contrast, immature faults, such as those on which intraslab-normal-fault earthquakes generally occur, are found in cold and intact lithosphere in which total fault displacement has been much less (from hundreds of metres to a few kilometres). Also, faults on which high τa oceanic strike-slip earthquakes occur are predominantly intraplate or at evolving ends of transforms. At subduction zones, earthquakes occurring on immature faults are likely to be more hazardous as they tend to generate higher amounts of radiated energy per unit of moment than earthquakes occurring on mature faults. We have identified earthquake pairs in which an interplate-thrust and an intraslab-normal earthquake occurred remarkably close in space and time. The intraslab-normal member of each pair radiated anomalously high amounts of energy compared to its thrust-fault counterpart. These intraslab earthquakes probably ruptured intact slab mantle and are dramatic examples in which Me (an energy magnitude) is shown to be a far better estimate of the potential for earthquake damage than MW . This discovery may help explain why loss of life as a result of intraslab earthquakes was greater in the 20th century in Latin America than the fatalities associated with interplate-thrust events that represented much higher total moment release.

Fault Rupture Process of the 20 September 1999 Chi-Chi, Taiwan, Earthquake

We modeled the source rupture process of the 20 September 1999 Chi- Chi, Taiwan, earthquake (Mw 7.6) using an integrated dataset of near-field strong ground motion and Global Positioning System observations. This large inland thrust earthquake produced a surface break more than 80 km long along the Chelungpu fault. The event triggered over 700 strong-motion instruments on the island of Tai- wan. This large ground-motion dataset provides the best opportunity to date to un- derstand earthquake source processes in a major earthquake. First, we develop a 3D curved fault model consistent with the surface rupture data and aftershock distribu- tion. The subsurface fault plane has an average dip of 30 to the east, and measures 96 km along strike and 40 km down-dip. Next we applied a genetic algorithm to reconstruct the source rupture process. The inversion results indicate a complex slip distribution with a generally broad triangular shaped slip zone that points down-dip. Most of the displacement occurred over a 15-km depth range in the northern segment of the fault, with the maximum slip exceeding 20 m. The average slip is 3.8 m, and the average rupture velocity is 2.6 km/sec. The mainshock slip distribution clearly complements the aftershock location distribution. Slip rise times range from 8 to 6 sec over most of the primary slip area and decrease slightly as rupture propagates south to north. This relatively long rise time resulted in a low dynamic stress drop and low peak ground accelerations observed at surface stations. The rake angle rotates across the rupture plane from predominantly dip-slip in the southern segment to mostly left-lateral oblique slip in the north. We find that geometrical irregularities over the fault plane played an important role in controlling the temporal rupture behavior. At fault bends, rupture tends to decelerate, the rake angle rotates to a near dip-slip orientation, and the shear stress increases as rupture tries to overcome the bending barriers. The total seismic moment from our inversion is 2.9 10 27 dyne cm, in good agreement with the Harvard and the U.S. Geological Survey moment estimation.

The dynamic rupture process of the 2001 Geiyo, Japan, earthquake

The 2001, Geiyo earthquake (Mw = 6.7) occurred as a normal fault event within the young subducting Philippine Sea plate. We investigate a dynamic rupture process of the earthquake and estimate the dynamic parameters. Firstly, the space‐time distribution of slip of the earthquake, i.e., the kinematic model, was investigated by inverting the near‐field strong ground motion data and teleseismic waveform data. Secondly, we reconstruct a dynamic model in constraint with the above kinematic model. The distribution of Dc is also estimated from the inverted slip rate time functions [Mikumo et al., 2003] and found to be less than 70 cm. We found that dynamic parameters are very heterogeneous. There exist two asperities. The peak stress drop on the first asperity is about 20 MPa, and 10 MPa on the second one. Very low or negative stress drop zone is found between two asperities. The strength excess, and fracture energy distribution are also heterogeneous.