Shallow Rupture Research Articles

Structural control on the Tohoku earthquake rupture process investigated by 3D FEM, tsunami and geodetic data.

The 2011 Tohoku earthquake (Mw = 9.1) highlighted previously unobserved features for megathrust events, such as the large slip in a relatively limited area and the shallow rupture propagation. We use a Finite Element Model (FEM), taking into account the 3D geometrical and structural complexities up to the trench zone, and perform a joint inversion of tsunami and geodetic data to retrieve the earthquake slip distribution. We obtain a close spatial correlation between the main deep slip patch and the local seismic velocity anomalies, and large shallow slip extending also to the North coherently with a seismically observed low-frequency radiation. These observations suggest that the friction controlled the rupture, initially confining the deeper rupture and then driving its propagation up to the trench, where it spreads laterally. These findings are relevant to earthquake and tsunami hazard assessment because they may help to detect regions likely prone to rupture along the megathrust, and to constrain the probability of high slip near the trench. Our estimate of ~40 m slip value around the JFAST (Japan Trench Fast Drilling Project) drilling zone contributes to constrain the dynamic shear stress and friction coefficient of the fault obtained by temperature measurements to ~0.68 MPa and ~0.10, respectively.

Constraining shallow slip and tsunami excitation in megathrust ruptures using seismic and ocean acoustic waves recorded on ocean-bottom sensor networks

Great earthquakes along subduction-zone plate boundaries, like the 2011 magnitude 9.0 Tohoku-Oki, Japan, event, deform the seafloor to generate massive tsunamis. Tsunami wave heights near shore are greatest when excitation occurs far offshore near the trench, where water depths are greatest and fault slip is shallow. The Tohoku event, featuring over 30 m of slip near the trench, exemplifies this hazard. Unfortunately the rupture process that far offshore is poorly constrained with land-based geodetic and even most seafloor deformation measurements, and seismic inferences of shallow slip are often nonunique. Here we demonstrate, through dynamic rupture simulations of the Tohoku event, that long-period guided waves in the ocean (specifically, leaking oceanic P-wave modes known as PL waves) can resolve the shallow rupture process and tsunami excitation near the trench. With predicted pressure changes of ∼0.1–1 MPa along most of the seafloor landward of the trench, and periods of several seconds, these PL waves should be observable with ocean-bottom pressure sensors and/or seismometers. With cabled sensor networks like those being deployed offshore Japan and in other subduction zones, these waves could be used to rapidly quantify shallow slip and near-trench seafloor uplift and improve local tsunami early warning systems.

The 2004 Aceh‐Andaman Earthquake: Early clay dehydration controls shallow seismic rupture

The physical state of the shallow plate‐boundary fault governs the updip extent of seismic rupture during powerful subduction zone earthquakes and thus on a first order impacts on the tsunamigenic hazard of such events. During the 2004 Mw 9.2 Aceh‐Andaman Earthquake seismic rupture extended unusually far seaward below the accretionary prism causing the disastrous Indian Ocean Tsunami. Here we show that the formation of a strong bulk sediment section and a high fluid‐pressured predécollement, that likely enabled the 2004 rupture to reach the shallow plate‐boundary, result from thermally controlled diagenetic processes in the upper oceanic basement and overlying sediments. Thickening of the sediment section to >2 km ∼160 km seaward of the subduction zone increases temperatures at the sediment basement interface and triggers mineral transformation and dehydration (e.g., smectite‐illite) prior to subduction. The liberated fluids migrate into a layer that likely host high porosity and permeability and that is unique to the 2004 rupture area where they generate a distinct overpressured predécollement. Clay mineral transformation further supports processes of semilithification, induration of sediments, and coupled with compaction dewatering all amplified by the thick sediment section together strengthens the bulk sediments. Farther south, where the 2005 Sumatra Earthquake did not include similar shallow rupture, sediment thickness on the oceanic plate is significantly smaller. Therefore, similar diagenetic processes occur later and deeper in the subduction zone. Hence, we propose that shallow seismic rupture during the 2004 earthquake is primarily controlled by the thickness and composition of oceanic plate sediments.

The February 6, 2013 Mw 8.0 Santa Cruz Islands earthquake and tsunami

The Santa Cruz Islands region has high seismicity near a 90° bend in the boundary between the Pacific and Australian plates. Southward, along the Vanuatu island arc, the Australian plate under-thrusts the Pacific plate in a well-defined subduction zone. Westward, a transpressional, predominantly transform boundary extends to the southern Solomon Islands subduction zone. The Santa Cruz Islands region has upper plate strike-slip and normal faulting, plate boundary under-thrusting, outer rise extensional faulting, and intraplate compressional faulting. On February 6, 2013 the largest under-thrusting earthquake (Mw 8.0) that has been instrumentally recorded in the region ruptured the megathrust. The epicenter (10.738°S, 165.138°E) is about 1° north of epicenters of prior large shallow under-thrusting events in Vanuatu in 1934 (M ~ 7.8) and 1966 (MS 7.9), and there is overlap of all three events' aftershock zones, but not their large-slip regions. At least 10 lives were lost, with 6 more missing, due to tsunami ~ 1.5 m high striking the town of Lata and several villages on the main Santa Cruz Island of Nendö (Ndeni) and a nearby small island Nibanga. Inundation of 500 m flooded the Lata airport. The tsunami was well-recorded by DART buoys spanning an unusually wide three-quadrant azimuthal aperture. Iterative modeling of teleseismic broadband P waves and the deep-water tsunami recordings resolves the slip distribution. There are two large-slip patches with a southeastward rupture expansion at about 1.5 km/s, with the second patch appearing to have ruptured with large slip at the trench. The event has relatively low short-period seismic wave energy release, but a typical overall moment-scaled total energy. The shallow rupture depth may be associated with the low level of short-period energy, and a near-total stress drop may account for a lack of underthrusting aftershocks among a highly productive aftershock sequence that included three events with Mw ≥ 7.0.

Time and Space Distribution of Coseismic Slip of the 2011 Tohoku Earthquake as Inferred from Tsunami Waveform Data

A multiple time window inversion of 53 high‐sampling tsunami waveforms on ocean‐bottom pressure, Global Positioning System, coastal wave, and tide gauges shows a temporal and spatial slip distribution during the 2011 Tohoku earthquake. The fault rupture started near the hypocenter and propagated into both deep and shallow parts of the plate interface. A very large slip (approximately 25 m) in the deep part off Miyagi at a location similar to the previous 869 Jogan earthquake model was responsible for the initial rise of tsunami waveforms and the recorded tsunami inundation in the Sendai and Ishinomaki plains. A huge slip, up to 69 m, occurred in the shallow part near the trench axis 3 min after the rupture initiation. This delayed shallow rupture extended for 400 km with more than a 10‐m slip, at a location similar to the 1896 Sanriku tsunami earthquake, and was responsible for the peak amplitudes of the tsunami waveforms and the maximum tsunami heights measured on the northern Sanriku coast, 100 km north of the largest slip. The average slip on the entire fault was 9.5 m, and the total seismic moment was 4.2×1022 N·m ( M w 9.0). The large horizontal displacement of seafloor slope was responsible for 20%–40% of tsunami amplitudes. The 2011 deep slip alone could reproduce the distribution of the 869 tsunami deposits, indicating that the 869 Jogan earthquake source could be similar to the 2011 earthquake, at least in the deep‐plate interface. The large tsunami at the Fukushima nuclear power station is due to either the combination of a deep and shallow slip or a triggering of a shallow slip by a deep slip, which was not accounted for in the previous tsunami‐hazard assessments. Online Material: Table of estimated slip for all subfaults at 0.5 min invervals.

Resistência ao cisalhamento de solos e taludes vegetados com capim vetiver

Taludes vegetados são mais resistentes contra movimentos de massa e erosão pela água. A vegetação contribui para a estabilidade do solo, principalmente pelo aumento da resistência ao cisalhamento por meio do reforço oferecido pelas raízes. As espécies vegetais que mais têm sido eficientes na estabilização de taludes instáveis são as gramíneas, pertencentes à família Poaceae. Um exemplar dessa família, capim vetiver (Chrysopogonzizanioides (L.) Roberty), tem se revelado muito eficaz nessa estabilização. Com o objetivo de avaliar a melhoria dos parâmetros de resistência ao cisalhamento oferecida ao solo pelas raízes do capim vetiver com o tempo de plantio, realizaram-se experimentos de cisalhamento direto por meio de amostras indeformadas de solo sem vegetação e cultivadas com capim vetiver na idade de um, dois, três e quatro anos de plantio. Também foram realizados experimentos de cisalhamento direto em talude do mesmo solo sem vegetação e com cobertura desse capim. Os resultados do ensaio de cisalhamento direto das amostras de talude de solo foram avaliados por três métodos de equilíbrio limite para verificação de um fator de segurança médio nos taludes. A partir dos ensaios de cisalhamento direto, determinaram-se os parâmetros de resistência ao cisalhamento (intercepto de coesão e ângulo de atrito interno), verificando que o capim vetiver proporcionou incremento de coesão aparente ao solo, sendo esse destacado após três anos de plantio do capim. Esse capim promoveu melhoria nos parâmetros de resistência com consequente elevação do fator de segurança de taludes e aumento da resistência ao cisalhamento de solos, o que auxiliou na estabilização de taludes em planos superficiais de ruptura.

The potential for a great earthquake along the southernmost Ryukyu subduction zone

Interseismic GPS data along the Hualien‐Suao coast (NE Taiwan) shows a pattern of strain accumulation that is consistent with a potential future large shallow earthquake along the southernmost Ryukyu subduction zone. The measured shortening rate parallel to the Ryukyu Trench is 80 mm/yr, about twice of the shortening rate perpendicular to the Ryukyu Trench. We invert for slip‐deficit rates and the geometric configuration of the plate interface. Our preferred fault model dips 10° northward and extends about 70 km from the Ryukyu Trench to a depth of 13 km. The slip‐deficit rate exhibits a left‐lateral motion of 78 mm/yr and a normal motion of 36 mm/yr on a 290°‐trending fault. The slip rate budget of the southernmost Ryukyu subduction zone is close to the plate convergence rate, suggesting the plate interface is fully locked. Assessments of seismic hazard in this region need to consider the potential threat from Mw 7.5∼8.7 tsunami earthquakes generated by shallow ruptures.

Shallow Dynamic Overshoot and Energetic Deep Rupture in the 2011 M w 9.0 Tohoku-Oki Earthquake

Strong spatial variation of rupture characteristics in the moment magnitude (M(w)) 9.0 Tohoku-Oki megathrust earthquake controlled both the strength of shaking and the size of the tsunami that followed. Finite-source imaging reveals that the rupture consisted of a small initial phase, deep rupture for up to 40 seconds, extensive shallow rupture at 60 to 70 seconds, and continuing deep rupture lasting more than 100 seconds. A combination of a shallow dipping fault and a compliant hanging wall may have enabled large shallow slip near the trench. Normal faulting aftershocks in the area of high slip suggest dynamic overshoot on the fault. Despite prodigious total slip, shallower parts of the rupture weakly radiated at high frequencies, whereas deeper parts of the rupture radiated strongly at high frequencies.

200‐m‐deep earthquake swarm in Tricastin (lower Rhône Valley, France) accounts for noisy seismicity over past centuries

AbstractIn the lower Rhône Valley (France), the Tricastin area was struck in 2002–2003 by an earthquake swarm with a maximum ML‐magnitude of 1.7. These shocks would have gone unnoticed if they had not occurred beneath habitations and close to the surface, some events being only 200‐m deep. A several months’ monitoring of the seismic activity by a 16‐station mobile network showed that earthquakes clustered along a N–S‐trending, at least 5‐km long, shallow rupture zone, with no corresponding fault mapped in the surface. Half of the seismic events occurred in a massive, c. 250‐m‐thick, Lower Cretaceous limestone slab that outcrops near by. Since the late eighteenth century, several much more severe earthquake swarms have struck Tricastin. The 1772–1773 and 1933–1936 swarms were prolific and protracted, with reports of numerous detonations and even damage. Obviously, the abnormal noises that caused panic in the past centuries can be explained by the shallowness of the phenomena, a 200‐m focal depth being perhaps a record value for tectonic earthquakes.

２００５年１０月８日パキスタン北部地震について

On Oct. 8, 2005, a large earthquake occurred near the Main Boundary Thrust in the northwestern region of Pakistan. The earthquake showed a shallow rupture with a reverse fault-type focal mechanism, which probably represents the relative plate consumption between the Indian and Eurasian plates. The history of the Himalayan collision and features of earthquakes in the collision zone are reviewed.

Thermal models of the Mexico subduction zone: Implications for the megathrust seismogenic zone

It has been proposed that the seismogenic zone of subduction thrust faults is primarily controlled by temperature or rock composition changes. We have developed numerical models of the thermal structure of the Mexico subduction zone to examine the factors that affect the temperature of the subduction thrust fault. Although the oceanic plates subducting beneath Mexico are young, the top of the oceanic plate at the trench is cool, because of the lack of a thick cover of insulating sediments. Marine heat flow observations suggest that hydrothermal circulation may further cool the oceanic plate. This results in a cool subduction thrust fault, where the brittle part of the fault extends to depths of over 40 km. At these depths, even slight frictional heating may have significant effects on temperature along the thrust fault, particularly for regions with a high convergence rate and shallow plate dip. With the addition of a small amount of frictional heating, the temperatures of the deep (30–40 km) thrust fault are increased by over 200°C. As the observed downdip limit of rupture in recent well‐constrained megathrust earthquakes is confined to depths above the intersection of the thrust fault and the continental Moho, a temperature of 350°C may control the downdip extent of the seismogenic zone. Thus, in order to be consistent with the observed shallow rupture areas, it is necessary to include a small amount of frictional heating, corresponding to an average shear stress of 15 MPa.

Predominance of Unilateral Rupture for a Global Catalog of Large Earthquakes

The manner in which an earthquake rupture propagates across a fault reflects both the initial properties of the fault and the dynamical stresses produced by the rupture. We quantify the propagation of an earthquake rupture using the second moments of the earthquake space-time distribution. In particular, the second moments provide a simple way to differentiate between approximately bilateral and predominantly unilateral ruptures. We determined the second moments for a catalog of M w ≥7 earthquakes that have occurred since 1994. The results show that approximately 80% of large shallow ruptures are predominantly unilateral. Our result agrees well with strong-motion inversions from previous studies on moderate and large earthquakes. The predominance of unilateral propagation in large earthquakes is explained by a simple characteristic earthquake model in which large events rupture an entire structurally defined fault segment and nucleation points are uniformly distributed along the fault. The predominance of unilateral rupture may also be enhanced by the dynamic stress field produced by contrasts in the elastic properties between the two sides of plate boundary faults. Manuscript received 16 November 2001.

Tsunami earthquakes possibly widespread manifestations of frictional conditional stability

Tsunami earthquakes, shallow events that produce larger tsunamis than expected given their surface wave magnitudes (Ms), typically have long durations and a source spectrum depleted in short period energy. Seven cases of underthrusting tsunami earthquakes provide information on the rupture processes, but little constraint on geographic distribution or frequency. We compare their rupture characteristics with smaller magnitude earthquakes on circum‐Pacific interplate thrust faults. Comparable moment release time histories are found for large tsunami earthquakes and for many smaller shallow subduction zone earthquakes, with significantly longer durations and additional source complexity than for events deeper than 15 km. Thus, very shallow interplate earthquake ruptures are scale invariant, with variable frictional properties on the plate interface controlling the depth dependent rupture process. Widespread occurrence of small shallow interplate earthquakes with long durations suggests that many subduction faults have frictional properties that may enable large tsunami‐generating earthquakes to occur; fortunately, large shallow ruptures are infrequent.

Shear response to an intrusive episode at Mt. Etna volcano (January 1998) inferred through seismic and tilt data

The space–time evolution of a seismic swarm (MDmax=3.7) occurring on 9–14 January 1998 at Mt. Etna volcano is analysed. The hypocentral pattern and most reliable focal mechanisms indicate that seismic activity during 10–14 January mainly occurred within a fault-zone located in the volcano's western flank. The fault showed an approximately NE–SW overall trend, and shallow rupture mode of right-lateral strike-slip type. It is noteworthy that earthquakes occurring during the initial phase (9 January) of the swarm showed prevalent dip–slip rupture mechanisms, which are consistent with tensile opening of an approximately NNW–SSE-trending fault-zone. This NNW–SSE structural lineament belongs to a regional lithospheric structure, which has also been identified as a main volcano-genic structural trend of Mt. Etna. The observed continuous tilt data are not explainable by the seismic slip associated with strain released along the NE–SW structure. However, an intrusive source would be compatible with the rupture mechanisms observed during the initial phase of the swarm and could explain the observed tilt. Therefore, we evidence that an intrusive episode occurred along the NNW–SSE structure starting 9 January and during the following days it forced the NE–SW fault system to work in a right-lateral mode. These mechanisms, leading to the February–November 1999 summit eruption, are analogous to the ones observed before the 1991–93 eruption.

The 1994 Java tsunami earthquake: Slip over a subducting seamount

On June 2, 1994, a large subduction thrust earthquake (Ms 7.2) produced a devastating tsunami on the island of Java. This earthquake had a number of unusual characteristics. It was the first recorded large thrust earthquake on the Java subduction zone. All of the aftershock mechanisms exhibit normal faulting; no mechanisms are similar to the main shock. Also, the large tsunami and the relatively low energy radiated by the main shock have led to suggestions that this earthquake might have involved slow, shallow rupture near the trench, similar to the 1992 Nicaragua earthquake. We first relocate the main shock and the aftershocks. We then invert long‐period surface waves and broadband body waves to determine the depth and spatial distribution of the main shock slip. A dip of 12°, hypocenter depth of 16 km and moment of 3.5×l020 N m (Mw 7.6) give the best fit to the combined seismic data and are consistent with the plate interface geometry. The source spectrum obtained from both body and surface waves has a single corner frequency (between 10 and 20 mHz) implying a stress drop of ∼0.3 MPa. The main energy release was preceded by a small subevent lasting ∼12 s. The main slip occurred at ∼20 km depth, downdip and to the NW of the hypocenter. This area of slip is collocated with a prominent high in the bathymetry that has been identified as a subducting seamount. We interpret the Java earthquake as slip over this subducting seamount, which is a locked patch in an otherwise decoupled subduction zone. We find no evidence for slow, shallow rupture. No thrust aftershocks are expected if the entire locked zone slipped during the main shock, but extension of the subducting plate behind the seamount would promote normal faulting as observed. It seems probable that such a source model could also explain the size and timing of the observed tsunami.

The 1994 Cacoosing Valley earthquakes near Reading, Pennsylvania: A shallow rupture triggered by quarry unloading

A mb(Lg) 4.6 mainshock on January 16, 1994, in the Cacoosing valley, 10 km west of Reading, southeastern Pennsylvania caused modified Mercalli VI‐VII intensities and the highest earthquake damage (∼$2 million) in the eastern United States since 1944. Aftershock hypocenters from a temporary local seismic network are confined to the upper 2.5 km of the crust. They are clustered around the periphery of a tabular 3 × 3 km zone that is interpreted to outline the mainshock rupture. This zone matches the nodal plane with reverse and left‐lateral slip (strike 135°, dip 54° southwest, and rake 55°) of a focal mechanism obtained from aftershock first motions and from mainshock waveforms. This rupture does not correlate with any of the faults mapped in the epicentral area, but it is parallel to the most prominent fracture set, including joints and small faults. Maximum possible strike‐slip accumulated on the causative fault is no more than several tenths of meters. A large carbonate rock quarry is centered above the rupture. We calculate a small, but significant (0.13 MPa) Coulomb stress increase caused by the quarry on the shallow portion of the rupture. Most of this increase was caused by pore pressure rise after the quarry was abandoned in December 1992 and flooded. Seismicity started may 1993. We conclude that the 1993–1997 Cacoosing Valley sequence is probably triggered by the quarry. About 200 km northeast of Cacoosing, another quarry in early Paleozoic carbonate rocks triggered the 1974 mb(Lg) 3.0 Wappingers Falls earthquake.

Constrains on the source of the 1755 Lisbon tsunami inferred from numerical modelling of historical data on the source of the 1755 Lisbon tsunami

The 1755 Lisbon tsunami was felt all over the North Atlantic, being one of the first major events of this kind relatively well documented by historical sources. However, in spite of the extensive research work on the historical reports by a considerable number of authors, the epicentre location of this event is still uncertain and its focal mechanism is still not well understood, implying a great uncertainty in the tsunami generating mechanism. The generally assumed epicentre, inferred from isoseismal maps, is located slightly north of the Gorringe Bank (SW Iberia) and the rupture mechanism has been assumed in the past to be similar to the well studied 1969.02.28 event. While all previous studies have used a seismic-based approach, this paper uses all that is known about the tsunami parameters at the coast — presented in a companion paper — to define the location and geometry of the tsunami source. For that purpose some backward ray-tracing techniques were developed and their results were used to define the initial fields in a number of shallow water simulations of the water height at the coastal locations where the most reliable historical data are available. The source parameters also took into account the estimated seismic energy released. The results obtained here suggest that the 1755 tsunami probably originated on the continental shelf, implying an epicentre area located between the Gorringe Bank and the Iberian coast, in a geodynamic context quite different from the one implied in the 1969.02.28 event. The amplitude of the initial movement in the source region, required by the shallow water simulations to account for the reported magnitudes, suggests an elongated but shallow rupture area, extending along the shelf. It is suggested that this location of the rupture would have significant implications in the geology of the region.

Estimation of seismic moment and slip distribution of the April 1, 1946, Aleutian tsunami earthquake

The 1946 Aleutian earthquake produced a tsunami of tsunami magnitude Mt=9.3, but the surface wave magnitude is only Ms=7.4, making it a tsunami earthquake. The discrepancy between the apparent size of the earthquake based on the seismic data and the size of the tsunami has been explained by several mechanisms, including a landslide and a slow earthquake, but there are few seismic data available to determine the correct mechanism. We study the generating mechanism of the tsunami using tsunami waveforms recorded on tide gauges. We have modeled the source of the 1946 Aleutian tsunami as the result of an underthrusting earthquake. We performed both forward and inverse modeling of the data using a finite difference calculation to compute synthetic tsunamis. We include both vertical and horizontal deformation of the ocean bottom due to faulting in the computation. The results of the inversion of the tsunami waveforms show that the slip on the fault is mainly concentrated in the shallow section of the fault near the epicenter. There is little slip in the area of the aftershocks, indicating that the aftershocks may represent an area of afterslip or triggered seismicity. The moment estimate of the earthquake is 23×1020 N m, or Mw=8.2. The majority of the waveforms are well‐explained by this fault model and slip distribution, including the Honolulu waveform, but the Hawaiian data also suggest that other effects, such as a landslide, may be necessary for explaining the abnormal tsunami amplitudes in Hawaii. The 1946 earthquake is similar to other tsunami earthquakes, such as the 1896 Sanriku and 1992 Nicaragua earthquakes, indicating that slow, shallow rupture may explain the disproportionally large tsunamis.

Rupture process of the MacQuarie Ridge Earthquake of May 23, 1989

Broadband body waves recorded at 15 digital seismic stations worldwide are used to study the rupture process of the May 23, 1989 Macquarie Ridge earthquake. The centroidal solution (strike 211°, dip 86°, rake 180°, and depth of 10 km below the seafloor) indicates shallow rupture with pure right‐lateral strike‐slip motion along the Pacific‐Australia plate boundary, in agreement with motion predicted by plate tectonic models. The total seismic moment is 13.4×1020 Nm, 80% of which was released in the first 24 s of the rupture process. Modeling favors a bilaterally propagating rupture with slightly different dip and rake for the northward and southward fault segments and similar moment release along both directions. The estimated fault length is quite short, about 90 km, and the derived stress drop of 180 bar and average displacement of 17 m are unusually high. The bathymetry in the epicentral region shows topographic segmentation of the ridge, possibly indicating fault segmentation which confines ruptures to short segments.

The foreshock sequence of the 1986 Chalfant, California, earthquake

AbstractThe ML 6.4 Chalfant, California, earthquake of 21 July 1986 was preceded by an extensive foreshock sequence. Foreshock activity is characterized by shallow clustering activity, including 7 events greater than ML 3, beginning 18 days before the earthquake, an ML 5.7 foreshock 24 hr before the main shock that ruptured only in the upper 10 km of the crust, and an “off-fault” cluster of activity perpendicular to the slip surface of the ML 5.7 foreshock associated with the hypocenter of the main shock. The Chalfant sequence occurred within the local short-period network, and the spatial and temporal development of the foreshock sequence can be observed in detail. Seismicity of the July 1986 time period is largely confined to two nearly conjugate planes; one striking N30°E and dipping 60° to the northwest associated with the ML 5.7 foreshock and the other striking N25°W and dipping 70° to the southwest associated with the main shock. Focal mechanisms for the foreshock period fall into two classes in agreement with these two planes. Shallow clustering of earthquakes in July and the ML 5.7 principal foreshock occur at the intersection of the two planes at a depth of approximately 7 km. The seismic moments determined from inversion of the teleseismic body waves are 4.2 × 1025 and 2.5 × 1025 dyne-cm for the principal foreshock and the main shock, respectively. Slip areas for these two events can be estimated from the aftershock distribution and result in stress drops of 63 bars for the principal foreshock and 16 bars for the main shock. The main shock occurred within an “off-fault” cluster of earthquakes associated with the principal foreshock. This cluster of activity occurs at a predicted local shear stress high in relation to the slip surface of the 20 July earthquake, and this appears to be the triggering mechanism of the main shock. The shallow rupture depth of the principal foreshock indicates that this event was anomalous with respect to the character of main shocks in the region.