Centroid Moment Tensor Solutions Research Articles

The 2007 Talala, Saurashtra, western India earthquake sequence: Tectonic implications and seismicity triggering

A damaging and widely felt moderate ( M w 5.0) earthquake occurred in the Talala region of Saurashtra, Gujarat (western India) on November 6, 2007. The highly productive sequence comprised about 1300 micro earthquakes ( M > 0.5) out of which 325 of M ⩾ 1.5 that occurred during November 6, 2007–January 10, 2008 were precisely located. The spatial aftershock distribution revealed a NE–SW striking fault in accordance with the centroid moment tensor solution, which in turn implies left-lateral motion. The orientation and sense of shear are consistent with similarly orientated geological fault identified in the area from satellite imagery and field investigation. The aftershocks temporal decay, b-value of frequency–magnitude distribution, spatial fractal dimension, D, and slip ratio (ratio of the slip occurred on the primary fault to the total slip) were examined with the purpose to identify the properties of the sequence. The high b-value (1.18 ± 0.01) may be attributed to the paucity of the larger ( M ⩾ 4.0) aftershocks and reveals crustal heterogeneity and low stress regime. The high p-value (1.10 ± 0.39), implying fast decay rate of aftershocks, evidences high surface heat flux. A value of the spatial fractal dimension ( D) equal to 2.21 ± 0.02 indicates random spatial distribution and source in a two-dimensional plane that is being filled-up by fractures. A slip ratio of 0.42 reveals that more slip occurred on secondary fault systems. The static Coulomb stress changes due to the coseismic slip of the main shock, enhanced off fault aftershock occurrence. The occurrence of a moderate earthquake ( M w 4.3) on October 5, 2008 inside a region of positive Coulomb stress changes supports the postulation on aftershock triggering. When the stress changes were resolved on a cross section including the stronger ( M4.8) foreshock plane that is positioned adjacent to the main fault, it became evident that the activity continued there due to stress transfer from the main rupture.

The Mw 7.6 Dusky Sound earthquake of 2009

The Mw 7.6 Dusky Sound earthquake of July 15th, 2009, was the largest magnitude earthquake in New Zealand since the devastating 1931 Hawke’s Bay event (Ms 7.8). The earthquake was sufficiently large to generate at least a 2.3 m wave at Passage Point. Despite its large magnitude, this event resulted in relatively minimal damage when compared to worldwide events of a similar size. This can be explained as a fortunate combination of the sparse population of the area and the specific physical characteristics of the earthquake. Centroid Moment Tensor (CMT) solutions define the rupture surface as a low-angle plane and finite fault inversions confirm the slip occurred on the interface between the eastward-subducting Australian plate and overriding Pacific plate, initiating at about 30 km depth and rupturing upward and southwestward to about 15 km depth. The oceanward rupture directivity likely contributed to the lower intensity of measured ground motion than might be expected for such a large, shallow event. The amount of radiated seismic energy from the earthquake was relatively small, and far fewer landslides were triggered from this event than from the 2003 Mw 7.2 Fiordland event.

Source Fault of the 2007 Chuetsu-oki, Japan, Earthquake

The 2007 Chuetsu-oki, Japan, earthquake is the world’s first major earthquake upon a source fault that extends beneath a nuclear power plant and is also characterized by difficulty determining the source fault plane. Centroid Moment Tensor solutions indicate an M w 6.6 reverse-faulting crustal earthquake with conjugate fault planes dipping to the northwest and southeast. Early results of aftershock locations suggest that either northwest-dipping plane or southeast-dipping plane can be the source fault plane of this earthquake. We carried out source inversions and empirical Green’s function simulations of observed seismograms; however, they resulted in similar waveform residuals for the two fault planes. We then determined the relative locations of earthquake asperities to the hypocenter using travel-time differences of strong-motion pulses and relocated the aftershocks observed by ocean bottom seismometers deployed in the source region. These results imply that slips mainly occurred on the southeast-dipping fault plane. This implication was later confirmed by results of reflection surveys. During the earthquake, the Kashiwazaki-Kariwa nuclear power plant experienced stronger ground motions than those anticipated at the time of design. The ground motions consist of three seismic pulses that correspond to three asperities. The first and second pulses arose from rupture propagation to the plant, while the compact asperity on the distant southeast-dipping fault plane and its S -wave radiation pattern are responsible for the significant third pulse.

Three-dimensional modeling of intraplate stresses in the epicentral zone of the 21 May 1997 Jabalpur earthquake of Mw 5.8, central India

Three-dimensional intraplate stresses induced by topography and crustal density heterogeneities have been modeled for a region in central India (latitude: 21–26°N, longitude: 77–82°E) that was affected by the 1927 Son-valley ( M w 6.4) and the 1997 Jabalpur ( M w 5.8) earthquakes. The modeled stresses show that these two events occurred in the zones of significant increase in gradients of the principal stress difference ( σ 1 − σ 3) contours. The highest value of these differential stresses (∼ 90 MPa) is obtained at 5 km depth beneath the epicentral zone of 1927 Son-valley earthquake, whereas, a value of 35 MPa is obtained at the focal depth (∼ 35 km) of the 1997 Jabalpur mainshock. Further, depth sections of estimated differential stresses delineate a south-dipping high stress body penetrating the whole crust (probably a mafic intrusive intruded during the Deccan volcanism episode) beneath a zone (bounded by Narmada north fault in the north and Narmada south fault in the south) within which all aftershocks including mainshocks of the 1997 Jabalpur and the 1927 Son-valley have occurred. The modeled direction of principal stresses, as well as inferred mode of faulting (at 5, 20, 35, and 40 km depths), correlates strongly with the long period (20–50 s) centroid moment tensor solutions for the broadband recordings of the M w 5.8 Jabalpur mainshock. Results suggest that estimated large intraplate stresses associated with an inferred mafic intrusive in the presence of high pore pressure (resulted from the dehydration of the serpentinite minerals) and deep brittle/ductile transition may contribute to the failure of rock along the pre-existing weak Narmada south fault leading to the earthquakes in the deep crust. It is proposed that the causal association of large intraplate stresses and the pre-existing deep crustal faults would control the future locales of earthquakes in the Narmada–Son lineament.

Determination of earthquake focal depths and source time functions in central Asia using teleseismic P waveforms

We developed a new method to determine earthquake source time functions and focal depths. It uses theoretical Green's function and a time‐domain deconvolution with positivity constraint to estimate the source time function from the teleseismic P waveforms. The earthquake focal depth is also determined in the process by using the time separations of the direct P and depth phases. We applied this method to 606 earthquakes between 1990 and 2005 in Central Asia. The results show that the Centroid Moment Tensor solutions, which are routinely computed for earthquake larger than M5.0 globally using very long period body and surface waves, systematically over‐estimated the source depths and durations, especially for shallow events. Away from the subduction zone, most of the 606 earthquakes occurred within the top 20 km of crust. This shallow distribution of earthquakes suggests a high geotherm and a weak ductile lower crust in the region.

A seismotectonic model for the Chortís Block

Abstract I propose a new seismotectonic model for the Chortís Block, at the northwestern corner of the Caribbean Plate. Shallow seismicity in the area clearly shows three zones of deformation: one along the North America–Caribbean Plate boundary and another along the Central America volcanic arc, and one in the area of the grabens of northern Central America. Analysis of Centroid moment–tensor solutions for shallow earthquakes in these three area show that T or tension axes are horizontal and trend away from the corner, and that P or compression axes for the plate boundary and the volcanic arc are also horizontal and trending towards the corner. Calculation of seismic moment release per unit volume reveals similar values for the volcanic arc and the plate boundary. The state of stress and similarity in seismic moment release suggest that the Chortís Block is being extruded towards the ESE. This is probably due to compression of the large North America and Cocos Plates that surround it.

Accurate Relative Earthquake Hypocenters Reveal Structure of the Burma Subduction Zone

The Burma arc links the Himalaya to the Andaman–Sumatra trench, and comprehension of the geological features in the region is vital to understand the active tectonic processes in the area. Subduction on the Sumatra trench produced the devastating 2004 Sumatra earthquake ( M w∼9.0), but the mechanisms accommodating relative India–Sundaland motion in Burma are still unknown. Previous seismological studies of the area use only earthquake catalog hypocenters, so structural details of subducted material remain poorly understood. Here, accurate relative hypocenters for 81 earthquakes are estimated using first arrivals picked from regional and teleseismic recordings. Depth determination is improved using measured travel-time differences between P onsets and depth phases ( pP and sP ). The results clearly illustrate a slab 21°–25° N dipping ∼25° at 40–80 km deep, ∼40° at 80–120 km deep, and ∼60° at 120–160 km deep, and its strike follows the Indo–Burman ranges. At latitudes >25° N earthquakes become more shallow, and a diffuse picture of hypocenters with an east–west lateral discontinuity at depth is observed, indicating lateral deformation of the slab. Previous studies suggest there is a transition at ∼90 km deep from shallower strike-slip earthquakes to deeper thrust-type events. Here, accurate depth estimates, combined with confirmation of two Global Centroid Moment Tensor solutions using body-wave modeling, shows that thrust and strike-slip earthquakes both occur deeper than 100 km in the Burma arc.

Seismological evidence for a slab detachment in the Tonga subduction zone

The Tonga Wadati–Benioff zone is characterized by a large seismicity gap beneath the Lau Basin that raises the question of the slab continuity between the shallow to intermediate part (60–300 km) and the deep part (400–700 km). To address this problem, we investigated the Wadati–Benioff Zone geometry and stress regime through a detailed analysis of the spatial distribution of moment tensors and variation of the stress tensor, using the global seismicity [Engdahl, E., Van der Hilst, R., Buland, R., 1998. Global teleseismic earthquake relocation with improved travel times and procedures for depth determination. Bull. Seism. Soc. Am. 88, 722–743.] and the Centroid Moment Tensor solutions (CMT) catalogs [Dziewonski, A.M., Chou, T., Woodhouse, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825–2852.]. The stress tensors were computed using the Gephart's program [Gephart, J.W., Forsyth, D., 1984. An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence. J. Geophys. Res. 89, 9305–9320.]. The stress inversion results indicate that between 21°S and 27°S, and depths down to 700 km the slab is under homogeneous down dip compressional stress regime, while north of 21°S we found strong variations of the stress orientations between the intermediate and deep portions of the slab. We also show that between 14° and 19°S, the stresses at intermediate depth (60–300 km) can be resolved into two slab parallel domains, a thin upper part of the slab that is under downdip compression and the lower part that is under downdip extension. This pattern of two zones with opposite mechanical behavior is characteristic of a subducted plate with a free lower limit that does not interact with the 670-km depth boundary. These results together with the large seismicity gap within the slab argue for a slab detachment.

Plate convergence at the westernmost Philippine Sea Plate

To understand the convergent characteristics of the westernmost plate boundary between the Philippine Sea Plate (PSP) and Eurasian Plate (EP), we have calculated the stress states of plate motion by focal mechanisms. Cataloged by the Harvard centroid moment tensor solutions (Harvard CMT) and the Broadband Array in Taiwan (BATS) moment tensor, 251 focal mechanisms are used to determine the azimuths of the principal stress axes. We first used all the data to derive the mean stress tensor of the study area. The inversion result shows that the stress regime has a maximum compression along the direction of azimuth N299°. This result is consistent with the general direction of the rigid plate motion between the PSP and EP in the study area. In order to understand the spatial variation of the regional stress pattern, we divided the study area into six sub-areas (blocks A to F) based on the feature of the free-air gravity anomaly. We compare the compressive directions obtained from the stress inversion with the plate motions calculated by the Euler pole and the Global Positioning System (GPS) analysis. As a result, the azimuth of the maximum stress axis, σ 1, generally agrees with the directions of the theoretical plate motion and GPS velocity vectors except block C (Lanhsu region) and block F (Ilan plain region). The discrepancy of convergent direction near the Ilan plain region is probably caused by the rifting of the Okinawa Trough. The deviation of the σ 1 azimuth in the Lanhsu region could be attributed to a southwestward extrusion of the Luzon Arc (LA) block between 21°N and 22°N whose northern boundary may be associated with the right-lateral NE–SW trending fault (i.e. Huatung Fault, HF) along the Taitung Canyon. Comparing the σ 1 stress patterns between block C and block D, great strain energy along HF may not be completely released yet. Alternatively, the upper crust of block C may significantly have decoupled from its lower crust or uppermost mantle.

Seismicity and state of stress within the overriding plate of the Tonga‐Kermadec subduction zone

To reassess the main tectonic units and to quantify the slip partitioning within the overriding plate of the Tonga‐Kermadec subduction zone, a seismotectonic study was performed using global seismicity and focal mechanisms catalogs. (1) New tectonic features were identified within the Lau Basin and the volcanic arc by remarkable shallow hypocenters alignments. (2) The Centroid Moment Tensor solutions catalog was processed in order to map the stress tensor variation in the upper plate. We found the tectonic features characterized by a diffuse seismicity are subjected to a composite stress regime and they are interpreted as diffuse immature plate boundaries controlled by the high thermal anomaly lying beneath the Lau Basin. (3) We quantified the margin‐parallel rates of motion using the azimuth of the maximum compressive stress component computed within the interplate zone. The results highlight a major tectono‐kinematic segmentation related to the subduction of the Louisville Seamount Chain.

Fault plane solutions in Sichuan-Yunnan rhombic block and their dynamic implications

Harvard Centroid Moment Tensor (CMT) solutions for earthquakes from 1977 to 2004 showed that the stress fields are obviously different in northwestern Sichuan sub-block (NWSSB), western parts of Central Yunnan sub-block (CYSB) and eastern part of CYSB. The characteristics of the mean stress fields in these three regions are obtained by fitting to CMT solutions. The stress state in NWSSB is characterized by its sub-horizontal tensile principal axis of stress (T axis) in roughly N-S direction and west dipping compressive principal axis of stress (P axis); the one in western part of CYSB is characterized by its ENE dipping T axis and sub-horizontal medium principal axis of stress (B axis) in roughly N-S direction; the one in eastern part of CYSB is characterized by its sub-horizontal P axis in roughly NNW-SSE direction and sub-horizontal T axis in roughly WSW-ENE direction. Finite element method simulation clearly shows that the Indian Plate imposes great extrusion on Sichuan-Yunnan rhombic block (SYRB) near Assam massif. The value of the simulated compressive principal stress decreases with the distance from Assam massif. The simulated directions of the T axes in SYRB form annular distribution encircling Assam. For a homogeneous elastic medium with free boundary conditions on the top and bottom surfaces as well as the displacement boundary conditions derived from the GPS observations on the lateral boundaries, the computation results are consistent with the Harvard CMT solutions in NWSSB and western part of CYSB, while inconsistent with the Harvard CMT solutions in eastern part of CYSB. The inconsistency in eastern part of CYSB can be reduced when it includes inhomogeneous elastic media. The stress states in NWSSB and western part of CYSB revealed by the Harvard CMT solutions are not local, which are mainly controlled by the boundary force on the whole region. On the other hand, the stress state in eastern part of CYSB given by the Harvard CMT solutions is local, which may be affected by local topography, material inhomogeneity, and the drag force underneath.

The 9th of July 1998 Faial Island (Azores, North Atlantic) seismic sequence

The Faial earthquake (ML 5.8) that occurred on the 9th of July, 1998, in the Azores region (north Atlantic), caused nine casualties and severe destruction affecting more than 5,000 people. The main shock was located at sea, 10 km NE of the Faial Island, and triggered a seismic sequence that lasted for several weeks and was characterized by an unusual high p-value of 1.40 for the modified Omori law. We present here the results of a joint inversion of hypocenters and 1D velocity model performed on the data collected by the permanent network complemented with a temporary network installed shortly after the occurrence of the main event. The 1D velocity model shows a heterogeneous upper crust, testified by the observed differences in site effects at the stations, while the middle crust from ∼2.5 to 8 km in depth is quite homogeneous. The Moho is located at a depth of about 12–13 km and the Vp/Vs ratio is found to be around 1.78. The events at depth are mainly concentrated in the middle-lower crust (8–12 km), while their spatial distribution shows a main cluster, visible after relocation, SSE trending. This direction of elongation is consistent with one of the fault planes (N151°E) of the centroid moment tensor (CMT) solution for the main shock. The same plane is the preferred main shock fault plane inferred after a Coulomb failure function analysis on the aftershock distribution. The main event relocation points to a focal depth shallower than 5 km. The aftershocks pattern shows that several fault systems were reactivated by the stress perturbation induced by the main shock. Besides the two main tectonic directions, trending WNW–ESE and NNW–SSE, observed in the tectonics of Faial, Pico, and S. Jorge, there is also evidence of a new tectonic direction trending WSW–ENE.

On the possibility of detecting complex sources of earthquakes from data of the centroid moment tensor catalog

In their analysis of the crustal stress-strain state and the study of strong earthquake sources, a number of authors use the parameter ɛ measuring the deviation of the focal mechanism from the double-couple model. Can one obtain reliable estimates of ɛ from regular determinations of the seismic moment tensor components made at world seismological centers and use these estimates to gain constraints on the rupture at the source? Comparing centroid moment tensor solutions of Harvard University with determinations made at the National Earthquake Information Center (United States) and analyzing some strong earthquakes, we arrive at a negative answer to this question. The values of ɛ and the consideration of determination uncertainties in seismic moment tensor components cannot provide a reliable idea of the source complexity, so that independent geological and geophysical information should be invoked.

Seismic Strain rate along the Middle America Trench reveals significant differences between Cocos-North America and Cocos-Caribbean convergence

SUMMARY Centroid-moment tensor solutions for thrust-faulting earthquakes in the last 27 yr are used to obtain and compare seismic strain rate associated to the subduction process along the Cocos‐North America and along the Cocos‐Caribbean convergent margins. In both cases, the eigenvectors of the strain-rate tensor align along radial and tangential directions with respect to their respective Euler Pole. Seismic strain rate along the Cocos‐North America plate boundary is about 2.8 × 10 −7 yr −1 , a value close to those previously found along the Japan and Aegean convergent margins. Strain rate along the Cocos‐Caribbean margin is one order of magnitude smaller, namely 3.7 × 10 −8 yr −1 . The relative plate velocity is also calculated using seismic moment tensors. We find a relative plate convergence of 1.13 ◦ Myr −1 for the Cocos‐North America plate margin, and 0.13 ◦ Myr −1 for Cocos‐Caribbean. This means that 80 per cent of the plate motion along the Cocos‐North America plate margin is expressed as thrust-faulting earthquakes, as compared to the velocity given by global plate motion models. In contrast, only about 10 per cent of the Cocos‐Caribbean relative motion is translated into thrusting events. Similar results are obtained when a 102-yr earthquake database is used. We suggest that differences in strain rate and percentage of plate motion are due to differences in the behaviour of the overriding plate and/or the Central America forearc sliver being detached from the Caribbean Plate and being incorporated into the Cocos Plate.

A tectonic stress map of China and adjacent regions from earthquakes occurred during 2000∼2004

Earthquake focal mechanism solutions provide the basic information about the present-day regional tectonics stress distribution, which controls the activities of crustal faults. Therefore, continued efforts for updating the database of earthquake focal mechanism solutions are quite valuable and important. The international lithosphere program initiated the ′′world stress map project′′ (Zoback, 1992) since 1980′s. During this project, worldwide studies and measurements of present-day regional tectonics stresses had been put together to establish the database of the global tectonics stresses, and to produce the ′′world stress map′′, which also included information about the present-day regional tectonics stresses of the East Asia. Based on the 1992 version of the world stress map, XU Zhong-huai (2001) had made several improvements, including earthquake centroid moment tensor (CMT) solutions of 2 993 shallow events, focal mechanism solutions according to the P-wave first motions of 404 earthquakes, and 47 bore-hole breakout measurements, and had produced the maps of the present-day regional tectonics stresses, and earthquake focal mechanism solutions of the east Asia. In this study we have tabulated all the available earthquake CMT solutions within China and adjacent regions from the Harvard University catalog for the 5-year period starting on January 1, 2000. Following Xu′s (2001) approach we have derived the maps of hypocenter distribution and earthquake focal mechanism solutions within China and adjacent regions for 2000~2004. Based on the analyses of these data we have discussed the seismic activity and the distribution of earthquakes within different tectonic regions and at different depths. This study has extended the database presented in XU (2001) for another 5 years to 2004.

Systematic determination of earthquake rupture directivity and fault planes from analysis of long-period P-wave spectra

Summary If an earthquake has a primarily unilateral rupture, the pulse width observed on seismograms will vary depending on the angle between the rupture direction and the takeoff vector to the station. We have developed a method to estimate the amount of pulse broadening from the spectrum and apply it to a long-period database of large, globally distributed earthquakes that occurred between 1988 and 2000. We select vertical-component P-waves at epicentral distances of 20°–98°. We compute the spectrum from a 64-s-long window around each P-wave arrival. Each spectrum is the product of source, receiver and propagation response functions as well as local source- and receiver-side effects. Since there are multiple receivers for each source and multiple sources for each receiver, we can estimate and remove the source- and receiver-side terms by stacking the appropriate P log spectra. For earthquakes deeper than ∼200 km, source effects dominate the residual spectra. We use our pulse-width estimates to determine the best rupture direction and to identify which nodal plane of the Harvard centroid moment tensor (CMT) solution is most consistent with this rupture direction for 66 events. In about 30 per cent of the cases, one of the two nodal planes produces a much better fit to the data and can be identified as the true fault plane. When results from previous studies are available for comparison, our rupture directions are usually consistent with their results, particularly for earthquakes with simple rupture histories.

Initial-rupture fault, main-shock fault, and aftershock faults: Fault geometry and bends inferred from centroid moment tensor inversion of the 2005 west off Fukuoka prefecture earthquake

Abstract The 2005 west off Fukuoka prefecture earthquake (M JMA = 7.0) occurred on March 20, 2005 in northwest Kyusyu, Japan. The fault geometry and rupture propagation of the main shock are investigated by applying the centroid moment tensor (CMT) inversion method to densely-distributed broadband seismic network data. An accurate distribution of small aftershocks is also examined by a double-difference method. The source fault of the main shock whose moment magnitude was 6.6 had a strike of 123°, and the rupture propagated in the southeast direction. We obtain forty-four CMT solutions of the main shock and aftershocks with a moment magnitude greater than 3.5. For the main shock, the fault strike estimated using P-wave first-motion polarities differs from that of the moment tensor solution by 13° in the counterclockwise direction. Some aftershocks with a moment tensor solution similar to the focal mechanism of the main shock derived from the P-wave first motions are distributed north of the epicenter of the main shock. The largest aftershock with a strike of 8° in the clockwise direction, which differs from that of the CMT solution of the main shock, was located south of the main shock. These changes in the fault strike play an important role in the initiation and termination of the rupture process of the main shock.

Automated Receiver Function Processing

The aim of this is article is twofold: first to introduce an automated processing system based on SOD (Standing Order far Data , http://www.seis.sc.edu/SOD/) (Owens et al. , 2004), and second to give some initial results and pitfalls from our experience with using this system for automated receiver function calculations. EARS, the EarthScope Automated Receiver Survey, aims to calculate bulk crustal properties for all stations within the continental United States that are available from the IRIS DMC using receiver functions and to distribute these results to interested research and education communities as a “product” of the USArray component of EarthScope. Because of the high level of automation that this requires, a natural extension of this effort is to calculate similar results for all stations in the IRIS DMC. To do this we have employed SOD , a FISSURES/DHI-based software package (Ahern, 2001a, b) that we have developed to do automated data retrieval and processing. We introduce EARS as an example of what we term “receiver reference models” They are standard analysis techniques applied to stations. These are analogous to the Harvard Centroid Moment Tensor solutions (which could be termed source reference models). An RRM need not be a definitive result but rather provides a standard, well known, globally consistent reference. This may be sufficient for many users, just as CMT's are sufficient for many, but may also be a starting point for more in-depth, focused studies. The key features of an RRM are that it be generated from a well known and widely accepted technique, produce results for most or all stations, and provide updated results as new sources of data become available. In addition, it must produce results that are of interest in a form usable by the community. Because automated processing proceeds, by definition, without a large …

Coseismic deformation of the Mid Niigata prefecture Earthquake in 2004 detected by RADARSAT/InSAR

Abstract Crustal deformation associated with the Mid Niigata prefecture Earthquake in 2004 (MJMA = 6.8) was detected using RADARSAT SAR interferometry. A displacement of −40 cm in the line-of-sight to the satellite was observed to the west of the main shock epicenter, and a displacement of 20 cm was observed to the east of the epicenter. Fault parameters estimated from the obtained deformation were in good agreement with those of the Centroid Moment Tensor solution for the main shock provided by the Japan Meteorological Agency. In the postseismic period, a steep gradient of deformation was found around the Obiro and the Western Muikamachi Basin faults, indicating that a fault-slip occurred in the shallow part of the fault. Since the seismicity in the shallow part of the Western Muikamachi Basin fault was very low, we believe that it must have been an aseismic slip. Crustal deformation associated with an aftershock that occurred on October 27 (MJMA = 6.1) was detected, and it was ascertained that this aftershock had ruptured the conjugate plane with respect to the Obiro fault. Since locations and/or mechanisms of these faults differed significantly, it was determined that at least three faults were involved in this series of earthquakes.

Space distribution of the contemporary stress field in the Izu-Bonin Wadati-Benioff zone by inversion of earthquake focal mechanisms

Abstract The study addresses the spatial distribution of the contemporary stress field and stress regime in the Kurile Wadati-Benioff zone (WBZ) based on homogeneous data of earthquake focal mechanism solutions (FMS) and the inverse technique by Gephart and Forsyth (1984) . The data set used consists of 829 Centroid Moment Tensor solutions (time period 1977–2010) and 38 FMS listed in previous studies for intermediate-depth and deep events that occurred prior to 1977. The detailed analysis of the spatial distribution of orientation of P (compression) and T (tension) axes of the individual FMS relative to the local geometry of the subducting slab allowed the outlining of 19 WBZ subvolumes along and across the arc for which the stress field parameters and stress regime (based on the orientations of the principal stresses in a slab's reference frame and the value of R) were evaluated. The stress inversion results show that the shallow portion of the slab (5 WBZ subvolumes), is characterized by sub-horizontal and close to strike-normal maximum compression σ1 and down-dipping minimum compression σ3, the stress regime is of general tension. A two-planar stress pattern with slab-parallel or in-slab σ1 and σ3 in the upper and lower planes, respectively, is observed at intermediate depth all along the arc. An exception of this pattern is found for the slab segment beneath Iturup island where the upper plane is ‘missing’ and the orientations of principal stresses in the depth range 61–140 km are similar to these for the lower plane in the slab segment located to the south. Five sub-volumes have been outlined within the Kurile slab at depth greater than 220 km, each with their own characteristic focal mechanisms and stress distribution. The subvolume V1 (depth range 225–380 km), stretching along the entire arc, is characterized by σ1 and σ3 of SW and ENE orientation, respectively, the stress regime being compressional. The results obtained for the deep portion of the slab indicate along-arc variations in the stress field. The southernmost deep segment of the slab (subvolume V2) is dominated by a tensional stress regime with strike-normal σ1 and close to strike-aligned trending NE σ3. The rest of the deep-seated WBZ subvolumes V3, V4 and V5, situated to the north of V2, are characterized by a compressional stress regime with slab-parallel or in-slab σ1 and slab-normal or strike-normal σ3. However, σ1 is close to slab-parallel only within V4 in the central part of the arc. In the subvolumes V3 and V5, located to the south and north of V4, σ1 rotates counterclockwise relatively to the slab-dip direction at about 50° and 60°, respectively. The results obtained indicate that the main geodynamic forces that drive the presently active processes at different depths along the Kurile Wadati-Benioff zone are the slab pull and ridge push at shallow depths, unbending of the slab at intermediate depths causing the observed two-planar stress pattern, and the mantle resistance in the central and northern deep-seated slab segments. The stress inversion results indicate also additional forces that participate in the contemporary dynamics within several WBZ subvolumes: lateral tension that modifies the σ3 orientation from slab-parallel into strike-aligned, trending NNE and NE in the lower plane of segment K1-2, in the intermediate-depth subvolume beneath Iturup island, and in the deep subvolume V2; forces modifying the orientation of σ1 within the subvolumes V1, V3 and V5 from slab-parallel into strike-aligned, trending SW, W and SW, respectively. The origin of these forces remains unsolved thus posing open questions for further investigations into the dynamics of the Kurile Wadati-Benioff zone.