Mainshock Hypocenter Research Articles

Anomalous depth dependency of the stress field in the 2007 Noto Hanto, Japan, earthquake: Potential involvement of a deep fluid reservoir

[1] We have elucidated depth variations in the stress field associated with the 2007 Noto Hanto, Japan, earthquake by stress tensor inversion using high-quality aftershock data obtained by a dense seismic network. Aftershocks that occurred above 4 km in depth indicated a strike-slip stress regime. By contrast, aftershocks in deeper parts indicated a thrust faulting stress regime. This depth variation in the stress regime correlates well with that in the slip direction derived from a finite source model using geodetic data. Furthermore, the maximum principal stress (σ1) axis was stably oriented approximately W20°N down to the depth of the mainshock hypocenter, largely in agreement with the regional stress field, but, below that depth, the σ1 axis had no definite orientation, indicating horizontally isotropic stress. One likely cause of these drastic changes in the stress regime with depth is the buoyant force of a fluid reservoir localized beneath the seismogenic zone.

Dissecting large earthquakes in Japan: Role of arc magma and fluids

AbstractWe synthesized information from recent high‐resolution tomographic studies of large crustal earthquakes which occurred in the Japanese Islands during 1995–2008. Prominent anomalies of low‐velocity and high Poisson's ratio are revealed in the crust and uppermost mantle beneath the mainshock hypocenters, which may reflect arc magma and fluids that are produced by a combination of subducting slab dehydration and corner flow in the mantle wedge. Distribution of 164 crustal earthquakes (M 5.7–8.0) that occurred in Japan during 1885–2008 also shows a correlation with the distribution of low‐velocity zones in the crust and uppermost mantle. A qualitative model is proposed to explain the geophysical observations recorded so far in Japan. We consider that the nucleation of a large earthquake is not entirely a mechanical process, but is closely related to the subduction dynamics and physical and chemical properties of materials in the crust and upper mantle; in particular, the arc magma and fluids.

Asperity and Barriers of the 2004 Mid-Niigata Prefecture Earthquake Revealed by Highly Dense Seismic Observations

Abstract A highly resolved velocity structure on the mainshock fault of the 2004 Mid-Niigata prefecture earthquake was elucidated using arrival times from aftershocks observed by an extremely dense network of temporary seismic stations. We evaluated the spatial relationship between the velocity structure and distributions of the slip, stress change, and aftershocks. Distributions of static stress drop on the fault were calculated from a kinematic slip model, incorporating the 3D crustal structure. A high-velocity body with low aftershock activity was in proximity to the mainshock hypocenter and extended to the northeast side of the fault. This high-velocity body roughly coincided with an asperity, where the amounts of coseismic slip and static stress drop were larger than those in the surrounding areas. In contrast, a zone of negative stress drop was observed on the shallow periphery of the high-velocity body, where the sediment thickness steeply increased southwestward and the aftershock activity was high. We suggest that the structural heterogeneities of the host rocks surrounding the fault damaged zone have significant potentials to control dynamic rupture processes of the mainshock fault.

Deep ruptures around the hypocenter of the 12 May 2008 Wenchuan earthquake deduced from aftershock observations

On 12 May 2008, a great earthquake of Mw 7.9 occurred in Wenchuan, China. To record its aftershocks and investigate the distribution of deep ruptures associated with the event, we deployed a temporary short-period seismic network around the epicenter soon after the earthquake had occurred. A total of 6036 aftershocks were located from 108 days of seismic data with a high signal-to-noise ratio, collected from July to October 2008. These aftershocks were further processed using a double-differential location method, and 2878 aftershocks were retained in the relocated clusters. According to the distribution of aftershocks, we refined the location of the main shock hypocenter to 31.001°N, 103.280°E, 17 km deep. The spatial distribution of aftershocks reveals interesting features of the deep ruptures: (1) most aftershocks are located in the upper crust at depths shallower than 20 km, beneath surface outcrop of the Pengguan massif, indicating that the upper crust is brittle and seismogenic and that the brittle–ductile transition is located at the transition between the middle and upper crust; (2) a NW–SE-striking low-seismicity zone crossing through Yingxiu Town divides the study area into two blocks with contrasting distributions of deep seismicity, suggesting different rupture characteristics either side of the line; and (3) upward extrapolation of aftershock clusters intersects with the southern and northern arms of the Zipingpu Reservoir, indicating that aftershocks and future seismicity along the ruptures around the reservoir that were activated by the Wenchuan earthquake will be influenced by reservoir impoundment/drainage for several decades to come.

Depth and region dependence of b-value for micro-aftershocks of the May 12th, 2008 Wenchuan earthquake and its tectonic implications

Micro-aftershocks with magnitude range of 1.5–4 around the Wenchuan earthquake epicenter, the southern part of the Longmenshan fault zone, exhibit good frequency-magnitude linear relationships, thus enabling b-value analysis. The average b-value for micro-aftershocks of M1.5–4 from July to December of 2008 in our local study region is about 0.88, similar to the b-value for all aftershocks of M3.0–5.5 from May, 2008 to May, 2009 along the whole Longmenshan fault zone. The similarity between the local and regional b-values possibly indicates that the southern part of the Longmenshan fault zone has similar seismogenic environment to the whole Longmenshan fault zone. Alternatively, it may also imply that b-values derived from all events without consideration of structural variation can not discriminate local-scale tectonic information. The present study shows that the b-value for the Wenchuan earthquake micro-aftershocks varies with different regions. The b-value in southwest of the Yingxiu town is higher than that in the northeast of the Yingxiu town. The high b-value in the southwest part where the Wenchuan earthquake main shock hypocenter located indicates that the current stress around the hypocenter region is much lower than its surrounding area. The b-values are also dependent on depth. At shallow depths of <5 km, the b-values are very small (∼0.4), possibly being related to strong wave attenuation or strong heterogeneity in shallow layers with high content of porosity and fractures. At depths of ∼5–11 km, where most aftershocks concentrated, the b-values become as high as ∼0.9–1.0. At the depth below ∼11 km, the b-values decrease with the depth increasing, being consistent with increasing tectonic homogeneity and increasing stress with depth.

Structural Heterogeneity in Northeast Japan and Its Implications for the Genesis of the 2004 and 2007 Niigata Earthquakes

Abstract We utilized 188,766 P -wave and 179,328 S -wave high-quality arrival-time data from 14,666 local earthquakes to determine the three-dimensional (3D) seismic velocity ( V P , V S ), V P / V S ratio, crack density ( e ), and bulk velocity ( V ϕ ) structures in and around the source areas of the 2004 and 2007 Niigata earthquakes (Japan Meteorological Agency magnitude [ M J ] 6.8) in Japan in order to understand the generation mechanisms of the Niigata mainshocks. The aftershock hypocenters were relocated by applying the double-difference location method simultaneously with the tomographic inversion, indicating a complicated fault system that includes not only two parallel dominant faults dipping to the northeast but also some conjugate faults crossing the Niigata mainshock hypocenters. In general, high velocity ( V P , V S ) with low e at 11 km depth is visible in the southeastern part of this area in contrast to anomalous low velocity with high crack density in the northwestern part along the Niigata-Kobe tectonic zone (NKTZ). These characteristics of the velocity and crack density are consistent with the spatial distribution of active faults along the NKTZ. The surface geological features that indurated pre-Neogene rock outcrops were observed in the southeastern area, whereas deep sedimentary basins were observed on the opposite side. Low V P and low V S zones with high V P / V S anomalies were imaged in and/or under the source areas, showing agreement with those revealed by previous studies. All the features of the velocity and V P / V S structures, together with the images of the crack density and bulk velocity, suggest the presence of fluids under the source areas. Such fluids might have weakened the mechanical strength of the fault zone of the source areas, thus triggering the 2004 and 2007 Niigata earthquakes. Our present study, together with other recent tomographic images, reveals that fluids penetrated into the source rocks from the lower crust and uppermost mantle as a result of dehydration of the subducting Pacific slab and thus may play a key role in the genesis of these large crustal earthquakes.

Observation of numerous aftershocks of an Mw 1.9 earthquake with an AE network installed in a deep gold mine in South Africa

Abstract This is the first report from the JAGUARS (JApanese-German Underground Acoustic Emission Research in South Africa) project, the overall aim of which is to observe ultra-small fracturing in a more or less natural environment. We installed a local (∼40-m span) network of eight acoustic emission (AE) sensors, which have the capability to observe up to 200 kHz at a depth of 3.3 km in a South African gold mine. Our specific objective was to monitor a 30-m thick dyke that remains as a dip pillar against active mining ∼90 m above our network. An M w 1.9 earthquake whose hypocenter was ∼30 m above the network occurred in the dyke. Although the mineowned geophone (4.5 Hz) network detected only five earthquakes in the surrounding 200×200×150-m3 volume within the first 150 h following the main shock, our AE network detected more than 20,000 earthquakes in the same period. More than 13,000 of these formed a distinct planar cluster (∼100×80 m2) on which the main shock hypocenter lay, suggesting that this cluster delineates the main shock rupture plane. Most of the aftershocks were presumably very small, probably as low as M ∼ −4. The aftershock cluster dipped ∼60°. This is consistent with normal faulting under a nearly vertical compression field, as indicated by nearly horizontal breakouts found in a borehole crossing the rupture plane.

Structural heterogeneity of the Longmenshan fault zone and the mechanism of the 2008 Wenchuan earthquake (Ms 8.0)

We determined detailed 3‐D images of P and S wave velocity (Vp, Vs) and Poisson's ratio (σ) in and around the Longmenshan (LMS) fault zone by using a large number of P and S wave arrival times from the aftershocks of the 2008 Wenchuan earthquake (Ms 8.0) and other local events. Our results show that the structure of the LMS fault zone north of the Wenchuan main shock is very different from that south of the main shock. The southern section of the LMS fault zone contains a broad zone with low‐Vp, low‐Vs, and high‐σ anomalies, while the northern segment exhibits more scattered heterogeneities, corresponding to most of the aftershocks which occurred there. A prominent low‐Vp, low‐Vs, and high‐σ anomaly exists directly beneath the Wenchuan main shock hypocenter, suggesting that in addition to compositional variations, fluid‐filled fractured rock matrices may exist in the LMS fault zone, which may have influenced the generation of the large Wenchuan earthquake. Our tomographic results provide sound seismic evidence for the hypothesis that an upward intrusion of lower crustal flow occurred along the LMS fault zone. In addition, most small earthquakes before the 2008 Wenchuan main shock occurred around the Guanxian‐Jiangyou fault, while the Wenchuan aftershocks were mainly concentrated on the Yingxiu‐Beichuan fault, suggesting that the rupture process of the Wenchuan earthquake may belong to an out‐of‐sequence thrusting event, a suggestion which is in good agreement with the results from geological surveys and also quite similar to the rupture processes of the 1999 Chi‐Chi earthquake (M 7.5) and the 2005 Kashmir earthquake (M 7.6). A few aftershocks occurred close to the blind Guangyuan‐Dayi fault in the Sichuan basin, suggesting that this blind fault was also ruptured by the Wenchuan earthquake, consistent with geological surveys.

稠密余震観測データを用いて推定した２００４年新潟県中越地震震源域の応力場

We have investigated a stress field in the source region of the 2004 Mid-Niigata Prefecture (Niigata-Chuetsu) earthquake (M=6.8) by using P-wave polarity data obtained by a dense temporal seismic observation. The earthquake took place in central Japan near the eastern border of the Niigata-Kobe Tectonic Zone characterized by E-W contraction. The purpose of this study is to get insight into the stress heterogeneities in the source area including quite complex fault system with two parallel, westerly dipping fault planes and one conjugate plane. About two-thirds of well-constrained 514 focal mechanisms are reverse-fault type with WNW-ESE trending P-axes, which is consistent with the regional stress field in the area. Minor strike-slip events are distributed in the northern and central part of aftershock zone. Ten normal-fault events that are rare in the northeastern Japan are scattered mostly outside the major earthquake faults and occurred in a short period, suggesting that they reflect local stress field. Stress tensors derived from the focal mechanisms exhibit the compressional stress field characterized by nearly horizontal maximum principal stress that is consistent with the average direction of P-axes in the whole aftershock zone. Horizontal rotation of maximum principal stress axis is evident from WNW-ESE in the northeastern part of aftershock zone to E-W in the southwestern part. The direction of intermediate principle stress axis also changes from NE-SW in the northeastern part to NNE-SSW in the southwestern part. The fold axes in the area show similar strike variation. An interesting feature of stress distribution found in this study is the change in dip angle of the maximum principle stress axes. The axes through the aftershock zone dip gently to WNW, but those for earthquake clusters of mainshock fault dip steeply in a similar direction. Judging from the stress ratio, the horizontal compression is dominant near the mainshock hypocenter while the state in the surrounding areas is relatively close to isotropic stress. Though the difference in dip angle may not significant by considering the estimation error of stress axes, the combined anomalies with the stress ratio suggest that the mainshock occurred in an area of local stress inhomogeneity. We further examined the spatial distribution of earthquakes that have inconsistent focal mechanisms with the estimated stress tensors. Such earthquakes are distributed in the northeastern edge of the aftershock zone, in the southern edge of easterly dipping fault plane, and in a shallow earthquake cluster that is located between the faults of mainshock and the largest aftershock. The location of these events indicates small-scale stress heterogeneities in the source area superposed on the larger-scale stress variation mentioned above.

Role of fluids in the initiation of the 2008 Iwate earthquake (M7.2) in northeast Japan

Seismic velocity (Vp, Vs) and Poisson's ratio (σ) images show that the hypocenter of the 2008 Iwate‐Miyagi earthquake (M7.2) is located in a distinctive zone with low‐Vp, slightly low‐Vs and low‐σ within the upper 10 km depths, where the mechanical strength might be weaker than the normal seismogenic layer with high velocity and low Poisson's ratio immediate below. A prominent low‐Vp, low‐Vs and high‐σ zone is revealed in the lower crust and uppermost mantle under the Iwate mainshock hypocenter, which may reflect fluids resulting from the dehydration of the subducting Pacific slab under northeast Japan. The aqueous component of fluids intruded into the source region may have reduced the mechanical strength of the rock matrix and so triggered the Iwate earthquake. Our results indicate that crustal fluids beneath the seismogenic layer can play an important role in the initiation of large crustal earthquakes.

Imaging heterogeneous velocity structures and complex aftershock distributions in the source region of the 2007 Niigataken Chuetsu-oki Earthquake by a dense seismic observation

Abstract The velocity structure and accurate aftershock distributions in the source region of the 2007 Niigataken Chuetsu-oki Earthquake (thrust type) are obtained by inverting the arrival times from 848 aftershocks observed by a dense seismic network deployed immediately after the mainshock (8 h later). Both the detailed velocity structure and the accurate aftershock distribution show lateral heterogeneity along the fault strike. In the northeast area, aftershocks are aligned along both the NW- and SE-dipping planes. These planes are conjugate to each other. The mainshock hypocenter is located close to the bottom of an approximately 50° NW-dipping plane, which indicates that the mainshock rupture could have initiated on the NW-dipping plane. The high-V p body beneath this aftershock alignment shows a convex upward shape. In contrast, from the center to the southwest area, most of the aftershocks are aligned along SE-dipping planes. The high-V p body beneath this aftershock alignment shows a convex downward shape. Based on these results, we suggest that the crustal structure in the source region is divided into two segments by a boundary zone situated between the northeast and southwest areas. It should be noted that this segment boundary zone is coincident with the complex aftershock zone where numerous conjugate fault planes exist. We propose that the mainshock rupture initiated near the bottom of the NW-dipping fault plane and ran to the southwest, then transferred at the segment boundary zone which has numerous conjugate fault planes to the SE-dipping plane.

Coseismic deformation of the 2007 Chuetsu-oki earthquake derived from PALSAR/InSAR and its fault model

AbstractThe 2007 Chuetsu-oki earthquake (Mj6.8) occurred offshore of Niigata Prefecture, northeast Japan, on 16 July 2007. I applied PALSAR/InSAR to detect coseismic deformation associated with the earthquake. The interferogram generated from descending data revealed a slant-range shortening exceeding 20 cm in the coastal area of Kashiwazaki adjacent to the epicenter and a maximum slant range extension of approximately 15 cm southeast of the slant-range shortening pattern. The interferogram generated from descending data is consistent with GPS displacements, but the non-crustal deformation component was large in the interferogram generated from ascending data. Based on a fault plane dipping to the southeast, a fault-slip distribution was estimated from the descending interferogram and GPS displacements. The largest fault-slip was located in the southwestern deeper part of the mainshock hypocenter. In the northern part of the focal region, the fault-slip was dominant at depths of 5–15 km, but it was limited to shallower depths in the southern part of the focal region. Such fault-slip distribution is consistent with the aftershock distribution. This consistency supports the assumption that the fault rupture in the mainshock has primarily occurred in the nodal plane dipping to the southeast.

Complex Faulting Associated with the 22 December 2003 Mw 6.5 San Simeon, California, Earthquake, Aftershocks, and Postseismic Surface Deformation

We use data from two seismic networks and satellite interferometric synthetic aperture radar (InSAR) imagery to characterize the 22 December 2003 M w 6.5 San Simeon earthquake sequence. Absolute locations for the mainshock and nearly 10,000 aftershocks were determined using a new three-dimensional (3D) seismic velocity model; relative locations were obtained using double difference. The mainshock location found using the 3D velocity model is 35.704° N, 121.096° W at a depth of 9.7±0.7 km. The aftershocks concentrate at the northwest and southeast parts of the aftershock zone, between the mapped traces of the Oceanic and Nacimiento fault zones. The northwest end of the mainshock rupture, as defined by the aftershocks, projects from the mainshock hypocenter to the surface a few kilometers west of the mapped trace of the Oceanic fault, near the Santa Lucia Range front and the >5 mm postseismic InSAR imagery contour. The Oceanic fault in this area, as mapped by Hall (1991), is therefore probably a second-order synthetic thrust or reverse fault that splays upward from the main seismogenic fault at depth. The southeast end of the rupture projects closer to the mapped Oceanic fault trace, suggesting much of the slip was along this fault, or at a minimum is accommodating much of the postseismic deformation. InSAR imagery shows ∼72 mm of postseismic uplift in the vicinity of maximum coseismic slip in the central section of the rupture, and ∼48 and ∼45 mm at the northwest and southeast end of the aftershock zone, respectively. From these observations, we model a ∼30-km-long northwest-trending northeast-dipping mainshock rupture surface—called the mainthrust—which is likely the Oceanic fault at depth, a ∼10-km-long southwest-dipping backthrust parallel to the mainthrust near the hypocenter, several smaller southwest-dipping structures in the southeast, and perhaps additional northeast-dipping or subvertical structures southeast of the mainshock plane. Discontinuous backthrust features opposite the mainthrust in the southeast part of the aftershock zone may offset the relic Nacimiento fault zone at depth. The InSAR data image surface deformation associated with both aseismic slip and aftershock production on the mainthrust and the backthrusts at the northwest and southeast ends of the aftershock zone. The well-defined mainthrust at the latitude of the epicenter and antithetic backthrust illuminated by the aftershock zone indicate uplift of the Santa Lucia Range as a popup block; aftershocks in the southeast part of the zone also indicate a popup block, but it is less well defined. The absence of backthrust features in the central part of the zone suggests range-front uplift by fault-propagation folding, or backthrusts in the central part were not activated during the mainshock.

Dynamic Triggering by Strong-Motion P and S Waves: Evidence from the 1999 Chi-Chi, Taiwan, Earthquake

High-frequency band-pass filtering of acceleration records from the 1999 Chi-Chi, Taiwan, earthquake ( M w 7.6) resolves the continuous signal into a series of relatively short-duration discrete-energy bursts. We hypothesize that these bursts originate near the individual stations as small, shallow events that have been dynamically triggered by the P and S waves generated by the Chi-Chi mainshock; however, we cannot rule out the possibility that they originate from some nonseismic source associated with the surface, such as buildings or trees. Bursts are observed only during the seismic signal and not in the pre- or postsignal noise. The bursts are not likely to be generated by the instruments because groupings of three colocated individual instruments record identical bursts. Also, we show that the bursts are not due to the band-pass filtering of instrumentally generated step functions. The hypothesis that they are local events is supported by the observation of bursts in the 40-Hz frequency band at distances up to 170 km from the epicenter. If the bursts originated on the Chi-Chi fault plane, as hypothesized by Chen et al. (2006), based on their analysis of recordings within 20 km from the Chelungpu fault, then they should not be observable at this distance, assuming any reasonable value of crustal attenuation. Assuming a local origin, we estimate an average local event magnitude of M w 0.2 and a source-receiver distance of approximately 1 km. We extended our analysis to lower stress levels by analyzing records from a smaller ( M w 5.3) event that was recorded by many of the same instruments used in the Chi-Chi analysis. For this event, bursts are observed only on the accelerograms from stations relatively close to the mainshock hypocenter. Analysis of the combined data set from both mainshocks suggests a stress threshold for triggering in the range 0.03–0.05 Mpa for S -wave triggering and 0.0013–0.0033 Mpa for P -wave triggering, consistent with prior observations of surface-wave triggering.

Highly resolved distribution of aftershocks of the 2007 Noto Hanto Earthquake by a dense seismic observation

Abstract The 2007 Noto Hanto Earthquake occurred on March 25, 2007, in the Noto Peninsula, central Japan. A half day after the main shock, we started installing temporary seismic stations in order to determine the precise locations of its aftershocks. Ten universities and two research institutes deployed 88 temporary seismic stations in and around the source area. The observation lasted for about 2 months. We relocated 1318 aftershocks with arrival time corrections at each station. The relocated hypocenters show relatively small errors—less than 0.2 km in the horizontal direction and less than 0.4 km depth. Most of the relocated hypocenters are about 2.0 km shallower than those determined by JMA. The distribution of the aftershocks forms a southeast-dipping plane. The main shock is located at the bottom part of their distribution. The precise aftershock distribution extends into a shallower area than the original, and it coincides with sea floor-ward extension of the active faults previously known from a sonic reflection survey. Heterogeneous distribution of the aftershocks on the fault plane shows low seismicity just above the main shock hypocenter, and middle-size aftershocks are distributed on the periphery of the main shock. A precursory event (M 4.4) that occurred 0.6 s before the main rupture is located close to the M 2.2 foreshock that occurred 12 min before it.

Magnetotelluric observations around the focal region of the 2007 Noto Hanto Earthquake (Mj 6.9), Central Japan

Abstract On 25 March 2007, a damaging earthquake (M j 6.9) occurred near the west coast of the Noto Peninsula, Central Japan. A wideband magnetotelluric (MT) survey was carried out in the onshore area of the source region immediately after the mainshock, with the aim of imaging the heterogeneity of the crustal resistivity structure. The final observation network had consisted of 26 sites. As a preparatory step for imaging three-dimensional features of the resistivity around the focal region, we constructed two-dimensional resistivity models along five profiles using only the TM mode responses, in order to reduce three-dimensional effects. Four profiles are perpendicular to the fault strike, and a fifth profile is parallel to the strike through the mainshock epicenter. Significant characteristics of the resistivity models are: (1) beneath the mainshock hypocenter, there is a conductive body which spreads to the eastern edge of the active aftershock region; (2) a resistive zone is located in the gap of the aftershock distribution between the mainshock hypocenter and the largest eastern aftershock; (3) one of the largest aftershock occurred at the boundary of the resistive zone described above. These results suggest that the deep conductors represent fluid-filled zones and that the lateral heterogeneity could have controlled the slip distribution on the fault plane.

Three-dimensional velocity structure in the source region of the Noto Hanto Earthquake in 2007 imaged by a dense seismic observation

Abstract The velocity structure and accurate aftershock distributions of the Noto Hanto Earthquake in 2007 (thrust type) are elucidated by inverting the arrival times from 917 aftershocks using double-difference tomography. P-wave velocity (V p ) of the hanging wall in the southeast appears to be higher than that of the footwall in the northwest, and the high-V p body of the hanging wall has a relatively high V p /V s ratio. Conversely, the low-V p body in the footwall appears to have a low V p /V s ratio at depths greater than 3 km. Aftershocks associated with the mainshock fault are roughly distributed along this velocity boundary between the hanging wall and footwall. Near-surface thin layers with significantly low V p and high V p /V s are imaged in a northwest direction from the mainshock epicenter. A likely explanation is that the mainshock fault plane was reactivated as a reverse fault in terms of the inversion tectonics due to the crustal shortening which initiated from the late Miocene. Both the mainshock hypocenter and the vertical alignment of aftershocks beneath it are located in the low-V p and low-V p /V s zones, indicating the potential presence of water-filled pores. Crustal stretching and shortening in and around the Noto Peninsula have created complex structures, including weak high-dip angle faults, almost vertical faults, and low velocity zones, which can potentially affect the seismic activities around the source region.

Three-dimensional velocity imaging of the Kachchh seismic zone, Gujarat, India

To understand the causative mechanism of the continued occurrence of earthquakes in Kachchh, Gujarat for the last six years, we estimated high-resolution three-dimensional Vp, Vs and Vp/Vs structures in the aftershock zones of the 2001 Mw7.7 Bhuj and 2006 Mw5.6 Gedi earthquakes. We used 13,862 P- and 13,736 S-wave high-quality arrival times collected from the seismograms of 2303 aftershocks recorded at 5–18 three-component seismograph stations during 2001–06. Seismic images revealed a marked spatial variation in the velocities (from − 20% to + 14% in Vp, from − 12% to 13% in Vs, and from − 12% to 12% increase in Vp/Vs) in the 0–34 km depth range beneath the Bhuj aftershock zone. Relatively more increase in Vp than Vs, resulting in an increase in Vp/Vs in the crust beneath the seismically active causative fault (North Wagad Fault, NWF) zone of 2001 Bhuj mainshock suggests a rigid, mafic crust beneath the region. They also delineate an increase of 8% in Vp and 14% in Vs, and a decrease of 4% in Vp/Vs in the almost vertical rupture zone of the 2006 Gedi earthquake extending up to 12 km depth. This high velocity body associated with the Gedi mainshock is inferred to be a gabbroic intrusive. The Banni region and the Wagad uplift are found to be associated with high velocity intrusive bodies (inferred to be mafic) extending from 5 to 35 km depth, which might have intruded during the rifting in early Jurassic (~ 160 Ma). Aftershock activity is mainly confined to the zones characterized by high Vp, high Vs and low Vp/Vs ratio, which might be representing the strong, competent and brittle parts of the fault zone/intrusive bodies that could accumulate large strain energy for generating aftershocks for more than six years. It is inferred that the crustal stress concentrations associated with the intrusive bodies are contributing significant perturbation to the crustal stress regime to generate the intraplate earthquakes in the Kachchh rift zone. A few patches of slow (Vp and Vs) and high Vp/Vs between 10 to 30 km depth have also been detected on the causative 45° south dipping north Wagad fault (NWF) for the 2001 mainshock, which may be attributed to the fluid filled fractured rock matrix. Interestingly, the 2001 Bhuj mainshock hypocenter is found to be associated with such a low velocity patch. Thus, it is inferred that the occurrence of 2001 Bhuj mainshock might have facilitated by the presence of fluids at hypocentral depth.

Aftershocks of the 1999 Chi-Chi, Taiwan, Earthquake: The First Hour

Within the hour immediately after the 1999 Chi-Chi mainshock, only 40 aftershocks were located by the Taiwan Central Weather Bureau Seismic Network (cwbsn) because of the power outage in more than half of the island and the limited dynamic range of the cwbsn high-gain instruments. Here, by analyzing 20 near- field, on-scale records from the Taiwan Strong-Motion Instrumentation Program (tsmip), we determined a catalog of 296 aftershocks with M L ≥3.4 within the first hour after the Chi-Chi mainshock. Focal mechanisms were also determined for 24 of these aftershocks. The frequency-magnitude relation obtained from the 296 aftershocks indicates that the catalog is complete above magnitude M L 4.3. Most of the aftershocks occurred in a small-slip region of the mainshock immediately to the north of the mainshock epicenter. Spatially, the aftershocks appear to migrate downward from the mainshock hypocenter. Later in the hour, the aftershocks began to concentrate in the fringe area of the main rupture. During the first hour, the b -value as defined by events in the 4.3 M L

Tomographic Imaging outside a Seismic Network: Application to the Northeast Japan Arc

The conventional local tomography can determine the 3D seismic velocity structure right beneath a seismic network, but it cannot determine the 3D structure outside a seismic network. In this study we show that such a limitation of tomography can be overcome if many earthquakes occur outside a seismic network. In the northeast Japan arc earthquakes occur actively from the Japan Trench to the Pacific coast. We detected and used sP -depth phase to relocate the suboceanic earthquakes accurately, and then used P - and S -wave arrival times from the relocated suboceanic events and the earthquakes under the northeast Japan land area to determine the 3D P - and S -wave velocity and Poisson’s ratio structures of the entire northeast Japan arc from the Japan Trench to the Japan Sea coast. Our results exhibit strong lateral heterogeneities under the forearc region. The mainshock hypocenters of large interplate earthquakes ( M 7.0–8.2) mainly cluster near the boundaries of high and low velocity and Poisson’s ratio above the subducting Pacific slab. A few large earthquakes are located in areas with high velocity and low Poisson’s ratio. These results suggest that lateral heterogeneities on the slab boundary can affect the rupture nucleation of large thrust earthquakes and the degree of interplate seismic coupling.