Showing papers on "Hypocenter published in 2001"

hypoDD -- A Program to Compute Double-Difference Hypocenter Locations

Abstract: HypoDD is a Fortran computer program package for relocating earthquakes with the double-difference algorithm of Waldhauser and Ellsworth (2000). This document provides a brief introduction into how to run and use the programs ph2dt and hypoDD to compute double-difference (DD) hypocenter locations. It gives a short overview of the DD technique, discusses the data preprocessing using ph2dt, and leads through the earthquake relocation process using hypoDD. The appendices include the reference manuals for the two programs and a short description of auxiliary programs and example data. Some minor subroutines are presently in the c language, and future releases will be in c. Earthquake location algorithms are usually based on some form of Geiger's method, the linearization of the travel time equation in a first order Taylor series that relates the difference between the observed and predicted travel time to unknown adjustments in the hypocentral coordinates through the partial derivatives of travel time with respect to the unknowns. Earthquakes can be located individually with this algorithm, or jointly when other unknowns link together the solutions to indivdual earthquakes, such as station corrections in the joint hypocenter determination (JHD) method, or the earth model in seismic tomography. The DD technique (described in detail in Waldhauser and Ellsworth, 2000) takes advantage of the fact that if the hypocentral separation between two earthquakes is small compared to the event-station distance and the scale length of velocity heterogeneity, then the ray paths between the source region and a common station are similar along almost the entire ray path (Fréchet, 1985; Got et al., 1994). In this case, the difference in travel times for two events observed at one station can be attributed to the spatial offset between the events with high accuracy. DD equations are built by differencing Geiger's equation for earthquake location. In this way, the residual between observed and calculated travel-time difference (or double-difference) between two events at a common station are a related to adjustments in the relative position of the hypocenters and origin times through the partial derivatives of the travel times for each event with respect to the unknown. HypoDD calculates travel times in a layered velocity model (where velocity depends only on depth) for the current hypocenters at the station where the phase was recorded. The double-difference residuals for pairs of earthquakes at each station are minimized by weighted least squares using the method of singular value decomposition (SVD) …

Delayed triggering of the 1999 Hector Mine earthquake by viscoelastic stress transfer

Abstract: Stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases1,2,3,4,5. The 1999 Hector Mine earthquake in southern California (magnitude 7.1) occurred only 20 km from, and 7 years after, the 1992 Landers earthquake (magnitude 7.3). This suggests that the Hector Mine earthquake was triggered in some fashion by the earlier event. But uncertainties in the slip distribution and rock friction properties associated with the Landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the Hector Mine hypocentre—with estimates varying from -1.4 bar (ref. 6) to +0.5 bar (ref. 7). More importantly, coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events. Here we present the results of a three-dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the Landers earthquake. Using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the Landers earthquake, our calculations suggest that lower-crustal or upper-mantle flow can lead to postseismic stress increases of up to 1–2 bar at the location of the Hector Mine hypocentre during this time period, contributing to the eventual occurrence of the 1999 Hector Mine earthquake. These results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard8,9,10,11.

Spatial distribution of focal mechanisms for interplate and intraplate earthquakes associated with the subducting Pacific plate beneath the northeastern Japan arc: A triple‐planed deep seismic zone

Abstract: The northeastern Japan arc is located in one of the most seismically active subduction zones in the world. In this study, we relocated hypocenters and determined focal mechanisms of small earthquakes (M≤5) beneath the arc in order to investigate in detail the stress distribution in and around the descending oceanic plate. In the hypocenter relocation we adopted a “source region station correction method” in which station corrections vary with hypocenter locations. We have developed a new focal mechanism determination technique named “master solution method,” which is analogous to the “master event method” in hypocenter determination. We applied the method to P and SH wave amplitude data to obtain 1106 focal mechanism solutions. From the new mechanism solutions and relocated hypocenters we found that there occur both low-angle thrust fault (LT) type and downdip compression (DC) type earthquakes at depths from 40 to 70 km near the aseismic front; the DC type events are underlying the LT-type events. Almost all the earthquake clusters are composed of LT-type events. The western limit of the region where LT-type events have occurred is subparallel to the trench axis, although it undulates considerably; it delineates the westernend of the active region of interplate seismicity. Furthermore, we found that normal fault (NF) type events also occur at depths from 70 km in the upper plane of the double-planed deep seismic zone, which is characterized mainly by DC-type event. These NF-type events are distributed only in a thin uppermost portion of the slab close to the plate boundary. Below these, in the lower plane, are downdip extension (DE) type events. This result indicates that the deep seismic zone in the northeastern Japan arc is not double-planed but triple-planed, even beneath the land area, which cannot be explained by any simple models.

HYPOSAT — An Enhanced Routine to Locate Seismic Events

Abstract: A program package, called HYPOSAT, has been under development that attempts to use the maximum information possible to estimate the hypocenter of a seismic source. The standard input parameters can be used: arrival times of first and later onsets with backazimuths and ray parameters (or apparent velocities). In addition, travel-time differences between different phases observed at the same station can be optionally used. The observed standard deviations are used to weight all input parameters and the inversion is done with a generalized matrix inversion code.

The 1994 Java tsunami earthquake: Slip over a subducting seamount

Abstract: 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.

Scale-dependence of seismic energy-to-moment ratio for strike-slip earthquakes in Japan

Abstract: We analyzed four pairs of a large (M_w ≈ 6) and a small (M_w ≈ 3.5 to 4) shallow strike-slip earthquakes to investigate the scale-dependence of the seismic energy-to-moment ratio, an important macroscopic parameter which reflects the basic physical process of seismic slip. These earthquakes occurred in the south-western part of Japan, and high-quality close-in records (epicentral distance < 50 km) are available for both the small and large earthquakes. The paired events have almost the same focal mechanism and hypocenter location. We used the spectral ratio of the paired events in order to remove the effects of attenuation along the wave propagation path and the station site response. We then estimated the seismic energy from the source spectra estimated from the spectral ratio. The energy-to-moment ratio increases with the earthquake size. This scale-dependence is very similar to that found earlier for earthquakes in Southern California.

A three‐dimensional crustal velocity model of the southwestern Alps from local earthquake tomography

Abstract: A temporary network of 65 short-period seismological stations was installed in the southwestern Alps during the second half of 1996. It complemented the permanent monitoring networks, obtaining an average interstation distance of ∼10 km. Travel time data from 446 local earthquakes and 104 quarry blasts were inverted simultaneously for hypocenter parameters and three-dimensional velocity structure. The P wave velocity model displays strong lateral contrasts both at shallow and deeper levels. A low-velocity anomaly stands out at shallow depths beneath the Digne and Castellane nappes in the southwestern part of the investigated area. Farther east, the Monviso ophiolitic massif appears to have a much larger extension at depth than previously assumed. The largest and strongest anomaly is located under the Dora Maira massif and the westernmost Po plain. It correlates with the well-known Ivrea body, which is classically interpreted as a wedge of Adriatic upper mantle. At the best resolved depths (10 and 15 km) it appears as a rather thin (10 to 15 km), north-south elongated, high-velocity (7.4 to 7.7 km s−1) anomaly with very sharp edges, extending to the south as far as 10 km north of the surface trace of the Frontal Penninic Thrust. Special care was taken with regard to the quantitative estimation of the resolution for the main anomalies using the inversion of synthetic travel time data.

Earthquake Doublet in Kagoshima, Japan: Rupture of Asperities in a Stress Shadow

Abstract: Two shallow moderate ( M ∼ 6) earthquakes occurred in the northwestern part of Kagoshima Prefecture, Kyushu, southern Japan. I discuss the effect of the stress change due to the first earthquake (26 March 1997) on the occurrence of the second earthquake (13 May 1997). The rupture characteristics of the two earthquakes are inferred in two steps to form the basis of the discussion. I first invert strong ground motion data to construct kinematic source models and then estimate the distribution of static stress drop from the derived dislocation distributions. The rupture process of the March event is simple and well described with rupture of a single asperity (patch of high stress drop). On the other hand, multiple asperities on conjugate faults ruptured during the May event. The maximum value of static stress drop for both earthquakes is about 4 MPa, and seems lower than those of other Japanese intraplate earthquakes. The hypocenter and the largest asperity of the May earthquake are located in a stress shadow caused by the March earthquake. Thus the rupture history of the May earthquake is difficult to explain with a static stress change model. Other mechanisms such as fluid migration and dynamic stresses were also investigated, but failed to explain the triggering. I propose the coupled effect of static change in shear stress and normal stress under the rate- and state-dependent friction law as a possible mechanism.

Three-dimensional seismic velocity structure beneath Aso Volcano, Kyushu, Japan

Abstract: Tomographic results for P- and S-wave velocity structure beneath the active Aso Volcano, Kyushu, Japan, using 800 well-recorded earthquakes and ten shots recorded by an eight-station seismic network, are presented. A 68% variance reduction was achieved upon simultaneous inversion for hypocenter and velocity structure. Well-resolved velocity anomalies associated with the active crater reveal heterogeneity up to 26% slower and 18% faster in P velocity, and up to 31% slower and 22% faster in S velocity, than the one-dimensional model. The largest anomaly is seen over the upper 11 km in the central and northern parts beneath the central cones. Two low-velocity regions are imaged. The first region, a 10×15-km region encompassing the upper 3 km centered near the caldera wall at Tateno Valley, is characterized by P velocities up to 19% slower (20% for S). The second low-velocity region is associated with the central cones and active magma conduit system at 6 km depth. Velocities as low as 4.3 km/s (up to 26%) in P and 2 km/s (31% slower) in S characterize the 7-km-wide volume. The magma chamber is roughly spherical in shape, centered at 6 km depth, flattens at 10 km depth, and is located between Mt. Kishima, Mt. Eboshi, and Mt. Naka, the present focus of magmatism. A sharp velocity contrast at the depth of 3 km, with high velocities to the southwest and lower velocities to the northeast, characterizes different abutting structures associated with the Oita-Kumamoto Tectonic Line.

Viscoelastic stress-triggering of the 1999 Hector Mine Earthquake by the 1992 Landers Earthquake

Abstract: The 1999 Mw 7.1 Hector Mine earthquake occurred only 20 km from the 1992 Mw 7.3 Landers earthquake fault. The close spacing between the two earthquakes suggests that the Landers earthquake trigger the Hector Mine event. Based on an elastic half-space model, scientists from the USGS, SCEC and CDMG (2000) have found a negative Coulomb stress change at the Hector Mine hypocenter due to Landers. This negative stress change is inconsistent with the hypothesis of static stress triggering. In this paper, I show evidence of stress triggering of the Hector Mine earthquake by Landers, due to a process governed by viscoelastic flow in the lower crust. This visoelastic flow has produced broad-scale postseismic rebound observed by GPS and InSAR measurements. The result of this study is that viscoelastic flow has significantly modified the regional stress field in the Mojave Desert after the Landers earthquake. The evolving stress field, including viscoelastic flow, has gradually moved the Coulomb stress change at the Hector Mine hypocenter to a positive level. The increase in Coulomb stress exceeded 1 bar right before the Hector Mine earthquake, bringing the Hector Mine ruptures to the proximity of catastrophic failure.

Seismic waveform attributes before and after the Loma Prieta earthquake: Scattering change near the earthquake and temporal recovery

Abstract: During 1987–1995 several clusters of nearly identical seismic events (multiplets) occurred near the Loma Prieta source region. These multiplets allow us to investigate and demonstrate spatial and temporal changes in seismic wave character associated with the 1989 Loma Prieta main shock. For seismogram pairs we use a moving window technique to compute coherencies depending on lapse time and frequency. Post-Loma Prieta events have reduced coherencies with pre-Loma Prieta events in a spatially limited region close to the Loma Prieta hypocenter, while other paths remain nearly unaffected. These changes gradually recover within a time interval of 5 years after the Loma Prieta earthquake. A possible explanation for the time dependence is coseismically opened cracks which cause scattering increase for wavefields after the Loma Prieta event. Postseismic relaxation processes such as crack healing, fluid diffusion, or after deformations lead to progressive closure of these cracks with time after the main shock. Thus the scattering properties of the local crust approach the pre-main shock state.

Aftershock Activity and Frequency-dependent Low Coda Qc in the Epicentral Region of the 1999 Chamoli Earthquake of Mw 6.4

Abstract: — On 28 March, 1999 (19:05:10.09, UT) a significant earthquake of M w 6.4 occurred in the Garhwal Himalaya (30.555°N, 79.424°E). One hundred and ten well-recorded aftershocks show a WNW-ESE trending northeasterly dipping seismic zone extending from a depth of 2 to 20 km. As the main shock hypocenter occurred at the northern end of this seismic zone and aftershocks extended updip, it is inferred that the main-shock rupture nucleated on the detachment plane at a depth of 15 km and then propagated updip along a NE-dipping thrust plane. Further, the epicentral distribution of aftershocks defines a marked concentration near a zone where main central thrust (MCT) takes a significant turn towards the north, which might be acting as an asperity in response to the NNE compression due to the underthrusting of Himalayan orogenic process prevalent in the entire region. Presence of high seismicity including five earthquakes of magnitude exceeding 6 and twelve earthquakes of magnitude exceeding 5 in the 20th century is presumed to have caused a higher level of shallow crustal heterogeneity in the Garhwal Himalaya, a site lying in the central gap zone of the Himalayan frontal arc. Attenuation property of the medium around the epicentral area of the 1999 Chamoli earthquake, covering a circular area of 61,500 km2 with a radius of 140 km, is studied by estimating the coda Q c from 48 local earthquakes of magnitudes varying from 2.5–4.8. These earthquakes were recorded at nine 24-bit REFTEK digital stations; two of which were equipped with three-component CMG40T broadband seismometers and others with three-component L4-3D short-period seismometers. The estimated Q o values at different stations suggest on average a low value of the order of (30 ± 0.8), indicating an attenuating crust beneath the entire region. The frequency-dependent relation indicates a relatively low Q c at lower frequencies (1–3 Hz) that can be attributed to the loss of energy due to scattering on heterogeneities and/or the presence of faults and cracks. The large Q c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneities are expected. An important observation is that the region north of MCT (more rigid highly metamorphosed crystalline rocks) is less attenuative in comparison to the region south of MCT (less rigid slightly metamorphosed rocks (sedimentary wedge)). The acceleration decays to 50% at 20 km distance and to 7% at 100 km. Hence, even 1g acceleration at the source may not cause significant damage beyond 100 km in this region.

Rupture history of September 30, 1999 intraplate earthquake of Oaxaca, Mexico (MW=7.5) from inversion of strong-motion data

Abstract: Near-source strong motions are inverted to estimate the rupture history of intraslab, normal-faulting September 30, 1999, Oaxaca, Mexico earthquake. Two focal mechanisms (Harvard and NEIC CMT solutions) are tested for the source geometry. The inversion with the NE dipping fault plane of the Harvard solution best matches the data (strike=295°, dip=50°, rake=−82°). We estimated the slip distribution on the fault and the associated rupture front propagation, as well as the rise time. The inversion results show that the rupture mainly propagated from ESE to WNW and slightly downdip, with an average rupture velocity of about 3 km/s. The rise time ranges between 1 and 2 s. The slip distribution on the fault is mainly concentrated in two interconnected patches with a maximum slip of 2.5 m located about 20 km and 40 km WNW of the hypocenter. Most of the slip is released at an average depth of 45 km. A smaller area with a maximum slip of 1.5 m is also observed close to the hypocenter. The total co-seismic moment released is equal to 1.8 × 1020 Nm.

Differential Evolution algorithm in the earthquake hypocenter location

Abstract: — A novel global optimizing algorithm — Differential Evolution (DE) — has appeared recently. This method is easy and advantageous when used for kinematic location of the earthquake hypocenter. The motivation for implementing a robust (i.e., global and nonlinear) optimizing algorithm for the location problem is to obtain better results than those from the classical (i.e., linearized) approach (such as the FASTHYPO, HYPOELLIPSE, HYPOCENTER solutions, among others). Better solutions have lower final misfits expressed as the common L2 norm. The features of the DE algorithm are studied on a set of synthetic location problems. The DE procedure is controlled by 3 internal parameters, which are easy to adjust, and the convergence properties are very good. Location results using DE are compared with the HYPO71 solutions for real earthquake data from the Gulf of Corinth region, Greece. The DE results are significantly better. The DE optimizing algorithm seems to be very promising both for the location problem as well as for other problems in geophysics.

The M6.1 earthquake triggered by volcanic inflation of Iwate Volcano, northern Japan, observed by satellite radar interferometry

Abstract: Ground uplift and earthquake swarm activity has continued around Mt. Iwate volcano, northern Japan, since February 1998. On September 3, 1998 a M6.1 earthquake occurred southwest of the volcano. Satellite radar interferometry acquired by JERS-1 reveals volcanic and coseismic deformation to the west of the summit, which is concordant with displacements observed by GPS. We analyzed geodetic observations including InSAR, GPS, and EDM to model the source of the surface deformation. The inferred model shows a 0.033km³ volumetric increase of an inflation (Mogi) source at a depth of 7.9km from April 1998 to the occurrence of the M6.1 earthquake. We find that the inflation increased Coulomb failure stress by 0.33MPa (7% of the coseismic stress drop) at the hypocenter. This result suggests the inflation promoted the rupture of a known Quaternary fault.

Micro-seismicity around the seaward updip limit of the 1946 Nankai Earthquake dislocation area

Abstract: We observed micro-seismicity around the seaward updip limit of the 1946 Nankai earthquake dislocation area by using pop-up type ocean bottom seismographs deployed for three months in 1998. At the subduction zone, we must consider strong lateral heterogeneity of the seismic velocity structure to locate earthquakes precisely. We constructed 3-D P- and S-wave velocity structure models based on the results of airgun-OBS seismic surveys along the Nankai Trough. We located 83 earthquakes using these 3-D models. These hypocenters show two groups of seismicity. One seismicity group locates in oceanic layer 3 within a subducting seamount. These earthquakes may reflect the deformation of the seamount caused by its subduction. The other seismicity group locates near the updip limit of the thermally modeled locked zone. These hypocenters are determined to be near the boundary between the accretionary prism and the subducting oceanic crust.

Fault dynamic rupture simulation of the hypocenter area of the thrust fault of the 1999 Chi‐Chi (Taiwan) Earthquake

Abstract: A 2D Discrete Element Model was employed to simulate the rupture propagation and near-source ground motion in the epicentral area of the 1999 Chi-Chi (Taiwan) earthquake. The observations show that the hanging wall side is characterized by larger particle motions than the footwall side. The simulation results reproduce the main features of the recorded ground motion and show that the particle velocity on the hanging wall side and on the footwall side are symmetric in the deeper fault region, however, as the crack approaches the free surface, the hanging wall side is characterized by larger particle motions than the footwall side. These results suggest that the difference in the particle motion on the hanging wall and on the footwall is due mainly to the asymmetric geometry of hanging wall and footwall. Thus, the model used leads to good approximations in the vicinity of the epicenter.

Source Processes of 15 September 1998 M 5.0 Sendai, Northeastern Japan, Earthquake and Its M 3.8 Foreshock by Waveform Inversion

Abstract: A moderate earthquake ( M 5.0) occurred on 15 September 1998 in Sendai City, northeastern Japan, causing slight damage in a portion of the city. It was located at about 12-km depth on the Nagamachi-Rifu fault, an active reverse fault passing through the center of the city. We investigate rupture processes of the earthquake and its largest foreshock ( M 3.8) by an empirical Green9s-function method. We use waveform data recorded by nearby dense strong-motion arrays and by short-period and broadband stations. Waveform inversion results for the mainshock show that the spatial extent of its rupture area is about 3 × 3 km 2 , which is roughly consistent with that of the aftershock area. Most of the moment release was contained in two asperities. Aftershocks precisely relocated by a homogeneous-station method show that their activity is high in areas with a relatively small amount of moment release of the mainshock. Rupture of the largest foreshock ( M 3.8) that occurred 6 min before the mainshock started approximately at the mainshock hypocenter. Its rupture area, with a radius of about 300 m, does not strongly overlap with the asperities of the mainshock.

Three-dimensional S wave velocity structure and Vp/Vs ratios in the New Madrid Seismic Zone

Abstract: A three-dimensional S wave velocity model for the New Madrid Seismic Zone (NMSZ) has been developed using nonlinear travel time tomography The inversion utilized 5544 S wave arrival times from 720 earthquakes recorded by digital, three-component stations deployed in the NMSZ over the time period 1989 through 1992 We imaged S wave velocity anomalies ranging from −5% to +8% relative to the starting one-dimensional velocity model Lowest S wave velocities are found south of Ridgley, Tennessee, in an area characterized by a high earthquake swarm rate and shallower than normal hypocenters Two centers of higher than average S wave velocity are located west of the Mississippi river, north of Caruthersville, Missouri The S wave model is similar to a P wave velocity model generated using the same earthquake data set The similarity in ray coverage in both the P and S wave solutions allowed calculation of Vp/Vs ratios Most of the seismicity in the NW trending central arm of the NMSZ is associated with normal Vp/Vs values that border regions with high Vp/Vs North of Ridgely, high Vp/Vs values are associated with higher than average compressional and shear wave velocities and are interpreted to be due to mafic intrusions along the axis and edges of the Reelfoot rift SE of Ridgley, the end of the central arm coincides with high Vp/Vs values that are due to a significant shear wave low-velocity zone and are interpreted to be the result of highly fractured and fluid saturated crust

The Rupture Process of the Mw 7.8 Cape Kronotsky, Kamchatka, Earthquake of 5 December 1997 and Its Relationship to Foreshocks and Aftershocks

Abstract: A M w 7.8 shallow subduction earthquake occurred on 5 December 1997 near Cape Kronotsky, Kamchatka Peninsula. Broadband P -wave inversions, carried out using two independent methods, allowed us to locate the position of the main asperities, one of high slip of up to 240 cm and a pair of lower slip, that ruptured during the mainshock. The mainshock hypocenter was located within the first asperity but not in the region of maximum slip. Most of the aftershock activity occurred within the low-slip asperities zone; the higher-slip asperity was characterized by low aftershock activity. All large aftershocks as well as the foreshocks ( M w ≥5.5) occurred outside of the asperities. The mainshock was preceded by a long-term series of single moderate-size events. Based on the spatial distribution of preceding events, foreshocks, aftershocks, and two main asperity zones broken during the mainshock, the following fault history of the M w 7.8 earthquake is proposed. There was an asperity zone below the Kronotsky Cape and its submarine continuation. This asperity was the site of concentration of the events preceding the mainshock, the single earthquakes of magnitude m b between 5.5 and 6.1 that occurred during the 35 years before the mainshock of 5 December 1997. The M w 5.8 earthquake of 9 February 1997, which was accompanied by aftershocks, finished this sequence of single events and marked a change in stress regime within the zone. A foreshock series occurred within the aftershock area of the 9 February earthquake, preparing the nucleus of rupture for the M w 7.8 event, which began at the periphery of the Kronotsky asperity and then broke it almost completely. The rupture continued its way to the southwestern asperities. However, the southwestern asperities were only partially broken, with the amplitude of slip half that for the first asperity. As a result, during the aftershock stage, the maximum activity occurred around these asperity zones. The region of the first asperity, which was completely broken by the mainshock rupture, had almost no aftershock activity.

Detecting premonitory seismicity patterns based on critical point dynamics

Abstract: We test the hypothesis that critical point dynamics precedes strong earthquakes in a region surrounding the future hypocenter. Therefore, we search systematically for regions obeying critical point dynamics in terms of a growing spatial correlation length (GCL). The question of whether or not these spatial patterns are correlated with future seismicity is crucial for the problem of predictability. The analysis is conducted for earthquakes with M > 6.5 in California. As a result, we observe that GCL patterns are correlated with the distribution of future seismicity. In particular, there are clear correlations in some cases, e.g. the 1989 Loma Prieta earthquake and the 1999 Hector Mine earthquake. We claim that the critical point concept can improve the seismic hazard assessment.

Analysis of Accelerations from the 1 October 1995 Dinar, Turkey, Earthquake

Abstract: The Dinar earthquake, 1 October 1995, occurred in southwestern Turkey and had a moment magnitude of 6.4 and a normal faulting mechanism. The earthquake caused 90 deaths, over 200 injuries, and a large amount of damage. It was recorded by seven strong-motion accelerographs. The strong-motion accelerograph at Dinar is on the edge of the surface projection of the fault. The record from Dinar is possibly unique in its proximity to the causative fault for this type of mechanism. The Dinar strong-motion station is in a small, stiff building on soft sediments with a shallow water table. The peak acceleration was 0.32 g on the horizontal component perpendicular to the fault trace. The earthquake was caused by rupture on the Dinar fault, which is a normal fault trending generally toward the south-southeast, with the hanging wall on the west. There was surface rupture along about a 12-km segment of this fault. The Dinar strong-motion station is at the south end of the fault, under 1 km from the nearest surface trace. The hypocenter was at the south end, beneath Dinar, with rupture propagating toward the north and away from the strong-motion site. A specific composite source model was found that reproduces the statistical characteristics of the ground motions and also approximately reproduces the low-frequency waveform in the strong motion. In this model the fault length, L, is 12.5 km, the width, W, is 17.3 km, and the moment M 0 is 4.72 × 10 25 dyne cm. The fault strikes 130° and dips 45° toward the southwest, from the surface to 12 km. For the composite parameters, the largest subevent ( R max ) is 4.0 km, the rupture velocity is 1.5 km/sec, and the subevent stress drop is 60 bars. After Anderson (1997) the composite source model parameters imply that the static stress drop was 36 bars, the radiated seismic energy in this event was 4.3 × 10 21 ergs, the apparent stress was 18 bars, and the Savage-Wood ratio was 1.0. Most of the seismic energy is released at depths greater than 2 km. The major asperity is beneath the location where synthetic aperture radar interferometry found the maximum surface deformation.

Hypocenter location accuracy and seismicity distribution in the Central Apennines (Italy)

Abstract: In this paper, we analyse the seismicity distribution in the Central Apennines (Italy) using the recordings of the Rete Sismometrica Marchigiana (RSM). In particular, the selected events are relocated using a 1-D model calculated by means of an inversion procedure. The robustness of the 1-D model and the location accuracy are tested. The capability of the RSM to well constrain crustal and subcrustal events in the studied area is discussed. We find that in the inner side of the chain the seismicity lies in the upper crustal layers, following the structural trend of the Apenninic belt. A W-deepening of the events is observed in the Apenninic foredeep, where the seismicity is mainly confined in the lower crust. This evidence implies the deepening of the brittle to ductile transition. Some well-located subcrustal events are found. Their locations seem to confirm the W-dipping subduction of the Adriatic lithosphere beneath the Apennines.

Sequence of strong intraplate earthquakes in the Kodiak Island Region, Alaska in 1999–2001

Abstract: A sequence of strong earthquakes was registered in 1999–2001 in the Kodiak Island region of the Alaska-Aleutian subduction zone. Two Mw 7 earthquakes occurred in December, 1999 and January, 2001 and an Mw 6.5 event occurred in July, 2000. These events and their aftershocks recorded by the regional seismograph network were relocated using a Joint Hypocenter Determination (JHD) method. Regional broadband data were used to obtain seismic moment tensors for the main shocks and their largest aftershocks. Relocation and moment tensor inversion results indicate that these events originated inside the subducting Pacific plate. The focal mechanisms indicate down-dip tension with the fault planes being nearly vertical and parallel to the strike direction of the subducting plate. The 1999 and 2000 events were located down-dip of the locked portion of the megathrust, while the 2001 event was located directly beneath it.

Seismological Constraints On Tectonics Of Southern And Central Alaska: Earthquake Locations And Source Mechanisms

Abstract: The major emphasis of this thesis is on investigations of earthquake locations and source mechanisms and what we can learn about Earth structure from them. I used a Joint Hypocenter Determination (JHD) method to improve the earthquake locations obtained after routine data processing. Over 15,000 subduction zone earthquakes in southern Alaska and over 3,600 crustal earthquakes in central Alaska with magnitudes ML ? 2 that occurred from 1988 to 2000 were relocated. I found that the relative earthquake locations can be improved with the use of the JHD relocation technique (30 – 60% reduction in RMS residuals). Thus, many details of the subduction zone geometry and crustal structure can be mapped. To constrain source characteristics, I use a moment tensor inversion method that simultaneously inverts for the source parameters and velocity structure. First, I apply this technique to the sequence of strong earthquakes in the Kodiak Island region, including December 6, 1999 and January 10, 2001 MW 7 events. Next, I expand this approach to moderate-sized ( ML ? 4) crustal earthquakes in central Alaska and calculate 38 moment tensors. I demonstrate that the moment tensor inversion of regional waveforms provides reliable results even when recordings from a single broadband station are used. A catalog of the moment tensors together with the focal mechanisms obtained using conventional P-wave first motion analysis is used to calculate principal stress directions in central Alaska. I find that the stress state in the crust is inhomogeneous and that the orientation of the maximum compressive stress changes from a SE-NW to SSW-NNE orientation from west to east across interior Alaska. One more topic of this thesis is the application of the array analysis to understanding characteristics of anomalous seismic phases observed in the records of the intermediate-depth Alaskan subduction zone earthquakes. I identified two secondary phases arriving with 1 – 3 s and 7 – 12 s delays after the first P-wave arrival. They are interpreted as S-to-P and P-to-S converted phases at the upper/lower surface of the subducted slab.

Earthquake arrival forecast system

Abstract: PROBLEM TO BE SOLVED: To provide earthquake arrival information in which a receiver position is reflected to various information media by a method wherein accurate operation detection processing results of earthquake data composed of earthquake intensity, position and time of a hypocenter obtained from outputs of seismometers arranged in plural portions at intervals of one to several km are transmitted, position setting is performed manually when a receiving point is a fixed position like a house, automatic position setting like GPS is performed in the case of a portable/movable position, and results of operation of the received earthquake data are obtained by using an hour meter together and converted in to earthquake arrival forecast information. SOLUTION: This earthquake arrival forecast system uses the seismometer and a unified detector capable of wireless transmission of each position and time information which are arranged in the vicinity of a hypocenter where an earthquake is generated, receives and processes data of the hypocenter and earthquake intensity through a center, performs alarm and information display with various kinds of information media, informs arrival of an earthquake, and makes it possible for receivers to take refuge in a safe place.

March 19, 1996 Artux Xinjiang Earthquake: A Simple Unilateral Rupture Event

Abstract: Using the Global Digital Seismograph Network (GDSN) broadband P wave records and the adaptive hybrid global search algorithm, the rupture process of the 19 March, 1996 Artux, Xinjiang earthquake(MS6.9) is examined. Aftershock data and results of geological and seismic field investigation are applied to constrain the real fault plane. This event occurred in the western part of the Keping Fault zone and is a unilateral-rupture thrust event with a small strike slip component. The fault plane has a strike of 252° and a dip of 30°. The rupture duration was 15 s and the focal depth was 13 km. The inferred slip distribution contains two major slip sources. A sub-event with a slip amplitude of 0.3m and a risetime of 0.8 s is located at the hypocenter. The maximum slip amplitude of 1.0m is associated with a 3.5 s risetime subevent located to the east of the first major slip source. The distance between the two major slip sources is about 25 km, and corresponds with the 30 km difference between the instrumental and macroseismic epicenters.

The Shape of the Philippine Sea Slab

Abstract: The shape of the Philippine Sea slab beneath southwest Japan was investigated using the hypocenter distribution and the fault plane solutions obtained by Japan Meteorological Agency. Two seismic belts were found in an equal depth section of the slab and these did not relate to the double seismic zone in the subducting slab. Then, the existence of a broken off piece of slab was proposed beneath Kii channel. This piece collided with Philippine Sea slab and created a high seismicity area at each side.Three events that were not located at a high seismic area in the slab were found. Two were considered to be the events that occurred at the near aseismic part of the slab. The other may have occurred at another broken off piece of the slab.

The 1996 artux,xinjiang,earthquake: a unilateral rupture event

Abstract: Using the broadband P wave records of the Global Digital Seismograph Network(GDSN) and the adaptive hybrid global search algorith,we investigate the rupture process of the 19 March,1996 Artux,Xinjiang,earthquake( M S=69) Slip vectors,risetime and rupture time for a finite fault are obtained and their errors is estimated by introducing a random weight method Aftershock data,field geology and seismic investigations are applied to constrain the fault plane This event occurred in the west part of the Kalpin fault zone and is a unilateral rupture thrust event with a small strike slip component The fault plane has a stricke of 252° and a dip of 30° The rupture duration is 15s and the focal depth is 13km We conclude that the inferred slip distribution contains two major slip sources A sub event with a slip amplitude of 03m and a risetime of 08s is located at the hypocenter The maximum slip amplitude of 10m is associated with a 35s risetime subevent located to the east of the first major slip source The distance between the two major slip sources is about 25km,and corresponds with the 30km difference between the instrumental and macroseismic epicenters [