Modelling of ground motion in the Higashinada (Kobe) area for an aftershock of the 1995 January 17 Hyogo-ken Nanbu, Japan, earthquake

We model the ground motion from an aftershock of the 1995 January 17 Hyogo-ken Nanbu (Kobe) earthquake to investigate basin edge effects on wave propagation in Higashinada ward, downtown Kobe. Point-source finite-difference seismograms calculated using a double-couple solution and a 2-D basin structure are compared with the ground-motion velocity seismograms recorded in a small station array deployed at sites within and outside the heavily damage zone in Higashinada ward. The comparison suggests that in the frequency range 0.1-2 Hz that was analysed, the observed spatial amplitude variation of the aftershock ground motion is attributable mainly to the basin edge effects. We found that the basin edge effect, caused by the superposition of the direct S wave and the basin-edge-diffracted waves, amplified the ground motion in a narrow zone that is offset by about 0.7 km from the basin edge.     

Applying the relative hypocentre location approach: where was the 1980 November 23 Irpinia earthquake?

The purpose of this work is to evaluate under what conditions it is feasible and with what accuracy it is possible to locate the nucleation point of a large earthquake, given the availability of aftershocks located with high precision by the deployment of a local network. We experiment with several approaches and apply them lo the location of the epicentre of the 1980 November 23 Irpinia earthquake ( M ^w= 6.9).
First we use local P ^g phases selected to optimize the azimuthal coverage, obtaining a well-constrained location with a small statistical error, which typically underestimates the true hypocentre uncertainty.
We then exploit the relative location technique, obtaining stable, almost coincident solutions under three conditions: (1) using multiple independent master events to derive an average epicentre; (2) fitting simultaneously the larger data set for all available master events, using a forward approach; (3) conducting an a priori evaluation of the statistics of station and master events to separate model uncertainties and improve the statistical accuracy of the relative locations. Moreover, only by introducing station statistics can we achieve the desired accuracy of ≅ 1 km in constraining the rupture nucleation point of this large earthquake, and we show that the application of the relative location technique to uncleaned, unweighted data for a single master event provides only a crude epicentre with a confidence ellipse deceivingly smaller than the true hypocentre uncertainty.
The revised epicentre for the 1980 November 23 Irpinia earthquake (48.803 °N-15.302°E) validates the class of multidisciplinary reconstructions of the source process such as the model of Valensise et al. (1989), based on the hypocentre of Westaway & Jackson (1987), and is shifted by almost 13 km to the NW of the epicentre recently proposed by Westaway (1992).     

The Zihuatanejo, Mexico, earthquake of 1994 December 10 (M= 6.6): source characteristics and tectonic implications

An analysis of the Zihuatanejo, Mexico, earthquake of 1994 December 10 ( M = 6.6), based on teleseismic and near-source data, shows that it was a normal-faulting, intermediate-depth ( H = 50 ± 5 km) event. It was located about 30 km inland, within the subducted Cocos plate. The preferred fault plane has an azimuth of 130°, a dip of 79° and a rake of −86°. The rupture consisted of two subevents which were separated in time by about 2 s, with the second subevent occurring downdip of the first. The measured stress drop was relatively high, requiring a Δσ of about a kilobar to explain the high-frequency level of the near-source spectra. A rough estimate of the thickness of the seismogenic part of the oceanic lithosphere below Zihuatanejo, based on the depth and the rupture extent of this event, is 40 km.
This event and the Oaxaca earthquake of 1931 January 15 ( M = 7.8) are the two significant normal-faulting, intermediate-depth shocks whose epicentres are closest to the coast. Both of these earthquakes were preceded by several large to great shallow, low-angle thrust earthquakes, occurring updip. The observations in other subduction zones show just the opposite: normal-faulting events precede, not succeed, updip, thrust shocks. Indeed, the thrust events, soon after their occurrence, are expected to cause compression in the slab, thus inhibiting the occurrence of normal-faulting events. To explain the occurrence of the Zihuatanejo earthquake, we note that the Cocos plate, after an initial shallow-angle subduction, unbends and becomes subhorizontal. In the region of the unbending, the bottom of the slab is in horizontal extension. We speculate that the large updip seismic slip during shallow, low-angle thrust events increases the buckling of the slab, resulting in an incremental tensional stress at the bottom of the slab and causing normal-faulting earthquakes. This explanation may also hold for the 1931 Oaxaca event.     

Determination of the fault plane using a single near-field seismic station with a finite-dimension source model

We explore the possibility of determining the actual fault plane of an earthquake from the inversion of near-source displacement seismograms of one station when a finite-dimension source is used instead of a point source model and when the complete displacement is taken into account, including near-field waves. Tests on synthetic seismograms and real data recorded at local distances show that this is possible even with a single, three-component station. A single accelerogram available for the Erzincan, Turkey, 1992 March 13, M _s = 6.8 earthquake is inverted and the solution found is compatible with other seismological studies and with the mechanism expected for the North Anatolian Fault.     

Three-dimensional geometrical ray theory and modelling of transmitted seismic energy of data from the Nevada Test Site

This paper presents a geometrically based algorithm for computing synthetic seismograms for energy transmitted through a 3-D velocity distribution. 3-D ray tracing is performed to compute the traveltimes and geometrical spreading (amplitude). The formulations of both kinematic and dynamic ray-tracing systems are presented. The two-point ray-tracing problem is solved by systematically updating the initial conditions and adjusting the ray direction until the ray intersects the specified endpoint. The amount of adjustment required depends on the derivatives of the position with respect to the given starting angles between consecutive rays. The algorithm uses derivatives to define the steepest-descent direction and to update the initial directions. The convergence rate depends on the complexity of the model.
Test seismograms compare favourably with those from a 2-D asymptotic ray theory algorithm and a 3-D Gaussian-beam algorithm. The algorithm is flexible in modelling arbitrary source and recorder geometries for various smoothly varying 3-D velocity distributions. The algorithm is further tested by simulating surface-to-tunnel vibroseis field data. Shear waves as well as compressional waves may be approximately included. Application of the algorithm to a data set from the Rainier Mesa of the Nevada Test Site produced a good fit to the transmitted (first arrival) traveltimes and amplitudes, with approximately 15 per cent variation in the local 3-D velocity.     

Coseismic deformation revealed by inversion of strong motion and GPS data: the 2003 Chengkung earthquake in eastern Taiwan

A moderate earthquake of   M _w= 6.8  occurred on 2003 December 10. It ruptured the Chihshang Fault in eastern Taiwan which is the most active segment of the Longitudinal fault as a plate suture fault between the Luzon arc of the Philippine Sea plate and the Eurasian plate. The largest coseismic displacements were 13 cm (horizontal) and 26 cm (vertical). We analyse 40 strong motion and 91 GPS data to model the fault geometry and coseismic dislocations. The most realistic shape of the Chihshang fault surface is listric in type. The dipping angle of the seismic zone is steep (about 60°–70°) at depths shallower than 10 km and then gradually decreases to 40°–50° at depths of 20–30 km. Thus the polygonal elements in Poly3D are well suited for modelling complex surfaces with curving boundaries. Using the strong motion data, the displacement reaches 1.2 m dip-slip on the Chihshang Fault and decreases to 0.1 m near surface. The slip averages 0.34 m, releasing a scalar moment of 1.6E26 dyne-cm. For GPS data, our model reveals that the maximal dislocation is 1.8 m dip-slip. The dislocations decrease to 0.1 m near the surface. The average slip is 0.48 m, giving a scalar moment of 2.2E26 dyne-cm. Regarding post-seismic deformation, a displacements of 0.5 m were observed near the Chihshang Fault, indicating the strain had not been totally released, as a probable result of near-surface locking of the fault zone.     

强震群活动构造环境比较研究

将1997-1998年新疆伽师6.0～6.6级强震群与国内外14例5.1～8.7级强震群的活动构造环境比较研究后发现,强震群持时25min～3a,强震群区的地震活动多具重复性,发震构造多为刚性地块共轭隐伏破裂。强震群为板内浅源地震。