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GEOLOGYEARTHQUAKES
955039 The finite extension of fractal geometry and power law distribution of shallow earthquakes: a geomecbanical effect P. Volant & J.-R. Grasso, Journal of Geophysical Research, 99(Bll), 1994, pp 21,879-21,889. A dome structure at an intermediate scale between laboratory analyses and tectonic processes was analysed. Local gas extraction induced about 1000 events within the dome recorded by a local network during 19 years. Two types of autosimilarity coefficients (b value and correlation dimension) are analyzed in three-dimensional (3-D) space. Three-dimensional fractal analysis of induced earthquakes allows definition of two distinct classes of events separated by a critical distance of 500 m. The first class shows a diffuse seismicity. The second class defines nests of seismic&y and attests a bifractal character. It is proposed that: 1) the finite extension of the earthquake power law is driven by the local setting and therefore is also a scale dependent process, 2) the geomechanical link between fractal behavior and fracture size, must also be found in the boundary values of the autosimilarity processes (slope breaks) rather than in the values of the power law exponents. (from Authors) 955040
Scaling of intraplate earthquake recurrence interval with fault length and implications for seismic hazard assessment R. Marrett, Geophysical Research Letters, 21(24), 1994, pp
circular canyon and an alluvium whose frequency domain solutions are available from the past publications. The results show that the proposed technique is reliable and therefore can be used to study the seismic response at irregular sites. (from Authors) 955043
Anisotropy and the splitting of PS waves Liqiang Su BEJ. Park, Physics of the Earth & Planetary Interiors, 86(4), 1994, pp 263-276. The PS wave is radially polarized in an isotopic spherically symmetric Earth owing to the P to SV conversion at the free surface. If we choose stations with weak SKS splitting to minimize the effect of the receiver, we can measure the anisotropy beneath the bounce point of the PS wave. With a
deep source and epicentral range of 90” < A < 125”, the PS phase can be distinguished from other phases. Owing to the limitation of recent large deep events with epicentral range between 90” and 125”, data from stations in North America and events beneath the Bonin Islands and the Fiji Islands are analysed. For events in these two regions, the bounce points of the PS wave are roughly located northwest of the Kuril Islands and near the Nova-Canton Trough in the central Pacific, respectively. Coupled-mode synthetics constructed with the ‘strong’ Born approximation can be used to model the interaction of the PS waves with anisotropic structure that has a horizontal axis of symmetry. (from Authors)
2637-2640. Consensus indicates that faults follow power-law scaling, although significant uncertainty remains about the values of important parameters. Combining these scaling relationships with power-law scaling relationships for earthquakes suggests that intraplate earthquake recurrence interval scales with fault length. Due to uncertainties regarding the recently recognized changes in scaling for large earthquakes, it is unclear whether recurrence interval varies as a negative or positive power of fault length for large faults. This question is of critical importance for seismic hazard assessment. (from Author) 955041 Non-plane geometries of seismic surface wavefields and their implications for regional surface-wave tomography W. Friederich, E. Wielandt & S. Stange, Geophysical Journal Znternationaf, 119(3), 1994, pp 931-948. Seismic surface waves exhibit much more complicated waveflelds than is commonly assumed. This was the conclusion after analysing 90 teleseismic events recorded at on average eight broad-band stations in Southern Germany. Large amplitude and phase fluctuations across the network are observed which are definitely not due to instrument
response or calibration problems. Experiments indicate that regional surface-wave tomography may work, if there is an excellent coverage of paths crossing the region and if each path is sampled several times. Under realistic conditions, that is on a sparse network with not much more than one velocity value per path, the imaging power of surface-wave tomography is very poor. (from Authors) 955042 SH-wave scattering and propagation analyses at irregular sites by time domain BEM H. Takemiya & A. Fujiwara, Bulletin - Seismological Society of America, 84(5), 1994, pp 1443-1455. This article presents numerical computations of two-dimensional transient responses of canyons and alluvium deposits with various configurations subject to vertically and obliquely incident SH waves. The time domain boundary element method (BEM) is used. Elementwise double integrals over space and time domains are performed analytically for the first time. The validity of the solution is examined for a semi-
955044
Nature of P-wave signals from Hindukusb earthquakes A. Ram, J. N. Tripathi 8c L. Yadav, Journal of Geodynamics, 19(2), 1994, pp 159-175. Several causes for complexity of P-wave signals from earthquakes in the Hindukush focal region and recorded at Gauribidanur seismic array in southern India, have been investigated. Visual inspection of the records of these earthquakes revealed that most of the events occurring at shallow depths had complex signatures as compared to the deeper events. The structure of the fault rock in the source region, the source functions itself, and the ray-scattering in the crustal-upper mantle near the source are the main responsibilities for the complexity of the Hindukush earthquake records at GBA since the transmission of the ray-tubes for all events studied is practically identical, with the exception of the very first near source part. (from Authors) 955045
Attenuation of coda waves in western Anatolia A. Akinci, A. G. Taktak & S. Ergintav, Physics ofthe Earth & Planetary Interiors, 87(1-2), 1994, pp 155-165. By analyzing the decay of coda wave amplitude, coda Q,Q=, were determined in Western Anatolia. For a lapse time of 30 s, the frequency dependence of Qc, is Qc = 50.7f.O’ and for a lapse time of 190 s it is Q&) = 183.2! 6. In this region, the exponential value of the Qc frequency dependence is practically constant, between 0.7 and 1.0. The obtained coda Qc values were compared with those estimated in other regions. (from Authors) 955046 On the derivation of coseismic displacement fields using differential radar interferometry: the Landers earthquake H. A. Zebker, P. A. Rosen, R. M. Goldstein, A. Gabriel & C. L. Werner, Journal of Geophysical Research, 99(BlO), 1994, pp 19,617-19,634. The paper presents a map of the coseismic displacement field resulting from the Landers, California, June 28, 1992, earthquake derived using data acquired from an orbiting high-resolution radar system. Results which are more accurate than previous space studies and similar in accuracy to those obtained by conventional field survey techniques are
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GEOLOGY:EARTHQUAKES
achieved. Data from the ERS 1 synthetic aperture radar instrument acquired in April, July, and August 1992 are used to generate a high-resolution, wide area map of the displacements. Comparison of the data with Global Positioning System and electronic distance measurement survey data yield a correlation of 0.96; thus the radar measurements are a means to extend the point measurements acquired by traditional techniques to an area map format. (from Authors) 955047 Analysis of coseismic surface displacement gradients using radar interferometry: new insights into the Landers ear&quake G. Peltzer, K. W. Hudnut & K. L. Feigl, Journal of
Geophysical Research, 99(Bll),
1994, pp 21,971-21,981.
The map of the cosemismic displacement field generated by interferometric processing of synthetic aperture radar (SAR) images taken before and after the June 28, 1992, Landers earthquake sequence brings new insights into the nature of deformation caused by these earthquakes. Complexities in the fringe pattern near the fault reflect short-wavelength variations of the surface rupture and slip distribution, and attest to large displacement gradients. In order to understand the observed fringe patterns, synthetic interferograms are computed in three simple cases: 1) rigid-body rotations about a vertical axis, 2) about a horizontal axis (tilt), and 3) distributed, simple shear. The orientation and spatial separation of interferometric fringes predicted by these models help constrain near-field deformation and rupture parameters. (from Authors) 955048 Modeling of periodic great earthquakes on the San Andreas Fault: effects of nonlinear crustal rheology Z. Reches, G. Schubert & C. Anderson, Journal of Geophysical Research, 99@11), 1994, pp 21,983-22,000.
The cycle of great earthquakes along the San Andreas fault is analysed with a finite element numerical model of deformation in a crust with a nonlinear viscoelastic rheology. The results are sensitive to crustal rheology. Models with quartzite-like rheology display profound transient stages in the velocity, displacement, and stress fields. The predicted transient zone extends about 3-4 times the crustal thickness on each side of the fault. Models with diabase-like rheology behave similarly to elastic models and exhibit no transient stages. The model predictions are compared with geodetic observations of fault-parallel velocities in northern and central California and local rates of shear strain along the San Andreas fault. (from Authors) 955049 Empirical Green’s function study of the January 17, 1994 Northridge, California earthquake D. S. Dreger, Geophysical Research Letters, 21(24), 1994, pp
2633-2636. The January 17, 1994, Northridge mainshock was analyzed using an empirical Green’s function approach to estimate the spatial and temporal distribution of fault slip. The location of the fault beneath the San Fernando valley as well as the updip directivity of the rupture contributed to the locally heavy damage due to this earthquake. (from Author) 955050 Seismische analyse van aardbevingen in Noord-Nederland (Seismic analysis of earthquakes in the northern Netherlands) H. W. Haak 8t T. De Crook, Scientific Reports - Royal Netherfan& Meteorological Institute, WR(94-Ol), 1994, 38
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The report forms part of a multi-disciplinary investigation of the relationship between gas extraction and earthquakes. The contribution of the Koninklijk Nederlands Meteorologisch Instituut (KNMI) was to carry out a seismic analysis of earthquakes in the North Netherlands. The most complete analysis of seismograms was carried out for the four strongest earthauakes in the Eleveld aas field. The KNMI also determined the maximum ear&quakes to be expected in the region and estimated the damage which could occur. The Institute also considered whether the placing of seismometers in earthquake detection boreholes could be improved, and a test was carried out near Finsterwolde. (P.Cooke) 955051 Determination of the fault slip distribution of the 1976 Tangshan earthquake by the finite element method Shaorong Zhao & Dingbo Chao, Journal of Geodynarnics, 19(2), 1994, pp 107-l 15.
A general method for the determination of the coseismic fault slip distribution by inversion of geodetic data is presented. One type of inverse problem and its solutions are investigated by the finite element and regularization methods. The coseismic fault slip vector is expressed by the solutions of the inverse problem of partial differential equations. The proposed method is used to determine the coseismic fault slip distribution on the Tangshan fault and Young’s modulus of the rock in the area around the coseismic fault, and the results show that the inverted fault slip distribution is in good agreement with the observed surface offsets on the fault caused by the earthquake. (Authors) 955052 Seismological
study for Weihai City (in Chinese)
Cai Kerning, Seismology & Geology, 16(l), 1994, pp 38-42. Weihai City lies on the eastern end of Shandong Peninsula; a part of the famous fault zone (Zhang Jiakou-Bohai-Weihai) goes across the city, where destructive earthquakes have occurred many times. This naner nresents data from seismology, hydrology and e&&e&g geology and a preliminary discussion and analysis of the fault zone is given. (English summary) 955053 B-type and explosion earthquakes observed at Asama volcano, central Japan M. Sawada, Journal of Volcanology & Geothermal Research, 63(3-4), 1994, pp 111-128.
B-type and explosion earthquakes recorded at Asama Volcano, central Japan, are analyzed. Waveforms in B-type events are gradational from a high-frequency B-type (BHtype) to a low-frequency B-type (BL-type). A simple P-wave source model for B-type and explosion earthquakes is presented. The P-wave source model can satisfactorily explain the distribution of initial motions and difference in the wave characteristics of these events. The waveform difference between BH-type and BL-type events is attributable to the different source depths. B-type and explosion events are due to gas explosion or gas emission. (from Author) 955054 High-frequency spectra of regional quakes and chemical explosions
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W. Y. Kim, D. W. Simpson & P. G. Richards, Bulletin Seismological Society of America, 84(5), 1994, pp 1365-1386. The high-frequency spectra of chemical explosions and earthquakes at local and regional distances in the northeastern US and in Norway are analysed to understand the seismic signal characteristics of single explosions, multiplehole instantaneous explosions, ripple-fired quarry blasts, and earthquakes. To discriminate instantaneous explosions from