Bering Sea Earthquake of February 21, 1991: Active faulting along the Bering Shelf Edge

Although fault‐bounded thrust sheets are common in the geological record, seismic evidence for their motion is sparse. The April 29, 1991, Racha earthquake (Ms = 7.0), the largest instrumentally recorded earthquake in the Greater Caucasus, is one of the largest recent earthquakes in continental thrust belts and provides evidence on mechanisms of thrust sheet motion. Using data from a deployment of Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) digital seismographs and various other instruments, we locate 1952 aftershocks occurring between May 7 and June 30, 1991. The aftershocks form a zone ∼70 km long and 10–25 km wide striking E‐W, following the Racha ridge at the southern boundary of the Greater Caucasus thrust system. Teleseismic body waves are inverted for source parameters of the mainshock and the two largest aftershocks. The solutions show thrust faulting with centroid depths of 3–10 km, comparable to depths of locally recorded aftershocks (∼2–12 km). The shallow‐dipping nodal plane, the aftershock distribution, and surface geology demonstrate that the main event was caused by faulting on a thrust system dipping NNE at 20°–31° bounding the southern slope of the Greater Caucasus. This fault system thrusts the Greater Caucasus structures south over the Dzhirula basement massif. The inferred fault geometry suggests that the active fault is either a detachment between sediments and Dzhirula basement or cuts through the basement at shallow depths. The 1500‐m‐high Racha ridge overlies the aftershock zone and is a likely consequence of repeated similar earthquakes. Hence the 1991 earthquake sequence shows that the western Greater Caucasus is accommodating plate convergence at a rate possibly comparable to the eastern Greater Caucasus (a few millimeters per year). Along‐strike geological discontinuities above and below the thrust surface correspond to the eastern end of the mainshock rupture area. No strong evidence for transfer structures could be found along strike, suggesting that differences in collisional style between the western and eastern Greater Caucasus may reflect differences in mechanical properties rather than differences in convergence rate. A June 15, 1991, event and its aftershocks, southeast of the primary aftershock zone along strike, show fault planes and slip vectors rotated ∼41° clockwise from the mainshock. This rotation is consistent with an along‐strike change in direction of the thrust front, near 44°E longitude, and demonstrates strong local structural or topographic control on slip direction. The rotation requires along‐strike shortening within the Greater Caucasus thrust system at a rate comparable to the rate of thrusting.

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Seismic source modeling

Beroza¹

1995

Reviews of Geophysics

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Seismic source modeling is taken to mean the modeling of seismograms to investigate kinematic properties of the earthquake source. Current trends such as the use of other data types (e.g. geodetic measurements of coseismic deformation) and the explicit consideration of source dynamics when modeling seismograms are beginning to blur this definition, but I will adhere to it in this review. I have divided seismic source modeling into teleseismic, regional, and near‐source modeling. Teleseismic modeling has the advantage of global coverage for earthquakes of magnitude 5.5 and larger, but does not provide much more than point‐source parameters. Regional‐distance modeling has the advantage of a more rapid assessment of earthquake source properties, but is complicated by relatively complex wave propagation. The coverage is also limited to areas of broadband regional networks ‐ typically continental areas of developed countries. Near‐source modeling has the potential for detailed imaging of source properties, but very few earthquakes occur in regions with sufficient instrumentation to allow this. I have also included a section on historical earthquakes because of the additional issues that seismologists face in working with pre‐WWSSN data and because of the important issues, such as earthquake recurrence, that require historical data. In addition to reviewing the progress in seismic source modeling during the last four years, I have also briefly reviewed some of the progress in understanding earthquake physics and hazards that has come about because of basic research in earthquake source modeling.

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Bering Sea Earthquake of February 21, 1991: Active faulting along the Bering Shelf Edge

Cited by 6 publications

References 24 publications

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Active thrust front of the Greater Caucasus: The April 29, 1991, Racha earthquake sequence and its tectonic implications

Seismic source modeling

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