The source parameters of moderate‐ to large‐sized earthquakes and the spatial distribution of earthquake hypocenters are used to investigate the active tectonics and three‐dimensional configuration of the subducted lithosphere in the Pamir‐Karakorum intracontinental convergence zone. The continental collision has resulted in intense deformation in the upper crust. The deformation is distributed over a broad area and is absorbed by thrust and strike‐slip faulting. Along the northern margin of the Pamirs, where the deformation due to continental collision is most intense, the Northern Pamir Thrust and several major fault zones mark the present boundary. Most of the earthquakes that occurred in these fault zones are shallow crustal events with focal mechanism solutions correlated with the tectonic features of the region. Along the Pamir Front, thrust faulting dominates, while on the western and eastern edges of the Pamirs the style of deformation is characterized by oblique thrusting with a component of strike‐slip motion. Thrust‐type events beneath the Tadjik Depression indicate that both the sedimentary rocks and the basement are involved in shortening. Right‐lateral strike‐slip motion is observed on the eastern edge of the Pamirs where they border the Tarim Basin and in the Talas‐Fergana‐Kun Lun fault zone. A southward dipping seismic zone beneath the Pamirs and Karakorum indicates that the Asian lithosphere has been subducted along the Pamir Front to a depth of at least 200 km. This interpretation is consistent with published geological and geophysical data. Focal mechanism solutions of some intermediate‐depth events beneath the Pamirs show strike‐slip faulting with approximately N‐S horizontally oriented P axes, indicating that nearly horizontal compression at the intermediate depth is the predominant mode of deformation. A 90‐km‐deep event beneath the Karakorum is interpreted as occurring in the subducted Asian lithosphere; the fact that the P axis of this event is oriented parallel to the descending plate suggests downdip compression within the Asian lithosphere. The overall tectonics of the region is interpreted as a consequence of the underthrusting Asian lithosphere being impinged upon by the shallow northward underthrusting Indian lithosphere.
The widespread existence of strong Lg attenuation in the Tibetan Plateau is further demonstrated by analysis of Lg spectra on many paths within the plateau and quantitative estimates of the Lg attenuation from low-frequency Lg signals for new, localized path geometries. Strong path-length-dependent shifts of the Lg spectra to lower corner frequencies with increasing distance are observed across the plateau, consistent with a low regional average 1-Hz Lg attenuation value, Q 0 , of about 125. There are clearly lateral variations within the plateau found in this and other recent studies, with localized areas having Q 0 values of 60-90, low enough to eliminate high-frequency Lg energy over path lengths of just several hundred kilometers, while some localized areas may have higher Q 0 values of up to 147 or higher. A Q 0 value of 103 is found in south-central Tibet, compatible with recent work by others for higher frequencies on very localized scales, and values from 83 to 147 are found in eastern Tibet. The lowest Q 0 estimates found in Tibet tend to be in areas for which there is evidence of volcanism and/or partial melting within the crust; however, the strong regional attenuation may have a contribution from scattering by small-scale crustal heterogeneity. The strong Lg attenuation in Tibet gives a new constraint for understanding the tectonic development of the plateau and presents challenges to seismic monitoring of the region for possible clandestine nuclear tests.
The installation of very broadband seismic stations makes it possible to recover the source parameters of small earthquakes (2.5 < ML < 5.0) which occur at local and regional distances. If the gross crustal structure along the travel path is known, it is possible to use the P, SV and SH displacement waveforms from a single station to determine the seismic moment tensor. Although the details of the crustal structure strongly affect the body waveforms at regional distances, the signature of the seismic source orientation on the waveform is robust at frequencies less than 1–3 Hz. We explore the trade‐offs between crustal model, hypocentral depth and filtering for a linear moment tensor inversion procedure. The procedure is tested on two small earthquakes which occurred in the Rio Grande Rift and were recorded at the IRIS/USGS station ANMO. The agreement between the single station moment tensor inversion fault plane parameters and those determined from local first motions is excellent.
The April 22, 1991, Valle de la Estrella, Costa Rica earthquake (M s = 7.6) was a back-arc thrusting event associated with the underthrusting of the Caribbean plate beneath Central America. A network of three PASSCAL-type, portable instruments was deployed to monitor the aftershock actiyity in southern Costa Rica 2 to 6 weeks after the main shock. The waveforms recorded on three-component midperiod seismometer• were used to recover source information for 15 small aftershocks (magnitudes between 3.2 and 4.4) with a linear moment tensor inverfion method. We conducted several tests to investigate the effects of unknown structure and event mislocation on source parameter recovery. The longer-period waveforms, in general, are less sensitive to the effects of the structural details so that the essential source information can be successfully extracted from the waveform data. The earlier part of the seismic waveforms has proven to be the most important carder of the source information. A gross cmstal model can be used to describe the structure for the source study. The small changes in the waveform character resulting from the mislocation of the events, or inexact Green's functions generated from the oversimplified crustal model, do not prohibit us from the recovery of the source orientation at local distances. In contrast, the determination of the focal depth is subject to uncertainty because of the lack of detailed structural information. Our focal mechanisms are generally in good agreement with P wave first-motion fault plane solutions determined from a local short-period network. The aftershocks show a clear spatial segmentation based on focal mechanism type. Most aftershocks near or southeast of the main shock were thrusting events with focal mechanisms similar to the main shock. In contrast, a cluster of aftershocks northwest of the main shock showed dominantly left-lateral, strike-slip motion on a northeasterly striking nodal plane. This suggests that a diffuse deformation zone exists in central Costa Rica and is characterized by left-lateral strike-slip motion. This diffuse, transcurrent deformation zone coincides with several geologic and geophysical features, and perhaps is a result of the slower subduction rate of the buoyant Cocos Ridge, than its adjacent segments along the Middle America Trench (MAT). The diffuse transcurrent boundary may intersect with the North Panama Deformed Belt (NPDB) near Limon, Costa Rica, and is very likely a plate boundary for the proposed Panama block.The later measures the amplitude coherency between the observed and predicted waveforms and requires polarity consistency between them. One major advantage of using a linear moment tensor inversion is that the solution is independent of the starting model. A detailed description of the procedure can be found in Fan and Wallace [ 1991 ].
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