Constraints on plate motions in southern Pakistan and the northern Arabian Sea from the focal mechanisms of small earthquakes

Abstract

The focal mechanism and depth were determined for nine small earthquakes (M0<1025 dyn cm, M<5.5) that occurred in southern Pakistan and the northern Arabian Sea from an analysis of the vertical component of Rayleigh waves in combination with limited first‐motion data. Focal parameters were determined from the Rayleigh waves by using an event‐pair method of analysis. For earthquakes that are located very close to each other (<≈ 50 km), the event‐pair method is able to remove a significant proportion of propagation effects at all periods in the range of interest (20–50 s). For events separated by more than ≈ 100 km the propagation effects are reduced for only the longer periods (≈ 40–50 s). The earthquakes that were studied provide evidence for a model of plate interactions in the vicinity of the southern Pakistan triple junction. The Owen fracture zone is a transform fault that accommodates right‐lateral motion between the Indian and Arabian plates. The plate boundary in the vicinity of the Murray ridge is also partially made up of transform segments that strike subparallel to the Owen fracture zone. Spreading centers may also exist in the vicinity of the Murray ridge but were not documented by seismic or other evidence. The slip azimuths for earthquakes along this boundary are significantly more northerly than those predicted by various regional and worldwide models of plate motion. The Arabian plate is being subducted beneath the Eurasian plate along the southern coast of Pakistan. Slip vectors for earthquakes along this boundary trend northnortheasterly in general agreement with predicted directions. Left‐lateral motion is documented along the boundary between the Indian and Eurasian plates in southern Pakistan. The predicted direction of relative motion between these plates is not significantly different from that observed. Two of the earthquakes studied appear to be intraplate in nature. The depth and focal mechanism of one intraplate event, which may have occurred within the accretionary prism along the Makran coast, however, are based on limited data. The other intraplate earthquake occurred within the sedimentary deposits along the western edge of the Indian subcontinent near the Indian‐Eurasian plate boundary.Appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document B83‐009; $2.50. Payment must accompany order.