Microearthquake Seismicity and Fault Plane Solutions in the Hindu Kush Region and Their Tectonic Implications (Paper 9B1560)

The nature of the Hindu Kush intermediate seismic zone was studied in two microearthquake investigations in 1976 and 1977. By testing several sources of uncertainty the precision of about 600 earthquake locations was estimated to be about 5 km in epicenter and 10 km in depth. Projections of the earthquake locations from several perspectives reveal several regions of aseismicity as well as a highly contorted nature of the active regions. Very little seismicity was recorded in the crust from 0- to 70-km depth. The part of the zone southwest of about 37°N, 71.5°E and shallower than about l60 km is broad and seems to dip north at progressively steeper angles from west to east. Fault plane solutions for this region do not reveal a simple consistent pattern. This region is separated from another active region to the northeast by a curved gap that is nearly 50 km wide. Northeast of this gap the zone dips to the southeast. In the western portion there is an aseismic region around 160-km depth that separates the contorted shallower zone from a narrow (15–20 km wide), consistently steep, deeper zone. As in island arcs, the fault plane solutions for the deeper events show T axes generally lying within the plane of seismicity and P axes perpendicular to the plane. In contrast to island arcs the T axes in general are not parallel to the dip direction, and there seems to be greater variation in the orientation of these axes. The entire western zone plunges to the west at about 20°, and most of the T axes plunge steeply to the west. In detail, the earthquakes tend to occur in clusters that leave aseismic gaps between the clusters. There is a distinct gap of about 15-km width near 70.7°E. This gap seems to separate events with fault planes solutions that in the west have westward plunging T axes and in the east have eastward or vertically plunging T axes. Although many of these features were not detected in previous studies of this region, the data from those studies are consistent with the dips, changes in dip, gaps, and breadth of the seismic zone. Both the variations in dip and breadth of the active zones and, for one gap, the difference in fault plane solutions of earthquakes on either side of it, make the role of the gaps as boundaries clear and suggest their long-term existence. We infer that the configuration of the Hindu Kush seismic zone could possibly be the result of the subduction of oceanic lithosphere from two separate, small basins in opposite directions. The age of the subducted lithosphere is probably greater than 70 m.y., and subduction probably has occurred over a relatively short duration. The rate of sub-duction probably has been between 20 mm/yr and 48 mm/yr. Correlations of seismic trends with surficial features suggest that in the south the Hindu Kush suture zone lies along or somewhere between the Panjer and Kunar faults and that in the north the Pamir suture zone lies near the Darvaz-Karakul fault. Finally, it seems that the protrusion of India into Eurasia has been a major factor in developing the present configuration of the zone. Appendix is available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Ave., N.W., Washington, DC 20009. Document J80-002; $1.00. Payment must accompany order.