Seismotectonics of Pakistan: A Review of results from Network Data and implication for the Central Himalaya.

Abstract

Microearthquakes detected by two telemetered seismic networks in northern Pakistan reveal a decollement style of tectonics for the Hazara and northwest Punjab region. The three-dimensional distribution of seismicity and composite fault-plane solutions delineate a nearly horizontal surface along which the decollement of sediments and metasediments is decoupled from the basement, Below the decoupling surface major faults trend northwest. These faults are recognized as basement structures that are associated with the Himalayan arc, but that extend towards the northwest beyond the Hazara-Kashmir syntaxis. During the last six years, the period of network observations, the seismicity is mostly associated with the basement faults, Seismic activity within the decollement is generally low and the decoupling surface is either aseismic or associated with very low seismicity. The results from northern Pakistan, together with recent seismicity along the Himalayan arc and information on great Indian earthquakes, are combined to derive a general model of Himalayan tectonics. A shallow thrust of gentle dip, termed the Detachment, underlies the Indo-Gangetic plain and the Lower Himalaya. Rupture of this fault, which separates the low-strength sedimentary wedge from the basement, results in great earthquakes that cause devastation over a wide area, but occur only infrequently. Down-dip from the Detachment, beneath the Higher Himalaya, the thrust assumes a steeper dip and juxtaposes basement material of similar properties. Earthquakes occur more frequently on this portion of the fault, termed the Basement Thrust, but are smaller than the Detachment events. They can, however, be damaging locally. The model suggests that much of the Indo-Gangetic plain underlain by the Detachment has a high seismic hazard. In the Hazara arc region of northern Pakistan the Detachment is associated with a thick layer of Infracambrian salt. In this region the Detachment may slip aseismically; however, slip by large infrequent earthquakes cannot be ruled out.

Results from a seismic network centered on Quetta show that the Chaman fault, and the fault associated with the Quetta earthquake of 1935, are currently active. The segments that ruptured during the most recent large earthquakes on these faults, 1892 on the Chaman fault and 1935 on the Quetta fault, are relatively quiet at present. On both faults the current seismicity is concentrated near the ends of the ruptures associated with these earlier events, and indicates left-lateral strike slip. In analogy with the San Andreas and Alpine fault systems, in, California and New Zealand, respectively, two distinct portions of the Chaman fault system are recognized. One, north of approximately 31^oN, trends obliquely to the regional slip vector and is probably characterized by infrequent great earthquakes. The other, south of 31 "N, has a strike subparallel to the regional slip vector and consists of broad zone of faults that rupture in large, but not great, earthquakes.