Seismic domains within the Gissar‐Kokshal Seismic Zone, soviet central Asia

Abstract

By evaluating seismological data from the belt of shallow intraplate seismicity known as the Gissar‐Kokshal seismic zone in Soviet Central Asia in conjunction with recent data on the shallow crustal structure and longer‐term geologic deformation, we show that certain size‐classes of earthquakes are characteristic of adjacent (and in some places overlapping) geologic/tectonic settings; we call these settings seismic domains. They are both geologically and seismologically distinct; i.e., they contain different rocks and structures and have different frequency‐magnitude relationships. For example, in terms of number of events, Gissar‐Kokshal seismicity is dominated by the microearthquakes that have occurred within the sedimentary rocks of the Vakhsh fold/thrust belt. These are all shallow and have occurred mostly within the frontal portions of the thrust sheets. More rarely, moderate‐size (M < 5.5) earthquakes have ruptured the thrust ramps. On the other hand, the major strain release in this region has accompanied the large (5.5 < M < 8.1) shallow earthquakes on basement faults in the southern Gissar range adjacent to the fold/thrust belt. Within these ranges there is relatively little microearthquake activity. That dichotomy indicates that a significantly different seismic hazard is associated with each of these areas, but the seismological distinction is obscured along the front of the fold/thrust belt, where the domain of small shocks in the sedimentary rocks overlaps the domain of large earthquakes on basement faults. To explain the localization of the large basement events along the southern flank of the fold/thrust belt adjacent to the fold/thrust belt, we hypothesize a mechanism of thrust sheet loading. Thrust loads are greatest in this region, since thrust sheets have overridden large basement block uplifts beneath the Illiac and Peter‐the‐First range faults.