PRECURSORY SEISMICITY CHANGES PRECEDING MODERATE AND LARGE EARTHQUAKES IN THE GARM REGION, CENTRAL-ASIA

This study reports on variations in seismic activity preceding moderat e and large earthquakes in a highly active portion of the India/Eurasi a collision zone. The study is based on the catalog of earthquake loca tions from the Complex Seismic Expedition (CSE) network in the Garm re gion, Central Asia. The local seismic network was established in the m id-1950s and recorded and located over 85,000 earthquakes from 1955 to 1988. Two main approaches were used to evaluate variations in seismic activity associated with 12 moderate- to large-magnitude (M(L) = 5.0 to 6.3) mainshocks that occurred in the study area: (1) a qualitative approach, involving examination of maps, cross sections, spacetime plo ts, and earthquake histograms for the area identified as the rupture a rea of the mainshock and (2) a quantitative approach based on magnitud e-frequency relations estimated for small space-time windows within th e seismic network. In addition to temporal variations in seismicity, t hree long-term gaps were identified: (1) in the western Peter the Firs t Range, near the city of Garm; (2) in the central Peter the First Ran ge; and (3) in the northeastern part of the study area, approximately coinciding with a gap identified by other authors using different appr oaches. The potential magnitude of future gap-filling events is estima ted to be in the range 6.7 to 7.0. Evidence of precursors has been fou nd to varying extent for 9 out of the 12 mainshocks studied. These inc lude quiescence or low activity before the mainshock, foreshock activi ty before the mainshock, and a sequence of low-activity-high-activity- quiescence before the mainshock. For the quantitative analysis, the le vel of seismic activity in small space-time windows is quantified by t he parameter A(2), defined as the A value at magnitude 2 in the Gutenb erg-Richter magnitude-frequency relation log N = A - bM. The A(2) valu es were calculated for 30 12 x 12-minute spatial cells with a moving t ime window of five years. From this analysis, only one earthquake, the M = 6.3 Dzhirgital earthquake of 26 October 1984, shows a significant decrease (by a factor of 45%) in activity level A(2). The results of the quantitative analysis also show strong spatial variations: the cen tral portion of the study area, which includes the Peter the First Ran ge fold-thrust belt, shows high, but relatively stable, seismic activi ty; the Tien Shan basement uplift in the north has a tendency of decre asing activity; and the northeastern part of the study area shows incr easing activity. The October 1984 earthquake is located near the trans ition between the two tendencies.