Source process of complex earthquakes by time-dependent moment tensor analysis

Abstract

Seismic source complexity, which may be due to fault geometry changes or discrete subevents, is difficult to quantify with conventional moment tensor inversion techniques. The time-dependent moment tensor (TDMT) inversion procedure is an attempt to extract a time varying source from teleseismic P-waves. The correctness and the limits of the procedure are tested by synthetic examples. To remove the constant moment tensor assumption of the conventional time-independent moment tensor (TIMT) approach, a higher degree of freedom of the linear inversion system is required. The inversion is performed over a range of depths, and the solution is decomposed into subevents with varying depths and source geometry by the temporal consistency of the individual moment tensor elements and the condition of causality. Analysis of the synthetic examples indicates that the inverted moment tensor is sensitive to the crustal structure, particularly the crustal thickness. The inverted focal mechanism and the shape of the moment tensor elements are stable with respect to the uncertainties of the epicenter locations. Source processes of three recent complex earthquakes were investigated by the TDMT analysis. The results show that the October 18, 1992 Colombia earthquake is a double event with similar focal mechanisms. The first subevent is a smaller initial phase about 8 sec long. The second subevent initiated right after the initial phase. A total of five subevents are recognized for the July 30, 1995 Northern Chile earthquake. Four subevents ruptured in the first 34 sec with similar dip-slip focal mechanisms while the fifth subevent has a strike-slip focal mechanism. The Solomon Islands earthquake on August 16, 1995 is dominated by two shallowly dipping, dip-slip subevents with about the same moment release. Both TIMT and TDMT techniques and the GIS (Geographic Information Systems) are combined to study the tectonics of the Northwestern Colombia region. The focal solutions of the strongest events in this area are recovered and the geological environment is mapped by GIS. The 1992 sequence likely ruptured the Murindo fault system. The prevailing focal mechanism in this area suggests that a northwest-southeast compressional stress regime has dominated in the past 20 years.