Determination of earthquake source parameters from waveform data for studies of global and regional seismicity

Abstract

It is possible to use the waveform data not only to derive the source mechanism of an earthquake but also to establish the hypocentral coordinates of the ‘best point source’ (the centroid of the stress glut density) at a given frequency. Thus two classical problems of seismology are combined into a single procedure. Given an estimate of the origin time, epicentral coordinates and depth, an initial moment tensor is derived using one of the variations of the method described in detail by Gilbert and Dziewonski (1975). This set of parameters represents the starting values for an iterative procedure in which perturbations to the elements of the moment tensor are found simultaneously with changes in the hypocentral parameters. In general, the method is stable, and convergence rapid. Although the approach is a general one, we present it here in the context of the analysis of long‐period body wave data recorded by the instruments of the SRO and ASRO digital network. It appears that the upper magnitude limit of earthquakes that can be processed using this particular approach is between 7.5 and 8.0; the lower limit is, at this time, approximately 5.5, but it could be extended by broadening the passband of the analysis to include energy with periods shorter that 45 s. As there are hundreds of earthquakes each year with magnitudes exceeding 5.5, the seismic source mechanism can now be studied in detail not only for major events but also, for example, for aftershock series. We have investigated the foreshock and several aftershocks of the Sumba earthquake of August 19, 1977; the results show temporal variation of the stress regime in the fault area of the main shock. An area some 150 km to the northwest of the epicenter of the main event became seismically active 49 days later. The sense of the strike‐slip mechanism of these events is consistent with the relaxation of the compressive stress in the plate north of the Java trench. Another geophysically interesting result of our analysis is that for 5 out of 11 earthquakes of intermediate and great depth the intermediate principal value of the moment tensor is significant, while for the remaining 6 it is essentially zero, which means that their mechanisms are consistent with a simple double‐couple representation. There is clear distinction between these two groups of earthquakes.

Toward automated directivity estimates in earthquake moment tensor inversion

“…Such stretching can be determined according to a predefined directivity vector with respect to ray takeoff angle to each station. We can then grid search the directivity vector in 3-D space with different stretched source time functions to each station rather than a common source time function to all stations as is typical in moment tensor inversions (Dziewonski et al 1981;Kanamori & Rivera 2008). In this manner, how properly the directivity effect is incorporated into the synthetic waveforms (convolution of Green's functions and stretched source time functions) relies on how precisely the RSTF is calculated.…”

Seismotectonics and rupture process of theMW 7.1 2011 Van reverse-faulting earthquake, eastern Turkey, and implications for hazard in regions of distributed shortening

“…BZSZ marks the Bitlis-Zagros Suture zone between Arabia and Eurasia. Modern earthquake focal mechanisms shown for magnitude > 5 from the Global Centroid Moment Tensor catalogue (Dziewonski et al 1981;Ekström et al 2012). Thrust mechanisms are shown in red, normal in blue and strike-slip in grey.…”

Testing stress shadowing effects at the South American subduction zone

“…Some of these uncertainties have been considered in previous studies. For instance, Rong et al (2003) used three different catalogues for the period of their analysis (1978 June 1 to 1998 December 31) to take into account the magnitude uncertainty, containing events (i) M s ≥ 7.0 from PDE (National Earthquake Information Center 1999), (ii) M other > =7.0 from other stations listed in the PDE catalogue, (iii) M w calculated from the seismic moment given in the CMT catalogue (Dziewoński et al 1981;Ekström et al 2012).…”