On the consistency of moment tensor source mechanisms with first-motion data

Abstract

Moment tensor source mechanisms for the largest twenty-nine shallow earthquakes from 1981 ( M s ≥ 6.5) have been determined using long-period Rayleigh-wave spectra and P-wave first-motions, recorded by the International Deployment of Accelerograms (IDA), Global Digital Seismic Network (GDSN), and World-Wide Standardised Seismic Network (WWSSN). These are compared with the centroid moment tensor mechanisms presented by Dziewonski and Woodhouse, which were obtained in a contrasting manner. In most cases, the source is well represented by a single double-couple mechanisms, with good agreement of scalar moment and orientation between the two studies. Apparent mismatches for other cases may be interpreted in terms of source complexity. We emphasise the objectivity of comparing moment tensor elements; apparent discrepancies in the moment and orientation of double-couple fault plane solutions are usually due to differences in the M xz and M yz moment tensor elements, which are poorly constrained by long-period data.