Moment tensor rate functions for the 1989 Loma Prieta Earthquake

The fault plane and overall coseismic slip of the 1989 Loma Prieta, California, earthquake (M s =7.1) are well determined [Plafker and Galloway, 1989]. Teleseismic waves can be used to determine the time history of moment release. We invert a data set of ten broadband P and SH waves for the most general point source description: the five moment tensor rate functions. The linear inversion also provides formal estimates of model uncertainty. While the moment tensor rate functions suggest a different focal mechanism for the first few seconds of the rupture process, it is not statistically significant at the 95% level. We can thus proceed to invert for one single time function (the source time function), and five scalars (the moment tensor). The major double couple that we find (strike 138°±6° clockwise from North, dip 76°±5°, slip angle 120°±10°) agrees with the results of Plafker and Galloway [1989]. The minor double couple is small (1%). The best point source depth is about 10 km. Several broadband P waves show that a small precursor occurred a few seconds before the main pulse of moment release. The duration of the source time function is 9 s, with a moment of 2±0.5 ×10 19 Nm. A bilateral rupture with velocity of 2.8 km/s and 9 s duration encompasses the aftershock region.