Bimaterial effects on earthquake source mechanics

Abstract

A material contrast exists on the San Andreas fault, with the Pacific plate to the SW more rigid than the North American plate to the NE. This thesis explores the effect of this material contrast on earthquake source mechanics. Two manifestations of the bimaterial effects in seismic observations are examined: 1. The existence of a systematic rupture propagation direction, and 2. Possible asymmetry in the distribution of immediate aftershocks.

In the first study, rupture directivity is quantified using the rupture lengths in the NW and SE directions along the San Andreas fault, and the rupture lengths are determined using the ratios of the spectra between nearby earthquakes. The directivity of nearly 900 events in a 3000 event catalog are reasonably well resolved. ∼40% of these events are roughly bilateral, although more than ∼80% of the 144 events classified as strongly unilateral rupture to the SE. The well-resolved rupture speed to the SE is also greater by roughly 10%. The preferential propagation to the SE is consistent with theoretical predictions. We also found that events with nearby foreshocks within several hours tend to rupture away from those foreshocks, indicating that asymmetry of prior stressing history can exert a stronger influence on rupture directivity than the material contrast.

To study the possible asymmetry in the distribution of immediate aftershocks, we searched for compound earthquakes which consist of two subevents separated by less than 0.3 second. About 450 candidate compound events were identified on the northern San Andreas and part of the Calaveras fault. Most delays between the two subevents cluster around the shear-wave transit time over the subevent separation. For subevents on the San Andreas fault which are separated by 0.7 to 2 times the estimated radius of the first subevent, more than twice as many second subevents occurred to the SE of the first as to the NW. The asymmetry of second subevent distribution is not present on the Calaveras fault, which has no significant material contrast in this region. One interpretation is that the extra SE subevents on the San Andreas fault are representative of the events "missing" from the "longer term" (10s-9hrs) aftershock population because they became part of the mainshock.