thumbnail

A case study of two M~5 mainshocks in Anza, California: Is the footprint of an aftershock sequence larger than we think?

Bulletin of the Seismological Society of America

By:
and
DOI: 10.1785/0120080268

Links

Abstract

It has been traditionally held that aftershocks occur within one to two fault lengths of the mainshock. Here we demonstrate that this perception has been shaped by the sensitivity of seismic networks. The 31 October 2001 Mw 5.0 and 12 June 2005 Mw 5.2 Anza mainshocks in southern California occurred in the middle of the densely instrumented ANZA seismic network and thus were unusually well recorded. For the June 2005 event, aftershocks as small as M 0.0 could be observed stretching for at least 50 km along the San Jacinto fault even though the mainshock fault was only ∼4.5 km long. It was hypothesized that an observed aseismic slipping patch produced a spatially extended aftershock-triggering source, presumably slowing the decay of aftershock density with distance and leading to a broader aftershock zone. We find, however, the decay of aftershock density with distance for both Anza sequences to be similar to that observed elsewhere in California. This indicates there is no need for an additional triggering mechanism and suggests that given widespread dense instrumentation, aftershock sequences would routinely have footprints much larger than currently expected. Despite the large 2005 aftershock zone, we find that the probability that the 2005 Anza mainshock triggered the M 4.9 Yucaipa mainshock, which occurred 4.2 days later and 72 km away, to be only 14%±1%. This probability is a strong function of the time delay; had the earthquakes been separated by only an hour, the probability of triggering would have been 89%.

Study Area

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
A case study of two M~5 mainshocks in Anza, California: Is the footprint of an aftershock sequence larger than we think?
Series title:
Bulletin of the Seismological Society of America
DOI:
10.1785/0120080268
Volume
99
Issue:
5
Year Published:
2009
Language:
English
Publisher:
Seismological Society of America
Description:
15 p.
First page:
2721
Last page:
2735
Country:
United States
State:
California
City:
Anza
Online Only (Y/N):
N
Additional Online Files(Y/N):
N