Js. Haase et al., CONSTRAINTS ON TEMPORAL VARIATIONS IN VELOCITY NEAR ANZA, CALIFORNIA,FROM ANALYSIS OF SIMILAR EVENT PAIRS, Bulletin of the Seismological Society of America, 85(1), 1995, pp. 194-206
Similar earthquake pairs recorded by the Anza Seismic Network in south
ern California are used as repeatable sources to place an upper limit
on temporal changes in seismic velocity which occurred in the vicinity
of the Anza seismic gap in the last 9 yr. Relative arrival times for
each pair of events are found using a cross-correlation method and rel
ative locations are calculated to verify that the pairs have nearly id
entical hypocenters. The time separation between events in these pairs
varies from less than a day to almost 7 yr. The longterm changes in s
eismic travel times, as measured from the pairs with the longest time
separation, are not significantly greater than the noise level estimat
ed from the short-time-separation event pairs. Almost all P-wave paths
show less than 0.06% (0.007 sec) change in travel time and all S-wave
paths have less than 0.03% (0.004 sec) change. Sensitivity tests plac
e an upper bound on traveltime changes that could be compensated by hy
pocenter mislocation at 0.2%. There is no evidence that localized stre
ss accumulation causes measurable changes in seismic velocity in the A
nza region.