TITLE:
Intermediate-Term Seismic Precursors to the Loma Prieta California Earthquake of 1989
AUTHORS:
Lynn R. Sykes, Felix Waldhauser, David P. Schaff
KEYWORDS:
Earthquakes, Precursors, Prediction, California, Faults, Plate Tectonics
JOURNAL NAME:
International Journal of Geosciences,
Vol.13 No.10,
October
24,
2022
ABSTRACT: We use precise locations of earthquakes to study
forerunning seismic activity to the 1989 Loma Prieta earthquake of magnitude
6.9 to the south of San Francisco, California, USA. Relocated shocks of
magnitude 4.3 to 5.4 and smaller micro-earthquakes define a distinct zone of
nearly the same orientation as the mainshock. That separate zone broke in the
15 months prior to the 1989 mainshock. That feature, which we call the Lake
Elsman fault zone, is identified as the site
of a prominent intermediate-term (yearly) precursor very close to the
coming 1989 mainshock. That zone experienced a relatively large stress decrease during the nearby great
earthquake of 1906. From the occurrence of the Lake Elsman shocks, we
deduce that stress drop was only restored in the 15 months prior to the 1989
main event. Those stresses are consistent
with little forerunning seismic activity in the region after 1906, later
increases just before the 1989 mainshock and a decrease in activity thereafter. The southern Santa Cruz mountains segment
of the San Andreas Fault zone, the location of the 1989 mainshock, had
not been the site of events of magnitude 5 and larger for many decades prior to
the occurrence of Lake Elsman earthquakes of magnitude 5.3 and 5.4 in 1988 and
1989. High-preci- sion locations readily available in real-time might be used to monitor
similar possible precursory activity very close to the San Andreas and other
transform faults.