HR: 1340h
AN: OS43B-0544    [Abstracts]
TI: Monitoring Local and Teleseismic Earthquakes Off--Shore San Diego(California) During an OBSIP Test Deployment
AU: * Laske, G
EM: glaske@ucsd.edu
AF: IGPP, Scripps Institution of Oceanography, UCSD, La Jolla, ca 92093-0225 United States
AU: Babcock, J
AF: IGPP, Scripps Institution of Oceanography, UCSD, La Jolla, ca 92093-0225 United States
AU: Hollinshead, C
AF: IGPP, Scripps Institution of Oceanography, UCSD, La Jolla, ca 92093-0225 United States
AU: Georgieff, P
AF: IGPP, Scripps Institution of Oceanography, UCSD, La Jolla, ca 92093-0225 United States
AU: Allmann, B
AF: IGPP, Scripps Institution of Oceanography, UCSD, La Jolla, ca 92093-0225 United States
AU: Orcutt, J
AF: IGPP, Scripps Institution of Oceanography, UCSD, La Jolla, ca 92093-0225 United States
AB: The Scripps OBS (Ocean Bottom Seismometer) team is one of three groups that provide instrumentation for the US National OBS Instrument Pool (OBSIP). The compact active source LC2000 instruments are being used successfully in numerous experiments, with excellent data quality and return rates. A set of five new passive seismic instruments was test--deployed from November 6th, 2003 through January 8th, 2004 in the San Diego Trough, about 1km below the sea surface, about 40km off--shore San Diego, California. These instruments are equipped with a Nanometrics Trillium 40s 3--component seismometer and a Cox--Webb differential pressure gauge. We recorded more than 30 teleseismic earthquakes suitable for a long-period surface wave study. The vertical--component seismometer recordings are of excellent quality and are often superior to those from similar sensors on land (Guralp CMG-40T). The signal--to--noise ratio on the DPGs depend strongly on the water depth and was expected to be low for the test deployment. Nevertheless, the December 22, 2003 San Simeon/ California earthquake was recorded with high fidelity and non--seismogenic signals are extremely coherent down to very long periods. We also recorded numerous local earthquakes. Many of these occurred off-shore and the OBSs were the closest stations by many tens of kilometers. For example, a magnitude 3.0 earthquake on the Coronado Banks Fault was recorded at station SOL in La Jolla at about 30km distance, with a signal-to-noise ratio too poor to pick the first arrival. The next closest stations were 60km and 80km away, while one of the OBSs was only 20km away. The co-deployment of DPGs allowed us to observe the first P arrival very clearly. We also recorded numerous events that were not recorded on land. About six months later, on June 15, 2004 the greater San Diego area was struck by a magnitude 5.2 earthquake on the San Clemente Fault, about 40km southwest of the OBS test deployment. Though no structural damage was reported, intensity 4 shaking occurred throughout the city, which prompted Amtrak and Sea World to shut down operations for inspections. These events are continous reminders that significant seismic hazard is caused by activity along the only poorly understood, off-shore faults in the California Borderland. Realtime seismic monitoring using cabled or moored seismic observatories is clearly needed.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 7255 Surface waves and free oscillations
DE: 7294 Instruments and techniques
DE: 3094 Instruments and techniques
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting