Four Anomalous Episodes in the Late 2003 Microseism Band Offshore Southern California

Four Anomalous Episodes in the Late 2003 Microseism Band Offshore Southern California Left: Location map of the Scripps OBS test deployment in 2003. Also shown are nearby land seismic stations and the Scripps Pier that collects oceanographic and meteorological data. a), c), d) sonograms for December 2003. b) Wave height measured at Scripps Pier. Most of the microseism signal at the seismometers can be found at twice the frequency of waves measured nearby (double--frequency peak).

The Scripps Ocean Bottom Seismometer (OBS) group has recently test-deployed their new passive seismic instruments. Five Nanometrics Trillium 40 3-component seismometers and Cox-Webb differential pressure gauges were deployed between November 8th, 2003 and January 8th, 2004, operating at a water depth of about 1000 m, 40 km offshore La Jolla, CA. Data were recorded continuously, at a sampling rate of 31.25 Hz.

In the OBS sonograms, we can identify four time periods of high microseismic activity that occurred in December 2003, around Julian days 339, 346, 352 and 360. We can classify these events into two categories. The spectral energy of the events on days 346 and 360 is extended to significantly higher frequencies than the other two events. Zoom-ins of these events exhibit Pierson-Moskowitz dispersion that is characteristic of ocean swell formation under increasing wind speeds. Their signals are much weaker on land station SOL, that is only 2 km from the beach, and on PFO that is 100 km inland. Comparisons with oceanographic (Scripps Pier and buoy records) and meteorological data confirm that these events are associated with North Pacific winter storms that approached the coast further north and then propagated south, generating significant waves within the Southern California Bight.

The two other events distinguish themselves by band-limited energy near the low-frequency end of the double-frequency microseism band. No dispersion is discernible in zoom-ins. Event 339 is clearly visible in the record of seismic station FFC in Canada and station KIP in Hawaii. Inspection of global wave amplitude plots published by the Fleet Numerical Meteorology and Oceanography Center reveals that a large storm approached the North American coast that day. Significant energy must have coupled into the solid earth by non-linear interaction of waves refracted by the coast. This energy then traveled as Rayleigh waves across the Pacific ocean and into the North American continent. The other event is very prominent on the KIP record but not discernible at FFC. The KIP record also exhibits the Pierson-Moskowitz dispersion suggesting that the microseisms were generated near Hawaii. Inspection of oceanographic and meteorological data and the wave amplitude plots draw a complicated picture. A North Pacific storm approached the North American coast but quickly dissipated and never reached the Southern California Bight. A large pressure trough expanded south, causing low atmospheric pressure and significant waves near Hawaii. We conclude that the microseisms were generated near Hawaii.


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