SCEC-USGS Workshop on LARSE II

On April 21 over 60 scientists attended a seminar at UCLA, sponsored by SCEC and the USGS, to review the current state of knowledge on the deep structure of the Los Angeles Region, how previous phases of LARSE (L.A. Region Seismic Experiment) have contributed to this knowledge, and how the next phase ("LARSE II") will contibute more data for study.

LARSE II will occupy a secondary line of LARSE I extending from Malibu to Lancaster with an active source seismic imaging experiment, in Fall 1999. Over 60 small explosions will be detonated along the 100 km profile, and will be recorded by over 1000 seismometers. 150 of the seismometers will be located on two lines which will extend off the line into Santa Monica. In addition, a high resolution experiment will be run in Santa Monica using a vibroseis truck as the active source rather than explosions.

The morning focused on the Santa Monica area, which received higher than expected damage during the Northridge earthquake, compared to other areas at the same distance from the epicenter. Several theories were presented as to the cause of the damage, including a possible deep structure related to the southern Santa Monica fault which acted as a lens to focus seismic energy in the damaged region. Other studies were presented that argued that the strong shaking could have resulted by focusing much nearer to the surface, related to the northern Santa Monica fault. The Santa Monica portion of LARSE II is designed to resolve the controversy. It is expected that results of the Santa Monica study will be applicable to other areas where seismic energy can be focused to cause unusual damage.

The afternoon was devoted to the broader goals of LARSE II, and results-to-date from LARSE I. As in the morning session, several overview presentations were given which detailed the evolution, structure, and seismicity of the area to be studied. LARSE II is very similar in scope to LARSE I, which was conceived to correct a deficit in our knowledge of seismogenic structures at depth. The objective is to have better understanding of crustal structure, specifically basins, upper mantle anomalies, and to contribute to the unified velocity model of southern California.

A summary of the meeting along with the agenda/speakers list is available at www.scec.org/news/98news/larse.html. Further information regarding the LARSE experiment, including details of the methods involved, is online at www.data.scec.org/larse.html.