Director's Report

As I was working on the introduction to this Annual Report, in the evening of October 30th, 2007, the San Francisco Bay Area - including my desk at home - was shaken by the strongest temblor since the $M$ 6.9 1989 Loma Prieta earthquake. It felt exactly as John Mitchell described in his impressions from the great Lisbon earthquake of 1755: ``a tremulous vibration followed by a wavelike undulation", but was generally a mild experience. Promptly, our real-time earthquake notification system indicated that it had occurred east of Alum Rock in the South Bay, and that its moment magnitude was $M_{w}$ 5.4. The shaking was felt widely, from Carson City in Nevada and Yosemite in the East, to Thousand Oaks near Los Angeles in the South and to Eureka in the North. Fortunately, nobody was hurt in this quake and the damage was very minor, surprisingly so. Owing to our real time estimation of rupture directivity, it quickly became apparent that the rupture propagated to the south-west, concentrating the strongest shaking in a sparsely populated area. This was in contrast to what was experienced last July, when an earthquake smaller by an order of magnitude, the $M_{w}$ 4.2, July 20th, 2007 Oakland earthquake, sent objects flying off shelves in Berkeley and San Francisco. The latter earthquake occurred in the heart of the urban area, and the rupture propagated to the north. These recent examples illustrate the complexity of earthquake ruptures, and how important it is to provide rapid and accurate information not only on the hypocentral location and the magnitude, but also on rupture directivity, which can significantly influence the distribution of the shaking. The near surface three-dimensional geometry of basins and the basement topography also play a role in amplifying or reducing local shaking, and the larger the earthquake, the more important this becomes. The ability to provide this kind of information in quasi-real time is the result of efforts that have been pursued at BSL for the last 15 years, progressively translating research developments into the operational environment. The Alum Rock earthquake was a perfect "dry run" test for our northern California real time system, operated jointly with the US Geological Survey at Menlo Park, and helped identify a few small issues with telemetry and hard-wired parameters in a real life, yet benign situation.

This earthquake also served to assess the performance of the Earthquake Early Warning system currently being developed (see section 2.17.), which successfully detected the earthquake and determined its magnitude to within 0.3 magnitude units.

During the night and days following the earthquake, members of our staff also spent considerable time providing information to the media - another regular activity which sets the BSL apart from most other Organized Research Units on the Berkeley Campus.


Berkeley Seismological Laboratory
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