Research Accomplishments

Chapter 13 documents the main research contributions of the past year. Research at the BSL spans a broad range of topics, from the study of microseismicity at the local scale to global deep earth structure, and includes the use of seismological, geodetic, and remote sensing (InSAR) techniques.

I wish to highlight several prominent research results of the past year. First, the investmed effort in the HRSN network is continuing to pay off. Nadeau and collaborators (see research report 1.) have discovered the presence of non-volcanic tremor activity on the San Andreas Fault at Parkfield. Previously, such tremors had been only observed in subduction zones, and an important role had been attributed to fluids in their generation. The new discovery, reported in Science, presents a challenge to this simple interpretation.

Second, at the other end of the spectrum of BSL research, the report by Cao et al.(see research report 22.) describes a convincing observation of the PKJKP wave, a seismic wave that travels through the inner core as a shear wave. Only three detections of this wave, whose existence is predicted from the solidity of the inner core, had previously been claimed, with controversial observations. In contrast, Aimin's observation, published in Science and based on data from the Gräffenberg array in Germany, is much clearer.

The 09/28/04 Parkfield earthquake resulted in several studies: a kinematic model of the earthquake, combining seismic and geodetic data (see research study 2.), comparison of the 2004 earthquake with previous repeating Parkfield earthquakes of 1922, 1934 and 1966 (see research report 3.), as well as studies of the earthquake rupture and afterslip using InSAR and GPS data (see research reports 4. and 5.). An intriguing swarm of small earthquakes near Orinda, CA, has provided a unique dataset for the study of earthquake scaling behavior (see research report 6.).

Studies devoted to the source and effects of earthquakes also include investigation of the generation mechanism of deep earthquakes (research report 9.), ground motion simulation of structures (research report 8.), and the development and testing of an early warning system (research report 10.). Dr Uhrhammer continues to lead the analysis and preservation of historical data (research report 11. and 12.).

BSL researchers have used microseismic noise data to study detailed structure of the Santa Clara Valley (research report 7.), have investigated further the oceanic origin of the low frequency ``hum" , and its relation to ocean storms (research reports 13., 14.). In collaboration with Peter Bromirski of UC San Diego, selected analog seismograms of the Berkeley historical collection have been scanned to study the relation of microseisms and wave climate, with important applications for climate studies (research report 12.).

We note an increasing interest in the study of structure and processes in volcanic areas: imaging the Newberry hotspot track (research report 15.), investigating fluid-influenced faulting in Long Valley (research report 16.), and fluid motion in Galeras Volcano, Colombia (research report 17.).

Geodetic studies also include the estimation of strong ground motions in real time from geodetic data (research report 20.), and the study of crustal deformation along the northern San Andreas Fault system (research report 13.38), and the tectonics of northeast Asia (research report 18.).

BSL researchers image earth structure at a variety of scales, investigating various theoretical improvements to the forward and inverse problems: 1) regional, in the Middle East (research report 21.) and in north America (research report 23.), in southeast Asia (research report 24.), in the deep mantle under the Pacific and African ``superplumes" (research report 25.), and 2) global: this year, an on-going study attempts direct inversion of seismic waveforms for lateral variations in temperature and composition (research report 26.) .

And finally, an investigation of physically consistent models of Europa is the first explicit study of planetary structures at BSL (research report 27.).

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