Research Accomplishments

Chapter 2 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. Productivity continues to be high: forty three papers in refereed journals, including two in Nature, have been published by BSL researchers in the last year, originating from 4 faculty members and their students, and one senior researcher.

The analysis of borehole microseismic data from the HRSN (Parkfield) network is continuing to provide exciting results. A highlight of this past year's research has been a study by Doug Dreger and collaborators (2.1.), in which they have reconciled the difference in reported stress drop estimates for small earthquakes, by applying Doug's finite source inversion method to microseism borehole observations. Bob Nadeau and collaborators (2.3., 2.6.) continue to analyze repeating earthquakes and tremor activity near Parkfield, finding intriguing patterns of changes in strain release associated with the M6 2004 Parkfield earthquake, and in particular a correlation between deep tremor activity and microseismicity in seismogenic zone above. On the other hand, Karl Kappler and collaborators (2.18.) show no clear precursory signal in electromagnetic data preceding or during the Parkfield earthquake.

Work on non-volcanic tremor in subduction zones has led Richard Allen to propose a relationship betweeen the recurrence period of these tremors and properties of the overriding plate, in particular topography (2.4.).

Richard Allen and his students have also continued to develop a methodology for earthquake early warning (2.17.), testing it in the framework of our real-time system. While the debate continues as to whether an earthquake knows when it starts how big it will become, an important by-product of this development is the ability to significantly speed-up the production of reliable "shake maps".

Doug Dreger and his students and collaborators have also worked on a variety of regional source and structure topics. In particular, they have finalized a methodology using moment tensor inversion to characterize different seismic sources, and in particular distinguish those with a significant non-double couple component, such as would be the case for nuclear tests and mine collapses (2.12.). In particular, this methodology has proven very useful in assessing the nature of the Utah mine collapse of Aug 6, 2007 (e.g. Concurrently, they have been testing crustal and basin models in the San Francisco Bay Area, and in particular the Santa Clara valley, using different approaches: microseismic noise (2.8.), 3D broadband waveform modeling using finite differences (2.9.), and inversion of teleseismic observations (2.10.). Finally, various approaches to determining the Lg attenuation structure in the San Francisco Bay Area have been compared, towards the determination of a robust model for this region (2.7.).

A number of studies focus on the seismicity and deformation in northern California. Peggy Hellweg 2.14. describes the unusual microearthquake sequence of October 2003 near Orinda, CA, while a recently begun study aims at characterizing moment tensors and spectra of slow events in the Mendocino region (2.16.). Using InSar data, Roland Bürgmann and his group have been characterizing creep along the Rodgers Creek Fault (2.19.) and the San Andreas Fault (2.21.). Earthquake and ground shaking potential has been analyzed in different fashions. Using continuous GPS data accumulated over the last 10-15 years on the BARD network, Nicolas Houlié and I (2.20.) point out to the consequences of asymmetric rheology across the Bay Area faults. The performance capabilities of the northern California seismic networks have been assessed through simulations of a repeat of the 1906 earthquake (2.15.). A recently begun study is searching for evidence of accelerating seismic moment release in northern and southern California by modelling the evolution of stress and seismicity in the region (2.2.). A new method based on coda spectral amplitude ratios shows promise in distinguishing earthquake sources, with consequences on earthquake scaling (2.5.). Finally, magnitude accuracy within CISN will soon be improved with the implementation of a California-wide consistent set of local magnitude ($M_L$) station corrections (2.13.).

Moving away from California, continuous and campaign style GPS data have been used to constrain the motion and deformation of the Indian Plate (2.22.), while Nicolas Houlié and Jean-Paul Montagner (IPG paris, France) 2.23. propose a method to track the long term deformation response of a volcano to changes in pressure inside the magma chamber.

BSL researchers have also contributed to larger scale regional and global structure studies. Mei Xue and Richard Allen 2.24. used teleseismic body wave travel time tomography to investigate the fate of the subducted Juan de Fuca plate and its interaction with the Yellowstone plume head. Barbara Romanowicz and her group have been investigating new approaches to waveform tomography. With post-doc Aimin Cao, we have been testing a promising method combining the advantages of the path average approximation and those of 3D Born scattering on the case of radially anisotropic upper mantle structure in southeast Asia (2.25.). We are making progress towards a global upper mantle model using the spectral element method in the forward computation of 3D synthetics (2.28.). We have further documented the presence of strong lateral variations at the base of the mantle using diffracted waves (2.29.). Using an array stacking method, Aimin Cao has been able to localize scatterers most likely associated with remnant slabs in the lowermost mantle under North America (2.30.). With post-doc Fabio Cammarano, we have been working towards understanding the respective contributions of variations in temperature and composition in the upper mantle using seismic waveforms and mineral physics data as constraints (2.27., 2.26.). Finally, we continue to study the earth's inner core, and, this year, have assembled a high quality dataset of near antipodal PKP travel time data to test for the existence of an innermost inner core (2.31.).

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