Romanowicz, B.

B: Unraveling the physical state of the upper mantle, including the transition zone, is one of the key factors for understanding the Earth's mantle dynamics. Knowledge of mantle temperature and composition is mainly based on the interpretation of seismological observations based on insights from mineral physics. Despite the progress made to image the 3-D seismic structure of the upper mantle, its interpretation in terms of physical parameters is still challenging and it requires a truly interdisciplinary approach. Due to the better knowledge of the elastic and anelastic properties of mantle minerals at high temperatures and pressures, such an approach is now becoming feasible. We propose a new waveform inversion procedure, based on a formalism previously developed at Berkeley for global elastic and anelastic tomography, and using our existing collection of long-period fundamental and higher mode surface waveforms. Here, we incorporate mineral physics data at an early stage of the process to directly map lateral variations in temperature and composition, using recent estimates of the temperature and composition derivatives of seismic velocities (∂lnV/∂lnT,C). Anelasticity introduces a non-linear dependence of the seismic velocities with temperature throughout the upper mantle, and phase-transitions confer a non-linear character to the compositional derivatives as well, therefore the kernels should be re-computed after each iteration of the inversion. We discuss ways to address the non-linearities, as well as uncertainties in the partial derivatives. In addition to constraining the lateral variations in temperature or composition, the models can have implications on the average structure of the upper mantle. The most-common accepted physical 1-D structure had problems to satisfactorily fit seismic travel time data, requiring a slower TZ to improve the fit. However, these data do not have sufficient coverage (and resolution) in the TZ. A complementary outcome of our models will be to shed light on whether the seismic data require a modification of the physical structure in the transition zone and if the three-dimensional heterogeneity introduces a significant shift of the average physical structure away from adiabatic pyrolite.

We report on the installation of a long-term buried ocean-floor broadband seismic station (MOBB) in Monterey Bay, California (USA), 40km off-shore, at a water depth of 1000 m. The station was installed in April 2002 using a ship and ROV, in a collaborative effort between the Monterey Bay Aquarium Research Institute (MBARI) and the Berkeley Seismological Laboratory (BSL). The station is located on the western side of the San Gregorio Fault, a major fault in the San Andreas plate boundary fault system. In addition to a 3-component CMG-1T seismometer package, the station comprises a current meter and Differential Pressure Gauge, both sampled at high-enough frequency (1 Hz) to allow the study of relations between background noise on the seismometers and ocean waves and currents. The proximity of several land-based broadband seismic stations of the Berkeley Digital Seismic Network allows insightful comparisons of land/ocean background seismic noise at periods relevant to regional and teleseismic studies. The station is currently autonomous. Recording and battery packages are exchanged every 3 months during scheduled one day dives. Ultimately, this station will be linked to shore using continuous telemetry (cable and/or buoy) and will contribute to the earthquake notification system in Northern California. We present examples of earthquake and noise data recorded during the first 6 months of operation of MOBB. Lessons learned from these and continued recordings will help understand the nature and character of background noise in regional off-shore environments and provide a reference for the installation of future off-shore temporary and permanent broadband seismic stations.