David Dolenc

Ground-Motion Modeling of the 1906 San Francisco Earthquake, Part I: Validation Using the 1989 Loma Prieta Earthquake

We compute ground motions for the Beroza (1991) and Wald et al. (1991) source models of the 1989 magnitude 6.9 Loma Prieta earthquake using four different wave-propagation codes and recently developed 3D geologic and seismic velocity models. In preparation for modeling the 1906 San Francisco earthquake, we use this well-recorded earthquake to characterize how well our ground-motion simulations reproduce the observed shaking intensities and amplitude and durations of recorded motions throughout the San Francisco Bay Area. All of the simulations generate ground motions consistent with the large-scale spatial variations in shaking associated with rupture directivity and the geologic structure. We attribute the small variations among the synthetics to the minimum shear-wave speed permitted in the simulations and how they accommodate topography. Our long-period simulations, on average, under predict shaking intensities by about one-half modified Mercalli inten- sity (MMI) units (25%-35% in peak velocity), while our broadband simulations, on average, under predict the shaking intensities by one-fourth MMI units (16% in peak velocity). Discrepancies with observations arise due to errors in the source models and geologic structure. The consistency in the synthetic waveforms across the wave- propagation codes for a given source model suggests the uncertainty in the source parameters tends to exceed the uncertainty in the seismic velocity structure. In agree- ment with earlier studies, we find that a source model with slip more evenly distributed northwest and southeast of the hypocenter would be preferable to both the Beroza and Wald source models. Although the new 3D seismic velocity model improves upon previous velocity models, we identify two areas needing improvement. Nevertheless, we find that the seismic velocity model and the wave-propagation codes are suitable for modeling the 1906 earthquake and scenario events in the San Francisco Bay Area. Online Material: Modified Mercalli intensities and velocity waveforms, and a movie of simulated wave propagation.

Identifying and removing noise from the Monterey ocean bottom broadband seismic station (MOBB) data

When compared to quiet land stations, the very broadband Monterey ocean bottom seismic station (MOBB) shows increased long-period background as well as signal-generated noise. Both sources of noise are unavoidable in shallow ocean bottom installations, and postprocessing is required to remove them from seismic observations. The long-period background noise observed for periods longer than 20 s is mainly due to infragravity waves and ocean currents. The shorter-period signal-generated noise, on the other hand, is due to reverberations of the seismic waves in the shallow sedimentary layers as well as in the water layer. We first present the steps that were taken prior to and during the instrument deployment to minimize instrument generated noise as well as to avoid noise due to water flow around the instrument. We then present results from two postprocessing methods that can be used to remove the long-period background noise, which both utilize the ocean bottom pressure signal locally recorded on a differential pressure gauge (DPG). One consists of subtracting the locally recorded ocean bottom pressure from the vertical seismic acceleration signal. In this case the frequency-independent scale factor is linearly estimated from the data. The other one makes use of the transfer function between the vertical seismic and pressure signal to predict the vertical component deformation signal. The predicted signal is then removed from the vertical seismic data in either frequency or time domain. We also present two methods that can be used to remove the signal-generated noise. One employs the empirical transfer function constructed from MOBB data and nearby land station data that do not show the signal-generated noise. The other one uses a synthetic transfer function computed by modeling the response of shallow layers at the MOBB location. Using either of the two transfer functions, most of the signal-generated noise can be removed from the MOBB data by deconvolution.

Microseisms Observations in the Santa Clara Valley, California

We have investigated ground-motion amplification in the Santa Clara Valley (scv) using microseisms observed during the 1998 deployment of 41 short-period seismometers. The Santa Clara Valley Seismic Experiment (scvse) (Lindh et al. , 1999; Fletcher et al. , 2003) recorded many local, regional, and teleseismic events. In our previous work we investigated the 3D velocity structure of the scv by modeling the teleseismic P waves recorded during the scvse(Dolenc et al. , 2005). To complement these results, we now focus on the microseisms that were recorded during the same period and relate these observations to local earthquake wave amplification. It is found that the seismic noise is related to the ocean wave heights measured on the weather buoy west of Half Moon Bay, California. The spectral ratio of the horizontal to vertical (h/v) microseisms at each scvse site is stable with time, and the period of the dominant peak in the h/v ratio is related to the basin depth. The results of this study show that seismic noise can be used in the assessment of the effects of deep sediments on long-period earthquake ground motions.

The MOBB experiment: A prototype permanent off‐shore ocean bottom broadband station

Technical accomplishments of the past 10 years in the design and deployment of sea floor broadband seismic systems are now making it possible to start addressing the issue of the limited coverage of the Earth that can be achieved through land-based installations, at the regional or global scale. In particular, the September 2002 Ocean Mantle Dynamics (OMD) workshop in Snowbird, Utah [OMD Workshop Committee, 2003] proposed the development of two “leap-frogging arrays” of about 30 broadband sea floor instruments to fill geophysically important target holes in ocean coverage for deployment periods of 1 to 2 years. The rationale for an off-shore (“Webfoot”) component of the SArray/Earth-scope “Bigfoot” array was also highlighted at this meeting, pointing out that the study of the North American continent should not stop at the ocean margin. The ocean floor environment is challenging for broadband seismology for several reasons. Broadband seismometers cannot be simply “dropped off” a ship with the expectation that they will produce useable data, particularly on the horizontal components. Several pilot experiments, [e.g., Montagner et al., 1994; OSN1, 1998; Suyehiro et al., 2002] have addressed the issue of optimal installation of ocean bottom stations, and in particular, have carried out comparisons between borehole, sea floor, and buried sea floor installations.

Basin Structure Influences on the Propagation of Teleseismic Waves in the Santa Clara Valley, California

We have investigated ground-motion amplification in the Santa Clara Valley (scv) using teleseismic P waves observed during the 1998 deployment of 41 short-period seismometers. The Santa Clara Valley Seismic Experiment (scvse) (Lindh et al. , 1999; Fletcher et al. , 2003) recorded many local and regional earthquakes and seven large ( M w > 6.4) teleseisms. Measured teleseismic P -wave arrival-time delays, relative P -wave amplification, and P -wave energy were used in the analysis. The relative P -wave amplification is found to correlate strongly with the arrival-time delays. In addition, the P -wave energy is found to correlate with the observed teleseismic delays. We also compared observed P -wave arrival-time delays and P -wave amplification with synthetics computed by using 3D finite-difference simulations of the teleseismic wave field to model these parameters using both the University of California, Berkeley (ucb) (Stidham et al. , 1999; Stidham, 1999) and the U.S. Geological Survey (usgs) (Brocher et al. , 1997; Jachens et al. , 1997) 3D velocity models. The results indicate that arrival-time delays on the order of ±0.25 sec correlate strongly with the reported basin depths in the two models. We find that the correlation between the arrival-time delays and basin depth is strongest for the usgs model. However, the ucb velocity model yields wave amplification that better matches the data. The finite-difference simulations indicate that, in general, the observations may be reproduced by either of the 3D velocity models, although refinements to the proposed 3D structure for the scv are needed.