Geology topics

Robert E. Westerlund

Publications and source records attributed to Robert E. Westerlund.

3 recordsLinked to original sources

USGS2012

Tidal variation of seismic travel times in a Massachusetts granite quarry

Conflicting results on tidal variation of seismic travel times exist in the literature. With improved methods, we have conducted a seismic survey at a Massachusetts granite quarry. The survey was conducted in the intervals (230d 23h, 231d11h) and (231d22h, 233d10h), 1983 (U.T.) along a 148 m baseline situated in nearly flat topography. The source for the present experiment was an air gun placed in a mud-filled pit. Travel times for the first five body wave extrema were analyzed. The results are: I. The 1st, 2nd, 4th, and 5th extrema show travel time variations; the 3rd extremum remains constant throughout the experiment. The magnitude of the fractional travel time variation, Δt/t, ranges from 0.5 to 0.9%. II. Changes in travel times of the 1st and 2nd extrema correspond to opposite changes in travel times of the 4th and 5th extrema. III. Two sets of nearly orthogonal joint systems are observed in the granite; the theoretical tidal strain in the direction perpendicular to the nearly vertical joints matches the travel time variations of the 1st and 2nd extrema whereas the tidal strain in the direction perpendicular to the nearly horizontal sheets matches the travel time variations of the 4th and 5th extrema, when a 4-hr delay is introduced for all the tidal strains. These results are interpreted in terms of the velocity changes of seismic rays as the two joint systems open and close due to the tidal stress.

Geophysical Research Letters↗

USGS1994

Seismic velocities and geologic logs from boreholes at three downhole arrays in San Francisco, California

The Loma Prieta earthquake of October 17, 1989 (1704 PST), has reinforced observations made by Wood and others (1908) after the 1906 San Francisco earthquake, that poor ground conditions (soft soil) increase the likelihood of shaking damage to structures. Since 1908 many studies (for example Borcherdt, 1970, Borcherdt and Gibbs, 1976, Borcherdt and Glassmoyer, 1992) have shown that soft soils amplify seismic waves at frequencies that can be damaging to structures. Damage in the City of San Francisco from the Loma Prieta earthquake was concentrated in the Marina District, the Embarcadero, and the China Basin areas. Each of these areas, to some degree, is underlain by soft soil deposits. These concentrations of damage raise important questions regarding the amplification effects of such deposits at damaging levels of motion. Unfortunately, no strong-motion recordings were obtained in these areas during the Loma Prieta earthquake and only a limited number (< 10) have been obtained on other soft soil sites in the United States. Consequently, important questions exist regarding the response of such deposits during damaging earthquakes, especially questions regarding the nonlinear soil response. Towards developing a data set to address these important questions, borehole strong-motion arrays have been installed at three locations. These arrays consist of groups of wide-dynamic-range pore-pressure transducers and three-component accelerometers, the outputs of which are recorded digitally. The arrays are designed to provide an integrated set of data on ground shaking, liquifaction-induced ground failure, and structural response. This report describes the detailed geologic, seismic, and material-property determinations derived at each of these sites.

California↗

USGS1994

In situ measurement of seismic shear-wave absorption in the San Francisco Holocene Bay Mud by the pulse-broadening method

By the pulse-broadening method, seismic absorption of San Francisco Holocene Bay Mud has been determined with the use of an impulsive shear-wave source at the surface, two receivers embedded in clay (Holocene Bay Mud), and digital seismic recording. The seismic anelastic quality factor is determined from 20 measurements to be 16.0 with a random error of ±0.9. Three sources of systematic error are examined. The fractional systematic error on the anelastic quality factor is estimated to be ∼ 11%. Details of experimental arrangement and error analysis are given.

Bulletin of the Seismological Society of America↗