High Resolution Seismic Imaging
The focus of the Hi-Res Seismic Imaging Group is to utilize active and passive methods of seismic imaging for earthquake hazards investigations. Active methods are used to map faults beneath the land surface, to characterize the type of fault, and determine fault rupture history. Both active and passive methods are used for site characterization and sedimentary basin studies in urban areas. Sedimentary basin studies include regional ground motion modeling that is incorporated into urban hazard maps.
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Our research under this project is divided into two unique efforts: (1) Northern California Urban basin imaging; (2) Shallow S-wave velocity estimation.
(1) The Santa Clara Valley of Northern California is a densely populated, broad flat region located within the central California Coast Ranges about 50 km southeast of San Francisco. The Valley is bounded by mountainous areas that contain the seismically-active Hayward-Calaveras and San Andreas fault systems on the east and west, respectively. Juxtaposed against these dynamic faults and mountains, the flat valley floor is striking contrast to the active tectonic environments on its margins. This study uses seismic reflection imaging to look for faults and active geologic structures that may be hidden below the flat valley floor. The 28 km of reflection profiles were collected on the Valley floor over the Evergreen basin and Cupertino Basins. The Evergreen Basin is a buried 8-km wide by 40-km long sedimentary valley that may be up to 5 km deep. The Cupertino Basin is broader and not as long. This study came about because there were complementary needs to better understand the Quaternary section for groundwater models needed by the Santa Clara Valley Water District, and to refine existing geologic and geophysical models of the basins that are used to model earthquake ground motions recorded by the USGS San Jose Dense Seismic Array.
(2) There is continuous and increasing demand for near-surface S-wave velocities (Vs) in earthquake hazard studies, but there is significant uncertainty about how best to measure Vs at typical depths of 30 m and deeper. Determining accurate shallow Vs structure is critical for estimating site response as well as developing realistic predictive ground motion models for integration into modern Urban Seismic Hazard Maps. It is important that a consensus standardized approach for acquiring and using shallow Vs be determined so that the earthquake-engineering community has confidence both in the surface methods and their Vs measurements. It is therefore critical that an assessment of the strengths and weaknesses of available acquisition methodologies be established. The High-resolution Seismic Imaging Group has conducted research in the use and reliability of many surface Vs acquisition techniques including active-source S-wave reflection/refraction using wood timber and vibroseis sources. We have also investigated many passive source methods including Refraction Microtremor (ReMi), Multi-channel Analysis of Surface Waves (MASW) and, most recently, Spatial Autocorrelation (SPAC). Results to date with each of these methods is promising, with no one method demonstrating superiority in all geologic conditions. Work on this task has been conducted throughout the United States and Puerto Rico.
Subsurface Structure of the Santa Rosa Plain, California, from High-Resolution Seismic-Reflection Data
We collected 13-km of high-resolution seismic-reflection data in two profiles on the Santa Rosa Plain to image basin structure and stratigraphy in this seismically-active area and provide key constraints for earthquake hazard assessment products. In particular, the seismic-reflection data will constrain the geometry and depth extent of the Trenton Ridge, the northeastern corner of the Cotati Basin, and the position of the Rodgers Creek fault in the urban area of Santa Rosa.
The Trenton Ridge is a completely concealed basement high that may contain an active fault and may partition the hydrologic response of the Santa Rosa Plain basin fill. This feature has been mapped using gravity data, which are excellent at defining the shape, but not the exact depth of the basement ridge. The seismic-reflection data will (1) constrain how close to the ground surface this impermeable basement ridge extends, (2) evaluate whether the basement ridge folds or truncates sedimentary layers (aquifers) that lie above the ridge, and (3) test whether the Trenton thrust fault extends east across the Plain.
Santa Rosa, California, experienced unexpectedly high building damage from the 1906 San Francisco earthquake and the 1969 Santa Rosa earthquake sequence (M5.6 and M5.7). Earthquake simulations of the 1906 earthquake also show higher and extended ground motions for the Santa Rosa area that correlate with reported higher 1906 Mercalli intensities (Boatwright et al., 2006; McPhee et al., 2007). The subsurface imaging research in the Santa Rosa area will lead to a better understanding of the underlying basin structure and the higher ground motions observed there. The imaging conducted in this project will better constrain geologic models and lead to improved ground motion simulations and earthquake hazard estimates. This research complements previous studies of the Santa Rosa area using gravity, magnetic, computer earthquake simulations, portable seismograph deployments, and borehole/velocity studies conducted over the past 3 years. Indeed, the proposed profile location will be constrained by the measurements made in these previous studies.
- John Boatwright, Howard Bundock, and Linda C. Seekins, 2006, Using Modified Mercalli Intensities to Estimate Acceleration Response Spectra for the 1906 San Francisco Earthquake: Earthquake Spectra, Volume 22, No. S2, pages S279–S295.
- D. K. McPhee, V. E. Langenheim, S. Hartzell, R. J. McLaughlin, B. T. Aagaard, R. C. Jachens, and C. McCabe, 2007, Basin Structure beneath the Santa Rosa Plain, Northern California: Implications for Damage Caused by the 1969 Santa Rosa and 1906 San Francisco Earthquakes: Bulletin of the Seismological Society of America, Vol. 97, No. 5, pp. 1449–1457, October 2007, doi: 10.1785/0120060269
Los Angeles Basin
Within the Los Angeles Basin region the USGS high-resolution seismic imaging group has focused on two areas of research in support of Urban Seismic Hazard Map projects and investigations: (1) Multi-scale high-resolution seismic-reflection imaging: (2) Urban earthquake ground motion effects.
To date, we have acquired high-resolution P-wave seismic reflection images across active fault systems (Palos Verde fault, Santa Monica Fault and Puente Hills blind thrust) to gain a better understanding of their structure and tectonic history. Seismic-reflection surveys have also been conducted in urban areas to map the geometry of the San Jacinto and San Bernardino basins and this imagery has been used to model basin effects on earthquake-wave propagation. Our group used a combination of high-resolution seismic-reflection profiling and ground motion characterization to model differential ground shaking and damage in San Fernando Valley following the Northridge earthquake. We research the modification and application of active and passive source techniques to acquire shallow shear-wave velocity structure data within urban areas that is used for estimating site response as well as developing realistic predictive ground motion models for integration into modern Urban Seismic Hazard and Shake Maps.
The densely populated Puget Lowland of Washington State and the Portland, Oregon metropolitan area are cut by several crustal faults capable of generating large earthquakes. The presence, locations, geometries, and slip rates of these faults are poorly known. This information is essential for reliable seismic hazard mapping, assessment, and mitigation. The goal of the high-resolution seismic imaging group is to obtain information on regional active faults and sedimentary basins for the characterization of Puget Lowland and western Oregon earthquake hazards. Not only is the information collected in this task useful for characterizing earthquake sources, it is also important for identifying structural offsets that can cause ground-motion focusing as well as the generation of basin surface waves. Our objectives also include incorporation of fault and velocity data into the urban hazard maps and the national databases so that it can be used in regional seismic hazard assessment and mapping.
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The study area is located at the southern end of the New Madrid seismic zone, about 80 km northwest of Memphis, which was seriously affected by great earthquakes of 1811-1812. The Mississippi River valley earthquakes of 1811-1812 rank among the most significant events in U.S. history. Industry and high-resolution seismic reflection data have successfully imaged localized post-Paleozoic deformation throughout the upper Reelfoot rift and New Madrid Seismic Zone, across such “features” as the Blytheville Arch, Crowley’s Ridge, Reelfoot fault, Crittenden County fault, Commerce geophysical lineament and the Bootheel lineament. Each of these features has documented (in most cases Quaternary) deformation associated with them, but it is unclear how they are interrelated in the context of long-term deformation across the Reelfoot rift and therefore their effect on earthquake hazard. To our knowledge, there is no location across the Reelfoot rift where disparate industry seismic reflection lines readily can be merged into a largely continuous transect to image the cumulative deformation. Additionally, most of the available industry data were acquired in the late 1970’s to early 1980’s before modern high-bandwidth recording technology, and because they were not designed to image post-Paleozoic strata they consistently provide an inadequate image of the critical upper-Tertiary and Quaternary strata. CoCORP transects from the late-1980’s cross significant continuous stretches of the rift, but suffer even more dramatically from poor shallow resolution because their target depth was the deep crust and upper mantle.
To address these gaps in imaging, we have begun an ambitious 5 year comprehensive investigation of long-term deformation rates along a single transect crossing the entire Reelfoot rift. Such a transect will for the first time image major tectonic features at a scale capable of resolving pre-Paleozoic to Quaternary deformation. As such, it will provide us with the first opportunity to understand how long-term deformation is partitioned across the failed rift system and to understand how the long-term tectonic history is affecting modern earthquake hazards. It could also provide a glimpse at currently unknown and potentially active tectonic faults for the first time. Since May 2006, and in collaboration the University of Memphis, and the University of Texas, we have collected 31 km of high-resolution data (5-m source/station interval) on the western side of the rift through Lepanto, Arkansas, across Crowleys Ridge, and across the western Reelfoot Rift lineament.
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Within the USGS Intermountain West Project, the high-resolution seismic imaging group is primarily focused on contributing critical seismological information to the Urban Seismic Hazard Maps that are being developed along the Wasatch Front, the Reno-Carson City urban corridor, and Las Vegas region. To date, we have acquired S-wave velocity information, critical for accurate ground motion characterization, using S-wave reflection/refraction, borehole, and SPAC micro tremor data along the Wasatch Front. Additionally, we have developed high-resolution P-wave seismic reflection images to give clearer pictures of the soil and bedrock geometry and active faults at several sites in both Salt Lake and Utah valleys.
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Puerto Rico lies along a tectonically active plate boundary which makes its urban centers and infrastructures susceptible to the effects of earthquake induced ground shaking. Ground motion and site amplification effects are strongly influenced by the physical properties of the near-surface geologic units through which earthquake energy passes. Previous studies have consistently shown that decreasing mean shear-wave velocity (Vs) in the near-surface geologic materials generally correlates with an increase in the average amplification of earthquake ground motion. Efforts are underway to develop seismic hazard, shake map and ground motion models for Puerto Rico’s urban areas. A key element for understanding local and regional seismic hazard parameters is the acquisition and determination of Vs data for the various rock and soil units. Little information currently exists about the shallow velocity structure of bedrock and soil units in and around the urban centers and PRSN broadband seismic instrument stations located throughout Puerto Rico.
The U.S. Geological Survey, Puerto Rico Seismic Network (PRSN), Puerto Rico Strong Motion Program (PRSMP) and the Geology Department at the University of Puerto Rico-Mayagüez (UPRM) are collaborating in a research investigation to study near-surface shear-wave (Vs) and compressional-wave (Vp) velocities in and around major urban areas of Puerto Rico. Methods using active source (hammer and timber) body wave (seismic refraction-reflection) and passive source (cultural noise) surface wave (refraction microtremor-ReMi) techniques are being used to acquire information about near-surface materials associated with the primary geologic units located within the urbanized areas of Puerto Rico. Geologic units surveyed included Cretaceous intrusive and volcaniclastic bedrock, Tertiary sedimentary and volcanic units, and Quaternary unconsolidated aeolian, fluvial, beach, and lagoon deposits.