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Thomas M. Brocher

Publications and source records attributed to Thomas M. Brocher.

At least 37 records · Page 2Linked to original sources
USGS2006

USGS science in Menlo Park -- a science strategy for the U.S. Geological Survey Menlo Park Science Center, 2005-2015

In the spring of 2004, the U.S. Geological Survey (USGS) Menlo Park Center Council commissioned an interdisciplinary working group to develop a forward-looking science strategy for the USGS Menlo Park Science Center in California (hereafter also referred to as "the Center"). The Center has been the flagship research center for the USGS in the western United States for more than 50 years, and the Council recognizes that science priorities must be the primary consideration guiding critical decisions made about the future evolution of the Center. In developing this strategy, the working group consulted widely within the USGS and with external clients and collaborators, so that most stakeholders had an opportunity to influence the science goals and operational objectives. The Science Goals are to: Natural Hazards: Conduct natural-hazard research and assessments critical to effective mitigation planning, short-term forecasting, and event response. Ecosystem Change: Develop a predictive understanding of ecosystem change that advances ecosystem restoration and adaptive management. Natural Resources: Advance the understanding of natural resources in a geologic, hydrologic, economic, environmental, and global context. Modeling Earth System Processes: Increase and improve capabilities for quantitative simulation, prediction, and assessment of Earth system processes. The strategy presents seven key Operational Objectives with specific actions to achieve the scientific goals. These Operational Objectives are to: Provide a hub for technology, laboratories, and library services to support science in the Western Region. Increase advanced computing capabilities and promote sharing of these resources. Enhance the intellectual diversity, vibrancy, and capacity of the work force through improved recruitment and retention. Strengthen client and collaborative relationships in the community at an institutional level. Expand monitoring capability by increasing density, sensitivity, and efficiency and reducing costs of instruments and networks. Encourage a breadth of scientific capabilities in Menlo Park to foster interdisciplinary science. Communicate USGS science to a diverse audience.

Circular↗
USGS2004

Geophysical investigation of the Denali fault and Alaska Range orogen within the aftershock zone of the October–November 2002, M = 7.9 Denali fault earthquake

The aftershock zone of the 3 November 2002, M = 7.9 earthquake that ruptured along the right-slip Denali fault in south-central Alaska has been investigated by using gravity and magnetic, magnetotelluric, and deep-crustal, seismic reflection data as well as outcrop geology and earthquake seismology. Strong seismic reflections from within the Alaska Range orogen north of the Denali fault dip as steeply as 25°N and extend to depths as great as 20 km. These reflections outline a relict crustal architecture that in the past 20 yr has produced little seismicity. The Denali fault is nonreflective, probably because this fault dips steeply to vertical. The most intriguing finding from geophysical data is that earthquake aftershocks occurred above a rock body, with low electrical resistivity (>10 Ω·m), that is at depths below ∼10 km. Aftershocks of the Denali fault earthquake have mainly occurred shallower than 10 km. A high geothermal gradient may cause the shallow seismicity. Another possibility is that the low resistivity results from fluids, which could have played a role in locating the aftershock zone by reducing rock friction within the middle and lower crust.

Alaska↗
USGS2004

Interpretation of the Seattle Uplift, Washington, as a passive-roof duplex

We interpret seismic lines and a wide variety of other geological and geophysical data to suggest that the Seattle uplift is a passive-roof duplex. A passive-roof duplex is bounded top and bottom by thrust faults with opposite senses of vergence that form a triangle zone at the leading edge of the advancing thrust sheet. In passive-roof duplexes the roof thrust slips only when the floor thrust ruptures. The Seattle fault is a south-dipping reverse fault forming the leading edge of the Seattle uplift, a 40-km-wide fold-and-thrust belt. The recently discovered, north-dipping Tacoma reverse fault is interpreted as a back thrust on the trailing edge of the belt, making the belt doubly vergent. Floor thrusts in the Seattle and Tacoma fault zones, imaged as discontinuous reflections, are interpreted as blind faults that flatten updip into bedding plane thrusts. Shallow monoclines in both the Seattle and Tacoma basins are interpreted to overlie the leading edges of thrust-bounded wedge tips advancing into the basins. Across the Seattle uplift, seismic lines image several shallow, short-wavelength folds exhibiting Quaternary or late Quaternary growth. From reflector truncation, several north-dipping thrust faults (splay thrusts) are inferred to core these shallow folds and to splay upward from a shallow roof thrust. Some of these shallow splay thrusts ruptured to the surface in the late Holocene. Ages from offset soils in trenches across the fault scarps and from abruptly raised shorelines indicate that the splay, roof, and floor thrusts of the Seattle and Tacoma faults ruptured about 1100 years ago.

Washington↗
USGS2004

The Catfish Lake Scarp, Allyn, Washington: Preliminary field data and implications for earthquake hazards posed by the Tacoma fault

The Tacoma fault bounds gravity and aeromagnetic anomalies for 50 km across central Puget lowland from Tacoma to western Kitsap County. Tomography implies at least 6 km of post-Eocene uplift to the north of the fault relative to basinal sedimentary rocks to the south. Coastlines north of the Tacoma fault rose about 1100 years ago during a large earthquake. Abrupt uplift up to several meters caused tidal flats at Lynch Cove, North Bay, and Burley Lagoon to turn into forested wetlands and freshwater marshes. South of the fault at Wollochet Bay, Douglas-fir forests sank into the intertidal zone and changed into saltmarsh. Liquefaction features found beneath the marsh at Burley Lagoon point to strong ground shaking at the time of uplift. Recent lidar maps of the area southwest of Allyn, Washington revealed a 4 km long scarp, or two closely spaced en-echelon scarps, which correspond closely to the Tacoma fault gravity and aeromagnetic anomalies. The scarp, named the Catfish Lake scarp, is north-side-up, trends east-west, and clearly displace striae left by a Vashon-age glacier. A trench across the scarp exposed evidence for postglacial folding and reverse slip. No organic material for radiocarbon dating was recovered from the trench. However, relationships in the trench suggest that the folding and faulting is postglacial in age.

Washington↗
USGS2003

Wide-angle seismic recording from the 2002 Georgia Basin Geohazards Initiative, northwestern Washington and British Columbia

This report describes the acquisition and processing of shallow-crustal wide-angle seismicreflection and refraction data obtained during a collaborative study in the Georgia Strait, western Washington and southwestern British Columbia. The study, the 2002 Georgia Strait Geohazards Initiative, was conducted in May 2002 by the Pacific Geoscience Centre, the U.S. Geological Survey, and the University of Victoria. The wide-angle recordings were designed to image shallow crustal faults and Cenozoic sedimentary basins crossing the International Border in southern Georgia basin and to add to existing wide-angle recordings there made during the 1998 SHIPS experiment. We recorded, at wide-angle, 800 km of shallow penetration multichannel seismic-reflection profiles acquired by the Canadian Coast Guard Ship (CCGS) Tully using an air gun with a volume of 1.967 liters (120 cu. in.). Prior to this reflection survey, we deployed 48 Refteks onshore to record the airgun signals at wide offsets. Three components of an oriented, 4.5 Hz seismometer were digitally recorded at all stations. Nearly 160,300 individual air gun shots were recorded along 180 short seismic reflection lines. In this report, we illustrate the wide-angle profiles acquired using the CCGS Tully, describe the land recording of the air gun signals, and summarize the processing of the land recorder data into common-receiver gathers. We also describe the format and content of the archival tapes containing the SEGY-formated, common-receiver gathers for the Reftek data. Data quality is variable but the experiment provided useful data from 42 of the 48 stations deployed. Three-fourths of all stations yielded useful first-arrivals to source-receiver offsets beyond 10 km: the average maximum source-receiver offset for first arrivals was 17 km. Six stations yielded no useful data and useful firstarrivals were limited to offsets less than 10 km at five stations. We separately archived our recordings of 86 local and regional earthquakes ranging in magnitude from 0.2 to 4.3 and 16 teleseisms ranging in magnitude 5.5 to 6.5.

British Columbia;Washington↗
USGS2003

Earthquake recordings from the 2002 Seattle Seismic Hazard Investigation of Puget Sound (SHIPS), Washington state

This report describes seismic data obtained during the fourth Seismic Hazard Investigation of Puget Sound (SHIPS) experiment, termed Seattle SHIPS . The experiment was designed to study the influence of the Seattle sedimentary basin on ground shaking during earthquakes. To accomplish this, we deployed seismometers over the basin to record local earthquakes, quarry blasts, and teleseisms during the period of January 26 to May 27, 2002. We plan to analyze the recordings to compute spectral amplitudes at each site, to determine the variability of ground motions over the basin. During the Seattle SHIPS experiment, seismometers were deployed at 87 sites in a 110-km-long east-west line, three north-south lines, and a grid throughout the Seattle urban area (Figure 1). At each of these sites, an L-22, 2-Hz velocity transducer was installed and connected to a REF TEK Digital Acquisition System (DAS), both provided by the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) of the Incorporated Research Institutes for Seismology (IRIS). The instruments were installed on January 26 and 27, and were retrieved gradually between April 18 and May 27. All instruments continuously sampled all three components of motion (velocity) at a sample rate of 50 samples/sec. To ensure accurate computations of amplitude, we calibrated the geophones in situ to obtain the instrument responses. In this report, we discuss the acquisition of these data, we describe the processing and merging of these data into 1-hour long traces and into windowed events, we discuss the geophone calibration process and its results, and we display some of the earthquake recordings.

Washington↗
USGS2002

Lower crustal deformation beneath the central Transverse Ranges, southern California: Results from the Los Angeles Region Seismic Experiment

We present a P wave velocity model derived from active source seismic data collected during the 1994 Los Angeles Region Seismic Experiment. Our model extends previously published upper crustal velocity models to mantle depths. Our model was developed by both ray tracing through a layered model and calculating travel times through a gridded model. It includes an 8-km-thick crustal root centered beneath the surface trace of the San Andreas fault, north of the highest topography in the San Gabriel Mountains. A simple mass balance calculation suggests that ∼36 km of north-south shortening across the San Andreas fault in the central Transverse Ranges could have formed this root. If north-south compression began when the “Big Bend” in the San Andreas fault formed at ∼5 Ma, 36 km of shortening implies a north-south contraction rate of ∼7.1 mm/yr across the central Transverse Ranges. If, instead, north-south compression began when the Transverse Ranges formed at 3.4–3.9 Ma, 36 km of shortening implies a contraction rate of 9.2–10.6 mm/yr. North of the San Andreas fault, the Mojave Desert crust has a low-velocity (6.3 km/s) mid and lower crust and a 28-km-deep Moho. South of the San Andreas fault, beneath the Los Angeles and San Gabriel Valley basins, there is a fast (6.6–6.8 km/s), thick (10–12 km) lower crust with a 27-km-deep Moho. Farther south still, the lower crust of the Continental Borderland is fast (6.6–6.8 km/s) and thin (5 km) with a shallow (22 km deep) Moho.

California↗
USGS2002

Implosion, earthquake, and explosion recordings from the 2000 Seattle Kingdome Seismic Hazards Investigation of Puget Sound (SHIPS), Washington

This report describes seismic data obtained in Seattle, Washington, March 24-28, 2000, during a Seismic Hazards Investigation of Puget Sound (SHIPS). The seismic recordings obtained by this SHIPS experiment, nicknamed Kingdome SHIPS, were designed to (1) measure site responses throughout Seattle and to (2) help define the location of the Seattle fault. During Kingdome SHIPS, we recorded the Kingdome implosion, four 150-lb (68-kg) shots, and a M w = 7.6 teleseism using a dense network of seismographs deployed throughout Seattle. The seismographs were deployed at a nominal spacing of 1 km in a hexagonal grid extending from Green Lake in the north to Boeing Field in the south. The Seattle Kingdome was a domed sports stadium located in downtown Seattle near the Seattle fault. The Seattle Kingdome was imploded (demolished) at 8:32 AM local time (16:32 UTC) on March 26 (JD 086), 2000. The seismic energy produced by implosion of the Kingdome was equivalent to a local earthquake magnitude of 2.3. Strong impacts produced by the implosion of the Kingdome generated seismic arrivals to frequencies as low as 0.1 Hz. Two shots located north of the Seattle fault, where the charges were detonated within the ground water column (Discovery and Magnuson Parks), were much more strongly coupled than were the two shots to the south of the Seattle fault, where the shots were detonated above the water table (Lincoln and Seward Parks). Thirty-eight RefTek stations, scattered throughout Seattle, recorded the M w = 7.6 Japan Volcano Islands earthquake (22.4°N, 143.6°E, 104 km depth) of 28 March 2000 (JD 088). This teleseism produced useful signals for periods between 4 and 7 seconds. Only a few recordings of small magnitude local earthquakes were made, and these recordings are not presented. In this report, we describe the acquisition of these data, discuss the processing and merging of the data into common shot gathers, and illustrate the acquired data. We also describe the format and content of the archival tapes containing the SEGY-formatted, common-shot gathers.

Washington↗
USGS2001

Density and velocity relationships for digital sonic and density logs from coastal Washington and laboratory measurements of Olympic Peninsula mafic rocks and greywackes

Three-dimensional velocity models for the basins along the coast of Washington and in Puget Lowland provide a means for better understanding the lateral variations in strong ground motions recorded there. We have compiled 16 sonic and 18 density logs from 22 oil test wells to help us determine the geometry and physical properties of the Cenozoic basins along coastal Washington. The depth ranges sampled by the test-well logs fall between 0.3 and 2.1 km. These well logs sample Quaternary to middle Eocene sedimentary rocks of the Quinault Formation, Montesano Formation, and Hoh rock assemblage. Most (18 or 82%) of the wells are from Grays Harbor County, and many of these are from the Ocean City area. These Grays Harbor County wells sample the Quinault Formation, Montesano Formation, and frequently bottom in the Hoh rock assemblage. These wells show that the sonic velocity and density normally increase significantly across the contacts between the Quinault or the Montesano Formations and the Hoh rock assemblage. Reflection coefficients calculated for vertically traveling compressional waves from the average velocities and densities for these units suggest that the top of the Hoh rock assemblage is a strong reflector of downward-propagating seismic waves: these reflection coefficients lie between 11 and 20%. Thus, this boundary may reflect seismic energy upward and trap a substantial portion of the seismic energy generated by future earthquakes within the Miocene and younger sedimentary basins found along the Washington coast. Three wells from Jefferson County provide data for the Hoh rock assemblage for the entire length of the logs. One well (Eastern Petroleum Sniffer Forks #1), from the Forks area in Clallam County, also exclusively samples the Hoh rock assemblage. This report presents the locations, elevations, depths, stratigraphic, and other information for all the oil test wells, and provides plots showing the density and sonic velocities as a function of depth for each well log. We also present two-way traveltimes for 15 of the wells calculated from the sonic velocities. Average velocities and densities for the wells having both logs can be reasonably well related using a modified Gardner’s rule, with p=1825v(1/4), where p is the density (in kg/m3) and v is the sonic velocity (in km/s). In contrast, a similar analysis of published well logs from Puget Lowland is best matched by a Gardner’s rule of p=1730v(1/4), close to the p=1740v(1/4) proposed by Gardner et al. (1974). Finally, we present laboratory measurements of compressional-wave velocity, shear-wave velocity, and density for 11 greywackes and 29 mafic rocks from the Olympic Peninsula and Puget Lowland. These units have significance for earthquake-hazard investigations in Puget Lowland as they dip eastward beneath the Lowland, forming the “bedrock” beneath much of the lowland. Average Vp/Vs ratios for the mafic rocks, mainly Crescent Formation volcanics, lie between 1.81 and 1.86. Average Vp/Vs ratios for the greywackes from the accretionary core complex in the Olympic Peninsula show greater scatter but lie between 1.77 and 1.88. Both the Olympic Peninsula mafic rocks and greywackes have lower shear-wave velocities than would be expected for a Poisson solid (Vp/Vs=1.732). Although the P-wave velocities and densities in the greywackes can be related by a Gardner’s rule of p=1720v(1/4), close to the p=1740v(1/4) proposed by Gardner et al. (1974), the velocities and densities of the mafic rocks are best related by a Gardner’s rule of p=1840v(1/4). Thus, the density/velocity relations are similar for the Puget Lowland well logs and greywackes from the Olympic Peninsula. Density/velocity relations are similar for the Washington coastal well logs and mafic rocks from the Olympic Peninsula, but differ from those of the Puget Lowland well logs and greywackes from the Olympic Peninsula.

Washington↗
USGS2000

Geophysical evidence for the evolution of the California Inner Continental Borderland as a metamorphic core complex

We use new seismic and gravity data collected during the 1994 Los Angeles Region Seismic Experiment (LARSE) to discuss the origin of the California Inner Continental Borderland (ICB) as an extended terrain possibly in a metamorphic core complex mode. The data provide detailed crustal structure of the Borderland and its transition to mainland southern California. Using tomographic inversion as well as traditional forward ray tracing to model the wide-angle seismic data, we find little or no sediments, low (≤6.6 km/s) P wave velocity extending down to the crust-mantle boundary, and a thin crust (19 to 23 km thick). Coincident multichannel seismic reflection data show a reflective lower crust under Catalina Ridge. Contrary to other parts of coastal California, we do not find evidence for an underplated fossil oceanic layer at the base of the crust. Coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge, which represents the transition to the western Transverse Ranges. On the shelf the Palos Verdes Fault merges downward into a landward dipping surface which separates "basement" from low-velocity sediments, but interpretation of this surface as a detachment fault is inconclusive. The seismic velocity structure is interpreted to represent Catalina Schist rocks extending from top to bottom of the crust. This interpretation is compatible with a model for the origin of the ICB as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks. The basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until ∼13 m.y. ago. Subduction of the young and hot Monterey and Arguello microplates under the Continental Borderland, followed by rotation and translation of the western Transverse Ranges, may have provided the necessary thermomechanical conditions for this extension and crustal inflow.

California↗
USGS2000

Report for explosion and earthquake data acquired in the 1999 Seismic Hazards Investigation of Puget Sound (SHIPS), Washington

This report describes the acquisition, processing, and quality of seismic reflection and refraction data obtained in the Seattle basin, central Puget Lowland, western Washington, in September 1999 during the Seismic Hazards Investigation of Puget Sound (SHIPS). As a sequel to the 1998 SHIPS air gun experiment (also known as 'Wet SHIPS'), the 1999 experiment, nicknamed 'Dry SHIPS,' acquired a 112-km-long east-west trending multichannel seismic reflection and refraction line in the Seattle basin. One thousand and eight seismographs were deployed at a nominal spacing of 100 meters, and 29 shot points were detonated at approximately 4 km intervals along the seismic line. The wide-angle seismic profile was designed to (1) determine the E-W geometry of Seattle basin, (2) measure the seismic velocities within the basin, and (3) define the basement structure underlying the Seattle basin. In this report, we describe the acquisition of these data, discuss the processing and merging of the data into common shot gathers, and illustrate the acquired profiles. We also describe the format and content of the archival tapes containing the SEGY-formatted, common-shot gathers. Data quality is variable, but useful data were acquired from all 29 shot points fired along the Dry SHIPS seismic line. The data show pronounced travel time delays associated with the low velocity sedimentary rocks filling the Seattle basin. Thirty-five REFTEK stations, deployed at 4 km intervals along the Dry SHIPS line, recorded 26 regional earthquakes and blasts and 17 teleseismic events, including the main shock and several aftershocks of the Mw=7.6 Chi-Chi (Taiwan) earthquake of 9/20/1999. The teleseismic recordings of the Chi-Chi (Taiwan) mainshock provide useful signals down to 10 second periods. They document a significant (factor between 5 and 10) focusing of compressional- and shear-wave energy by the Seattle basin at periods between 1 and 2 seconds relative to 'bedrock' sites east and west of the basin. Signal durations in the Seattle basin were also substantially increased relative to 'bedrock' sites in the Olympic peninsula and Cascade foothills.

Open-File Report↗