Tom Brocher

• PhD
• Professor Emeritus at United States Geological Survey

The purpose of this report is to provide a compilation of structural retrofits and replacements of older buildings in the San Francisco Bay Area that have either been completed since 1989 or that are in progress as of October 2018. For the purposes of this report, all or parts of nine Bay Area counties were included: Alameda, Contra Costa, Marin, N...

The epicenter of the 16 July 1936 M 6 Milton-Freewater earthquake, also known as the State Line earthquake and the largest historical earthquake in northeastern Oregon or southeastern Washington, is uncertain. Various studies place the epicenter of the earthquake, which was widely felt in eastern Washington, northeastern Oregon, and northern Idaho,...

Reported aftershock durations, earthquake effects, and other observations from the large December 14, 1872 earthquake in central Washington are consistent with an epicenter near Entiat, Washington. Only near Entiat were aftershocks reported lasting for more than 3 months. Modal intensity data described in this paper are consistent with an Entiat ar...

We investigate spatial and temporal relations between an ongoing and prolific seismicity cluster in central Washington, near Entiat, and the 14 December 1872 Entiat earthquake, the largest historic crustal earthquake in Washington. A fault scarp produced by the 1872 earthquake lies within the Entiat cluster; the locations and areas of both the clus...

We determine Modified Mercalli (Seismic) Intensities (MMI) for nine onshore earthquakes of magnitude 4.5 and larger that occurred in central and western Washington between 1989 and 1999, on the basis of effects reported in postal questionnaires, the press, and professional collaborators. The earthquakes studied include four earthquakes of M5 and la...

When tectonic plates collide, the thrusting of one plate over the other (a process called subduction) leads inevitably to the world’s largest earthquakes, powerful tsunamis, explosive volcanic eruptions, and massive landslides, both on land and offshore. As recent subduction zone earthquakes and tsunamis in Sumatra (magnitude [M] 9.1, 2004), Chile...

A 700 km² rectangular seismicity cluster near Entiat, Washington, is a prolific source of seismicity east of the Cascade arc, producing mainly thrust focal mechanisms. The M6.5 to M7 1872 Chelan earthquake was one of the largest historic earthquakes in Washington. Its epicenter lies within the cluster, but the cluster’s constant rate of...

Paleomagnetic and GPS data indicate that Washington and Oregon have rotated clockwise for the past 16 Myr. Late Cenozoic and Quaternary fault geometries, seismicity lineaments, and focal mechanisms provide evidence that this rotation is accommodated by north-directed thrusting and strike-slip faulting in Washington, and SW- to W-directed normal fau...

The attenuation of seismic intensity with distance for 15 magnitude M4.8 to M6.8 earthquakes between 1949 and 2015 shows significant variation along the Cascadia forearc. Felt intensities for these earthquakes were taken from NOAA’s U.S. Earthquake Intensity Database (1635-1985), USGS Open-File Reports, and a 2015 Did You Feel It? report. I follow...

Paleomagnetic and GPS data indicate that Washington and Oregon have rotated clockwise for at least 16 Myr. Neogene and Quaternary fault geometries, seismicity lineaments, and earthquake focal mechanisms provide evidence that in the west this rotation is accommodated by N-S shortening along thrust faults and NNW-trending strike-slip faults, and in t...

Both longer- and shorter-term seismicity rates suggest that earthquakes in the Cascadia subduction system follow a Gutenberg-Richter distribution, in which M8.5 to M9 earthquakes occur at a rate predicted by smaller earthquakes, rather than a characteristic distribution. Cumulative plots of felt earthquakes in the fore-arc, arc, and back-arc of Ore...

The Mw 6.0 South Napa earthquake, which occurred at 10:20 UTC 24 August 2014 was the largest earthquake to strike the greater San Francisco Bay area since the Mw 6.9 1989 Loma Prieta earthquake. The rupture from this right‐lateral earthquake propagated mostly unilaterally to the north and up‐dip, directing the strongest shaking toward the city of N...

Executive Summary. Through discussions between the Federal Emergency Management Agency (FEMA) and the U.S. Geological Survey (USGS) following the South Napa earthquake, it was determined that several key decision points would be faced by FEMA for which additional information should be sought and provided by USGS and its partners. This report addres...

Recent research indicates that the shallow mantle of the Cascadia subduction margin under near-coastal Pacific Northwest, USA is cold and partially serpentinized, storing large quantities of water in this wedge-shaped region. Such a wedge probably formed to the south in California during an earlier period of subduction. We show by numerical modelin...

Introduction The magnitude 9.2 Great Alaska Earthquake that struck south-central Alaska at 5:36 p.m. on Friday, March 27, 1964, is the largest recorded earthquake in U.S. history and the second-largest earthquake recorded with modern instruments. The earthquake was felt throughout most of mainland Alaska, as far west as Dutch Harbor in the Aleutian...

We simulate long-period (T > 1.0-2.0 s) and broadband (T > 0.1 s) ground motions for 39 scenario earthquakes (M(w) 6.7-7.2) involving the Hayward, Calaveras, and Rodgers Creek faults. For rupture on the Hayward fault, we consider the effects of creep on coseismic slip using two different approaches, both of which reduce the ground motions, compared...

We summarize recent observations of long-period (0.5 - 2 s) seismic waves from seven local and regional earthquakes in the magnitude range 3.5 - 6.5, made using accelerograph arrays in the Sacramento/San Joaquin Delta, in the San Leandro basin, and in the Livermore Valley. The observations were made on both isolated accelerometers (both NetQuakes a...

We simulate long-period (T > 1:0-2:0 s) and broadband (T > 0:1 s) ground motions for 39 scenario earthquakes (M w 6.7-7.2) involving the Hayward, Calaveras, and Rodgers Creek faults. For rupture on the Hayward fault, we consider the effects of creep on coseismic slip using two different approaches, both of which reduce the ground motions, compared...

We obtain a seismic velocity model of the California crust and uppermost mantle using a regional-scale double-difference tomography algorithm. We begin by using absolute arrival-time picks to solve for a coarse three-dimensional (3D) P velocity (V_P) model with a uniform 30 km horizontal node spacing, which we then use as the starting model for a f...

We investigated the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic underplating. In the 1980’s and early 1990’s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-lon...

We present a three-dimensional (3D) tomographic model of the P wave velocity (Vp) structure of northern California. We employed a regional-scale double-difference tomography algorithm that incorporates a finite-difference travel time calculator and spatial smoothing constraints. Arrival times from earthquakes and travel times from controlled-source...

The Seattle fault is one of several east-trending compressional structures in the Puget Lowland (PL), seemingly at odds with the northeasterly oriented compression along the Juan de Fuca subduction zone. The existence of these faults is thought to be related to the northward movement of a strong Oregon forearc block. A weaker PL block accommodates...

Last October 21st marked the 140th anniversary of the M6.8 1868 Hayward Earthquake, the last damaging earthquake on the southern Hayward Fault. This anniversary was used to help publicize the seismic hazards associated with the fault because: (1) the past five such earthquakes on the Hayward Fault occurred about 140 years apart on average, and (2)...

We compute ground motions in the San Francisco Bay area for a suite of 35 magnitude 6.7--7.2 scenario earthquake ruptures involving the Hayward fault. The suite of scenarios encompasses variability in rupture length, hypocenter, distribution of slip, rupture speed, and rise time. The five rupture lengths include the Hayward fault and portions there...

A key question in earthquake hazard analysis is whether individual faults within fault zones represent independent seismic sources. For the Seattle fault zone, an upper plate structure within the Cascadia convergent margin, evaluating seismic hazard requires understanding how north-side-up, bedding-plane reverse faults, which generate late Holocene...

This article presents new empirical compressional and shear-wave velocity (Vp and Vs) versus depth relationships for the most common rock types in northern California. Vp versus depth relations were developed from borehole, laboratory, seismic refraction and tomography, and density measurements, and were converted to Vs versus depth relations using...

We estimate the ground motions produced by the 1906 San Francisco earthquake making use of the recently developed Song et al. (2008) source model that combines the available geodetic and seismic observations and recently constructed 3D geologic and seismic velocity models. Our estimates of the ground motions for the 1906 earthquake are consistent a...

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 chara...

We investigate the crustal structure and tectonic evolutionof the North American continent in Alaska, where the continenthas grown through magmatism, accretion, and tectonic under-plating.In the 1980s and early 1990s, we conducted a geological andgeophysical investigation, known as the Trans-Alaska CrustalTransect (TACT), along a 1350-km-long corri...

We present a digital elevation model (DEM) constructed from newly acquired high-resolution LIght Detection and Ranging (LIDAR) data along the Hayward Fault in Northern California. The data were acquired by the National Center for Airborne Laser Mapping (NCALM) in the spring of 2007 in conjunction with a larger regional airborne LIDAR survey of the...

October 21, 2008 marks the 140th anniversary of the M7 1868 Hayward earthquake. This large earthquake, which occurred slightly before 8 AM, caused extensive damage to San Francisco Bay Area and remains the nation's 12th most lethal earthquake. Property loss was extensive and about 30 people were killed. This earthquake culminated a decade-long seri...

The 1868 Hayward earthquake has been overshadowed by the subsequent 1906 San Francisco earthquake that destroyed much of San Francisco. Nonetheless, a modern recurrence of the 1868 earthquake would cause widespread damage to the densely populated Bay Area, particularly in the east Bay communities that have grown up virtually on top of the Hayward f...

GeoEarthScope acquired more than 1500 square km of airborne LiDAR data in northern California, providing high-resolution topographic data of most of the major strike-slip faults in the region. The coverage includes the San Andreas Fault from its northern end near Shelter Cove to near Parkfield, as well as the Rodgers Creek, Maacama, Calaveras, Gree...

A deep (317 m) borehole constructed with 6 piezometers was drilled adjacent to a dual-stage extensometer (under construction) near the modern San Francisco Bay shore in San Lorenzo, California for the purpose of monitoring pore-fluid pressure changes that may result from a proposed aquifer storage and recovery (ASR) program. Continuous lithological...

We report on progress towards the development of a state-wide tomographic model of the P-wave velocity for the crust and uppermost mantle of California. The dataset combines first arrival times from earthquakes and quarry blasts recorded on regional network stations and travel times of first arrivals from explosions and airguns recorded on profile...

Recent observations indicate that the Seattle sedimentary basin, underlying Seattle and other urban centers in the Puget Lowland, Washington, amplifies Iong-period (1-5 sec) weak ground motions by factors of 10 or more. We computed east-trending P- and S-wave velocity models across the Seattle basin from Seismic Hazard Investigations of Puget Sound...

[1] A new three-dimensional P wave velocity model for the greater San Francisco Bay region has been derived using the double-difference seismic tomography method, using data from about 5,500 chemical explosions or air gun blasts and approximately 6,000 earthquakes. The model region covers 140 km NE-SW by 240 km NW-SE, extending from 20 km south of...

The Hayward Fault System is considered the most likely fault system in the San Francisco Bay Area, California, to produce a major earthquake in the next 30 years. To better understand this fault system, we use microseismicity to study its structure and kinematics. We present a new 3D seismic-velocity model for the eastern San Francisco Bay region,...

We present a new three-dimensional (3-D) P-wave velocity model for the upper-crustal struc-ture beneath the Strait of Georgia, southwestern British Columbia based on non-linear tomo-graphic inversion of wide-angle seismic refraction data. Our study, part of the Georgia Basin Geohazards Initiative (GBGI) is primarily aimed at mapping the depth of th...

Regional 3-D seismic velocity models used for broadband strong motion simulations must include compressional-wave velocity (Vp), shear-wave velocity (Vs), intrinsic attenuation (Qp, Qs), and density. Vs and Qs are the most important of these parameters because the strongest ground motions are generated chiefly by shear- and surface-wave arrivals. B...

This paper presents the first regional three-dimensional P wave velocity model for the Northern Cascadia Subduction Zone (SW British Columbia and NW Washington State) constructed through tomographic inversion of first-arrival traveltime data from active source experiments together with earthquake traveltime data recorded at permanent stations. The...

The northward-vergent strand of the active Seattle thrust fault zone in the Puget Lowland of Washington State has experienced at least one large magnitude earthquake in the late Holocene. Previous researchers have asserted that this fault is genetically related to the formation of the Seattle basin, a 60 km wide, 35 km long and ~9 km deep basin imm...

Detailed, realistic models of the subsurface are needed for predicting damage patterns from future earthquakes and simulating other phenomena affecting human safety and well being. The simple models used in the past are no longer adequate. In support of a planned simulation of the ground shaking from the Great 1906 San Francisco earthquake, we cons...

We present a new regional 3D seismic velocity model for Northern California for use in strong motion simulations of the 1906 San Francisco and other earthquakes. The model includes compressional-wave velocity (Vp), shear-wave velocity (Vs), density, and intrinsic attenuation (Qp, Qs). These properties were assigned for each rock type in a 3D geolog...