Seismic survey probes urban earthquake hazards in Pacific Northwest

Fisher, Michael A.; Brocher, Thomas M.; Hyndman, R. D.; Tréhu, Anne M.; Weaver, Craig S.; Creager, K. C.; Crosson, Robert S.; Parsons, Tom; Cooper, Alan K.; Mosher, D.; Spence, G. D.; Zelt, B. C.; Hammer, Philip T. C.; Brink, Uri S. ten; Pratt, Thomas L.; Miller, Kate C.; Childs, J. R.; Cochrane, Guy R.; Chopra, Sundeep; Walia, R.

doi:10.1029/99eo00011

Cited by 36 publications

(34 citation statements)

References 10 publications

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“…Our data set comprises P and S wave arrival times for three types of events: regular intraplate earthquakes (within the fore-arc crust and subducting Juan de Fuca plate), low-frequency earthquakes (LFE) located near or at the plate boundary at depths of 30-40 km and air gun shots from the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS) experiment (Brocher et al, 2001;Fisher et al, 1999), described in more detail below. The study region encompasses southern Vancouver Island along with the Juan de Fuca and Georgia Straits (latitude between 47.758N and 508N and longitude between 125.258W and 122.258W).…”

Section: Methodsmentioning

confidence: 99%

Seismicity, Metamorphism, and Fluid Evolution Across the Northern Cascadia Fore Arc

Savard

Bostock

Christensen

2018

Geochem Geophys Geosyst

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We invert traveltime data from regular seismicity and low‐frequency earthquakes (LFEs) on southern Vancouver Island to map fore‐arc structure and seismogenesis. Tomographic images reveal high Poisson's ratios associated with a previously mapped, dipping low‐velocity zone inferred to be overpressured, upper oceanic crust of the Juan de Fuca plate, where LFEs and other slow‐slip phenomena occur. Low Poisson's ratios (∼0.225) in the fore‐arc continental crust above the mantle wedge are accompanied by high Vp (∼6.65 km/s) and high levels of clustered microseismicity. This crustal anomaly is positioned above the slab, where focused expulsion of fluids is inferred based on independent evidence, suggesting an association between composition, fluids, and seismogenesis. We propose that the anomalous crust harbors a high percentage (≤15%) of quartz, characterized by extremely low Poisson's ratio of 0.1. Earlier studies have proposed that excess quartz in the crust is precipitated from fluids fluxed from the subducting slab. In contrast, we posit that quartz is produced and concentrated in situ by metasomatism in the fore‐arc crust catalyzed through focused ingress of slab‐derived fluids at high‐pore pressure. These fluids enable microseismic activation of high‐angle thrust faults through a fault‐valve mechanism that concentrates quartz via pore‐pressure cycling. Larger M ≥ 6 events like those recently proposed to occur along the Leech River Fault may nucleate deep in the brittle‐ductile transition as a result of these reactions. The fore‐arc crust in warm subduction settings may thus suffer extreme exposure to prolonged fluid flow, an inference with relevance to the genesis of orogenic gold deposits.

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Section: Methodsmentioning

confidence: 99%

Seismicity, Metamorphism, and Fluid Evolution Across the Northern Cascadia Fore Arc

Savard

Bostock

Christensen

2018

Geochem Geophys Geosyst

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“…To address this question, we analyzed data collected during the 1998 SHIPS experiment, in which 50 three-component seismometers were distributed throughout the Puget Lowland and over the largest sedimentary basins in the region ( Fig. 1) Fisher et al, 1999). This three-component receiver array was the first and only such array to have a broad, uniform distribution throughout the Puget Lowland, and it therefore provided an important data set for site-response studies.…”

Section: Introductionmentioning

confidence: 99%

Site Response and Attenuation in the Puget Lowland, Washington State

Pratt

Brocher

2006

Bulletin of the Seismological Society of America

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Simple spectral ratio (SSR) and horizontal-to-vertical (H/V) siteresponse estimates at 47 sites in the Puget Lowland of Washington State document significant attenuation of 1.5-to 20-Hz shear waves within sedimentary basins there. Amplitudes of the horizontal components of shear-wave arrivals from three local earthquakes were used to compute SSRs with respect to the average of two bedrock sites and H/V spectral ratios with respect to the vertical component of the shear-wave arrivals at each site. SSR site-response curves at thick basin sites show peak amplifications of 2 to 6 at frequencies of 3 to 6 Hz, and decreasing spectral amplification with increasing frequency above 6 Hz. SSRs at nonbasin sites show a variety of shapes and larger resonance peaks. We attribute the spectral decay at frequencies above the amplification peak at basin sites to attenuation within the basin strata. Computing the frequency-independent, depth-dependent attenuation factor (Q s,int ) from the SSR spectral decay between 2 and 20 Hz gives values of 5 to 40 for shallow sedimentary deposits and about 250 for the deepest sedimentary strata (7 km depth). H/V site responses show less spectral decay than the SSR responses but contain many of the same resonance peaks. We hypothesize that the H/V method yields a flatter response across the frequency spectrum than SSRs because the H/V reference signal (vertical component of the shear-wave arrivals) has undergone a degree of attenuation similar to the horizontal component recordings. Correcting the SSR site responses for attenuation within the basins by removing the spectral decay improves agreement between SSR and H/V estimates.

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“…Since 1980, numerous seismic reflection‐refraction experiments have been conducted across the margin of Oregon, Washington, and British Columbia to explore the complex velocity and tectonic structure of the Cascadia convergent margin [ Spence et al , 1985; Green et al , 1986; Taber and Lewis , 1986; Clowes et al , 1987; Calvert and Clowes , 1990, 1991; Hyndman et al , 1990; Tréhu et al , 1994; Calvert , 1996; Miller et al , 1997; Flueh et al , 1998; Parsons et al , 1998, 1999; Gerdom et al , 2001]. During the Seismic Hazards Investigations in Puget Sound (SHIPS) experiment, conducted in March 1998, onshore‐offshore wide‐angle data and multichannel reflection data were collected in northwestern Washington State and southwestern British Columbia [ Brocher et al , 1999; Fisher et al , 1999]. The objectives were (1) to define the geometry of deep structures that control earthquake occurrence, including the megathrust fault that produces great earthquakes, (2) to provide detailed controls on seismic velocity crustal structure and on crustal faults, and (3) to define sedimentary basins that may affect strong motions during earthquakes.…”

Section: Introductionmentioning

confidence: 99%

Crustal structure beneath the Strait of Juan de Fuca and southern Vancouver Island from seismic and gravity analyses

et al. 2003

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[1] Wide-angle and vertical incidence seismic data from Seismic Hazards Investigations in Puget Sound (SHIPS), gravity modeling, and seismicity are used to derive twodimensional crustal models beneath the Strait of Juan de Fuca. The Eocene volcanic Crescent-Siletz terrane is significantly thicker than previously recognized and extends from near the surface to depths of 22 km or greater. For the northern strait, a weak midcrustal reflector, dipping east from 12-to 22-km depth, is inferred from wide-angle reflections. A stronger deeper reflector, dipping eastward from 23-to 36-km depth, is associated with the top of ''reflector band E,'' a zone of high reflectivity on coincident Multichannel Seismic (MCS) data, interpreted as a shear zone. A high-velocity zone (7.60 ± 0.2 km s À1 ) between these reflectors is interpreted as a localized slice of mantle accreted with the overlying Crescent-Siletz terrane. For the southern strait, no deep high-velocity layer is observed and the E-band reflectivity is weaker than to the north. A strong deep reflector, interpreted as the oceanic Moho dips eastward from 35 to 42 km. Seismicity within the subducting slab occurs mainly above the inferred oceanic Moho. Gravity modeling, constrained by the wide-angle seismic models and seismicity, is consistent with the inferred large thickness of Crescent-Siletz and high-density rocks (3030 kg m À3 ) in the lower crust.

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Seismic survey probes urban earthquake hazards in Pacific Northwest

Cited by 36 publications

References 10 publications

Seismicity, Metamorphism, and Fluid Evolution Across the Northern Cascadia Fore Arc

Seismicity, Metamorphism, and Fluid Evolution Across the Northern Cascadia Fore Arc

Site Response and Attenuation in the Puget Lowland, Washington State

Crustal structure beneath the Strait of Juan de Fuca and southern Vancouver Island from seismic and gravity analyses

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