Program for earthquake hazards assessment in the Pacific Northwest

Shedlock, Kaye M.; Weaver, Craig S.

doi:10.3133/cir1067

Cited by 6 publications

(5 citation statements)

References 38 publications

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“…The Cascadia subduction zone has received considerable attention recently, both because of a large uncertainty about the seismic hazard it poses to the inhabitants of the Pacific Northwest region [e.g., Heaton and Hartzell, 1987;Atwater, 1987;Grant et al, 1989;Shedlock and Weaver, 1991] shelf, the age and composition of the sediments and intrusive rocks overlying the Siletz basement are constrained by a deep industry test hole located along the profile [Snavely, 1987].…”

Section: Introductionmentioning

confidence: 99%

A seismic reflection profile across the Cascadia Subduction Zone offshore central Oregon: New constraints on methane distribution and crustal structure

Tréhu¹,

Lin²,

Maxwell³

et al. 1995

J. Geophys. Res.

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In 1989, we conducted an onshore/offshore seismic experiment to image the crustal structure of the Cascadia forearc. In this paper, we discuss the processing and interpretation of a multichannel seismic reflection profile across the continental margin that was collected as part of this effort. This profile reveals several features of the forearc that were not apparent in an earlier, coincident reflection profile. One of the most important of these features is a very strong bottom simulating reflection (BSR) beneath the midslope region that is nearly continuous from water depths of about 1500 m to 600 m, where it appears to crop out on the seafloor. The pressure and temperature conditions at the BSR derived from our observations are remarkably consistent with the experimentally determined phase diagram for a methane hydrate/seawater system over a broad range of temperatures and pressures, assuming hydrostatic pressure and the temperature gradiant measured near the base of the continental slope during Ocean Drilling Program (ODP) leg 146. Interval velocities and reflection coefficients derived from the data indicate that the BSR represents a contrast between sediment with a small amount of hydrate overlying sediment containing free gas, consistent with results obtained during leg 146. Although the regional distribution of the anomalously strong BSR beneath the midslope is poorly known, we speculate that it may be related to apparent slope instability. The data also provide constraints on the thickness and geometry of the Siletz terrane, which is the basement beneath the shelf and acts as the subduction zone backstop. A deep reflection, which might mistakenly be interpreted to be Moho if coincident large‐aperture data were not available, is interpreted to be the base of the Siletz terrane. A “recently” active strike‐slip (?) fault zone that overlies the seaward edge of the Siletz terrane suggests that the Siletz terrane controls the location of decoupling of the subduction complex from the rest of the forearc.

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Section: Introductionmentioning

confidence: 99%

A seismic reflection profile across the Cascadia Subduction Zone offshore central Oregon: New constraints on methane distribution and crustal structure

Tréhu¹,

Lin²,

Maxwell³

et al. 1995

J. Geophys. Res.

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“…Resolution of seismic-safety issues in the region requires reasonable estimates of the magnitude, extent, and recurrence of prehistoric plate-interface events during the late Holocene (last 5000 years; Shedlock and Weaver, 1989). A probable locked plate interface, a convergence rate of 30-40 mm/yr, and the young age (<10 Ma) of the subducting crust suggest plate-interface events of Mw 8-9 are plausible (Heaton and Kanamori, 1984).…”

Section: Introductionmentioning

confidence: 99%

The potential for great earthquakes in Oregon and Washington; an overview of recent coastal geologic studies and their bearing on segmentation of Holocene ruptures, central Cascadia subduction zone

Nelson¹,

Personius²

1991

Open-File Report

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Fundamental questions in earthquake hazards research in the Pacific Northwest are concerned with the magnitude and recurrence of great earthquakes in the Cascadia subduction zone (CSZ). Geologic work of the last few years has produced convincing evidence of coseismic subsidence along the Washington and Oregon coasts. Regional subsidence recorded by estuarine deposits suggests that plate-interface earthquakes of at least Mw 8 (>100-km-long ruptures) occurred during the late Holocene in northern Oregon and southern Washington. Differences in the types of coastal marsh sequences between northern and south-central Oregon, however, suggest that the Oregon coastline did not subside due to regional deformation south of about 44 N. North of this latitude, the coast may intersect the seaward edge of a zone of coseismic subsidence that may continue southeastward onshore. Alternatively, the CSZ may be segmented near 44-45 N; a segment boundary at this location would indicate that plate-interface events near Mw 8 along the central CSZ are more frequent than larger (Mw 9) events. South of this boundary in the Coos Bay region, the tectonic framework developed through mapping and dating of marine and fluvial terraces indicates that episodes of abrupt marsh burial in south-central Oregon are best interpreted as the product of deformation on local structures. Some of the local deformation could be associated with moderate earthquakes (Ms<6). At most sites in south-central Oregon, however, it is still unclear whether the observed deformation is related to local faulting and folding during moderate-magnitude earthquakes, to regional deformation during great plate-interface earthquakes, or to both.

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“…The calculated probabilities, however, will have large uncertainties because the earthquake age estimates are imprecise. If these uncertainties are taken into account using the procedure of Savage [1994], the probability of an earth- [Shedlock and Weaver, 1991;Rogers, 1994]. One need only look to Kobe, Japan, for an example of the devastation that can be wrought by a crustal earthquake that was smaller than three quakes in southern British Columbia and Washington in this century.…”

Section: Earthquake Hazardmentioning

confidence: 99%

Evidence for large earthquakes at the Cascadia Subduction Zone

Clague¹

1997

Reviews of Geophysics

181

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Abstract. Large, historically unprecedented earthquakes at the Cascadia subduction zone in western North America have left signs of sudden land level change, tsunamis, and strong shaking in coastal sediments. The coastal geological evidence suggests that many of the earthquakes occurred at the boundary between the overriding North American plate and the subducting Juan de Fuca plate. This hypothesis is consistent with geodetic measurements and the results of geophysical modeling, which indicate that part of the plate boundary is locked and accumulating elastic strain that will be released during a future large earthquake. Arguments based on potential amounts of seismic slip and likely rupture areas suggest that most or all of the plate boundary earthquakes were magnitude 8 or larger events. The last earthquake or series of earthquakes, about 300 years ago, ruptured the entire 1000-km length of the subduction zone; if it was a single quake, it probably exceeded magnitude 9. Other earthquakes may have ruptured one or more segments of the subduction zone Or may have occurred on faults in the North American plate. Recurrence intervals are uncertain because of difficulties in identifying and dating earthquakes. In southwestern Washington state, intervals for the seven most recent earthquakes average about 500 years but range from less than 200 years to 700-1300 years. Future research on Cascadian plate boundary earthquakes will probably focus on (1) the relation between plate boundary and crustal earthquakes, (2) earthquake magnitude, (3) the areal extent and severity of seismic ground motions, (4) ages and number of past plate boundary earthquakes, and (5) land level changes preceding earthquakes.

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Program for earthquake hazards assessment in the Pacific Northwest

Cited by 6 publications

References 38 publications

A seismic reflection profile across the Cascadia Subduction Zone offshore central Oregon: New constraints on methane distribution and crustal structure

A seismic reflection profile across the Cascadia Subduction Zone offshore central Oregon: New constraints on methane distribution and crustal structure

The potential for great earthquakes in Oregon and Washington; an overview of recent coastal geologic studies and their bearing on segmentation of Holocene ruptures, central Cascadia subduction zone

Evidence for large earthquakes at the Cascadia Subduction Zone

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