Seismicity of Washington and Oregon

Ludwin, R.S.; Weaver, Craig S.; Crosson, Robert S.

doi:10.1130/dnag-csms-neo.77

Cited by 46 publications

(51 citation statements)

References 44 publications

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“…The shallow 1959 M 7.3 Hebgen Lake, Montana, earthquake was followed by a vigorous aftershock sequence (Stover and Coffman, 1993). In contrast, there were no aftershocks following the large intraslab events beneath Puget Sound in 1949 (magnitude 4.5 detection threshold) and 1965 (magnitude 2.5 detection threshold) (Ludwin et al, 1991). Only four small aftershocks (codalength magnitude 1.0 detection threshold) occurred after the 2001 M 6.8 intraslab Nisqually earthquake.…”

Section: Shallow Crustal Earthquakes East Of the Cascadesmentioning

confidence: 84%

“…Although earthquakes west of the Cascades occur throughout the crust and within the Juan de Fuca plate, all Washington earthquakes east of the crest of the Cascade Range since 1970 are located in the crust (Ludwin et al, 1991), and there is no tectonic basis to suppose that large pre-1970 earthquakes might be deeper.…”

Section: Shallow Crustal Earthquakes East Of the Cascadesmentioning

confidence: 99%

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The December 1872 Washington State Earthquake

Bakun¹

2002

Bulletin of the Seismological Society of America

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The largest historical earthquake in eastern Washington occurred on 15 December 1872. We used Modified Mercalli intensity (MMI) assignments for 12 twentieth-century earthquakes to determine attenuation relations for different regions in the Pacific Northwest. MMI attenuation for propagation paths east and west of the Cascade Mountains differs significantly only for epicentral distances greater than about 225 km. We used these attenuation relations and the MMI assignments for the 15 December 1872 earthquake to conclude that its epicentral region was east of the Cascade Mountains near Lake Chelan, Washington, and most probably near the south end of Lake Chelan. The intensity magnitude, M I , is 6.8 and moment magnitude, M, is 6.5-7.0 at the 95% confidence level.

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Section: Shallow Crustal Earthquakes East Of the Cascadesmentioning

confidence: 84%

Section: Shallow Crustal Earthquakes East Of the Cascadesmentioning

confidence: 99%

The December 1872 Washington State Earthquake

Bakun¹

2002

Bulletin of the Seismological Society of America

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“…Focal mechanisms of shallow earthquakes in the Puget Sound region indicate a consistent pattern of north-south compression in the upper plate . In contrast, the focal mechanisms of events in the subducting slab indicate down-dip tension (LUDWIN et al, 1991;MA and LUDWIN, 1987). This significant difference in tectonic stress orientation indicates that the plates are not mechanically coupled.…”

Section: Seismicity Of the Pacific Northwestmentioning

confidence: 99%

Intermittent Criticality and the Gutenberg-Richter Distribution

Bowman

Sammis

2004

Pure appl. geophys.

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In recent years there has been renewed interest in observations of accelerating moment release before large earthquakes, as well as theoretical descriptions of seismicity in terms of statistical physics. Most aspects of these works are encompassed by a concept called intermittent criticality in which a region alternately approaches and retreats from a critical -point. From this perspective, the evolution of seismicity in a region is described in terms of the growth and destruction of correlation in the stress field over the course of the seismic cycle. In this paper we test the concept of intermittent criticality by investigating the temporal evolution of the Gutenberg-Richter distribution before and after two successive M ‡ 5.0 earthquakes in western Washington State. The largest event in this distribution, M max , is observed to systematically increase before each event, producing accelerating moment release, and then to subsequently decrease. Associated variations in the b-value are minimal. This is the predicted result if M max is a measure of the correlation length of the regional stress field.

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“…Both the N-S strike of ubiquitous anticlines and thrust faults and the northwest strike of active left-lateral strike-slip faults in the accretionary complex offshore Oregon indicate that the maximum horizontal compressive stress is generally oriented E-W seaward of the backstop [Goldfinger et al, 1992[Goldfinger et al, , 1997. In contrast, the orientation of maximum horizontal compressive stress in the onshore forearc is N-S [Werner et al, 1991;Ludwin et al, 1991]. Moreover, northwest trending, left-lateral strike-slip faults that are common in the accretionary prism appear to terminate eastward near the seaward edge of the Siletz terrane [Goldfinger et al, 1997].…”

Section: The Siletz Terranementioning

confidence: 99%

Crustal structure beneath the central Oregon convergent margin from potential‐field modeling: Evidence for a buried basement ridge in local contact with a seaward dipping backstop

Fleming¹,

Tréhu²

1999

J. Geophys. Res.

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Abstract. Models of magnetic and gravity anomalies along two E-W transects offshore central Oregon, one of which is coincident with a detailed velocity model, provide quantitative limits on the structure of the subducting oceanic crust and the crystalline backstop. The models indicate that the backstop-forming western edge of the Siletz terrane, an oceanic plateau that was accreted to North America-50 million years ago, has a seaward dip of less than 60ø3. Seismic, magnetic, and gravity data are compatible with no more than 2 km of subducted sediments between the Siletz terrane and the underlying crystalline crust of the Juan de Fuca plate. The data also suggest the presence of a N-S trending, 200-km-long basaltic ridge buried beneath the accretionary complex from about 43øN to 45øN. Although the height and width of this ridge probably vary along strike, it may be up to 4 km high and several kilometers wide in places and appears to be locally in contact with the Siletz terrane beneath Heceta Bank. Several models for the origin of this ridge are discussed. These include: a sliver of Siletz terrane detached from the main Siletz terrane during a late Eocene episode of strike-slip faulting; imbrication and thickening of subducted oceanic crust in place; an aseismic ridge rafted in on the subducting oceanic crust during the past 1.2 million years; and a series of ridges and/or seamounts rafted in over a longer period of time and transferred from the subducting plate to the overlying plate. The last model is the most consistent with the complicated history of local uplift, subsidence, and slope instability recorded in the ridges, basins, and banks of this part of the margin. We speculate that the massive seaward dipping western edge of the Siletz terrane in this region inhibits subduction of seamounts and sediments, resulting in fomation of buried ridge as the accumulated flotsam and jetsom of subduction. This process may also be responsible for thickening of lower accretionary complex material, oversteepening of slopes leading to massive slumping, and north-south extension through strike-slip faulting in the accretionary complex to the west of the buried ridge. Regardless of its origin, the ridge may currently be acting as an asperity inhibiting subduction.

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Seismicity of Washington and Oregon

Cited by 46 publications

References 44 publications

The December 1872 Washington State Earthquake

The December 1872 Washington State Earthquake

Intermittent Criticality and the Gutenberg-Richter Distribution

Crustal structure beneath the central Oregon convergent margin from potential‐field modeling: Evidence for a buried basement ridge in local contact with a seaward dipping backstop

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