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Date Permissions Signed
Date of Award
Master of Science (MS)
Engebretson, David C.
Burmester, Russell F.
Easterbrook, Don J., 1935-
Field, John J. (John Jacob), 1961-
Records show that thousands of earthquakes have occurred in the northern Puget Lowland since 1969. At least fifteen of these events have been greater than magnitude 4, and one (Deming mag. 5.2, 1990) was the second largest recorded shallow crustal event in Washington for the last hundred years. Despite the evidence that suggests that the potential for great earthquakes capable of severe damage in the northern Puget Lowland is very real, the seismicity in the area is poorly understood. This is in part due to the wide scattering and apparently random assortment of recorded events. This thesis attempts to create a better understanding of the structures along which these earthquakes occur. A better understanding the active structures can help to better evaluate the full potential for destructive earthquakes.
Conclusions in this thesis are based primarily on historical seismic records from the Washington Regional Seismic Network, and include earthquake activity from 1969 to 1995. Focal mechanisms from the Washington Regional Seismic Network and Western Canadian Telemetered Network databases are for seismic events that were recorded at enough stations to meet quality standards. The records used were limited to events with foci within the upper 30 km of the North American crust and lie within the boundaries of the northern Puget Lowland.
Depth distributions of earthquake foci were used to determine the extent and location of seismic deformation. Results show that events are concentrated within two general depth intervals in the crust. This bimodal distribution includes a shallow concentration between the surface and 5 km depth and another concentration at approximately 17+/- 3 km. Depth distribution across the region suggests that a wedge-shaped body is being deformed seismically. This wedge-shaped body roughly conforms to low temperature regions defined by geothermal modeling for the region, suggesting a direct correlation between low heat flow and seismic deformation.
This thesis introduces a modified technique of fault classification that can be used to analyze focal mechanisms for a region as a group, rather than just an event-by-event basis. The plunges of the pressure and tension axes from focal mechanisms were used to classify earthquakes by fault type. The results were consistent with the study region being in a transpressive regime, with 74% of the events being clearly reverse and strike-slip. However, deformation is not uniform. A depth distribution plot shows a change from predominantly reverse faulting above 20 km to predominantly strike-slip faulting below 20 km. This could be the result of a swapping of σ2 and σ3 due to an increased lithostatic load, the result of faulting along preexisting zones of weakness, or the result of spatial distribution of both.
Maps of focal mechanisms of a single fault type helped to define possible trends in an otherwise seemingly random assortment of focal mechanisms. Right lateral strike-slip faulting between 20 and 30 km along the trend of the southern Whidbey Island fault was most apparent. Shallower than 20 km, the stresses appear diffused into the northern part of the study region that forms the wedge.
A statistical Chi-square analysis for planar distributions of earthquake foci was developed and used over the entire region to investigate possible correlations with maps of focal mechanisms. Planar distributions that met statistical criteria that indicated they are significant show some interesting patterns. The eastern portion of the Devil's Mountain fault had planar distributions that aligned with focal mechanisms showing shallow strike slip and reverse faulting. Clear reverse faulting around 15 km depth along the trace of the Vedder Mountain lineament was also observed.
Northern Puget Lowland, Washington Regional Seismic Network
Western Washington University
Subject – LCSH
Seismic networks--Washington (State)--Puget Sound Lowland; Earthquake hazard analysis--Washington (State)--Puget Sound Lowland; Faults (Geology)--Washington (State)--Puget Sound Lowland; Geology, Structural--Washington (State)--Puget Sound Lowland; Earthquakes--Washington (State)--Puget Sound Lowland
Puget Sound Lowland (Wash.)
Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.
Roberts, Lori K. (Lori Kelly), "Seismic Characterization of the Northern Puget Lowland, Washington" (1999). WWU Graduate School Collection. 839.