First Advisor

Curt D. Peterson

Date of Publication

1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.) in Environmental Sciences and Resources: Geology

Department

Environmental Science and Management

Language

English

Subjects

Earthquakes -- Oregon, Faults (Geology) -- Oregon, Bays -- Oregon

DOI

10.15760/etd.1146

Physical Description

3, xi, 167 leaves (some folded): ill. 28 cm.

Abstract

Marsh paleoseismological studies were conducted in four bays (Necanicum, Nestucca, Siletz, and Yaquina) along the northern Oregon coast and compared with completed studies in two other bays (Netarts and Alsea). Coseismically buried peats were identified in all bays, based on 1) abrupt contacts, decreases in organic content, increases in sand content, increases in beach sand, and changes in diatom assemblages, all from the peat to the overlying sediments, 2) distinct sandy layers and key plant macrofossils, such as Triglochin, above the buried peat, and 3) widespread correlation of the buried peats within the bay. The stratigraphy and the ages and depths of the top six coseismically buried peats were compared between bays. The following similarities were noted: 1) All bays recorded five burial events in the top 2.6 meters within the last 2200 years. 2) Six burial events were recorded in six bays in the top 3.0 meters, except Alsea Bay (3.3 m), and all six events occurred within the last 2600 years except Yaquina (2780 years). 3) The depth to the top of each buried peat in the bays is consistent, falling within discrete ranges, except for the top two events at Yaquina. 4) Distinct sandy layers (tsunami-deposited) are present over the topmost buried peat in all bays except Yaquina and over the 4th in all bays except Yaquina and Nestucca. 5) Distinct tsunami-deposited sandy layers are absent over the third buried peat in Netarts, Nestucca, Siletz, Alsea, and possibly Yaquina, but present at Necanicum. The evidence strongly suggests synchroneity of coseismic events between the Necanicum River and Alsea Bay (a distance of 175 km), with the exception of the 2nd and 6th event. The 6th coseismic event would be synchronous between Alsea and Netarts, a distance of 105 km. The support for synchroneity of the 2nd event is weak. Synchroneity of coseismic burial events on the northern Oregon coast would argue for paleomagnitudes of at least 8.1 Mw, given a minimum rupture width of 50 km and a rupture length of 105 km. The paleomagnitudes were determined using the moment magnitude equation, Mw = 2/3 IOg10 Mo - 10.7 where Mo = shear modulus x rupture area x seismic slip. The seismic slip is estimated from a minimum recurrence interval of 300 years and a minimum convergence rate of 3.5 cm/yr.Marsh paleoseismological studies were conducted in four bays (Necanicum, Nestucca, Siletz, and Yaquina) along the northern Oregon coast and compared with completed studies in two other bays (Netarts and Alsea). Coseismically buried peats were identified in all bays, based on 1) abrupt contacts, decreases in organic content, increases in sand content, increases in beach sand, and changes in diatom assemblages, all from the peat to the overlying sediments, 2) distinct sandy layers and key plant macrofossils, such as Triglochin, above the buried peat, and 3) widespread correlation of the buried peats within the bay. The stratigraphy and the ages and depths of the top six coseismically buried peats were compared between bays. The following similarities were noted: 1) All bays recorded five burial events in the top 2.6 meters within the last 2200 years. 2) Six burial events were recorded in six bays in the top 3.0 meters, except Alsea Bay (3.3 m), and all six events occurred within the last 2600 years except Yaquina (2780 years). 3) The depth to the top of each buried peat in the bays is consistent, falling within discrete ranges, except for the top two events at Yaquina. 4) Distinct sandy layers (tsunami-deposited) are present over the topmost buried peat in all bays except Yaquina and over the 4th in all bays except Yaquina and Nestucca. 5) Distinct tsunami-deposited sandy layers are absent over the third buried peat in Netarts, Nestucca, Siletz, Alsea, and possibly Yaquina, but present at Necanicum. The evidence strongly suggests synchroneity of coseismic events between the Necanicum River and Alsea Bay (a distance of 175 km), with the exception of the 2nd and 6th event. The 6th coseismic event would be synchronous between Alsea and Netarts, a distance of 105 km. The support for synchroneity of the 2nd event is weak. Synchroneity of coseismic burial events on the northern Oregon coast would argue for paleomagnitudes of at least 8.1 Mw, given a minimum rupture width of 50 km and a rupture length of 105 km. The paleomagnitudes were determined using the moment magnitude equation, Mw = 2/3 IOg10 Mo - 10.7 where Mo = shear modulus x rupture area x seismic slip. The seismic slip is estimated from a minimum recurrence interval of 300 years and a minimum convergence rate of 3.5 cm/yr.

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