In this thesis, we present the first detailed, long-term seismotectonic studies of oceanic
ridge-transform systems. The proximity of the Juan de Fuca plate to a network of
broadband seismic stations in western North America provides a unique synergy of
interesting tectonic targets, high seismicity, and recording capabilities. Our main tools are
earthquake source parameters, determined by robust waveform modeling techniques, and
precise earthquake locations, determined by joint epicenter relocation. Regional broadband
data are used to invert for the source moment tensors of the frequent, moderate-sized (M ≥
4) earthquakes; this analysis began 1994. We include Harvard centroid moment-tensors
available since 1976 for larger (M ≥ 5) earthquakes.
Two studies comprise the main part of this thesis. In the first, we determine the current
tectonics of Explorer region offshore western Canada. Earthquake slip vector azimuths
along the Pacific-Explorer boundary require an independent Explorer plate. We determine
its rotation pole and provide a tectonic model for the plate's history over the last 2 Ma.
Plate motion changes caused distributed deformation in the plate's southeast corner and
caused a small piece in the southwest corner to transfer to the Pacific plate. Capture of the
plate fragment indicates that preserved fragments not necessary represent entire
microplates.
In the second study, we investigate seismicity and source parameters along the Blanco
Transform Fault Zone (BTFZ). The deformation style-- strike slip and normal faulting--
correlates well with observed changes in BTFZ's morphology. We infer that Blanco Ridge
probably consists of two fault segments, that several parallel faults are active along BTFZ's west part, and that Cascadia Depression possibly is a short spreading center. The slip
distribution along the BTFZ is highly variable, although seismicity could account for the
full plate motion rate along the entire BTFZ.
The final part is a short study where we locate an earthquake in the tectonically active
Mendocino triple junction region offshore northern California using land and offshore data.
The precise location may be useful as a master event for relocating other earthquakes. / Graduation date: 1999
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29264 |
| Date | 23 October 1998 |
| Creators | Braunmiller, Jochen |
| Contributors | Nabelek, John L. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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