Analysis of the 1978 Brooks Peninsula, Vancouver Island earthquakes
| dc.contributor.author | Spindler, Christopher William Leonard | en_US |
| dc.date.accessioned | 2024-08-15T18:24:46Z | |
| dc.date.available | 2024-08-15T18:24:46Z | |
| dc.date.copyright | 1991 | en_US |
| dc.date.issued | 1991 | |
| dc.degree.department | Department of Physics and Astronomy | |
| dc.degree.level | Master of Science M.Sc. | en |
| dc.description.abstract | Two earthquakes occurred on the Brooks Peninsula on June 2nd and July 25th, 1978. They were the largest events to have been recorded in the region. Both were followed by extensive aftershock sequences and a field program was mounted to monitor part of the first sequence. Using the field program data for reference, the main shock epicentres were relocated and the teleseismic and regional location biases identified. The main shock epicentres were coincident within location uncertainties and located on the NW shore of the peninsula at a depth of between 10 and 20 km. Teleseismic epicentre solutions, using the Jeffreys Bullen model and all available data, misplaced the epicentres to the NE by 12 km. This was corrected by using only data from Delta greater or equal to 20°. The regional bias was on the order of 4-6 km to the east or west, depending on whether the Canadian Shield or Georgia Straits model was being used. The focal mechanisms, determined using a combination of first-motion and surface wave data, were found to be essentially identical. The preferred fault plane displayed sinistral, normal strike-slip faulting along a NE striking fault, dipping 30°-45° northwest. The other plane displayed dextral motion on a steeply dipping, north striking fault. There was no strong evidence to favour either plane as the fault plane. The pressure axes of the mechanisms were oriented NE-SW, consistent with present convergence vector of the Explorer plate with North America. Using the surface wave data, the seismic moments were found to be 1. 60 X 10^24 dyne·cm for June and 1.19 X 10^24 dyne·cm for July. The surface wave magnitudes were redetermined using additional data (June: 5.2, July 5.1) and agreed with the I.S.C. values. The stress drops were calculated from the far field displacement spectra and averaged 16 bars for both events. The rupture surface radii for June and July were found to be 3.5 km and 3.2 km respectively. Other earthquakes in the Brooks Peninsula region, spanning the period from 1978 to 1987, were relocated using the corrections developed from the aftershock data. The events were found to be concentrated off the NW shore of the peninsula and not spread over the entire region, as the original G.S.C. data indicated. The orientations of the pressure axes of the focal mechanisms suggest that the Brooks Peninsula earthquakes are directly related to the relative motions of the Explorer and North American plates. If this is also true of the regional seismicity, then the data imply that there is coupling between between the plates in the region, possibly due to the subsidence of the peninsula. The placement of the events in the overlying plate further implies that the plates are locked at the interface and that the stress across the interface has been transmitted into the interior of that plate. The relatively low stress drops indicate that the earthquakes did not take place on a new or well cemented fault and that motion on this fault is not uncommon. | |
| dc.format.extent | 185 pages | |
| dc.identifier.uri | https://hdl.handle.net/1828/19767 | |
| dc.rights | Available to the World Wide Web | en_US |
| dc.title | Analysis of the 1978 Brooks Peninsula, Vancouver Island earthquakes | en_US |
| dc.type | Thesis | en_US |
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