Digital strong ground motion recordings of the June 1997 Strait of Georgia earthquake

Cassidy, John F.; Rogers, G C; Weichert, D. H.; Little, T E

doi:10.4095/209892

Cited by 2 publications

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“…Note that these data sets all represent "weak" ground motion, with peak ground accelerations of 2.5-5.3% gravity (g), 0.5-1.5% g, 0.2-2.4% g, and 0.3-3.5%g, respectively. Details of these data sets are given in [9], [10], [11], and [12], respectively. Analysis of these data sets, summarized in this article, are presented in [13], [14], and [15].…”

Section: The Data Setsmentioning

confidence: 99%

Variation in ground shaking on the Fraser River delta (Greater Vancouver, Canada) from analysis of moderate earthquakes

Cassidy

Rogers

2004

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The thick, soft soils of the Fraser River delta, just south of Vancouver, Canada, are home to critical infrastructure such as one of North America's busiest port facilities, Canada's second busiest airport, and key transportation and power-transmission facilities for 2-3 million people. This area is one of the most seismically active regions in Canada. We have utilised recordings of recent moderate (1996 M=5.1 at 180 km distance, 1997 M=4.3 at 40 km distance) and large (2001 M=6.8 at 230 km distance) earthquakes to examine site response in the greater Vancouver, region, with an emphasis on the Fraser River delta. These suites of accelerograms have relatively low amplitudes (0.015g for the 1996 records, 0.024g for the 1997 records, and 0.035g for the 2001 records). The 1997 data set is significant as it contains the first three-component recordings made on bedrock in greater Vancouver, and the 2001 data set is significant as it contains longer-period energy. We compute spectral ratios to estimate the site response for each of the soil sites. We find frequency-dependent amplification of up to 12 times (relative to competent bedrock) near the edge of the delta. Here, the amplification occurs over a relatively narrow frequency range of 1.5-4 Hz (0.25-0.67 s period). Near the centre of the delta (where the soft soils are thickest) peak amplification of 4-10 times (relative to bedrock) is measured. Relative to firm soil, the amplification ranges from 2-5 for the thick soil delta centre sites, and 2-6 for the delta edge sites. At higher frequencies, little or no amplification or even slight attenuation is observed. The Geological Survey of Canada is currently deploying a dense urban seismograph network (~1km spacing) which crosses the northern edge of the Fraser delta to address varying site response in more detail.

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Section: The Data Setsmentioning

confidence: 99%

Variation in ground shaking on the Fraser River delta (Greater Vancouver, Canada) from analysis of moderate earthquakes

Cassidy

Rogers

2004

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“…For more information on the strong-motion data sets and their interpretation, see Cassidy et al (1998) and Cassidy and Rogers (1999).…”

Section: The Data Setmentioning

confidence: 99%

Characterization of Active Faulting Beneath the Strait of Georgia, British Columbia

Cassidy¹,

Rogers²,

Waldhauser³

2000

Bulletin of the Seismological Society of America

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Southwestern British Columbia and northwestern Washington State are subject to megathrust earthquakes, deep intraslab events, and earthquakes in the continental crust. Of the three types of earthquakes, the most poorly understood are the crustal events. Despite a high level of seismicity, there is no obvious correlation between the historical crustal earthquakes and the mapped surface faults of the region. On 24 June 1997, a M L ‫ס‬ 4.6 earthquake occurred 3-4 km beneath the Strait of Georgia, 30 km to the west of Vancouver, British Columbia. This well-recorded earthquake was preceded by 11 days by a felt foreshock (M L ‫ס‬ 3.4) and was followed by numerous small aftershocks. This earthquake sequence occurred in one of the few regions of persistent shallow seismic activity in southwestern British Columbia, thus providing an ideal opportunity to attempt to characterize an active near-surface fault. We have computed focal mechanisms and utilized a waveform cross-correlation and joint hypocentral determination routine to obtain accurate relative hypocenters of the mainshock, foreshock, and 53 small aftershocks in an attempt to image the active fault and the extent of rupture associated with this earthquake sequence. Both Pnodal and CMT focal mechanisms show thrust faulting for the mainshock and the foreshock. The relocated hypocenters delineate a north-dipping plane at 2-4 km depth, dipping at 53Њ, in good agreement with the focal mechanism nodal plane dipping to the north at 47Њ. The rupture area is estimated to be a 1.3-km-diameter circular area, comparable to that estimated using a Brune rupture model with the estimated seismic moment of 3.17 ‫ן‬ 10 15 N m and the stress drop of 45 bars. The temporal sequence indicates a downdip migration of the seismicity along the fault plane. The results of this study provide the first unambiguous evidence for the orientation and sense of motion for active faulting in the Georgia Strait area of British Columbia.

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Digital strong ground motion recordings of the June 1997 Strait of Georgia earthquake

Cited by 2 publications

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Variation in ground shaking on the Fraser River delta (Greater Vancouver, Canada) from analysis of moderate earthquakes

Variation in ground shaking on the Fraser River delta (Greater Vancouver, Canada) from analysis of moderate earthquakes

Characterization of Active Faulting Beneath the Strait of Georgia, British Columbia

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