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

Abstract

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.

Quaternary Rupture of a Crustal Fault beneath Victoria, British Columbia, Canada

“…1 and 2A) (see Cassidy et al, 2000;Mosher et al, 2000;Balfour et al, 2011). Several previous authors suggest that this fault, which places Jurassic-Cretaceous schists of the Leech River Complex to the north against Eocene basalts of the Metchosin Formation to the south ( Fig.…”

Earthquake Ground Motion and 3D Georgia Basin Amplification in Southwest British Columbia: Shallow Blind-Thrust Scenario Earthquakes

“…Slip decreases up-dip of the 5 km hypocenter and reaches zero value at 1.25 km depth, that is, a blind-thrust event. The preferred direction of rupture here is down-dip based on temporal aftershock sequences (Amadi, 1992;Dragovich et al, 1997;Cassidy et al, 2000). A uniform 3:0 km=s rupture velocity is used, as determined for the Northridge event (Wald et al, 1996), which is ∼80% of the local V S (Graves and Pitarka, 2004) in the 3D Georgia basin model used here.…”

“…(b) Cross-section plots of clustered seismicity and corresponding lists of the largest events (from left to right, modified from Cassidy et al, 2000, and Dragovich et al, 1997. Italicized events in list are not shown.…”

Wide-angle seismic recording from the 2002 Georgia Basin Geohazards Initiative, northwestern Washington and British Columbia

“…From Julian Day 138 to 139 the Tully collected 20 lines in the Georgia Strait. Between Julian Day 139 and 141 the Tully collected 20 lines in the epicentral region of a 1997 earthquake (Cassidy et al, 2000). From Julian Day 141 to the end of the survey on Julian Day 146, the Tully collected 100 lines in a 5-km by 10-km 3-D seismic survey of a pockmark field thought to have formed along an active crustal fault.…”

“…In the past decade seismic hazards posed to northwestern Washington and southern British Columbia by crustal faulting have been recognized (e.g., Pratt et al, 1997;Cassidy et al, 2000;Mosher et al, 2000;Johnson et al, 2001). The existence of crustal faults capable of large (M~7) magnitude earthquakes within Puget Lowland has been inferred and mapped using a variety of methods including paleoseismic, seismicity, seismic reflection, and potential field geophysical data (Atwater and Moore, 1992;Bucknam et al, 1992;Johnson et al, 1994Johnson et al, , 1996Pratt et al, 1997;Wells et al, 1998;Blakely et al, 2002;ten Brink et al, 2002).…”