This thesis presents the results of three independent but related studies of aspects of the Mw 7.8 2012 Haida Gwaii earthquake, which was the second largest Canadian earthquake in recorded history. This event ruptured an area of roughly 150 by 40 km on a gently northeast-dipping thrust fault off the west coast of Moresby Island, British Columbia. This event was felt over 1600 km away from the epicentre, and produced tens of thousands of aftershocks. Adjacent to the mainshock fault plane is the Queen Charlotte Fault, the site of the largest event recorded in Canada: the 1949 Ms 8.1 strike-slip earthquake whose rupture extended as far south as this 2012 event and roughly as far north as an Mw 7.5 strike slip event which occurred on 5 January 2013. The 2012 thrust event was a surprise to some members of the seismological community as it ruptured a slab offshore of a major strike slip boundary. This earthquake therefore presents an excellent opportunity to constrain the tectonics and seismic hazard off the northwest coast of British Columbia.
Herein a Coulomb stress transfer analysis is performed using finite fault models which incorporate both seismological and geodetic data. Static stress changes are projected onto optimally-oriented fault planes, determined using regional tectonic stresses in addition to mainshock stress; nodal planes, determined by aftershock centroid moment tensors; and onto the Queen Charlotte Fault. I find that aftershocks are generally consistent with Coulomb stress changes using optimal planes and known nodal planes, although the latter have slightly higher percentages of events consistent with triggering. I find that the Queen Charlotte Fault experienced stress changes greater than the empirically-determined threshold for triggering. This is particularly important as the southern extent of this fault is believed to lie in a seismic gap going back at least 116 years. With added stress from the mainshock and a lack of post-mainshock seismicity occurring in this seismic gap, it is a likely location for future earthquakes on this portion of the plate boundary.
To obtain estimates of rupture parameters, an empirical Green's function technique and directivity analysis is performed. This method constrains rupture kinematics of the mainshock using systematic azimuthal variations in relative source time functions. My results indicate a rupture that progressed mainly to the northwest and updip. Subevent analysis confirms the existence of at least two subevents, with the first being roughly twice as large as the second. The results herein are similar to those found using finite fault inversion, but are better able to explain observed surface wave amplification at Alaskan seismic stations. My findings help support the idea that strong surface wave shaking may have resulted in delayed-onset dynamic triggering of the 2013 Craig event, through an unknown but intermediate mechanism that accounts for the two-month hiatus.
Finally, an attempt was made to relocate all offshore aftershocks for this sequence by improving locations for events during a two-week ocean bottom seismometer deployment. This dataset includes a wider range of source-station azimuths and decreases the minimum source-receiver distance, relative to locations that only use land stations. My locations therefore represent the best-constrained depths for M greater than or equal to 3 offshore aftershocks occurring during the two-week deployment, and help constrain reasonable depth estimates from other relocation techniques. For events located using only ocean bottom seismometers I determine the time residual between observed and predicted phase arrivals at land stations to be used as a correction for all aftershocks recorded at land stations through the entire aftershock sequence. Although I was not able to find consistent time residuals I present suggestions for future implementations of this technique to this dataset, and discuss challenges associated with location of offshore earthquakes in regions with sparse regional seismic networks.
All of these findings contribute to a more thorough understanding of this 2012 earthquake, as well as the tectonics of southern Haida Gwaii. I pay particular attention to identification of hazard within a seismic gap south of Moresby Island, and the northwest rupture directivity of the 2012 mainshock. / Graduate / 0373 / tiegan.hobbs@gmail.com
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/5509 |
| Date | 06 August 2014 |
| Creators | Hobbs, Tiegan Elizabeth |
| Contributors | Cassidy, John Francis, Dosso, Stanley Edward |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | Available to the World Wide Web, http://creativecommons.org/licenses/by-nc-nd/2.5/ca/ |
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