2012 Haida Gwaii Mw 7.7 Earthquake Response - Ocean Bottom Seismometer Relocation and Geophone Orientation Analysis and Quality Control of Wide-Angle P-wave Refraction Data.

Riedel, Michael , Côté, M. M., Neelands, P. J., Middleton, G., Standen, G., Iuliucci, R., Ulmi, M., Stacey, C. D., Murphy, R., Manning, D., Brillon, C. and Spence, G. D. (2014) 2012 Haida Gwaii Mw 7.7 Earthquake Response - Ocean Bottom Seismometer Relocation and Geophone Orientation Analysis and Quality Control of Wide-Angle P-wave Refraction Data. . Geological Survey of Canada, Open File , 7632 . , 79 pp. DOI 10.4095/295551.

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Abstract

Canada's second largest instrumentally recorded earthquake occurred on October 27th, 2012, off the west coast of Moresby Island, Haida Gwaii. Analysis of seismic waveforms and the pattern of aftershocks indicate that it was a thrust earthquake with a magnitude of Mw = 7.7. To accurately locate earthquakes, recording stations need to be positioned such that they surround the epicenter and the sound speed at which seismic waves travel through the earth's crust must be known. The land stations on Haida Gwaii are all to the east of the aftershock sequence, thus making these offshore earthquake locations uncertain and making depths for those events almost impossible to calculate accurately. Only sparse information from seismic refraction work conducted in the early 1980ies on the offshore velocity structure around the epicenter is known. Therefore, a series of 14 ocean bottom seismometers (OBS) were deployed for the duration of 1 month between December 6, 2012 and January 5, 2013 to record a portion of the aftershock sequence. An active source seismic program was conducted in January 2013 prior to the OBS recovery to acquire information on the sediment- and crustal structures to aid in the earthquake location analyses. However, the initial refraction data were not recorded on any of the 14 OBS as the batteries on all of the OBS stations had drained by the time of the survey. An extra set of six OBS was re-deployed and a total of four single-channel seismic profiles were acquired across these six OBS stations for a smaller-scale detailed refraction velocity experiment. Using the active source seismic data, each of the six OBS of the second deployment were re-located on the ocean floor, which is a critical pre-requisite for any velocity analysis. The OBS instruments drifted on average by 200m to the NW of the deployment drop-position. New offset information for the relocated OBS stations were calculated, and used for an initial exemplary but non-ray-tracing based 1D refraction velocity analysis on selected OBS stations to demonstrate the general utility of the OBS refraction data. Additional particle motion analyses were conducted for five of the six OBS stations of the second refraction survey to define the geophone orientation. One of these six stations (OBS-5) appeared to have identical horizontal components and therefore could not be used for a geophone-orientation determination. Long-range refraction seismic arrivals from the active-source experiment with a single 520 cubic inch G-gun were identified on some of the Haida Gwaii land-stations with offset ranges of up to 58 km. This information can potentially be used for a deep-crustal refraction velocity analysis. At this stage, only basic information on these land-station data is documented for potential future analyses.

Document Type: Report (Research Report)
Keywords: Haida Gwaii Mw 7.7 Earthquake ; seismicity
Open Access Journal?: Yes
DOI etc.: 10.4095/295551
Date Deposited: 05 Oct 2015 09:45
Last Modified: 05 Oct 2015 09:45
URI: http://oceanrep.geomar.de/id/eprint/29915

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