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A re-examination of the August 22, 1949 Queen Charlotte earthquake

University of British Columbia

Using previously unavailable horizontal seismograms recorded at Sitka, Alaska, thirty-eight new aftershock locations were determined for the August 22, 1949 Queen Charlotte earthquake (Ms=8.1). The aftershock zone was found to extend from 300 km to the north of the epicenter to 190 km south of the epicenter, yielding a total aftershock zone of 490 km. This aftershock zone implies that a previously suggested seismic gap to the north of the Ms=8.1 earthquake (Rogers, 1983) does not exist. The aftershock distribution suggests a time variation of the rupture sequence, with the aftershocks clustering first to the north, and then to the south of the epicenter. The directivity function and differential phases were analyzed at three stations. The results imply a unilateral rupture propagating to the .northwest for 265 km at a rupture velocity between 3.1 km/s and 3.5 km/s. The difference between the radiation length and the aftershock zone implies that the radiation fault length does not represent the full rupture fault length. The non-equivalence of the radiation fault length and the rupture fault length suggests that the displacement offset along the fault was uneven, with the largest displacement occurring in the zone indicated by the radiation fault length. An attempt was made to derive the mechanism solutions for the two largest aftershocks using their azimuthal surface wave radiation patterns. It was concluded that the use of this technique to obtain focal mechanism solutions is ineffective for this area and time in history when station coverage was sparse and the quality of instrument calibrations poor.

Text

2010-05-10

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http://hdl.handle.net/2429/24553

Geophysics

University of British Columbia

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