Repeating Earthquakes on the Queen Charlotte Fault near Haida Gwaii, B.C.

Abstract:

Major transcurrent plate boundaries can host both low-frequency earthquakes (Shelly, 2015) and repeating regular earthquakes (Nadeau et al., 1995), manifesting distinct, depth-dependent frictional regimes that may be separated by as little as 3 km. An improved understanding of the physical properties that lead to either style of seismicity will result from comparative analysis of both phenomena including their spectral and recurrence characteristics (Plourde et al., 2015). Triggered tremor has been observed on the Queen Charlotte Fault (QCF) (Aiken et al., 2013) and in this study we document the occurrence and characteristics of repeating regular earthquakes in the same region. The QCF is a major transcurrent fault located off the northwest coast of North America, and is part of the plate boundary between the North American and Pacific Plates. This region has produced large earthquakes in 1949 (M8.1) and more recently in 2012 (M7.8). Due to poor station coverage and data quality, traditional methods of locating earthquakes are not applicable to the repeating earthquakes of interest. Instead, we have implemented an algorithm to locate local (i.e., < 100 km distance to epicenter) earthquakes using a single, three-component seismogram. This algorithm relies on P-wave polarization and, through comparison with larger local events in the Geological Survey of Canada catalogue, is shown to yield hypocentral locations accurate to within 5-10 km. A total of 24 unique families of repeating earthquakes has been identified to date. Their epicenters locate directly on the trace of the QCF and their depths are shallow (i.e., 5-15 km), consistent with the proposed depth of the QCF. Analysis of temporal recurrence leading up to the 2012 M7.8 event reveals a non-random pattern, with an approximately 15 day periodicity. Further analysis is planned to study whether this behaviour persists after the 2012 event, and whether the recurrence time of the repeating events can be related to their moment. We will also investigate whether the repeating earthquakes relate to previously identified triggered tremor, and aim to gain insight into the effects of the 2012 event on the stress field and frictional properties of the plate boundary.