Moment tensors, state of stress and their relation to post-glacial rebound in northeastern Canada

Abstract

SUMMARY In the stable craton of northeastern Canada moderate seismicity as well as glacial isostatic adjustment (GIA) has been observed. We investigate five earthquakes with moment magnitudes between 3.6 and 4.1 that have occurred in northern Hudson Bay since 2007, which may be triggered by GIA. Focal mechanisms of the earthquakes are determined using a waveform‐fitting procedure for surface waves, in which the best double‐couple mechanism is obtained through a grid search over strike, dip, and rake. All events exhibit a thrust‐fault mechanism, in general agreement with mechanisms of previously analysed earthquakes in that area. Stress‐inversion results incorporating our new mechanisms as well as previously published results exhibit significant differences from previously calculated stresses from GIA models. For the first time, maximum horizontal stress direction SHmax is compared to the modelled stress values for northeastern Canada. We find that the maximum horizontal stress direction strikes roughly NNW–SSE, in contrast to previously inferred NE‐SW orientation of SHmax. Our results indicate that the local stress field in northern Hudson Bay deviates from regional stresses resulting from the existence of a roughly E–W oriented zone of faulting, which is not included in existing GIA models.

Stress and fault parameters affecting fault slip magnitude and activation time during a glacial cycle

“…Observation and models thus indicate that GIA transformed a stable area into a tectonically active zone; however, models show that main activity terminates after only one or two earthquakes. Present‐day observations show that seismicity still exists in formerly glaciated regions, but the magnitudes are mostly below 4.0 [e.g., Lund et al , ; Bungum et al , ; Steffen and Wu , ; Steffen et al , ]. Present‐day activity may actually be long‐lived aftershock sequences [e.g., Stein and Liu , ].…”

Crustal anisotropy beneath Hudson Bay from ambient noise tomography: Evidence for post‐orogenic lower‐crustal flow?

“…Within the Bay the maximum stress direction is generally NE‐SW, although stress‐field directions have changed in the last 9000 years due to glacial rebound stress following the last ice age [ Wu , 1996]. More recent studies based on moment‐tensor inversion from local earthquakes have refined the picture of the stress field in northern Hudson Bay [ Steffen et al , 2012]. Using data from five earthquakes, Steffen et al [2012] show NNW‐SSE directed maximum horizontal stress direction, which is inconsistent with the principal anisotropic fast directions from our study.…”

A revised crustal stress orientation database for Canada

“…New data are also added since data published elsewhere and from a previously unavailable dataset that had been maintained by the Alberta Geological Survey (AGS). The published data are compiled from a literature investigation of the following articles by Adams (1987), Balfour et al (2011, Bell and Bachu (2003), Buchbinder (1990), Du et al (2003), Eisbacher and Bielenstein (1971), Hamid (2008), Hurd and Zoback (2012), Konstantinovskaya et al (2012), Kim (2003), Kim et al (2006), Ma et al (2008), Mazzotti and Townend (2010), Michael and Buschkuehle (2008), Ruppert (2008), Steffen et al (2012), StLouisEQcenter (2010), Yassir and Dusseault (1992), and Zakharova and Goldberg (2014). The update adds up to 514 new records in Canada (Fig.…”