Teleseismic and near-field analysis of the Nahanni earthquakes in the Northwest Territories, Canada

Abstract

The analysis of the Nahanni earthquakes of October 5, 1985 (MS 6.6), and December 23, 1985 (MS 6.9), will have important implications for the assessment of seismic hazards in intraplate environments. To maximize the information available to seismic engineers, broadband data recorded teleseismically are analyzed jointly with strong-motion data recorded in the near field. The time-domain analysis of teleseismic data yields the source mechanisms, depths, and complexities of rupture of each earthquake. Both earthquakes occurred as shallow thrusts with centroid depths (6 to 7 km) and shallowly dipping fault planes that correspond well with the aftershock distributions obtained from a local survey run by the Canadian Geological Survey. The shallow nodal plane for the October 5 earthquake dips 30° to the WSW, while the shallow nodal planes of the subevents for the December 23 earthquake dip an average of 23° to the WSW. The October 5 earthquake has an impulsive initial rupture, followed by a weak subevent of longer duration but smaller moment release. The December 23 earthquake exhibits more complexity, being comprised of three subevents of similar size. The subevent delays derived from the teleseismic analysis are used to help interpret arrivals in records of ground velocity recorded in the near field of the December 23 earthquake. The rupture geometries inferred from the joint near- and far-field analysis suggest that the rupture processes were unusually complicated and that the 2g peak that occurs late in one of the near-field records could be a localized phenomenon. Spectral analyses of the teleseismic P waves yield the following source parameters for the October 5 and December 23 earthquakes, respectively: the seismic moments are 1.2 and 1.8 × 1026 dyne-cm, the radiated energies are 1.8 and 2.8 × 1021 dyne-cm, and the dynamic stress drops are 65 and 50 bar. The acceleration source spectra of both earthquakes exhibit an intermediate slope (| üα(ω) | ∝ω) from 0.03 to 0.3 Hz, suggesting that the earthquakes represent the failure of asperities. Extrapolating the teleseismic P-wave spectra to estimate the near-field S-wave spectra yields good fits to the acceleration spectra from two strong motion records, but underestimates the spectra from a third strong motion record with the strongest, but possibly localized, accelerations.