The 1 May 2017 British Columbia‐Alaska Earthquake Doublet and Implication for Complexity Near Southern End of Denali Fault System

• Published in: Geophysical research letters 2018-06, Vol.45 (12), p.5937-5947
• Main Authors: He, X., Ni, S., Zhang, P., Freymueller, J.
• Format: Article
• Language: English
• Subjects: Basins; Complexity; Denali fault system; Dipping; earthquake doublet; Earthquakes; Fault lines; Focal plane; Geological faults; Geological hazards; Global positioning systems; GPS; Modelling; Planes; Regional analysis; Rivers; Rupture; rupture directivity; Rupturing; Seismic activity; Seismic hazard; Seismic waves; Slip; Solutions; Water pollution; Waveforms
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL078014

On 1 May 2017, two MW6.2 earthquakes occurred near the border of northwestern British Columbia and Alaska separated by about 2 hr in time. Despite their close distance (~10 km), the two events have different focal mechanisms, with the first featuring a reverse focal mechanism and the second strike slip. Both focal plane solutions are inconsistent with the nearby southeastern Denali fault system. To resolve their ruptured fault planes, we invert for the earthquake point source parameters and analyze rupture directivity via regional and teleseismic waveforms. We also model the near‐field GPS data and relocate the aftershocks to determine the fault planes. The results indicate that the first event ruptured updip along a steep SW‐dipping fault and the second event ruptured to the ESE along a left‐lateral fault. We infer that the earthquake doublet was related to the regional stress field and slip on the active Duke River fault. The involved faults are associated with transpression caused by the oblique collision of the Yakutat block. Plain Language Summary The Denali fault is one of the most prominent structural features in North America and has been assigned high hazard in probabilistic seismic hazard maps. But, the seismic risk near the southeastern end of Denali fault is not well understood. On 1 May 2017, two MW6.2 earthquakes occurred near the border of northwestern British Columbia and Alaska, with an interval of less than 2 hr. The two earthquakes are located within ~10 km, but their focal mechanisms are remarkably different, with the first featuring a reverse focal mechanism and the second strike slip, suggesting tectonic complexity in this region. In this paper, we resolved the ruptured planes and rupture direction of two events via modeling seismic waveforms and near‐field GPS data. We found that the first event ruptured upward along a very steep SW‐dipping fault, which is very unusual as compared with most reverse earthquakes with dip angle less than 45°. The second event ruptured to the ESE along a left‐lateral fault, and it probably occurred on an unmapped fault. This study helps understand the complex seismotectonics near the termination of large strike‐slip faults and implies that the potential seismic hazard around southeastern Denali fault system cannot be neglected. Key Points Two M6+ earthquakes (EQ 1, EQ 2) with different focal mechanisms occurred near the southeastern termination of the Denali fault system We invert for the point source pa