M 7.6 - 99 km SE of Sand Point, Alaska

  • 2020-10-19 20:54:38 (UTC)
  • 54.602°N 159.626°W
  • 28.4 km depth

Finite Fault

 Contributed by US last updated 2020-10-21 03:19:57 (UTC)
  • The data below are the most preferred data available
  • The data below have been reviewed by a scientist

Scientific Analysis

We have chosen to model slip on the moderately dipping right-lateral strike-slip fault striking toward the NNW because this nodal plane fits the data better and is consistent with the early aftershock distribution. We note that the depth to the top of the slab at the location of the epicenter is about 26 km, yet our model strongly prefers slip at shallower depths. Rupture extending through the top of the slab is non-physical and is likely indicative of some greater rupture complexity (e.g. multi-fault rupture) not considered in this model.

Data Process and Inversion

We analyzed 83 teleseismic broadband P waveforms, 18 broadband SH waveforms, and 106 long period surface waves selected based on data quality and azimuthal distribution. Waveforms are first converted to displacement by removing the instrument response and are then used to constrain the slip history using a finite fault inverse algorithm (Ji et al., 2002). We begin modeling using a hypocenter matching or adjusted slightly from the initial NEIC solution ( location = 54.7°N, 159.7°W; depth = 40.1 km ), and a fault plane defined using either the rapid W-Phase moment tensor (for near-real time solutions), or the gCMT moment tensor (for historic solutions).

Result

This result is based on the moment tensor nodal plane ( strike = 346.0°; dip = 56.0° ). The seismic moment release based upon this plane is 2.9e+20 N-m (Mw = 7.6) using a 1D crustal model interpolated from CRUST2.0 (Bassin et al., 2000).

Cross-section of Slip Distribution

Slip distribution map image
Cross-section of slip distribution. The strike direction is indicated above each fault plane and the hypocenter location is denoted by a star. Slip amplitude is shown in color and the motion direction of the hanging wall relative to the footwall (rake angle) is indicated with arrows. Contours show the rupture initiation time in seconds.

Surface Projection

Finite fault basemap
Surface projection of the slip distribution superimposed on GEBCO bathymetry. Thick white lines indicate major plate boundaries [Bird, 2003]. Gray circles, if present, are aftershock locations, sized by magnitude.

Moment Rate Function

Moment rate function image
Source time function, describing the rate of moment release with time after earthquake origin, relative to the peak moment rate (listed in the top right corner of the plot). The red dashed line represents the interpreted end of the event.