Wave Propagation Analysis of the SP Headwave Observed in the Charlevoix Seismic Zone and Its Application for Constraining Source Depth

Date: 4/25/2019

Time: 01:45 PM

Room: Pike

The Charlevoix seismic zone (CSZ) occurs along the ancient St. Lawrence rift zone in southeastern Quebec at the location of a Devonian impact crater. The crater is superimposed on major basement faults. Over 200 earthquakes are recorded each year in the CSZ and hypocenters have a bimodal distribution with peaks at 10 and 25 km depths. We observe a large, secondary P-wave arrival before the S-wave after a critical epicentral distance with a constant time difference relative to the first P wave arrival. We interpret the seismic phase as an SV to P (SP) head wave that propagates near the surface in the relatively homogeneous Charlevoix crust. Even though the amplitude of the SP headwave is smaller than the S wave amplitude, it could be confused with the S wave near the critical distance and misinterpreted as seismic anisotropy. To explore the characteristics of the SP headwave, we perform a wave propagation study for local earthquakes in the CSZ through computation of synthetic seismograms using wavenumber integration and generalized ray theory methods. In particular, we investigate the origin of the SP headwave using relative phase travel times and particle motions determined from a polarization filter. We also address the characteristics of the headwave for different source mechanisms, its response to different source depths and complex near-surface structures such as a low-velocity zone. We also investigate its use as an additional phase to constrain the focal mechanism. The travel time of the SP head wave relative to S offers a sensitive constraint on source depth for events in the seismic zone, validating earthquake locations. The formation of this seismic phase is of importance in understanding the structure of the CSZ and constraining earthquake source parameters.

Presenting Author: Oluwaseun I. Fadugba

Authors