Aftershock Evolution of the 2017 M5.5 Pohang Earthquake: A Possible Post-Seismic Relaxation within Heterogeneous Fault System

Session: Earthquake Source Parameters: Theory, Observations and Interpretations
Type: Oral
Date: 4/30/2020
Time: 08:45 AM
Room: 120 + 130
Description: 

Physical mechanisms involved in foreshock-mainshock-aftershock sequences are described by complex characteristics of tectonic stress field, fault heterogeneity and fault geometries. The 2017 M_W 5.5 Pohang earthquake sequence occurred as the reactivation of an almost critically stressed fault system affected by the fluid injections. For better understanding the fault behavior in response to perturbed stress, the characteristics of earthquake sequence should be comprehensively analyzed. In this study, we analyzed waveform data recorded by temporary seismic stations and developed an aftershock catalog for the 2017 Pohang earthquake sequence using two different detection methods. We then examined the statistical parameters of aftershocks (b and p values) and mapped their spatio-temporal distributions. Aftershock distribution shows that there exist at least four fault segments and three largest earthquakes (M>4) occurred on the contact boundaries between two fault segments. We infer that stress concentration at junctions of two fault segments resulted in the relatively large earthquakes and the reactivation of multiple fault segments may explain high portions of non-double-couple component, which are observed for the three largest earthquakes. Aftershock zone expanded from the hypocenter of the mainshock to NE and SW directions by constant speeds in both directions with logarithmic time scale, which indicates a possible post-seismic deformation or a cascade of earthquake triggering mechanisms. With local heterogeneity, observed b values were generally low throughout the aftershock area, which is consistent with the observation of the critically stressed fault system. The p values are varying in strike direction, which may be related to the different expansion speeds to NE and SW directions in logarithmic time scale. Investigation of fault properties using plenty of aftershocks not only provides information on circumstantial characteristics of the fault system but can be applied to evaluate seismic hazard in the study area by using the obtained statistical parameters.

Presenting Author: Jeong-Ung Woo

Authors