Recently, scientists examined the differences in coseismic velocity changes observed during earthquakes using ambient seismic noise, shedding light on seismic hazard assessments.
The study focused on post-earthquake variations caused by the 2016 Gyeongju and Pohang earthquakes in South Korea, alongside two significant seismic events recorded across the sea—the 2016 Kumamoto and Tottori earthquakes in Japan. These four earthquakes offer unprecedented insights, particularly as South Korea has limited past experience with destructive seismic activity.
Research indicates substantial variances between the two countries' seismic responses, where velocity reduction concerns arise largely due to the magnitude of earthquakes. The Pohang and Gyeongju events (M 5.4 and M 5.8) presented different outcomes compared to Kumamoto (M 7.3) and Tottori (M 6.6). While the velocity decrease related closely to higher magnitude tremors, the South Korean earthquakes exhibited patterns influenced by factors such as aftershock activity and local noise.
By utilizing moving window cross-spectral analysis, the researchers correlated seismic signals from records and identified deviations in velocity changes across various frequencies. This technique allowed for filtering noise interference and offered more reliable outcome measurements than previous techniques.
One notable observation was the significant asymmetry found when analyzing cross-correlation directions—an outcome not previously documented for seismic events within Korea. The authors noted, “The average velocity decrease between forward and reverse order pairs correlated well with earthquake magnitude, except in the case of the Korean earthquake.” This intriguing finding contributes to the academic conversation around the role of geographic and geological differences between regions when assessing seismic impacts.
The study also brought to light the impact of aftershocks on measurement accuracy, as velocity changes were obscured by long-term interference from these smaller tremors. Continuous monitoring of ambient seismic noise has become increasingly significant for post-event assessments—especially for places previously untouched by substantial earthquake events.
“We examined and compared two earthquakes,” the team explained, “which, though geographically close, display distinct seismicity patterns.” This research not only enhances the scientific community's narrative on seismic studies but also emphasizes the necessity for more proactive data collection methods from regions like South Korea, where earlier data was scarce.
On concluding their comprehensive analysis, the authors underscored the importance of consistent monitoring for future earthquake hazard predictions. Their findings highlight the pressing need for designing improved measurement systems able to discern minor changes even as seismic activity fluctuated greatly.
With increasing seismic activity across both regions, the discoveries affirmed by this study provide useful groundwork to build future studies and may also guide policymakers as they formulate disaster preparedness strategies.