Triggered Seismicity

A Magnitude 8.2 earthquake struck Chignik, Alaska at 2021-07-29 06:15 am UTC. In this study I Investigate the triggered seismicity due to this earthquake at 4 sites along US west coast - 1) Mount Rainier 2) Geysers 3) Coso and 4) Long Valley Caldera

Mount Rainier

Mount Rainier is an active volcano located in the central washington. There have been several reports of triggered seismicity caused by dynamic stress changes from surface waves resulting from local and large distant earthquakes. Waveforms of one hour window since the origin of earthquake from all the stations lying within 100 Kms of Mount Rainier peak were downloaded, resulting in combined 34 stations and all the three channels from 'UO','UW' and 'CC' networks. 10 of these stations were Broadband and the rest were short period stations.

When the waveforms were bandpass filtered between (2-8Hz), a single consistent event appeared to arrive near the timing of arrivals of Rayleigh waves. This observation was consistent in several stations, the spectrograms of the waveforms futher confirmed the arrival of these event. The probability of arrival of an earthquake in a given hour near Mount Rainier was determined to be less than 1 percent additionaly confirming that this event was a triggered tremor due to surface waves from the Chignik earthquake This event was not found to be reported by any catalog. No delayed response or change in seismicity was observed at this site.

Geysers

Geysers geothermal field had a very clear evidence of triggered seismicity. waveforms were downloaded for 16 statios lying within 100 Kms of the geothermal field. All the stations were from BK(Berkeley) seismic network and consisted of short period sensors.

A series of 26 tremors were observed in the span of 10 minutes from 6:25am to 6:35am UTC, they were split in two groups which were seprated by a interval of 100s. First group contained 14 tremors which were slightly large in amplitude than the amplitudes of tremors in other group. There appears to be one to one correspondence between the Rayleigh wave phases with negative ground amplitude of more than 0.5mm and the triggered earthquakes indicating a threshold limit. The first group was large enough to be reported in the 'NEIC' catalog. These events had a maximum magnitude of 3 and their locations showed linear trend suggesting a common source. The probability of 6 or more events in such a short span of time is close to 0 assuming poisson distribution. No delayed response was observed.

Coso

Coso geothermal field did not show any clear and consistent evidence of triggered seismicity. data from 27 stations within the field were analyzed. 26 were from CI (Southern California) network, one was from NN(Nevada) seismic network. All the stations had a short period sensors. No consistent observation near the time of arrivals of Rayleigh waves were seen. There were however, swarm of earthquakes were observed in some stations. eg CI.ISA, It remained unclear that there was any triggered seismicity during the passage of Rayleigh waves. No events were reported either in 'NEIC' or 'ISC' catalog during this time. The catalog however showed a rise in seismicity nearly 9 hours after the mainshock suggesting there could be delayed response as the probability of 6 events in the given hour is less than 1 percent.

Long Valley Caldera

Long Valley Caldera also did not show any clear evidence of the triggered arrivals. Data was obtained from 17 stations; 4 from BK(Berkeley), 2 from CI(California), 1 from IM(International Mischellaneous), 5 from NC (Northern California) and rest were from NN(Nevada) Network. No consistent observation around the arrival of Rayleigh waves was found. The stations present on the eastern side of caldera however, showed the evidence of rigorous swarm activity going before and after the event. Seismicity in this swarm appeared to decrease after the mainshock. None of the earthquakes in this swarm were reported in 'NEIC' and 'ISC' catalog during one hour before and after the mainshock.

References

West, Michael, John J. Sánchez, and Stephen R. McNutt. "Periodically triggered seismicity at Mount Wrangell, Alaska, after the Sumatra earthquake." Science 308, no. 5725 (2005): 1144-1146.

Prejean, S. G., D. P. Hill, E. E. Brodsky, S. E. Hough, M. J. S. Johnston, S. D. Malone, D. H. Oppenheimer, A. M. Pitt, and K. B. Richards-Dinger. "Remotely triggered seismicity on the United States west coast following the M w 7.9 Denali fault earthquake." Bulletin of the Seismological Society of America 94, no. 6B (2004): S348-S359.

Hill, David P., P. A. Reasenberg, A. Michael, W. J. Arabaz, G. Beroza, D. Brumbaugh, J. N. Brune et al. "Seismicity remotely triggered by the magnitude 7.3 Landers, California, earthquake." Science 260, no. 5114 (1993): 1617-1623.