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Stanford University researchers identified more than 182,000 small seismic events in six years in South Korea—135,000 of which were related to mining explosions.
In a presentation at the Seismological Society of America (SSA)’s 2023 Annual Meeting, Jeong-Ung Woo and colleagues explained that after using machine learning techniques to detect tiny earthquakes in seven years’ worth of data collected from 421 seismic stations in the country, they found distinct patterns in event times and locations that allowed them to identify which microseismic events were associated with mining operations.
Microseismicity usually refers to earthquakes of magnitude 2.0 and smaller.
Woo noted that South Korea is a relatively quiet country, seismically speaking, because it is located in the middle of a tectonic plate. However, seismicity between 2016 and 2022 was mostly occurring in the daytime, because mining operations are usually occurring in the daytime.
Seismicity patterns also varied between summer and winter, due to how different sunrise and sunset times affected mining operations. The records show a distinct drop in seismic events on Sundays when the mines are traditionally closed.
Natural seismicity doesn’t follow such regular timing, so microseismicity could be used as “very strong evidence to discriminate earthquakes and explosions without many physical techniques,” Woo said.
The researcher and his team also compared their data with satellite images of mining locations, confirming that the seismic events identified as coming from mining blasts overlapped the location of operations.
There were unusual clusters of seismicity at the mining locations, between sunset and sunrise, that didn’t fit the timing of mining operations.
“These could be considered mining-related events, or we need to figure out what else could be happening,” Woo said, such as whether mining blasts might be triggering natural seismicity in the area.
In his view, the microseismicity data could help seismologists pinpoint previously unidentified active faults or look more closely at earthquake aftershock sequences.
South Korea experienced the largest earthquakes in its instrumental history in the past seven years: the magnitude 5.8 Gyeongju earthquake in 2016 and the magnitude 5.4 Pohang earthquake in 2017.
Woo and his group pointed out that the Pohang earthquake may have been triggered by water injected into rock layers at a geothermal plant.
In the wake of the development of early-warning systems for these unexpected earthquakes, the country increased its number of seismic stations, creating the dense network that the Stanford researchers used in their study.
“The new microseismicity data, reveals previously unreported seismic swarms and activated faults in South Korea as well as capturing characteristic mining activities,” Woo said.
