Volcanic earthquake foreshocks during the 2018 collapse of Kīlauea Caldera

Abstract

SUMMARY The summit collapse of the Kīlauea Caldera—due to magma chamber drainage being directed to the Volcano's lower east rift zone—was accompanied by 50 large, nearly identical magnitude Mw 5 earthquakes between 29 May and 2 August 2 2018. I have examined the seismicity associated with these 50 primary earthquakes, and find that the typical pattern of earthquake aftershocks decaying in number and magnitude is not evident. Rather, immediately after the primary shock there is a hiatus of one-to-several hours before the associated earthquakes grow in number and magnitude up to the next primary shock. In essence, the associated seismicity consists of thousands of foreshocks. The magnitude of completeness is estimated at ML = 2.5. The trend of foreshocks does not fit an Omori power-law model. Rather, the pattern of foreshocks (number per hour) is fit well by a semi-Gaussian curve, which initially grows rapidly and slows hours prior to the primary earthquakes. The Gaussian fits (${r^2} > 0.98$) for three different magnitude thresholds are self-similar with a common half-width, $\sigma \sim 13$ hr. The pattern of foreshock seismic moments aligned with and stacked for the 50 primary events is fit by an exponential trend, growing at the mean (stacked intervals) rate of 17 per cent per hour. The power of foreshocks measured within each interval also grows with time—the total foreshock power per interval (J hr–1) increases by a factor of 18 through the first half of the sequence (May 29 through June 26), and then declines by half through to the end.