The rupture process of the 2018 Mw 6.9 Hawaíi earthquake as revealed by a genetic algorithm-based source imaging technique

Continuous volcanic activity since 1983 within Kīlauea's East Rift Zone, known as the Púu ´Ō´ō eruption, has altered the landscape of the region and caused damage in nearby communities. In late April 2018, a renewed period of increased volcanic output and seismicity began near the Kīlauea summit and in the East Rift Zone. Associated with this period of unrest was a M_w 6.9 earthquake on 4 May 2018 near the south flank of Kīlauea. The epicentral location and focal mechanism suggest that this earthquake occurred on the low-angle décollement that separates the overlying volcanic edifice of Kīlauea from the underlying oceanic crust. In this study, the source properties of this earthquake are investigated using a novel genetic algorithm-based back-projection approach that selects a network of seismic stations that reduces artifacts that arise from using a large distribution of seismic stations. This approach is applied to northern circum-Pacific stations that include Hi-net in Japan and the Transportable Array in Alaska. Significant variation in rupture direction and speed is observed during this event. The dominant feature of the earthquake in terms of both rupture length and energy release is a slowly propagating western rupture offshore of the southern flank of Kīlauea. The area surrounding this western segment has little background and aftershock seismicity and is known to host slow-slip events. The high-frequency radiation and location of the dominant western rupture suggests that small asperities within the slow slip region achieved seismic slip rates as the rupture propagated southwest into this region from its hypocenter. The last major earthquake to occur in this region was the 29 November 1975 M_w 7.7 Kalapana earthquake, which initiated 4 km from the 2018 epicenter. The 2018 and 1975 events both had anomalously long durations, low rupture speeds, and a lack of aftershocks, rupture properties that are possibly related to the soft sediments that compose the décollement near Kīlauea. Imaging the rupture properties of large events such as the 2018 M_w 6.9 earthquake may provide insights on the relationship between flank earthquakes and volcanic activity in Kīlauea, mitigating future volcanic and seismic hazards in the region.