A decade of precursory seismicity at Mauna Loa: 2013–2022
The 2022 eruption of Mauna Loa was preceded by nearly 10 years of seismic unrest. Using the earthquake catalog of the Hawaiian Volcano Observatory, we describe and explore many aspects of unrest between January 1, 2013, and the start of the eruption on November 28, 2022 (UTC). Earthquakes associated with unrest occurred above an inflating dike at shallow depths under and to the south of Moku‘āweoweo and in the cluster of seismicity to the west of Moku‘āweoweo associated with a buried rift zone. Earthquakes also occurred in a broad region at the base of the crust, likely associated with a zone of magma accumulation. Unrest is broken down into four phases of increasing intensity with time and into individual geographic regions. While activity occurring at shallow depths under the caldera often alternated with activity on the west flank in the first two phases of unrest, the third phase of unrest featured increased rates of earthquake activity in both areas acting concurrently. We also document a slow shift in seismicity across multiple years from the south of Moku‘āweoweo to an area under the caldera that likely represents a change in the location of the intrusion through time. Broadly speaking, unrest observed in advance of the 2022 eruption occurred in the same areas as the unrest preceding eruptions in 1975 and 1984. Furthermore, precursory unrest prior to eruptions in 1975, 1984 and 2022 lasted years and culminated in an intense phase of seismic activity lasting on the order of hours that represented the final dike propagation to the surface. Concurrent earthquake activity under Moku‘āweoweo and the west flank of Mauna Loa, paired with a shift in earthquakes under Moku‘āweoweo were the two most notable long-term precursors to an eruption; however, neither provided an actionable timeline of an impending eruption. The decade of precursory seismicity prior to the 2022 eruption is similar to long precursory phases prior to eruptions in 1975 and 1984 and underscores the challenges of informing on eruption hazards in a timely manner. By combining seismic activity with other geophysical fields such as geodesy and gravity, we present a conceptual model to aid in the interpretation of future unrest at Mauna Loa.