You can’t have missed the dramatic images coming out of Hawaii of glowing lava and burning homes, thanks to the current activity of Kilauea. The Island of Hawaii—more popularly known as the Big Island—has also experienced a series of earthquakes. The largest of these quakes struck on May 4 and had a magnitude of M6.9, and like most of Hawaii’s temblors, it was linked to volcanic activity.
The Hawaiian Islands were formed during the last ~30 million years as the Pacific plate moved northwestward over the isolated Hawaiian hotspot, where a rising thermal plume reaches the earth’s surface, resulting in volcanic eruptions.
Right now this plume is under the Big Island, which has five major volcanoes: Hualalai, Kilauea, Kohala, Mauna Kea, and Mauna Loa (Figure 1), so most of the volcanic and associated seismic activity occurs beneath and around the Big Island—particularly in the south and southeast, around Mauna Loa and Kilauea, which are some of the most active and productive volcanoes on Earth. Each eruption causes the island to grow upward, and outward toward the sea.
The Hawaiian Islands experience thousands of earthquakes each year, despite being far away from an active tectonic plate boundary. Most of these earthquakes are associated with the movement of magma through conduits and cracks or high angle normal faulting at the volcanic edifice, and tend to be smaller (i.e., less than magnitude 6.0). During the last 150 years, however, more than 40 earthquakes with magnitudes greater than 6.0 have also been recorded.
The majority of these quakes in the shallow crust can be explained by gravity-driven, rapid seaward sliding of the mobile southern flanks (see Figure 2). These flanks travel along weak detachment boundaries separating the younger volcanic rocks from the older oceanic crust underneath. Slip on these interfaces serves to release strain that builds up over decades due to volcanic activity, making room for more magma. The M6.9 earthquake on May 4 was an example of one of these quakes, and the largest since 1975.
Extensive volcanic activity preceded the May 4 earthquake, which is likely to have been triggered by overpressure following the intrusion of magma into dike(s). According to the preliminary USGS estimates, the shallow dipping interface slipped by as much as 3 meters. Data acquired by continuously operating GPS stations at the surface show horizontal displacements as high as ~0.8 meters (Figure 1). Peak ground acceleration near populated areas closest to the epicenter was ~0.1-0.2g. This is in line with expected values but lower compared to other intra-plate earthquakes due to the high temperature of the surrounding rocks (which is likely to result in low stress-drop).
Since the mainshock, a number of aftershocks have been recorded and activity is anticipated to slowly decay over time. Although property damage from this earthquake was minimal, it is a stark reminder of the hazard posed by volcanoes not just in Hawaii but also in similar settings around the world.
