Density structure of the island of Hawai'i and the implications for gravity-driven motion of the south flank of Kīlauea Volcano

Abstract

SUMMARY The discovery that large landslides dissected the Hawaiian Islands, scattering debris over thousands of square kilometres of seafloor, changed our ideas of island growth and evolution. The evidence is consistent with catastrophic flank collapse during volcano growth, and draws our focus to the currently active island of Hawai'i, the volcanoes Mauna Loa and Kīlauea, and particularly to the actively mobile south flank of Kīlauea. Both the weight distribution and pressure within an extensive magmatic system are perceived to affect stability, but the role of gravitational body forces and island density distribution has not been quantitatively assessed. We use seismic velocities derived from tomography to model the density distribution of the island of Hawai'i and find that olivine-rich melts and rocks in Hawaiian volcanoes result in a close association of seismic velocity and density. The resultant density model reproduces more than 95 per cent of the observed gravity disturbance signal wherever tomographic control exists and provides a basis for evaluating the body forces from gravity. We also find that if the decollement is weak, then gravitational body forces can produce slip that explains most seismo-tectonic and volcano-tectonic structural features of Kīlauea. Where the decollement is in a state of incipient slip from this weight distribution, fluctuations in magma pressure can trigger accelerated slip on the decollement. Yet this is only true of the south flank of Kīlauea. Though weight and magma distributions produce significant forces driving the west flank of Mauna Loa seaward, this flank is relatively stable. Stability over most of the last decade indicates a strong foundation beneath the west flank of Mauna Loa, perhaps as a result of past landslides that scraped clay-rich sediments from the decollement.