Longer vs shorter-term behavior of Mauna loa Volcano Hawaii

Abstract

Almost all eruptions are preceded by periods of unrest characterized by anomalies including seismicity, deformation, and degassing of the volcano. Understanding unrest allows one to unravel the behavior and operating principles within a volcano, which is an intermediate and fundamental step to forecast eruptions. Surface deformation is a crucial indicator of the unrest state of a volcano. Geodetic (InSAR, GPS, Tilt, Leveling, EDM) and seismic data characterized recent very short periods (up to a very few years) of unrest at Mauna Loa(Hawaii). However, a medium-term (decades) and a longer (thousands of years or more) reconstruction of the volcano’s evolution are lacking. Mauna Loa has a summit caldera, from which two well-developed rift zones depart, and minor radial eruptive fissures (only on the NW flank), whereas the SE flank is unstable. I first studied the selective distribution of the radial fissures, and thus of shallow magma transfer, on the NW flank of Mauna Loa, using a numerical modelling approach. The modelling was devoted at understanding the conditions controlling the selective distribution of the radial fissures, on the longer-term (thousands of years). I simulated quiescent periods, with instability (of very few cm/yr) on the SE flank only; then I simulated volcano unrest with shallower and/or deeper magma intrusions along the rift zones that override the flank motion or, if the latter increases(for large flank earthquakes), become similar. The results suggest that the selective occurrence of the radial fissures may be explained by the competition between two processes: a) dike intrusion along the rift zones (especially along the NE Rift), promoting the development of radial dikes along the NW flank; b) flank slip, inhibiting the development of the radial dikes on the SE flank. The opening of a kinked rift zone promotes the local circumferential extension necessary to promote the feeding of radial fissures by dikes on the NW flank. An analysis of the historical radial eruptions at Mauna Loa in the last 200 years shows how these are immediately preceded by eruptions along the rift zones, confirming the modeling results.