Hydrothermal Eruption During the 2017 6.5M Leyte Earthquake: the How’s, Why’s, and Lessons Learned

Abstract

Common earthquake hazards include ground rupture, landslides, liquefaction, and tsunamis. However, the M6.5 earthquake that hit the Leyte Geothermal Field in Eastern Philippines last July 6, 2017 triggered the formation of a basin-like feature that extends 235 m in diameter and 19 m deep, removing an estimated 215,000 cu.m. of ground material. This breached part of the access road towards some critical facilities including the wellhead of one of the producers that is just 28 m away from the edge of the basin rim. Field investigation shows the presence of biased burning and oriented deformation of surrounding vegetation whilst there is an apparent grading of sediments that have formed around the rim. Locals also report the formation of a dark plume immediately above the area which happened just shortly after the first ground shaking due to the earthquake. These observations are consistent with the occurrence of a hydrothermal eruption, and combined with a landslide and a debris avalanche, the eruption crater was enhanced then breached, resulting to its present-day morphology. The sequence of events suggests that the frictional heating from the slipping fault during the earthquake may have triggered an increase in the subsurface pressure underneath existing hydrothermal manifestation that eventually resulted to the eruption. This illustrates a complex effect from large earthquakes occurring within geothermal fields, and thus should be considered in the geological hazard assessment and mitigation planning.