Slow earthquakes on the flank of Kilauea volcano, Hawai'i

Following the detection of 'silent' or slow earthquakes (SE) using strainmeters, continuous GPS (CGPS) observations have led to the discovery of SE afterslip events after conventional subduction zone earthquakes in Japan, a SE in Cascadia unassociated with a conventional earthquake, and recognition that Cascadia SEs are periodic and accompanied by seismic tremor. The first SE detected in an intraplate setting occurred in November 2000 following intense rainfall at the Hilina slump (HS) on Kilauea volcano's south flank. However, because it most likely occurred in a well-hydrated landslide wedge shallower than ~10 km, it cannot be explained by theories being developed for SEs in subduction zones which postulate that water liberated by metamorphic phase changes at depths of 35-40 km promote fault slip and seismic tremor. Previous work suggested that the extreme rainfall that occurred just prior to the SE may have triggered the November 2000 event. Here, we present 8 years of CGPS data from the HS and identify 3 additional SEs which stand out from other transient motions in the time series because of their similar location and spatial coherence. None of the newly identified SEs is associated with heavy rainfall. Each event is very similar to the November 2000 SE, suggesting a common source; furthermore, the events are separated by 774 (+/- 7) day periods. SE source parameters other than moment are not well-constrained by the CGPS network. Using the January 2005 event for reference, we calculate moment and equivalent moment magnitudes and explore how SE moment release compares with seismic moment release during the same period.