Intermediate-term seismic precursors to the 2007 Father's Day intrusion and eruption at Kilauea Volcano, Hawai'i

For two days beginning on June 17 (Father's Day), 2007, and following a four-year-long period of summit inflation, magma intruded into Kilauea's east rift zone, resulting in a small eruption just north of Makaopuhi Crater (~7 km west of the long-lived Pu'u O'o vent). On the basis of concurrent summit deflation and observations of lava chemistry and temperature, the June 17-19 Father's Day event has been interpreted as the result of forcible intrusion driven by high magma pressure at the summit, as opposed to a passive response to rifting. The Father's Day event was preceded by a) two shallow oblique strike-slip M4+ earthquakes along the outermost caldera faults on 24 May 2007, and b) a strong swarm of shallow volcano-tectonic (VT) earthquakes beginning on June 17 and signaling the onset of intrusion into the ERZ. Little is known, however, about any intermediate-term precursors that may have occurred between these two sets of earthquakes. We analyzed continuous and event-detected seismic data recorded by the Hawaiian Volcano Observatory permanent seismic monitoring network during the first half of 2007 and observe a) a subtle increase in the rate of seismic moment release beginning in April 2007, and b) a subtle decrease in seismic event rate beginning in early June 2007, both of which appear to correspond to changes in shallow (<5 km BSL) and intermediate-depth (>5 km BSL) seismicity. Most located events during this period occur in the upper southwest and east rift zones; however, relocation of newly-detected low-magnitude events also indicates the presence of a 'ring' of seismicity centered on the southeast caldera rim and a cluster of events with epicenters near Kilauea Iki crater. Additional analyses will indicate whether these features are unique to the months preceding the Father's Day event or whether they represent long-term features of Kilauea's seismicity. Finally, Interferometric Synthetic Aperture Radar (InSAR) analyses were used to examine the M4.7 and M4.1 earthquakes that occurred on May 24. Interferograms and preliminary forward models indicate that surface deformation during these events cannot be satisfactorily explained with the type of faulting indicated by the focal mechanisms, suggesting that there may have been other phenomena, such as magma intrusion, involved in these earthquakes.