DEEP FAULT PLANE GEOMETRY INFERRED FROM MULTIPLET RELATIVE RELOCATIONBENEATH THE SOUTH FLANK OF KILAUEA

Dense microearthquake swarms occur in the upper south flank of Kilauea , providing multiplets composed of hundreds of events. The similarity of their waveforms and the quality of the data have been sufficient to provide accurate relative relocations of their hypocenters. A simple and efficient method has been developed which allowed the relative rel ocation of more than 250 events with an average precision of about 50 m horizontally and 75 m vertically. Relocation of these events greatly improves the definition of the seismic image of the fault that genera tes them. Indeed, relative relocations define a plane dipping about 6- degrees northward, although corresponding absolute locations are widel y dispersed in the swarm. A composite focal mechanism, built from even ts providing a correct spatial sampling of the multiplet, also gives a well-constrained northward dip of about 5-degrees to the near-horizon tal plane. This technique thus collapses the clouds of hypocenters of single-event locations to a plane coinciding with the slip plane revea led by previous focal mechanism studies. We cannot conclude that all s outh flank earthquakes collapse to a single plane. There may locally b e several planes, perhaps with different dips and depths throughout th e south flank volume. The 6-degrees northward-dipping plane we found i s too steep to represent the overall flexure of the oceanic crust unde r the load of the island of Hawaii. This plane is probably an importan t feature that characterizes the basal slip layer below the upper sout h flank of Kilauea volcano. Differences in seismicity rate and surface deformations between the upper and lower south flank could be related to the geometry of this deep fault plane. The present work illustrate s how high precision relative relocations of similar events in dense s warms, combined with the analysis of geodetic measurements, can help t o describe deep fault plane geometry. Systematic selection and extensi ve relative relocation of similar earthquakes could be attempted in ot her well-instrumented, highly seismic areas to provide reliable basic information, especially useful for understanding of earthquake generat ion processes.