Seismic studies of the island of Hawaii and Loihi Seamount

Abstract

Models for flank dynamics of Hawaiian volcanoes relate seismicity in the Hawaiian islands to volcanic edifice growth. In most models. tectonic earthquakes occurring along a subhorizontal detachment surface or weak zone at the base of the volcanic edifice are the principal means of releasing accumulated flank stresses. The persistence of and damage caused by basal sliding earthquakes underscore the importance of understanding these events. Not only will this knowledge yield better models of volcanic growth, but it has important implications for seismic hazard assessment. Focal mechanism and slip analysis for a set of small basal sliding earthquakes show that the southern third of the island of Hawaii is composed of at least three rigid blocks that slide horizontally along the edifice basal interface. Relative motion between these blocks may be accommodated by oblique opening across their shared boundaries. Differences in fault-plane solutions determined from short-period local-seismic-network and long-period teleseismic data for recent large Hawaiian earthquakes suggest that the rupture processes in large basal sliding earthquakes may represent complex patterns of stress release within the volcanic edifices. Rupture may initiate as small events that trigger the large-scale energy release at the bases of the volcanic edifices. the type of energy release often observed in large Hawaiian earthquakes. These triggering events may activate rupture surfaces that differ from those along which the long-period moment release occurs and, thus, may represent release of a local stress concentration superposed upon the regional stress field. Finally, seismic data from a Loihi swarm are examined to determine a crustal velocity model for Loihi Seamount and to assess systematic mislocation incurred by locating Loihi earthquakes with only the stations of the Hawaiian Volcano Observatory permanent seismic network. An insufficient number of ocean bottom seismometers, poor site locations, and poor coupling to the bottom precluded realization of these goals. Information learned from these studies combined with that from studies of the temporal and spatial patterns of seismicity and stress release within the volcanic edifices will yield a much more complete understanding of volcanic growth and deformation processes.