HR: 1340h
AN: S13D-1101    [Abstracts]
TI: Tomographic Imaging of Active Hawaiian Volcanoes
AU: * Okubo, P G
EM: pokubo@usgs.gov
AF: Hawaiian Volcano Observatory U. S. Geological Survey, P. O. Box 51, Hawaii National Park, HI 96718 United States
AU: Nakata, J S
EM: jnakata@usgs.gov
AF: Hawaiian Volcano Observatory U. S. Geological Survey, P. O. Box 51, Hawaii National Park, HI 96718 United States
AU: Villasenor, A
EM: antonio@ija.csic.es
AF: Inst. Ciencias Tierra Jaume Almera, CSIC, Barcelona, 08028 Spain
AU: Benz, H M
EM: benz@usgs.gov
AF: U. S. Geological Survey Mail Stop 966, P. O. Box 25046, Denver, CO 80225 United States
AB: In the past 25 years, seismic travel time tomography has become a widely, if not routinely, used tool taking advantage of earthquake arrival times, recorded on regional microearthquake monitoring networks, to image earth structure. When applied in volcanically active regions, tomographic studies have revealed the locations of magma chambers, pathways, and other features whose presence and properties will need to be incorporated into physically reasonable models of volcanic structure and process. We have expanded our calculations to include over 30 years of earthquake arrival time data cataloged at the Hawaiian Volcano Observatory. The large datasets extend from 1970 through 2003, and span Mauna Loa's 2 most recent eruptions in 1975 and 1984. Our results are consistent with those of earlier calculations and other investigators. The summit calderas and rift zones of the active volcanoes are associated with high velocity signatures, interpreted to suggest the presence of olivine cumulates developed via repeated eruptive and intrusive activity in these regions. In addition, fault zones beneath the southeastern flanks of both volcanoes are associated with velocity contrasts that extend quite deep into the volcanic edifice, possibly to the basal decollement. We also look to the identification of possible time-varying changes in volcanic structure. A distinct P-wave high-velocity anomaly under Mauna Loa deepens, from beneath its summit caldera and upper southwest rift zone into the southeast flank. Its associated P-wave speeds are comparable to those of features imaged in earlier calculations and which we have interpreted as magma bodies. At depths greater than 9 km below sea level, the lateral velocity heterogeneity weakens, but seismicity in the Kaoiki fault system beneath Mauna Loa's southeast flank does appear to localize along velocity contrasts. Prior to the 1975 eruption, the Mauna Loa high-velocity feature is principally centered beneath the summit caldera and is most apparent above depths of 3 km below sea level. Between 1975 and 1984, the anomaly is at its largest and extends to depths of 9 km. Since 1984, this feature has decreased in lateral extent but also exhibits greater velocity contrasts at shallower depths. Determining the possible relationships of these changes to eruptive processes will require additional volcanological monitoring data, as well as further testing and modeling using the tomographic results.
DE: 7260 Theory and modeling
DE: 7280 Volcano seismology (8419)
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2004 AGU Fall Meeting