HR: 1330h
AN: V23A-10    [Abstracts]
TI: Locations of Long-Period Seismic Events Beneath the Soufriere Hills Volcano, Montserrat, W.I., Inferred from a Waveform Semblance Method
AU: * Taira, T
EM: taka@dtm.ciw.edu
AF: DTM, Carnegie Institution of Washington, 5241 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Linde, A T
EM: linde@dtm.ciw.edu
AF: DTM, Carnegie Institution of Washington, 5241 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Sacks, I S
EM: sacks@dtm.ciw.edu
AF: DTM, Carnegie Institution of Washington, 5241 Broad Branch Rd. NW, Washington, DC 20015 United States
AU: Shalev, E
EM: shalev@duke.edu
AF: Earth and Ocean Sciences, Duke University, 109A Old Chemistry, Durham, NC 27708 United States
AU: Malin, P E
EM: malin@duke.edu
AF: Earth and Ocean Sciences, Duke University, 109A Old Chemistry, Durham, NC 27708 United States
AU: Nielsen, J M
EM: janmn@duke.edu
AF: Earth and Ocean Sciences, Duke University, 109A Old Chemistry, Durham, NC 27708 United States
AU: Voight, B
EM: voight@ems.psu.edu
AF: Dept. of Geosciences, Penn State University, 503 Deike Building, University Park, PA 16802 United States
AU: Hidayat, D
EM: hidayat@geosc.psu.edu
AF: Dept. of Geosciences, Penn State University, 503 Deike Building, University Park, PA 16802 United States
AU: Mattioli, G S
EM: mattioli@uark.edu
AF: Dept. of Geosciences, University of Arkansas, 113 Ozark Hall, Fayetteville, AR 72701 United States
AB: Analysis of long-period (LP) seismic events provides information about the internal state of a volcano because LP events are attributed mainly to fluid dynamics between magma and hydrothermal reservoirs in its volcano (e.g., Chouet, 1992). We analyzed LP events recorded by three borehole seismic stations (AIRS, OLVN, and TRNT) at Soufriere Hills Volcano (SHV), Montserrat, W.I., during the period from March to June 2003. Borehole stations were deployed by the Caribbean Andesite Lava Island Precision Seismo-geodetic Observatory project (e.g., Shalev et al., 2003; Mattioli et al., 2004) and equipped with three-component short-period velocity seismometers with a sampling rate of 200 Hz. We selected 61 LP events with high signal-to-noise ratios. Almost all of the selected LP events are characterized by dominant periods in a range of 0.3 to 2.0 sec and durations of about 30 sec. Several LP events appear to be generated by a single source, based on the strong similarity in their waveforms. We first identified a family of LP events based on the dimensionless cross-correlation coefficient (CCC) of their spectral amplitudes of a period in a range of 0.2 to 2.0 sec, under the assumption of a fluid-driven crack model (Chouet, 1986). Seven LP events are identified as a family of LP events with high CCCs, particularly CCCs at AIRS in the vertical component greater than 0.88 in each event. This result suggested that these LP events are probably due to a repeated excitation of an identical source mechanism. We next attempted to estimate the locations of the identified a family of LP events by a waveform semblance method (Kawakatsu et al., 2000; Almendros and Chouet, 2003). To apply the above method, we searched the seismic phases with a rectilinear polarization from LP events, by performing a complex polarization analysis (Vidale, 1986). These phases are identified as averaged particle motion ellipticities of all stations in a time window less than 0.50. Incident angles of the detected phases are rather shallow and range from about 30 to 70 degree. These particle motions point approximately to a shallow source located beneath the SHV lava dome. Assuming the detected phases to be P-wave motions, we conducted the waveform semblance method for 0.3 to 2.0 sec bandpassed seismograms containing these phases. Waveform semblances are calculated in a 1.2 sec time window with sliding increments of 0.4 sec, assuming a constant P-wave speed (3.56 km/sec) appropriate for the SHV (Rowe, Thurber, and White, 2004). Our model space consists of 14 x 20 x 10 nodes with node spacing of 500 m, extending from -5 to 2 km in the east-west direction, from -5 to 5 km in the north-south direction, and from -1 to 4 km in depth. The coordinated center is fixed at the dome (16.712N, 62.176 W) and at sea level. The source location is defined as the grid where the waveform semblance reaches its maximum. Note that location uncertainties are about 1.0 km in all directions. As a result, we found that all the analyzed sources of LP events are located about 2.0 km north of the dome and 3.0 km deep.
DE: 1734 Seismology
DE: 7280 Volcano seismology (8419)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly