HR: 1340h
AN: G53B-0881    [Abstracts]
TI: High-Frequency Sampling From Shallow Borehole Tiltmeters Extends the Monitoring Capabilities at Stromboli Volcano, Italy
AU: * Sherrod, D R
EM: dsherrod@usgs.gov
AF: U.S. Geological Survey, Cascades Volcano Observatory 1300 SE Cardinal Court, Vancouver, WA 98683 United States
AU: Krug, R M
EM: ralf.krug@pinntech.com
AF: Pinnacle Technologies, 600 Townsend Street, Ste 160W, San Francisco, CA 94103 United States
AU: Ripepe, M
EM: mripepe@geo.unifi.it
AF: Dip. Scienza della Terra, Universita' di Firenze Via LaPira 4, Firenze, 50121 Italy
AU: Gengo, R
EM: ricca_lofoforo@virgilio.it
AF: Dip. Scienza della Terra, Universita' di Firenze Via LaPira 4, Firenze, 50121 Italy
AU: Marchetti, E
EM: marchetti@steno.geo.unifi.it
AF: Dip. Scienza della Terra, Universita' di Firenze Via LaPira 4, Firenze, 50121 Italy
AB: In September 2004 we installed an electronic borehole tiltmeter at the 600-m altitude on Stromboli volcano, Aeolian Isles, Italy. A second tiltmeter was installed in June 2005. The two tiltmeters are set about 5 m deep on the east and south flanks, about 1 km from the active vents in the Sciara del Fuoco, and record data at 1-sec intervals. Earth tides are resolvable in the data of one instrument (the other is somewhat noisier). With a 25-sec low-pass filter, the noise level at short periods (less than 1 hour) is low enough to detect events as small as 3 nanoradians. Virtually all explosions from the vents, as recorded by seismometers at the summit, are also seen as tilt events. An inflationary tilt lasting as long as 5 min precedes each explosion. The explosions themselves are recorded as 15s to 60s signals with measured amplitudes as large as 0.2 microradians. This deformation signal is heavily attenuated by the filter, however, and likely is ten to one hundred times larger than measured on the basis of lab simulations of a similar tilt-step response. The events can be classified as one of many common types of very long-period seismic events. Radial tilt axes for both types of VLP events point to the vent area, and their point of crossing coincides with a zone of semblance mapped previously by B.A. Chouet and colleagues under the Sciara del Fuoco, about 250 m northwest of the vents. Although the main goal for the installation was to extend the monitoring capabilities of the instrumental network on Stromboli into the 1-sec to 10-min range, the tiltmeters are recording longer-period events; for example, several VLP events have been followed by a 0.75 microradian deflation lasting 30 min. The resolution is poorer for these events than for the explosions, owing to the noise that characterizes the longer time period. No secondary local sources of volcano-tectonic deformation have been recognized since the June 2005 installation.
DE: 1200 GEODESY AND GRAVITY
DE: 1209 Tectonic deformation (6924)
DE: 8400 VOLCANOLOGY
DE: 8419 Volcano monitoring (7280)
DE: 8494 Instruments and techniques
SC: Geodesy [G]
MN: Fall Meeting 2005