HR: 14:00h
AN: G43A-02 INVITED     [Abstracts]
TI: Signal Coherence and Improved Bandwidth in Kilometer-Scale Water-Pipe Tilt-Meters for Monitoring Slow Earthquakes
AU: * Bilham, R
EM: bilham@colorado.edu
AF: CIRES and Geol Sci, 2200 Colorado Avenue, Boulder, CO 80309-0216 United States
AU: Suszek, N
EM: suszek@colorado.edu
AF: CIRES and Geol Sci, 2200 Colorado Avenue, Boulder, CO 80309-0216 United States
AU: Flake, R
EM: rex@caliente.geology.cwu.edu
AF: Geol Sciences Central Washington University, 400 University Avenue, Ellensburg, WA 98926 United States
AU: Szeliga, W
G43A-02 AF: CIRES and Geol Sci, 2200 Colorado Avenue, Boulder, CO 80309-0216 United States
AU: Melbourne, T
EM: tim@caliente.geology.cwu.edu
AF: Geol Sciences Central Washington University, 400 University Avenue, Ellensburg, WA 98926 United States
AB: Slow earthquakes have been detected by GPS networks in numerous subduction zones but signals are frequently close to detection levels. Although strain-meters and tilt-meters possess a thousandfold higher resolution (~ 1 nstrain & 1 nrad), noise levels in these instruments tend to be site specific and it is sometimes considered necessary to instal clusters to distinguish tectonic signal from local noise. This approach to strain measurement can more than double the cost of initial installation. We report here first results from a half-km-long water pipe tiltmeter in which a test for signal coherence is an inherent product of the geometry of the instrument. An appealing feature of water-pipe tiltmeters is that they cost 25% less than a borehole strain-meter, assume long good long term stability within days of installation, and unlike the decade-longevity of borehole systems, have an indefinite life span. In a Michelson tilt-meter, tilt of the earth's surface is manifest as a rise in water level at one end of the pipe and an equal and opposite reduction in water level at the other. In newly installed tiltmeters in the Cascadia region we have introduced a central transducer that effectively provides two 250-m-long independent measures of tilt in each 500 m long pipe, and hence a measure of signal coherence for little extra cost. Data from each sensor are telemetered via radio modem to a remote computer at rates of 1-6 samples/minute. Initial results from four 500 m long water pipes installed in the Cascadia region, reveal that a secular drift level of better than 0.1 microradian/yr is established within a week of installation and that the two half-tiltmeters track each other closely at all periods. Noise levels are frequency dependent and vary form 0.2 nrad at hourly periods to 100 nrad at yearly periods. Atmospheric and aperiodic ocean loading appears to be the largest souce of noise at periods of several days to weeks in the bandwidth where slow earthquakes are anticipated. One disadvantage of long water pipe tilt meters is that they cannot faithfully record tilts with periods shorter than their fundamental resonant modes (20-30 minutes). We report first results from a hybrid tiltmeter installed in the Andaman islands that uses a 2.5 cm diameter pipe within a 15 cm half-filled water pipe to extend the useful bandwidth to 2 minutes. Water level changes in the two independent tiltmeters are monitored by meniscus float sensors. The meniscus float consists of a 15 cm diameter polypropylene disk weighing 31 gm, perforated by more than 200 holes, and held at the water surface by surface tension equivalent to that experienced by a 1.4 m diameter float. The vertical position of the float is monitored relative to a 10 m deep vertical pile using a 3 mm range LVDT.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Geodesy [G]
MN: Fall Meeting 2005