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Earthquake Monitoring with Gravity Meters: Case Studies from the November 2006 and January 2007 Kuril Islands Earthquakes

Publisher –
SEG/EAGE 2010 Summer Research Workshop, Low Frequencies: Their value and challenges, August 15-20, 2010, Snowbird, Utah

Authors –
Tim N. Niebauer, Jennifer L. Hare, Jeff MacQueen, Daniel Aliod, and Olivier Francis

Paper – [pdf] ENV_EarthquakeMonitoringGphone

Introduction
Relative gravity meters are sensitive instruments capable of detecting small changes of the earth’s gravity field with a precision of a few parts per billion (109) over time scales of one second. They are often used to characterize earth-tides that vary with diurnal and semidiurnal periods. Recently, a superconducting gravity meter was successfully used to record low frequency gravest seismic modes (< 1 mHz) excited by the 2004 (M> 9) Sumatra-Andaman earthquake (Rosat et al., 2005; Ferreira et al., 2006). High frequency and high amplitude signals such as the S and P body waves and the Rayleigh and Love surface waves associated with earthquakes have traditionally been the purvey of seismometers. Seismometers are usually optimized to record seismic frequencies (0.1-10Hz) and are designed not to saturate during large amplitude signals. Gravity meters, on the other hand, are usually optimized to filter out seismic noise and often are too sensitive to faithfully record the high amplitude waves associated with the first arrival of an earthquake.
Recently, these difficulties have been overcome with the introduction of a new type of gravity meter (gPhone) with both large dynamic range and high sensitivity. In this paper we examine records from two earthquakes: the November 15, 2006, magnitude 8.3 Kuril Islands (Japan) earthquake, and the January 13, 2007, magnitude 8.2 Kuril Islands earthquake. In order to determine the efficacy of using gravity meters for earthquake monitoring, we compared and contrasted data from three types of instruments: a Streckeisen STS-2 long period seismometer, a GWR superconducting gravity meter (SG), and six Micro-g LaCoste (MGL) gPhone gravity meters. Our results are encouraging. While still limited to vertical component waves, time-series records from improved gravity instruments may yield new information that can be used to study and understand crustal seismic velocities, attenuation, and dispersion, as well as the 1D density model of the earth. Their low frequency content augments traditional seismic data providing enhanced interpretation and inversion capabilities as well.