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MONITORING OTHER SEISMIC SOURCES 2006 IRIS 5-YEAR PROPOSAL Excitation of Earth’s Incessant Free Oscillations by Atmosphere-Ocean-Seafloor Coupling Junkee Rhie, Barbara Romanowicz • University of California, Berkeley The observation on long period seismograms of continuously excited free oscillations of the Earth was first made by japanese scientists in 1998, during intervals free of significant earthquakes. Since then, attention has focused on elucidating the physical mechanism responsible for them. The sum of all small earthquakes remaining in the records cannot explain the observed level of excitation. The mechanism must be shallow, as fundamental modes appear to be preferentially excited. It shows seasonal variability, with peaks in the northern and southern hemisphere winters. Stochastic mechanisms involving turbulent motion in the atmosphere have been proposed as well as random distribution of sources around the world. An al the computation of spectra or correlations of signals across long time series, do not have the spacial and temporal resolution to locate these sources. Results of application of a grid search method to locate the source of the low frequency hum using 6 hours of data from the BDSN array, F-NET and 10 additional quiet GSN stations, on a day without earthquakes (January 31, 2000). In each panel, the data have been narrow bandpass filtered with a different center period. Dark red areas indicate 5 x 5 degrees areas which yield maximum amplitudes of the corresponding network stacks. Resolution is lost at periods greater than 200 sec. We have developed an array-based method to detect and locate sources of very long period surface wave energy, utilizing the dispersive properties of Rayleigh waves. Our basic approach uses data from two large aperture arrays of very long period seismometers (BDSN in California and F-NET in Japan). We stack the data after projection to the center of each array, and look for directions of arrival of maximum amplitude in the stacks as a function of back-azimuth, taking into account the array response. We show that, for each array, there is a well defined preferential direction, which is stable over one season but changes significantly from winter to summer. The fluctuation as a function of time of the maximum stack amplitudes are correlated across the two arrays and point to the northern Pacific ocean in the northern hemisphere winter and the southern Oceans in the summer, correlating with changes in the global distribution of maximum wave height. The addition of other stations equipped with STS-1 seismometers (GSN and GEOSCOPE) allows us to apply a grid search method to more precisely determine the loci of the multiple sources during a particularly stormy day in the north Pacific ocean. We infer that the background oscillations originate primarily in the oceans, and are caused by a non-linear coupling mechanism involving the atmosphere (winds), the oceans (transfer of energy from ocean waves to infragravity waves) and the seafloor (transfer of energy to elastic waves). Rhie, J. and B. Romanowicz, Excitation of earthʼs incessant free oscillations by Atmosphere-Ocean-Seafloor coupling, Nature, 431, 552-556, 2004. 36
2006 IRIS 5-YEAR PROPOSAL MONITORING OTHER SEISMIC SOURCES Kinematic Modeling and Complete Moment Tensor Analysis of the Anomalous, Vertical CLVD Bardarbunga, Iceland, Event Hrvoje Tkalcic • Lawrence Livermore National Laboratory Douglas S. Dreger • University of California, Berkeley Gillian R. Foulger • University of Durham, United Kingdom Bruce R. Julian • U.S. Geological Survey, Menlo Park Using a complete moment tensor inversion method and the seismic data from the HOTSPOT PASSCAL experiment acquired from the IRIS data center, we investigated the September 29, 1996, volcanic event of Mw = 5.6 originated beneath the Badarbunga caldera in Iceland. The corresponding moment tensor is characterized by a significant non-double-couple component (NDC) previously reported in the Harvard centroid moment tensor catalog (CMT) and confirmed by analysis of long-period and intermediate surface wave data. The deviatoric inversion performed by using Iceland HOTSPOT Project stations, yields a NDC solution with a 67% vertically oriented compensated linear vector dipole (CLVD) component, while the full moment tensor solution shows a similar, 66% CLVD component, 32% of double-couple component (DC) and a small volumetric contraction (ISO) of 2%. Statistical tests confirm that CLVD is a stable component of the moment tensor, while ISO is statistically insignificant. Using an elastic finite difference code with a large number of equidistantly distributed point sources, we simulated various rupture scenarios on the walls of a conical surface of the Bardarbunga caldera in order to compare them with the observations. Suites of seismograms for each independent run were produced at locations corresponding to HOTSPOT stations. We then inverted these synthetic data to investigate what portion of the original source information can be recovered by the moment tensor inversion (Figure 1). We were able to identify physical characteristics of a rupture scenario that produces synthetics resembling the observed data to a quite high level of detail. For example, we obtained the best results for the ruptures extending along one-half perimeter of the caldera, while one-quarter or fulllength perimeter ruptures were unlikely scenarios. We found that the rupture velocity, which took place at Bardarbunga, could have been a super-shear one, and we hypothesize that it could have been triggered by a compressional wave field that spread throughout the volume of the caldera. Full-waveform moment tensor inversion for a half-cone rupture scenario. The observed waveforms are shown by solid lines while the synthetic data are shown by dashed lines. The resulting mechanism is dominated by a vertically oriented strong compensated linear vector dipole. The caldera in finite source modeling is assumed to be a part of a conical surface, dipping at 45 degrees outward, and the rupture is assumed to take place along a segment between 3 and 5 km of depth. A finite source is described with a number of point sources and the rupture is simulated by initiating increment displacements at multiuple locations on the caldera walls. 37
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