NEW_Accomplishments.indd - IRIS

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GLOBAL MANTLE STRUCTURE 2006 IRIS 5-YEAR PROPOSAL New Measurements of Radial Mode Eigenfrequencies Gabi Laske • IGPP, Scripps Inst. of Oceanography Radial mode eigenfrequencies are commonly thought to be measured with great ease and precision. The reason for this is that these modes have no geographic pattern so one should be able to measure frequencies from a spectrum observed at any station in the world. Yet, radial modes often seem inconsistent with spherical Earth models that fit all other mode frequencies. It turns out that radial modes are sometimes strongly coupled. The strongest coupling is predicted to be with neighboring l = 2 modes, sometimes more than 50 mHz away. The coupling is caused by the Earthʼs hydrostatic ellipticity and aspherical structure of harmonic degree 2. Mode coupling due to ellipticity alone can cause a frequency shift for the radial modes by more than 4 mHz (e.g. 11 S 0 and higher). Given that mode frequencies can be measured to within 0.1 microHz, this shift is significant, and some singlets of l = 2 modes have indeed been misidentified as the radial mode in the past. Including the spectra of the June 23, 2001, Southern Peru and the November 14, 2001, China Earthquakes we have reanalyzed radial mode eigenfrequencies and obtained a mode dataset that is internally more consistent than previous ones. Only small perturbations to PREM are needed to fit the new data, together with the current best estimates of “Reference Normal Mode Data” (available on the Reference Earth Model web site: //mahi.ucsd.edu/Gabi/rem.html). The smallest perturbation to isotropic PREM that is consistent with our radial mode data requires no 220 km discontinuity but there exists a trade-off between higher shear velocities and densities in the lid and lower values in the asthenosphere below. Slightly higher velocities are also required above the CMB and the inner core boundary. The great December 26, 2004, Andaman-Sumatra earthquake still awaits detailed mode analysis. However, new frequency estimates for the gravest radial mode, “breathing modeʼʼ 0 S 0 have been reported with error bars smaller than the coupling effects from neighboring modes. This stressed the importance of having a state-of-the-art global seismic network running as well as, investigating possible systematic effects in the measurements very carefully. a) Spectra obtained for coupled-mode synthetic seismograms when only selfcoupling is considered. Effects that removed singlet degeneracy come from Earth’s rotation and hydrostatic ellipticity, mantle model S16B30 and hypothetical l = 2 structure in the upper 200 km of the inner core. Core structure split the mode anomalously (grey bar marks effects from rotation/ellipticity alone). b) When mode coupling between 27 S 2 and 11 S 0 is considered. Coupling significantly shifts the frequency of 11 S 0 . Also note that the amplitudes are affected, as observed by Park (1990). c) Old and new radial mode data and residuals with respect to isotropic PREM and our new model. Zurn, W., Laske, G., Widmer-Schnidrig, R. and F. Gilbert. Observation of Coriolis coupled modes below 1mHz, Geophys. J. Int., 143, 113-118, 2000. 154
2006 IRIS 5-YEAR PROPOSAL GLOBAL MANTLE STRUCTURE The Global Short-Period Wavefield Modeled with a Monte Carlo Seismic Phonon Method Peter Shearer • University of California, San Diego Paul Earle • U.S. Geological Survey, Golden At high frequencies (~1 Hz), much of the seismic energy arriving at teleseismic distances is not found in the main phases (e.g., P, PP, S, etc.) but is contained in the extended coda that follows these arrivals. This coda results from scattering off small-scale velocity and density perturbations within the crust and mantle and contains valuable information regarding the depth dependence and strength of this heterogeneity as well as the relative importance of intrinsic versus scattering attenuation. Most analyses of seismic coda to date have concentrated on S-wave coda generated from lithospheric scattering for events recorded at local and regional distances. Here we examine the globally averaged vertical-component, 1-Hz wavefield (>10 degree range) for earthquakes recorded in the IRIS FARM archive from 1990 to 1999. We apply an envelope-function stacking technique to image the average timedistance behavior of the wavefield for both shallow(< 50 km) and deep (> 500 km) earthquakes. Unlike regional records, our images are dominated by P and P-coda owing to the large effect of attenuation on PP and S at high frequencies. Modeling our results is complicated by the need to include a variety of ray paths, the likely contributions of multiple scattering, and the possible importance of P-to-S and S-to-P scattering. We adopt a stochastic, particle-based approach in which millions of seismic “phonons” are randomly sprayed from the source and tracked through the Earth. Each phonon represents an energy packet that travels along the appropriate ray path until it is affected by a discontinuity or a scatterer. Discontinuities are modeled by treating the energy normalized reflection and transmission coefficients as probabilities. Scattering probabilities and scattering angles are computed in a similar fashion, assuming random velocity and density perturbations characterized by an exponential autocorrelation function. Intrinsic attenuation is included by reducing the energy contained in each particle as an appropriate function of travel time. We find that most scattering occurs in the lithosphere and upper mantle, as previous results have indicated, but that some lower mantle scattering is likely also required. A model with 3% to 4% RMS velocity heterogeneity at 4-km scale length in the upper Comparisons between the envelope stack for shallow event data (heavy line) with the predictions of the phonon method applied to the scattering model (thin line). (a) Peak P-wave amplitude as a function of source-receiver distance. (b) Coda envelopes in 5 degree range bins plotted as a function of time from the direct P arrival. Amplitudes are normalized to the same energy in the first 30 s. mantle and 0.5% RMS heterogeneity at 8-km scale length in the lower mantle (with intrinsic attenuation of Qα = 450 above 200 km depth and Qα = 2500 below 200 km) provides a reasonable fit to both the shallow and deep earthquake observations, although many tradeoffs exist between the scale length, depth extent and strength of the heterogeneity. Shearer, P.M. and P.S Earle, The global short-period wavefield modelled with a Monte Carlo seismic phonon method, Geophys. J. Int., 158, 1103-1117, 2004. 155
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Cornerstone Facilities for Seismolo
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Cornerstone Facilities for Seismolo
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2006 IRIS 5-YEAR PROPOSAL ACCOMPLIS
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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2006 IRIS 5-YEAR PROPOSAL EARTHQUAK
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