NEW_Accomplishments.indd - IRIS

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SURFACE OF THE EARTH: NORTH AMERICA 2006 IRIS 5-YEAR PROPOSAL Imaging the Earth With Dense Arrays of Broadband Seismometers Stéphane Rondenay • Massachusetts Institute of Technology One of the current challenges faced by solid-earth seismologists is the development of new tools for high-resolution imaging of the lithosphere and underlying mantle. While tomographic methods using transmitted body waves and surface waves have proven to be powerful tools for volumetric imaging of seismic anomalies, a wealth of complementary information on internal discontinuities can be obtained from secondary scattered waves in the coda of body waves. The relative arrival time and amplitude of these secondary waves are indicative of the position and magnitude of discontinuities in elastic parameters of the subsurface. The teleseismic P wavefield is of particular interest as it benefits from generally higher signal-to-noise ratio than other (later arriving) primary phases. Scattered waves, notably P-to-S conversions, recorded at single stations have thus been utilized for over two decades as a basis for 1-D modelling of planar discontinuities in the lithosphere. In recent years, a substantial increase in availability and fidelity of broadband seismometers has led to new opportunities for multichannel processing of scattered teleseismic body waves recorded at dense recording arrays, allowing for 2D/3D imaging of subsurface structure. Many of these approaches are similar to industryoriented (e.g., migration) techniques and generate analogous high-resolution depth sections of the subsurface, down to the mantle transition zone. Recently developed methods form a continuum with respect to the level of complexity adopted in the treatment of the scattering problem. On one end of the spectrum, images may be produced by simple stacking of normalized P-to-S conversion records (i.e., receiver functions ), which are binned according to common conversion points (CCP) and mapped to depth. Finer resolution can be achieved through the stacking of singly scattered wavefields along diffraction hyperbolae to recover relative scattering intensity/potential at individual points through a 2D or 3D model space (Revenaugh, 1995; Ryberg and Weber, 2000; Sheehan et al., 2000). Moving to higher levels of complexity, methods involving inversion/backprojection operators (e.g., High-resolution, teleseismic imaging of the lithosphere beneath the Abitibi 1996 broadband array (IRIS-PASSCAL, Lithoprobe). a)Map of study area. b) CCP receiver function profile. Red and blue pulses denote discontinuities with positive and negative downward velocity gradients, respectively. Black lines show interpretation. Abbreviations: GF = Grenville Front, M = Moho. c) S-velocity perturbation profile obtained by 2D Generalized Radon Transform inversion (Rondenay et al., 2005), with red to blue colour scale representing negative (slower) to positive (faster) velocity perturbations. Bostock and Rondenay, 1999; Bostock et al., 2001; Rondenay et al., 2005) and full 3D waveform inversion of scattered teleseismic body waves recover either the scattering potential or estimates of localized material property perturbations with respect to an a priori background model. Bostock, M. G., and S. Rondenay, Migration of scattered teleseismic body waves, Geophys. J. Int., 137, 732-746,1999. Bostock, M. G., S. Rondenay, and J. Shragge, Multiparameter two-dimensional inversion of scattered teleseismic body waves, 1, Theory for oblique incidence, J. Geophys. Res., 106, 30,771-30,782, 2001. Rondenay, S., M. G. Bostock, and J. Shragge, Multiparameter two-dimensional inversion of scattered teleseismic body waves, 3, Application to the Cascadia 1993 data set, J. Geophys. Res., 106, 30,795-30,808, 2001. Rondenay, S., M. G. Bostock, and K. M. Fischer, Multichannel inversion of scattered teleseismic body waves: practical considerations and applicability, in Seismic Earth: Array Analysis of Broadband Seismograms, edited by A. Levander and G. Nolet, Geophysical Monograph Series, AGU, in press, 2005. Shragge, J., M. G. Bostock, and S. Rondenay, Multiparameter two-dimensional inversion of scattered teleseismic body waves, 2, Numerical examples, J. Geophys. Res., 106, 30,783-30,794, 2001. 88
2006 IRIS 5-YEAR PROPOSAL SURFACE OF THE EARTH: NORTH AMERICA Imaging Upper Mantle Structure Beneath the Illinois Basin Heather Bedle, Suzan van der Lee • Northwestern University The Illinois Basin is an oval-shaped cratonic basin which covers parts of southern Illinois and Indiana, western Kentucky, Tennessee, and Missouri. The shallow structure of the Illinois Basin is well-defined and provides constraints which may allow the effects of crustal structure to be removed from seismic waveforms during tomographic inversions. We used IRIS data to test if better constraints of crustal heterogeneities will allow better resolution of the upper mantle S-velocity structure beneath the Illinois Basin. This improved imaging of the crust and the upper mantle S-velocities provides insights into the development of the basin, and addresses the issue of whether it is possible to improve seismic waveform tomography on a regional scale by incorporating crustal constraints. Fundamental and higher mode waveforms from regional events were obtained from the IRIS DMC and were then inverted to image the three-dimensional upper mantle S-velocity structure under the Illinois basin. The method of partitioned waveform inversion, PWI, (Nolet, 1990; van der Lee and Nolet, 1997) was applied to the waveforms. Utilizing six mid-continent events, a total of 60 seismograms from the IRIS/IDA Network, the IRIS/USGS Network, the Cooperative New Madrid Seismic Network, the PEPP-Indiana network and PASSCAL experiments were fitted in this study (Figure 1a), incorporating external crustal constraints. These constraints were then inverted along with the constraints from S-velocity model NA00 (van der Lee, 2002). The linear inversions resulted in S-velocity model IL05. Figure 1b represents the deviation of calculated S-velocities for IL05 from the standard one-dimensional Earth model, MC35, for a profile across the Illinois Basin. Both NA00 and IL05 image a slightly thinner seismic lithosphere beneath the Illinois Basin. A thinner seismic lithosphere is also observed in both models south of the Illinois Basin, which may be related to the original upwelling of the mantle during the formation of the Reelfoot Rift. a.) Event to station ray paths used for the waveform fitting. The Illinois Basin is outlined in blue. Events are red circles and stations are red triangles. b.) Profile across the Illinois Basin shows IL05 S-velocity differences with 1D earth model MC35. Nol J. Geophys. Res., 95, 8499-8512, 1990. van der Lee, S., High-resolution estimates of lithospheric thickness from Missouri to Massachusetts, USA, Earth Planet. Sci. Lett., 203, 15-23, 2002. van der Lee, S. and G. Nolet, Upper mantle S velocity structure of North America, J. Geophys. Res., 102, 22815-22838, 1997. 89
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Cornerstone Facilities for Seismolo
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Cornerstone Facilities for Seismolo
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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