Missouri to Massachusetts Array of Broadband Seismometers Slicing Into the Earth: Seismic Mapping with the Missouri-to-Massachusetts Broadband Deployment

Introduction

Regional arrays of seismometers provide a powerful means of mapping the details of deep-Earth structure. Our understanding of the geological processes at work within our planet depends upon our ability to examine them; seismology remains the best tool available. However, spatial aliasing due to the less-than-optimal distribution of global seismometers has long made it difficult to determine deep-Earth structure from teleseismic waves. The temporary deployment of portable broadband seismometers can help by providing high-resolution windows into our planet. Patterns of global mantle convection create seismically observable features such as anisotropy at the top and bottom of the mantle, topography of upper mantle discontinuities, and heterogeneous structure at the core-mantle boundary. We hope to provide additional constraints on the style of mantle convection by quantifying these observations.

April, 1996, marked the end of a 1-year deployment of 18 seismometers as part of the Missouri-to-Massachusetts (MOMA) IRIS PASSCAL Broadband Deployment. The seismometers were evenly spaced between permanent stations CCM (Cathedral Caves, Missouri) and HRV (Harvard, Massachusetts), providing a 20-station array spanning 1740 km with a 91.5 km average interstation distance (Figure 1). The geometry of these stations is providing important information not only about the crust and lithosphere of the Eastern United States, but also of a 2-D slice across the entire mantle and into the core.

Deployment

The deployment of the MOMA stations was logistically challenging. Sixteen of the 18 temporary stations were borrowed from the IRIS PASSCAL pool, and because of shortages in instrument availability, a variety of types of broadband sensors, clocks and diSKS were used. The deployment began in the heavy snows of mid-January (1995) with an incomplete set of sensors, and was completed by late March using sensors borrowed from the Joint Seismic Program.

Eighteen evenly-spaced locations between stations CCM and HRV were initially targeted, and strong attempts were made to site the stations within 5 km of these initial targets. Vaults were usually placed on bedrock, though this was not possible for many of the western stations where layers of glacial loess are many tens of meters thick. Cloudy winter skies sometimes prevented the solar panels from recharging the deep-cycle batteries. Stations had to be visited often. The spring and early summer of 1995 were very rainy, so large drains had to be installed at several sites to prevent recurrent flooding. Despite problems with instrument failure, negative power budgets and flooding, the average MOMA station uptime was very high, and overall data quality is excellent.

Two data streams were recorded: 20 sps and 1 sps. While the 1 sps stream is redundant, since all information is contained in the 20 sps stream, having the 1 sps stream allows for convenient examination of the data and facilitates analyses that only require long-period data (such as with surface and core-diffracted waves). The data are in the process of being converted into SEED volumes through the use of newly-available database software written by PASSCAL engineers, and will be available from the IRIS Data Management Center in April, 1997.

Core-Mantle Boundary

The lowermost mantle has been the site of several exciting seismological discoveries over the past decade. The rock in the D'' region, the few hundred kilometers of mantle overlying the core, contains a greater degree of lateral variation than any part of the Earth other than its surface. There is a pattern to the long-wavelength variation in D'' velocities that greatly resembles the locations of Mesozoic paleosubduction, suggesting some degree of mass transport between the upper and lower mantles. Many regions display a discontinuous velocity increase atop D''. The depth of this increase varies by location, and there is some suggestion that it may be different for P and S waves. Because D'' may be both a conductive thermal boundary layer between the core and mantle as well as a chemical boundary layer containing differentiated mantle phases, more than one discontinuity is possible. The core-mantle boundary (CMB) causes significant seismic scattering. This could be a result of small-scale heterogeneity within D'' or in CMB topography; there is even evidence of an extremely slow velocity layer at the bottom of D''.

The MOMA array was designed to examine several of the current questions about the CMB. Its stations recorded roughly 53% of the world's large earthquakes in the a distance range of 100° - 140°, nearly double that of an average global station location. Most of these earthquakes came from the southwest Pacific. This range is ideal for core phases that diffract along and refract across the CMB. Waves from earthquakes in the Tonga/Kermadec region arrive nearly along-strike to the MOMA array. Figure 2a shows core-refracted S_diff waves from transverse MOMA seismograms for a Kermadec earthquake on July 3, 1995. The ray parameter (slowness) of the S_diff arrivals is the slope of the arrival times with respect to distance, and is a direct result of the velocity structure at the very base of the mantle. The change in the shape of the diffracted waves can also be used to model the vertical structure of D'' through an analysis of the rate of decay of amplitudes during diffraction as a function of frequency [Valenzuela et al., 1996]. The velocity structure N710 shown in Figure 2b obtained by forward-modeling the SH_diff amplitude decay, shown in Figure 2c. This preliminary model a discontinuous increase at a depth of 2710 km, 180 km above the CMB, underlain by a parabolic decrease. It is similar to the discontinuous increase found by Garnero et al. [1993], though with very different data and techniques. A preliminary analysis of the variation of this slowness of the core-diffracted P_diff waves for the Kermadec earthquake as a function of frequency shows the presence of a minimum value at a mid-frequency range is indicative of fast velocities at the top of D'', underlain and overlain by slower velocities, supporting the type of model shown in Figure 2b.

We are also examining the polarization of core-diffracted shear waves as an indication of anisotropy at the base of the mantle, in the manner of Maupin [1994]. Several other CMB studies are making use of the SKS phase, that travels through the mantle as an S wave but through the core as a P wave: (1) differential times of core-diffracted and core-refracted waves provide constraints on velocities above and below the CMB [Garnero and Helmberger, 1995; Wysession, 1996]; (2) an analysis of SKS waves that partially diffract around the core has revealed an ultra-slow velocity D'' layer north of its identification by Garnero and Helmberger [1996]; and (3) relative amplitudes of multiple SKS phases examine the S velocities at the very base of the mantle [Silver and Bina, 1992]. Work is also underway to invert the entire body wave train across the array for a long-wavelength image of a 2D slice through the mantle.

Subducting Slab Velocity Structure

The seismic velocity structure of subducting lithospheric slabs is fundamental to our understanding of mantle dynamics. The overall geometry and extent of subducting lithosphere is a powerful constraint on patterns of convective flow in the mantle. For instance, recent travel-time-based images of subducting slab velocity anomalies indicate a wide range of slab morphology within the mantle transition zone, suggesting that interactions between subduction flow and mantle structure are highly variable. Theoretical studies demonstrate that broadband waveforms should provide a strong complement to travel-time data in the resolution of slab structure, but the sparse distribution of permanent broadband stations has in general hindered use of waveforms to constrain slabs in the transition zone and lower mantle. Broadband waveforms recorded by the MOMA array will improve resolution of the geometry and extent of subducting slabs beneath the Caribbean, South America, and the northwest Pacific Ocean.

For example, two intermediate-focus earthquakes in Colombia generated body-wave phases that have excellent signal-to-noise ratios at stations across the array. These phases largely miss the lithosphere currently subducting beneath South America, but travel through a slab-like high-velocity anomaly that has been imaged in the lower mantle beneath the Caribbean and interpreted to be the subducted Farallon plate [Grand, 1994; and others]. The data contain strong core-reflected shear wave travel-time anomalies that are consistent with the existence of the Caribbean anomaly to great depth in the lower mantle, and they also manifest large variations in amplitude and waveform complexity. We plan to model body waveforms from these and other South and Central American events with synthetic seismograms to constrain both radial and lateral variations in the upper mantle and D''. This analysis should considerably enhance resolution of the width and dip of the lower mantle Caribbean slab anomaly.

Upper Mantle Structure and Crust

The geometry of MOMA is ideally suited to provide a transect of the crust and upper mantle across the Eastern United States. As seen in Figure 1, the stations extend from the stable midcontinent craton across the Appalachians to the Eastern terranes. This represents a large variation in crustal and lithospheric structure. MOMA can detail the transition of the lithosphere among these very different tectonic regions, including a variation in the depth to the MOHO. In addition, we can resolve variations in the depths of upper mantle discontinuities, that are the result of thermal variations associated with mantle convection.

The CORE method [Clarke, 1993] is one technique we are using to address these questions. This is a tool for the analysis of body-wave seismograms that not only allows interpretation of arrival times, but also provides a framework for waveform inversion. Whole body-wave trains from multiple seismograms can be inverted to reveal a three-dimensional image of mantle velocities, including discontinuity locations. In addition, receiver functions are being generated for the MOMA stations, providing information about the thickness and velocity of the crust and additional constraints on the depths of mantle discontinuities.

Surface waves travelling across the MOMA stations can be inverted for upper mantle structure because of variations in group and phase velocity dispersion. Large earthquakes both in the Southwest Pacific and in Western North America (Northern Mexico, Southern California, Texas) have occurred nearly along-strike, providing an opportunity to make a 2-D image along a slice through the crust and mantle. An example is shown in Figure 3 for the West Texas Earthquake of April 14, 1995 [Al-eqabi et al., 1996]. Figure 3a shows the propagation of the body and surface wavetrains across MOMA as seen on the vertical component. Figures 3b,c show an example of a waveform inversion for the vertical component record at station MM04, in Le Raysville, PA, using an algorithm of Nolet [1990]. The solid line represents the data and the dashed line depicts the model, both before (3b) and after (3c) inversion. The best fitting model for the West Texas - MM04 path is shown as a solid line in Figure 3d, with the initial model represented as a dashed line [Iyer and Hitchcock, 1989]. The final model has a deeper MOHO depth (38 km) and faster sub-crustal velocities. Work is underway to produce a 2-D model of the crust and mantle beneath MOMA using the West Texas and other earthquakes in the manner of van der Lee [1995].

Upper Mantle Anisotropy

We are using an analysis of shear wave splitting to constrain the extent and orientation of seismic anisotropy beneath the eastern United States [Fouch et al., 1996], which can be an indication of lithospheric genesis and convective flow directions. The MOMA array provides detailed sampling of variations in anisotropic mantle structure across several different lithospheric provinces: the interior North American craton, the Grenville Province, the Appalachian Plateau and Range, and the complex orogenic zones of New England. A number of large, deep earthquakes at a variety of back azimuths yield very clear teleseismic core phases such as SKS that provide well-constrained splitting parameters (fast polarization direction, ¢, and splitting time, *t) at 15 of the 18 MOMA stations (Figure 1). Stations at which we have not yet documented splitting are marked by crosses, indicating an absence of anisotropy beneath the station, very weak anisotropy that cannot be detected, or anisotropy with a fast direction that is parallel or perpendicular to the back azimuths of events analyzed.

The observed splitting parameters represent anisotropy on nearly vertical paths from the core-mantle boundary to the station. Our data are the first measurements of shear wave splitting in Ohio, Indiana, and Illinois, and greatly increase lateral resolution of shear wave splitting in northern Pennsylvania and southern New York. Images of upper mantle shear wave velocity structure contain high velocities to depths of more than 300 km beneath the cratonic interior of the North American continent [Grand, 1994; van der Lee, 1995]. The eastern margin of this fast continental root intersects the MOMA array in the vicinity of MM07 - MM05 and coincides with a shift in the pattern of observed fast directions. From MM07 to the west , the fast directions are within 20° of the direction of absolute plate motion (∼WSW) (Figure 1). In contrast, the fast directions are more variable at stations MM06 - MM01, located within the Paleozoic Appalachian orogenic belt. From MM06 east to MM03, ¢ rotates from W to ∼NW, and from MM02 east to HRV,¢ repeats this pattern, rotating from WSW to ∼NW. These splitting measurements in general are consistent with the findings of previous studies over length scales of ∼100 km [Levin et al., 1995; Barroul et al., 1996; and others]. However, the more densely spaced MOMA stations in northern Pennsylvania and southern New York document a much stronger variation in fast direction orientation than is apparent in previous studies.

The magnitude of the splitting times (0.5 - 1.4 s) indicates that a substantial portion of the observed splitting originates from anisotropy at subcrustal depths. Assuming that the anisotropy is controlled by strain-induced preferred orientation of upper mantle minerals like olivine, the variations in fast direction observed across the MOMA array are strong evidence that shearing in the asthenosphere parallel to the absolute motion of the North American plate is not the sole source of the anisotropy. We are investigating a variety of models to explain the splitting observed. In one, the translation of a mechanically strong North American continental root drives asthenospheric flow around the continental root and produces an asthenospheric fabric and fast direction orientation parallel to the root margin. In this model, some of the complexity in the observed fast directions from MM05-MM01 could be related to asthenospheric flow around irregularities in root morphology. Another model we plan to evaluate is one where the anisotropy is largely located in the lithosphere and reflects ancient deformation beneath the western segment of the array and more recent and complicated strain events (the Appalachian orogeny, and rift events in the Jurassic and Triassic related to the opening of the Atlantic Ocean) beneath the eastern segment of the array. A third possible explanation is that the variable fast directions, particularly over the eastern segment of the array, reflect the superposition of significant asthenospheric and lithospheric anisotropy.

Acknowledgments

We would like to thank the many people at IRIS who made this experiment possible, and in particular, Paul Friberg, Carl Ebeling, Sid Hellman, Gennady Pratusevich, Tom Jackson, John Weber, Bob, Busby, Tim Ahern, and Rick Benson. We thank Tom Owens for helpful advice about station deployments and Francis Wu for his help in locating the MM04 site. We thank the many land owners, private and public, who allowed us to dig up their back yards.

References

Al-eqabi, G. I., M. E. Wysession, P. J. Shore, K. M. Fischer, and T. J. Clarke, Upper mantle structure under the northeastern United States from the inversion of regional and teleseismic Rayleigh waves recorded by the MOMA array, Seismol. Res. Lett., 67, 29, 1996.

Barroul, G., P.G. Silver, and A. Vauchez, Seismic anisotropy in the eastern U.S.: Deep structure of a complex continental plate, submitted to J. Geophys. Res., 1996.

Clarke, T. J., The complete ordered ray expansion 2: Multi-phase body wave tomography, Geophys. J. Int., 115, 435-444, 1993.

Fouch, M. J., K. M. Fischer, M. E. Wysession, and T. J. Clarke, Shear wave splitting beneath the Eastern United States, Abstracts from the 8th Ann. IRIS Workshop, Blaine, WA, 56, 1996.

Garnero, E. J., and D. V. Helmberger, Seismic detection of a thin laterally varying boundary layer at the base of the mantle beneath the central Pacific, Geophys. Res. Lett., 23, 977-980, 1996.

Garnero, E. J., D. V. Helmberger, and S. Grand, Preliminary evidence for a lower mantle shear wave velocity discontinuity beneath the central Pacific, Phys. Earth Planet. Int., 79, 335-347, 1993.

Grand, S. P., Mantle shear structure beneath the Americas and surrounding oceans, J. Geophys. Res., 99, 11,591-11,621, 1994.

Iyer, H. M. and T. Hitchcock, Upper-mantle velocity structure in the continental U.S. and Canada, in Pakiser,L. C., and W. D. Mooney, Geophysical framework of the continental United States: Boulder, Colorado, Mem. Geol. Soc. Am., 172, 681-710, 1989.

Levin, V., W. Menke, and A. Lerner-Lam, Seismic anisotropy in northeastern U.S. as a source of significant teleseismic P traveltime anomalies, submitted to Geophys. J. Int., 1996.

Maupin, V., On the possibility of anisotropy in the D'' layer as inferred from the polarisation of diffracted S waves, Phys. Earth Planet. Int., 87, 1-32, 1994.

Nolet, G., Partitioned waveform inversion and two dimensional structure under the Network of Autonomously Recording Seismographs. J. Geophys. Res., 95, 8499-8512, 1990.

Silver, P. G., and C. R. Bina, An anomaly in the amplitude ratio of SKKS/SKS in the range 100-108° from portable teleseismic data, Geophys. Res. Lett., 20, 1135-1138, 1993.

Valenzuela, R. W., M. E. Wysession, G. I. Al-eqabi, P. S. Shore, T. J. Clarke, and K. M. Fischer, Determination of lateral and radial velocity structure of the base of the mantle from diffracted waves recorded using the Missouri-to-Massachusetts IRIS PASSCAL Array, Eos Trans. AGU, 77(17), Spring Meeting suppl., S178, 1996.

Wysession, M. E., Large-scale structure of the core-mantle boundary from core-diffracted waves, Nature, 382, 244-248, 1996.

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