Project Summary
Intellectual Merit. We propose to improve understanding of the lithosphere-scale structure of
North America's stable continental interior (Midcontinent cratonic platform) by addressing seven
major issues: (1) How does the lithosphere beneath intracratonic uplifts (domes and arches) differ
from that beneath intracratonic basins? (2) How did the lithosphere of the cratonic platform
assemble during the Proterozoic? (3) What is the manifestation of rifts and other fault-and-fold
zones at depth? (4) What characteristics distinguish lithospheric regions with a propensity for
intraplate seismicity from regions that do not? (5) What lies beneath the anorogenic graniterhyolite province? (6) How did the Midcontinent platform of the United States transform into a
craton and what provides the craton's strength? (7) What features of the lithosphere are associated
with sharp gradients displayed on maps of potential-field data?
We propose to deploy a 3-D, broadband, passive seismic array centered over the
depocenter of the Illinois Basin. The array was designed to provide enhanced station density or 3
to 1 relative to the Earthscope TA. The design builds on the experience of the first major
EarthScope FA experiment (SNEP) that was set up in the southern Sierra Nevada. SNEP
unambiguously demonstrated the imaging capabilities of a dense 3-D array, and showed how
existing imaging methodologies can provide important insight into lithospheric-scale processes.
In order to ensure that we are able to extract the best results from our data, and thus go beyond
the simple production of "redite-bluite" seismic images, we have built the foundation for a largerscale interdisciplinary collaboration of researchers at a recent EarthScope Workshop on
Geoscience Issues of the Midcontinent.
Our study area includes some of the best examples of cratonic platform lithosphere
structures to be found anywhere. Specifically, it spans the Ozark Dome and the Illinois Basin
(including the abrupt boundary between them along which the Precambrian/Cambrian
unconformity displays over 7.5 km of structural relief), as well as the Rough Creek Graben and
other major fault zones including the Wabash Valley Seismic Zone. It also crosses major
Proterozoic sutures and the enigmatic Eastern Granite-Rhyolite province. The seismic
experiment we propose will provide sufficient resolution to determine if distinct crustal or uppermantle features relate to these features, and thus will provide a basis for testing models put forth
to explain these features. We will occupy 120 stations over a 3-year period coinciding with the
deployment of the TA; site occupation will roll in four distinct stages using a maximum of 60
simultaneously operating stations. The main data analysis tools we propose to use as part of this
project are: (1) travel-time tomography using teleseismic P and S waves; (2) wave-field imaging
with P receiver functions using CCP stacking methods and a recently developed 3-D plane-wave
migration method; and (3) complementary wavefield imaging of S-wave receiver functions. We
will also conduct a detailed analysis of seismicity recorded on this dense array, to better locate
earthquake sources and characterize faults. Estimates from previous work in the area suggest we
can expect of the order of 100 local earthquakes and 12,000 mining explosions during the
recording period.
Broader Impacts. The Midwest does not have a strong tradition of earth-science awareness, so
our project provides a unique opportunity to reach a new audience. We propose an integrated
education and outreach component, involving: (1) In-service teacher workshops, to expand the
pool of master teachers knowledgeable about EarthScope science; (2) a teacher Research Fellows
program to include high school science teachers in summer research teams; (3) Involvement of
students and the public in EarthScope-related activities using already established connections
with schools, pre-college research symposia, and State Survey outreach programs (field trips;
open houses; public presentations); (4) Development of material for "Active Earth" displays for
regional outreach, and for incorporation in widely used introductory geoscience textbooks
(authored by one of the projects co-PIs); (5) an undergraduate REU program; and (6) graduate
student education at all institutions.
PROJECT DESCRIPTION
Perspectives on the Continental Interior: A Geoscience Frontier
The stable continental interior ("cratonic platform") of North America is a relatively
unexplored frontier for EarthScope. Indeed, one might argue that advances in understanding the
structure and dynamics of the continental interior from USArray may prove to provide a bigger
jump in understanding than did those made in the Cordillera, because so little is known about the
deeper structure of the lithosphere of the US Midcontinent. Many first-order problems about the
region remain to be addressed. One of the founding principles of USArray was that increased
resolution made possible by the array would improve understanding of the processes that created
and shaped the continent. The resolution gap in the stable interior is vast compared to the
Cordillera. In the continental interior, there are almost no high-resolution models for regions
outside of the New Madrid region. And even the most recent tomographic models (e.g. Bedle
and van der Lee, 2006) have an order of magnitude lower resolution than local models that
existed in numerous places in the western US prior to EarthScope. One of the major
contributions of USArray to date is that it has provided a framework in which a patchwork of
higher resolution models can be integrated with results using the regular geometry of the new
array. The Midcontinent, in contrast, lacks even that patchwork to build upon.
Our proposal seeks to produce a transformational understanding of cratonic structure, and
the processes that produced it, by establishing a high-density passive seismic array experiment.
This experiment will provide an integrated, high-resolution view across a swath of the continental
interior that includes definitive examples of major intracratonic structures (basins, domes, sutures,
geochemical boundaries, rifts, fault zones, and anorogenic granite provinces). Our results will
provide insight into the relation of these features to characteristics of the deeper crust and upper
mantle, and will provide a foundation to tie together results from decades of geological research - much of which resides in state geological survey archives -- into a coherent picture. It will also
set the stage for future active-source and magnetotelluric studies that can help relate upper-crustal
features to deeper ones that are less tied to the transportable array.
Scientific Focus, and Problems to be Addressed
Cratons are the relatively stable and long-lived portions of continents whose overall
fabric and metamorphic grade has not changed significantly during at least the past half-billion
years of Earth history. They comprise over a quarter of the continental area of the Earth.
Geoscientists divide cratons into shields, regions in which Precambrian crust crops out over a
broad area, and cratonic platforms, regions in which a veneer of Phanerozoic sedimentary strata
up to 7 km thick buries Precambrian basement. The portion of North America's craton that lies
within the United States is almost entirely a cratonic platform with low topographic relief.
Because of its cover, the at-depth structure of this region remains very poorly understood. What
is known about it comes from compilations of drill-hole data, potential-field studies, a few
available seismic-reflection profiles, and low-resolution tomographic studies. As a result, so
many first-order questions about this substantial volume of lithosphere remain unanswered that a
focused Earthscope experiment in the region will undoubtedly make a major contribution. The
EarthScope transportable array (TA) will sweep into the USA Midcontinent portion of the craton
during 2011 and provides a unique and invaluable opportunity to address several of these
questions. However, many of the key features of the craton that need to be analyzed have
dimensions on the order of tens of kilometers, and thus cannot be resolvable by TA data alone.
We propose to use instruments of the flexible array (FA) to densify instrument spacing within a
key region of the cratonic platform (Fig. 1). The densification of the array will provide sufficient
resolution to provide substantial new insight into the formation and behavior of the lithosphere of
a cratonic platform.
The specific region that we propose to analyze (Figures 1 and 2) extends across two of
the largest epeirogenic structures in the world (the Ozark Dome and the Illinois Basin), two
possible major sutures associated with the Proterozoic assembly of the craton (the 1.6 Ga crustal
boundary and the Grenville Front), several important rifts and fault-and-fold zones (including the
Reelfoot Rift and the Rough Creek Graben), one of North America's most active regions of
intraplate seismicity (including the Wabash Valley seismic zone), and one of the world's largest
"anorgogenic" igneous province (the 1.47 Ga Eastern Granite-Rhyolite province). Data from this
experiment (Fig. 1) will be used to address the following issues related to the structure and
dynamics of cratonic North America.
Fig. 1. Geometry of proposed flexible array (FA) experiment. Upper map is a shaded relief map of topography and the
lower right map shows the location of the upper map in reference to a map of faults and depth to the top of Precambian
rocks. Triangles show the idealized location of TA stations and circles show approximate locations of 120 FA stations
we propose to add to supplement the TA in this region. The approximate location of the Grenville Front is shown to
illustrate how the arrays spans Precambrian outcrop in Missouri to the Grenville Front.
How does the lithosphere beneath intracratonic uplifts (domes and arches) differ from
that beneath intracratonic basins? The surface of lithospheric basement in cratonic platforms is
not horizontal (even where overlain by flat topography) but rather is affected by broad (> 100-km
wavelength), first-order warps that rise or subside in pulses over geologic time (e.g., Park and
Jaroszewski, 1994; van der Pluijm and Catacosinos, 1996; Howell and van der Pluijm, 1999).
They are long-lived, in the sense that intracratonic uplifts (domes and arches) host thin,
condensed sections whereas intracratonic basins host thick, fairly complete sections (Heidlauf, et
al., 1986; Leighton et al., 1991; Bond and Kominz, 1991; Kolata and Nelson, 1991; McBride et
al., 2003). Subsidence analysis indicates that, in general, when basins are subsiding, uplifts are
rising (Tranel, 2005). Our proposed study area spans the Ozark dome (presently manifested by
a broad plateau), the Illinois basin, and a portion of the Cincinnati arch. Notably, the structural
relief between the Precambrian/Cambrian unconformity exposed at crest of the Ozark Plateau and
the same horizon on the floor of the Illinois Basin, only 100 km to the east, is over 7.5 km,
comparable to orogenic relief between the Ganges and the summit of Mt. Everest (Figure 2;
Marshak et al. 2005). Our proposed analysis will allow us to compare the lithosphere beneath
these examples of epeirogenic provinces to determine if behavior during the past half-billion
years correlates with lithospheric scale structure. Debate has not been resolved as to whether
epeirogenic provinces of cratonic platforms are flexural, are a manifestation of lithospheric
buckling, are related to the existence of uncompensated crustal or mantle loads (positive beneath
uplifts and negative beneath basins) that move at times when the lithosphere weakens, or are
associated with sub-lithospheric mantle anomalies (e.g., Marshak and van der Pluijm, 2003). We
will focus in particular on the character of the abrupt boundary between the Ozark Plateau and the
Illinois basin and the northern edge of the Rough Creek graben as it is likely that the greatest
contrasts will be observed across these boundaries. The study area also encompasses the abrupt
boundary separating the portion of the Illinois basin that has subsided the most (the Fairfield subbasin) from portions to the north that have subsided less.
Figure 2. (a) Oblique, vertically exaggerated, shaded-relief map of the central Midcontinent, emphasizing the
structural relief on the Precambrian/Cambrian contact between the Ozark Plateau and the Illinois Basin. (b)
Schematic tectonic map of crustal provinces and regional structures in the central USA.
Seismic images obtained in this study will be able to determine if there are significant
contrasts in lithospheric properties. This is testable by comparison of 3D wavespeed estimates
from tomography and detailed crustal thickness estimates from P receiver function analysis to
geologic data. For example, we expect to produce higher-resolution images of the diffuse lowvelocity body revealed by the Bedle and vanderLee (2005) tomography model (Fig. 3), and we
can produce a real estimate of crustal thickness to compare to the well known features of this
basin.
How did the lithosphere of the cratonic platform assemble during the Proterozoic?
Although the crust of the cratonic platform was assembled in the Precambrian it is now nearly
completely covered with younger sediments. As a result the detailed geometric relationship of
different blocks of lithosphere that form this region remains virtually unknown. Recent
compilations suggest that the boundary between the Mazatzal and Yavapai terranes (Fig. 2) trends
northwest and through the northern boundary of Illinois (Van Schmus et al., 2007; Holm et al.,
2007; Sims et al., 2008). Which way the suture dips at depth is not known. It may or may not dip
southeastward beneath the Illinois basin. A second, poorly known boundary, cuts diagonally
through the center of the Illinois basin. This boundary, which is based on ages measured from
limited deep core samples of granite, appears to delineate the southeastern edge of 1.6 Ga crust
(Van Schmus et al., 2007). Whether or not this boundary can be imaged remains to be
determined. The eastern end of the proposed study area includes the Grenville Front, a shear
zone delineating the northwestern limit of the 1.1 Ga Grenville Orogen (e.g., Baranoski et al.,
2009). Imaging this relatively well-known boundary, will provide a basis for interpretation of
other potential sutures within the proposed study area. In both cases, the increased resolution
made possible by the proposed FA experiment is essential to address these issues. Lateral
resolution of structures with existing array imaging techniques is close to the average station
spacing. Since 70 km is roughly twice the thickness of the crust and is comparable to our best
estimates of lithospheric thickness, this is a serious limitation. The dense spacing of our proposed
FA experiment is essential to resolve details of crustal thickness (P receiver functions),
lithospheric-scale dipping sutures (3D migration methods), and variations in bulk material
properties in different blocks that separate sutures (P and S wave tomography). Fig. 3 clarifies
this scale variation by illustrating the size of these structures relative to the depth of the sediments
and crustal images produced from a variety of other techniques.
Fig. 2. Study areas in perspective with selected previous studies. The center cross-section illustrates features that we
can expect to see within the study area that serve as a working model derived from previous data. The two top sections
and that on the right are from studies based on seismic reflection data (Okure and McBride, 2006; Bear, 1990; and
Culotta et al., 1990). These sections illustrate the major difference in scale between features resolved by reflection data
and passive array imaging as proposed here. The lower left panel illustrates the opposite resolution character. This is a
section from the IL05 S wave tomography model of Bedle and van der Lee (2006). The location of the IL05 section is
shown relative to our study area in the map at the lower right. Note that Moho depths, the Grenville Front, and the
depth of the lithosphere asthenosphere boundary (LAB) in this section are virtually unknown. The LAB depth can be
inferred from IL05, but crustal depths are only extrapolations.
What is the manifestation of rifts and other fault-and-fold zones at depth? The proposed
study area includes major rifts, which preserve thick sections of strata, and Midcontinent faultand-fold zones that may represent inverted rifts and be manifestations of "Laramide-style"
basement-involved faulting active during Paleozoic continental-margin orogenies (Kolata &
Nelson, 1991; Marshak and Paulsen, 1996; Marshak et al., 2000; McBride and Nelson, 1999).
Seismic-reflection data also have revealed enigmatic zones of reflectors at depth, interpreted to
be buried caldera complex (McBride et al., 2003). By combining tomography results with highresolution P receiver function images we will be able to determine if there are deeper
manifestations of the fault zones (e.g., do they overlie velocity anomalies, or localized changes in
crustal thickness). The densification of instruments will also allow for better location of
earthquake sources, which may permit delineation of fault dip in the upper crust. Knowledge of
fault dip direction is essential for the determination of the strain significance of faulting.
What characteristics distinguish lithospheric regions with a propensity for intraplate
seismicity from regions that do not? Seismicity in intracratonic regions occurs far less frequently
than that at plate boundaries, but it does occur and can pose substantial seismic risk (e.g.,
Hamburger and Rupp, 1988; Bear et al., 1997; McKenna et al., 2007; Tuttle et al., 2002; Stein
and Mazzotti, 2007). At least five moderate-sized (M~5) earthquakes have taken place in the past
half-century in our proposed study area, which lies to the north of the significantly more infamous
New Madrid Seismic zone. In addition, evidence from paleoseismology indicates that the zone
has hosted at least six large, prehistoric earthquakes over the past 20Ka. (Obermeier et al., 1991;
Munson et al., 1995). However, the distribution of intraplate seismicity is not uniform. A
significant concentration of seismicity occurs in the Wabash Valley (Nuttli, 1979; Braile et al.,
1982; 1986; Nelson and Lumm, 1984; Sexton et al., 1986; Bear et al., 1997a; Pavlis et al., 2002;
Kim, 2003; Pavlis et al., 2002; Eagar et al., 2006). Our proposed study area traverses the Wabash
Valley zone and also straddles a subtle boundary (approximately the latitude of St. Louis)
between the relatively seismic and relatively aseismic portions of the Illinois basin. Our analyses
will allow us to determine if there are changes in lithospheric structure across this boundary and
provide insight into the question of what determines the spatial distribution of seismicity. A more
detailed understanding of seismogenesis in such intracratonic settings relies on improved models
of lithospheric structure and rheology of intraplate seismic zones. Various models for this
seismicity have been proposed, including: seismicity occurs in zones of weakness (Sykes, 1978);
lithospheric weakening due to high heat flow (Liu and Zoback, 1997); gravitational instabilities
due to high-density crustal bodies (Pollitz et al., 2001); variations in lithospheric thickness
(Schulte and Mooney, 2005); manifestation of glacial unloading (Wu and Mazzotti. 2007;
Grollimund and Zoback, 2001) or of poroelastic effects associated with changes in river discharge
(Costain et al., 1987); transient processes in the lower crust (Kenner and Segall, 1997); or is
driven dynamically by the sinking of the underlying Farallon slab in the asthenosphere (Bedle and
van der Lee, 2006; Forte et al., 2007). Our project will permit evaluation of the options by
producing 3-D models of Earth structure that can be compared with more precise (albeit small)
catalogs of earthquake hypocenters made possible by the dense recording array and historical
seismicity.
What lies beneath the anorogenic granite-rhyolite province? A carapace of granite and
rhyolite cover forms the upper part of the crust throughout much of the central United States
(Fig.2; Van Schmus et al., 1996). Fundamental debate remains as to whether these rocks
originated in association with subduction, rifting and extensional collapse, or some other
mechanism. Distinguishing among such alternatives requires better constraints on the thickness
of the granite-rhyolite carapace, and on whether there is correlative zone below the rhyolite from
which the magma was derived. This project can help distinguish among alternative hypotheses
for the origin of the granite-rhyolite province by relating 3-D images from tomography to current
estimates of regional extent of these rocks. In particular, a working hypothesis is that the
wavespeed of the crust and upper mantle would be different under the granite-rhyolite province
than areas not impacted by this event.
How did the Midcontinent platform of the United States transform into a craton and what
provides the craton's strength? The process by which orogenic crust transforms into craton
remains one of the great enigmas in geoscience (e.g. Hinz, 1996). The proposed study area
contains crust that has undergone such a transition relatively recently (post 1.6 Ga). Our
proposed FA experiment will allow us to evaluate this issue with improved resolution in a region
of the lithosphere where we have every reason to believe there could be rapid variations in
lithospheric properties. For example, results from MOMA (Li et al., 2002) suggest lithospheric
thickness decreases significantly across the Grenville Front. This issue is complicated by the fact
that there are multiple measures for estimating the lithosphere asthenosphere boundary (LAB)
from seismic data (Romanowitz, 2009) that are not internally consistent. In particular, inferences
from tomographic methods like that of Bedle and van der Lee (2009) (Fig. 3) are not consistent
with inferences from P (Li et al, 2002) and/or S receiver functions. Receiver functions have the
best potential for high resolution mapping of lithospheric thickness, but as Romanowitz (2009)
points out the current methods lack an unambiguous mineral physics link to explain the velocity
reversal commonly interpreted as the LAB. This is particularly problematic for cratons where
there is little data and the cratonic "root" may have a drastically different signature.
We will address this issue using 3-D S receiver function images cross-validated with
tomography models. S wave receiver functions have been used extensively in a series of recent
papers (e.g. Kumar et al., 2005a,b,2006; Landes et al.,2007) to define a seismic LAB. The more
commonly used P receiver functions are problematic for imaging the LAB because crustal
multiples back project to the same depths. S-to-P data, in contrast, do not suffer from this
problem but come at the cost of being harder to estimate reliably and having a lower frequency
content due to the lower frequency content of S waves relative to P waves. We propose to
address this issue using S receiver function imaging at two scales: (1) a S-to-P, three-dimensional
image within the study area, and (2) a comparable continental scale image volume from the entire
TA. Pavlis will be working on the later under a recent grant from the Earthscope program. The
higher resolution result from this experiment is then a natural add on and will allow us to directly
address this issue. Given the disconnect between lithospheric thickness estimated from
tomography and receiver function data (Romanowicz, 2009), it will also be important to crossvalidate the receiver function results with our own tomography results and regional scale results
by other investigators (see collaboration letters in supplement).
What features of the lithosphere are associated with sharp gradients displayed on maps
of potential-field data? Sharp gradients in geophysical field measurements cross our proposed
study area (Braile et al., 1982; Hildenbrand and Ravat, 1997). Some of these, for example, the
Commerce Geophysical Lineament (Hildenbrand et al., 2002; McBride et al., 2002), are spatially
associated with known fault zones or align with Precambrian structural grain. However, not all
display such associations, and thus may be related with as yet unknown features at greater depth.
Imaging deeper crustal and mantle levels will allow us to test this concept and understand the
fundamental nature of potential-field fabrics in cratonic platforms. Because none of the PIs
assembled for this project are potential-field specialists, this will be addressed with collaboration
made possible by the regional collaborations described below (see also support letters).
Surprisingly, decades after the advent of plate tectonics, we have no universally accepted
theory to explain controls on tectonism in cratonic platforms, even though these regions account
for over 25% of continental lithosphere. The limitation on understanding comes largely from
lack of data on the deeper character of the crust and mantle beneath these regions. Such data may
become available through EarthScope, so the unique opportunity to organize a coordinated
scientific agenda for studying a particularly illustrative example of cratonic platform is not to be
missed. Below, we provide the details of our proposed instrumentation plan, a work plan for
setting it up, and a description of the specific types of analyses that we will do with the data that
we collect. We anticipate that our results will set the stage for a new generation of focused
studies, including work with active-source seismology, magnetotellurics, and geodynamic
modeling, which will serve to link the at-depth images that we obtain more directly with uppercrustal features, and lead to a better understanding of the dynamic evolution of the region. We
also describe an ambitious program of education and outreach that has the potential to
disseminate the excitement of this grand scientific experiment to a broader audience throughout
the Midwest.
Fig. 4. SNEP interpreted receiver function cross section showing lithospheric foundering beneath the western Sierra Nevada
(Frassetto et al., 2010). Red colors between 30 and 40 km depth represent positive conversions produced at the Moho, while blue
represents negative conversions at the boundary between the lithosphere and asthenosphere. Locations of nearby earthquakes
projected onto the profile are shown as circles. GV = Great Valley, SNB = Sierra Nevada batholith, WL = Walker Lane. SNEP
stations plotted as blue triangles across the mountain range, with the location of transect shown on 40s phase velocity map (left)
illustrating high lower crustal and upper mantle velocities of the western Sierran foothills and low velocities beneath Long Valley
(Gilbert et al., 2009.
Experiment Design
Geophysical observations of the central portion of North America provided observations
of crustal structure [e.g. Braile et al., 1982; Bear et al, 1997; McBride et al., 2002; French et al.,
2009] that primarily focused on shallow features (Fig. 3). Comparison to existing images of largescale features in the mantle offer enticing glimpses into the potential to relate crustal structures to
mantle features, and thus provide insight into the complex evolution of the continent (e.g. Li et
al., 1998; Li et al., 2002; van der Lee et al., 2002; Bedle and van der Lee, 2006). With this in
mind we designed our experiment based on three principles. (1) Improving understanding of
North America's continental-interior (cratonic platform) lithosphere requires resolution at a scale
better than what is currently possible with the TA but broader than the scale that can be derived
from surface geology and seismic-reflection imaging. (2) The structure of the cratonic platform is
unambiguously three-dimensional so techniques based on linear arrays of receivers cannot
effectively address most of the problems described above. (3) To obtain the best data, the array
should intentionally avoid areas with thick glacial deposits or with thick accumulations of
unconsolidated sediments (e.g., the Mississippi embayment). The area that we have selected will
provide bedrock sites for most stations (based on M. Hamburger’s experience in GPS monitoring
in this region), providing better quality data than might be obtainable from other potential study
areas in the Midcontinent.
Building on the success of the Sierra Nevada EarthScope Project (SNEP; Fig. 4), we
propose to deploy a FlexArray component of USArray to study lithospheric structure in the area
shown in Fig. 1. The SNEP array demonstrated the ability of the FlexArray to image crustal
structures and connect them to features in the mantle beneath the Sierra Nevada. For our new
project, we propose to supplement the TA to decrease station spacing to 25 km, which is
comparable to that used in the SNEP deployment. The geometry of the SNEP array has proven to
be effective for imaging lithospheric structure at lower crustal and upper mantle depths. Stations
spaced 25 km apart, or less, leads to areas of the crust and upper mantle being sampled by
multiple stations and allows for the lateral extent of crustal features to be determined to within
~10 km (see SNEP Fig.).
There are approximately 30 TA stations within the footprint of the proposed FA
deployment. Correspondence with the IRIS-PASSCAL instrument center (see supplemental
documents) indicates 50 to 60 instruments could be available within a timeline to match the
arrival of the TA in western Missouri. Our design is based on a 60 element array rolling as close
as possible with the TA. Scheduling constraints will get us slightly out of phase because at best
only 25 broadband instruments would be available to us by summer 2011 and the remainder
would not be available until early 2012. But the east-west dimension of this profile is
approximately matched with the TA roll schedule. This is the basic reason for our proposed 3
year deployment since this area spans approximately a 2 year TA roll schedule with the added
year needed to match the 18 month nominal station recording time. The total number of proposed
stations is 120 to match this roll schedule.
Data Management
For a long list of reasons we plan to use the cell modem telemetry that is available for FA
experiments. In this region that means that most, but probably not all, stations will provide a near
real-time data stream. We plan to build an extended virtual network using neighboring TA
stations and as many broadband ANSS stations of the New Madrid network as possible. The FA
real time data will be streamed directly to IRIS in a manner similar to the TA. We are completely
familiar with the Antelope system used for FA telemetry and will use it to set up an automated,
real-time detection and event association/location mechanism. This system will aid our data
processing strategy allowing us to build a complete database of events actually detected in near
real-time. This will also provide important feedback to identify data problems quickly so field
teams can be sent to fix problems. A graduate student assistant will be responsible for review of
real-time arrival picks, producing a catalog of preliminary event locations, and maintaining the
real-time and archival database.
Because of incomplete cell phone coverage and a host of other factors, real time data
streams are never complete. Hence, we will need to assimilate late date from two main sources:
(1) media downloads from station service runs, and (2) regional stations not accessible in real
time but available from IRIS DMC. We have done this multiple times with other experiments.
Indiana University has a useful facility for doing this processing called the data capacitor, which
provides a scratch space of 500 TB. The merged data will be transmitted to IRIS as definitive
data using established procedures with which both Pavlis and Gilbert are familiar.
Data Analysis
Earth Structure. When the merged data are assembled we will measure P and S arrival
times with the recently developed dbxcor (Pavlis and Vernon, 2010) program. This program,
which was developed to streamline this type of processing for the TA, will significantly enhance
the speed and quality of teleseismic body wave arrival time measurements. We have found that
the arrival time database produced by this processing is also an effective way to winnow out
poor-quality data to reduce the effort required for P and S receiver function estimation. Standard
receiver function estimates will be produced using existing data flows based on the iterative
method of Ligorria and Ammon (1999). We also intend to experiment with alternative
algorithms Pavlis is developing under current Earthscope support. The most promising is a
multichannel generalization of the Ligorria and Ammon method.
Body-wave tomography models will be estimated by multiple methods in collaboration
with the larger regional collaborative group (see supplementary support documents). We have
used S. Roecker’s code successfully in the past (Yang, 2004; Wu, 2004; Bravo and Pavlis, 2007).
We plan, however, to compare results with a modification by Cecily Wolfe (used in Wolfe et al.,
2009) of the popular Vandecarr code. Her modifications provide the option of using finite
frequency kernels instead of the conventional ray theory approach. We will use both options as
the finite-frequency theory is based on an approximation that is poor for this type of data at
crustal depths (Wolfe, written communication). We also plan to use surface wave tomography to
help further refine the depth extent of lithospheric features. We intend to utilize the two-plane
method of Forsyth and Li (2005) adapted for finite-frequency kernels (Yang and Forsyth, 2006).
This approach helped to illuminate detailed variations in the depth extent of lithospheric structure
in the Sierra Nevada (Fig 4). Gilbert is familiar with this processing technique and will lead this
effort.
P receiver function data will be analyzed by a set of conventional and nonconventional
methods. Conventional methods we will use include: (1) the Zhu and Kanamori (2000) method
which provides estimates of average crustal Vp/Vs and crustal thickness by superimposing
primary and crust-mantle free surface multiples; and (2) CCP stacking (Dueker and Sheehan,
1997, 1998) focused on the crust-mantle boundary with the enhancement described by Frassetto
et al. (2010). We will also use the recently developed 3D, plane-wave imaging algorithm (Pavlis,
in review; Fig. 6a) based on the theoretical framework of Poppeliers and Pavlis (2003a,b). This
method is well suited to this experimental geometry as is capable of resolving much more steeply
dipping structures than are possible with the now conventional CCP method. With this approach
we may, for example, be able to directly image the suture at the Grenville Front and possible
lithospheric-scale boundaries associated with the 1.6 Ga discontinuity. A more regional synthesis
with TA data can provide links possible lithospheric discontinuities associated with the Mazatzal
and Yavapai terrane boundaries (Fig. 2).
S receiver function data are more problematic, but as noted above provide a critical
means to test questions about lithospheric structure of the craton. We have experience with this
processing with data from the Bolivar project (Landes and Pavlis, in review) and from the STEEP
project in Alaska (Bauer et al., 2008). There are some important technical issues regarding how
this analysis can be done more efficiently and reliably. There is not sufficient space to address
these issues and they are largely outside the scope of this project, which is focused on earth
science issues and not data processing methodologies. For example, the generalized iterative
method noted above for P receiver function estimation may be adaptable to this purpose. The key
point is some technical work will be required make processing data for S receiver functions more
robust. We have existing processing flows that allow us to construct CCP stacks of S receiver
functions in 3D, but for this project we would use a revision of the plane wave code (under
development) that would allow 3D back projection or plane wave migration of S receiver
function data.
Seismicity. Analysis of intraplate seismicity in the Illinois Basin and environs will be an
important additional scientific product of this experiment. The deployment of 150 broadband
sensors (counting TA) in the Ozark/Illinois Basin/Rough Creek Graben area will significantly
augment existing seismograph networks in the region. The deployment will enable highprecision earthquake location and possible first-motion source mechanism determination. Based
on our experience with previous deployments in the region, we anticipate reduction of the
nighttime detection threshold to M1.5 within the network and 2.0 for surrounding areas of the
Illinois Basin. Day-time magnitude thresholds are expected to increase by 0.3-0.5 magnitude
units (Pavlis et al. 2002; Webb et al., 2006). Based on current seismicity rates (see Fig. 5 of
Pavlis et al., 2002) we can expect to record ~40 earthquakes per year, or ~120 earthquakes in the
full 3-year period. Precise hypocentral locations, magnitudes, and earthquake mechanisms for
local events may allow us to identify the location, geometry, and level of activity associated with
seismogenic faults. In addition, the enhanced sensitivity of the network will allow us to examine
the unusual nest of microearthquakes in the Wabash Valley seismic zone reported by Pavlis et al.
(2002) and Eager et al. (2006) which they speculated was an induced swarm and hence may no
longer be active.
An easily unappreciated challenge for seismicity studies in this area is discrimination. A
detailed study of events detected by broadband stations in southernmost Indiana (Webb et al.,
2006) in a 251 day period in 2002 found: 207 larger teleseismic events, 28 regional events
(mostly New Madrid), 2 local earthquakes, and 5 events that were unclassified. In the same time
period there were approximately 8000 mining explosions. Fortunately, however, the discrimination problem is generally fairly unambiguous as explosions in this region nearly always have a
distinctive Rg phase that can be readily identified. We have found this as an ideal introductory
research project for an undergraduate intern or teachers who want to learn about seismology.
Hence, we propose to utilize undergraduate interns and EarthScope teacher summer research
fellows (see ’Broader Impacts’ section) to assist with this event identification processing.
Data Integration
We have assembled an interdisciplinary team of scientists to go beyond redite-bluite
seismic images and apply the results of the experiment to address broader earth science questions.
Because the study area and the technologies we propose to apply are intrinsically 3D appropriate
forms of 3D visualization will be essential to properly interpret the results. Fig. 5 illustrates two
of the basic tools we will use to accomplish this. Pavlis has developed tools to georeference and
assemble diverse data components into a single scene that can be viewed with a well supported,
public-domain visualization package called paraview (http://www.paraview.org). The example
shown in Fig. 5a shows we can superimpose 3D volumes, map line data derived from the generic
mapping tools (GMT), point data (e.g. earthquake hypocenters), and arbitrarily complex 3D
surfaces. Paraview allows one to integrate such diverse data in a single scene that can be
explored interactively on a desktop machine even with a large set of 3D data objects like those
shown in true 3D geometry. Fig. 5b also shows the other critical component we will need for
integration – GIS-based compilations of map data. The compilations show in Fig. 5b were
prepared in association with the EarthScope in the Midcontinent Workshop held in Urbana, IL, in
April 2010. Work is ongoing at the ISGS to complete this compilation and they will be served to
the public through the Geospatial Data Clearinghouse at the Illinois State Geological Survey and
mirrored at a similar site at the Indiana Geological Survey that allows for easy access through a
user-friendly web-based GIS interface (http://inmap.indiana.edu/index.html). We will work to
assimilate all our results into these systems for distribution of results to other scientists and the
broader public. This is an excellent example of how collaboration with the Indiana and Illinois
Geological Surveys can strengthen the broader impacts of this project.
Fig. 5. Examples of current data integration capabilities. (a) 3D visualization of the western US comparing a series of
3D objects derived from TA data. The rainbow colored slice is a section through the P tomography model of Burdick et
al. (2009). The red-blue image is a slice through the 3D P receiver function image from Pavlis (in review). These are
displayed with a 3D model of the top of the Juan de Fuca/Farallon slab displayed as a wireframe with 2.5 Ma contours
based on the current plate motion of Juan de Fuca – North America (NUVEL1A). (b) compilation of multiple data sets
for the Illinois Basin by the Illinois Geological Survey.
Experimental Plan
An FA deployment of 120 stations is ambitious, but borders on commonplace today.
This is approximately 1/3 larger than the number of stations deployed in SNEP, but we will be
working in far easier terrain logistically. Pavlis and Gilbert have been in involved in multiple
experiments of comparable scale and are well prepared to execute this type of experiment. The
schedule shown in Fig. 6 is based on initial communication with the PASSCAL Instrument
Center (see letter in supplementary documents) that indicates we could obtain 20 instruments in
summer 2011 and the remainder in early 2012 with a phased removal beginning in the early fall
of 2013 to winter 2014.
Fig. 3. Project schedule showing main tasks required to complete the seismic experiment at the top and E&O activities
on the lower part of the chart.
The Illinois State Geological Survey will provide access to their southern Illinois field
office in Carbondale as a forward field station for this project. This will facilitate shipping and
deployment from locations relatively remote from our base facilities in Champaign, Illinois; West
Lafayette, Indiana; and Bloomington, Indiana. For work in the eastern part of the project area, we
will use facilities of Indiana University and the Indiana Geological Survey in the Bloomington,
Indiana area.
Site preparation work will be conducted during the summers of 2011 and 2012 by student
teams from Purdue University and Indiana University. Teams consisting of a graduate student
leader and two undergraduate helpers will install approximately 40 vaults per team per season.
During the summer of 2011 we will use a team from Purdue and a team from Indiana to make
sure that we will be ready for the instruments when they arrive. During the summer of 2012 we
will need only a single site preparation team.
We plan to install the first set of 20 instruments in summer 2011. A required training
course will be held at Bloomington, Indiana, prior to the first installations. All inexperienced
participants would then take part in the initial station installs to apply course content. The
remaining installs would take place in stages according to the plan outlines in Fig. 6 using teams
from each of the institutions. The rolling characteristic of this experiment means the timelines
can be viewed as nearly constant activity with periods of more intense activity in summer and
when equipment first arrives and stations are scheduled to roll from west to east. The constant
background is dominated by two activities: (1) repairing failed equipment, and (2) service runs.
The former is possible because real time telemetry will allow us to know when failures occur. To
avoid schedule conflicts that would increase down time costs for such maintenance trips are
spread between the institutions. The second is necessary even with real-time telemetry for two
reasons. First, there will inevitably be some stations without cell coverage. Second, the auxiliary
recording devices currently available for the FA fill to capacity in approximately 120 days at the
planned sample rate of 40 Hz. Thus we plan four-month service intervals to avoid loss of data.
We are familiar with executing large experiments like this and emphasize it is
manageable only if there is good communication among the different institutions and
responsibilities of different parties are clear. We are taking advantage of the logistical strengths
of the different types of institutions involved in the project. The Illinois and Indiana Geological
Surveys have long-standing contacts with local officials and landowners. We will leverage these
contacts in the permitting process. State surveys also have professional staff available for
extended field assignments during winter months when students are not as available for
fieldwork. We will use these staff to deploy the large groups of instruments that we anticipate to
have available early in 2012 and then move them again the following winters. On the other hand,
we will use student workers whenever practical providing them with valuable field experiences.
Data analysis responsibilities will be divided between the research groups at Purdue and
Indiana University according to interests and expertise and will be determined relative to our
most important scientific goals. We have scheduled group meeting for the entire research team
twice a year. The proximity of all involved facilitates gathering for daylong meetings to discuss
deployment issues, seismic observations, and results as the data set matures. The tectonic analysis
and integration of data sets will take place at these group meetings and include all the PIs and
their students. These meetings will also provide an opportunity for students involved on the
project to get to know the students at the other institutions. Tectonic analysis will be carried out in
concert with 3-D visualization analysis so that relationships among various geological features
stand out and can be addressed. The locations of these meetings will rotate or occasionally take
place in a location equidistant from Bloomington, West Lafayette, and Urbana. Finally, to further
facilitate group dynamics among students we have budgeted for more extended student exchanges
between the three locations for one person-week per year.
Broader Impacts
Because of the intimate relation of this project to the arrival of the EarthScope TA, this
project presents a timely opportunity to enhance geoscience education and public awareness in
the region. The Midcontinent has not, traditionally, been a region of in which public awareness
of geoscience is high — EarthScope provides a major opportunity to remedy this situation. An
important strength of the PI team assembled for this project is that we have the foundations for
building a high-impact, integrated education and outreach program linked to the project: (1) both
the Indiana and Illinois Geological Surveys have comprehensive geoscience education and
outreach programs upon which our program can build; (2) Hamburger and Pavlis have been
leaders for more than a decade in a highly successful seismographs in schools program
(Hamburger and Pavlis (2003); see http://www.indiana.edu/~pepp); and (3) Marshak is the author
of two of the most widely used undergraduate geology textbooks and is in a position to
immediately place EarthScope-related methods and scientific results in undergraduate education
nationwide. The commitment to education and outreach as a core component of this project is
underscored by the fact that 13% of the budget is devoted to an E&O program. This funding will
support six distinct efforts.
(1) Professional Development Workshops. We will develop in-depth professional
development workshops for in-service teachers solicited from the four-state area covered by our
experiment (Missouri, Illinois, Indiana, and Kentucky). We propose a week-long workshop in
Year 1, followed by shorter ‘refresher’ workshops in years 3 and 4. The workshops will be
directed at earth science, physics, and environmental science instructors at the middle- and highschool level who are particularly interested in introducing authentic scientific research into their
classrooms. The goal of these workshops is to produce master teachers in the four-state study
area who will “spread the gospel” about EarthScope in the midcontinent to a broad audience—
and who will work with us to bring EarthScope data and science into the classroom. We will
follow the format of previous workshops conducted at Indiana and Purdue, and will incorporate
exercises and instructional materials from IRIS’s highly successful ‘Seismographs in Schools’
workshops (see support letter from IRIS). Indiana University has agreed to provide continuing
education credits at no cost to participants (see supplements). Recruitment, workshop
organization, and curricular materials will be coordinated with Robert Nelson (Illinois State
University), who has initiated a similar professional development program for the State of
Illinois.
(2) EarthScope Student Research Symposia. A unique aspect of our PEPP Earthquake
Science program has been an annual student-teacher research symposium (now in its tenth year),
which brings together about 30-40 high-school students from regional high schools to learn about
seismology and share the results from their own independent research, mentored by PEPP highschool teachers. This has proven to be an important connection to a pool of science-oriented
students including many from underrepresented groups. We propose to expand these workshops
to include a wider range of schools and to focus research projects on data acquired through the
EarthScope project. We will develop a clearinghouse for suggested research projects that make
use of dense seismic data from USArray, provide resources to help teachers mentor student
researchers, and will focus activities during our student research symposia around the USArray
experiment.
(3) Participation of Teacher and Student Interns. This project affords a special
opportunity to provide research experience for undergraduate students and science teachers. The
undergraduate internship will be done in collaboration with the IRIS REU program (see IRIS
support letter). In addition to providing critical support for data analysis, this training will allow
us to broaden the impact to a much larger group. The teacher interns will be able to use these
skills to mentor their own students in seismology research. For the student interns, we anticipate
using the internship to recruit students from other institutions—particularly including
underrepresented minority students—in our research efforts, which coincides with one of the top
priorities of the IRIS E&O program (see support letter from IRIS).
(4) Active Earth Displays. In order to further expand the visibility and impact of
EarthScope deployments in our study area, we will collaborate with the IRIS Education &
Outreach program to develop a new material on Midcontinent seismicity for their “Active Earth”
museum display. Active Earth is a web-based interface that allows for widespread dissemination
seismological information and real-time data incorporated into a museum style display. Our
midcontinent module will include a series of interactive pages related to earthquakes, crustal
structure, seismogenesis, and earthquake hazards in the midcontinent. A broad range of users of
the Active Earth displays, in turn, can incorporate these. We also request funds for acquisition of
two Active Earth touch-screen kiosks that could be used as ‘traveling museum exhibits’ for this
region. The displays will initially be installed at the offices of the state Geological Surveys, but
will be distributed to other educational institutions for periods of 2-3 months. We have already
made preliminary contacts with a number of interested museums, government organizations, and
educational institutions that are interested in hosting an Active Earth display.
(5) Legacy Deployment of Educational Seismographs. Funding of this project would aid
us in efforts to extend the USArray ‘Adopt-a-Station’ and Siting Outreach programs to help retain
some of the USArray stations in the Midcontinent once the Transportable Array leaves our area in
2013-14. This effort could also result in an effective, long-term monitoring network for the
region that could fill in persistent gaps in regional monitoring networks. We will work with
participating organizations to offer a range of options for adoption, ranging from (a) full adoption
of a USArray station (cost ~ $30-37K); (b) acquisition of a networked PEPP digital broadband
station (cost ~$3-6K) to (c) acquisition of low-cost IRIS ‘Seismographs in Schools’ instruments
(~$600); or (d) other commercial alternatives (see http://www.indiana.edu/~usesn) and IRIS
Seismographs in Schools (http://www.iris.edu/hq/sis). We can build on educational materials
already in place for the ‘Buyers Guide’ at the US Educational Seismology Network (see
http://www.indiana.edu/~usesn). Funding for these legacy stations is not requested here. Support
for this project, however, will aid our cause in working with local and state funding agencies for
the future. Operation of educational seismographs will be an explicit focus of the professional
development workshops in years 3 and 4 and could fold into this plan as it develops.
(6) Regional Seismic Hazard Workshops. The Indiana Geological survey has an ongoing
program of outreach to teacher groups and emergency management groups that will provide an
outlet for products of this project. The most effective means of reaching large numbers is to take
the outreach programs to the schools and communities. Regional outreach will incorporate a 3pronged approach: 1) a day science teachers workshop, which incorporates the materials from
the in-service workshops described above; 2) a “general assembly” program on earthquakes and
seismicity in the Midcontinent for students in the same school corporation; and 3) an evening
“town hall” program with a specific focus on elected officials and the first responders (fire, law
enforcement, etc.) in the community. We request support or the E&O coordinator of the Indiana
Geological Survey to allow him to develop appropriate materials and for travel connected to this
activity. Support will be leveraged with matching funds from the Indiana Department of
Homeland Security (see letter of support).
Michael Hamburger and Walter Gray (E&O Coordinator for the Indiana Geological
Survey) will lead the E&O activities for this project. Pavlis and Parke will assist as needed.
Central US Working Group
This is part of a series of proposals submitted as part of an informal, regional collaboration
described in supplementary document titled “Collaboration and Coordination Statement”. The
critical point is this is an agreement for free an open exchange of data between collaborators with
protection mechanism for students involved in the research. See the supplement for details.
Results from Prior NSF support
CMG0327827 Imaging Earth Structure with Elastic Waves by Application of the Inverse
Scattering Series (Jan. 1, 2004-Dec. 31, 2008, $246,496). This project was funded by the
Opportunities for Research Collaborations Between the Mathematical Sciences and the
Geosciences (CMG) program. It was a partnership between Art Weglein at the University of
Houston and Indiana University.
Pavlis, G. L. (in review). Three-dimensional, wavefield imaging of broadband seismic array data,
Computers and Geosciences, accepted with changes subject to editorial approval.
Pavlis, G L. (in review). Three-dimensional wavefield imaging of data from the USArray: New
constraints on the geometry of the Farallon Slab, Geosphere, reviewed acceptable with revisions.
Pavlis, G. L. and F. L. Vernon (2010). Array processing of teleseismic body waves with the USArray,
Computers and Geosciences, doi 10.1016/j.cageo.2009.10.008.
Fan, Chengliang (2006). Extracting P-primary Transmission and Reflection Responses from Teleseismic
Data, Ph.D. dissertation, Indiana University.
Fan, C., G. L. Pavlis, A. B. Weglein, and B. G. Nita (2006). Removing free surface multiples from
teleseismic transmission data and reconstructing the reflection response using reciprocity and the inverse
scattering series, Geophysics,71, S171-S178.
Weglein, A. B., B. G. Nita, K. Innanen, E. Otnes, S. A. Shaw, F. Liu, H. Zhang, A. C. Ramirez, J., G. L.
Pavlis, and C. Fan (2006), Using the inverse scattering series to predict the wavefield at depth and the
transmitted wavefield without an assumption about the phase of the measured reflection data or backpropagation in the overburden, Geophysics, 71, SI125-SI137.
EAR-0454554, Collaborative Research: Lithospheric Foundering Beneath the Sierra Nevada, H.
Gilbert and with G. Zandt, 1/1/05-12/31/08, $380,525, Deployment of 80 seismic stations in the
Sierra Nevada. Publications to date: numerous abstracts and:
Gilbert, H., Jones, C., Owens, T.J., and Zandt, G., 2007, Imaging Sierra Nevada Lithospheric Sinking: Eos,
Tran. AGU, v. 88, p. 225&9.
Frassetto, A., G. Zandt, H. Gilbert, T. J. Owens, and C. H. Jones, Improved imaging with phase-weighted
common conversion point stacks of receiver functions, Geophys. J. Int., 182, 368-374, 2010.
Frassetto, A. M., G. Zandt, H. Gilbert, T. J. Owens, C. H. Jones, Lithospheric Structure of the Sierra
Nevada from Receiver Functions and Implications for Lithospheric Foundering, Geosphere, in review,
2009.
Assessing diffusive differentiation during igneous intrusion using integrated theoretical,
experimental and field studies, $270,442 (C. Lundstrom, PI; Marshak, co-PI), NSF EAR 0609726
6/06-6/09. This project has funded work on temperature gradient-based experiments as well as
measurements of non-traditional isotope systems in both experiments and a field study. The
results provide new insight into the process of forming granitic rocks through the process of
thermal diffusion. A number of important findings are already published and several more are
either in review or in preparation, including:
F. Huang, C.C. Lundstrom, J. Glessner, A. Ianno, A. Boudreau, J. Li, E. C. Ferré, S. Marshak, & J.
DeFrates, 2009, Chemical and isotopic fractionation of wet andesite in a temperature gradient: GCA 73,
729-749.
C.C. Lundstrom, 2009, Hypothesis for origin of convergent margin granitoids and Earth's continental crust
by thermal migration zone refining, GCA 73, 5709-5729, 2009.
X. Ding, W. Sun, F. Huang, C. Lundstrom, & J. Li, 2009, High mobility and fractionation of Nb and Ta
under a large thermal gradient, Intern. Geol. Rev. 51, 473-501.
F. Huang, Lundstrom, C.C., Glessner, J.G.J, Ianno, A., & Zhang, Z.F. 2009, Magnesium isotopic
composition of igneous rock standards measured by MC-ICP-MS, Chem. Geol. 268, 15-23.
F. Huang, P. Chakraborty, C.C. Lundstrom, C.E. Lesher, C. Holmden, J.J.G. Glessner, and S. Kieffer,
2008, Isotopic fractionation in silicate melts by thermal diffusion, Nature 464, 396-400. C.C. Lundstrom,
S. Marshak, J. DeFrates and J. Mabon, 2010, Alternative processes for developing fabric and mineral
compositional zoning in intrusive rocks. Intl. Geology Review.
F. Huang, C.C. Lundstrom, 2007, 231Pa excesses in arc volcanic rocks: Constraint on melting rates at
convergent margins, Geology 35, 1007-1010, 2007.
F. Huang, L. Gao, C. C. Lundstrom, 2008, The effect of assimilation and fractional crystallization on Useries disequilibria in arc lavas, GCA 72, 4136-4145, 2008.