geodetic observations of active intraplate

7/29/2019 Geodetic Observations of Active Intraplate

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Seismological Research Letters Volume 81, Number 5 September/October 2010 699doi: 10.1785/gssrl.81.5.699

E

geoet Oervton of atve intrpltecrutl deformton n the wh Vlley

sem Zone n the southern illno bnGerald A. Galgana and Michael W. Hamburger

Gerald A. Galgana1,2 and Michael W. Hamburger1

Onlinematerial: Figures showing observed and predicted GPSvelocity vectors, including a tabulation of observed GPS veloci-ties and modeled principal strain rates.

INTRODUCTION

Seismically active intraplate continental settings are charac-terized by relatively low strain rates, but they nonetheless dis-play evidence of active faulting and seismicity. Examples ofthese tectonic environments include seismically active areasof northern Eurasia, central India, Australia, and the U.S.mid-continent (e.g., Sykes 1978; Johnston 1996). ey arecharacterized by low heat ow (e.g., Kusznir and Park 1984),relatively thick lithosphere, and presumably low strain rates(~10101012yr1) (Gordon 1998). Yet in only a handful ofthese environments have geodetic measurements been used toprovide constraints on the rates and directions o present-daydeormation in these intraplate settings. We present precisegeodetic observations and modeling results for one type-exam-ple o a continental, intraplate deormation zone: the WabashValley seismic zone (WVSZ) in the central United States. Weuse high-precision campaign-based GPS data from a networkof GPS sites in the WVSZ to address evidence for present-daytectonic strain in an intraplate seismogenic zone. Tis studybuilds on the results o Hamburger etal. (2002), extendingtheir preliminary one-year observation period (199798) toan eleven-year observation cycle (19972008). e observedevidence or active crustal deormation o the region is thenmodeled using a ault-and-block approach, which treats deor-

mation as described by nite, rigid crustal blocks boundedby planar aults (e.g., atcher 1995; McClusky etal. 2001;apponnier et al. 2001; McCarey 2002). e main scien-tic questions delve into the nature o active deormation inthis intraplate zone and address the role o ault systems in thedeformation of the North American continent.

TECTONIC SETTING

Te Wabash Valley seismic zone (WVSZ) is a broad, seismi-cally active area located in southern Illinois, southwestern

Indiana, and westernmost Kentucky. Situated within theWVSZ is the Wabash Valley ault system (WV FS), a seriesof parallel NNE-trending normal faults close to the WabashRiver Valley, which marks the Indiana-Illinois border. TeWVSZ is hypothesized to be a northeastern extension of theNew Madrid seismic zone because of its location at the north-northeastern tip o the Reeloot ri and the presence o deeplysubsided structural lows within the zone (Braile, Hinze, etal.1982; Braile, Keller etal. 1982; Sexton etal. 1986). However,the WVSZ is truncated at its southern end by the CottageGrove fault and Rough Creek fault system (Figure 1), and sev-eral o the basement structures associated with the WVSZ ter-minate there (Bear etal. 1997). e WVSZ is associated witha signicant concentration o recently recorded low to moder-ate magnitude (mb< 5.5) earthquakes (Figure 1). In addition,paleoseismic studies indicate the occurrence o at least sevenstrong (mb> 6) prehistoric earthquakes within the last 20,000years (i.e., Obermeier etal. 1991; Munson etal. 1995).

Te Wabash Valley ault zone is demarcated by basement-penetrating aults that traverse Paleozoic sediments romSSW to NNE. Seismic reection data show grabens withwell-dened fault systems, in some cases extending to at leastseven kilometers depth (Bear etal. 1997; McBride and Kolata1999; McBride etal. 2007). e Commerce geophysical linea-ment (CGL) (Figure 1) is expressed by a line of strong mag-

netic and gravity anomalies that traverses the area rom SSWto NNE and is coincident with the general strike of existingnormal faults on the surface (Hildenbrand and Ravat 1997;Langenheim and Hildenbrand 1997). e Cottage GroveRough CreekShawneetown fault system (Figure 1) runsalmost perpendicular to this system o aults, trending almostwest to east; Hicks Dome, a cryptovolcanic structure of prob-able Permian age (Denny etal. 2008), is located to the westwhile the La Salle fold system (Figure 1), a NNE-trendingseries o ault-cored monoclinal olds, is located in the north

1. Department of Geological Sciences, Indiana University,Bloomington, Indiana

2. Now at Lunar and Planetary Institute, Universities Space ResearchAssociation, Houston, Texas


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of the WVSZ (Hamburger and Rupp 1988; McBride 1997;McBride etal. 2007). e present average stress eld is domi-nated by east-west to ENE-WSW compression (Hamburgerand Rupp 1988; Zobacketal. 1989).

e well-known New Madrid seismic zone (NMSZ),located southwest of the WVSZ, is a zone of high seismicitymost notably the three M > 7 earthquakes of 181112 (Nuttli1979; Johnston 1996)located within the Mississippi embay-ment. Te embayment is a structural trough that containsthick Tertiary and Cretaceous strata, overlain by Quaternaryalluvium (Figure 1) (Crone 1981). Evidence gathered from seis -mic and potential eld data indicate that the NMSZ is coinci-

dent with the northeast-trending Cambrian-age Reelfoot ri,dened by a system o high-angle normal aults that truncatebasement rocks (Ervin and McGinnis 1975; Nelson and Zhang1991). Seismicity in the NMSZ is dened by a few plane-like structures (Stauder 1982; Himes etal. 1988; Chiu etal.1992); motion is mainly right-lateral slip along the two mainNE-SW fault segments and reverse slip along the interveningNW-SE segment (Stark 1997; Csontos and van Arsdale 2008).Earthquake repeat times in this region are estimated to rangefrom 200 to 800 years, with a 500-year average recurrence timebetween earthquakes (uttle etal. 2002). e New Madridand Wabash Valley seismic zones are separated by a zone o

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Figure 1. Seismotectonic map o the U.S. mid-continent , showing the Wabash Valley seismic zone and the New Madrid seismic zone.

Historic and recent seismic events in the central United States are plotted, with white dots representing historical seismic events

(Nuttli 1979) and black dots showing recent, instrumentally recorded seismic events rom the Center or Earthquake Research and

Inormation catalog.


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intensive deformation associated with the Cottage GroveShawneetown fault systems in southern Illinois (Nelson andKrausse 1981; Nelson 1991) and the Rough Creek graben inwestern Kentucky, hypothesized to be a continuation of theReelfoot ri, comprising the northern extension of the failedri system (Kolata and Nelson 1991; Marshak and Paulsen1996). Te aults bounding the graben have been reactivatedmultiple times including the late Paleozoic era (Nelson1991).

Based on high-resolution seismic proles, the motion along theRough Creek fault underwent reverse orientation (Paleozoic toearly Mesozoic), then strike-slip and normal sense o motions(Nelson 1991).

GPS observations in the NMSZ have been the subject ofmany recent papers and vigorous discussions on their signi-cance or earthquake hazard evaluation. Initial estimates overy high deormation rates in the region based on geodeticobservations (Liu etal. 1992) have been dramatically reduced asnew data have been collected (e.g., Weber etal. 1998; Newmanetal. 1999). Similarly, higher-precision continuous GPS datahave also been used to suggest high near-eld deormation rates(Smalley et al. 2005), whereas subsequent reanalysis of lon-

ger data series (Calais etal. 2006) suggests that the observedmotions remain well within the North American plate motionuncertainties. Tese studies have important implications orseismic hazard analysis, as the low deormation rates imply sig-nicantly longer recurrence rates in order to reconcile paleo-seismic ault slip estimates with geodetic data and earthquakemagnitude- frequency relationships. On the other hand, somegeodynamic models (e.g., Kenner and Segall 2000) suggest thatintraplate aulting mechanisms need not ollow the same cycleo earthquake recurrence patterns as plate boundary systems.In addition, other models such as glacial isostatic loading havebeen investigated as possible time-variable triggers o mid-con-tinent deformation and earthquakes in North America (Wuand Johnston 2000; Grollimund and Zoback 2001; Sella etal. 2007). e ndings from a continentwide continuous GPSnetwork (Calais etal. 2006) suggested that most o the system-atic deormation in the region can be interpreted as a result oglacial isostatic adjustment (GIA). e observed patterns ofresidual surace velocities are similar to some o those predictedby the geodynamic models of Peltier (1994, 1996).

Geodetic Data Acquisition and Processing Methode GPS network that provides the basis of this researchextends over an area of approximately 100,000 km2, with 48campaign stations distributed across southern Illinois, south-

ern Indiana, and western Kentucky. In addition, we observeda dense 19-station network in the Shawnee National Forestarea in southern Illinois, near the intersection o the WabashValley fault system and the Rough Creek graben (Table S1 inthe online material). e survey benchmarks are a mix of exist-ing Coast and Geodetic Survey benchmarks and new bench-marks, including both stainless steel pins driven into bedrockand stainless steel rods situated in unconsolidated materials(Hamburger etal. 2002). Some stations are part o the HighAccuracy Reference System (HARN), used as primary con-

trols for the U.S. geodetic network. Campaign observationswere generally done on a 36- to 48-hour observing epoch,with 30-second sampling intervals. e set of observationsper campaign was usually completed in approximately twoto three weeks during summers in 1997, 1998, 2000, 2002,and 2007. Supplementary campaigns over the dense Shawneenetwork started in the year 2000, ollowed by campaigns in2003 and 2008. We also combine the velocities derived from

nine sites of the GAMA (GPS Array for Mid-America) net-work in the New Madrid seismic zone (Smalley etal. 2005)and four selected stations from the National Geodetic SurveysContinuously Operating Reference System (CORS) GPS net-work to spatially extend our observations and generate jointstrain inversions with our GPS network (Table S1). In additionto densication, these continuous GPS (CGPS) stations pro-vide additional stability by acting as redundant baseline con-trols and additional positional tie points linked to global andcontinental geodetic reerence rames.

We utilize the GAMIT soware (King and Bock 1997;Herringetal. 2006a) to estimate loosely constrained positionso survey sites through double-dierenced daily phase observa-

tions derived from seven campaign observations in 1997, 1998,2000, 2002, 2003, 2007, and 2008. Position estimates are cal-culated by combining the campaign daily phase observationswith sampled continuous data from 11 IGS (InternationalGlobal Navigation Satellite System, formerly the InternationalGPS Service), four CORS, and nine GAMA GPS sites (usingdata subsets during the campaign period only), along withatmospheric zenith delay, satellite orbit, and Earth orienta-tion parameters, obtained from the NASA Jet PropulsionLaboratory (NASA-JPL), the US Naval Observatory (USNO),and IGS, respectively. Coordinates were rotated into the StableNorth America Reference Frame (SNAR F version 1.0, Blewittetal. 2005), the recently developed geocentric reference framethat minimizes the motion of ducial sites within the NorthAmerican tectonic plate and takes into account the eects oglacial isostatic anomalies (GIA). To account for site-depen-dent noise primarily coming rom multipath errors, we appliedelevation-dependent noise models depending on the phaseobservations. In the processing, we also accounted or eectsemanating rom long-wavelength site motions due to earthsolid-body tides derived rom the International Earth Rotationand Reference System Service or IERS 2003 (Herring etal.2006a).

Daily positions of these data from GAMIT were thendetermined using the GLOBK soware (Herringetal. 2006b),

with respect to a network of eight relatively stable IGS stations.Position estimates are then made using a seven-parameterHelmert transormation (translation, rotation, and scale) tominimize relative motion between sites and reerence stations.Finally, velocity estimates were made rom least-square linearts through Kalman ltering of daily positions and covariancematrices of campaign stations and regional continuous GPSsites or each o the observation periods. Tis procedure is simi-lar to the technique described in McCareyetal.(2007). Wealso added similar 14-day seasonal (days 195 to 205) observa-


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tions within the years 2004 and 2005 to improve velocity esti-mation of the continuous IGS, CORS, and GAMA networks.Tis is necessary to eliminate short-period variability in thelong-term deormation signals that may be caused by localized,non-tectonic motions. e GLOBK soware was then used todetermine and tie these position and velocity estimates withrespect to a network of around 120 IGS and CORS stationsdening the SNARF. We also converted the station velocities

based on a local station well outside the seismically active area othe WVSZ (i.e., with respect to station BLO1 in Bloomington,Indiana). Tis is to urther assess velocity trends due to localdeormation without the possible velocity bias introduced bythe North American plate motions. We analyzed the resultingtime series or every station and included random walk noiseor all the component motions o all campaign and continuoussites. Te applied noise is based on station velocity requency dis-tributions as well as reported random walk motions (e.g., rommonument instability) rom empirical data (e.g., see Williams2003; Williams etal. 2004). We then generate the velocityeld based on the GPS observations (Table S1), to be used asmain input or various strain inversions. For this research, we

utilize two sets: one that includes all o the Shawnee stations(or local-scale strain inversions), and one that includes onlya 10-station subset o the Shawnee stations. Tis strategy isimplemented so as not to include localized deormation withinthe heavily aulted Shawnee/Fluorspar district in the generalblock motions, and it reduces the likelihood o overweighingthe dense Shawnee network in the regional strain estimates.

MODELING

We use the hypothesis of Marshak and Paulsen (1997) as ageneral approach or modeling. Tat is, we assume that thecentral United States can be characterized as a system of jos-tling blocks throughout the Phanerozoic, separated by faultsand olds, and that these deorming zones control present-daydeormation patterns. With this in mind, we treat tectonicstructures surrounding the Wabash Valley (and New Madrid)areas to be potentially active aults. Tese bounding structuresinclude the Wabash Valley fault system, the Rough CreekPennyrile fault system, the Cottage GroveShawneetown faultsystem, and the Reeloot ri. Te intervening areas would becomposed o relatively stable tectonic blocks between thesestructures. o model tectonic motions, we represent the areaby a series o rotating elastic blocks on a spherical Earth, imple-mented through the soware DEFNODE (McCarey 1995).

Tese tectonic blocks are separated by aults, dened in turn bydiscrete, near-vertical planes (dip = 89, depth = 20 km, withault patches dened or every 1 km along strike and along dip)along which motion is assumed to take place through elasticslip during earthquakes. Block motions can be constrained bypublished Euler poles and rotation rates, observed GPS veloci-ties, and, where available, earthquake slip vectors.

Te large-scale motion o blocks is dened by relative rota-tion along Euler poles, with local deormation inuenced byrictional coupling along bounding ault planes. Block motions

are dened either by purely rigid rotation, by a combination origid rotation and elastic strain due to ault locking, or evenby including uniorm internal block deormation. Models oblock motions, ault slip rates, and ault coupling parametersare estimated through inversions relating surace motion withmotion at depth using elastic hal-space dislocation models(Okada 1985, 1992). Fault-locking parameters are estimatedbased on integration o coupling eects along small nite ault

patches, which are dened by nodes along the block-bound-ing ault planes. Te best-t model is obtained by comparingobserved and predicted motions, with the errors minimized byleast squares through the simulated annealing/simplex mini-mization technique. For details regarding mathematical equa-tions describing this modeling approach, we reer the reader tothe more elaborate discussions of McCarey (2002).

RESULTS

GPS Velocity PatternsObserved GPS velocities in our study area display a small butsystematic northward pattern of motion in the SNARF ref-

erence rame (Figure 2A). Most o the stations in the WVSZnetwork show a consistent NNW to northerly trend at ratesranging rom less than 1 mm yr1 to 4 mm yr1, whereas sta-tions of the GAMA network show a general trend of north-to-northwest-directed motion at slightly lower velocities, less than1 mm yr1 (i.e., stations PTGV, NWCC, STLE, and MCTY).More variability in site velocities is observed in this area: sta-tion RLAP (whose data were processed through 2005 only,since unusually large motions were recorded in 2006, which aresuspected to have been aected by local, non-tectonic deor-mation sources [Calais and DeMets 2008]) shows apparentsouthwest-trending velocity, while PIGT and CVMS move tothe south and northeast, respectively (Figure 2A). e majorityo the observed stations have statistically signicant velocitieswith respect to their ormal error estimates, and very low northto east velocity component correlations (able S1).

Figure 2B shows the velocities with respect to a localreerence rame, i.e., with respect to station BLO-1, locatedin Bloomington, Indiana. Tat station was chosen because oits location near the northeastern (stable) margin o the studyarea, its consistent observation through all o the measurementcampaigns, and its high-quality bedrock benchmark and goodsky visibility. In this reerence rame, the systematic northwest-ward translation o the network disappears, and only internalmotions remain. Te velocities or most o the stations are

within their 95% condence ellipses. e few exceptions (e.g.,NOL-1, SEBR, and KY02 in Kentucky; RUSH and W231 inIndiana; PIGT in Arkansas; and R LAP in Tennessee) show noobvious systematic spatial patterns. While these individual sitevelocities are mostly within their error estimates, these subtlemotions may still carry some inormation about systematicstrain within the network; our analysis of geodetic strain ispresented in the ollowing sections.

e velocities for stations in the Shawnee National Forestnetwork (Figure 3A) also show a generally northward trend


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with respect to stable North America, although more vari-

ability and larger error estimates are observed due to shorterobservation times. When referenced to station BLO1 (inBloomington, Indiana), thus removing possible systematicmotions with respect to the North American reference frame(Figure 3B), the station velocities are signicantly reduced buttrend northward at < 1 mm yr1, with more scatter in the east-west direction. Velocities or stations in the Shawnee networkrelative to station BLO1 show generally 1 to 2 mm yr1 system-atic eastward motion. Our observed northerly velocity eld (inthe North American reference frame) appears to have a similar

orientation with the observed (Calais etal. 2006) and modeled

(Sella etal. 2007) northerly velocity trends of stations locatedin Indiana and Illinois. On the other hand, those velocities donot appear to be consistent with the spatially averaged residualvelocity eld determined by Calais etal. (2006), which sug-gests generally southerly velocities or the region within andsouth of the Great Lakes region. However, we note that thatvelocity eld includes almost no points in southern Illinois,Indiana, or western Kentucky, and thus the discrepancy mightbe strongly aected by the addition o these velocities into theresidual velocity eld.

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Figure 2. Observed regional velocity feld o the southern Illinois basin and New Madrid regions, based on GPS campaigns and

continuous observations (19972007). Velocities with 95% error ellipses (with random walk component accounting or monument

instability) are plotted in the Stable North American Reerence Frame (SNARF V1.0) (Figure 2A) and with respect to local station BLO1

in Bloomington, Indiana (Figure 2B, next page). Box with dotted line indicates the location o the Shawnee National Forest GPS stations

(some stations removed or clarity), entirely shown in Figures 3A and 3B.


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Inversionso assess tectonic deormation in the WVSZ, we run mod-els that treat the area as tectonic blocks that are separated byfaults. We explore progressively more complex parameter-

izations o the study area. Initially, we run models with tworegions separated by a single fault (the NNE-SSW trendingWV FS); then we subdivide two more regions by adding anEW-trending fault zone (representing the Cottage GroveRough Creek fault system). We also assess the contribution ofinternal strain in these two- and our-block models by allow-ing the blocks to deorm internally. Finally, we assess the over-all pattern o regional deormation without considering theault system, by calculating an average strain rate eld andthen by determining spatial variations in the strain eld.

Tectonic Blocks with Elastic Strain along Faults

Two-blockModelWe begin with a simple two-block elastic model with an ide-

alized NNE-SSW trending fault (representing the WVFS)dividing the region (Figure 4 and Figure S1 in the online mate-rial). Tis ault alignment reects the structural predominanceo the WVFS (Figure 1) and ollows the general approach usedin the previous study (Hamburger etal.2002). Tis approachis used to test the hypothesis o active aulting along the WVFSor the subparallel Commerce geophysical lineament, whichwould be maniested by relative motion o the western block(i.e., southern Illinois) with respect to the eastern block (i.e.,southwestern Indiana and western Kentucky). For this two-

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Figure 2. (Continued).


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Figure 3. Shawnee National Forest and Fluorspar area GPS network velocity feld. Data rom the 2000, 2003, and 2008 campaigns

are combined with the 19972007 datasets or the large network. A) Observed campaign GPS velocities with 95% error ellipses are

plotted with respect to stable North America. B) Observed GPS velocities plotted with respect to local stat ion BLO1.


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block system, with the western and eastern Wabash blocks(WWAB and EWAB, respectively) divided by a NNE-SSW-trending WVFS, the inversion shows 0.73 0.14 mm yr1southwest motion or block WWAB with respect to EWAB(Figure 4). Elastic strain curves across the WVFS-NMSZ donot dier signicantly rom zero, with no signicant changein overall block velocities or block rotation crossing this struc-ture. Coupling estimates along this fault plane indicate zerolocking, but cannot be resolved given the low slip rate acrossthe ault. Te errors reported by the two-block model are wellwithin the error level of GPS observations (e.g., ~2 mm yr1).Tus, these observations suggest that there is no signicant,measurable systematic strain accumulation along the WVFSor Commerce Geophysical Lineament.

Four-blockModelWe add an EW-trending boundary to represent the CottageGroveRough Creek (CGRC) fault system. In this case, twofault systems (WVFS and CGRC) intersect near southern

Illinois and are almost perpendicular at the point o intersec-tion. Tis approach is used to test whether there is signicantrigid motion between blocks separated by these major fault sys-tems and to look specical ly at the relative motions between thenorthern and southern blocks. Te our-block model (Figure5 and Figure S2 in the online material), which shows fourblocks (INDY, ILLI, MISO, and KTKY divided by the NNE-SSW-trending WVFS-NMSZ fault line and the EW-trendingCGRC fault system), indicates a similar trend in velocities asthe previous test. With respect to INDY, ILLI (measured at the

block centroid) appears to move southeast (counter-clockwise)at a rate of 0.10 0.22 mm yr1, block MISO moves at 0.61 0.42 mm yr1 in a southwesterly direction, and block KTKYmoves northeastward at 0.32 0.28 mm yr1 with counter-clockwise block rotation (Figure 5). ese velocities translate tole-lateral strike-slip motion along the WVFS between INDYand ILLI, transpression along the Rough Creek fault (betweenINDY and KTKY), and transtension along the Cottage Grovefault (between ILLI and MISO). e changing motion alongthe Cottage GroveRough Creek fault system (i.e., extensionalto transpressional) results rom the rotation o the two south-ern blocks. However, these velocity and ault slip values are sta-tistically insignicant, as the values are all not distinguishablefrom zero with respect to the error levels at the 95% condenceinterval. Velocity proles across the aults show insignicantchanges of ~0.1 to 0.3 mm yr1 across dened blocks. Couplingestimates rom best-t models reveal almost no resolvable lock-ing along both aults.

Strain Inversions rom Internally Deorming Regions

ests or principal strain rates and directions through inver-sions o velocity residuals within selected regions (or polygons)were also implemented using the DEFNODE code (McCarey1995). Inversions are run using dierent geographic subsetsrepresenting spatial groups with generally similar observedGPS velocities. In eect, this technique changes the samplingresolutions (through spatial windows) to determine patterns ocontinuous deormation in specic areas within the WVSZ.

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Figure 4. Block velocities rom the two-block model, plotted with respect to the eastern Wabash block (EWAB).


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Moreover, this approach tests or strain consistency and/or possible internal deormation within individual blocks.Initially, we run models that do not consider aults and treatthe entire study area as 1) a single deorming block to deter-mine the average regional strain, and 2) a multiblock systemto determine the strain variability within the WVSZ region,based on subareas dened by dierent tectonic environments.Ten, we assess the internal strain rates or the two- and our-block systems described in the preceding section (able S2in the online material). Tese inversions determine whetherinternal strain signicantly contributes to the overall patterno regional deormation.

AverageRegionalStrainModelInversions using the residual Wabash GPS velocities within asingle deorming polygon (i.e., solving or internal strain rateswith the poles to North America xed) reveal signicant prin-cipal strain rates: we obtain a strain eld with 1.60 0.60 nsyr1 NE-SW extension and 1.40 0.70 ns yr1 NW-SE com-

pression within the entire study area (Figure 6). Tis test showsthe average principal strain rates and directions or the studyarea. e compressive strain axis is rotated at ~45 comparedto the almost E-W orientation of the principal stress axis foundby Zoback and Zoback (1980), presumably due to the eect ofobservations in Kentucky near the Rough Creek graben.

SpatiallyVariableRegionalStrainModelTo further examine spatial variability of the strain eld, we useinternally deorming multiple polygons to model the deorma-

tion eld o subareas by enclosing structurally homogeneousareas within regular quadrilaterals. In this case, the bound-aries are not necessarily aults or geologic discontinuities,but are based on the observed average velocity trends withinsubregions o the geodetic network. Tis approach resulted inthe construction o a total o eight tectonic regions, with vepolygons comprising the southern Illinois-Indiana-Kentuckyarea, and three polygons comprising the New Madrid area(Figure 7). In the multiple-block model, we obtain 3.14 2.36ns yr1 E-W compression and 2.04 ns yr1 2.28 ns yr1 N-Sextension in the SCWA block (south-central W VFS). ere is2.36 2.68 ns yr1 WNW-ESE compression and 3.42 2.33ns yr1 NNE-SSW extension in the NCWA block (north-central Wabash Valley block). We observe 4.26 3.11 ns yr1NW-SE extension and 0.73 2.61 ns yr1 NE-SW extension insouthwestern Indiana (NEWA block) and 2.61 3.43 ns yr1NW-SE compression and 3.67 + 5.32 ns yr1 NE-SW exten-sion in the WWAB block in southern Illinois. Block SEWA inKentucky experiences 2.46 3.73 ns yr1 of nearly E-W exten-

sion and 1.24 2.96 ns yr1

nearly N-S extension. e centralNew Madrid block (CNMZ) experiences 4.66 7.67 NE-SWcompression and 11.23 5.91 NW-SE extension (Table S2).We ignore results in the two southern blocks (i.e., WNMZand ENMZ) due to larger errors resulting from the sparse GPSstation coverage in those areas. Tese results indicate inerredstrain rates o the same magnitude as the estimated error levels,hence indicating marginal signicance. However, we note thatthe results or the multiple block system have principal strainrate directions similar to those obtained rom the our-block

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model (Table S2). e N97E principal compressive strain axiso the lithosphere near the WVFZ in southern Illinois (i.e.,block SCWA) agrees with the map of stress patterns inferredrom structural and geophysical data (i.e., principal compressivestress axes oriented at N8590E inferred from joints, hydro -fractures, and focal mechanisms (Nelson and Bauer1987) andN96EN101E from borehole breakouts (Bauer and Nelson2005; Heidbach etal. 2008) and the N80E orientation of theprincipal compressive stress axis from continentwide geophysi-cal studies (i.e., Zobacketal. 1989).

Two-blockStrainModelWe then utilize the block geometries determined rom previ-ous inversions, taking into consideration the actual ault geom-etries as block boundaries. In these inversions, we invert orblock strain rates to determine the amount o internal deor-mation experienced by regional blocks comprising the area. eresults o our strain inversions suggest statistically signicanttectonic strain in a number o areas within our geodetic net-

work. e dual-block strain model (Figure 8) shows that theIndiana area (EWAB block) is dominated by 4.71 0.96 ns yr1NW-SE extension, and 1.13 0.85 ns yr1 NE-SW extension,while the Illinois area (WWAB block) is dominated by 3.03 0.72 ns yr1 WNW-ESE compression and 0.99 0.53 ns yr1NNE-SSW extension. Inversion results for both blocks indi-cate statistical signicance at the 95% level, based on the num-ber and spatial distribution o stations. Te results suggest thatthere is a general change o strain patterns across the WVFZ.NW-SE extension in Indiana changes to WNW-ESE compres-

sion in Illinois while NE-SW compression in Indiana gives wayto NNE-SSW extension in Illinois. e strain inversions forthe two-block model indicate that the principal strain rate esti-mates are signicantly above the error estimates (able S2).

Four-blockStrainModelTe our-block system (Figure 9) also demonstrates statisticallysignicant regional strain patterns. Our results (Table S2) indi-cate that 4.50 1.42 ns yr1 WNW-ESE extension dominatesthe Indiana (INDY) block, with 0.69 1.42 ns yr1 NNW-SSE extension. e Illinois block (ILLI) is dominated by 2.07 1.16 ns yr1 compression NW-SE and 1.34 1.18 ns yr1NE-SW extension. e Kentucky block (KTKY) is dominatedby 4.97 1.56 ns yr1 of nearly north-south extension and 1.27 1.95 ns yr1 of nearly east-west extension, while the Missouriblock (MISO) has relatively larger 11.59 6.77 ns yr1 WNW-ESE compression and 1.12 7.94 ns yr1 NNE-SSW extension(inversion for MISO has low statistical signicance becauseo sparsely distributed stations). Te obtained principal strain

rates imply that internal deormation o the other three blocksis marginally signicant at the 95% condence interval.

DISCUSSION

Comparisons o Block ModelsWe compare the dierent inversion results using chi-squareandF-ratio tests (Stein and Gordon 1984). We used these teststo examine the quality of t for two- and four-block models,considering: 1) congurations o rigid block rotations and elas-

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tic ault-locking strain, and 2) combined rigid rotation, elasticault-locking strain, and internal block strain. Te our-blockmodel with internal strain was ound to be the best-t model,with a reduced chi-square value o 1.96. Te other models wereound to be statistically inerior in comparison, based on theF-ratio tests (able 1). Te preerred model suggests that inter-

nal strain is a sizeable component o regional deormation, asopposed to rigid rotation only or rigid rotation combined withault-locking strain. Te preerred model also suggests that~0.20.7 mm yr1 motion occurs along the E-W ault system(i.e., CGRC) as well as ~0.20.6 mm yr1 motion along theNNE-SSW-trending WVFS, but provides little evidence forpossible elastic strain accumulation along these ault zones.

Continuous Deormation Patterns in the WVSZTe regional average strain rate based on the single block inver-sion (Figure 6) indicates that deormation in the region is dom-inated by NW-SE compression and NE-SW extension. On theother hand, spatial variability o the strain rates is shown bythe multiple block model (Figure 7): the southern Illinois side(block WWAB) experiences mainly NW-SE compression andNE-SW extension; the block along the length of the WabashValley fault system (block NCWA) experiences mostly com-pression normal to the fault axes and extension a long it, sug-gesting relatively low resolved strain on the fault planes; theShawnee area (block SCWA) in southern Il linois appears to beunder transpressional strain; the Indiana area (block NEWA)experiences mostly NW-SE extension; and the Kentucky area(block SEWA) has N-S and E-W extension.

Te observed deormation in the WVSZ can be interpretedby our distinct geodynamic models: 1) a result o gradual accu-mulation o regional tectonic strain, 2) the ar-eld inuenceof viscoelastic strain in the aermath of the large New Madridearthquakes, 3) eects of glacial unloading, or 4) jostling tec-tonic blocks. Te rst scenario o strain accumulation is difcult

to reconcile with the orientation and spatial variability o thetectonic strain eld obtained here. We do not nd evidence or asingle, uniorm strain eld that might result rom application oa uniorm stress eld to a homogeneous continental lithosphere.However, that neglects the inuence o signicant variationsin elastic properties associated with heterogeneous structure inthe regione.g., ault terminations, accommodation zones, andchanges in structural grain. Alternatively, the source o tectonicstrain could involve spatially and temporally variable regionaltectonic stresses associated with long-term viscoelastic eectsassociated with lower crust and mantle relaxation (and result-ing tectonic strain) associated with the 181112 NMSZ earth-quakes (e.g., Li etal. 2005). Our initial modeling results (com-bining realistic fault ruptures associated with the New Madridearthquakes in an elastic l ithosphere over a viscoelastic astheno-sphere) predict that a signicant increase on strain and seismic-ity rates can persist in a long period as a result of the 181112NMSZ earthquakes (Hamburger etal. 2007). However, suchpredictions are strongly model-dependent and predict very low(< 1 mm/yr) velocities or this region. Based on the current lim-ited accuracy of GPS measurements at this strain level and thesmall number o larger magnitude earthquakes (>M 5.0) in theregion, this interpretation remains ambiguous.

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A possible third class of models that might explain ourobserved GPS velocities involves exural models of glacial iso-static adjustment (GIA) in response to Holocene melting ofcontinental glaciers in the northern United States and Canada.Glacial unloading has been identied as a probable source ofsurface deformation in eastern Canada and New England(Mazzotti etal. 2005) and has been proposed as a cause forseismic events in New Madrid (Grollimund and Zoback 2001).

e exural response of the continental lithosphere predicts alarge area of upli over the area of maximum ice cover (cen-tered near Hudson Bay), an area o subsidence in the ice-reeregion to the south, and a hinge line, located somewhere in theGreat Lakes Region (Calais etal. 2006). Our observed andmodeled velocity patterns show similar trends when comparedto observed (residual) horizontal velocities of GPS stations inthe region (Calais etal. 2006) and predicted horizontal veloci-ties resulting rom glacial isostatic rebound models (Sella etal.2007). In their models, the maximum upli near the center(assumed at 55N, 75W) is predicted to be ~10 mm yr1, withsubsidence at ~1.4 mm yr1 near the forebulge; the hinge lineis determined to about 1,500 km from the GIA center. A verti-

cal deormation gradient o ~1 mm yr1 just south of the GIAcenter is predicted between 1,000 and 2,000 km from the GIAcenter, located in northeastern Canada (Calais etal. 2006). Tecorresponding predicted horizontal motions are highly depen-dent on the mantle viscosity structure, with predicted strainrates on the order o 109 yr1. Our observed northerly velocitytrends could thus comprise a ar-eld response, whereby suracevelocities point radially toward the upli center in northeast-ern Canada. Our block models show similar velocity trends tothose predicted by one o the two-layered mantle earth models(Sella etal. 2007). In general, we believe that these two latterexplanations of deformation (i.e., 12 mm yr1 N-NW motion

as a combined eect o long-term post-seismic viscoelasticrelaxation and glacial isostatic unloading) could both be rec-onciled with active regional seismicity and the observed GPSvelocity eld.

Independent o the undamental cause o this deorma-tion, our block models also suggest that the long-term tectonicmodel of Marshak and Paulsen (1996, 1997) and Marshaketal.(2003) on jostling tectonic blocks within the U.S. mid-conti-nent may provide a plausible model or representing active tec-tonics o intraplate seismic zones. ectonic blocks that slowly

rotate, separated by aults, are shown by our two- and our-block models. Te regionalized block strain inversions (multi-ple-block model) indicate the presence o heterogeneous strainelds within the WVSZ. Unfortunately, we can still provideonly limited constraints on more localized crustal motions,given that the GPS velocities remain marginally signicant ascompared to the errors and most stations are near the boundso geodetic error and detection at velocities o ~1 mm yr1.

CONCLUSIONS

While the accurate determination of our GPS site (and block)velocities depend strongly on the quality o selected ducialstations that couple with the reerence rame (i.e., SNARF),improved ambiguity resolution and meticulous error analy-sis has denitely improved the detection capability o ourtechnique since the initial set of observations in 199798(Hamburger etal. 2002). Our ndings show that strain accu-mulation is very slow in this region, conrming the results oprevious studies in nearby NMSZ (Weber etal. 1998; Newmanetal. 1999; Calais etal. 2005, 2006; Calais and Stein 2009).

Based on our models, we interpret the strain patterns in thesouthern Illinois basin as dominated by WNW-ESE compres-sion and NNE-SSW extension, while the Indiana area experi-ences mostly WNW-ESE extension. e Kentucky area expe-riences mostly NNE-SSW extension. Geodetic measurementsmade within the dense Shawnee National Forest network(designed to ocus on the Fluorspar area and spatially resolvethe deormation eld in this heavily aulted area) indicate thatthis area experiences mostly E-W compression and N-S exten-sion. We note that the GPS observations resolve the compres -sional nature o the WVFS and the structurally ragmentednature o the Fluorspar area correlates well with the observed

velocity eld. While the strain rates in these areas exhibit sys-tematic trends, the magnitudes remain marginal comparedto current GPS accuracy and correspond well with low strainrates (~109yr1) attributed to stable intraplate regions. Tus,for slowly deforming regions, use of campaign-style GPS obser-vations remains signicantly more useul than higher spatialcoverage satellite-based Intererometric Synthetic ApertureRadar (InSAR) measurement techniques (e.g., Wright 2002),which remain incapable o providing surace velocity resolu-tion at the subcentimeter level.

TABLE 1Summary o tests or block confguration o the central United States

Model Description Data Parameters DOF Total 2 Reduced 2 Prob. (%)

A 4 blocks with internal strain 154 27 127 249.12 1.96 -

B 2 blocks 154 9 145 310.91 2.14 10.05

C 2 blocks with internal strain 154 15 139 291.59 2.10 18.38

D 4 blocks 154 27 139 302.28 2.17 13.41

The probability (Prob.) column indicates the degree to which the current model is statis tically similar to the best ft model(Model A).


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Several improvements are needed to obtain the ull spec-trum o tectonic deormation in the WVSZ. First, the highlyvariable observed vertical motion patterns or the Wabashcampaign stations need to be reprocessed and incorporatedin a comprehensive regional velocity eld. Accurate verticalvelocities might provide some constraints on the consistencyo the deormation eld, i.e., whether the deormation eld ispredominantly tectonic deormation or related to continen-

tal GIA eects. Second, improved precision of observationscan be readily obtained by extending the observation of exist-ing campaign GPS networks in the region, e.g., reoccupationof the New Madrid regional campaign network (i.e., Newmanetal.1999) aer an eleven-year hiatus is likely to produce sig-nicant improvements in velocity estimation. Tis can greatlyenhance the spatial coverage of strain in the U.S. mid-continentby improving the ar-eld constraints on the deormation eld.Last, the spatial distribution of campaign and continuous GPSstations should be increased to improve quantitative estimates opresent-day tectonic deformation in the U.S. mid-continent.

ACKNOWLEDGMENTS

We acknowledge the valuable help extended by the UNAVCOengineering and data archiving teams. We are indebted toKaj Johnson, who provided great help in analyzing the GPSvelocity eld and its associated tectonic patterns, as well as inimproving the manuscript. Terry Stigalls valuable presenceenabled the completion o the eld surveys, along with helpfrom Qizhi Chen, Mark Bauer, John Rupp and the sta of theIndiana Geological Survey, Altair Galgana, and the many stu-dents who took part in the survey. Francisco Gomez and histeam from the University of Missouri graciously assisted inthe August 2008 Shawnee eld observations. We thank BobKing (MIT) for his help in using GAMIT-GLOBK and RobMcCarey (RPI) for the DEFNODE modeling program. Weused the Generic Mapping Tools (GMT) from Wessel andSmith (1991). We thank Eric Calais, Gary Pavlis, Al Rudman,and Bruce Douglas or helpul discussions. We acknowledgethe thoughtul insights and valuable comments suggested byBrendan Crowell and Luciana Astiz, which greatly improvedthe manuscript. is research was supported by the U.S.Geological Survey National Earthquake Hazards ReductionProgram (NEHRP) grant No. 07HQGR0062 to MH; G.G.is supported in part by a postdoctoral ellowship rom the LPI(USRA). is is LPI contribution No. 1555.

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Lunar and Planetary InstituteUniversities Space Research Association

3600 Bay Area Boulevard,Houston, Texas 77058 U.S.A.

[email protected]

(G. A. G.)

Department of Geological SciencesIndiana University

Bloomington, Indiana 47405 [email protected]

(M. W. H.)

geodetic observations of active intraplate

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