Seismicity of the Ste. Genevieve Seismic Zone Based on Observations from the Earthscope OIINK Flexible Array

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Seismicity of the Ste. Genevieve Seismic Zone Based on Observations from the EarthScope OIINK Flexible Array

Article in Seismological Research Letters · November 2014 DOI: 10.1785/0220140079

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Online Material: Tables of OIINK catalog information and seismic zones. The WVSZ has received a great deal of attention focal mechanisms. in the last two decades, following the discovery of major paleoliquefaction features by Obermeier et al. (1991) and Munson INTRODUCTION et al. (1992) and the occurrence of the Mt. Carmel M w 5.4 earthquake on 18 April 2008 (Herrmann et al., 2008; Yang et al., Although far away from active plate boundaries, continental 2009; Hamburger et al., 2011). In this study, we will compare interiors are seismically active and include significant seismic some of the seismogenic analyses of the SGSZ with the NMSZ zones (Sykes, 1978). The best known of these zones in the cen- and the WVSZ. For direct comparisons, these three seismic zones are defined as 2°×2° rectangular regions (illustrated in Fig. 1). tral United States is the New Madrid seismic zone (NMSZ; The research reported in this paper is a study of opportu- Nuttli, 1973a, 1983). The NMSZ has been a particular focus – – of attention for seismic studies in the central United States due nity, made possible by the deployment of the Ozarks Illinois Indiana–Kentucky (OIINK) seismic network, which is a flexible to the three large M >7:0 earthquakes in 1811–1812 (e.g., w array component of EarthScope’s USArray project embedded Nuttli, 1973a; Dunn et al., 2010; Page and Hough, 2014). However, there is evidence of significant potential for large within the Transportable Array (TA). The OIINK experiment has broader goals to study the Earth’s structure linked to cratons earthquakes outside the NMSZ, such as in the Wabash Valley in general and to intracontinental basins, including the Illinois seismic zone (WVSZ; Nuttli, 1979; Obermeier et al., 1991; basin. This study is based on an early phase of the experiment Munson et al., 1992; Pavlis et al., 2002; Herrmann et al., 2008; with a mix of broadband and short-period sensors. The im- Yang et al., 2009; Hamburger et al., 2011), the region near Ma- proved station coverage and the high-quality records give us a rianna, Arkansas, southwest of the NMSZ (Tuttle et al., 2006), the region around the Meers fault, southwestern Oklahoma unique opportunity to detect more earthquakes with more ac- curate source parameters and to improve our understanding of (e.g., Luza et al., 1987; Kelson and Swan, 1990), and the region the seismogenic processes in this region. northwest of the NMSZ around the Ste. Genevieve fault zone (Heinrich, 1937, 1949; Nuttli, 1973b). This study focuses on what we refer to as the Ste. Gen- DATA evieve seismic zone (SGSZ), the seismically active region associated with the Ste. Genevieve fault zone (Fig. 1). The Ste. Network Design Genevieve fault zone is one of the northwest-trending fault The full OIINK experiment involves the deployment of 140 systems that cut across southwestern Illinois and eastern Mis- broadband seismic stations in three distinct phases, with an souri (Nelson, 1995; Marshak and Paulsen, 1996, 1997). It has average station spacing of about 25 km. The first phase in- been mapped for approximately 190 km along strike from the volved a deployment of 23 stations, including 14 short-period northeastern flank of the Ozark dome in southeastern Missouri stations and 9 broadband stations, operated between July 2011 into southwestern Illinois (Fig. 1)(Nelson and Lumm, 1985; and June 2012. The second phase operated from June 2012 to Nelson, 1995; Harrison and Schultz, 2002). late 2013, with the deployment of 70 broadband stations ex- Characterizing the seismicity of the SGSZ is significant tending from southeastern Missouri through southern Illinois. due to its proximity to the densely populated St. Louis met- The final phase, which involved moving the entire array to ropolitan area, yet the region has received little attention com- western Kentucky and southern Indiana, began in August pared to the neighboring New Madrid and Wabash Valley 2013 and is currently ongoing. This paper focuses on the data doi: 10.1785/0220140079 Seismological Research Letters Volume 85, Number 6 November/December 2014 1285 −92° −90° −88° −86° (a) −92° −90° −88° −86° 40° 40° 40° 40° IL IL IN Illinois Basin IN

SGFZ

38° MO 38° 38° MO 38° WVFS

OD RCG KY KY

Outline of 36° Mississippi OIINK BB 36° TN Embayment AR TN OIINK SP 36° AR 36° USArray TA RR m b ≥ 5.0 CNMSN BB GSN BB Earthquakes (after 1811) −92° −90° −88° −86° −92° −90° −88° −86°

Depth of Precambrian (b) −92° −90°− 88° −86° basement top (m) −5000 −2500 0 40° 40° IL SGSZ Illinois Basin ▴ Figure 1. Epicenters of mD CERI ≥2:0 historical earthquakes that IN occurred between 1 January! 1974 and 13 October 2013 from the New Madrid earthquake catalog distributed by the University of WVSZ SGFZ Memphis Center for Earthquake Research and Information (CERI). 38° MO 38° WVFS

Outlines of major structures were modified from Buschbach and OD RCG Kolata (1991). Stars are the mb ≥5:0 earthquakes that occurred KY Outline of after 1800 (prior to 1973, Nuttli, 1983; thereafter, National Earth- Mississippi Embayment quake Information Center catalog). (Geological structures are OD, Ozark dome; NMSZ, New Madrid seismic zone; RR, Reelfoot rift; 36° TN 36° AR By OIINK only RCG, Rough Creek graben; SGSZ, Ste. Genevieve seismic zone; Shared RR WVSZ, Wabash Valley seismic zone. States are IL, Illinois; KY, Ken- NMSZ By CERI only tucky; TN, Tennessee; AR, Arkansas; MO, Missouri.) −92° −90° −88° −86° from the first phase operation from 29 July 2011 to 8 June ▴ Figure 2. (a) Seismic stations used in this study and (b) earth- 2012, during which we have complete data coverage. quake epicenters from the Ozarks-Illinois-INdiana-Kentucky We combined our data with the data from 38 USArray (OIINK) catalog. (OIINK BB, OIINK broadband stations; OIINK SP, TA stations, 8 broadband Cooperative New Madrid Seismic OIINK short-period stations; USArray TA, USArray Transportable Network (CNMSN) stations, and the Global Seismographic Array broadband stations; CNMSN BB, Cooperative New Madrid Network (GSN) station CCM. Hereafter, this composite net- Seismic Network broadband stations; GSN BB, Global Seismo- work is referred to as the OIINK network (Fig. 2a). graphic Network broadband stations; OD, Ozark dome. Other ab- breviations are as listed in Fig. 1.) In (b), earthquakes reported by OIINK and by CERI are plotted separately for different sources to Earthquake Discrimination emphasize the benefit of the OIINK network in earthquake detec- The study area of southeastern Missouri and southwestern Illi- tion inside the SGSZ. The earthquakes reported by CERI during the nois is host to a large number of active surface and underground OIINK catalog time period that were not recorded by OIINK network coal mines and quarries. Consequently, a critical issue in data are grouped as CERI only. processing is to discriminate earthquakes from blasts. The waveform of a blast typically has a low-frequency P wave and a high- frequency coda of surface waves with relatively large amplitudes, 16:00 to 23:00 UTC. We will refer to this time period as the blast time window. known as the Rg phase (Hutchenson and Herrmann, 1993). In addition, the origin times of blasts or explosions are chiefly con- We employed a conservative approach to ensure the qual- centrated during local daylight hours. The discrimination rule ity of the OIINK earthquake catalog: earthquakes occurring used in this study is based on these characteristics of the blasts. within the blast time window were accepted only if they were As a test, we visually scanned the continuous data from 29 July detected by both OIINK and the Center for Earthquake Re- 2011 to 17 August 2011. During this period, only 2 out of the search and Information (CERI) and do not fit with the char- 771 identified local events (or 0.25%) were classified as natural acteristics of blasts mentioned above. For events outside the blast earthquakes. More than 70% of the blasts occurred roughly from time window, we applied the discrimination rules described

1286 Seismological Research Letters Volume 85, Number 6 November/December 2014 Table 1 DATA ANALYSIS: METHODS AND The Regional Earth Velocity Model Used for Locating OBSERVATIONS Earthquakes in This Study Earthquake Spatial Distributions Top Depth (km) P Velocity (km=s) S Velocity (km=s) Figure 1 shows the earthquake epicenters that occurred be- 0 5.6 3.28 tween 1974 and 2013 in our study area from the New Madrid 2 6.15 3.6 earthquake catalog. Earthquakes inside the NMSZ are densely 20 6.7 3.92 concentrated along the Reelfoot rift. Epicenters in the SGSZ 40 8.18 4.78 are more sparsely distributed but tend to concentrate near the 97 8.37 4.9 Ste. Genevieve fault zone (Figs. 1 and 2b). Epicenters in the WVSZ, in contrast, tend to be more diffusely distributed through southern Illinois and Indiana and do not show a clear above and located all events we classified as earthquakes. This concentration around the Wabash Valley fault system. approach results in the risk of missing some small-magnitude In the New Madrid catalog, all of the earthquakes prior to earthquakes that occur during the blast time window but reduces 1995 have fixed depths. Thus, for focal depth analysis, we se- the risk of falsely identifying blasts as earthquakes. In this study, lected the subset of the New Madrid catalog containing only we focus on the subset of the entire event catalog that contains earthquakes that occurred after 1 January 1995 with estimated only local earthquakes. This subset is referred to as the OIINK depths and with depth uncertainties less than 5.0 km. From catalog. both the short-term OIINK catalog (Fig. 3a,e) and the long- term New Madrid catalog (Fig. 3b,f ), earthquakes within the SGSZ are sparsely distributed with depth from the surface to Earthquake Catalogs about 22 km. In this range, the hypocenters are relatively uni- OIINK Catalog formly distributed compared to those in the NMSZ (Fig. 3c,g), The OIINK catalog includes 130 local earthquakes inside the with a clear concentration from 5 to 12 km depth. Although study area (Fig. 2b and Ⓔ Table S1, available in the electronic poorly constrained, the mb ≥5:0 earthquakes from historical supplement to this article). We use the Antelope Datascope records from the Nuttli (1983) and National Earthquake In- programs dbdetect and dbgrassoc to automatically detect seismic formation Center catalogs (see Data and Resources) inside the phase arrivals and to provide preliminary event locations based SGSZ are generally located above 15 km depth (Fig. 3d,e). In on the IASP91 global Earth model (Kennett and Engdahl, the WVSZ, there is a cluster of earthquakes that occurred at 1991). The range of signal-to-noise ratio used in dbdetect for about 12–20 km depth that are the aftershocks of the 2008 event detection is 2.5–4.0. A network detection is declared Mt. Carmel earthquake (Fig. 3d,h). Other than that, the earth- when at least six stations have detections consistent with an quakes in the WVSZ are nearly uniformly distributed from the event within the map area of Figure 2. We review the detected surface to about 30 km depth (Fig. 3h). events manually through dbloc2. During the review process, – the events are located by dbgenloc (Pavlis et al., 2004) using Frequency Magnitude Relationship a regional velocity model (Table 1). We associate each event The parameters used in Mlrichter were estimated based on with the New Madrid earthquake catalog produced by CERI earthquake records in California and are not directly applicable at the University of Memphis. For detections by both OIINK to the central United States. A better calibration is helpful for and CERI, the OIINK solutions are accepted for events inside further analysis that involves earthquake magnitudes. We therefore calibrate mL OIINK by correlating them with the matching the SGSZ and the CERI solutions are accepted for other re- ! gions. Finally, we compute local magnitudes for earthquakes in events that have mD CERI determined by CERI in the New Ma- drid earthquake catalog! (Fig. 4). By applying a linear least-square the OIINK catalog using the Antelope program, Mlrichter, regression, we estimated the following relationship between which is based on the traditional local earthquake magnitude mL OIINK and mD CERI: algorithm using the maximum amplitudes of high-frequency ! ! body waves (Richter, 1935). Those magnitudes are labeled mD CERI ≈ 0:677 × mL OIINK 0:595: 1 as mL OIINK. ! ! ! Equation (1) was used to convert mL OIINK to mD OIINK for Reference Catalog earthquakes in the OIINK catalog. ! ! We selected the unified New Madrid earthquake catalog dis- The standard relationship between earthquake frequency tributed by CERI as the reference catalog in our analysis. The and magnitude is New Madrid instrumental earthquake catalog from 1 January 1974 to 8 June 2012 is used in this study for a direct compari- log10 N a − b × M 2 son with our independent analysis. This truncated New Ma- drid catalog contains about 8460 earthquakes in our map area, (Gutenberg and Richter, 1954), in which N is the cumulative as shown in Figure 2. The duration magnitudes in this catalog number of earthquakes with magnitude ≥M,anda and b are were determined by CERI and are labeled as mD CERI. empirically derived constants. Constant a in equation (2) reflects !

Seismological Research Letters Volume 85, Number 6 November/December 2014 1287 (a) (b) (c) (d) OIINK cata. in SGSZ NM cata. in SGSZ NM cata. in NMSZ NM cata. in WVSZ 0 0 0 0

5 5 5 5

10 10 10 10

15 15 15 15

Depth (km) 20 20 20 20

25 25 25 25

30 30 30 30 0 10 20 0 10 20 0 10 20 01020 Percentage (%) (%) (%) (%) (e) (f) (g) (h) OIINK cata. in SGSZ NM cata. in SGSZ NM cata. in NMSZ NM cata. in WVSZ 0 0 0 0

5 5 5 5

10 10 10 10

15 15 15 15

Depth (km) 20 20 20 20

25 25 25 25 2008 Mt. Carmel Cluster 30 30 30 30 −91 −90 −89 −91 −90 −89 −90.5 −89.5 −88.5 −89 −88 −87 Longitude Longitude Longitude Longitude

▴ Figure 3. Depth distributions for earthquakes in the (a, b, e, and f) SGSZ, (c and g) the NMSZ, and (d and h) the WVSZ, based on the OIINK and the New Madrid (NM) catalogs. In (e)–(h), earthquake hypocenters were projected onto longitudinal profiles with error bars showing depth uncertainties. Stars are the projections of the mb ≥5:0 earthquakes as in Figure 1.

the seismicity level or earthquake rate in a region (Gutenberg 4 and Richter, 1954). To compare the frequency–magnitude Linear Least- distributions of multiple earthquake catalogs with various time Square Fit durations for different regions, we rewrite equation (2) as 3.5 ) D-CERI

m N log a′ − b × M; 3 10 T × A 3

in which N, M, and b are the same as in equation (2), a′ is the 2.5 normalized seismicity level, T is the catalog duration in years, and A is the area of the target region in km2.Equations(2) and (3) y = 0.677 x + 0.595 share the same coefficient b. R 2 = 0.861 2 The magnitude of completeness is defined as the mini- CERI Duration Magnitude ( mum magnitude of complete recording (Wiemer and Wyss, 2000). We can visually estimate the magnitude of completeness 1.5 1 2 3 4 5 from the frequency–magnitude distribution by estimating the

OIINK Local Magnitude (mL-OIINK ) minimum magnitude that satisfies equation (3). The OIINK catalog nominally contains 316 days of earth- ▴ Figure 4. Magnitude calibration between mD CERI and mL OIINK. quake records. However, the events not associated with the The dashed line shows the linear least-square! fit. ! New Madrid catalog within the blast time window are

1288 Seismological Research Letters Volume 85, Number 6 November/December 2014 Table 2 Earthquake Catalog Information Used in Frequency–Magnitude Distribution Analysis Event Counts Magnitude Range* Starting Date Ending Date SGSZ NMSZ SGSZ NMSZ (yyyy/mm/dd) (yyyy/mm/dd) Days† Years† OIINK Catalog 15 — 1.5~2.4 — 2011/7/29 2012/06/08 223.83 0.61 New Madrid Catalog 482 5792 0.2~4.8 0.2~5.0 1974/01/01 2012/06/08 14039 38.46

*For the OIINK catalog, mD OIINK is used based on equation (1). †For the OIINK catalog, the! reduced time duration is used. discarded. Theoretically, therefore, the catalog for the blast Madrid catalog is complete to no lower than about magnitude time window is more incomplete compared to that for the time 2.3 in this region. Given a b-value of about 1.0, the improve- period outside this window. For the purpose of earthquake fre- ment of 0.5 in magnitude unit translates to over three times quency analysis, we used a reduced duration of 223.83 days by more earthquakes recorded by the OIINK network in the same excluding the hours during the blast time window of 16:00 to time period. 23:00 UTC (Table 2). In the SGSZ, considering the relatively small sample size of Earthquake Focal Mechanisms the OIINK catalog, we compute the 95% confidence range of Several methods have been developed to determine earthquake the earthquake frequency based on a Poisson distribution for focal mechanisms, such as the methods based on P-wave and S- positive integers (the vertical bars in Fig. 5). The observed wave first-motion polarities and amplitudes (Khattri, 1973), P/ number of earthquakes is used as the mean of the Poisson dis- S amplitude ratios (Kisslinger et al., 1981), and moment tensor tribution. The results indicate that the earthquake frequency in inversions (Dziewonski et al., 1981; Herrmann, et al., 2011). In the NMSZ is about 3.4 times higher than that in the SGSZ, addition, orientations of the P axis given in the focal mecha- which is approximately comparable with that of the WVSZ. nism solutions provide information on the direction of the The b-values for the three seismic zones are about 1.0 (Fig. 5). maximum compressive stress (Zoback, 1992). The magnitude of completeness is estimated based on Fig- We use first-motion focal mechanisms to analyze the faulting types and the contemporary state of stress in the ure 5 and is used as an approximation of the earthquake de- SGSZ. We visually estimate the double-couple solutions inter- tection limit of the catalog. In the SGSZ, the OIINK catalog has actively through a Python computer program, fmfocal.py, by a detection limit of approximately 1.8. In comparison, the New identifying orthogonal nodal planes that minimize the number of discrepant first motions within compressional and dilata- tional quadrants. The nodal plane parameters and the P axis −1.5 NMSZ (m ) are computed by the program. Before picking P-wave first mo- D−CERI SGSZ (m ) tions, we apply a 1.0 Hz high-pass filter to the short-period b -value 1.0 D−CERI SGSZ (m ) records and a 1.0–10.0 Hz band-pass filter to the broadband −2.5 D−OIINK WVSZ (m ) D−CERI records. The quality of the focal mechanism solution is graded by the percentage of inconsistencies between the solution and ) − 2 −3.5 (multiplier 3.4) the first-motion observations. By this grading scheme, a low in-

.km consistency rate means relatively high reliability of the solution. − 1 We were able to determine reliable focal mechanisms for 7 −4.5 of the 16 earthquakes located inside the SGSZ (Fig. 6 and Ⓔ

log(N.yr Table S2). In addition, we collect the focal mechanism solutions of 57 M w ≥2:75 earthquakes that occurred from 1962 OIINK NM −5.5 M = 1.8 c SGSZ Mc SGSZ = 2.3 to 2013 inside our study area from the Saint Louis University (SLU) focal mechanism catalog (Fig. 7). Following the conven- tion used in the SLU catalog, horizontal projections of the P −6.5 1 2 3 4 5 axes are plotted with bars scaled by the plunges of P axes. Magnitude Earthquakes inside the SGSZ are dominated by strike-slip faulting mechanisms with some dip-slip components (Figs. 6 ▴ Figure 5. Earthquake frequency–magnitude distributions for and 7). The average P axis in this region inferred from the focal the SGSZ, the NMSZ, and the WVSZ. Mc, magnitude of complete- mechanisms is trending northeast–southwest (Fig. 8). On aver- OIINK NM ness; McSGSZ , Mc of the OIINK catalog in the SGSZ; McSGSZ, Mc of age, the trends of the P axes for the SGSZ, the NMSZ, and the the New Madrid earthquake catalog in the SGSZ. The two dashed WVSZ are 66:4° 23:3°, 83:9° 20°, and 81:1° 9:9°, respec- lines with b ≈ 1:0 are plotted as the reference lines. tively. Comparing with that in the SGSZ, the average trends of

Seismological Research Letters Volume 85, Number 6 November/December 2014 1289 Event ID: 2 Event ID: 5 Event ID: 47 Event ID: 49

6% 4% 27% 0%

Event ID: 51 Event ID: 93 Event ID: 142

Compressional

Double-Couple

6% Percentage of Inconsistencies 10% 7% 11%

▴ Figure 6. First-motion polarities of earthquakes in the OIINK catalog and the inferred double-couple focal mechanisms. the P axis in the other two seismic zones are closer to east–west DISCUSSION (Fig. 8). The average plunge of the P axis in the SGSZ is around 15°. A similar value is observed in the NMSZ. In contrast, in the Earthquake Detection WVSZ, the average plunge of P axes is about 4°. The New Madrid earthquake catalog is the most complete earthquake catalog for the southern Illinois basin. As shown in Figure 5, the earthquake detection limit of the New Madrid earthquake catalog is about m 1.5 inside the NMSZ. However, −92° −90° −88° −86° D the detection abilities are significantly poorer for regions out- 40° OIINK Center 40° From OIINK SGSZ IL side the NMSZ. The earthquake detection limit of the New From SLU Illinois Basin IN Madrid catalog increases to about mD 2.3 in the SGSZ and closer to 3.0 in the WVSZ. This is undoubtedly a result of 51 142 WVSZ 47 the large variation in station coverage by the current regional networks. The station density in the NMSZ likely competes for 93 SGFZ 2 38° MO WVFS 38° the highest density in the entire United States, with an average 5 station spacing of about 10 km; outside the concentration of 49 RCG KY OD seismicity in the New Madrid region, the station density is rel-

Outline of atively low, with average station spacing up to about 100 km. Mississippi Embayment Therefore, an unfiltered plot of the New Madrid earthquake 36° AR P−axis Projection 36° catalog potentially overemphasizes seismicity within the NMSZ. TN Horizontal The paucity of seismic stations in the SGSZ has directly RR NMSZ Vertical limited the earthquake-detection ability in this region. In addi-

−92° −90° −88° −86° tion, the occurrence of a large number of coal mines and quarries that produce blasts makes it difficult to discriminate the ▴ Figure 7. Earthquake focal mechanisms in the central United microearthquakes from a background of blasts (Braile et al., States from the Saint Louis University (SLU) focal mechanism 1982). This is effectively a form of additional bias that could catalog and the OIINK catalog. P-axis horizontal projections are cause an underestimation of the seismicity rate if not handled plotted as solid bars with lengths scaled by the relative degrees carefully. During the 20-day test period, less than 1% of the of plunging. local events were classified as natural earthquakes based on

1290 Seismological Research Letters Volume 85, Number 6 November/December 2014 (a) (b) (c) SGSZ N NMSZ N WVSZ N 330 30 330 30 330 30

300 60 300 60 300 60

W E W E W E

240 120 240 120 240 120

210 150 210 150 210 150 S S S

▴ Figure 8. Lower hemisphere stereonet equal-area projections of the P axis for (a) the SGSZ, (b) the NMSZ, and (c) the WVSZ. The data were plotted using the computer program Stereonet (Allmendinger et al., 2013; Cardozo and Allmendinger, 2013). The concentrations of the data points were plotted as contours using the method of Kamb (1959). For the SGSZ, we merged the focal mechanisms from the OIINK catalog with those from the SLU catalog. the discrimination rules in Hutchenson and Herrmann (1993). sition from upper crust to middle crust. In the SGSZ, earth- In this study, we discarded most of the data from 16:00 to 23:00 quake hypocenters are nearly uniformly distributed above UTC to make data processing tractable and to avoid biasing our about 22 km (Fig. 3). This pattern is inconsistent with either estimates. Consequently, we suggest that our seismicity results of the two models in Klose and Seeber (2007) but similar to are highly reliable, which is supported by the good agreement the distribution pattern in the WVSZ when we exclude the of the frequency–magnitude distribution of our data with the 2008 Mt. Carmel earthquake aftershock cluster. On the other longer term record (Fig. 5). hand, the sample sizes for those two seismic zones are relatively For the parameters used in the processing scheme we em- small, and this conclusion should be viewed as tentative. In ployed, the detection limit of the OIINK catalog is at most contrast, in the NMSZ, the region from about 5 km to about magnitude 1.8 inside the SGSZ. This is about 0.5 units lower 12 km is the most seismogenic layer, contributing most of the than that of the New Madrid catalog in this region. This im- earthquake activity (Fig. 3). provement is equivalent to about three times more earthquakes Braile et al. (1982, 1986) asserted that the Ste. Genevieve than those located by the regional network. Therefore, the im- fault zone is the northwestern extension of the Reelfoot rift. proved station coverage by the OIINK network enhanced our However, Nelson and Lumm (1985) have argued that geologi- ability to detect small earthquakes in the southern Illinois basin cal evidence is insufficient to support this idea. Our study sug- by compressing one year of recording to be equivalent to about gests that the SGSZ is seismically distinct from the NMSZ. In three years of recording by the regional network. comparison with the NMSZ, the earthquakes in the SGSZ are slightly more spatially scattered, extend over a greater depth Earthquake Activity: Spatial Concentration and range, occur at lower activity levels, and exhibit a rotated prin- Seismicity Rate cipal stress orientation (Figs. 1, 2, 4, and 7). Based on the dif- Heinrich (1937, 1949) contended that the region around the ferences in seismicity patterns, we argue that the SGSZ should Ste. Genevieve fault zone is seismically active. The distribution be treated as a separate seismically active region instead of the of epicenters is statistically correlated with the general strike of northwestern extension of the NMSZ. the Ste. Genevieve fault zone (Amorèse, 2003). The results of Nuttli (1973b) argued that the region around the Ste. this study provide new support for those arguments (Figs. 1 Genevieve fault zone from western Illinois to east central Mis- and 2). In the SGSZ, no earthquakes are recorded below a souri is one of the regions in the central United States that has depth of about 22 km (Fig. 3). This is consistent with the maxi- the highest probability of suffering damaging earthquakes. mum focal depth of 25 5 km for intraplate earthquakes From our analysis, the normalized frequency–magnitude distri- predicted by Chen and Molnar (1983). This indicates a seis- butions suggest that the seismicity rate in the SGSZ is approx- mogenic upper crust in contrast to the relatively aseismic lower imately one-third that of the concentrated seismicity in the crust. Klose and Seeber (2007) concluded that some of the sta- NMSZ. This rate is clearly nonzero and is comparable with the ble continental regions have well-developed bimodal distribu- rate in the WVSZ estimated by Pavlis et al. (2002). This result is tions with earthquakes confined within the upper third and the important given the proximity of the SGSZ to the St. Louis lower third of the crust, while other stable continental regions metropolitan area. Paleoliquefaction surveys have been con- present approximately unimodal distributions within the tran- ducted to reveal prehistoric earthquake activity in this region

Seismological Research Letters Volume 85, Number 6 November/December 2014 1291 (Tuttle et al., 1999; Tuttle and U.S. Geological Survey, 2005). ering of the detection limit from a magnitude of about 2.3 to Although evidence is insufficient to support the occurrence of 1.8. This improvement significantly increased the number of any significant prehistoric earthquakes, we suggest that the seis- earthquakes recorded inside this region. mic hazard in the SGSZ could be underappreciated compared The earthquake epicenters in the SGSZ are diffusely spread to the NMSZ. along a trend approximately following the Ste. Genevieve fault zone. The hypocenters in the SGSZ are nearly uniformly dis- Seismogenic Mechanisms and State of the Stress Field tributed with depth from the surface to about 22 km. The seis- In the SGSZ, the relatively low seismicity rate and the paucity micity rate in this region is about one-third of that in the of seismic stations before the deployment of the OIINK net- NMSZ and is comparable with that of the WVSZ. work have limited the number of focal mechanism determina- Earthquakes in the SGSZ are dominated by strike-slip tions. The very dense coverage of the OIINK network provided mechanisms under a horizontal to subhorizontal compressive the opportunity to determine the focal mechanisms for small stress regime. The average maximum horizontal compressive events in this region using the standard first-motion method. stress in the SGSZ trends approximately northeast–southwest, The contemporary seismogenic processes in the SGSZ are contrasting with that observed in the NMSZ and the WVSZ. dominated by strike-slip mechanisms under horizontal to sub- This variation may be associated with local stress concentrations horizontal compressive stresses, as indicated by P axes with low around the major pre-existing faults in those regions interacting plunges (Figs. 7 and 8). In addition, dip-slip components are with the stable cratonic lithosphere beneath the Illinois basin observed from some focal mechanism solutions in this region under the regional northeast–southwest stress field. (Fig. 7). Two oblique normal-faulting earthquakes occurred in the SGSZ (Fig. 7), one of which is located close to the DATA AND RESOURCES northwestern end of the Ste. Genevieve fault zone and the other on the western flank of the Ozark dome. Those two nor- The Ⓔ OIINK local earthquake catalog produced by this study mal-faulting solutions, together with the large deviation of the and the focal mechanisms determined by this study are avail- P-axis orientations, suggest that the contemporary seismogenic able in the electronic supplement to this article. Other earth- processes in the SGSZ are possibly controlled by localized pre- quake catalogs used in this study are the National Earthquake existing structures superimposed on the regional, predomi- Information Center catalog (http://earthquake.usgs.gov/ nantly shear stresses. As a stress indicator, the average P axis regional/neic/; last accessed October 2013), the New Madrid inside the SGSZ from the OIINK solutions is oriented approx- earthquake catalog released by the Center for Earthquake Re- imately northeast–southwest, approximately consistent with search and Information (CERI) at the University of Memphis the borehole breakout data and the hydraulic fracture data (http://www.memphis.edu/ceri/seismic/catalogs/index.php; last in and around this area (Heidbach et al., 2008, 2010). accessed October 2013), and the Saint Louis University (SLU) Superimposed on the background stress field in central focal mechanism catalog (http://www.eas.slu.edu/eqc/eqc_mt/ North America with maximum compressive stresses trending MECH.NA/MECHFIG/mech.html; last accessed October 2013). northeast–southwest (Zoback and Zoback, 1980; Heidbach Information about other networks involved in this study et al., 2010; Hurd and Zoback, 2012), variations in the state can be found from the following resources: the Cooperative of mean stress fields are observed across the SGSZ, the NMSZ, New Madrid Seismic Network (CNMSN) (http://www.eas.slu. and the WVSZ (Fig. 8). In the SGSZ, the inferred average maxi- edu/eqc/eqcnetinfo.html;lastaccessedFebruary2014),theGlobal mum compressive stress is oriented about 15°–20° counterclock- Seismographic Network (GSN) (http://earthquake.usgs.gov/ wise relative to the east–west direction observed in the monitoring/gsn; last accessed February 2014), and seismic sta- WVSZ and the NMSZ (Heidbach et al., 2008, 2010), with tions in New Madrid region (http://folkworm.ceri.memphis. near-horizontal plunge (Fig. 8). Mazzotti and Townend edu/REQ3/html/station.html; last accessed February 2014). (2010) analyzed the rotation between the focal mechanism de- Information about active surface and underground coal termined stress field and that determined by borehole measure- mines and quarries in Missouri and Illinois can be found on ments in ten seismic zones in central and eastern North the Missouri Mine Maps (http://dnr.mo.gov/geology/geosrv/ America. They proposed the rotation mechanism to be the in- geores/minemaps.htm; last accessed February 2014) and the teraction between long-wavelength stress perturbations and local coal mines in Illinois Viewer (http://www.isgs.illinois.edu/? stress concentrations. In this study, the regional variations of the q=ilmines; last accessed February 2014), respectively. inferred stress field in the three seismic zones in central North America may be attributed to the local stress concentrations due ACKNOWLEDGMENTS to the interactions of the stable cratonic lithosphere beneath the Illinois basin and its surrounding structures. Instrumentation for the Ozarks–Illinois–Indiana–Kentucky (OIINK) experiment was provided by the Program for the CONCLUSIONS Array Seismic Studies of the Continental Lithosphere (PASS- CAL) Instrument Center of the Incorporated Research Insti- The composite OIINK network used in this study significantly tutions for Seismology (IRIS) Consortium. The OIINK improved the station coverage in the SGSZ, resulting in low- experiment is supported by National Science Foundation

1292 Seismological Research Letters Volume 85, Number 6 November/December 2014 Grants EAR-1053354 to Indiana University, EAR-1053248 to Heinrich, R. R. (1949). Three Ozark earthquakes, Bull. Seismol. Soc. Am. Purdue University, and EAR-1053551 to University of Illinois 39, 1–8. Herrmann, R. B., H. Benz, and C. J. Ammon (2011). Monitoring the under the EarthScope program. Great thanks to all of the earthquake source process in North America, Bull. Seismol. Soc. Am. OIINK team members for their contributions to the deploy- 101, 2609–2625, doi: 10.1785/0120110095. ment of OIINK stations. We are especially grateful to all of Herrmann, R. B., M. Withers, and H. Benz (2008). The April 18, 2008 the landowners for allowing us to install and operate stations Illinois earthquake: An ANSS monitoring success, Seismol. Res. on their properties. Thanks to Yinzhi Wang for helping on the Lett. 79, 830–843, doi: 10.1785/Gssrl.79.6.830. Hurd, O., and M. D. Zoback (2012). 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