Showing papers in "Seismological Research Letters in 1997"

Modification of Empirical Strong Ground Motion Attenuation Relations to Include the Amplitude and Duration Effects of Rupture Directivity

Abstract: Rupture directivity effects cause spatial variations in ground motion amplitude and duration around faults and cause differences between the strike-normal and strike-parallel components of horizontal ground motion amplitudes, which also have spatial variation around the fault. These variations become significant at a period of 0.6 second and generally grow in size with increasing period. We have developed modifications to empirical strong ground motion attenuation relations to account for the effects of rupture directivity on strong motion amplitudes and durations. The modifications are based on an empirical analysis of near-fault data. The ground motion parameters that are modified include the average horizontal response spectral acceleration, the duration of the acceleration time history, and the ratio of strike-normal to strike-parallel spectral acceleration. The parameters upon which the adjustments to average horizontal amplitude and duration depend are the fraction of the fault rupture that occurs on the part of the fault that lies between the hypocenter and the site, and the angle between the fault plane and the path from the hypocenter to the site. Since both of these parameters can be derived from the hypocenter location and the fault geometry, the model of rupture directivity effects on ground motions that we have developed can be directly included in probabilistic seismic hazard calculations. The spectral acceleration is larger for periods longer than 0.6 second, and the duration is smaller, when rupture propagates toward a site. For sites located close to faults, the strike-normal spectral acceleration is larger than the strike-parallel spectral acceleration at periods longer than 0.6 second in a manner that depends on magnitude, distance, and angle. To facilitate the selection of time histories that represent near-fault ground motion conditions in an appropriate manner, we provide a list of near-fault records indicating the rupture directivity parameters that each contains.

Equations for Estimating Horizontal Response Spectra and Peak Acceleration from Western North American Earthquakes: A Summary of Recent Work

Abstract: In this paper we summarize our recently-published work on estimating horizontal response spectra and peak acceleration for shallow earthquakes in western North America. Although none of the sets of coefficients given here for the equations are new, for the convenience of the reader and in keeping with the style of this special issue, we provide tables for estimating random horizontal-component peak acceleration and 5 percent damped pseudo-acceleration response spectra in terms of the natural, rather than common, logarithm of the ground-motion parameter. The equations give ground motion in terms of moment magnitude, distance, and site conditions for strike-slip, reverse-slip, or unspecified faulting mechanisms. Site conditions are represented by the shear velocity averaged over the upper 30 m, and recommended values of average shear velocity are given for typical rock and soil sites and for site categories used in the National Earthquake Hazards Reduction Program's recommended seismic code provisions. In addition, we stipulate more restrictive ranges of magnitude and distance for the use of our equations than in our previous publications. Finally, we provide tables of input parameters that include a few corrections to site classifications and earthquake magnitude (the corrections made a small enough difference in the ground-motion predictions that we chose not to change the coefficients of the prediction equations).

Empirical Response Spectral Attenuation Relations for Shallow Crustal Earthquakes

Abstract: Using a database of 655 recordings from 58 earthquakes, empirical response spectral attenuation relations are derived for the average horizontal and vertical component for shallow earthquakes in active tectonic regions. A new feature in this model is the inclusion of a factor to distinguish between ground motions on the hanging wall and footwall of dipping faults. The site response is explicitly allowed to be non-linear with a dependence on the rock peak acceleration level.

Model of Strong Ground Motions from Earthquakes in Central and Eastern North America: Best Estimates and Uncertainties

Abstract: Ground-motion attenuation equations for rock sites in central and eastern North America are derived, based on the predictions of a stochastic ground-motion model. Four sets of attenuation equations are developed ( i.e. , 2 crustal regions × 2 magnitude scales). The associated uncertainties are derived by considering the uncertainties in parameter values, as well as those uncertainties associated with the ground-motion model itself. Comparison to data shows a reasonable agreement. Comparison to other attenuation functions for the region shows consistency with most attenuation functions in current use.

Strong Ground Motion Attenuation Relationships for Subduction Zone Earthquakes

Abstract: We present attenuation relationships for peak ground acceleration and response spectral acceleration for subduction zone interface and intraslab earthquakes of moment magnitude M 5 and greater and for distances of 10 to 500 km. The relationships were developed by regression analysis using a random effects regression model that addresses criticism of earlier regression analyses of subduction zone earthquake motions. We find that the rate of attenuation of peak motions from subduction zone earthquakes is lower than that for shallow crustal earthquakes in active tectonic areas. This difference is significant primarily for very large earthquakes. The peak motions increase with earthquake depth and intraslab earthquakes produce peak motions that are about 50 percent larger than interface earthquakes.

Attenuation Relationships for Shallow Crustal Earthquakes Based on California Strong Motion Data

Abstract: Attenuation relationships are presented for peak acceleration and response spectral accelerations from shallow crustal earthquakes. The relationships are based on strong motion data primarily from California earthquakes. Relationships are presented for strike-slip and reverse-faulting earthquakes, rock and deep firm soil deposits, earthquakes of moment magnitude M 4 to 8+, and distances up to 100 km.

Empirical Near-Source Attenuation Relationships for Horizontal and Vertical Components of Peak Ground Acceleration, Peak Ground Velocity, and Pseudo-Absolute Acceleration Response Spectra

Abstract: Based on extensive user feedback, it has been determined that there are several errors and ambiguities that need to be corrected in the original manuscript (Campbell, 1997). Some of the errors are the result of typographical errors by the publisher, while others are omissions to the original equations that occurred as an oversight. I also offer some additional guidance on the use of the attenuation relations for calculating ground motion parameters for normal-faulting earthquakes and for sites classified simply as “soil” or “rock.” I hope that these corrections and additional guidance will make the attenuation relations more understandable and easier to use. Equation (1) should be corrected as follows: when dSEIS ≥ HSEIS \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[d\_{SEIS}=\frac{1}{2}[H\_{TOP}+H_{BOT}-W\mathrm{sin}({\alpha})]\] \end{document} otherwise \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\ d\_{SEIS}=H\_{SEIS}\] \end{document}(1) where HTOP (km) is the depth to the top of the fault and HSEIS (km) is the depth to the top of the seismogenic part of the crust. The original equation mistakenly assumed that the depth to the top of the fault was equal to the depth to the seismogenic crust. Equation (2) should be corrected as follows: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\ \mathrm{log}{\ }W=-1.01+0.32M_{W}\] \end{document}(2) The original equation mistakenly omitted the log term. Based on the revision to Equation (1), Table 1 and the related discussion should be corrected as follows: View this table: TABLE 1 Recommended Average Values of dSEIS …

Some Comparisons Between Recent Ground- Motion Relations

Abstract: We provide an overview of new ground-motion relations for eastern North America (ENA) developed over the last five years. The empirical-stochastic relations of Atkinson and Boore (1995) are compared to relations developed by the Electric Power Research Institute (EPRI, 1993; also Toro et a/., 1994), Frankel et al. (1996), and the consensus ENA ground-motion values as reported by SSHAC (1996). The main difference between our relations and those of EPRI or Frankel is in the low-frequency amplitudes (f 2) than California values at high frequencies. The alternative soil correction leads to the conclusion that our ENA relations are moderately lower (factor<2) than the California relations at low frequencies, and moderately higher at high frequencies. Both of these conclusions imply that ground-motion relations or time series for earthquakes in one region cannot be simply modified for use in engineering analyses in another region.

Don't Call it Stress Drop

Abstract: O ne of the most battle-weary parameters in seismology today is stress drop. Seismologists argue over everything about this parameter, including its name, meaning, measurement, and scaling with magnitude. The original concept of stress drop was introduced as a static measure of final fault slip, as a fraction of fault dimension (Δσ ∼ u/r ), and was estimated from measurements or inferences of these geologically-based parameters (as, for example, by Kanamori and Anderson, BSSA, 1975, pp. 1,073–1,095). Stress drop became an important earthquake source parameter following Brune's classic paper ( JGR , 1970, pp. 4,997–5,009), showing that the radiated far-field spectrum of shear waves could be interpreted in terms of a simple point-source model with just two source parameters—seismic moment ( M ) and stress drop (Δσ). This paved the way for measurments of stress drop from seismic signals. In Brune's model, the acceleration source spectrum has a simple `omega-squared' shape...

Stochastic Point-Source Modeling of Ground Motions in the Cascadia Region

Abstract: A stochastic model is used to develop preliminary ground motion relations for the Cascadia region for rock sites. The model parameters are derived from empirical analyses of seismographic data from the Cascadia region. The model is based on a Brune point-source characterized by a stress parameter of 50 bars. The model predictions are compared to ground-motion data from the Cascadia region and to data from large earthquakes in other subduction zones. The point-source simulations match the observations from moderate events ( M M > 7.5) in other regions; motions are overpredicted near the earthquake source and underpredicted at large distances (> 100 km). The discrepancy at large magnitudes suggests further work on modeling finite-fault effects and regional attenuation is warranted. In the meantime, the preliminary equations are satisfactory for predicting motions from events of M

SEA96--A New Predictive Relation for Earthquake Ground Motions in Extensional Tectonic Regimes

Abstract: We present a new predictive relation for horizontal peak ground acceleration and 5% damped pseudo-velocity response spectrum appropriate for predicting earthquake ground motions in extensional tectonic regimes. This new empirical relation, which we denote “Sea96,” was originally derived by Spudich et al. (1996) as part of a project to estimate seismic hazard at the site of a proposed nuclear waste repository at Yucca Mountain, Nevada. Because of the length and relative inaccessibility of that report, we are briefly presenting the Sea96 relation and its derivation here. We developed our relation based on data from extensional regime earthquakes having moment magnitude M > 5.0 recorded at distances less than 105 km. Extensional regions are regions in which the lithosphere is expanding areally. This areal expansion is the result of applied forces that yield a state of stress for which Sv > SHmax > SHmin , where Sv , SHmax , and SHmin represent principal...

Liquefaction Evidence for Holocene and Latest Pleistocene Seismicity in the Southern Halves of Indiana and Illinois: A Preliminary Overview

Abstract: Clastic dikes filled with sand and gravel, interpreted to be of seismic liquefaction origin, occur throughout much of the southern halves of Indiana and Illinois. Nearly all of the liquefaction features originated from earthquakes centered in the southern halves of Indiana and Illinois, rather than further south in the nearby source region for the great 1811–12 New Madrid earthquakes. At least eight earthquakes strong enough to induce liquefaction have occurred during the past 20,000 years. At present, the best dated paleoliquefaction evidence lies in Indiana. More paleoliquefaction features have been found in Illinois, but studies there have not yet narrowly bracketed the age of the liquefaction evidence at most sites. Estimated magnitudes of at least two prehistoric events are higher than M 7, which greatly exceeds the largest historic earthquake of M 5.5 that has originated in Indiana and Illinois. The strongest prehistoric earthquakes have been centered in the general vicinity of strongest historic seismicity, in and near the Wabash Valley seismic zone in the lower Wabash Valley of Indiana and Illinois. However, in several cases prehistoric events greater than M 6 have struck far outside the Wabash Valley seismic zone, in areas where there has been little or no historic seismicity. Numerous other strong paleoearthquakes could have struck elsewhere in the southern halves of Indiana and Illinois but not be recognized because of the lack of liquefiable deposits in many areas.

Taiwan Rapid Earthquake Information Release System

Abstract: INTRODUCTION Interest in rapid access to earthquake information has grown enormously in the past few years (Gee et al. , 1996; Teng et al. , 1997; Wu et al. , 1997). In addition to satisfying the public and media needs, rapid notification programs provide valuable information for rapid earthquake disaster response, thereby mitigating the loss. Recognizing the importance of rapid earthquake information for seismic hazard mitigation, efforts to design and implement systems to provide rapid earthquake information have expanded over the last 10 years (Heaton, 1985; Nakamura and Tucker, 1988; Buland and Person, 1992; Romanowicz et al. , 1993; Bakun et al. , 1994; Espinosa Aranda et al. , 1995; Lee et al. , 1996; Shin et al. , 1996; Teng et al. , 1997; Wu et al. , 1997). In 1989, a telemetered, digital, short-period seismograph network of 75 three-component field stations was installed in Taiwan (see Figure 1) using Teledyne Geotech 1-Hz short-period sensors (Teledyne S13). The signals...

Source Time Functions

Abstract: INTRODUCTION Seismologists have always striven to determine earthquake parameters as soon as possible after an earthquake occurs. Now there are several research groups that routinely “broadcast” their rapid determinations of earthquake location, focal mechanism, size, and depth. We report on our efforts to produce and “broadcast” source time functions of earthquakes. The steady march of technical progress in seismology has provided first for the rapid determination of epicenter and magnitude (e.g., from the National Earthquake Information Center [NEIC] and various regional networks). Technical requirements for this effort are that several stations have their output telemetered to some location where the arrivals can be picked and that at least one of these stations be well-calibrated so that magnitude can be estimated. This technical level and service has been available for several decades. In some cases, this procedure has been completely automated with earthquake alarms issued with no human intervention. It is...

Seismic Interpretation of the Deep Structure of the Wabash Valley Fault System

Abstract: Interpretations of newly available seismic reflection profiles near the center of the Illinois Basin indicate that the Wabash Valley Fault System is rooted in a series of basement-penetrating faults. The fault system is composed predominantly of north-northeast-trending high-angle normal faults. The largest faults in the system bound the 22-km wide 40-km long Grayville Graben. Structure contour maps drawn on the base of the Mount Simon Sandstone (Cambrian System) and a deeper pre-Mount Simon horizon show dip-slip displacements totaling at least 600 meters across the New Harmony fault. In contrast to previous interpretations, the N-S extent of significant fault offsets is restricted to a region north of 38° latitude and south of 38.35° latitude. This suggests that the graben is not a NE extension of the structural complex composed of the Rough Creek Fault System and the Reelfoot Rift as previously interpreted. Structural complexity on the graben floor also decreases to the south. Structural trends north of 38° latitude are offset laterally across several large faults, indicating strike-slip motions of 2 to 4 km. Some of the major faults are interpreted to penetrate to depths of 7 km or more. Correlation of these faults with steep potential field gradients suggests that the fault positions are controlled by major lithologic contacts within the basement and that the faults may extend into the depth range where earthquakes are generated, revealing a potential link between specific faults and recently observed low-level seismicity in the area.

Geophysical Setting of the Wabash Valley Fault System

Abstract: Interpretation of existing regional magnetic and gravity data and new local high-resolution aeromagnetic data provides new insights on the tectonic history and structural development of the Wabash Valley Fault System in Illinois and Indiana. Enhancement of short-wavelength magnetic anomalies reveal numerous NW- to NNE-trending ultramafic dikes and six intrusive complexes (including those at Hicks Dome and Omaha Dome). Inversion models indicate that the interpreted dikes are narrow (≤3 m), lie at shallow depths ( Based on the interpretation of both the regional magnetic and gravity data and the high-resolution magnetic data, we propose that the shallow faults and deep-seated rift structures in the Wabash Valley terminate at or near the Rough Creek-Shawneetown Fault System. The Grayville Graben (∼20 km wide, ∼700 m maximum basement relief, and et al. , this volume]) underlying the Wabash Valley developed during rifting, perhaps in response to stress concentrations generated by a bend in the Reelfoot-Rough Creek-Rome rift system. We therefore hypothesize that although the Reelfoot Rift and Rough Creek Graben represent tectonic intraplate structures of large areal extent (>500 km long and generally >50 km wide) and with deep basins (locally >3 km thick), the ancestral Wabash Valley faults express, in comparison, minor tectonic structures and probably do not represent a failed rift arm. There is a lack of any obvious relation between the Wabash Valley Fault System and the epicenters of historic and prehistoric earthquakes. Five prehistoric earthquakes lie conspicuously near structures associated with the Commerce geophysical lineament, a NE-trending magnetic and gravity lineament lying oblique to the Wabash Valley Fault System and possibly extending over 600 km from NE Arkansas to central Indiana.

Estimated Magnitudes and Accelerations Associated with Prehistoric Earthquakes in the Wabash Valley Region of the Central United States

Abstract: Seismic hazards in the central United States are typically based on occurrences of earthquakes in the New Madrid Seismic Zone. However, paleoliquefaction evidence shows that large prehistoric earthquakes also occurred in the Wabash Valley region of Indiana and Illinois. A geotechnical study of the soil conditions at paleoliquefaction sites there is used to estimate both the magnitudes and accelerations of the prehistoric earthquakes. This study covers an area of the Wabash River drainage approximately 250 km north to south and 180 km east to west, in southern Indiana and Illinois. In situ soil strength parameters were measured at 22 sites. The measured strength parameters are used in conjunction with liquefaction susceptibility analyses to estimate moment magnitude ( M ) and peak surface accelerations of four separate paleoearthquakes. In addition, site response studies based on the Atkinson and Boore (1995) model of bedrock motions for eastern North American earthquakes are used to develop attenuation relations. This allows a comparison of the results of the geotechnical study with a seismological prediction of peak surface accelerations due to the paleoearthquakes. Using a regionally appropriate relationship between magnitude and maximum distance to liquefaction effects (Obermeier et al. , 1993) leads to preliminary magnitude estimates of M 6.8, M 6.9, M 7.2, and M 7.8 for the four paleoearthquakes. Also, a method we developed, an energy-stress approach, is used to estimate the peak surface accelerations at these magnitudes that would be required to induce the observed liquefaction effects. Next, the acceleration estimates are compared with peak surface motions predicted by a seismological attenuation model for various magnitudes in order to determine the best-fitting magnitude. For the four paleoearthquakes studied, the geotechnical and seismological estimates of surface accelerations most closely agree for M 6.9, M 7.1, M 7.3, and M 7.8. Thus, two different methods yield basically the same paleomagnitudes, and therefore provide our best estimates.

Shear-Wave Velocity Survey of Seismographic Sites in Eastern Canada: Calibration of Empirical Regression Method of Estimating Site Response

Abstract: Reliable information concerning the predominant site effects on ground motions can be obtained from low-cost shear-wave refraction surveys using a sledgehammer as an energy source. For the south and southeast Ontario region, the velocity structure can be determined to a depth of approximately 70 m. Near-surface shear-wave velocities of hard-rock sites range from 1.7 to 3.1 km/sec, with an average value of approximately 2.6 km/sec. Typical soil sites have shear-wave velocities of 250–700 m/sec near the surface. Empirical methods of determining the relative values of frequency-dependent amplification are commonly employed. These methods fit the observed earthquake spectra with a regression model that decomposes the recorded spectrum into source, path, and site terms. By regression analysis of large amounts of recorded data, the average site term for a particular station can be determined. We calibrated such an empirical technique against the theoretical responses based on the velocity structure obtained from a detailed field survey. The empirical-regression approach and the theoretical-response approach provide reasonably consistent estimates of amplification at hard-rock sites. We conclude that the amplification for an average hard-rock site is about a factor of 1.3. At soil sites, empirical-analysis and theoretical-response results agree as to the frequency of the fundamental resonance peaks. The maximum theoretical amplification values generally exceed those indicated from the empirical analyses; theoretical amplifications by as much as factors of 6 and 7 were calculated, while empirical amplifications were below a factor of 3. Thus use of theoretical site responses may be conservative.

Structural Style, Regional Distribution, and Seismic Implications of Midcontinent Fault-and-Fold Zones, United States

Abstract: Paleozoic/Mesozoic strata of the United States continental interior contain arrays of steeply dipping faults and associated monoclinal forced folds. Though these Midcontinent fault-and-fold zones clearly were active in pulses during the Phanerozoic, we suggest that they initiated during episodes of Proterozoic extensional tectonism. Based on fault-trace orientation, we divide Midcontinent fault-and-fold zones into two sets—one trending N to NE and the other trending W to NW. These sets effectively break the upper crust into blocks that jostle with respect to each other in response to changes in stress state. Notably, many W- to NW-trending fault-and-fold zones link along strike to define semi-continuous NW-trending deformation corridors. One of these, the 200 km-wide Transamerican tectonic zone (TTZ), traces over 2,500 km from Idaho to South Carolina. Seismic events occur in association with fault-and-fold zones, presumably because the zones persist as crustal weaknesses and/or stress risers. Significantly, seismicity most frequently occurs where N- to NE-trending fault-and-fold zones cross the TTZ, suggesting that intracratonic strain in the United States currently concentrates at or near intersecting fault zones within this corridor.

Boundary Separating the Seismically Active Reelfoot Rift from the Sparsely Seismic Rough Creek Graben, Kentucky and Illinois

Abstract: The Reelfoot rift is the most active of six Iapetan rifts and grabens in central and eastern North America. In contrast, the Rough Creek graben is one of the least active, being seismically indistinguishable from the central craton of North America. Yet the rift and graben adjoin. Hazard assessment in the rift and graben would be aided by identification of a boundary between them. Changes in the strikes of single large faults, the location of a Cambrian transfer zone, and the geographic extent of alkaline igneous rocks provide three independent estimates of the location of a structural boundary between the rift and the graben. The boundary trends north-northwest through the northeastern part of the Fluorspar Area Fault Complex of Kentucky and Illinois, and has no obvious surface expression. The boundary involves the largest faults, which are the most likely to penetrate to hypocentral depths, and the boundary coincides with the geographic change from abundant seismicity in the rift to sparse seismicity in the graben. Because the structural boundary was defined by geologic variables that are expected to be causally associated with seismicity, it may continue to bound the Reelfoot rift seismicity in the future.

Calibration Studies at TXAR

Abstract: Calibration studies at TXAR (Lajitas, Texas) used a modified version of the correlation method described by Cansi et al. (1993) in order to estimate azimuth and horizontal phase velocity of 144 events for which USGS m b values were available. Modifications to the correlation method include the Fourier interpolation of the data by a factor of 8 to obtain a virtual sample rate of 320/sec, use of an L1 norm (least absolute deviation) to obtain estimates of the azimuth and phase velocity, and a moving window display to indicate those portions of the waveform that show strongest correlation across the array. Corrected phase velocities normally associated with Pn (less than 8.6 km/s) are generally seen for events at epicentral distance as far as 2,000 km. Upper mantle refracted first arrivals (P) with corrected phase velocities greater than 8.6 km/s are generally observed for epicentral distances beyond about 1,600 km. Phase identification is essential in order to select a suitable magnitude scale. Based on the 144 well-located events by USGS and using the Denny et al. (1987) formula, the most reliable magnitude estimates are as follows: For horizontal phase velocity less than 8.6 km/sec: m b ( D ) = log A + 2.4 (log D ) - 3.95 + C , with C = +0.3 For horizontal phase velocity greater than 8.6 km/sec: m b ( D ) = log A + 2.4 (log D ) - 3.95 + C , with C = −0.50 The M -discontinuity beneath TXAR was determined to the first order to strike along an azimuth of 109 degrees (NW-SE) and dip 11 degrees to the northeast. This result is consistent with the tectonic setting for the area.

Seismic Evidence of Quaternary Faulting in the Benton Hills Area, Southeast Missouri

Abstract: Two reflection seismic profiles at English Hill, across the southern edge of the Benton Hills escarpment, southeast Missouri, establish that geologic structures at English Hill are of tectonic origin. The lowland area to the south of the escarpment is relatively undisturbed. The geology at English Hill is structurally complex, and reflection seismic and geologic data indicate extensive and episodic faulting of Paleozoic, Cretaceous, Tertiary, and Quaternary strata. The individual faults have near-vertical fault surfaces with maximum vertical separations on the order of 15 m. They appear to be clustered in north-northeast trending zones that essentially parallel one of the dominant Benton Hills structural trends. These observations suggest that previously mapped Quaternary faults at English Hill are deep-seated and tectonic in origin. This paper documents recent faulting at English Hill and is the first time late Quaternary, surface-rupture faulting has been recognized in the middle Mississippi River Valley region outside of the New Madrid seismic zone. This has important implications for earthquake assessment in the midcontinent.

Proterozoic Structure, Cambrian Rifting, and Younger Faulting as Revealed by a Regional Seismic Reflection Network in the Southern Illinois Basin

Abstract: Four high-quality seismic reflection profiles through the southern Illinois Basin, totaling 245 km in length, provide an excellent regional subsurface stratigraphic and structural framework for evaluation of seismic risk, hydrocarbon occurrence, and other regional geologic studies. These data provide extensive subsurface information on the geometry of the intersection of the Cambrian Reelfoot and Rough Creek rifts, on extensive Proterozoic reflection sequences, and on structures (including the Fluorspar Area Fault Complex and Hicks Dome) that underlie a transitional area between the well-defined New Madrid seismic zone (to the southwest) and a more diffuse area of seismicity in the southern Illinois Basin. Our principal interpretations from these data are listed here in order of geologic age, from oldest to youngest: 1. Prominent Proterozoic layering, possibly equivalent to Proterozoic (∼1 Ga) Middle Run Formation clastic strata and underlying (1.3–1.5 Ga) volcanic rocks of the East Continent rift basin, has been strongly deformed, probably as part of the Grenville foreland fold and thrust belt. 2. A well-defined angular unconformity is seen in many places between Proterozoic and Cambrian strata; a post-Grenville Proterozoic sequence is also apparent locally, directly beneath the base of the Cambrian. 3. We infer a major reversal in Cambrian rift polarity (accommodation zone) in the Rough Creek Graben in western Kentucky. 4. Seismic facies analysis suggests the presence of basin-floor fan complexes at and near the base of the Cambrian interval and within parts of a Proterozoic post-Grenville sequence in several parts of the Rough Creek Graben. 5. There is an abrupt pinchout of the Mount Simon Sandstone against crystalline basement beneath the Dale Dome (near the Texaco no. 1 Cuppy well, Hamilton County) in southeastern Illinois, and a more gradual Mount Simon pinchout to the southeast. 6. Where crossed by the seismic reflection line in southeast Illinois, some faults in the Wabash Valley Fault System produce discrete offset in Ordovician and younger strata only; one of the Wabash Valley faults cuts the top of the Precambrian on this seismic profile. 7. The data show clear evidence of late Paleozoic reverse faulting along both boundaries of the Rough Creek Graben in western Kentucky, although significant unreactivated Cambrian rift-bounding faults are also preserved. 8. Chaotic reflection patterns in the lower and middle Paleozoic strata near Hicks Dome, southern Illinois, are related to a combination of intrusive brecciation, intense faulting, and alteration of carbonate strata by acidic mineralizing fluids, all of which occurred in the Permian. 9. Late Paleozoic(?) reverse faulting is interpreted on one flank of the Rock Creek Graben, southern Illinois. 10. Permian and Mesozoic(?) extensional faulting is clearly imaged in the Fluorspar Area Fault Complex; neotectonic studies suggest that these structures were reactivated in the Quaternary.

A New Digital Accelerograph Network for El Salvador

Abstract: INTRODUCTION There are few countries whose geography and history have been so affected by earthquake and volcanic activity as El Salvador, the smallest republic in Central America (Figure 1). The capital, San Salvador (Figure 2), has the unenviable claim of being the Latin American city most frequently damaged by earthquakes. Since 1700, San Salvador has been severely damaged by earthquakes on at least 14 occasions (Harlow et al., 1993). The last destructive earthquake to affect San Salvador occurred on 10 October 1986, causing about 1,500 deaths and extensive damage over much of the city, as well as an economic loss equivalent to 31% of El Salvador's GNP (Coburn and Spence, 1992). El Salvador has been the focus of several studies of seismicity and seismic hazard, and the San Salvador earthquake of October 1986 generated renewed interest in the area. Three major hazard studies have produced seismic zonifications of El Salvador...

Investigating Possible Earthquake-Related Structure Beneath the Southern Illinois Basin from Seismic Reflection

Abstract: The relationship between seismicity and faults observed on seismic reflection profiles from the New Madrid Seismic Zone (NMSZ) in the central Mississippi Valley has been intensively studied for the past 15 years. However, comparable studies relating reflector sequences and earthquakes in the southern Illinois Basin, located northeast of the NMSZ, have not been undertaken. This study investigates the possible relationship between the source parameters of the November 9, 1968, magnitude ( m bLg ) 5.5 earthquake (a NNE-trending, previously interpreted west-dipping reverse fault at 21.2 ± 5.4 km depth) in southern Illinois, and a zone of moderately dipping reflectors in crystalline (?) basement observed on a nearby high-quality seismic reflection profile. The 1968 event was the twentieth century9s largest magnitude earthquake in the southern Illinois region. The zone of dipping basement reflectors is part of a broad prominent sequence, in which reflectors are subhorizontal or inclined with a strong west-dipping component, that appears beneath the Wabash Valley Fault System and extends to the west beneath the Illinois Basin where it steepens and plunges deeper into the crust over the 1968 hypocenter. More than one interpretation of the dipping reflector zone is admissible, including intrusion of igneous sills or thrust faults or both within a localized shear zone. The dipping reflector zone cannot be traced from the basement into the overlying Phanerozoic sedimentary section or associated directly with any particular previously mapped fault. If a tectonic interpretation is correct, the correlation between the 1968 reverse fault event and the reflector zone may mean that such quakes are nucleating along a blind compressional structure in the crystalline basement of southern Illinois, possibly analogous to the recent destructive southern California earthquakes.

Proterozoic Sequences and Their Implications for Precambrian and Cambrian Geologic Evolution of Western Kentucky: Evidence from Seismic-Reflection Data

Abstract: Analyses of two seismic-reflection lines in western Kentucky indicate the presence of two Proterozoic, unconformity-bounded sequences. One is autochthonous and of probable Late Proterozoic age; the other is allochthonous and of probable Middle Proterozoic age. Reflector patterns and apparent relationships to similar sequences elsewhere in the region suggest that the two sequences are of continental-rift origin. The two Proterozoic sequences lie beneath and adjacent to rocks of the Cambrian rift sequence in the Rough Creek Graben. The oldest sequence, the pre-Grenville sequence, was apparently folded and thrust faulted by the Grenville compressional event, implying that it is older than ∼0.975 Ga (Middle Proterozoic). Two seismic-reflection pattern types are present in the western Kentucky data that may relate to the Middle Run (lithic arenite) and volcanic sequences defined farther east near the Grenville Front. The presence of imbricate, thrust-belt geometries in the pre-Grenville sequence extends the known westward limit of Grenville compressional structures into western Kentucky. The younger, post-Grenville sequence is less deformed and was apparently formed after the Grenville compressional event; several lines of evidence indicate that it is Late Proterozoic (0.7 to 0.6 Ga) in age. This probable siliciclastic and volcanic-rift sequence is represented by only thin remnants in western Kentucky and has no equivalent near the Grenville Front in southwestern Ohio and central Kentucky. Rocks of the better documented Cambrian rifting event belong to the thick, pre-Knox sequence in the Rough Creek Graben of western Kentucky and lie unconformably above these earlier sequences. A previously undocumented, northward-thickening interval within the lower part of the Cambrian pre-Knox sequence is recognized north of the Rough Creek Graben.

Construction of Regional Ground Truth Databases Using Seismic and Intrasound Data

Abstract: INTRODUCTION The Comprehensive Test Ban Treaty (CTBT) verification community is concerned with estimating capabilities to correctly identify the sources of regional seismic events whose magnitudes are in the range 2.5 < mb < 4.0. This interest has developed, in part, from the recognition that commercial explosions such as quarry or mine blasts can account for a substantial fraction of the regional seismic activity in this magnitude range in many nations (Richards et al. , 1992). Consequently, a variety of seismic methods have been developed to identify the sources of regional seismic signals generated by commercial explosions as well as those generated by earthquakes (cf., Baumgardt and Ziegler, 1988; Bennett et al. , 1989; Dysart and Pulli, 1990; Harris, 1991; Hedlin et al. , 1989; Stump and Reinke, 1988). Experience has demonstrated that successful application of these methods within one region does not guarantee their global applicability. Therefore, it is necessary to evaluate the...

Earthquake Occurrence in the Reno-Carson City Urban Corridor

Abstract: INTRODUCTION The Reno-Carson City urban corridor is the second most populated region in Nevada and lies in one of the most seismically active parts of the state. This has prompted the development of an earthquake scenario (dePolo et at. , 1996) to assist with earthquake preparedness and emergency response planning within the corridor's communities. As part of this effort, we have estimated probabilities of a potentially damaging earthquake affecting the scenario area (see Figure 1) over a 50-year time period. This paper briefly describes local historical earthquakes of magnitude ≥ 6 and compares their occurrence rates with b -value curve extrapolations from the instrumental time period and with preliminary estimates based on local fault activity rates. HISTORICAL EARTHQUAKES Thirteen earthquakes of magnitude 6 or greater have occurred in the scenario region since 1850 (see Figure 1 and Table 1). These events are briefly described in the Appendix. For many of the earlier...

New Madrid Seismicity, Gravity Anomalies, and Interpreted Ancient Rift Structures

Abstract: Seismicity data for the New Madrid seismic zone recorded by a regional seismograph network since 1974 delineate prominent epicentral trends associated with the Reelfoot rift in northeastern Arkansas, northwestern Tennessee, and southeastern Missouri. Additional epicentral trends and more diffuse clusters of epicenters have also become visible during the 20-plus years of microearthquake monitoring. Several of these trends appear to be spatially correlated with geophysical anomalies, particularly gravity anomaly data. The geophysical anomalies are interpreted to be caused primarily by crustal structure variations, some of which are related to ancient rifting. The epicentral patterns and associated gravity anomalies extend considerably beyond the interpreted Reelfoot rift structure, particularly to the north. A substantial amount of new data, particularly the precise epicenter information, and recent geophysical studies have significantly improved our understanding of the causes of earthquakes in the Reelfoot segment of the New Madrid seismic zone. However, the geographical extent of the seismic zone, detailed information on the structure of the crust in the seismically active areas, and a refined seismotectonic model for the New Madrid area remain to be determined.

Potential for Earthquake Rupture and M 7 Earthquakes Along the Parkfield, Cholame, and Carrizo Segments of the San Andreas Fault

Abstract: Introduction The 1857 Fort Tejon earthquake—the largest in California's history—occurred along the south central San Andreas Fault (SAF; Figure 1). It was apparently preceded by earthquakes at Parkfield, California (Sieh, 1978a); the two most widely felt foreshocks occurred in central California with intensity distributions similar to the 1966 Parkfield earthquake (Figure 2). The SAF also slipped at Parkfield during the main shock (Agnew and Sieh, 1978; Sieh, 1978b). Given that historic precedent, we have examined the possibility that coseismic slip along the Parkfield segment of the SAF will either trigger a larger event or that slip will continue further southeast and the event will be larger than the “typical” Parkfield earthquakes of M ∼6 (throughout this paper, M will refer to moment magnitude). A graphic comparison of roughly end-member rupture and intensity scenarios is shown in Figure 2, in which the dawn and sunrise 1857 foreshock intensities are compared with...