James B. Harris

Comparing shear-wave velocity profiles inverted from multichannel surface wave with borehole measurements

Recent field tests illustrate the accuracy and consistency of calculating near-surface shear (S)-wave velocities using multichannel analysis of surface waves (MASW). S-wave velocity profiles (S-wave velocity vs. depth) derived from MASW compared favorably to direct borehole measurements at sites in Kansas, British Columbia, and Wyoming. Effects of changing the total number of recording channels, sampling interval, source offset, and receiver spacing on the inverted S-wave velocity were studied at a test site in Lawrence, Kansas. On the average, the difference between MASW calculated V s and borehole measured V s in eight wells along the Fraser River in Vancouver, Canada was less than 15%. One of the eight wells was a blind test well with the calculated overall difference between MASW and borehole measurements less than 9%. No systematic differences were observed in derived V s values from any of the eight test sites. Surface wave analysis performed on surface data from Wyoming provided S-wave velocities in near-surface materials. Velocity profiles from MASW were confirmed by measurements based on suspension log analysis. ⓒ 2002 Elsevier Science Ltd. All rights reserved.

Identification of possible Quaternary deformation in the northeastern Mississippi Embayment using quantitative geomorphic analysis of drainage-basin asymmetry

To investigate neotectonism in the Mississippi Embayment east of the New Madrid seismic zone, we identified geomorphic domains that show evidence of ground tilting during Quaternary time. Transverse basin profiles were converted to two-dimensional vectors that denote channel position with respect to basin divides. These basin-asymmetry vectors record the net direction and degree of lateral migration of trunk streams. More than 2500 vectors were measured and spatially averaged within 400 km 2 bins. This field of 300 mean vectors delineates several domains that show preferred directions of stream migrations possibly driven by ground tilting. The timing of stream migration was interpreted using across-valley distributions of Quaternary alluvial terraces. Comparison of our mean vector field with subsurface structures suggests that some domain boundaries may be related to reactivated faults. Late Quaternary activity is suggested for two northeast-striking faults of the southeastern Reelfoot Rift margin. We acquired two seismic profiles showing near-surface faulting beneath scarps that follow the domain boundary associated with one of these northeast-striking faults (Big Creek fault zone). Reelfoot thrust seismicity ends on the south against this fault, suggesting that the rift margin has dextral slip accommodating northeastward movement of the thrust hanging wall. Our vector field also suggests late Quaternary movement on the Reelfoot thrust and on two other northwest-striking faults, here termed the Hatchie River fault and the Wolf River fault. Several other weak domains may imply minor elements of neotectonism. Our results demonstrate that morphometric analysis of drainage-basin asymmetry can be an effective reconnaissance tool within neotectonic settings.

Neotectonics of the southeastern Reelfoot rift zone margin, central United States, and implications for regional strain accommodation

The northeast-striking New Madrid fault system of central North America has been described as a right-lateral strike-slip system with a left-stepping restraining-bend thrust. The fault system has one of the highest rates of seismic energy release in an intraplate setting, and it has been regarded as a zone of significant earthquake hazard. The New Madrid fault system is the central part of the wider Reelfoot rift fault system, a northeast- striking basement fault zone. Although component faults of the New Madrid system are well defined by microseismicity, recent geodetic surveys suggest that little if any strain is accumulating on the principal (southern) strike-slip arm of the fault system. Seismicity and geologic data show that the “restraining-bend” thrust continues past the southern strike-slip arm to the southeastern Reelfoot rift margin. Thus, we suggest that earthquakes defining the southern arm of the New Madrid fault system are primarily aftershocks of an earthquake on that arm during the last sequence of great earthquakes (A.D. 1811– 1812), and it is the southeastern rift-margin fault system that is currently accommodating right-lateral strain along the boundary of the thrust block. This interpretation is consistent with recent geodetic results. The southeastern rift margin coincides with a 150-km-long linear topographic scarp from near Memphis to the Tennessee-Kentucky line, and S-wave reflection profiles, auger data, and a trench excavation reveal late Wisconsin–early Holocene surface faulting and late Holocene liquefaction associated with this fault-line scarp. Variation in sense of throw along strike and flower-structure geometry suggest that this is a strike-slip fault. Recognition of this rift margin as an important element of active tectonism in the Mississippi embayment has broad implications for assessment of the seismic hazard of this and similar intraplate settings. Temporal shifts in strain accommodation may give rise to short-term seismicity patterns and/or geodetic velocities that do not reveal long-term tectonic patterns.

Downhole seismic logging for high‐resolution reflection surveying in unconsolidated overburden

Downhole seismic velocity logging techniques have been developed and applied in support of high‐resolution reflection seismic surveys. For shallow high‐resolution reflection surveying within unconsolidated overburden, velocity‐depth control can sometimes be difficult to achieve; as well, unambiguous correlation of reflections with overburden stratigraphy is often problematic. Data obtained from downhole seismic logging can provide accurate velocity‐depth functions and directly correlate seismic reflections to depth. The methodologies described in this paper are designed for slimhole applications in plastic‐cased boreholes (minimum ID of 50 mm) and with source and detector arrays that yield similar frequency ranges and vertical depth resolutions as the surface reflection surveys. Compressional- (P-) wave logging uses a multichannel hydrophone array with 0.5-m detector spacings in a fluid‐filled borehole and a high‐frequency, in‐hole shotgun source at the surface. Overlapping array positions downhole result...

Quaternary faulting in the southern Mississippi embayment and implications for tectonics and seismicity in an intraplate setting

The recently recognized Saline River fault zone in the southwestern Mississippi embayment (strike = 135°) is characterized by moderate historic seismicity. We document paleoseismicity (possibly strong) on the Saline River fault zone that is outside the known region of neotectonism in the northern embayment. Six surface faults were excavated within the Saline River fault zone, and all faults displace marine Eocene units and fluvial Pliocene–Pleistocene units; five show post-Wisconsin loess movement (thermoluminescence [TL] age = 23.6 ± 3 ka), and three deform middle to late Holocene eolian silt (TL age = 5.1 ± 0.6 ka and 3.6 ± 0.5 ka) as fault-tip flexures. Along the principal excavated fault, fault-plane braiding and plunging drag folding suggest a strong component of left slip, and subsidiary faults show both normal and reverse displacement (<1m) of Pliocene−Pleistocene, Wisconsin, and Holocene units. We interpret these faults as components of a left-slip flower structure. A shallow seismic reflection profile acquired across the principal fault and a subsidiary fault shows Eocene stratigraphy separated in a reverse sense below 120 m depth and separated in a normal sense above 120 m, supporting a strike-slip interpretation. In addition, the only focal plane mechanism available in the area indicates left slip with a small reverse component. We conclude that the Saline River fault zone is a Paleozoic–Mesozoic basement fault zone reactivated as a left-slip system in a Quaternary east-west compressive stress field. Field evidence suggests that earthquakes with several meters of strike slip may be characteristic, and the width of a liquefaction field on the Saline River fault zone suggests an event of magnitude 5.5–6. Considering a similarity of strike and sense of Holocene movements of the Meers fault of southern Oklahoma and of the Saline River fault zone, there may be additional active fault zones that pose seismic hazards concealed by sediments in southern North America.

Higher Mode Observation By the MASW Method

A higher mode (the first overtone) of high frequency (5–30 Hz) surface waves was observed by using the multi-channel analysis of surface waves (MASW) method at three boreholes located in unconsolidated sediments in the Fraser River Delta, near Vancouver, British Columbia. Each site has a unique near-surface shear (S)-wave velocity (Vs) structure as verified from downhole Vs measurements. The relative dominance of higher mode energy is examined in association with source distance as well as Vs structure. Our examination indicates that energy of higher modes tends to become more significant as the source distance becomes greater. It also reveals that the dominance may be related to a Vs structure: a greater dominance as Vs changes little with depth, or Vs has an overall low value, or a combination. The dependency on the source distance is observed to be stronger than that on the Vs structure. Attempts are made to explain the dependency by referring to one or a combination of three factors: attenuation, the nearfield effects, and the intrinsic nature of surface waves. Inclusion of higher mode during a surface wave measurement for near-surface (<30 m) application can be either an advantage or a disadvantage, depending on the specific type of application and the method used during the data acquisition and processing steps. It is, therefore, important to recognize through field observations those conditions both favorable and unfavorable to the generation of higher modes of high-frequency surface waves.

Site effects at a vertical accelerometer array near Paducah, Kentucky

Abstract The downhole vertical accelerometer array VSAP near Paducah, KY, consists of three-component accelerometers at the surface, the top of the McNairy Formation (−41 m), and the top of the Paleozoic bedrock (—99 m). The array is at the northern end of the Mississippi Embayment, and it was installed to verify the ground-motion modeling for the site, assuming a significant earthquake in the New Madrid Seismic Zone. Accelerograms from 4.2 and 2.0 mb earthquakes were used to check aspects of the modeling pertaining to linear behavior of the soil column, and to review the soil column models derived by drilling and geotechnical methods and through the use of high-resolution P- and SH-wave seismic refraction in reflection techniques. Results of the study indicate that for the linear case the soil column models derived by the two techniques are equivalent, and that the most important boundary in the soil column, with respect to amplification of the ground motions, is the interface between the limestone bedrock and soil.

Shallow seismic reflection investigations of neotectonic activity in the Lower Mississippi Valley

Summary Shallow shear wave seismic reflection methods were used to help interpret the significance of neotectonic surface deformation at three sites in the Lower Mississippi Valley. The seismic data were acquired with a hammer and mass energy source and processed using a standard sequence for shallow reflection data. The interpreted profiles show a range of shallow structural styles that includes reverse faulting, fault propagation folding, and reactivated normal faulting. Determining the style and extent of near-surface structural deformation is critical in evaluating the seismic potential of an area.

Investigation of the shallow subsurface near the Paducah Gaseous Diffusion Plant using SH‐wave seismic methods

Over 17 km of shallow, high-resolution SH-wave reflection and refraction data were collected near the Paducah Gaseous Diffusion Plant (PGDP) in Paducah, Kentucky (Fig. 1). The data have been used to characterize the shallow subsurface structure neighboring the PGDP. This study indicated that the subsurface structure influences the flow of groundwater in the study area. The study also revealed that deformation is present in the Quaternary Lower Continental deposits, which includes the regional gravel aquifer (RGA). Of interest to the characterization of the structure within the area is the presence of a graben, imaged in the Mississippian bedrock over a distance of roughly 0.8 km. The trend of this graben follows that of the Fluorspar Area Fault Complex (FAFC) and of several faults in southernmost Illinois that also display evidence of Quaternary deformation.

Seismic investigation of near-surface geological structure in the Paducah, Kentucky, area: application to earthquake hazard evaluation

Abstract Seismics method were used to evaluate shallow geological conditions at 33 sites in the vicinity of Paducah, Kentucky. A combined set of P- and S-wave seismic refraction and reflection soundings were used, in addition to local borehole information, to produce structure maps of (1) a shallow ( 160 m deep). Shear-wave velocity contrasts across the shallow unconformity were generally 2-to-1 while the contrast at the top of the Paleozoic bedrock exceeds 5-to-1. These seismic boundaries have been determined to be very important in modelling and interpreting earthquake ground motion amplification in the Paducah area. The quality and accuracy of the data, and the cost effective nature of the methods, suggest that other communities in areas at risk to damage from seismic activity, with foundation conditions comparable to Paducah, might benefit from similar characterization in order (1) to identify seismically hazardous, near-surface, geological conditions, and (2) to develop geological models that could be used in computer simulations of site response.