Buried Stream Channels Research Articles

Surface rupture history and 18 kyr long slip rate along the Pazarcık segment of the East Anatolian Fault

We investigate the palaeo earthquakes and slip rate on the Pazarcık segment of the East Anatolian Fault, which was involved in the surface rupture of the 6 February 2023 Pazarcık–Kahramanmaraş earthquake (M w 7.7) and provided insights into the long-term behaviour of this major continental fault. Palaeoseismological data from two trench sites reveal evidence for at least five surface ruptures in the Holocene Period. The historical earthquake of AD 1114 is verified at both trench sites but the following event of AD 1513 is identified at only one site. In addition, the age difference of the older events shows that historical activity is separated by much longer periods of relative quiescence that range from 500 to 1000 years, which suggests quasiperiodic earthquake occurrence on sub-segments of the Pazarcık segment. Our fault-parallel trenches revealed 101 ± 5 m offset in the last 18 kyr and 51 ± 1 m offset in the last 9 kyr on a buried stream channel and the actual channel of the same stream respectively. The correlation of the maximum and abandonment age of the channel with measured offsets revealed a 5.6 mm a −1 long-term slip rate of the fault. Supplementary material: A table and figures providing details of radiocarbon ages and trench sites are available at https://doi.org/10.6084/m9.figshare.c.6850786

Multidisciplinary site investigations: refraction microtremor surveys

We present three case studies from recent site investigations that have utilised geophysical data to supplement traditional geotechnical investigations. The refraction microtremor (ReMi) method, which measures the shear-wave velocity of the subsurface soil profile, is used to enhance our overall understanding of geotechnical site conditions. Interpolation of the closely spaced one-dimensional velocity-depth profiles along linear arrays allows two- or three-dimensional velocity-versus-depth representations to be produced, thereby mapping lateral variations and extending subsurface characterisations between more expensive spot borehole measurements. The ReMi technique provides a non-invasive and cost-effective way of estimating vertical soil/rock shear-wave versus depth profiles and therefore is an effective reconnaissance tool for targeting key areas for further, more expensive intrusive investigation method. This paper examines the contribution ReMi shear-wave velocity assessments can make towards enhancing subsurface geological and geotechnical models to mitigate risk from unforeseen ground conditions. Case studies demonstrate the benefits of incorporating the shear-wave velocity estimates from ReMi into the geotechnical investigations. These include identifying the thickness of basalt flows, identifying the location of buried stream channels, characterising palaeo-topographical features, identifying areas of low velocity which may be prone to liquefaction, and assessing the thickness and velocity variations within geological units between borehole and test pit locations. The objective is not to replace traditional geotechnical investigations but allow more meaningful ground models to be developed.

Application of geophysical approach to highway pavement failure: a case study from basement complex terrain southwestern Nigeria

Geophysical survey involving Schlumberger Vertical Electrical Sounding, 2-D imaging dipole–dipole technique and ground magnetic method were carried out along Ibadan-Ife Highway located in the Precambrian Basement Complex of Southwestern, Nigeria to examine the geological factors responsible for Highway pavement failure. This was with the view to detailing the subsurface geoelectric sequence, mapping the subsurface structural features within the sub-grade soil and delineating the bedrock relief as a means of establishing the cause(s) of the road pavement failure. The geoelectric sections, 2-D resistivity structures and modeled magnetic profiles revealed that the stable segments were founded on a shallow/outcropping basement near homogeneous substratum devoid of major geological features while linear features suspected to be fault/fractured zones, buried stream channels, bedrock depressions and lithological contacts were identified beneath the failed segments of the highway pavement. The geoelectric sections generally identified four geologic layers comprising the topsoil, weathered layer, partly weathered/fractured basement and fresh bedrock. Thus, the failed portions of the road are probably precipitated by very thick and low resistive substratum (clay) beneath the highway pavement and the identified suspected linear features are the major geological factors responsible for the highway pavement failure.

Variable slip-rate and slip-per-event on a plate boundary fault: The Dead Sea fault in northern Israel

We resolved displacement on buried stream channels that record the past 3400years of slip history for the Jordan Gorge (JGF) section of the Dead Sea fault in Israel. Based on three-dimensional (3D) trenching, slip in the past millennium amounts to only 2.7m, similar to that determined in previous studies, whereas the previous millennium experienced two to three times this amount of displacement with nearly 8m of cumulative slip, indicating substantial short term variations in slip rate. The slip rate averaged over the past 3400years, as determined from 3D trenching, is 4.1mm/yr, which agrees well with geodetic estimates of strain accumulation, as well as with longer-term geologic slip rate estimates. Our results indicate that: 1) the past 1200years appear to significantly lack slip, which may portend a significant increase in future seismic activity; 2) short-term slip rates for the past two millennia have varied by more than a factor of two and suggest that past behavior is best characterized by clustering of earthquakes. From these observations, the earthquake behavior of the Jordan Gorge fault best fits is a “weak segment model” where the relatively short fault section (20km), bounded by releasing steps, fails on its own in moderate earthquakes, or ruptures with adjacent segments.

Spatial Analysis of High-Resolution Radar Rainfall and Citizen-Reported Flash Flood Data in Ultra-Urban New York City

New York City (NYC) is an ultra-urban region, with over 50% impervious cover and buried stream channels. Traditional flood studies rely on the presence of stream gages to detect flood stage and discharge, but these methods cannot be used in ultra-urban areas. Here we create a high-resolution radar rainfall dataset for NYC and utilize citizen and expert reports of flooding throughout the city to study flash flooding in NYC. Results indicate that interactions between the urban area and land–sea boundary have an important impact on the spatial variability of both heavy rainfall and flooding, sometimes in contrast to results obtained for other cities. Top days of daily and hourly rainfall exhibit a rainfall maximum over the city center and an extended region of higher rainfall downwind of the city. The mechanism for flooding appears to vary across the city, with high groundwater tables influencing more coastal areas and high rain rates or large rain volumes influencing more inland areas. There is also a strong relationship between sewer type and flood frequency, with fewer floods observed in combined sewer areas. Flooding is driven by maximum one-hour to one-day rainfall, which is often substantially less rain than observed for the city-wide daily maximum.

English

Seismic refraction study has been carried out on Gurara Dam Phase II proposed axis with the aim of determining its feasibility for the founding of a dam. The field techniques involved off-end/reverse and split spread shootings on five profiles. The study site is situated on the Precambrian Crystalline Basement Complex rocks of North Central Nigeria. Velocity values obtained from the shots vary between 183 ms-1 in unconsolidated soil materials and 6522 ms-1 in the fresh bedrock. The overburden is presumably characterized by low to moderately high velocity varying between 183 and 1235 ms-1. Fractured/partially weathered basement rocks within the study area are presumably defined by velocity ranging from 543 to 1667 ms-1. The fresh bedrock is presumably characterized by velocity in excess of 2000 ms-1. The results of the seismic refraction study conducted on the proposed dam axis showed that some portions of the western flank of the axis present noticeable seepage characteristics within the bedrock especially on profiles 1 and 2. Profile 4 on the eastern bank of Gurara River present a downward displacement of the bedrock. Profile 5 also on the eastern bank is characterized by significant fracturing of the bedrock with possibilities for seepage. The general characteristics of the materials within the investigated sections of Gurara Phase II dam study area indicate that they are competent and suitable. However, the existence of the delineated features susceptible to seepage requires some form of remediation to prevent seepage from the area proposed for reservoir through fractures and buried stream channels. Key words: Seismic refraction, dam axis, velocity, overburden, seepage, fresh bedrock.

SEAM Update

At the end of January, SEAM Corporation issued the first request-for-bids (RFB) on numerical simulations for its Phase II consortium “Land Seismic Challenges.” The consortium has been working on three separate models covering different settings, and challenges, for seismic exploration on land. The first model to be completed, called the Unconventional Model, represents subtle features of shale-gas shale reservoirs embedded in a complex stratigraphic section and overlain by the buried topography of a typical midcontinent basin. The Arid Model places these same shale-gas reservoirs and stratigraphic section beneath a 500-m thick near-surface zone with features characteristic of desert areas—such as karsts, buried stream channels or wadis, with unconsolidated, low-velocity surface sediments juxtaposed with outcropping, high-velocity refractors. Finally, the Foothills Model contains complex structural features, typical of fold-and-thrust zones, and is topped by near-surface layer consisting of extreme topography and rapid lateral variations often found in mountainous regions.

Spatial Variability of Hydraulic Properties and Sediment Characteristics in a Deep Alluvial Unsaturated Zone

Statistical analysis and interpretation of heterogeneous sediment hydraulic properties is important to produce reliable forecasts of water and solute transport dynamics in the unsaturated zone. Most field characterizations to date have focused on the shallow 2‐m root zone. We characterized the geologic and hydraulic properties of a 16‐m‐deep, alluvial vadose zone consisting of unconsolidated sediments typical of the alluvial fans of the eastern San Joaquin Valley, California. The thickness of individual beds varies from <5 cm for some clayey and silty floodplain material to >2.5 m for large sandy deposits associated with buried stream channels. Eight major geologic units (lithofacies) have been identified at the site. Unsaturated hydraulic properties were obtained from multistep outflow experiments on nearly 100 sediment cores. Multivariate analysis of variance and post hoc testing show that lithofacies and other visual‐ and texture‐based sediment classifications explain a significant amount of the spatial variability of hydraulic properties within the unsaturated zone. Geostatistical analysis of hydraulic parameters show spatial continuity of within‐lithofacies variability in the horizontal direction in the range of 5 to 8 m, which is approximately an order of magnitude larger than spatial continuity in the vertical direction. Low nugget/sill ratios suggest that 1‐ to 10‐m sampling intervals are adequate for detection of horizontal spatial structure. The existence of thin clay or silt layers within lithofacies units results in only moderate spatial continuity in the vertical direction, however, suggesting inadequate sampling frequency for hydraulic parameter variogram development in that direction.

Late Holocene activity of the Dead Sea Transform revealed in 3D palaeoseismic trenches on the Jordan Gorge segment

Three-dimensional excavations of buried stream channels that have been displaced by the Jordan Fault, the primary strand of the Dead Sea fault zone in northern Israel, demonstrate that late Holocene slip has been primarily strike–slip at a minimum rate of 3 mm/yr. The palaeoseismic study was carried out in the Bet-Zayda Valley, the delta of the Jordan River at the north shore of the Sea of Galilee. The site was chosen where a north-striking scarp with up to 1-m vertical expression crosses the flat valley. One group of trench excavations was located where a small stream crosses the scarp. The active stream, which is incised into the scarp, is not offset by the fault. However we found two palaeo channels about 2 m below the surface offset sinistrally 2.7±0.2 m by the fault and two younger nested channels offset 0.5±0.05 m. Based on radiocarbon dates we attribute the last 0.5 m rupture to the earthquake of October 30, 1759. The older offset of 2.2 m most probably occurred in the earthquakes of May 20, 1202. These two events correlate with the findings at Ateret, about 12 km north of Bet-Zayda, where the 1202 earthquake produced 1.6 m of lateral displacement in E–W-striking defence walls of a Crusader castle, and an Ottoman mosque was offset 0.5 m in the earthquake of 1759. In the second group of trenches some 60 m farther south we found another offset channel. Its northern margin is displaced 15 m sinistrally whereas the southern margin shows only 9 m of sinistral offset. The dip slip component is 1.2 m, west side down. The different amounts of margin offset can be explained by erosion of the southern margin during the first 6 m of displacement. Additional slip of 9 m accrued after the stream had been abandoned and buried by a 2-m-thick lacustrine clay layers. Radiocarbon dates on organic residue provide the age control which indicates that the 15 m of slip has accrued over the past 5 kyr, yielding a short-term slip rate of 3 mm/yr for the late Holocene. It is possible that our study covers only part of the fault zone, hence we regard this mean slip rate to be a minimum for the DST. Based on other palaeoseismic studies the best estimate for Quaternary slip rate is 4±1 mm/yr.

The nature and location of gassy sediment sections in the continental shelf and slope in the northwestern Gulf of Mexico

In the northwestern Gulf of Mexico continental shelf and upper slope gassy sediments are a pervasive phenomena and an important consideration relative to engineering and acoustic activities on the sea floor. An examination of seismic data from over a thousand M.M.S. geohazard reports and core logs of 1,670 foundation boreholes drilled to an average subbottom depth of 125 m on the continental shelf and upper slope in the northwestern Gulf, has revealed that gassy sediment sections are most abundant near the Mississippi River Delta, in the sediment fill of buried stream channels that were eroded during the early and late Wisconsinan, and in Miocene and Plio–Pleistocene depocenters on the continental shelf and upper slope. Out of the 1,670 bore holes examined 1,158 (68%) contained indications of gassy sediments most of which is of biogenic origin. Large patches of gassy sediments exist, some exceeding 10 km in size but most are less than 500 m. The examination of 500 piston cores, up to 40 meters in length, taken on the mid and lower continental slope areas were almost void of gassy sediments as the result of the halokeiesis of allocthonous salt.

Paleoseismic studies of the San Francisco Peninsula segment of the San Andreas Fault zone near Woodside, California

A late Holocene slip rate is proposed for the San Francisco Peninsula segment of the San Andreas fault zone at the Filoli Center, located between the town of Woodside and Upper Crystal Springs Reservoir, California. At this site the primary trace of the San Andreas fault zone traverses N35°W across the head of a large alluvial fan, offsetting buried stream channel deposits. One deposit, with a minimum calibrated accelerator mass spectrometry (AMS) radiocarbon age of 2070±120 years B.P., is offset 30±2 m, yielding an average geologic slip rate of 17±4 mm yr−1 for the late Holocene. A stratigraphically lower channel sequence, with a minimum calibrated AMS radiocarbon age of 330±200 years B.P., is offset 4.1±0.5 m. We interpret that this older sequence records both the penultimate earthquake on the peninsula segment, accompanied by 1.6±0.7 m of dextral slip, and the ∼2.5 m of slip that occurred in 1906. On the basis of radiocarbon analyses and historical descriptions we propose that the pre‐1906 earthquake occurred in June 1838. Our interpretations suggest that the San Francisco Peninsula segment of the San Andreas fault zone may rupture in earthquakes of at least two magnitudes: (1) a large‐magnitude (≈Mw 8) event with a recurrence of ∼250–320 years that involves the North Coast, San Francisco Peninsula, and Santa Cruz Mountains segments, as occurred in 1906 and (2) a Mw 7.0–7.4, 1838‐type event that ruptures only the San Francisco Peninsula segment. The apparent difference in slip rate of 7 ± 7 mm ‐yr−1 between the North Coast segment (24±3 mm yr−1) and the peninsula segment (17±4 mm yr−1) suggests a possible slip rate gradient on the northern California section of the San Andreas fault.

Electrical resistivity mapping of the buried stream channel of the Canopic Branch in the western Nile Delta, Egypt : M. M. El-Gamili, F. F. Shaaban & O. A. El-Morsi, Journal of African Earth Sciences, 19(1–2), 1994, pp 135–148

Electrical resistivity mapping of the buried stream channel of the Canopic Branch in the western Nile Delta, Egypt : M. M. El-Gamili, F. F. Shaaban & O. A. El-Morsi, Journal of African Earth Sciences, 19(1–2), 1994, pp 135–148

Estimating aquifer transmissivities — on the value of auxiliary data

Groundwater quality modelling relies heavily on the knowledge of preferential flowpaths such as buried stream channels and their distribution within the aquifer. This paper examines the extent to which these patterns may be identified by including auxiliary data, such as transverse electric resistances or specific capacities, when estimating the transmissivity field. The analyses are based on two hypothetical aquifers. The first involves a high transmissivity flowpath. The second is a realization of a correlated random field with the same spatial moments as the organised case. Monte Carlo simulations and cokriging estimates are used to analyse the effect of the number of samples and their correlation with transmissivity on the width of the capture zone of a well. Results indicate that, in the organised case with no auxiliary information, the estimated widths are substantially biased. This bias can be reduced significantly by including auxiliary data, even when poorly correlated to transmissivity. Auxiliary data also reduce the scatter (i.e. standard deviation) of the estimated widths significantly, which is a measure of the accuracy of the estimates. In the example used here, 70 samples of auxiliary data, which are correlated to transmissivity by r = 0.6, outweigh the information from 12 additional pumping tests. For the case of the correlated random field, the benefit of using auxiliary data is much less pronounced both in terms of removing the bias and in terms of accuracy (i.e. standard deviation). It is concluded that auxiliary data are useful for estimating transmissivity fields in the context of groundwater quality modelling, particularly when channelised flow is to be expected.

Electrical resistivity mapping of the buried stream channel of the Canopic branch in the western Nile Delta, Egypt

Buried stream channels, which can often be mapped accurately by resistivity, are favoured targets for exploration. Horizontal profiling, electrical soundings, or both, are generally used. In the western Nile Delta, the electrical sounding method was applied using a Schlumberger electrode array with the maximum AB distance being 200 m. The field survey was conducted along profiles extending NE-SW, perpendicular to the expected historical Canopic buried stream channel. About 107 vertical electrical soundings (VES) were measured along eleven profiles. The (VES) field curves were interpreted using the automatic interpretation method of Zohdy and Bisdorf (1989) in which a layered model is obtained directly from a digitized sounding curve. The interpreted results were correlated with borehole data to delineate the main lithological units and to help construct geoelectrical cross-sections based on layer thicknesses and their corresponding ranges in litho-resistivity. The lithological information from borehole data, surface geology and the present layer resistivities indicate three major lithofacies: Holocene clay and silt at the top, Pleistocene sands, and then gravelly sands and gravels (El-Tahrir gravels) at the bottom. From the thickness of the riverine topmost clay-silt facies and the paleotopograph of the Pleistocene sands, the buried stream channels can be delineated. It is evident that two streams existed for the defunct Canopic branch. These defunct streams are discussed and correlated with the historical records.

Statistical Characteristics of Gassy Sediment Sections in the Northwest Gulf Of Mexico: ABSTRACT

Examination of seismic data from over 1000 MMS geohazard reports and core logs of 1670 foundation boreholes (drilled to an average subbottom depth of 125 m), has revealed that gassy sediment sections are most abundant near the Mississippi River delta, in buried stream channels eroded during the early and late Wisconsinan, and in Miocene and Pliocene-Pleistocene depocenters on the shelf and upper slope. Of all the boreholes that we examined, 1158 (68%) contained indications of gassy sediments. The {open_quotes}average{close_quotes} Mississippi River delta related gassy sediment section occurs at a water depth of 53 m; the top of the section lies at a subbottom depth of 10 m; its thickness is 12 m; and the sediment water content is 45%, with liquid and plastic limits of 73% and 24%, respectively. For the nondeltaic areas of the continental shelf (areas west of 90{degrees}W), the average gassy sediment section lies at a water depth of 37 m, its depth below the sea floor is 37 m, its thickness is 8 m, the water content is 40%, and the liquid and plastic limits are 68 and 27%. For the upper continental slope in the northwestern Gulf of Mexico, the average gassy sediment section ismore » found at a water depth of 265 m, is 25 m below the sea floor, and has a thickness of 60 in, a water content of 40%, and a liquid and plastic limit of 62 and 32%. The median areal extent of gassy sediment sections within the entire shelf and upper slope of the northwestern Gulf area ranges from 238 to 546 m. Although large patches of gassy sediments exist, with some exceeding 10 km in size, most are less than 500 m.« less

Geoelectric stratigraphy and subsurface evaluation of quaternary stream sediments at the Cooper Basin, NE Texas

AbstractAn electrical resistivity survey of a portion of the Cooper Reservoir Basin in northeast Texas has been used to delineate the major lithostratigraphic units of Quaternary flood plain deposits and subsurface paleogeomorphic features of interest to geoarchaeologists. This investigation was conducted to evaluate the fluviatile deposits along the South Sulphur River which flows through Cooper Basin, to identify the basin morphology, and to detect buried geomorphic features. Buried stream channels were considered to be important targets since buried prehistoric site locations are thought to be related to patterns of fluvial facies. Two general techniques were used in the survey. First, soundings were conducted over borehole locations to correlate lithologies with resistivity variations. Second, multiple resistivity profiles were established along traverses across the flood plain. By repeating each traverse but with increased probe separations, a composite of profiles was developed yielding electrical cross‐sections of the subsurface. These cross‐sections were then interpreted in terms of the lithologic stratigraphy in the basin. Sounding data were evaluated using empirical methods, and profile data were evaluated qualitatively. the sounding data defined the major resistivity horizons which correlate to the primary sedimentary units in the Cooper Basin. Profile data, used to construct resistivity cross‐sections, revealed a series of buried erosional features interpreted as channel segments and allowed for the estimation of relative time boundaries within sediments buried under the flood plain as the basin evolved.

Monitoring and Mathematical Modeling of Contaminated Ground‐Water Plumes in Fluvial Environments

ABSTRACTGround‐water monitoring to delineate a contaminant plume in fluvial hydrostratigraphic units often is uncertain. Fluvial deposits consist typically of interbedded layers of sands, silts and clays, with buried stream channel deposits of sands or gravels. The channel deposits are often interpreted erroneously to be discontinuous between test holes and in cross section due to their sinuosity. Erroneous conclusions pertaining to the areal continuity of these geometrically complex deposits are inevitable unless the investigator thoroughly understands the depositional environment(s).The hydraulic conductivity of buried stream channel deposits may be several orders of magnitude higher than the matrix materials in which they are enclosed. The higher hydraulic conductivity of buried stream channel deposits has potentially significant ramifications with respect to ground‐water monitoring to delineate the geometry of a contaminant plume migrating through these deposits.Ground‐water monitoring at uranium mill waste disposal sites located in fluvial environments began on a significant scale in about 1977. A uranium mill tailing disposal site located in such an environment in central Wyoming is among the first sites monitored. Thirty‐seven monitor wells were constructed at the site to delineate a seepage plume originating from one of the tailing ponds. This case history illustrates the need for a detailed under—standing of the hydrostratigraphy at a waste disposal site in order to interpret the meaning of ground‐water quality data effectively. Water quality data from monitor wells located on a hit or miss basis often are misleading. The hydrostratigraphic horizon from which a water quality sample is collected must be well defined before the sample analyses can be interpreted quantitatively.

Relevance of Cratonic Erosional Unconformities and Sedimentary Veneers to Mineral Exploration in Weathered Terranes: ABSTRACT

On stable platforms, erosional intervals may persist for great lengths of time either as continuously exposed planation surfaces or, when buried by sedimentary covers, as unconformities. On the West Australian craton, for example, erosional unconformities and thin sedimentary veneers are closely comparable in attitude and altitude. Repeated cycles of weathering, stripping or exhumation, and burial of shields constitute a morphogeodynamic pattern, a cratonic regime, which accounts for the slow but progressive lowering of cratonic erosion surfaces. Because phases of intense chemical weathering initiated in the later Mesozoic and continuing in Tertiary times tend to mask the presence of buried paleogeomorphic surfaces, specialized techniques are required for detection of deg aded (weathered) unconformities. Application of stratigraphic principles to weathered zones and micromorphological analysis of paleosolic and weathered rock fabrics, as well as interpretation of geochemical and sedimentological data, facilitate reconstruction of paleoenvironments. Stone lines, saprolitic fabrics, gravel-clay interfaces, reverse weathering differentials, and etched or embayed skeleton grains showing the effects of epidiagenetic alteration are key to the detection of unconformities in strongly weathered cratons. Differentiation of soil-stratigraphic layers from sedimentary deposits requires proof of pedogenic existence and is in large part based on interpretation of boundaries between them, i.e., pedologic, lithologic, and geomorphic discontinuities. Paleogeomorphic recons ructions incorporating unconformities have practical application in mineral exploration End_Page 568------------------------------ because they provide a rational basis for pedogeochemical sampling techniques applicable to the search for base metals, the detection of deep leads (buried stream channels containing placer deposits), and telethermal uraniferous deposits associated with calcreted valley fills. End_of_Article - Last_Page 569------------

Variable rates of Late Quaternary strike slip on the San Jacinto Fault Zone, southern California

Three strike slip displacements of strata with known approximate ages have been measured at two locations on the San Jacinto fault zone. Minimum horizontal offset between 5.7 and 8.6 km in no more than 0.73 m.y. northeast of Anza indicates 8–12‐mm/yr average slip rate since late Pleistocene time. Two measures of more recent displacement are based on trenching studies of stratigraphic offsets on the Coyote Creek fault in western Imperial Valley. Horizontal slip of 1.7 m has been calculated for the youngest sediment of Lake Cahuilla since its deposition 275–510 yr B.P. The corresponding slip rate is 2.8–5.0 mm/yr. Right lateral offset of 10.9 m measured on a buried stream channel older than 5060 yr B.P. but younger than 6820 yr B.P. yields average slip rates for the intermediate time periods, 400± to 6000± yr B.P., of 1–2 mm/yr. The average rates of slip for these three time intervals suggest a relatively quiescent period for the San Jacinto fault zone from about 4000 B.C. to about 1600 A.D. To the extent that long‐term variations in seismic activity of major strike slip faults elsewhere are known, fluctuations in slip rate for the San Jacinto fault would not appear to be abnormal. If the San Jacinto and adjacent segments of the San Andreas fault alternately assume dominant roles in absorbing motion between the Pacific and American plates, perhaps even more recently than 400 yr ago, the San Andreas fault south of the Transverse Ranges expressed most of the motion but has since become relatively inactive.

An Evaluation of Some Geophysical Methods for Water Exploration in the Piedmont Area

AbstractThe Piedmont is underlain by igneous and metamorphic rocks, from which it is difficult to locate quantities of ground water adequate for municipal and industrial purposes. Ground‐water occurrence and movement in the Piedmont is controlled by topography, recharge and discharge, rock type, fracture systems, and type and thickness of saprolite.Approximately 12 square miles in the Heflin area, Cleburne County, Alabama, were included in this study. Fractures within bedrock are the most effective avenues for ground‐water movement in this area, but domestic supplies can be produced from saprolite and alluvium overlying bedrock. Thick saprolite contributes to bedrock‐fracture production by absorbing water from streams and rainfall and releasing it slowly to underlying fractures.Seismic methods were used with geologic, hydrologie and topographic data to locate and define favorable areas for drilling. As shown on the seismic sections, faults or fracture systems appear to correlate with depressions on bedrock surfaces associated with thick saprolite zones. Seismic data indicate twenty‐five prospective areas for test drilling.Resistivity data were used to confirm seismic depth to bedrock, and to locate gravelly zones in buried stream channels. Some of the buried alluvium is thick enough to be considered a potential aquifer.Twenty‐five prospective areas are indicated by geophysical work in the Heflin area. A proposal to drill 10 wells to evaluate the most promising of these areas is presently being considered.