Lorraine W. Wolf

The Earthquake Potential of the New Madrid Seismic Zone

The fault system responsible for New Madrid seismicity has generated temporally clustered very large earthquakes in A.D. 900 100 years and A.D. 1450 150 years as well as in 1811-1812. Given the uncertainties in dating liquefaction features, the time between the past three New Madrid events may be as short as 200 years and as long as 800 years, with an average of 500 years. This advance in understanding the Late Holocene history of the New Madrid seismic zone and thus, the contemporary tectonic behavior of the associated fault system was made through studies of hundreds of earthquake-induced liquefaction features at more than 250 sites across the New Madrid region. We have found evidence that prehistoric sand blows, like those that formed during the 1811-1812 earthquakes, are probably com- pound structures resulting from multiple earthquakes closely clustered in time or earthquake sequences. From the spatial distribution and size of sand blows and their sedimentary units, we infer the source zones and estimate the magnitudes of earth- quakes within each sequence and thereby characterize the detailed behavior of the fault system. It appears that fault rupture was complex and that the central branch of the seismic zone produced very large earthquakes during the A.D. 900 and A.D. 1450 events as well as in 1811-1812. On the basis of a minimum recurrence rate of 200 years, we are now entering the period during which the next 1811-1812-type event could occur.

Through thick and thin: A new model for the Colorado Plateau from seismic refraction data from Pacific to Arizona Crustal Experiment

We have constructed a model for the crust and uppermost mantle beneath the Colorado Plateau by combining newly acquired seismic refraction/wide-angle reflection data from the 1989 Pacific to Arizona Crustal Experiment (PACE) cross profile with data collected by Roller (1965). A combination of forward modeling methods was used to model travel times and relative amplitudes of crustal and upper mantle phases. The salient features of the model for the Colorado Plateau (hereafter referred to as the plateau) are (1) an upper crust having an average velocity of 6.1 km/s with a low vertical velocity gradient, (2) midcrustal discontinuities at depths of 30 to 38 km beneath the plateau, (3) a lower crust that varies in thickness (5 to 16 km) and has an average velocity of 6.8 km/s, (4) a transitional lower crust/upper mantle boundary, and (5) a crustal thickness of 45 ± 3 km. A comparison of our new model for the Colorado Plateau with recent models for the Basin and Range shows that the upper crusts of the two provinces are similar but that the lower crust of the plateau is thicker, particularly in the plateaus central region. These observations are consistent with the idea that the Basin and Range could have formed from crust like that of the Colorado Plateau.

CHARACTERIZATION OF COPPER BIOREDUCTION AND BIOSORPTION BY A HIGHLY COPPER RESISTANT BACTERIUM ISOLATED FROM COPPER-CONTAMINATED VINEYARD SOIL

Copper is an essential but toxic heavy metal that negatively impacts living systems at high concentration. This study presents factors affecting copper bioremoval (bioreduction and biosorption) by a highly copper resistant monoculture of Pseudomonas sp. NA and copper bioremoval from soil. Seven bacteria resistant to high concentration of Cu(II) were isolated from enrichment cultures of vineyard soils and mining wastes. Culture parameters influencing copper bioreduction and biosorption by one monoculture isolate were studied. The isolate was identified by 16S rRNA gene sequence analysis as a Pseudomonas sp. NA (98% similarity to Pseudomonas putida, Pseudomonas plecoglossicida and other Pseudomonas sp.). The optimal temperature for growth was 30 degrees C and bioremoval of Cu(II) was maximal at 35 degrees C. Considerable growth of the isolate was observed between pH 5.0 and 8.0 with the highest growth and biosorption recorded at pH 6.0. Maximal bioreduction was observed at pH 5.0. Cu(II) bioremoval was directly proportional to Cu(II) concentration in media. Pseudomonas sp. NA removed more than 110mg L(-1) Cu(II) in water within 24h through bioreduction and biosorption at initial concentration of 300mg L(-1). In cultures amended with 100mg L(-1), 20.7mg L(-1) of Cu(II) was biologically reduced and more than 23mg L(-1) of Cu(II) was biologically removed in 12h. The isolate strongly promoted copper bioleaching in soil. Results indicate that Pseudomonas sp. NA has good potential as an agent for removing copper from water and soil.

New evidence for a large earthquake in the New Madrid seismic zone between A.D. 1400 and 1670

In an integrated geological, archaeological, and geophysical study in the New Madrid seismic zone of the southeastern United States, we documented a prehistoric sand blow and related feeder dikes at an archaeological site near Steele, Missouri. Archaeological analysis combined with radiocarbon dating suggest that the earthquake-induced features formed between A.D. 1400 and 1670. This paleoseismic study provides the best evidence to date for a large earthquake occurring in the zone within ∼400 yr prior to the 1811–1812 New Madrid earthquake sequence. To determine an optimal location for excavating at the study site, we mapped surficial artifact density and conducted geophysical surveys. In doing so, we were able to reveal critical relationships for constraining the age of the prehistoric earthquake with minimal impact to the archaeological site.

Analysis of fluid pressure propagation in heterogeneous rocks: Implications for hydrologically-induced earthquakes

Mathematical models are developed to study the propagation of excess pore pressure in heterogeneous and fractured rocks. For a homogeneous rock, the time needed for a pore pressure front to migrate downward is directly proportional to the square of the depth, and inversely proportional to the permeability of the rock. Variational studies show that pore pressure propagation is highly influenced by the permeability heterogeneity of the crust. The results argue for the importance of accounting for geologic complexity when using mathematical models to estimate the extent of downward transmission of increases in hydraulic heads. The models presented in the paper are used to estimate a range of possible hypocentral depths of periodic seismicity observed near Mt. Ogden on the Alaska-British Columbia border. The time lag between these earthquakes and hydrologic loading is on the order of days or weeks, indicating a quick response of seismicity to the increased surface water input from rainfall or glacial discharge, if such a causal relationship exists. Our models estimate that the hypocentral depths of these earthquakes could be on the order of several kilometers, if a high degree of vertical interconnectivity of fractures exists.

Crustal structure of the active margin, south central Alaska: An interpretation of seismic refraction data from the Trans‐Alaska crustal transect

Seismic refraction and wide-angle reflection data from the U.S. Geological Surveys Trans-Alaska Crustal Transect are used to investigate the upper crustal structure of southcentral Alaska and to develop a model using two-dimensional asymptotic ray theory. The data considered here are from the N-S 126-km Cordova Peak refraction profile, which was positioned to cross the Chugach and Prince William terranes. These two tectonostratigraphic terranes form part of a large accretionary complex related to the Alaska subduction zone. Interpretation of data from the Cordova Peak profile indicates systematically higher velocities in Chugach terrane rocks as compared with Prince William terrane rocks at comparable depths. Unconsolidated sediment and glacial ice overlie Chugach terrane rocks Of unusually high compressional velocities (4.7–7.0 km/s) in the upper 10 km of crust. A northward thickening metasedimentary and metavolcanic flysch sequence, having an average velocity of 6.0 km/s, is underlain by metavolcanic mafic basement rocks having an average velocity of 6.6 km/s. The upper 10 km of the Prince William terrane have compressional velocities ranging from 3.7 to 6.2 km/s and are correlated with clastic sedimentary and volcanic rocks which are overlain by younger terrigenous sedimentary rocks and underlain by mafic to ultramafic Prince William terrane basement rocks. The upper crustal velocity structures of the Chugach and Prince William terranes are distinct to at least 10 km depth, which suggests that the Contact fault zone is a boundary which extends to at least 10 km. Midcrustal layers beneath the Chugach terrane contain two velocity reversals which may indicate the presence of off-scraped oceanic sediment, overpressurized zones, shear zones, or some combination of these possibilities. Data from energy sampling midcrustal layers beneath the Prince William terrane do not provide similar evidence for low-high velocity zones in that area.

Geochemical, Microbiological, and Geophysical Assessments of Anaerobic Immobilization of Heavy Metals

Abstract Bioremediation methods that precipitate contaminants in situ as solid (mineral) phases can provide cost-effective options for removing dissolved metals in contaminated groundwater. The current field-scale experiments demonstrate that indigenous bacteria can be stimulated to remove metals by injection of electron-donating substrates and nutrients into a contaminated aquifer. Groundwater at the investigation site is aerobic and contains high levels of lead, cadmium, zinc, copper, and sulfuric acid (pH = 3.1) derived from a car-battery recycling plant. During the experiments, lead, cadmium, zinc, and copper were almost completely removed by precipitation of solid sulfide phases, as pH increased from 3 to ∼ 5 and Eh dropped from +400 mV to −150 mV. X-ray and transmission electron microscopy (TEM) analyses of filtered material from the treated groundwater indicated the presence of newly formed nanocrystalline metal sulfides. Genetic sequencing indicated that the principal species of sulfate-reducing bacteria involved in the bioremediation process was Desulfosporosinus orientis. Geochemical modeling shows that oxidation of added substrates and subsequent bacterial sulfate reduction produced desired geochemical conditions (i.e., decreasing Eh and increasing pH) for the precipitation and sorption of metal sulfides. Geophysical survey results suggest that bioremediation lowers electrical conductance of groundwater and possibly increases the magnetic susceptibility of porous media. This study demonstrates that integrated geochemical, geophysical, and microbiological analyses, combined with theoretical modeling, can successfully track and predict the progress of subsurface bioremediation.

Geophysical reconnaissance of earthquake-induced liquefaction features in the New Madrid seismic zone

Abstract Electrical resistivity and electromagnetic induction surveys performed at a site in the New Madrid seismic zone in the central United States demonstrate the ability of geophysical instrumentation to determine the location, size and orientation of earthquake-induced liquefaction features in the subsurface. Liquefaction features, including sand blows and sand dikes, are common within the Late Pleistocene and Holocene floodplain of the Mississippi River. These features and their relationship to host sediments provide important information about historic and prehistoric earthquakes and their source parameters, such as timing, epicentral location and magnitude. Following the geophysical surveys, two excavations were made and documented for a paleoseismic study. Sediment samples of soils, Native American occupation horizons, and liquefaction features within the trenches were collected for sedimentological and archeological analyses and compared with the geophysical observations. Measurements of the heights of cotton plants growing at the site were also taken, since growth appeared to reflect variations in the texture and thickness of soils developed on fluvial deposits and sand blows. The excavations, along with sedimentological and agricultural data, provided a means for calibrating sediment characteristics with the geophysical interpretations and for developing criteria to distinguish anomalies due to facies changes from anomalies related to liquefaction features. Disruption of the local sedimentological trends by an en echelon arrangement of sand dikes and related sand blows is seen in the combined geophysical and agricultural data. Results of the surveys indicate that constant-spread resistivity profiling (Wenner array) was an effective method for locating and mapping shallow sand dikes, some with widths of less than 50 cm. The electromagnetic induction method (EM-31), while less sensitive to dike locations, was useful in characterizing depositional facies changes by their differences in electrical conductivity. Data from the study site support the interpretation that the earthquake-induced liquefaction features occurred near the boundary of a facies change, which may have constituted a zone of weakness along which excess pore fluids and sand escaped.

Possible cause for an improbable earthquake: The 1997 MW 4.9 southern Alabama earthquake and hydrocarbon recovery

Circumstantial and physical evidence indicates that the 1997 Mw 4.9 earthquake in southern Alabama may have been related to hydrocarbon recovery. Epicenters of this earthquake and its aftershocks were located within a few kilometers of active oil and gas extraction wells and two pressurized injection wells. Main shock and aftershock focal depths (2–6 km) are within a few kilometers of the injection and withdrawal depths. Strain accumulation at geologic rates sufficient to cause rupture at these shallow focal depths is not likely. A paucity of prior seismicity is difficult to reconcile with the occurrence of an earthquake of Mw 4.9 and a magnitude-frequency relationship usually assumed for natural earthquakes. The normal-fault main-shock mechanism is consistent with reactivation of preexisting faults in the regional tectonic stress field. If the earthquake were purely tectonic, however, the question arises as to why it occurred on only the small fraction of a large, regional fault system coinciding with active hydrocarbon recovery. No obvious temporal correlation is apparent between the earthquakes and recovery activities. Although thus far little can be said quantitatively about the physical processes that may have caused the 1997 sequence, a plausible explanation involves the poroelastic response of the crust to extraction of hydrocarbons.

Geophysical surveys of earthquake-induced liquefaction deposits in the New Madrid seismic zone

We explore the effectiveness and limitations of electrical and electromagnetic methods in imaging buried, earthquake-induced liquefaction deposits. Geophysical surveys conducted at liquefaction sites in the New Madrid seismic zone (NMSZ) in the central United States demonstrate that these subsurface-imaging techniques can be useful tools in paleoseismology. Paleoseismological studies of liquefaction features provide one of the few means for estimating recurrence intervals of large earthquakes in the NMSZ, a region with widespread evidence of strong ground shaking but short instrumental record. Noninvasive geophysical methods minimize ground disturbance during these studies, an attribute of particular importance when the studies are conducted at federally protected archaeological sites. Surveys such as those described here can be used to locate buried liquefaction deposits and to site trenches for detailed geologic studies.