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NARROW
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all geography including DSDP/ODP Sites and Legs
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Maine
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Groundwater recharge as the trigger of naturally occurring intraplate earthquakes
Abstract I explore the hypothesis that most intraplate earthquakes and their aftershock sequences are triggered by pore-fluid pressure increases. As proposed in this paper, data from the magnitude 5.7 Virginia earthquake of 23 August 2011 show that this is a two-step process. (1) First, from areas where there is greater than normal meteoric recharge, pore-fluid pressure diffusion by means of Biot slow waves transfers more pore-fluid pressure towards a future hypocentre. Here the cumulation of Biot slow waves produces a steady increase in pore-fluid overpressure until a main shock is triggered. (2) Then, aftershocks occur in the zone reaching from the depth of the main shock to a depth of a few kilometres below the land surface, preferring to localize in a weaker, pervasive anisotropic crustal fabric, in response to locally increased permeability and pore-fluid pressure transients caused by the main shock. The primary corrosive agent responsible for reducing the strength of silicate minerals in this upper crustal zone is water, so that quartz-rich crust tends to have lower values of Poisson’s ratio. I show here that increases in pore-fluid overpressure from normal groundwater recharge can start crack dilation leading to fracturing and the creation of new permeability. Previous chemical analyses across the Central Virginia Piedmont that hosted the 2011 Virginia shock show high upper crustal quartz content. This proposed two-step model for a main shock-aftershock sequence explains why intraplate earthquakes are rarely correlated with recognizable brittle faults at the Earth’s surface. Supplementary material: A biography of John Costain is available at https://dx.doi.org/10.6084/m9.figshare.c.2854324.v3
Finite element simulation of an intraplate earthquake setting—Implications for the Virginia earthquake of 23 August 2011
The 23 August 2011 M 5.7 intraplate earthquake occurred in the Central Virginia seismic zone near Mineral, Virginia (USA), far from the nearest plate boundaries. I suggest here that this earthquake, as well as others occurring in this region since 1774, was triggered by pore-fluid pressure diffusion associated with groundwater recharge. Using finite element modeling (FEM) estimates are made of the magnitude and timing of pressure diffusion with respect to the time of the earthquake. Two scenarios are considered: (1) the diffusion took place along vertical to near-vertical diffusion paths, or (2) the diffusion was restricted to a hydraulically transmissive fracture zone that was later illuminated by thousands of aftershocks. Both scenarios have merit. The fracture zone may not have been entirely generated by the main shock. For either model a fractured crust is assumed to be recharged at the surface of the Earth by groundwater recharge with subsequent pore-fluid pressure diffusion propagating to the hypocenter. The transient behavior of pore-fluid pressure is examined at the focal depth of 8 km. These results are compared with the duration and timing of base flow (groundwater recharge) as estimated from a hydrograph separation. The delays of the peaks in pore-fluid pressure diffusion as computed by the FEM simulations are found to be consistent with the start and duration of groundwater recharge with respect to the timing of the Virginia earthquake.
Review : Research Results in Hydroseismicity from 1987 to 2009
Intraplate Seismicity, Hydroseismicity, and Predictions in Hindsight
Seismic reflection evidence for the evolution of a transcurrent fault system: The Norumbega fault zone, Maine
Abstract The pre-Jurassic rocks of Corridor E-3 as shown in the Main Display, West Sheet, reveal the tectonic history of the middle Atlantic margin of the North American continent during the interval Late Proterozoic through Tertiary. The history is graphically shown on the main display and is also summarized in the conclusions of this paper. This corridor differs from other eastern margin corridors in four important respects; 1) there is a large uplift of IGa Grenville basement in the eastern Piedmont at this latitude. 2) Only one suture (early Taconic, Cambrian - Late Ordivician) is recognized in the exposed Appalachians, and that separates the Carolina (Avalon) magmatic terrane from the Laurentian passive margin. 3) The Chopawamsic/ James Run volcanic belt is recognized as a part of Carolinia/Avalonia, and is not a different island arc. 4) The eastern margin of Laurentia (and its upper bounding surface, the early Taconic suture) extends in the subsurface below the coastal plain at least 50 kilometers east of Richmond in one model, or may reach the continental edge in another. Bird and Dewey (1970) produced the first comprehensive modern tectonic model that included the central and southern Appalachians. It was essentially an extrapolation of northern Appalachian and Newfoundland data into the southeast. However, a model based primarily on northern Appalachian geology didn't seem to fit the central and southern Appalachians and, in 1972 Robert D. Hatcher, Jr., attempted the first comprehensive tectonic model for the southern Appalachians. His model proposed that the eastern Piedmont volcanics, (Charlotte,
Crustal structures and the eastern extent of lower Paleozoic shelf strata within the central Appalachians: A seismic reflection interpretation
Inversion
INTRODUCTION The estimation of P- and S-wave seismic velocities and densities of earth layers from the data sets recorded at the surface has long been one of the main research interests in reflection seismology. Although structural interpretations of the subsurface with sophisticated data processing and interpretation techniques have been facilitated, there remains a lot more to be done in obtaining quantitative information about the seismic parameters to lead to lithological interpretation that will open the ways to direct hydrocarbon exploration, groundwater exploration, and understanding the deep earth crust.
Post-Paleozoic activity
Abstract Post-Paleozoic tectonic activity in the Appalachian orogen (including the concealed basement of the passive margin) is primarily a consequence of the breakup of Pangaea and the opening of the Atlantic Ocean. It embraces a major tectonic cycle that is marked by Late Triassic-Early Jurassic rifting of the Alleghanian-Variscan orogen and by Middle Jurassic to Recent drifting of the newly forming passive margin.
Geophysical characteristics of the Appalachian crust
Abstract This chapter reviews the geophysical data in the U.S. Appalachians–including gravitational and magnetic fields, refraction and reflection seismology, terrestrial heat flow, and electrical properties. An even treatment of the various kinds of geophysical data is neither attempted nor justified. Instead, emphasis and bias are placed primarily on geophysical data that have been useful for the interpretation of the tectonic history of the orogen; of these, seismic reflection data have had the greatest impact on the development and testing of tectonic models in the U.S. Appalachians and elsewhere because of their greater resolution.
Tectonic Setting of Triassic Half-Grabens in the Appalachians: Seismic Data Acquisition, Processing, and Results
Abstract The locations and morphologies of Mesozoic half- grabens in the southeastern United States Pied-mont are generally believed to have been controlled by reactivation along older Paleozoic ductile- deformation fault systems. We suggested earlier (Costain etal., 1987a; Qoruhetal., 1988a) that the localization may have been controlled in part by the development of a strike-slip duplex (SSD) that extends from the Brevard-Bowens Creek fault zone, its “floor” on the northwest, to near its “roof” at the eastern Piedmont fault system beneath the Atlantic coastal plain (=300 km), and from central Virginia southwest to the Pine Mountain belt and the Towaliga and Goat Rock faults (= 1,000 km). Included in the SSD is the entire metamorphic core of the southern Appalachians except for the Blue Ridge. Seismic data from central Virginia as well as reprocessed COCORP data from Georgia reveal antiformal and synformal shapes of regional extent that may have formed by reactivation of the same Paleozoic mylonite zones during Mesozoic crustal extension. Excellent onshore seismic-reflection data have been recovered from the Culpeper, Virginia basin, the buried Jedburg basin near Summerville, South Carolina, and the South Georgia basin in Johnson and Laurens counties, Georgia. Data from the buried Jedburg basin indicated that processing single-sweep data instead of stacking source arrays results in better lateral resolution than conventional processing methods.
Comment and Reply on "Hydroseismicity—A hypothesis for the role of water in the generation of intraplate seismicity"
Abstract The geothermal resources in the eastern United States are liquid-dominated, low-temperature systems, oriented toward nonelectric power applications such as space heating and industrial processes (Toth, 1980; John Hopkins University, 1981). Evaluation of the geothermal resource potential in the eastern U.S. takes place in a geologic framework quite unlike that of the western U.S., where geothermal energy is primarily used in the generation of electric power. The resource in the East must be both large and favorably located with respect to potential utilization; fortunately, a large fraction of our energy consumption currently is devoted to space heating. The high costs associated with drilling for, pumping, and circulating warm water make it important to locate the highest temperatures at the shallowest depths. Systematic efforts to estimate the geothermal resources of the United States have been made by the U.S. Geological Survey (Muffler, 1979; Sammel, 1979; Reed, 1983). Muffler and Cataldi (1978) proposed the use of a consistent terminology for geothermal resource assessment. The “geothermal resource base” is defined as all of the thermal energy in the Earth's crust under a given area, measured from the mean annual temperature. The “accessible resource base” is that part of the resource base shallow enough to be tapped by production drilling, and it is divided into “useful” and “residual” components. The “useful” component is defined as thermal energy that could be extracted at costs competitive with other forms of energy at some specified future time. This useful component is the subject of this section.