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Mark D. Zoback

Publications and source records attributed to Mark D. Zoback.

45 records · Page 3Linked to original sources
USGS1981

State of stress and intraplate earthquakes in the United States

Recently compiled data on the state of stress have been used to define stress provinces in the conterminous United States in which the orientation and relative magnitude of the horizontal principal stresses are fairly uniform. The observed patterns of stress constrain mechanisms for generating intraplate lithospheric stresses. Coupled with new information on geologic structure and tectonism in seismically active areas of the Midcontinent and East, these data help to define some characteristics common to these areas and to identify key questions regarding why certain faults seem to be seismically active.

Science↗
USGS1980

Recurrent intraplate tectonism in the New Madrid seismic zone

For the first time, New Madrid seismicity can be linked to specific structural features that have been reactivated through geologic time. Extensive seismic reflection profiling reveals major faults coincident with the main earthquake trends in the area and with structural deformation apparently caused by repeated episodes of igneous activity.

Arkansas, Illinois, Kentucky, Missouri, Tennessee↗
USGS1979

Magnitude of shear stress on the San Andreas fault: Implications of a stress measurement profile at shallow depth

A profile of measurements of shear stress perpendicular to the San Andreas fault near Palmdale, California, shows a marked increase in stress with distance from the fault. The pattern suggests that shear stress on the fault increases slowly with depth and reaches a value on the order of the average stress released during earthquakes. This result has important implications for both long- and short-term prediction of large earthquakes.

California↗
USGS1979

Preliminary report on drilling in and near the San Andreas fault zone, California; Dry Lake Valley No. 1

The purpose of drilling into the San Andreas fault zone is threefold. First, recovery of material from depth will permit determination of the composition and constitutive properties of both solid and fluid phases. Second, it will enable measurements of the physical state of the fault zone to be made (that is, the state of in-situ stress, pore pressure, and temperature). Finally, it will allow emplacement of instruments at depth in the fault zone as part of experiments related to earthquake prediction. This report briefly describes the drilling history and preliminary analysis of fault zone materials from site Dry Lake Valley (DLV) No. 1 (Fig. 1). The site is located at Lat. 36028.09'N, Long. 12103.32'W. This site was chosen in an attempt to drill a 1 km deep well that would reach earthquake foci. Although the entire central section of the San Andreas fault is characterized by moderate seismicity and aseismic creep, Dry Lake Valley has particularly shallow earthquakes as well as a fairly high creep rate (18-20 mm/year, S. Schultz, pers. comm.). Fig. 2 is a longitudinal section along the San Andreas fault showing high precision locations of magnitude one or greater earthquakes that occurred in 1973-1975 (from W. Ellsworth, per. comm.). The depth accuracy of the earthquakes shown is about ± 0.5 km, and it is obvious from Figure 2 that Dry Lake Valley earthquakes are extremely shallow. As shown in Figure 1 the general geology of the central section of the San Andreas fault consists of Tertiary to Jurassic sedimentary and volcanic rocks overlying Cretaceous age Gabilan granite of the Salinian Block on the west side of the fault and Cretaceous-Jurassic sedimentary assemblages of the Franciscan Complex to the east of the fault. Dibblee (1979) summarizes the geology along the central San Andreas fault. At the Dry Lake Valley site as much as 2 km of Pliocene-Miocene marine sediments (Etchegoin and Santa Margarita formation) overlie Franciscan rocks on the northeast side of the San Andreas fault. On the southwestern side of the fault, 1-2 km of Pliocene-Miocene valley sediments and granitic conglomerate overlies the Gabilan granite.

California↗
USGS1979

Preliminary analysis of clay gouge from a well in the San Andreas fault zone in central California

Drilling into the San Andreas fault zone has begun in central California as part of the U.S.G.S. program of fault zone studies. The purpose of drilling into the fault zone is threefold. First, it will allow recovery of material from depth in order to determine the composition of both solid and fluid phases. Second, it will allow determination of the physical state of the fault zone, that is, the state of stress, pore pressure, and temperature. Finally, it will allow emplacement of instruments at depth in the fault zone for monitoring experiments related to short-term earthquake prediction. In an attempt to drill to the depth of earthquake focii, the initial drill site was chosen at a locale known as Dry Lake Valley (Fig. 1). Although the entire central section of the San Andreas fault is characterized by moderate seismicity and aseismic creep, Dry Lake Valley has particularly shallow earthquakes as well as a fairly high creep rate (18-20 mm/year). Figure 2 is a longitudinal section along the San Andreas fault. The high precision earthquake locations shown are magnitude one or greater events that occurred in 1973-1975 (from W. Ellsworth, pers. comm.). The depth estimates are accurate to about + 0.5 km. It is obvious from the figure that earthquakes in the section of the fault near the well are quite shallow.

California↗
USGS1977

Preliminary stress measurements in central California using the hydraulic fracturing technique

Use of the hydraulic fracturing technique for determining in situ stress is reviewed, and stress measurements in wells near the towns of Livermore, San Ardo, and Menlo Park, California are described in detail. In the Livermore well, four measurements at depths between 110 and 155 m indicate that the least principal compressive stress is horizontal and increases from 1.62 to 2.66 MPa. The apparent direction of maximum compression is N 70?? E (??40??). At the San Ardo site the least principal stress is that due to the overburden weight. At depths of 240.2 and 270.7 m the minimum and maximum horizontal stresses are estimated to be 11.4 and 22.5 MPa, and 12.0 (??1.1) and 15.8 (??3.3) MPa, respectively. From an impression of the fracture at 240.2 m, the direction of maximum compression appears to be about N 15?? E. The rock in the Menlo Park well is too highly fractured to yield a reliable measurement of the horizontal stresses. The data indicate, however, that the least principal stress is vertical (due to the overburden weight) to a depth of 250 m. ?? 1977 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1977

Laboratory hydraulic fracturing experiments in intact and pre-fractured rock

Laboratory hydraulic fracturing experiments were conducted to investigate two factors which could influence the use of the hydrofrac technique for in-situ stress determinations; the possible dependence of the breakdown pressure upon the rate of borehole pressurization, and the influence of pre-existing cracks on the orientation of generated fractures. The experiments have shown that while the rate of borehole pressurization has a marked effect on breakdown pressures, the pressure at which hydraulic fractures initiate (and thus tensile strength) is independent of the rate of borehole pressurization when the effect of fluid penetration is negligible. Thus, the experiments indicate that use of breakdown pressures rather than fracture initiation pressures may lead to an erroneous estimate of tectonic stresses. A conceptual model is proposed to explain anomalously high breakdown pressures observed when fracturing with high viscosity fluids. In this model, initial fracture propagation is presumed to be stable due to large differences between the borehole pressure and that within the fracture. In samples which contained pre-existing fractures which were 'leaky' to water, we found it possible to generate hydraulic fractures oriented parallel to the direction of maximum compression if high viscosity drilling mud was used as the fracturing fluid. ?? 1977.

International Journal of Rock Mechanics and Mining↗