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S.T. Harding

Publications and source records attributed to S.T. Harding.

5 recordsLinked to original sources
USGS2012

Shallow structure and deformation along the San Andreas Fault in Cholame Valley, California, based on high-resolution reflection profiling

The mapped active traces of the San Andreas fault are separated by a 1-km-wide right-stepping offset in Cholame Valley. The geometry of this offset, defined in other strike-slip systems as a releasing bend or a dilational jog, has resulted in the formation of a pullapart basin. Various researchers have inferred that this offset served as a rupture terminus for earthquakes on both strands of the San Andreas fault (1966 Parkfield and 1857 Fort Tejon); thus, this en echelon offset may represent a barrier to the propagation of rupture between two segments of the San Andreas fault. We collected 18 km of high-resolution seismic reflection data specifically designed to image the San Andreas fault zone in the shallow crust surrounding this offset. Four short profiles (≤ 3.3 km long) of Mini-Sosie reflection data (1 s of two-way travel time ≈ 1.5 km deep) were collected perpendicular to the San Andreas fault; three of these profiles were tied by a 7-km-long profile that trended northwest through Cholame Valley, subparallel to the San Andreas fault. A zone of incoherent energy, narrow at the surface but widening with depth, underlies the mapped active traces of the San Andreas fault and abruptly terminates shallow reflections on both sides. The reflection profiles and available well data indicate that west of the mapped active traces of the San Andreas fault the shallow subsurface structure of the crust consists of thin (≤400 m thick), offset packages of reflections, laterally coherent on the scale of tens of meters, overlying deformed clastic sedimentary rocks. East of the San Andreas fault, the structure of the shallow crust in southern Cholame Valley is characterized by thick packages of reflections, laterally coherent on the scale of kilometers, overlying the Franciscan complex. All of the strata east of the fault (within Cholame Valley) dip toward the San Andreas fault and the offset, into an approximately 1-km-deep sedimentary basin abutting the south strand of San Andreas fault. The offset in Cholame Valley is characterized by a gentle downwarping of sediments into the offset, the presence of many small faults and discontinuous reflections (consistent with areally distributed shear) between the traces of the main fault, localized subsidence abutting the main strike-slip fault, the formation of a basin, near the offset, that is about as deep as the jog is wide, and the southward propagation of the deformation associated with the offset. Strain field modeling based on simple geometries of the San Andreas and associated faults successfully predicts the general features of the observed topography and subsurface structure of southern Cholame Valley, including subsidence and basin formation near the offset, a discontinuous San Andreas fault plane, and at least one fault in southeastern Cholame Valley.

Journal of Geophysical Research↗
USGS1986

Seismic-reflection profiles of the New Madrid seismic zone-data along the Mississippi River near Caruthersville, Missouri

The tectonic setting and the earthquake hazards of the New Madrid seismic zone in the northern Mississippi embayment have been the subject of intensive study the past several years as part of the Earthquake Hazard Reduction Program of the USGS (U.S. Geological Survey). These efforts have improved our knowledge of the geologic history, general tectonic setting, and patterns of modern seismicity (see McKeown and Pakiser, 1982) in the New Madrid seismic zone. Identifying and characterizing the specific structures that may be responsible for the seismicity is the focus of continuing research. Unfortunately, these structures are buried by the veneer of poorly consolidated Upper Cretaceous and Tertiary sediments that overlie competent rocks in the embayment. In such a geologic setting, seismic-reflection profiling is one of the most valuable techniques available to obtain useful information on the distribution of faults, and on the local and regional structure. Three major seismic-reflection programs have been conducted by the USGS in the New Madrid seismic zone. The first program consisted of 32 km of conventional Vibroseis profiling designed to investigate the subsurface structure associated with scarps and lineaments in northwestern Tennessee (Zoback, 1979). A second, more extensive Vibroseis program collected about 250 km of data from all parts of the New Madrid seismic zone in Missouri, Arkansas, and Tennessee (Hamilton and Zoback, 1979, 1982; Zoback and others, 1980). The profiles presented here are part of the third program that collected about 240 km of high-resolution seismic-reflection data from a boat along the Mississippi River between Osceola, Ark., and Wickliffe, Ky. (fig. 1). The data for profiles A, B, C, and D were collected between river miles 839-1/2 and 850-1/2 from near the Interstate-155 bridge to upstream of Caruthersville, Mo. (fig. 2). Profiles on this part of the river are important for three reasons: (1) they connect many of the land-based profiles on either side of the river, (2) they are near the northeast termination of a linear, 120km-long, northeast-southwest zone of seismicity that extends from northeast Arkansas to Caruthersville, Mo. (Stauder, 1982; fig. 1), and (3) they cross the southwesterly projection of the Cottonwood Grove fault (fig. 1), a fault having a substantial amount of vertical Cenozoic offset (Zoback and others, 1980).

Missouri↗
USGS1984

Relationship of late Quaternary fault scarps to subjacent faults, eastern Great Basin, Utah

High-resolution seismic reflection profiles in the Sevier Desert basin in western Utah help clarify the relationship between some late Quaternary fault scarps and subjacent faults. A profile crossing the Clear Lake fault confirms that Holocene(?) displacement has occurred on a high-angle normal fault that is directly connected to the Sevier Desert detachment. If detachment faults can act as seismogenic source zones, then the young movement on the normal fault suggests that the Sevier Desert detachment may be unstable and perhaps the source of future earthquakes. A swarm of late Quaternary scarps near the Drum Mountains overlies a network of steep faults in competent rock that have had recurrent movement. Many subsurface faults are not associated with scarps, suggesting that selected faults ruptured during the last episode of surface faulting.

Geology↗