Spot cores from Phase 1 drilling of the main borehole at the San Andreas Fault Observatory at Depth (SAFOD) were mapped to characterize the mesoscale structure and infer paleostress at depth. Cores were oriented by comparing mapped structures with image logs of the borehole. The upper core (1476-1484 m measured depth, MD) is a medium-grained, weakly foliated, hornblende-biotite granodiorite containing leucocratic phenocrysts and lenses. Principal structures are sub-vertical veins, shallow dipping shears, and natural fractures of unknown kinematics. The features are compatible with horizontal extension and right-lateral, normal, oblique-slip on faults striking approximately parallel to the SAF. The lower core (3055.6-3067.2 m MD) has massivebedded, pebble conglomerates and coarse to fine grained arkosic sandstones grade into siltstones. Principal structure features are conjugate shears and two minor faults. The fracture fabric is consistent with strike-slip faulting and a maximum principal compressive paleostress at ~80° to the SAF plane. This paleostress is essentially parallel to the current in situ stress measured in the main borehole and to paleostresses inferred from fracture fabrics in exhumed faults of the San Andreas system to the south. The similarity between the current state of stress and paleostress states supports the suggestion that the maximum principal compressive stress direction is, on average, at high angles to the SAF and that the fault has been weak over geologic time.

The San Andreas Fault (SAF) near Parkfield, CA moves by a combination of aseismic creep and micro-earthquake slip. Measurements of in situ stress orientation, stress magnitude, and heat flow are incompatible with an average shear stress on the SAF greater than approximately 20 MPa. To investigate the micro-mechanical processes responsible for the low strength and creeping behavior, gouge samples from the 3 km-deep scientific borehole near Parkfield (the San Andreas Fault Observatory at Depth, SAFOD) are sheared in a triaxial rock deformation apparatus at conditions simulating those in situ, specifically a temperature of 100°C, effective normal stress of 100 MPa, pore fluid pressure of 25 MPa, and a Na-Ca-K pore fluid chemistry. The 2 mm-thick gouge layers are sheared to 4.25 mm at shear rates of 6.0, 0.6, 0.06, and 0.006 mu m/s. The mechanical data are corrected for apparatus effects and the strength of the jacketing material that isolates the sample from the confining fluid. Experiments indicate that gouge is extremely weak with a coefficient of friction of 0.14, and displays velocity and temperature strengthening behavior. The frictional behavior is consistent with the inferred in situ stress and aseismic creep observed at SAFOD. The low frictional strength likely reflects the presence of a natural fabric characterized by microscale folia containing smectite and serpentinite.

Thesis (Ph. D.). / Title from document title page. Includes bibliographical references. Available in PDF format via the World Wide Web.

Thesis (Ph. D.)--California Institute of Technology, 1954. / Includes bibliographical references (p. 141-147).

Thesis (M.S.)--University of Massachusetts Amherst, 2009. / Includes bibliographical references (p. 61-67).

No description available.

No description available.

Thesis advisor: Alan Kafka / Thesis advisor: Seth Kruckenberg / The western United States (WUS) is an area of high seismic activity. The Juan de Fuca, Pacific, and North American plates all meet in this area, resulting in zones of subduction and strike-slip faulting, as well as other styles of faulting, all of which make it prone to frequent, as well as large magnitude earthquakes. In this study the WUS encompasses the area between 30° to 52°N and 110° to 131°W. The diverse seismicity and tectonics of the area makes the study of seismo-tectonic processes in the WUS important not only in terms of basic geoscience, but also in terms of earthquake hazards. Understanding earthquake processes in this region is critical because of the potential for devastating earthquakes to occur along the Pacific-Juan de Fuca-North American plate boundary system. Large WUS earthquakes do not, however, only occur along these plate boundaries. They can also happen in intraplate environments within the WUS. The WUS includes three distinct tectonic regions for which this study compares and contrasts characteristics of seismic activity: the Cascadia subduction region, the San Andreas strike-slip region, and a continental extension/intraplate region to the east of the major plate boundaries referred to here as the “Western Intraplate Hinterland Region”. To help make these comparisons, the method of “Cellular Seismology” (CS; Kafka, 2002, 2007), is used here to investigate similarities and differences in the extent to which past earthquakes delineate zones where future earthquakes are likely to occur in the WUS and its various tectonic sub-regions. The results of this study show that while there seems to be a “signal” of CS predictability being dependent on tectonic region, that signal is subtle in most cases, meaning that there is not a significant difference in the level of CS predictability between the regions stated here. This means we can apply CS predictability studies widely across different regions, however, it also counterintuitively suggests that tectonic understanding of a region does not necessarily elucidate how well past seismicity predicts spatial patterns of earthquakes in a region.

Graduation date: 2000

Recovered core samples from the San Andreas Fault Observatory at Depth (SAFOD), located near Parkfield, CA, offer a unique opportunity to study the products of faulting and to learn about the mechanisms of slip at 3 km depth. Casing deformation reflects active creep along two strands of the San Andreas Fault (SAF) at SAFOD. The two fault strands are referred to as the Southwest Deforming Zone (SDZ) at 3194 m measured depth (MD) and the Central Deforming Zone (CDZ) at 3301 m MD. The SDZ and CDZ contain remarkably similar gouge layers, both of which consist of a clay-bearing, ultrafine grain matrix containing survivor clasts of sandstone and serpentinite. The two gouges have sharp boundary contacts with the adjacent rocks. We have used X-ray Computed Tomography (XCT) imaging, at two different sampling resolutions, to investigate the mesoscale and microscale structure of the fault zone, specifically to characterize the shape, preferred orientation, and size distribution of the survivor clasts. Using various image processing techniques, survivor clast shape and size are characterized in 3D by best-fit ellipsoids. Renderings of survivor clasts illustrate that survivor clasts have fine tips reminiscent of sigma type tails of porphyroclasts observed in myolonites. The resolution of the XCT imaging permits characterization of survivor clasts with equivalent spherical diameters greater than 0.63 mm. The survivor clast population in both the SDZ and CDZ gouge layers have similar particle size distributions (PSD) which fit a power law with a slope of approximately -3; aspect ratio (major to minor axis ratios) distributions also are similar throughout ranging between 1.5 and 4, with the majority occurring between 2-2.5. The volume- and shape- distributions vary little with position across the gouge zones. A strong shape preferred orientation (SPO) exists in both creeping zones. In both the SDZ and CDZ the minor axes form a SPO approximately normal to the plane of the San Andreas Fault (SAF), and the major axes define a lineation in the plane of the SAF. The observation that the size-, shape- and orientation-distributions of mesoscale, matrix-supported clasts are similar in the SDZ and CDZ gouge layers, and vary little with position in each gouge layer, is consistent with the hypothesis that aseismic creep in the SDZ and CDZ is achieved by distributed, shearing. The consistency between the SPO and simple-shear, strike-slip kinematics, and the marked difference of PSD, fabric, cohesion and clast lithology of the gouge with that of the adjacent rock, is consistent with the hypothesis that the vast majority of the shear displacement on the SAF at SAFOD is accommodated within the gouge layers and the gouge displays a mature, nearly steady-state structure.