Three‐dimensional imaging of steeply dipping structure near the San Andreas fault, Parkfield, California

Abstract

Shot gathers from the Parkfield, California, deep crustal seismic reflection line, recorded in 1977 by COCORP, reveal coherent events having horizontal to reverse moveouts. These events were migrated using a multioffset three-dimensional Kirchhoff summation method. This method is a ray-equation back projection inversion of the acoustic wave field. which is valid under the Born, WKBJ, and far-field assumptions. Migration of full-wave acoustic synthetics, having the same limitations in geometric coverage as the COCORP survey, demonstrates the utility of the imaging process. The images obtained from back projection of the survey data suggest that the Gold Hill fault carries ultramafic rocks from the surface to 3 km depth at a dip greater than 45 degrees. where it joins the San Andreas fault, which may cut through more homogeneous materials at shallow depths. To the southwest, a 2 km Tertiary sedimentary section appears to terminate against a nearvertical fault. The zone between this fault and the San Andreas may be floored at 3 km by flat-lying ultramatics. Lateral velocity inhomogeneities are not accounted for in the migration but, in this case, do not seriously hinder the reconstruction of reflectors.

Interpretation of ambiguous reflection data by imaging processes: An example profile crossing steep submarine valley at the sea bottom

“…A number of studies demonstrated that prestacked Kirchhoff depth migration may efficiently be used in regions characterized by complex tectonic structures to determine geometry of complex tectonic structures that involve faulting, folding, etc. (Louie et al, 1988;Louie and Qin, 1991;Honjas, 1995;Kanbur et al, 2000;Kanbur, 2006;Kanbur et al, 2007). Therefore, prestack migration algorithm used in this study is Kirchhoff summation technique in order to better and correctly image complex tectonic structures related to Northern strand under the westernmost Marmara Sea.…”

“…Though the horizontal layers in depths of the earth are easily displayed in the seismic time section (Yılmaz, 1987), the events of steeply dipping and folded layers shown in the reflection data are not correctly displayed in the section due to the limitations of the conventional stacking techniques (Le Louie et al, 1988). Such data also require an extensive examination of the reflections appearing individually in the shot gathers.…”

“…Therefore, some other advanced methods, such as prestack Kirchhoff depth migration, is also used to process the data in the multi-offset form of seismic experiment; in addition to stacking, post-stack migration is used to overcome the problem and to provide additional information. Several studies have shown that pre-stack Kirchhoff depth migration technique may image the steeply dipping structures and give significant contribution to the conventional stacking process technique in tectonically complex areas (Louie et al, 1988;Louie and Qin, 1991;Honjas, 1995;Kanbur et al, 2000;Kanbur, 2006;Kanbur et al, 2007).…”

“…Revealing geometry of a vertical structure is one of the most important problems in the conventional stacking process used as an imaging process (Louie et al, 1988). Though the horizontal layers in depths of the earth are easily displayed in the seismic time section (Yılmaz, 1987), the events of steeply dipping and folded layers shown in the reflection data are not correctly displayed in the section due to the limitations of the conventional stacking techniques (Le Louie et al, 1988).…”

Heterogeneous slip and rupture models of the San Andreas fault zone based upon three-dimensional earthquake tomography

“…This has been a result of the availability until recently of only one-dimensional crustal velocity models in earthquake seismology, and from the difficulty of imaging localized fault zones, with their often severe vertical and lateral velocity gradients, by conventional reflection and refraction techniques (Feng and McEvilly, 1983;McBride and Brown, 1986;Louie et al, 1988). Such surveys provide images of two-9 dimensional slices of the crust, and the cost of attempting surveys at high resolution for large distances along the strike of a major fault zone would be prohibitive.…”