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3D/4D mapping of the San Andreas Fault Zone

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TASK 3 - Geophysical/subsurface geologic mapping

New geologic mapping will be supported by concurrent gravity and magnetic interpretations in cases of ambiguity or where critical relationships are concealed. New subsurface geologic mapping is needed along most of the fault, and will follow the philosophy of starting with the surface geologic map and progressively carrying critical contacts into the subsurface. This work will depend heavily on modeling of potential field anomalies, detailed analysis of seismic tomography models, analysis of micro-seismicity, and integration of drill hole data. Construction of the 4D geologic map will require integration of the results of tasks 1) and 2) within a framework that explicitly incorporates time-histories of along-fault, cross-fault and vertical deformation. This task will be sequenced geographically, with the Bay Region section (between the San Andreas/Calaveras junction and the San Andreas/San Gregorio junction) being the focus of the initial effort.

Statement of Problem: The problem addressed by Task 3 is the compilation, acquisition, reduction, and interpretation of geophysical and related data needed to define the geologic units in the subsurface along the San Andreas Fault. The primary types of information will be gravity data, aeromagnetic surveys, tomographic velocity models, active-source seismic profiles, double-diffence seismicity, electrical soundings, and drill-hole data. Many data already exist, but need to be compiled. Other data will need to be collected.

Objectives: The objective of work under this Task is to prepare a complete and diverse set of data relating to the subsurface geology along the San Andreas Fault on which to base construction of the 3D geologic map. The data will be assembled and prepared in ways that enhance the joint inversion of different types of data. Gravity data will be compiled according to uniform standards. Individual aeromagnetic surveys will be merged into a common map at a nominal height of 300 m above terrain. All profile and spatial data will be geographically referenced and projected according to a common standard projection.

New gravity data will be needed along the more remote stretches of the SAF, and new aeromagnetic surveys will be collected at three isolated locations to complete modern coverage of the fault.

An analysis of aeromagnetic data in terms of boundary identification will be made along the San Andreas Fault to identify all automatically determined boundaries that are relevant to the location of fault strands. This will be a stand-alone data set available for comparison with other data in any future studies of the SAF.

Methodology: Standard methods will be used to collect and prepare the gravity and aeromagnetic data. Modern aeromagnetic surveys cover most of the San Andreas Fault included in this study. Data are lacking over the northern Carrizo Plain, between Priest Valley and San Juan Bautista, and over the northernmost Gualala block north of Black Point. Gravity coverage is good over most of the study area, but will require additional collection in remote parts of the fault and to address specific problems.

The catalogue of seismicity relocated by the double-difference technique will be examined and vetted. Active-source seismic interpretations mostly will come from completed studies, thus requiring literature search and compilation of results.

Drill-hole data will be compiled, including geophysical logs, and geographically referenced. Sparse electrical soundings will be treated in a manner similar to the seismic profiles.

An exciting new tool for defining subsurface geology along the central California and Bay region sections of the fault is available in the form of 3D velocity models derived by seismic tomography. We have experimented with defining velocity interfaces by automatically identifying points of maximum velocity gradient (the volumetric equivalent of maximum horizontal gradient techniques that are a mainstay of potential field interpretations) and connecting these locally with geologic contacts. Although the techniques need to be refined, the initial results are extremely encouraging--they were used to define the boundaries of the Permanente Terrane in the Parkfield area.

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