Seismic imaging of active faults

Seismic reflection profiling, which uses sound waves to look into the Earth, is a valuable tool for examining faults beneath the surface. The methodology was developed primarily by the oil industry to explore for oil and gas, but it is now being used in the earthquake hazards community to determine the subsurface geometry of faults. Examining a fault’s geometry can tell us what type of fault it is and how fast it is moving, both of which are valuable pieces of information for estimating how dangerous the fault might be.

Recent projects I have been involved with include:

The Seattle fault and other faults beneath the Puget Lowland of Washington State

Pratt, T.L., Johnson, S.Y., Potter, C.J., Stephenson, W.J., and Finn, C., 1997, Seismic Reflection Images beneath Puget Sound, Western Washington State: The Puget Lowland Thrust Sheet hypothesis, Journal of Geophysical Research, v. 102, no. B12, p. 27,469-27,489. (pdf) (BIG FILE!)

This paper presented the first comprehensive interpretation of crustal faults beneath the densely populated Puget Lowland of Washington State. In this paper we argue that the faults form an interconnected network forming a north-directed thrust sheet. The Seattle fault is hypothesized to be a low-angle thrust fault, in contrast to previous interpretations of the fault as steeply dipping. The paper was the driving force behind the recent SHIPS seismic experiments and was used in designing those experiments. The paper has important implications for seismic hazard estimates, such as revising estimates of the total area of the faults in the region and the structural interpretation of those faults.

Interpretation of active faults beneath Cook Inlet, Alaska

Haeussler, Peter, Bruhn, Ronald L., and Pratt, T.L., 2000, Quaternary Deformation in Upper Cook Inlet Basin, Alaska, Geological Society of America Bulletin, v. 112, p. 1414-1429. (pdf)

Although active, shallow faults have always been assumed to lie beneath and around the city of Anchorage, Alaska, there had never been a study carried out to identify and characterize these faults. In this project we examined seismic reflection data donated by ARCO, Alaska, to examine the geometry of faults in the region and to determine whether they cut young geologic strata. The paper describes several such faults, and compiles evidence for recent motion on those faults.

Shallow faults and the Columbia River paleochannel, Portland, Oregon.

Pratt, T.L., Odum, J., Stephenson, W., Williams, R., Dadisman, S., Holmes, M., and Haug, B., 2001, Late Pleistocene and Holocene tectonics of the Portland Basin, Oregon and Washington, from high-resolution seismic profiling, Bulletin of the Seismological Society of America, v. 91, p. 637-650. (pdf)

It had been an open question as to whether active, shallow faults lie beneath the city of Portland, Oregon, and this work provides the first solid evidence for such faults. In this project, we acquired high-resolution seismic reflection profiles across hypothesized faults on land and beneath the rivers. The data show what appears to be several faults displacing post-glacial (post 14,000 year old) sediments at the location of the hypothesized East Bank fault near the mouth of the Willamette River. Other potential faults are imaged, although the evidence is ambiguous. In addition to the faults, the data show an 80-m-deep, 1.5-km-wide paleochannel meandering beneath the present Columbia River and the land to the south. The data also show a strong reflector associated with an unconformity; this reflector could significantly amplify ground motions by resonance within the shallow sediments.

Thrust faults beneath Los Angeles, California

27. Dolan, J.F., and Pratt, T.L., 1997, Shallow expression of the Santa Monica fault zone, west Los Angeles, California, from high-resolution seismic reflection imaging and trenching, Geophysical Research Letters, v. 24, p. 2051-2054. (pdf)

31. Pratt, T.L., Dolan, J.F., Odum, J.K., Stephenson, W.J., Williams, R.A., and Templeton, M.E., 1998, Multi-scale imaging of active fault zones for hazard assessment: A case study of the Santa Monica fault zone, Los Angeles, California, Geophysics, v. 63, no. 2, p. 1-11. (pdf)

49. Pratt, T.L., Shaw, J. H., Dolan, J. F., Christofferson, S., Williams, R. A., Odum, J. K., and Plesch, A., 2002, Shallow seismic imaging of folds above the Puente Hills blind-thrust fault, Los Angeles, California, Geophysical Research Letters, v. 29, no. 9, p. 18-1 – 18-4 (May 8, 2002). (pdf)

50. Shaw, J. H., Plesch, A., Dolan, J.F., Pratt, T.L., and Fiore, P., 2002, Puente Hills blind-thrust system, Los Angeles, California, Bulletin of the Seismological Society of America, v. 92, p. 2946-2960. (pdf)

Thrust faults are a recently recognized threat to the city of Los Angeles. These papers describe the use of high-resolution seismic imaging, in conjunction with geologic data, to document the recent history and slip rates of two of these faults directly beneath urban Los Angeles. The Santa Monica fault runs along the base of the Santa Monica Mountains along the northern edge of the Los Angeles valley. The Puente Hills fault lies directly beneath downtown Los Angeles and was responsible for the 1971 San Fernando earthquake. These studies provide the first estimates of slip rates on these faults.

Young faults beneath the northern Panama Canal, Republic of Panama

Pratt, T.L., Holmes, M., Schweig, E.S., Gomberg, J., and Cowan, H.A., 2003, High resolution seismic imaging of faults beneath Limón Bay, northern Panama Canal, Republic of Panama, Tectonophysics, v. 368, p. 211-227. (pdf)

The work described in this paper was part of an earthquake hazard assessment of Gatún Dam, the 2.5 km long, earthen dam that impounds Gatún Lake and is the key component of the strategically and economically crucial Panama Canal. Eugene Schweig, Joan Gomberg, Hugh Cowan and I synthesized available geologic and geophysical data to write a final report of recommendations for the hazard assessment. My part of the project was to determine whether shallow faults lie beneath or near the dam. I performed high-resolution, marine and land seismic profiling that unequivocally found a number of shallow faults within a few km of the dam, and provided equivocal evidence for faulting beneath the dam. These faults cut the shallowest strata, Late Miocene Gatún Formation, and possibly local Holocene muds. Relatively little geologic work has been done in the geologically complex Panama region, so our work added significantly to the geologic database. In particular, the faults discovered during our work give insights into the stress regime during the Late Miocene, during which time the Isthmus of Panama was rising and lowering to open and sever the connections between the two oceans.