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Reply to "Comment on ‘Faulting Apparently Related to the 1994 Northridge, California, Earthquake and Possible Co-Seismic Origin of Surface Cracks in Potrero Canyon, Los Angeles County, California,’ by R. D. Catchings, M. R. Goldman, W.H.K. Lee, M. J. Rymer, and D. J. Ponti," by Brian J. Swanson, Allan E. Seward, Perry L. Ehlig, and James E. Slosson

¹ USGS-MS 977
345 Middlefield Road
Menlo Park, California 94025
catching@usgs.gov

The first 300 words of the full text of this article appear below.

	Introduction

 
Swanson et al. (2002), herein referred to as the "comment authors," take issue with numerous aspects of our 1998 article (Catchings et al., 1998) on Potrero Canyon, Los Angeles County, California. The comment authors take considerable liberty in paraphrasing our conclusions and discounting our interpretations, principally because they believe that our seismic images do not represent the geologic structure beneath Potrero Canyon. In particular, the comment authors contend that our subsurface images are inconsistent with surface geologic mapping. The comment authors attempt to support their objections by suggesting a combination of reflection artifacts and errors in our velocity model result in an "inaccurate seismic profile." In response to their contentions, we show a published map, two published cross sections, and two sets of published borehole data that are consistent with our seismic image and the known geologic structure of the Potrero Canyon area. We present velocity measurements from a borehole along our seismic profile that are consistent with our velocity model and inconsistent with the comment authors’ assumed velocities. Furthermore, we show that the comment authors’ assumptions about our acquisition and processing techniques are based on inadequate research and lack of familiarity with seismological techniques. Below, we address 16 objections by the comment authors on a topical basis.

	Geologic Structure

 
Comment: "Catchings et al. (... the authors) stated that Potrero Canyon is believed to be within or overlie a broad syncline that is faulted along its axis. They interpreted this syncline to be present in their seismic profiles based on the shallow-dipping (0°–20°) reflectors shown in their figures 5 and 6, but they did not provide a geologic cross section at the seismic line to verify this. In contrast to the author’s statements and seismic profile, however, Winterer and Durham (1962) did not show a syncline below . . . [Full Text of this Article]

	Geologic Dips

 

	Objection to Use of Section EE'

 

	Section FF'

 

	Field Evidence for Faulting in Potrero Canyon

 

	Geodetic and Seismological Evidence for Faulting in Potrero Canyon

 

	Faults Interpreted from the Seismic Profiles

 

	Correlation with Published Cross Sections

 

	Geophysical Modeling, Processing, and Acquisition Methods

 

	Velocity Modeling

 

	Reflection Images

 

	Summary