Finding Active Faults Using Earthquake Focal Mechanisms, Geomorphic Analysis and Field Work (SLAM)

Vincent Cronin, Baylor University
Jordan Dickinson, Baylor University

The seismo-lineament analysis method (SLAM) uses an earthquake focal mechanism and a DEM or topographic map, along with geomorphic analysis, to spatially correlate the earthquake with the causative fault.

What is a seismo-lineament? Each double-couple focal mechanism has two nodal planes that are normal to each other, one of which coincides with the fault that generated the earthquake. Uncertainties in hypocenter location and nodal-plane orientation combine to form the uncertainty volume associated with each nodal plane. A seismo-lineament is the intersection of that uncertainty volume with the ground surface. Each seismo-lineament is a swath across the ground surface within which the trace of the causative fault is likely to be found if [a] the nodal plane coincides with the fault plane (i.e., it is not the auxiliary plane), [b] the earthquake occurred in the upper crust, [c] the fault is approximately planar from the hypocenter to the ground surface, and [d] the fault is emergent at the ground surface. SLAM does not require the earthquake to have actually ruptured the ground surface; however, a small earthquake must have occurred along a fault that deformed the ground surface sometime during its displacement history.

Seismo-lineament boundaries are usually mapped using an open-source code written in Mathematica, although a description of the underlying algorithm enables the boundaries to be defined manually on a topographic map. After the seismo-lineament boundaries are defined, geomorphic analysis is conducted using a DEM-based hillshade image illumined perpendicular to the nodal-plane strike, to identify any geomorphic features that might have developed due to fault displacement. Subsequent fieldwork is undertaken to locate exposures of faults or fault-related geomorphic features.

In addition to its scientific utility, SLAM provides students with a meaningful experience in the use of scientific data (earthquake focal mechanisms and DEMs), geomorphic analysis, hypothesis generation, and field work to test the hypotheses. SLAM has been used by undergraduate and graduate students to yield reasonable spatial correlations between recorded earthquakes and faults. We have spatially correlated earthquakes with known active faults, with faults that were not known to be active, and with faults that had not previously been mapped. In the future, seismo-lineament swaths might be used to design aerial lidar surveys conducted to identify faults through recognition of characteristic structural geomorphology. More information about SLAM is available via http://bearspace.baylor.edu/Vince_Cronin/www/SLAM/.