Fault Parameter Constraints Using Relocated Earthquakes: A Validation of First-Motion Focal-Mechanism Data

Abstract

We estimate the strike and dip of three California fault segments (Calaveras, Sargent, and a portion of the San Andreas near San Jaun Bautistia) based on principle component analysis of accurately located microearthquakes. We compare these fault orientations with two different first-motion focal mechanism catalogs: the Northern California Earthquake Data Center (ncedc) catalog, calculated using the fpfit algorithm (Reasenberg and Oppenheimer, 1985), and a catalog created using the hash algorithm that tests mechanism stability relative to seismic velocity model variations and earthquake location (Hardebeck and Shearer, 2002). We assume any disagreement (misfit >30° in strike, dip, or rake) indicates inaccurate focal mechanisms in the catalogs. With this assumption, we can quantify the parameters that identify the most optimally constrained focal mechanisms. For the ncedc/fpfit catalogs, we find that the best quantitative discriminator of quality focal mechanisms is the station distribution ratio (stdr) parameter, an indicator of how the stations are distributed about the focal sphere. Requiring stdr > 0.65 increases the acceptable mechanisms from 34%–37% to 63%–68%. This suggests stations should be uniformly distributed surrounding, rather than aligning, known fault traces. For the hash catalogs, the fault plane uncertainty (fpu) parameter is the best discriminator, increasing the percent of acceptable mechanisms from 63%–78% to 81%–83% when fpu ≤ 35°. The overall higher percentage of acceptable mechanisms and the usefulness of the formal uncertainty in identifying quality mechanisms validate the hash approach of testing for mechanism stability. Online material: 3D visualization of relocated earthquakes and accuracy of focal mechanisms.