Using S/P Amplitude Ratios to Constrain the Focal Mechanisms of Small Earthquakes

We test whether S-wave/P-wave amplitude ratio data can improve the computed focal mechanisms of small earthquakes, using events from two southern California aftershock sequences. The observed S/P ratios are generally consistent with the expected mechanisms, implying that S/P ratios can in fact be useful in constraining the focal mechanisms of small events. However, we also find that noise in the observations leads to scatter in the S/P ratios of factors of 2, and sometimes higher. This scatter limits the usefulness of the S/P ratios in two ways: (1) the focal mechanism cannot simply be fit to S/P amplitude data alone without accounting for the noise in a more sophisticated focal mechanism inversion process; (2) while the amplitude ratios may improve poorly constrained mechanisms, they are less useful in refining solutions that are already relatively well constrained.