Abstract and subjects
We quantify the total attenuation, kappa, and the attenuation component due to near-surface site effects, kappa(0), in a region in northern New Mexico using data recorded by the Los Alamos Seismic Network. The area is characterized by low seismicity, where most of the well-recorded earthquakes have magnitudes between 1 and 2. This magnitude range poses a challenge for commonly used kappa methods because the high-frequency attenuation cannot be confidently isolated from the bandwidth in which the corner frequency roll-off occurs. We determine through synthetic experiments that estimates of kappa within this range have quantifiable biases that depend on source (corner frequency), site (kappa magnitude), and data quality characteristics (fitting bandwidth), which can be used to correct estimated kappa from three commonly used kappa methods. Using 412 recorded earthquakes, we show that a bias correction results in kappa distributions and kappa(0) estimates that are more consistent between the three methods, suggesting that the bias correction results in kappa values with higher fidelity. Using the bias-corrected kappa, we find kappa(0) between 0.038 and 0.049 s within the Valles Caldera and between 0.026 and 0.066 s on Los Alamos National Laboratory property, values near those commonly used in the western United States. We find that a main limitation in the quality of kappa(0) is the small number of usable waveforms at some stations, which will to improve as more earthquakes are recorded. This contrasts with other aspects, such as fitting bandwidth and source and path variability, which are unlikely to change in the future and will ultimately be the limiting factor in kappa(0) resolution. Overall, our results suggest that the bias-correction scheme presented here could potentially be used in other regions where small-magnitude earthquakes are prevalent. However, future work should look to verify that bias-corrected kappa estimates show consistency with those retrieved from higher magnitude earthquakes.