Uncertainty and Spatial Correlation in Station Measurements for mb Magnitude Estimation

Abstract

The body-wave magnitude (mb) is a long-standing network-averaged, amplitude-based magnitude used to estimate the magnitude of seismic sources from teleseismic observations. The U.S. Geological Survey National Earthquake Information Center (NEIC) relies on mb in its global real-time earthquake monitoring mission. Although waveform modeling-based moment magnitudes are the modern standard to characterize earthquake size, mb is important because (1) in many cases, waveform modeling is not possible (e.g., low signal-to-noise events), (2) mb is applicable over a broad range of magnitudes, ∼M 4–7, and (3) there is a many decades-long history of estimating mb magnitudes. We use the NEIC Preliminary Determination of Epicenters earthquake catalog to investigate the uncertainty in NEIC station mb measurements. We show that mb measurements are spatially correlated, which can bias event mb, and we describe an empirical relation between this spatial correlation and station-to-station distance. We further describe an approach to mitigate bias from the spatial correlation. Accounting for the spatial covariance of observations can change the event mb from −0.15 to 0.07 mb units (10th to 90th percentile) for smaller events (mb≤4.5). These smaller events have the largest mb standard deviations ranging from 0.05 to 0.15 mb units (10th to 90th percentile).