Exploring the Coda Envelope Technique for Moment Magnitude Estimation of Small Earthquakes in Southern Kansas

Abstract:

An earthquake’s magnitude is a conceptually simple measure of its size. Yet, this conceptual simplicity belies a complexity of magnitude types and associated methods employed in practice. Of the many flavors of magnitude commonly reported in seismic catalogs, only moment magnitude (M_w) is clearly tied to a physical quantity of the earthquake source. Although M_w has become the established standard for moderate and large earthquakes, difficulty in reliably measuring seismic moment for small (M<4) earthquakes has meant magnitudes for these events remain plagued by a patchwork of inconsistent measurement scales. This presents a significant problem, because magnitudes of small earthquakes are increasingly relied upon for applications such as seismic hazard forecasting and regulation of industrial practices related to induced seismicity. Therefore, routine computation of reliable M_w for small earthquakes remains an important unrealized goal in earthquake science.

Among proposed techniques, coda envelope analysis has been demonstrated as perhaps the most stable technique for deriving M_w of small earthquakes (e.g. Mayeda et al., BSSA, 2003). This is because the seismic coda, which consists of scattered seismic waves, is minimally affected by the seismic radiation pattern that strongly affects direct waves. Once calibrated, even a single station can provide a reliable estimate of M_w, derived from measurements of the amplitude and shape of the seismic coda envelope made in multiple narrow-band windows. Software to aid this processing is under active development at Lawrence Livermore National Laboratory.

To explore the potential and impact of adopting M_w at smaller magnitudes, we examine a dataset of M 2.5-4.0 earthquakes recorded by a dense broadband network deployed by the U.S. Geological Survey in southern Kansas from 2015-2019. Preliminary results from a small sample suggest that preferred network magnitudes (typically M_L or M_{b_lg}) for these events are broadly consistent with our estimated M_ws, yet large discrepancies occasionally occur. Continued analysis will provide a sample large enough for statistical comparisons to identify potential biases. We also consider the challenges and potential solutions for adopting the coda envelope M_w approach in routine network operations.