Coda-Derived Source Parameters of Shallow Earthquakes in Mainland Alaska

Abstract We use Coda envelope calibration to derive moment magnitude and apparent stress measurements for shallow earthquakes across Alaska. We base this on a set of ∼70 reference moment magnitude (3.6<Mw<5.2) from the National Earthquake Information Center and 13 reference apparent stress (σa) estimates obtained from the coda spectral ratios method. We use the Coda Calibration Tool to correct for the low Q observed at surface stations, which allows us to produce nearly identical source spectra across the region, including spectra from colocated surface and borehole stations. After validating the calibration, we use it to calculate Mw for 700 shallow earthquakes and apparent stress for 200 events. This dataset allows us to develop a new conversion relationship between Mw and ML using an orthogonal regression method that accounts for uncertainty in both ML and Mw. This conversion is valid for shallow events in mainland Alaska. We observe a modest magnitude dependence in apparent stress, in agreement with previous studies. When considering spatial variation across the state, we accounted for this magnitude dependence by calculating ψa—a version of apparent stress (σa) with the magnitude effect removed. We further introduce φa—a magnitude and depth-detrended version of σa. This measure accounts for the magnitude and the slight depth dependence observed in the multiple linear regression model. The final detrended values reveal spatial variations in apparent stress across mainland Alaska, with relatively higher values in south-central Alaska, northern Alaska (Kaktovik region), and two events located on the Tintina fault.