Geophysical Research Letters
Supporting Information for
Automated Detection and Location of Microseismicity at Mount St. Helens with a Large-N
Geophone Array
Steven M. Hansen1,* and Brandon Schmandt1
1. Department of Earth and Planetary Sciences, University of New Mexico
*. Corresponding author. Email address: stevehansen@unm.edu
Contents of this file
Text S1
Figures S1 to S2
Additional Supporting Information (Files uploaded separately)
Caption for Dataset S1
Caption for Dataset S2
Introduction
This supplement contains details pertaining to the event coda magnitude estimation
methodology in Text S1 and Figure S1. Plots comparing hypocenters from the reverse-time
imaging (RTI) with the Pacific Northwest Seismic Network (PNSN) earthquake catalog are
shown in Figure S2. In the main text we refer to the historical rate of seismicity detected by the
PNSN in the source image volume and this is shown in Figure S2d. Additionally, a dataset which
contains the hypocenter information for all of the detected events is included.
S1. Magnitude Methodology
The coda magnitudes of the detected events were calculated by measuring the temporal
duration of the median trace envelop from the closest 200 stations to the event hypocenter.
The duration was defined as the width of the envelope at 10% of the envelope maximum
relative to the pre-event trace amplitudes (Fig S1a). The 10% amplitude threshold was chosen
such that events occurring in close temporal proximity (e.g., Fig. S1a) would not result in
overlapping event durations. Coda magnitudes were calculated using
1
Mi  a *log10  i   b
(0.1)
where M i is the magnitude of the i-th event and  i is the event duration measurement. The
magnitude constants a and b where calibrated to the 42 catalog earthquakes that were
detected by the RTI analysis (see main text). The measured durations and the catalog
magnitudes form a system of equations via (0.1) which was solved for the model parameters a
and b using an iteratively reweighted least-squares algorithm [Aster et al., 2013] (Fig. S1b).
References
Aster, R. C., Borchers, B., & Thurber, C. H. (2013). Parameter estimation and inverse problems. Academic
Press.
Figure S1. Coda magnitude calculation and catalog calibration. A) An example of coda duration
measurements for two of the catalog events that occurred during the July 28th earthquake
cluster. The blue line shows the median trace envelope, calculated using the time-aligned
traces from the 200 stations closest to the event hypocenter. The black lines show the duration
measurements. B) The results of applying the iteratively reweighted least-squares inversion to
determine the magnitude constants. Dot locations show the catalog magnitude versus the
coda magnitude resulting from the inversion. Marker color denotes the final weighting applied
to the data points and the black line shows a one-to-one relation.
2
Figure S2. Comparisons between the RTI hypocenters and the PNSN catalog values. A) Stick
diagram showing the PNSN epicenter locations for the 42 detected earthquakes (black) relative
to the RTI epicenters (red). B) Histogram showing the difference between the RTI event depths
and the associated PNSN depths. C) Histogram showing the difference between the RTI origin
times and the PNSN times. D) The historical PNSN detection rate for earthquakes occurring
within the source image volume. Origin times are binned using a moving window that is 11 days
3
long, the approximate length of the dataset used in the RTI analysis. The red bar spans the June
1st to August 30th time period in 2014 where increased PNSN scrutiny occurred.
Data Set S1. A list of the event hypocenters and coda magnitudes for the detected
earthquakes. The PNSN event ID number is given for events associated with catalog
earthquakes, otherwise a null value of -1 is used. The file format is comma delimited text file
and the first line contains the header.
Data Set S2. Results from the bootstrap hypocenter error calculations. The standard deviation
(sig) and mean (mu) are reported for the estimated parameters.
4