4.1 Initial single bin inversion (Strategy 1)
First, we perform a single inversion of all the event spectra to solve
for stress drops for all individual earthquakes in our dataset. We use
the hybrid SNSS approach to calculate a single ECS to correct for the
source spectra of all the earthquakes (no consideration of spatial
varying attenuation), and use a constant shear wave velocity (assuming
constant rupture velocity) to compute stress drops from corner
frequencies and moments with Equation 4 (Figure 3).
To quantify any dependence of stress drop on seismic moment, we
calculate the median stress drop in overlapping Mw bins of 1 Mw unit
width centered at 0.5 unit intervals. We then use linear regression to
find the best fitting trend and R-squared value (squared correlation
coefficient) in two separate intervals with a relatively large number of
events: (i) Mw0.75-1.75, in which stress drop increases with moment, and
(ii) Mw1.75-2.75, in which there is negligible dependence of stress drop
with moment, see Table 1 and Figures 3B and S6.
We use a similar approach to quantify any dependence of stress drop on
depth, by calculating the median stress drop in 1 km depth intervals.
The results of the single inversion with no correction for depth
dependent velocity or attenuation indicate an increase in stress drop
with depth, both between 1.5 and 6.5 km, and between 6.5 and 11.5 km
(Table 1 and Figure 3A).