2.2 USArray waveforms
To confirm SB19’s imaging results, we analyze 59,517
transverse-component displacement waveforms from 337 global earthquakes
(Figure 2) recorded by stations from the USArray and other regional
networks in the forty-eight conterminous United States. The earthquakes
are shallower than 35 km, so the direct S wave and the depth phase sS
form a single pulse at long periods. The earthquakes have moment
magnitudes smaller than 7.0 so rupture complexity does not affect
long-period waveforms strongly. The epicentral distances are between 60°
and 110° and waveforms have been filtered using a bandpass Butterworth
filter with corner frequencies of 20 mHz and 80 mHz. We align the
waveforms on the peak S-wave displacement and normalize them, so the S
waves have the same polarities and maximum displacements of +1. In all
waveforms, the S-wave displacement is at least six times larger than the
signal in the 100-second window prior to the S wave onset. The maximum
and the root-mean-square displacement in the window [30 s, 220 s]
after the S-wave arrival time are more than six times and three times
smaller than the peak S-wave displacement, respectively. We remove
earthquakes with fewer than 20 seismograms left after these quality
control steps.