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.