Corey, Robert

Derek reported that S240420aw was likely caused by scattering noise so I looked into the problem. The figure shows that a large seismic pulse (~2 orders of magnitude above background) started something swinging at 1.44 Hz, with a Q of about 300. The scattering shelf's cutoff slowly dropped in frequency over ten minutes,  so there were plenty of chirps reaching different frequencies.  It is tempting to guess that the source of scattering is a TMS, which have transverse resonances of about 1.4 Hz, but I checked and the Qs of TMS motion were much lower than the Q of 300 of the scatter source.  The time of the seismic pulse matches that of the M2.8 earthquake in Richland on Friday night, 24 surface km from the site and 8 km deep. There is quite a bit of light scattered, but at normal ground motion levels I would expect the scattering noise shelf to reach only about 5 Hz, so I don’t think this scattering noise source affects us during normal operation. Andy L. and Beverly B. noticed similar scattering noise back in 2017 ( 37947 ) for a smaller nearby quake.