Continuous Aftershock Hum for over Ten Days Following the 2019 Ridgecrest, California, Earthquakes Observed with Borehole Seismometers

Abstract Seismograms from two borehole seismometers near the 2019 Ridgecrest, California, aftershock sequence do not return to pre-mainshock noise levels for over ten days after the M 7.1 Ridgecrest mainshock. The observed distribution of root mean square amplitudes in these records can be explained with the Reasenberg and Jones (1989) aftershock occurrence model, which implies a continuous seismic “hum” of overlapping aftershocks of M > −2 occurring at an average rate of 10 events per second after ten days, which prevents observing the background aseismic noise level at times between the body-wave arrivals from cataloged and other clearly observed events. Even after the borehole noise levels return at their quietest times to pre-mainshock conditions, the presence of overlapping low-magnitude earthquakes for 80 days is implied by waveform cross-correlation results provided using the matrix profile method. These results suggest a hidden frontier of tiny earthquakes that potentially can be measured and characterized even in the absence of detection and location of individual events.