SEISMIC SIGNALS OF STORM-GENERATED FLASH FLOODS TRAVELING ALONG BLACK CANYON CREEK, ARIZONA

Numerous monsoon rainstorms struck the watershed of central Arizona’s intermittent Black Canyon Creek between July and September 2018, followed by remnants of tropical cyclone Rosa in early October 2018. Runoff from these storms generated at least twenty discrete water pulses – described here as “rapid creek refills” to encompass all abrupt increases in stream discharge, including flash floods – which flowed along the southern stretch of the creek, past the Black Canyon Creek augmented temporary seismic network.

The seismic portion of the network consisted of twelve short-period stations, each with a three-component Sercel L-22 2-Hz sensor and a RefTek RT 130 digitizer, and one broadband station with a three-component Nanometrics Trillium Compact Posthole 120s sensor and a Nanometrics Centaur digitizer. Two Spypoint Solar trail cameras photographed the creek once per minute during daylight. A Davis Vantage Vue weather station recorded precipitation and windspeed near the network’s center.

Thirteen creek refills were photographed by one or both trail cameras; another seven were inferred by overnight increases in photographed creek level and verified by examination of stream-discharge data from the United States Geological Survey “Agua Fria at Rock Springs” gauge. We present our initial analysis of seismic signals accompanying these events.

Most creek refills generated a pulse of wide-bandwidth seismic noise which rose rapidly to a peak amplitude as the floodcrest approached a station, then decayed to background-noise levels over the course of several hours. The peak signal amplitude typically migrated downstream at an average of roughly 2 m/s. At the seismic stations nearest the creek, this signal was strongest between ~15 and ~85 Hz. More distant stations only observed the lower-frequency portion of this band, and at the farthest station, over 500 meters away from the creek, only the highest-discharge Rosa-induced refill produced a plausibly flood-related signal above background noise. The flood signals were distinct from earthquake, mine-blast, and air- and road-traffic noise in duration, envelope shape, and frequency content. However, rainstorms produced high-amplitude seismic noise over a broad frequency range, obscuring simultaneous flood signals at most seismic stations.