United States stream gauges show no detectable median discharge change in rain-free windows after 1980-1990 earthquakes; evidence above 1 J/m3 is too sparse to test

Question: Does earthquake shaking change the discharge of United States streams in rain-free periods, and does the change have a threshold or a dose response? Approach: Earthquake time is independent of weather. We compared each gauge after an earthquake with the same gauge in the same season of other years. We used 94 earthquakes (M >= 5.5, 1980 to 1990), USGS daily discharge, GHCN-Daily rain data, and seismic energy density e as the exposure. A plan and two amendments were written before outcomes were computed. Filters kept 1,276 gauge-event pairs from 22 earthquakes and 711 gauges. Findings: At e >= 0.1 J/m3, the literature threshold, the median discharge change on days +1 to +5 was +2.2 percent (124 pairs, 13 events; placebo p = 0.23). The slope of the median effect against log10 e was -0.009 per decade (95 percent interval -0.113 to +0.054; placebo p = 0.77). For e of 0.01 to 0.1 J/m3 the 95 percent interval was -0.9 to +2.1 percent. For e of 0.1 to 1 J/m3 it was 0.0 to +7.6 percent. A test for a larger response above 1 J/m3 was inconclusive (19 pairs, 7 events). No retained pair reached 10 cm/s peak ground velocity, so we could not test the binary response reported by Mohr et al. (2018). Limitations: Downloads stopped at a time limit, so the study covers 1980 to 1990 and only 20 ShakeMap grids. Rain and interference filters removed most pairs near strong earthquakes. A median test has low power when only a minority of gauges respond. Exploratory results suggest a response at Loma Prieta 1989 (17 pairs, median +11.5 percent), but this is a post-hoc look at one event.

streamflow response to earthquakes · seismic energy density · natural experiment · placebo test · null result · hydrology · NWIS daily discharge · GHCN-Daily · ShakeMap