Seismometer Records of Ground Tilt Induced by Debris Flows

Abstract

ABSTRACT A change in surface loading causes the Earth’s surface to deform. Mass movements, such as debris flows, can cause a tilt large enough to be recorded by nearby instruments, but the signal is strongly dependent on the mass loading and subsurface parameters. Specifically designed sensors for such measurements (tiltmeters) are cumbersome to install. Alternatively, broadband seismometers record translational motion and also tilt signals, often at periods of tens to hundreds of seconds. Their horizontal components are thereby the most sensitive to tilt. In this study, we show how to obtain tilt caused by the passing by of debris flows from seismic measurements recorded within tens of meters of the flow and investigate the usefulness of this signal for flow characterization. We investigate the problem on three scales (1) large-scale laboratory experiments at the U.S. Geological Survey debris-flow flume, where broadband seismometers and tiltmeters were installed for six 8–10  m3 experiments, (2) the Illgraben torrent in Switzerland, one of the most active mass wasting sites in the European Alps, where a broadband seismometer placed within a few meters of the channel recorded 15 debris-flow events with volumes up to 105  m3, and (3) Volcán de Fuego, Guatemala, where a broadband seismometer recorded two lahars. We investigate how the tilt signals compare to debris-flow parameters such as mean normal stresses, usually measured by expensive force plates, and debris-flow height. We model the elastic ground deformation as the response of an elastic half-space to a moving surface load. In addition, we use the model with some simplifications to determine the maximum debris-flow heights of Volcán de Fuego events, where no force plate measurements are available. Finally, we address how and under what assumptions the relatively affordable and straightforward tilt measurements may be utilized to infer debris-flow parameters, as opposed to force plates and other complicated instrument setups.