Nonseismic Short‐Duration Events Offshore Cascadia: Characteristics and Potential Origin

Ocean‐bottom seismometers (OBS) often record nonearthquake signals termed short‐duration events (SDEs) that are typically recorded on one station with durations of ∼1 s and frequencies between 4 and 30 Hz. Their origin has been attributed to animals bumping the sensor and a variety of physical mechanisms, including the passage of methane bubbles through sediments and across the seafloor. We analyzed records from OBSs deployed on the Cascadia margin and Juan de Fuca plate for the occurrence and characteristics of SDEs. SDEs are observed on all OBSs at rates that vary by over two orders of magnitude. The rates vary significantly at any given depth but on average decrease with increasing depth. The rates are low on buried sensors. For some unburied sensors, the rates are dependent on diurnal and/or tidal cycles. Strongly diurnally dependent rates for several OBSs on the continental shelf and upper slope with more SDEs during the day, are evidence of a biological mechanism for some SDEs. The presence of SDEs on buried sensors requires a physical mechanism for others. Because the decrease of SDE rates with depth is broadly consistent with both the expected variation in benthic animal densities and the emission of methane bubbles from the seafloor, it is not possible to infer the relative importance of biological and physical mechanisms. Experiments with cameras observing the sensor could be supported by cabled observatories and would provide a means to address this issue.