Contrasting T-Phase and P-Wave Patterns from the 2015 and 2017 Eruptions of the Submarine Volcano Kick-‘em-Jenny: Influence of Cardinal Direction on Recorded First Phase Arrival

Abstract

ABSTRACT Kick-‘em-Jenny (KeJ)—the most active volcano in the Lesser Antilles and located about 8 km north of Grenada—is the only known, live submarine volcano in the region. Unrest and eruption episodes are approximately decadal. Typically, a KeJ episode, which may consist of several eruptions, is short-lived and rapidly returns to quiescence, with virtually no background seismicity. Although two previous eruptions broke the surface of the sea, and some have had seismic effects felt in northern Grenada and sometimes Martinique, instrumental recordings of T phases were considered to provide firm evidence of eruptions. T phases are hydroacoustic waves that propagate efficiently in the Sound Fixing and Ranging channel allowing long-range recording. The most recent episodes occurred on 29 April 2017 and 23, 24 July 2015, with no reported surface evidence. The 2015 hr-long seismic signals, among the longest durations ever recorded, manifested differences in the first phase arrivals at some of the same recording stations suggesting a fundamental difference in T-phase wave trains’ generation at KeJ. The single eruption in the 2017 volcanic episode had a duration similar to those prior to the 2015 eruptions; however, except for the four stations in the immediate vicinity of KeJ, the detected signals propagated only as T waves. Real-time and posteruption deductions that the episodes occurred from different cardinal directions on the volcano were found to be supported by the findings from bathymetric analyses done shortly after the episodes. This characteristic enhances the KeJ profile, possibly improving real-time activity assessments in future KeJ episodes, with a view to strengthening hazard and risk communication.