Amplitude-duration and other discriminants for seismically recorded hydroacoustic phases

J. Talandier and E.A. Okal

We expand to a considerably enlarged dataset the investigation of the duration-amplitude criterion D introduced by Talandier and Okal [2001] for the purpose of discriminating between earthquakes (D < 0) and explosions (D > 0) as sources of T phases recorded at teleseismic distances by seismic T-phase stations. In the case of earthquakes, we confirm that "hotspot earthquakes" taking place in intraplate volcanic edifices (Hawaii) can feature D > 0 when located close to a steep conversion slope. By contrast, all subduction and transform events have D < 0, as do genuine intraplate ("abyssal") earthquakes. For subduction events, we further show that D correlates well with parameters characterizing the slowness of the source, such as the energy-to-moment ratio, and the T-Phase-Energy-Flux-to-moment ratio. We tentatively reproduce, on an independent dataset, the observation by Dziak [2001] of preferential excitation of T waves by strike-slip events, at least at small magnitudes. We also show that the 1998 tsunamigenic PNG underwater landslide had a significantly deficient D with respect to the mainshock and main aftershocks. An enlarged dataset of 77 documented underwater explosions fails to produce a single negative D. In a pattern reminiscent of hotspot earthquakes, underground explosions can feature positive or negative D. We further discuss advanced discrimination criteria, such as the exponent of the power-law fall-off of the ground velocity spectrum of the T phase, found to be much higher (in absolute value) for earthquakes (including hotspot events) than for explosions, the goodness-of-fit of such laws (found to be high only for earthquakes), and the dispersive nature of the spectrum at the lowest SOFAR frequencies, found to be characteristic of explosions. The combination of the duration-amplitude discriminant D and of the advance criteria allows the correct identification of the nature of practically all sources in our dataset. We discuss the case of a presumed underwater landslide and of a volcanic explosion, both of which occurred near the shores of the Big Island of Hawaii in December 2002.