Hybrid numerical modelling ofT-wave propagation:application to the Midplate experiment

Abstract

Summary A hybrid method, coupling a ray tracing method and a finite difference approach, is proposed for modelling T‐wave propagation from an underwater source to an on‐land seismic station. The long‐range hydroacoustic wavefield, estimated in the SOFAR channel by the Maslov approach, shows many triplications of propagation with an increasing number of caustics as the range increases. Ray tracing approaches lead to a straightforward analysis of the SOFAR propagation: we find that the duration and the amplitude of the hydroacoustic T waves generated by a source close to the SOFAR axis may be respectively eight times longer and almost seven times higher than the duration and the amplitude of hydroacoustic T waves generated by a source close to the SOFAR limits. The finite difference modelling handles the complex hydroacoustic–seismi c T‐wave conversion on atoll shores with an illustration of seismic T waves recorded during the Midplate experiment in 1989. Two different seismic stations, FGA on the Fangataufa Atoll and DIN on the Mururoa Atoll, both in French Polynesia, have recorded the seismic T waves due to an underwater chemical blast at a distance greater than 900 km. Synthetic seismograms computed by our proposed hybrid method are close enough to the real data for quantitative interpretation. We believe that the input model structure is accurate enough to allow such analysis of the seismic T waves. The numerical simulation shows that the seismic T waves recorded at both stations are mainly composed of P phases and Rayleigh phases. The simulation shows that the seismic T‐wave duration is often linked to the source depth, although other factors (the continental slope or the distance between the top of the continental slope and the seismic station) may also affect the signal duration.

Identification Criteria for Sources of T Waves Recorded in French Polynesia

“…(OKAL and TALANDIER, 1986). Finally, the conversion slope will often be shallowdipping, which requires a complex conversion mechanism involving several reverberations in the water before entrapment by the SOFAR channel can take place, hence the emergent nature of the wavetrain (TALANDIER and OKAL, 1998;PISERCHIA et al, 1998).…”

T‐waves excited by S‐waves and oscillated within the ocean above the Southeastern Taiwan Forearc

“…Thus, it may be reasonable to assume that in thier study the T-waves were converted from the P-waves. Briefly put, T-waves trapped within the SOFAR channel require a small incident angle on the horizontal for Pwaves (Ewing and Worzel, 1948;Piserchia et al, 1998), while T-waves can be converted from S-waves when the incidence angle of the S-waves is nearly vertical.…”

Operational Processing of Hydroacoustics at the Prototype International Data Center

“…It is expected that they will record signals that have primarily traveled through the oceans, converting into elastic waves at the ocean island interface. The performance of T-phase stations is not well known, particularly at high frequencies (>10 Hz) (e.g., TALANDIER and OKAL, 1998;PISERCHIA et al, 1998;STEVENS et al, 1998;HANSON et al, 1997). They were conceived as a costsaving measure because hydroacoustic stations are very expensive, largely due to the The planned IMS network has 6 hydrophone stations and 5 T-phase stations.…”

“…For example, an earthquake generated signal which propagates through the ocean and is then recorded on a T-phase station might have been called``PTP''. The problem with this approach is that the mode of propagation on land is not known and probably includes compressional, shear and surface type waves (PISERCHIA et al, 1998;OKAL and TALANDIER, 1997;STEVENS et al, 1998). It would be similar to separating the regional seismic Lg phase into its various components.…”

“…However, due to the scarcity of in-water explosions, the identi®cation of H phases relies mostly on features extracted from broadband timeseries simulations. These are generated using the normal-mode model KRAKEN modi®ed by an interpolation scheme that makes broadband time series simulation a practical alternative (PORTER and REISS, 1985;LANEY, 1994).…”