On the mechanism of conversion of seismic waves to and from T waves in the vicinity of island shores

High-frequency seismic records from the Polynesian Seismic Network are used to investigate in detail the processes of conversion of acoustic (T-wave) energy from and to seismic waves at island shores. On the source side, we study the seismic-to-acoustic conversion based on T phases from Hawaiian events recorded at Polynesian stations located on the coral platter; on the receiver side, we study the acoustic-to-seismic conversion based on T phases from marine sources recorded across the Polynesian islands. In both instances, our results underline the importance of steep slopes (typically 50°) in allowing an efficient conversion between P waves in the island structure and T waves in the water column. These slopes can be coral reefs or the heads of young, presumably unconsolidated, basalt flows. Under this geometry, modeling based on raytracing indicates that the seismic record of the T phase consists of a P wave at distances from the conversion point greater than 9 km; at shorter distances, S waves and surface waves are generated. In the absence of a steep slope, only surface waves are present. These models can be used to compute precise and predictable travel-time station corrections, allowing the use of seismometers located inland to exploit the superb detection and location capabilities of T waves.