Mechanism of the 1975 Kalapana, Hawaii, earthquake inferred from tsunami data

We investigated the source mechanism of the 1975 Kalapana, Hawaii, earthqua ke (M-S = 7.2) by modeling the tsunamis observed at three tide-gauge statio ns, Hilo, Kahului, and Honolulu. We computed synthetic tsunamis for various fault models. The arrival times and the amplitudes of the synthetic tsunam is computed for Ando's fault model (fault length = 40 km, fault width = 20 km, strike = N70 degrees E, dip = 20 degrees SE, rake = -90 degrees, fault depth = 10 km, and slip = 5.6 m) are similar to 10 min earlier and 5 times smaller than those of the observed, respectively. We tested fault models wi th different dip angles and depths. Models with a northwest dip direction y ield larger tsunami amplitudes than those with a southeast dip direction. M odels with shallower fault depths produce later first arrivals than deeper models. We also considered the effects of the Hilina fault system, but its contribution to tsunami excitation is insignificant. This suggests that ano ther mechanism is required to explain the tsunamis. One plausible model is a propagating slump model with a 1 m subsidence along the coast and a 1 m u plift offshore. This model can explain the arrival times and the amplitudes of the observed tsunamis satisfactorily. An alternative model is a wider f ault model that dips 10 degrees NW, with its fault plane extending 25 km of fshore, well beyond the aftershock area of the Kalapana earthquake. These t wo models produce a similar uplift pattern offshore and, kinematically, are indistinguishable as far as tsunami excitation is concerned. The total vol ume of displaced water is estimated to be similar to 2.5 km(3). From the co mparison of slump model and the single-force model suggested earlier from s eismological data we prefer a combination of faulting and large-scale slump ing on the south flank of Kilauea volcano as the most appropriate model for the 1975 Kalapana earthquake. Two basic mechanisms have been presented for explaining the deformation of the south flank of Kilauea: (1) pressure and density variation along the rift zone caused by magma injection and (2) gr avitational instability due to the steep topography of the south flank of K ilauea. In either mechanism, large displacements on the south flank are inv olved that are responsible for the observed large tsunamis.