Tsunami source of the 2004 off the Kii Peninsula earthquakes inferred from offshore tsunami and coastal tide gauges

Tsunamis from the 2004 off the Kii Peninsula earthquakes (M 7.1 and 7.4) were recorded on offshore tsunami gauges, a GPS tsunami gauge and eight bottom-pressure gauges, as well as coastal tide gauges located south of Honshu and Shikoku. The maximum amplitudes on the GPS and bottom-pressure gauges were several to ten cm, while those on tide gauges were up to 0.9 m. We first computed tsunami waveforms from the earthquake source models proposed Yamanaka (2004) and Yagi (2004) from seismic waveform analysis, and compared them with the observed waveforms. For the first event (foreshock), both models produce similar waveforms with the observations. For the second event (mainshock), the waveforms computed from the Yamanaka model is closer to the observed waveforms, but there are still discrepancies between the observed and computed waveforms. We then performed tsunami waveform inversions to estimate the water height distributions in the source area. The foreshock source is ∼1600 km² with the maximum water height of 0.2 m. The estimated tsunami source area for the mainshock, ∼3600 km² with the maximum of 0.6 m, extends ∼60 km toward northwest and ∼40 km southwest from the epicenter along the aftershock distribution, suggesting that multiple faulting was involved in the mainshock.