Previous studies have suggested submarine landslides as sources of the tsunami that damaged coastal areas of Palu Bay after the 2018 Sulawesi earthquake. Indeed, tsunami run-up heights as high as 10 m determined by field surveys cannot be explained by the earthquake source alone although the earthquake is definitely the primary cause of the tsunami. The quantitatively reexamined results using the earthquake fault models reported so far showed that none of them could fully explain the observed tsunami data: tsunami waveforms inferred from video footage and the field survey run-up tsunami height distribution. Here, we present probable tsunami source models including submarine landslides that are consistent with the observed tsunami data. We simulated tsunamis generated by submarine landslides using a simplified depth-averaged two-dimensional model. The estimated submarine landslide model consisted of two sources in the northern and southern parts of the bay, and it explained the observed tsunami data well. Their volumes were 0.02 and 0.07 km 3. The radius of the major axis and the maximum thickness of the initial paraboloid masses and the maximum horizontal velocity of the masses were 0.8 km, 40 m and 21 m/s in the northern bay, and 2.0 km, 15 m and 19 m/s in the southern bay, respectively. The landslide source in the northern bay needed to start to move about 70 s after the earthquake to match the calculated and observed arrival times.

The M w 6.9 earthquake off Fukushima Prefecture, Japan, of 22 November 2016 was followed by a tsunami that struck the Japanese coast from Hokkaido in northern Japan to Wakayama Prefecture in western Japan. We compared the performance of a seismologically deduced single-fault model, a seismologically deduced finite fault slip model (FFM), an optimized single-fault model based on tsunami data, the FFM with horizontal shift, and the tsunami waveform inversion models of the previous studies considered for this earthquake regarding reproduction of tsunami waves by tsunami computations. It is important to discuss how these models work well because it is sometimes desirable to obtain an earthquake source model to estimate tsunami waves with a simple process obtained with limited data from the viewpoint of tsunami prediction. The seismologically deduced FFM has an advantage in terms of the information of slip regions of fault plane and was superior to the seismologically deduced single-fault model, especially in predicting amplitudes of tsunami waves. This means that when only with seismic data, the FFM could narrow the range of forecast of tsunami amplitude. In the comparison of models optimized with tsunami data, the single-fault model showed the almost equivalent performance of the tsunami waveform inversion models of previous studies regarding the waveform coincidence with observations and the horizontal location at the negative peak of the initial sea surface displacement. In case the main generation region of the tsunami is concentrated in one place, the tsunamis can be expressed by a single-fault model by conducting the detailed grid search. We also confirmed that the centroid location of centroid moment tensor (CMT) solution and the absolute location of the FFM were not necessarily suitable to express tsunamis, while the moment magnitude, the focal mechanism, the centroid depth of CMT solution, and the relative slip distribution of the FFM were effective to represent tsunamis. Since this event occurred at the shallow depth, the speed of tsunami wave is particularly slow. Therefore, it would be advisable to pay attention to the horizontal uncertainty to apply seismologically obtained solution to tsunami forecast, especially when a tsunami occurs in shallow water.

Around the Ogasawara Islands, only a few seismic stations in the area can be used to determine the hypocenters of regional earthquakes; thus, hypocenter location precision tends to be low. To more precisely determine hypocenter locations, we deployed a temporary seismic observation network of pop-up ocean bottom seismometers around the Ogasawara Islands from July to October 2015. We identified a double seismic zone in the 70-200 km depth range associated with the subducting Pacific slab. The slab-normal distance between the two planes of the double seismic zone is about 30-35 km, similar to such distances observed along the Japan and Mariana trenches. Furthermore, we found unusual seismicity in the mantle wedge at 20-50 km depth beneath the Ogasawara trough that might be related to structure formed at the onset of the oceanic slab subduction. The hypocenters determined from the ocean bottom seismometer observation were horizontally separated by a few tens of kilometers from hypocenters published by the Seismological Bulletin of Japan. USGS locations (Preliminary Determination of Epicenters) seem to be offset westward about 30 km compared with the locations determined in this study.

To enhance the tsunami warning operation system in the Philippines caused by earthquakes in and around the country, staff members of the Japan Meteorological Agency (JMA) joined the SATREPS program in 2012 to help building a tsunami simulation database in the Philippine Institute of Volcanology and Seismology (PHIVOLCS), which storesmultiple results of tsunami simulations such as estimated tsunami arrival times and heights at coasts for multiple hypothetical earthquakes of various hypocenter locations and magnitudes. The procedure to construct a database consists of several steps starting from setting assumed fault parameters and others, proceeding to tsunami simulations and data creation to be stored in the database, and as the last step, creating a searching system which picks results from the database according to the location and magnitude of an earthquake. As of July 2014, the PHIVOLCS has stored the results of tsunami simulations conducted for more than 30,000 assumed faults for local tsunamis. The searching system is also prepared which enables to get a quick grasp of expected tsunami features quantitatively. With this database and the searching system, the PHIVOLCS is in near future to issue initial tsunami warnings based on the information of estimated tsunami arrival times and heights immediately after the hypocenter location and magnitude of an earthquake are determined. When the necessary coordination with related organizations as well as the public education for the system and warning messages are ready, the PHIVOLCS will start the enhanced local tsunami warning operation.

We used pop-up ocean bottom seismometers deployed in 2013 and 2014 to investigate the southern limit of seismic activity within the Philippine Sea plate south of the Nankai Trough axis off the Kii Peninsula. The hypocenter distribution we determined included microearthquakes with magnitudes lower than 1.5 that were not detected by the land-based seismic network. Hypocentral depths ranged from 5 to 15 km below sea level and there were few earthquakes more than 100 km south of the axis of the Nankai Trough. We therefore infer that the southern limit of microearthquake activity in this region is about 100 km south of the trough axis.