Satoko Murotani

Fault slip distribution of the 2014 Iquique, Chile, earthquake estimated from ocean‐wide tsunami waveforms and GPS data

We applied a new method to compute tsunami Greens functions for slip inversion of the 1 April 2014 Iquique earthquake using both near-field and far-field tsunami waveforms. Inclusion of the effects of the elastic loading of seafloor, compressibility of seawater, and the geopotential variation in the computed Greens functions reproduced the tsunami traveltime delay relative to long-wave simulation and allowed us to use far-field records in tsunami waveform inversion. Multiple time window inversion was applied to tsunami waveforms iteratively until the result resembles the stable moment rate function from teleseismic inversion. We also used GPS data for a joint inversion of tsunami waveforms and coseismic crustal deformation. The major slip region with a size of 100 km × 40 km is located downdip the epicenter at depth ~28 km, regardless of assumed rupture velocities. The total seismic moment estimated from the slip distribution is 1.24 × 1021 N m (Mw 8.0).

Scaling of characterized slip models for plate-boundary earthquakes

We characterized source rupture models with heterogeneous slip of plate-boundary earthquakes in the Japan region. The slip models are inferred from strong-motion, teleseismic, geodetic, or tsunami records. For the identification of asperities in the slip models, we found that the area of subfaults retrieved with slips of >1.5 times the total average slip provides a size approximately equivalent to the characterized asperity by Somerville et al. (1999). We then carried out regression analyses of the size and slip for the rupture area and asperity. The obtained scaling relationship to the seismic moment indicates that rupture area S, average slip D, and combined area of asperities Sa are 1.4, 0.4, and 1.2 times larger, respectively, than those of crustal earthquakes. In contrast, the ratios of the size and slip between the asperities and rupture area (Sa/S and Da′/D) are the same for plateboundary earthquakes as for crustal earthquakes. The above analyses indicate that plate-boundary and crustal earthquakes share similar source characteristics.

Short-term spatiotemporal variations in the aftershock sequence of the 2004 mid-Niigata prefecture earthquake

We deployed 56 temporary seismic stations within approximately a month after the occurrence of the 2004 mid-Niigata prefecture earthquake. Using manually-picked arrival data obtained from the temporary and surrounding permanent seismic stations, 1056 aftershocks have been relocated. Based on the spatiotemporal variations in the relocated aftershocks, the cluster activities associated with the mainshock and some large aftershock events are identified. The aftershocks associated with the mainshock, the largest occurred on the two steep west-dipping planes at an angle of 60° and approximately 5 km away. In contrast, the aftershocks following the event on Oct. 27 are aligned on east-dipping plane at a low angle of 25°. It is further observed that the aftershock area extended in both northeastward and southwestward directions at a later stage. The triggered seismicity around the northeast edge was more significant than that around the southwest edge. This difference could be understood by the discrepancy in the shear stress level accumulated at the dynamic shear rupture due to the mainshock.

Fault size and depth extent of the Ecuador earthquake (M w 7.8) of 16 April 2016 from teleseismic and tsunami data

The April 2016 Ecuador Mw 7.8 earthquake was the first megathrust tsunamigenic earthquake along the Ecuador-Colombia subduction zone since 1979 (Mw 8.2 with 200 deaths from tsunami). While there was no tsunami damage from the 2016 earthquake, small tsunamis were recorded at Deep-ocean Assessment and Reporting of Tsunami and tide gauges. Here we designed various fault models with and without shallow-slip area and compared the computed teleseismic and tsunami waveforms with the observations. While teleseismic inversions were indifferent about inclusion or exclusion of the shallow slip, tsunami waveforms strongly favored the slip model without shallow slip. Our final slip model has a depth range of 15–44 km, and its western shallowest limit is located at the distance of ~60 km from the trench. Maximum and average slips were 2.5 and 0.7 m, respectively. The large-slip area was 80 km (along strike) × 60 km (along dip) in the depth range of 15–35 km.

Rupture process of the 1946 Nankai earthquake estimated using seismic waveforms and geodetic data

The rupture process of the 1946 Nankai earthquake (MJMA 8.0) was estimated using seismic waveforms from teleseismic and strong motion stations together with geodetic data from leveling surveys and tide gauges. The results of joint inversion analysis showed that two areas with large slip are more confined than in previous studies. In our inversion, we assumed spatially varying strike and dip angles and depth of each subfault by fitting those to the actual complex shape of the upper surface of the Philippine Sea plate in the Nankai Trough region. As a result, we calculated the total seismic moment, M0 = 5.5 × 1021 Nm; the moment magnitude, Mw = 8.4; and a maximum slip of 5.1 m, occurring at a point south of Cape Muroto. The estimated slip distribution on the west side of the fault plane appears somewhat complicated, but it explains well the vertical deformations at Tosashimizu and in the vicinity of Inomisaki. Arguments have been made that the westernmost part slipped slowly after the earthquake over a period of days or months as an afterslip because the seismic waveforms can be largely explained without the slip in this part. However, in order to explain the displacement recorded by the tide gauge at Tosashimizu, we conclude that the westernmost part slipped simultaneously with the earthquake. Splay faulting, which was suggested in previous studies, is not required in our model to explain the seismic waveforms and geodetic data.

Source model of the 16 September 2015 Illapel, Chile,Mw8.4 earthquake based on teleseismic and tsunami data: ILLAPEL (CHILE) TSUNAMI OF 16 SEPTEMBER 2015

Teleseismic data were provided by the Incorporated Research Institutions for Seismology (http://www.iris.edu/wilber3/find_event). Tide gauge data can be found at the Intergovernmental Oceanographic Commission website (http://www.ioc-sealevelmonitoring.org/). DART records were provided by NOAA (http://nctr.pmel.noaa.gov/Dart/). Earthquake catalogs by the USGS National Earthquake Information Center (http://earthquake.usgs.gov/earthquakes/search/) and Global Instrumental Earthquake Catalogue (1900-2009) of International Seismological Centre Global Earthquake Model (http://www.globalquakemodel.org/what/seismic-hazard/instrumental-catalogue/) were used in this study. We used the GMT software for drawing the figures [Wessel and Smith, 1998]. This article benefited from constructive review comments by Costas E. Synolakis (University of Southern California, USA) and Yuichiro Tanioka (Hokkaido University, Japan) for which we are grateful. We acknowledge financial supports from the Japan Society for the Promotion of Science.