ALBERT

Description: A multiple time window inversion of 53 high-sampling tsunami waveforms on ocean-bottom pressure, Global Positioning System, coastal wave, and tide gauges shows a temporal and spatial slip distribution during the 2011 Tohoku earthquake. The fault rupture started near the hypocenter and propagated into both deep and shallow parts of the plate interface. A very large slip (approximately 25 m) in the deep part off Miyagi at a location similar to the previous 869 Jogan earthquake model was responsible for the initial rise of tsunami waveforms and the recorded tsunami inundation in the Sendai and Ishinomaki plains. A huge slip, up to 69 m, occurred in the shallow part near the trench axis 3 min after the rupture initiation. This delayed shallow rupture extended for 400 km with more than a 10-m slip, at a location similar to the 1896 Sanriku tsunami earthquake, and was responsible for the peak amplitudes of the tsunami waveforms and the maximum tsunami heights measured on the northern Sanriku coast, 100 km north of the largest slip. The average slip on the entire fault was 9.5 m, and the total seismic moment was 4.2X10 (super 22) N.m (M (sub w) 9.0). The large horizontal displacement of seafloor slope was responsible for 20%-40% of tsunami amplitudes. The 2011 deep slip alone could reproduce the distribution of the 869 tsunami deposits, indicating that the 869 Jogan earthquake source could be similar to the 2011 earthquake, at least in the deep-plate interface. The large tsunami at the Fukushima nuclear power station is due to either the combination of a deep and shallow slip or a triggering of a shallow slip by a deep slip, which was not accounted for in the previous tsunami-hazard assessments.

Description: The 11 March 2011 Tohoku earthquake (05:46:24 UTC) involved a massive rupture of the plate‐boundary fault along which the Pacific plate thrusts under northeastern Honshu, Japan. It was the fourth‐largest recorded earthquake, with seismic‐moment estimates of 3–5×1022  N·m (Mw 9.0). The event produced widespread strong ground shaking in northern Honshu; in some locations ground accelerations exceeded Embedded Image. Rupture extended ∼200  km along dip, spanning the entire width of the seismogenic zone from the Japan trench to below the Honshu coastline, and the aftershock‐zone length extended ∼500  km along strike of the subduction zone. The average fault slip over the entire rupture area was ∼10  m, but some estimates indicate ∼25  m of slip located around the hypocentral region and extraordinary slip of up to 60–80 m in the shallow megathrust extending to the trench. The faulting‐generated seafloor deformation produced a devastating tsunami that resulted in 5–10‐km inundation of the coastal plains, runup of up to 40 m along the Sanriku coastline, and catastrophic failure of the backup power systems at the Fukushima Daiichi nuclear power station, which precipitated a reactor meltdown and radiation release. About 18,131 lives appear to have been lost, 2829 people are still missing, and 6194 people were injured (as reported 28 September 2012 by the Fire and Disaster Management Agency of Japan) and over a half million were displaced, mainly due to the tsunami impact on coastal towns, where tsunami heights significantly exceeded harbor tsunami walls and coastal berms. The 2011 Tohoku event is the largest earthquake known to have struck Japan, a country having a very long documented earthquake history. Seismic‐hazard assessments based on instrumentally observed and historically documented events led most researchers to expect earthquakes no larger than about M 8.5 in the region, thus the Mw 9.0 event and huge tsunami caught many by surprise. Large …

Description: Along the northern Kuril trench, a great interplate earthquake occurred in November 2006 followed by an outer-rise normal fault event in January 2007. Surface wave magnitudes (M (sub S) 7.8 and 8.2) indicate that the 2007 event was larger, while the Global CMT solutions (M (sub w) 8.3 and 8.1) indicate that the 2006 event was larger. Tsunamis from both events were recorded at tide gauge stations in Japan, Russia, and the United States, as well as deep-ocean assessment and reporting of tsunamis (DART) systems and cabled tsunami sensors installed on deep-ocean bottom. Inversion of 52 tsunami waveforms indicates that the 2006 tsunami source was about 200 km long, extending from the epicenter to northeast. The largest slips of 4-7 m are estimated on the northeastern subfaults. For the 2007 event, inversion of 32 waveforms shows that the tsunami source was about 120 km long with the largest slip of 3.5 m on the northeastern subfault. The agreements between the observed and synthetic tsunami waveforms are generally good for both events, not only within the inversion time windows but also for the later phases, which were not used in inversions. The slip distributions yield the seismic moment of Formula (M (sub w) 8.1) for the 2006 event and Formula (M (sub w) 7.9) for the 2007 event. The seismic moment of the 2006 event was larger than the 2007 event, from comparison of tsunami data, regardless of fault model.

Description: Tsunami source of the 2004 Sumatra-Andaman earthquake was estimated from a joint inversion of tsunami waveforms recorded on tide gauges and sea surface heights captured by satellite altimetry measurements. The earthquake, the largest in the past 40 years, caused devastating tsunami damage to countries around the Indian Ocean, but the tsunami source, in particular, its northern end, was not well resolved. Although aftershocks and crustal deformation extended from off northwestern Sumatra Island through the Nicobar Islands to the Andaman Islands, some seismic-wave analyses indicated a shorter source length, several hundred kilometers. We used tsunami waveforms recorded at 12 tide gauge stations around the source and the sea surface heights measured by three satellites: Jason1, TOPEX/Poseidon, and Envisat. We numerically computed tsunami propagation using realistic bathymetry; more than 66,000 depth points were digitized from nautical charts and combined with the ETOPO2 data. Inversion of satellite data indicates that the tsunami source extended to the Andaman Islands with a total length of 1,400 km, but such a model produces much larger tsunami waveforms than observed at Indian tide gauge stations. Inversion of tide gauge records and the joint inversion indicate that the tsunami source was about 900 km long. The largest slip, about 13 to 25 m, was located off Sumatra Island and the second largest slip, up to 7 m, near the Nicobar Islands. The slip distribution is similar for different rupture velocities and rise times, with a slow velocity of 1 km/sec and a rise time of 3 min yielding the largest variance reduction.

Description: The 2004 Indian Ocean tsunami reached Syowa Station, Antarctica, approximately 12.5 hr after the December Sumatra-Andaman earthquake. We have analyzed the tsunami signals recorded on ocean-bottom pressure gauges, broadband seismometers (STS-1), and a superconducting gravimeter (SG). We calculated the sea level variation, tilt, and gravity changes induced by the tsunami and compared these results to observations. From this comparison we confirmed the loading and gravity effects of the tsunamis on the STS-1 (horizontal components) and the SG records at Syowa Station. The magnitudes of these effects given as root mean square amplitudes are as follows: for the tilt effects obtained from 20-hr-long STS-1 records at frequencies in the range 0.3-0.6 mHz, 5 and 8 mu Gal (10 (super -8) m/sec (super 2) ) in the east-west and north-south directions, respectively; and for the gravity effect obtained from the SG records for the same time period of 20 hr at frequencies in the range 0.1-0.2 mHz, 0.2 mu Gal. By using detailed bathymetry around Syowa Station, the synthetic amplitudes similar to the observed were obtained, although the waveforms of synthetic and observation are not always consistent.