When an earthquake occurs, a certain amount of time elapses before destructive seismic energy hits nearby population centers. Though this time is measured on the order of seconds, depending on the proximity of the rupture to a given city or town, a new public safety program in Japan is taking advantage of the fact that seismic energy travels slower than electronic communication.In this program, the Japan Meteorological Agency (JMA) rapidly determines the hypocenter (earthquake epicenter and focal depth) and magnitude of the earthquake by using real-time data from stations near the hypocenter. The distribution of strong ground shaking is anticipated quickly, and then the information is delivered immediately to government officials, representatives from various industries, members of the news media, and individuals before strong ground shaking reaches them. For example, on receiving the warning, the control room of a railway company can send an emergency notice to all train drivers to stop their trains immediately, elevators in buildings can be triggered to stop at the nearest floor and open their doors automatically, and surgeons can temporarily suspend their surgical operations to avoid risk to patients on operating tables.This innovative new service, called Earthquake Early Warning (EEW), started nationwide in Japan and became fully operational in October 2007. This service is definitely different from earthquake prediction. Although it is currently impossible to be aware of earthquakes before their occurrence (earthquake prediction), EEW operates with the assumption that it is possible to warn people located at a certain distance from the hypocenter before strong ground shaking reaches them.Even though the interval between the delivery of EEWs and the time when strong shaking reaches people is relatively short (counted in seconds), EEWs can be a useful and powerful tool for mitigating an earthquake disaster by giving people enough time to take appropriate safety measures in advance of strong shaking. Determining Hypocentral Parameters and Anticipating Seismic IntensityEarthquakes occur when stressed rock moves through brittle rupture. Two types of seismic waves are radiated from the hypocenter: One is the P wave, which travels at about 7 kilometers per second, and the other is the S wave, which travels at about 4 kilometers per second.EEW technology not only takes advantage of the relatively slow velocity of the seismic waves as compared with instantaneous electronic communication, but it also uses the difference in arrival time between P and S waves. The S wave is slower than the P wave, but the amplitude of the S wave is usually 3-10 times larger than that of the P wave. This generally means that stronger shaking is observed along the S wave.The hypocenter and magnitude of an earthquake are determined as quickly as possible using only early parts of the P waves at a few stations close to the hypocenter. Using information about the hypocenter and magnitude, the arrival time of the S waves and seismic intensit...

Satellite altimetry measurements of sea surface heights for the first-time captured the Indian Ocean tsunami generated from the December 2004 great Sumatra earthquake. Analysis of the sea surface height profile suggests that the tsunami source, or the seafloor deformation, of the great earthquake propagated to the north at an extremely slow speed of less than 1 km/sec on average for the entire 1300-km-long segment along the northern Sumatra-Nicobar-Andaman Trench. The extremely slow propagation speed produces a very long duration of tens minutes, longer than earthquake source duration estimated (480-500 sec) from short-period P-wave radiation. The satellite altimetry data requires a total seismic moment of 9.86 × 10 22 Nm (Mw=9.3). This estimate is approximately 2.5 times larger than the value from long-period surface wave analysis but nearly the same as that from the ultra-long-period normal mode study. The maximum amount of slip (∼30 m) is identified in an offshore region closest to the northern most part of Sumatra where the largest tsunami run-up heights were observed.

The first sign of magma accumulating beneath Miyakejima, an island volcano in the northern Izu islands, Japan, came at around 18:00 on 26 June 2000, when a swarm of earthquakes was detected by a volcano seismic network on the island. Earthquakes occurred initially beneath the southwest flank near the summit and gradually migrated west of the island, where a submarine eruption occurred the next morning. Earthquakes then migrated further to the northwest between Miyakejima and Kozushima, another volcanic island and developed to the most intense earthquake swarm ever observed in and around Japanese archipelago. To better image how the initial magma intrusion occurred, we relocated hypocenters by using a station-correction method and a doubledifference method. The relocated epicenters are generally concentrated near the upper bound of dyke intrusions inferred from geodetic studies throughout the initial stages of the 2000 eruption at Miyakejima from 26 to 27 June 2000. As for seismic activity westward off Miyakejima in the morning on 27 June, hypocenters from both a nationwide seismic network that were relocated by the double-difference method, and those from the volcano seismic network relocated by the station-correction method, formed a very shallow cluster that ascended slowly with time as it propagated northwestward from Miyakejima. This suggests that the dykes have both a radial and upward component of movement.

Permanent ocean bottom observation system including four stations equipped with three seismometer component sets succeeded in recording many T waves. Most of the T waves observed are not associated with seismic body waves. Among several characteristics of T waves studied in this paper, the most prominent feature is a high degree of linear polarization of wave trajectory along the direction of propagation and a prograde elliptic particle motion in the vertical radial plane. This characteristic was applied in the estimation of the source direction of T waves and determination of the orientation of the seismometers. Weak separation of wave train like dispersion, in that high frequency signals precede low frequency signals was also recognized, although no satisfactory explanation was available in this study.An investigation about the source distribution and the observed intensity of T waves supports the assumption that T waves recorded by the OBS must be guided waves propagated in the SOFAR channel. Because T waves are easily interrupted in their propagation by the shallow sea where the SOFAR channel is truncated, a special mode of propagation such as multiply reflected water waves between the water surface and the sea bottom is not plausible for a long range propagation.

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