For forecasting of tsunamis, information on the hypocenters such as location and magnitude of earthquakes are essentially required. To determine hypocenters, JMA collects seismic waveform data from global seismological networks such as IRIS/USGS* and IRIS/IDA** through the Internet. The least square method is applied for determining hypocenters with observed arrival time of P and S waves and theoretical arrival time calculated from the IASPEI91 which is used as the travel timetable. Since a depth of an earthquake is one of the determining factors in tsunami generation, JMA utilizes arrival time of various reflected phases such as depth phases (pP, sS, PcP etc) for more reliable hypocenter determination.

Although the Internet is a useful mean for international communications, availability of data is not always secured because of various communication troubles including uncontrollable ones. To avoid the serious consequences of unavailability of data via the Internet, JMA also obtains earthquake parameters from the seismic array system of Matsushiro Seismological Observatory (Nagano, Japan) and from the large aperture array comprised of Japanese seismological observation networks. However, lack of seismic waveform data via the Internet could lead to delayed issuance and affect the accuracy of tsunami forecast.

* IRIS: Incorporated Research Institutions for Seismology, http://www.iris.edu/hq/
   USGS: United States Geological Survey, http://earthquake.usgs.gov/
** IDA: International Deployment of Accelorometers, http://ida.ucsd.edu/