Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

The U. S. Geological Survey’s Databases of Seismic Events 143Most of the earthquake databases are on-line computer files and associated softwarethat now or can soon be accessed by all users of USGS computers. The NEICdatabase of global earthquakes, for example, is routinely accessed on data telephonesby USGS seismologists in Menlo Park and in Reston, Virginia. It may alsobe possible in the future to make these databases available to non-USGS users, ifproblems of funding, computer availability and computer security can be resolved.In October 1979, the authors began a “National Earthquake Catalog Project”.The goal of this project is to consolidate the various earthquake lists and catalogsinto a uniform US. national earthquake catalog with references and estimates of parameteruncertainties. Work in Menlo Park concentrates on California earthquakesand earthquakes in the rest of the United States are handled in Golden. Thepresent paper briefly describes (1) databases that are developed for this project,and (2) databases that we utilize but are developed by other projects in the U.S.Geological Survey and other institutions.2. Database ContentThe USGS currently has five principal databases. Three of the principal databasescontain computed or assigned parameters from three categories of earthquakes:(1) the local-regional database mostly contains small events recorded byUSGS and cooperating seismograph networks; (2) the national database containsall published hypocenters of earthquakes recorded in or near the U.S.; and (3) theglobal database contains published hypocenters of earthquakes larger than aboutmb 3.0 recorded throughout the world.The above three databases contain the origin time, location, depth, and sourcereference for the included events. Magnitudes, if determined, are also listed. Most ofthe earthquakes have been located with arrival-time data from seismograph stations,but a few thousand events, including nearly all historically important earthquakesbefore 1895, have been located only from published descriptions of damages andintensities. In all, intensity or damage information is available for perhaps 40,000earthquakes from 1177 B.C. to the present.The fourth principal database, and by far the largest, is the archival files. Thesefiles comprise magnetic tapes of digital waveform data recorded by the Global DigitalSeismograph Network (GDSN), and the USGS California Seismic Networks,several magnetic tapes of phase data edited from the International SeismologicalCentre Bulletins for 1964-present1 results of special studies by USGS seismologists,and global lists of fault plane and moment tensor solutions for many earthquakeslarger than mb 5.3 that have occurred since 1977. The waveform data (1980-1984)from the Global Digital Seismograph Network have been edited and reduced fromabout 1825 network-day tapes to 92 event tapes with earthquakes 2 mb 5.5. Althoughmagnetic tapes of digital waveform data from recent earthquakes have beendrastically edited, it is not now feasible to keep these data on line at USGS. Theedited phase data, (which currently totals 200 megabytes and is increasing by about1 megabyte per month), may be placed on-line at the NEIC computer as soon as thenecessary disk storage becomes available. Digital waveform data recorded by theUSGS California Seismic Networks are systematically collected since 1984 at a rateof about 100 megabytes of data per day. These waveform data are kept off-line on