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

144 J. N. Taggart, W. H. K. Lee, and K. L. Meagher6250 BPI magnetic tapes, but can be loaded back on the computer. The associatedphase data are collected at a rate of about 0.5 megabyte of data per day, and arekept on-line in the USGS Menlo Park computer.The fifth principal database contains non-instrument a1 descriptions of Californiaearthquakes which are compiled from existing literature. Information are extractedand arranged chronologically by earthquake dates. This effort is very similar tothat described by Usami (1985) and Xie (1985), except that in our case, we put thedata (mostly text descriptions) on computer for easier access, cross checking, andre- arrangement.3. Database FormatThe systematic collection of seismic events for the databases began with olderevent lists (pre-1964), which were verified against published values and correctedwhere necessary. Events after 1963 did not have to be verified or corrected becausethese data were calculated on digital computers and stored on magnetic tape essentiallyfree of transcription errors. The event lists were then reformatted to thatof one or more database files, and, finally, these files were merged into operationaldatabases.Individual records in all of the Geological Survey’s database files consist of ASCIIcodedcharacter strings, which require very little decoding time on the computers.Although records could have been stored as strings of binary constants, which wouldrequire less than half the space of the character strings, they would have requiredmuch more decoding time.Figure 1 shows records of the same earthquake in three of the primary databaseformats. The local-regional databases are on-line files with fixed record lengths of80 characters (figure la) that accommodate output from the HYP071, HYPOEL-LIPSE, HYPOINVERSE and related computational hypocenter programs. Abouthalf of the data in these databases comprise confidence ellipsoid parameters andsemi-quantitative estimates of the quality of the hypocenters.The global database (figure lb) is on-line indexed files with fixed record lengths of100 characters. The national database (figure lc) contains on-line primary indexedfiles with fixed record lengths of 168 characters. The national database also containssecondary files with intensity (36 characters), additional magnitude (84 characters),and hypocenter statistics (80 characters) data for some events, which are indexed asin the primary file. The date-origin-time indices in the global and national databaseshelp the user to begin at any event in the files. Thereafter, the records are checkedsequentially to determine whether various selection criteria have been matched.Figure 2 shows the selection criteria for events in the global and national databases.The geographic search modes are mutually exclusive, and with two exceptionsthe event parameters must match all of the specified options for an event to beselected. The first exception to this requirement is that events without recordeddepths will be included if a range of shallow depths is selected. The second exceptionis that events without magnitudes that occurred before 1963 will be included whena range of magnitudes less than M 6.0 is selected.