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

290 R. G. Valenzuela and L. C. GarciaThe first volume contains a listing of seismic events from 1589 to 1864. Thisperiod covers the years when there was yet no organized seismological service inthe archipelago. Earthquake records were in various formats, but nevertheless,informative and useful.The second volume contains data from 1865 to 1900. The year 1865 marked thebeginning of seismology in the Philippines. During this period and up to 1906,a local intensity scale of I-VI was used. It is notable that in 1884, the ManilaObservatory was subsidized by the Spanish government as the Central WeatherBureau of the Philippines, which included seismological work.The third volume covers events from 1901 to 1942. In 1901, the Manila Observatory,while continuing to function as the Weather Bureau was re-organizedunder the American Regime. From this year onward, more accurate observationswere made with the addition of better instruments. Reports were classified intoinstrumental and non-instrumental bulletins published periodically. The originalRossi-Fore1 Intensity Scale replaced the local intensity scale. Repetti revised theRossi-Fore1 to a scale of I to IX in 1934. As a consequence of the Japanese invasion,the Manila Observatory ceased its function a8 a government institution in 1942, butcontinued its operation by the Jesuits.The fourth volume covers events from 1948 to 1983. Fkom the restoration ofthe Weather Bureau after the war, information on only one earthquake was foundfor 1948. Instrumental records in other countries between 1942 and 1947, however,showed earthquakes occurred during this period.The present compilation of earthquake data cannot be considered complete.There are possibilities of finding data in Spain, e.g. in the National Library atMadrid and the Archivo de Indios (Archive of the Indies) in Sevilla, and in othercountries which have had relations, in one way or another, with the Philippines.3. Destructive EarthquakesFrom the list of events of the nearly 400 years of written seismic history, a catalogueof 63 destructive earthquakes was prepared. This includes records which givevery long narratives of damages on natural and man-made structures.Forty of these destructive events are listed in Table 1. Each event has a ModifiedMercalli intensity re-assessed macroseismic descriptions. Figure 1 shows the epicentersof these listed events. Many of these epicenters were determined only from theisoseismal maps. Others were obtained from the work of Lomnitz (1974).4. Preparation of Isoseismal MapsEarthquake intensities in given localities were evaluated using the 1956 versionof the Modified Mercalli Intensity Scale for the purpose of drawing isoseismal maps.Maso (1895) gave the original local scale in use prior to the Rossi-Fore1 IntensityScale. Su (1985) suggested the correlation of the adapted Rossi-Fore1 Scale and theModified Mercalli Scale. This is shown in Table 2.In the process of evaluating intensities and preparation of isoseismal maps, somedifficulties were encountered such as: (a) Inapplicability of the descriptions to localstructures - both Rossi-Fore1 and Modified Mercalli scales were developed in foreigncountries where the engineering structures axe different to conditions in the Philippines.(b) Assessing earthquakes based only on reported intensities - the levels of