Estimation of the magnitudes and epicenters of Philippine historical earthquakes

Abstract

The magnitudes and epicenters of Philippine earthquakes from 1589 to 1895 are estimated based on the review, evaluation and interpretation of historical accounts and descriptions. The first step involves the determination of magnitude–felt area relations for the Philippines for use in the magnitude estimation. Data used were the earthquake reports of 86, recent, shallow events with well-described effects and known magnitude values. Intensities are assigned according to the modified Mercalli intensity scale of I to XII. The areas enclosed by Intensities III to IX [ A(III) to A(IX)] are measured and related to magnitude values. The most robust relations are found for magnitudes relating to A(VI), A(VII), A(VIII) and A(IX). Historical earthquake data are obtained from primary sources in libraries in the Philippines and Spain. Most of these accounts were made by Spanish priests and officials stationed in the Philippines during the 15th to 19th centuries. More than 3000 events are catalogued, interpreted and their intensities determined by considering the possible effects of local site conditions, type of construction and the number and locations of existing towns to assess completeness of reporting. Of these events, 485 earthquakes with the largest number of accounts or with at least a minimum report of damage are selected. The historical epicenters are estimated based on the resulting generalized isoseismal maps augmented by information on recent seismicity and location of known tectonic structures. Their magnitudes are estimated by using the previously determined magnitude–felt area equations for recent events. Although historical epicenters are mostly found to lie on known tectonic structures, a few, however, are found to lie along structures that show not much activity during the instrumented period. A comparison of the magnitude distributions of historical and recent events showed that only the period 1850 to 1900 may be considered well-reported in terms of magnitude distribution. Each earthquake is evaluated for its ‘quality’ of determination based on the number of intensity reports. Earlier than 1850, the data collected are few and most earthquakes had fewer than ten reports. Good quality reports began to be collected from 1850, partly correlative to an increase in the number of towns and partly to the start of a systematized collection of earthquake accounts by the Manila Observatory. Parameters of these well-reported earthquakes may be used for conducting various seismological studies. Examples of how the parameters of poorly reported events were arrived at are also discussed.