Earthquake cycle in Western Taiwan: Insights from historical seismicity

We attempt at providing new insights about the earthquake behaviour in western Taiwan, based on a comparison between historical information and present-day instrumental records. We provide a consistent picture of the earthquake history during the last four centuries and draw some inferences in terms of seismic cycle. Before instrumental seismic observation in Taiwan started at the end of the 19th century, ancient written earthquake records are available from the ancient Chinese governments and the public concerning the 17th, 18th and 19th centuries. Distribution of casualties and property damage, indicating seismic intensity, can be estimated from archives. Using the Central Weather Bureau (CWB) intensity scale we calibrate the intensity-magnitude relationships from the instrumental seismicity recorded from 1995 to 2005, showing that within the range of historical uncertainties and earthquake depths in western Taiwan, 0-40 km, the depth is not critical in these relationships. With estimated intensities, the magnitudes of historical earthquakes can be evaluated based on a single average empirical relationship between M_L, the local magnitude, and I₀, the epicentral intensity: M_L = 0.08 I₀² - 0.04 I₀ + 3.41. Three types of diagrams are then proposed to describe the historical seismicity. The first type involves simple representation of earthquake events according to time and magnitude. The second type involves cumulative plot of the released elastic energy with time, as calculated from reconstructed magnitudes. The third type of diagram shows the evolution of cumulative seismic strain release with time, based on a Benioff's law indicating that the release of elastic strain related to an earthquake is proportional to the square root of the dissipated energy. These curves highlight inferences of historical seismicity analysis in terms of earthquake frequency and seismic cycle duration in the different segments of the front belt and foreland zones of western Taiwan, with large contrasts suggesting different levels in earthquake hazard.