From Source to Damage: A Case study of the M7.2 October 2013 Bohol Earthquake

Abstract:

The island of Bohol, in central Visayas, Philippines, endured a devastating earthquake of magnitude 7.2 in the morning of October 15, 2013. This inland earthquake occurred as a result of movement along a previously unknown thrust fault now called the North Bohol Fault. Surface rupture, coastal uplift and analysis of SAR data all indicate surface deformation spread over a rupture area extending along the 60 km length of the island of Bohol. In contrast, the NEIC finite fault solution, shows slip concentrated in a tight asperity offshore and to the SW of the island. About 230 people were killed by the earthquake and over 70,000 buildings damaged. Because of the extensive damage and the wide spread of intensities inferred to have shaken the island, the Bohol earthquake presents an important opportunity to improve knowledge of building fragility for the Philippines and similar countries. However, this requires: (1) a detailed source model for accurate estimation of earthquake ground motion, and (2) a reliable statistical description of building damage. To this end we developed an improved source model through joint inversion of seismic and SAR data, as well as empirical Green's functions. We also undertook a statistical building survey of damaged as well as undamaged buildings, collecting data for over 18,000 structures in rural and urban districts spread throughout the island. These data have been correlated with the intensities derived from different finite source models. We show how this allows us to place observational constraints on building fragility functions, thereby improving earthquake risk and impact forecasts for future earthquakes.