Estimation of Peak Ground Acceleration from Horizontal Rigid Body Displacement: A Case Study in Port-au-Prince, Haiti

Abstract

The M 7.0 Haiti earthquake of 12 January 2010 caused catastrophic da- mage and loss of life in the capital city of Port-au-Prince. The extent of the damage was primarily due to poor construction and high population density. The earthquake was recorded by only a single seismic instrument within Haiti, an educational seis- mometer that was neither bolted to the ground nor able to record strong motion on scale. The severity of near-field mainshock ground motions, in Port-au-Prince and elsewhere, has thus remained unclear. We present a detailed, quantitative analysis of the marks left on a tile floor by an industrial battery rack that was displaced by the earthquake in the Canape Vert neighborhood in the southern Port-au-Prince metropolitan region. Results of this analysis, based on a recently developed formula- tion for predicted rigid body displacement caused by sinusoidal ground acceleration, indicate that mainshock shaking at Canape Vert was approximately 0:5g, correspond- ing to a modified Mercalli intensity of VIII. Combining this result with the weak- motion amplification factor estimated from aftershock recordings at the site as well as a general assessment of macroseismic effects, we estimate the peak acceleration to be ≈0:2g for sites in central Port-au-Prince that experienced relatively moderate damage and where estimated weak-motion site amplification is lower than that at the Canape Vert site. We also analyze a second case of documented rigid body dis- placement, at a location less than 2 km from the Canape Vert site, and estimate the peak acceleration to be approximately 0:4g at this location. Our results illustrate how observations of rigid body horizontal displacement during earthquakes can be used to estimate peak ground acceleration in the absence of instrumental data.