Seismic Microzonation of Port-au-Prince: Synthesis

The final synthesis report, dated December 2013, of the seismic microzonation of Port-au-Prince: sector 1 of a national microzonation program that the BRGM, the BME and the LNBTP proposed after the 12 January 2010 earthquake, which identified seven priority sectors. It covers about 80 square kilometers of urban Port-au-Prince across the communes of Port-au-Prince, Tabarre, Delmas, Pétion-Ville and Cité Soleil, carried out under a research contract for MTPTC on Haitian state funds through MPCE, with CNIGS collaboration and data from the State University of Haiti.

The report is written for the people who will use the maps rather than for specialists in the method. Chapter 1 gives the context, the report structure and the composition of the microzonation team; chapter 2 defines what a seismic microzonation is and what it can be used for, distinguishing regional from local hazard, lithological site effects, topographic site effects, surface rupture on an active fault, soil liquefaction and landslides; chapter 3 sets out the project, its objectives and the general methodology; chapter 4 synthesizes the data collected and the specific investigations that had to be carried out; chapter 5 presents the results and maps for each hazard, with the precise definitions of what is represented, the limits of validity, the precautions of use and recommendations for construction and planning; chapter 6 gives general recommendations on use.

The central result is that seismic hazard is high everywhere in the agglomeration, whatever the site, but the expected effects differ by sector. Regional hazard is higher toward Pétion-Ville than toward Cité Soleil because of the greater proximity of the active Pétion-Ville-Tiburon fault system. The broken topography of the south carries strong landslide hazard over wide areas, with topographic site effects on crests, slope edges and steep gradients, and the geological mapping also found recent faults there whose activity and mapping remain to be established. On the flatter coastal terrain and the Cul-de-Sac plain the dominant effects are lithological instead: poorer foundation soils, high liquefaction hazard and amplification of shaking through lithological site effects.

Mapping is at 1/25,000 across the study area with 1/10,000 zooms on the town centers of Port-au-Prince and Pétion-Ville. Response spectra are computed at 5 percent damping for a return period of 2,475 years, corresponding to a 2 percent probability of occurrence in 50 years, with soil classes keyed to mean VS30. Soil bearing capacity is classified in bands from weak to moderate through moderate to good, and the report notes that weak-to-moderate bearing soils should lead to special foundations. Fault traces and activity rates are described as preliminary, with average fault lengths of 15 to 20 km. Rockfall is put at under 10 percent of the territory, and slope instability is concentrated on low-gradient ground of 5 to 10 degrees and 10 to 20 degrees underlain by marl, marly limestone and Miocene silts.

The report states five intended uses: supplying ground-motion parameters to design offices and project owners; supporting the application of the seismic building rules in force; providing base data for risk prevention plans and seismic risk scenarios; framing urban planning; and serving preventive information and awareness. It carries the BRGM reference RC-63100-FR and cites the CNBH 2012 among its references.