Revision of the geological context of the Port-au-Prince metropolitan area, Haiti: implications for slope failures and seismic hazard assessment

International audienceFollowing the earthquake of 12 January 2010 in the Port-au-Prince area, the Haitian government, in close cooperation with BRGM, the French geological Survey, decided to undertake a seismic microzonation study of the metropolitan area of the capital in order to take more fully into account the seismic risk in the urbanization and planning of the city under reconstruction. As the first step of the microzonation project, a geological study has been carried out. Deposits of Miocene and Pliocene formations in a marine environment have been identified. These deposits are affected by the Enriquillo-Plantain Garden N80° E fault system and N110° E faults. Tectonic observations and morphological analysis indicate Quaternary activity of several faults mapped in the area of Port-au-Prince. These faults have a N110° trend and show a reverse-sinistral strike-slip motion. Moreover, on the basis of these geological results and of new topographical data, a hazard assessment of ground movements has been made. Along with the map of active faults, the hazard map of ground movements is an integral component of the seismic microzonation study

Site effect assessment of the gros-morne hill area in port-au-prince, haiti, part B: Mapping and modelling results

This paper presents the general results in terms of maps, as well as geological and numerical models of a site effect study, that aimed at a better understanding of the ground motion amplification on the Gros-Morne hill, in the southeastern part of Port-au-Prince, Haiti, which might have influenced the 2010 event damage pattern in that area. These maps and models are based on multiple geophysical–seismological survey outputs that are presented, in detail, in Part A of this publication. Those outputs include electrical resistivity tomography sections, P-wave velocity profiles, S-wave logs, estimates of the fundamental resonance frequency for many locations, as well as earthquake recordings at three sites and associated site amplification assessment for the top of the hill. Related results are discussed in Part A with respect to outputs and interpretations that had been published earlier by other research teams for the same site. Our results only partly confirm the strong seismic amplification effects highlighted by some of the previous studies for this hill site, which had been attributed to the influence of local topographic and soil characteristics on seismic ground motion. Here, we focus on the imaging of different site effect components over the entire survey area; we present maps of shear wave velocity variations, of changing fundamental resonance frequencies, and of related estimates of soft soil/rock thickness, of peak spectral amplitudes, and of ambient ground motion polarization. Results have also been compiled within a 3D surface–subsurface model of the hill, which helps visualize the geological characteristics of the area, which are relevant for site effect analyses. From the 3D geomodel, we extracted one 2D geological section along the short-axis of the hill, crossing it near the location of Hotel Montana on top of the hill, which had been destroyed during the earthquake, and has now been rebuilt. This cross-section was used for dynamic numerical modelling of seismic ground motion, and for related site amplification calculation. The numerical results are compared with the site amplification characteristics that had been estimated from the ambient vibration measurements and the earthquake recordings. © 2018 by the authors. Licensee MDPI, Basel, Switzerland

Plate boundary segmentation in the northeastern Caribbean from geodetic measurements and Neogene geological observations,

International audienceThe Caribbean–North America plate boundary in the northeastern Caribbean shows a remarkable example of along-strike transition from plate boundary–normal subduction in the Lesser Antilles, oblique subduction with no strain partitioning in Puerto Rico, and oblique subduction/collision with strain partitioning further west in Hispaniola. We show that this segmentation is well marked in the interseismic strain, as measured using space geodetic data, and in the Neogene deformation regime, as derived from geological observations. Hence, interseismic segmentation, which reproduces the geological segmentation persistent over a long time interval, is inherited from the geological history and long-term properties of the plate boundary. This result is relevant to the assessment of seismic hazard at convergent plate boundaries, where geodetic measurements often show interseismic segmentation between fully–and partially–coupled plate interface regions

Is the local seismicity in western Hispaniola (Haiti) capable of imaging northern Caribbean subduction?

International audienc

Site Effects in Port‐au‐Prince (Haiti) from the Analysis of Spectral Ratio and Numerical Simulations

International audienceTo provide better insight into seismic ground motion in the Port‐au‐Prince metropolitan area, we investigate site effects at 12 seismological stations by analyzing 78 earthquakes with magnitude smaller than 5 that occurred between 2010 and 2013. Horizontal‐to‐vertical spectral ratio on earthquake recordings and a standard spectral ratio were applied to the seismic data. We also propose a simplified lithostratigraphic map and use available geotechnical and geophysical data to construct representative soil columns in the vicinity of each station that allow us to compute numerical transfer functions using 1D simulations. At most of the studied sites, spectral ratios are characterized by weak‐motion amplification at frequencies above 5 Hz, in good agreement with the numerical transfer functions. A mismatch between the observed amplifications and simulated response at lower frequencies shows that the considered soil columns could be missing a deeper velocity contrast. Furthermore, strong amplification between 2 and 10 Hz linked to local topographic features is found at one station located in the south of the city, and substantial amplification below 5 Hz is detected near the coastline, which we attribute to deep and soft sediments as well as the presence of surface waves. We conclude that for most investigated sites in Port‐au‐Prince, seismic amplifications due to site effects are highly variable but seem not to be important at high frequencies. At some specific locations, however, they could strongly enhance the low‐frequency content of the seismic ground shaking. Although our analysis does not consider nonlinear effects, we thus conclude that, apart from sites close to the coast, sediment‐induced amplification probably had only a minor impact on the level of strong ground motion, and was not the main reason for the high level of damage in Port‐au‐Prince

Thin sheet numerical modelling of continental collision

We study the effects of incorporating surface mass transport and the gravitational potential energy of both crust and lithospheric mantle to the viscous thin sheet approach. Recent 2D (cross-section) numerical models show that surface erosion and sediment transport can play a major role in shaping the large-scale deformation of the crust. In order to study these effects in 3D (planform view), we develop a numerical model in which both the dynamics of lithospheric deformation and surface processes are fully coupled. Deformation is calculated as a thin viscous layer with a vertically-averaged rheology and subjected to plane stresses. The coupled system of equations for momentum and energy conservation is solved numerically. This model accounts for the isostatic and potential-energy effects due to crustal and lithospheric thickness variations. The results show that the variations of gravitational potential energy due to the lateral changes of the lithosphere–asthenosphere boundary can modify the mode of deformation of the lithosphere. Surface processes, incorporated to the model via a diffusive transport equation, rather than just passively reacting to changes in topography, play an active role in controlling the lateral variations of the effective viscosity and hence of the deformation of the lithosphere.This work is supported by the University College London, the Netherlands Research Centre for Integrated Solid Earth Science (ISES) and the Spanish Ministry research projects BTE2002-02462 and REN2001-3868-C03-02/MAR. The authors also benefited from NATO grant EST.CLG.978922.Peer reviewe

Transpressional rupture of an unmapped fault during the 2010 Haiti earthquake

On 12 January 2010, a Mw 7.0 earthquake struck the Port-au-Prince region of Haiti. The disaster killed more than 200,000 people and caused an estimated $8 billion in damages, about 100% of the country’s gross domestic product1. The earthquake was initially thought to have ruptured the Enriquillo–Plantain Garden fault of the southern peninsula of Haiti, which is one of two main strike-slip faults inferred to accommodate the 2 cm yr−1relative motion between the Caribbean and North American plates2, 3. Here we use global positioning system and radar interferometry measurements of ground motion to show that the earthquake involved a combination of horizontal and contractional slip, causing transpressional motion. This result is consistent with the long-term pattern of strain accumulation in Hispaniola. The unexpected contractional deformation caused by the earthquake and by the pattern of strain accumulation indicates present activity on faults other than the Enriquillo–Plantain Garden fault. We show that the earthquake instead ruptured an unmapped north-dipping fault, called the Léogâne fault. The Léogâne fault lies subparallel to—but is different from—the Enriquillo–Plantain Garden fault. We suggest that the 2010 earthquake may have activated the southernmost front of the Haitian fold-and-thrust belt4 as it abuts against the Enriquillo–Plantain Garden fault. As the Enriquillo–Plantain Garden fault did not release any significant accumulated elastic strain, it remains a significant seismic threat for Haiti and for Port-au-Prince in particular