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Le but de ce blog est d'eduquer et de discuter a propos des desastres naturels avec un focus sur l'activite et la vulnerabilite sismique, de reporter des informations generales relatives au tremblement de terre d'Haiti du 12 janvier 2010 et aux tremblements de terre du monde. Il met l'accent sur les efforts de reconstruction d'Haiti et la necessite d'utiliser des techniques de conception des structures de batiments et construction parasismique dans la construction des infrastructures physiques.

Haitilibre.com / Les dossiers

Sunday, October 31, 2010

Haiti Reconstruction & Seismology

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Latest updates on the Haiti earthquake

I) Unraveling Complexity of Haiti Quake Reveals Hidden Faults and Future Hazards

Released 10/10/2010
The January 2010 M7.0 earthquake that devastated Haiti’s economy and caused over 200,000 casualties also resulted in significant uplift of the ground surface along Haiti’s coastline, and involved slip on multiple faults, according to a study published online in Nature Geoscience.

Because the earthquake did not involve slip near the surface of the Earth, the study suggests that it did not release all of the strain that has built up on faults in the area over the past two centuries, and so future surface rupturing earthquakes in this region are likely.

The paper also suggests that similar events may be hidden from the prehistoric earthquake record both in Haiti and in other similar tectonic settings such as the San Andreas fault in California.

Gavin Hayes, a U.S. Geological Survey geophysicist, along with colleagues from USGS, California Institute of Technology, NASA's Jet Propulsion Laboratory (JPL), and the University of Texas at Austin, used a combination of seismological observations, geologic field data and satellite geodetic measurements to analyze the earthquake source. Initially the Haiti earthquake was thought to be the consequence of movement along a single fault, which accommodates the motion between the Caribbean and North American plates.

By modeling the patterns of surface deformation, the team was able to assess which fault was responsible. Their results showed that the earthquake may not have been caused by the simple rupture of a single fault, but instead may have involved a complex series of faults.

The pattern of surface deformation was dominated by movement on a previously unknown, subsurface thrust fault, named the Léogâne fault, which did not rupture the surface.

II) Haiti earthquake shaking amplified by local landforms
Released: 10/17/2010 1:00:00 PM
PORT-AU-PRINCE, Haiti — The severe damage and loss-of-life caused by the devastating January 2010 M7.0 earthquake in Haiti was exacerbated by amplification of shaking due to local geological conditions and landforms in Port-au-Prince, according to a study published online today in Nature Geoscience.

Following the earthquake, Susan Hough, a U.S. Geological Survey geophysicist, led a team from the USGS and the Haitian Bureau des Mines et de l’Energie that deployed a total of nine portable seismometers in Port-au-Prince to investigate the variability of shaking throughout the city. The aftershock recordings captured by the seismometers revealed that ground motions were amplified by the relatively young and soft rocks that underlie the valley in which the city is situated. The strongest observed amplifications were along a narrow, steep foothill ridge in the district of Petionville.

Shaking in any earthquake can be amplified significantly by local geological conditions. Amplification due to topographic features, such as ridges, has been considered less important than amplification due to near-surface geological structure.

In Haiti, the zone where high shaking amplification was observed corresponded with a swath of high damage during the January mainshock. A number of substantial structures in this region collapsed catastrophically, including several United Nations Development Programme offices and several large hotels.

The instrument deployment was undertaken as a partnership effort between the USGS and the Bureau des Mines et de l’Energie, which continue to work together to establish a permanent seismic monitoring network in Haiti.

The study underscores the need to consider seismic provisions in the rebuilding effort, and suggests that topographic effects should be considered when detailed hazard zone maps are made for other regions.
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Haiti Quake Risk may still be high U.S Geological Survey

Released 10/29/2010
The fault initially thought to have triggered January’s devastating earthquake in Haiti is likely still under considerable strain and continues to pose a significant seismic hazard, according to a study published online in Nature Geoscience Sunday.

U.S. Geological Survey geologist Carol Prentice led a team of scientists to Haiti immediately after the earthquake to search for traces of ground rupture and to investigate the geology and paleoseismology of the area.

Using geological field observations, and interpretations of satellite imagery, aerial photography, and light detection and ranging, the researchers sought evidence of deformation from the 2010 quake and determined the main strand of the Enriquillo-Plantain Garden Fault did not rupture in the January quake, as was initially thought.

They also documented evidence of geologically-young ground ruptures on the EPG Fault, which they believe may have formed during earthquakes that struck Haiti in 1751 and 1770. Because the EPG Fault did not rupture the surface in January, little, if any, accumulated strain on that fault was released during the quake and the hazard remains high.

The EPG Fault is a tectonic plate boundary similar to the San Andreas Fault in California. The January Haiti quake was similar in many respects to the 1989 Loma Prieta earthquake in the San Francisco Bay Area of California. Like the Haiti quake, the Loma Prieta quake did not produce primary surface rupture and did not occur on the main San Andreas Fault. However the fault that ruptured during the 1989 quake is part of a complex set of nearby faults whose movement is driven by the plate-boundary tectonics, much like the setting of the Haiti earthquake.

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