mardi 9 mars 2010

Haïti-séisme/Rapport technique d'une équipe d'ingénieurs américains USGS/EERI, février 2010 (*)

USGS/EERI Advance Reconnaissance Team: TEAM REPORT
V 1.1

February 23, 2010

A field reconnaissance in Haiti by a five-member team with expertise in seismology and earthquake engineering has revealed a number of factors that led to catastrophic losses of life and property during the January 12, 2010, Mw 7.0 earthquake. The field study was conducted from January 26 to February 3, 2010, and included investigations in Port-au-Prince and the heavily damaged communities to the west, including Léogâne, Grand Goâve, Petite Goâve, and Oliver.

Seismology: Despite recent seismic quiescence, Haiti has suffered similar devastating earthquakes in the historic past (1701, 1751, 1770 and 1860). Haiti had no seismograph stations during the main earthquake, so it is impossible to estimate accurately the intensity of ground motions. Nonetheless, the wide range of buildings damaged by the January 12, 2010 earthquake suggests that the ground motions contained seismic energy over a wide range of frequencies. Another earthquake of similar magnitude could strike at any time on the eastern end of the Enriquillo Fault, directly to the south of Port-au-Prince. Reconstruction must take this hazard into account.

The four portable seismographs installed by the team recorded a series of small aftershocks. As expected, the ground motions recorded at a hard rock site contained a greater proportion of high frequencies than the motions recorded at a soil site. Two of the stations continue to monitor seismic activity.

A thorough field investigation led the team to conclude that this earthquake was unlikely to have produced any surface rupture.

Geotechnical Aspects: Soil liquefaction, landslides and rockslides in cut slopes, and road embankment failures contributed to extensive damage in Port-au-Prince and elsewhere. A lack of detailed knowledge of the physical conditions of the soils (e.g., lithology, stiffness, density, and thickness) made it difficult for us to quantitatively assess the role of ground-motion amplification in the widespread damage.

Buildings: The Haitian Ministry of Statistics and Infomatics reported that one-story buildings represent 73% of the building inventory. Most ordinary, one-story houses have roofs made of sheet metal (82%), whereas most multi-story houses and apartments have roofs made of concrete (71%). Walls made of concrete/block/stone predominate both in ordinary houses and apartments.

It appears that the widespread damage to residences, and commercial and government buildings was attributable to a great extent to the lack of attention in design and construction to the possibility of earthquakes. In many cases, the structural types, member dimensions, and detailing practices were inadequate to resist strong ground motions. These vulnerabilities may have been exacerbated by poor construction practices. Reinforced concrete frames with concrete block masonry infill appeared to perform particularly poorly. Structures with light (timber or sheet metal) roofs performed better compared with structures with concrete roofs and slabs.

The seismic performance of some buildings was adequate, and some of the damaged buildings appeared to have had low deformation demands. These observations suggest that structures designed and constructed with adequate stiffness and reinforcing details would have resisted the earthquake without being damaged severely.
A damage survey of 107 buildings in downtown Port-au-Prince indicated that 28% had collapsed and another 33% were damaged enough to require repairs. A similar survey of 52 buildings in Léogâne found that 62% had collapsed and another 31% required repairs.

Bridges: There was no evidence of bridge collapses attributable to the earthquake. Most bridges in Port-au-Prince are simple box culverts consisting of 2.0 to 2.5 meter (6 to 8 ft) deep box girders. However, in several cases the roadway settled differentially between the approaches and the section spanning the culvert. Multi-span bridges on primary routes are engineered structures that experienced some damage but are still serviceable.

Port Facilities: The main port in Port-au-Prince suffered extensive damage during the earthquake, inhibiting the delivery of relief supplies. The collapse of the North Wharf appears to have been caused by liquefaction-induced lateral spreading. The westernmost 120 meters (400 ft) of the South Pier collapsed, and approximately 85% of the vertical and batter piles supporting the remaining section were moderately damaged or broken. The remaining section of pier was shut down to vehicle traffic following additional damage that occurred during an aftershock. The collapse of a pile-supported pier at the Varreux Terminal resulted in the deaths of about 30 people working on the pier at the time of the earthquake. Less severe damage, including a small oil spill, occurred at a marine oil terminal located near Port-au-Prince.

Damage to Institutions: The functioning of the government and social infrastructure was seriously deteriorated by the loss of personnel, records and facilities. Such losses occurred in numerous clinics, hospitals, police stations, schools, universities, palaces, ministries and churches. These losses have compromised the recovery and reconstruction efforts.

Satellite Imagery: The use of remote sensing data, including satellite and aerial imagery, proved highly effective in assisting damage assessment, evaluating the extent of landslides, and guiding rescue and recovery efforts. Light Detection and Ranging (LIDAR) technology has been effective to create three-dimensional images for damage assessment and rebuilding operations.

Conclusions: The massive human losses can be attributed to a lack of attention to earthquake-resistant design and construction practices, and the poor quality of much of the construction. The historic pattern of earthquakes in Haiti indicates that an earthquake of magnitude 7 or larger could strike southern Haiti near Port-au-Prince at any time. Reconstruction must therefore be based on sound, simple and cost-effective engineering practice for all possible natural hazards. These principles must be clearly communicated to the citizens of Haiti. Additional fact gathering is needed, both to quantify the January 12th fault rupture and earthquake history (inputs to calculations of future earthquake probabilities), and to more comprehensively evaluate damage to buildings and infrastructure, so as to inform decisions about reconstruction.
(*) Le rapport a 64 pages et contient de nombreuses photos documentées. Pour le lire cliquez sur: USGS/EERI Team report.

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