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Rupture energy moved up the slope of the fault surface raising the block on the Port au Prince side of the fault as the fault slipped 5 meters left laterally. The upward motion of the rupture caused very high ground accelerations in the soft, weak, wet sedimentsaround the harbor of Port au Prince.


Figure 1. Cross-section of slip distribution. The strike direction of fault plane is indicated by the black arrow and the hypocenter location is denoted by the red star. The slip amplitude are showed in color and motion direction of the hanging wall relative to the footwall is indicated by white arrows. Contours show the rupture initiation time in seconds.

Help the earthquake victims here.

NASA space shuttle topographic mapping shows a deep topographic slash in the center of the image that is the trace of the Enriquillo Plaintain Garden fault. NASA feature article

The USGS's computer generated large scale intensity map.
The shaking and damage was heaviest close to the 25 mile long fault rupture.

The long-term deformation rate of this fault is 0.7cm/yr - over a third but under a half of the total motion of the Caribbean plate relative to the north American plate. The recurrence period of this earthquake (provided that earthquakes have recurrence periods), which had up to 5 meters of offset is therefore about 700 years, on average. Previous earthquakes on this fault that devastated Port au Prince over 200 years ago were centered on different segments of the fault to the east of Port au Prince.

Gavin Hayes of the USGS determined the dynamics of the earthquake from the seismic records.

Gavin Hayes from the USGS's National Earthquake Information Center in Boulder, CO, has used dozens of seismic recordings to compute the actual movement along the Enriquillo-Plaintain Garden fault. According to his calculations, the fault ruptured exactly underneath the visible scarp over a length of approximately 25 miles.

Photo of Port au Prince, fault zone highlighted in yellow after the earthquake taken from a small plane that evacuated the injured.

Google imagery from space, rotated 180 degrees to make shading appear more natural to the eye, shows the fault trace and the uplifted block on the Port au Prince side of the fault.

In the center of the image, running from the center left is a river offset left-laterally about 8km - about 5 miles - by the Enriquillo Plaintain Garden fault.

Just released detailed Google imagery taken after the earthquake shows near total destruction on the uplifted soft sediments in Port au Prince along the fault on the uplifted side.

Poor construction practices played a role in the mass destruction near the fault, but, in the San Fernando Valley of California, where construction practices were good and building codes were enforced, high local ground accelerations destroyed buildings built to code. Seismic energy was focused towards the surface as the fault rupture moved upwards.

In the Port au Prince earthquake, weak soils and focused seismic energy caused structures to be subjected to severe shaking. A study of Turkish earthquakes(Note this USGS study of Turkey indicates a high seismic hazard for major Turkish cities.), explains what probably happened in Haiti.

NEAR-FIELD IMPULSE-TYPE GROUND MOTIONS
In the vicinity of an active fault system, the wave propagation pattern of ground motion is strongly affected by radiation pattern, directivity, rupture model, stress drop and also by stratigraphy, geo-morphology and lateral scatterers. At longer periods (T >1.0 sec), near-fault ground motions within the diameter of less than 15 km from fault rupture are exposed to high amplifying effects of the earthquake faulting mechanism and orientation of the site.

Particularly, when the rupture propagates in forward direction toward the site, and the direction of slip on the fault is aligned with the site, ground motions oriented in this forward rupture directivity path may follow certain radiation patterns and generate long period pulses perpendicular to strike of the fault. Ground motions having such a distinct pulse-like characters (i.e., fling) arise in general at the beginning of the seismogram, and their effects tend to increase the long-period portion of the acceleration response spectrum (Galesorkhi and
Gouchon, 2000).

This type of ground motion may generate large energy demands that force the structures to dissipate such a high energy with few large displacement excursions. During such an instantaneous energy demand and release sequence, structures may experience large amplitude plastic cycles that may yield high levels of inter-story drift. In this condition, the risk of brittle failure for poorly detailed systems is seriously enhanced (Manfredi et al, 2000). The detrimental effects of such phenomenon have been recognized during several worldwide earthquakes including 1992 Erzincan, 1992 Landers, 1994 Northridge, 1995 Kobe and finally 1999 Kocaeli and Düzce earthquakes.

Update of previous diary.

Royal Caribbean cruise lines has donated one million dollars to relief efforts in Haiti. I have reviewed the detailed Google images and subsequent reports on relief efforts and concluded that Royal Caribbean has done all they can under the circumstances.

Originally posted to FishOutofWater on Wed Jan 27, 2010 at 04:22 PM PST.

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