Enriqillo Fault

The northern Caribbean plate boundary near Haiti consists of two principal surface faults - the Septrional fault in the north and the Enriquillo/Plantain-Gdn fault in the south.

The slip rate on the Enriquillo fault that slipped on 12 January has been estimated by Manaker and others to be about 8 mm/yr. A damaging sequence of earthquakes occurrred near or on the fault in 1751 and 1770. Hence the slip deficit locally prior to the earthquake had amounted to about 1.7 m, close to the mean slip that was released by the 12 Jan earthquake. i.e. the earthquake could have, and indeed was foreseen (Manaker, Calais, Mann, Prentice), although its exact timing was uncertain.

Segments of the fault both to the east and west are likely to slip in the next several decades. The timing of these future ruptures is currently unknown.

The earthquake occurred on an east-west, strike-slip fault with a minor amount of convergence. Rupture was roughly 65 km long with mean slip of 1.8 m. According to preliminary models the rupture propagated to the west with most of the slip over in the first 15 s, and with maximum slip near the epicenter exceeding 4 m.

The epicenter was roughly 8 km deep. No clear evidence for surface rupture exists. Aftershocks delineate the east west extent of the rupture.

 

 

 

 

Haiti Earthquake

Field reconnaisance report 21-25 January

email rogerbilham@gmail.com for password until 2 Feb.

Mainshock 12 Jan 2010 at 04:53:10 PM local time

Prelim. Epicenter 18.457 deg N, 72.533 deg W, Mw =7.0

Fatalities

The urban agglomeration of Port-au-Prince has expanded rapidly in the past decade, and before the earthquake exeeded 2 million. The death toll was estimated at 200,000 (18 Jan) and the official count as of 24 January was 150,000.

With a Richter magnitude of M=7 the Haiti earthuquake is classed as a major earthquake, however, no previous M=7 earthquake has resulted in more than 35k fatalities (see Figure above). At 150k, the deathtoll is the fifth largest of any previous earthquake in history. The large death-toll is caused by the almost complete absence of earthquake resistance in most of the structures in Port-au-Prince and the surrounding towns and villages.

A recent article discussing the seismic future of cities may be downloaded here. It forecasts a rising deathtoll from earthquakes, especially in the developing nations, where earthquake resistance building codes are either absent or not enforced.

After the Earth Quakes by Susan Hough and Roger Bilham discusses the increasing problem of urban earthquakes.