Haiti-Earthquakes : Probability of 80 percent for an earthquake of magnitude 5 or greater in the next 90 days

USGS Updates Assessment of Earthquake Hazard and Safety in Haiti and the Caribbean / Released: 2/23/2010

Published by AlterPresse on February 24, 2010

The U.S. Geological Survey (USGS) issued an update to its Jan. 21, 2010, statement, which includes the aftershock probabilities for the next 30-to-90-day period and for the overall year.

U.S. Geological Survey Director Marcia McNutt asked the team of USGS earthquake scientists to continue to provide an evaluation of the earthquakes facing Haiti now and in the future. Here is the updated statement in its entirety from the USGS:

This statement revises and updates the statement issued by the USGS on Jan. 21, 2010.

The magnitude-7 earthquake of Jan. 12, 2010, near Port-au-Prince, Haiti, has generated a sharp increase in concerns about the potential for future earthquakes in Haiti and the surrounding region. These concerns extend to understanding the causes of the earthquake hazard and learning what can be done to ensure seismic safety in the future. The purpose of this statement is to convey our best judgment on these subjects.

Aftershocks: The aftershock activity will continue for many months, although the frequency of events should diminish with time. Nevertheless, the threat of additional damaging earthquakes within the sequence remains. Based on the characteristics of the aftershock sequence observed so far, we estimate the probabilities of future aftershocks, as of February 23, 2010, as follows:

Precautions: Any aftershock above magnitude 5.0 will be widely felt and has the potential to cause additional damage, particularly to vulnerable, already damaged structures. Everyone in the Port-au-Prince area must maintain awareness with regard to their personal earthquake safety. Individuals should always have in mind what action to take if the ground starts to shake. Open spaces are generally safe. If indoors, drop to the floor, take cover under a sturdy desk or table, and wait for the shaking to stop. Do not go outdoors until the shaking stops.

Only qualified engineers can determine if a damaged building is safe for reoccupation. Until engineering assistance arrives, a general rule to follow is: If it does not look safe, it probably is not safe. Entry into or reoccupation of obviously damaged structures should be avoided.

Short-term concerns: The geologic fault that caused the Port-au-Prince earthquake is part of a seismically active zone between the North American and Caribbean tectonic plates. The earthquake undoubtedly relieved some stress on the fault segment that ruptured during the event, but the extent of rupture along the fault is unclear at this time. Fault slip models, preliminary radar surface deformation measurements, and examination of satellite and airborne imagery for surface rupture suggest that the segment of the Enriquillo fault to the east of the January-12 epicenter and directly adjacent to Port-au-Prince did not slip appreciably in the earthquake. This implies that the Enriquillo fault zone near Port-au-Prince still stores sufficient strain to be released as a large, damaging earthquake during the lifetime of structures built during the reconstruction effort. In historic times, Haiti has experienced multiple large earthquakes, apparently on adjacent faults. Field studies and ground observations of fault offsets during this earthquake and in past events are essential to evaluate the potential for future earthquakes in proximity to Port-au-Prince.

Long-term concerns: Over the past three centuries, earthquakes comparable to or stronger than the recent one have struck Haiti at least four times, including those in 1751 and 1770 that destroyed Port-au-Prince.
We have estimated the probabilities of a future large earthquake on the Enriquillo fault. These estimates are based on techniques developed for earthquake hazard assessments in the United States. Our estimates indicate a probability range of 5 percent to 15 percent, less than one chance in six, for an earthquake of magnitude 7 on the Enriquillo fault near Port-au-Prince during the next 50 years. The range of probabilities is due to uncertainties in our current understanding of the seismicity and tectonics of the region. Further study of the historical seismicity and the geological characteristics of the Enriquillo fault zone will help to reduce the uncertainty.

For comparison, the probability of a magnitude 7 or greater earthquake within the next 50 years on the Hayward-Rodgers Creek fault in the eastern portion of the San Francisco Bay region of California is about 15 percent.

For the future: Given the estimates of earthquake hazard cited above, buildings in Port-au-Prince and environs will continue to be at risk from strong earthquake shaking. These risks can be minimized through the use of earthquake-resistant design and construction practices that apply the results of a comprehensive earthquake hazard assessment. It has been shown that the benefits of losses avoided through earthquake mitigation practices outweigh the costs.

The probability estimates given above fall far short of a thorough seismic hazard assessment that takes into account all of the relevant information on the geology, tectonics, and seismicity of a region. Such assessments include maps of the levels of ground shaking expected over various time periods. Seismic hazards assessments, coupled with site-specific studies of rock and soil conditions and landslide susceptibility, are essential for land use planning and safe, cost-effective earthquake resistant construction.

Regional concerns: The experience of the Port-au-Prince, Haiti, earthquake reveals a need for better understanding of the nature and extent of earthquake and tsunami hazard in the Caribbean region. This entire region is seismically active due to the relative motion between tectonic plates and is prone to damaging earthquakes: It is a small-scale “ring of fire” similar to that encircling the Pacific Ocean. Historical earthquakes greater than magnitude 7 have occurred in Puerto Rico, Jamaica, Dominican Republic, Martinique, and Guadeloupe. Along the northern coast of Venezuela, the juncture of the Caribbean and South American plates has caused damaging earthquakes in the vicinity of Trinidad and Tobago. Earthquake safety policy, including building codes throughout the region, should be based on thorough seismic hazard assessments.