By U.S. Geological Society
RESTON, Va.—The aftershock sequence of the magnitude-7 earthquake
that struck near Port-au-Prince, Haiti, on Jan. 12, 2010, will continue
for months, if not years. The frequency of events will diminish with
time, but damaging earthquakes will remain a threat.
U.S. Geological Survey Director Marcia McNutt asked a team of USGS
earthquake scientists to provide an evaluation of the earthquakes
facing Haiti now and in the future. Here is the statement in its
entirety from the U.S. Geological Survey:
Earthquake Hazard and Safety in Haiti and the Caribbean Region
The magnitude-7 earthquake of January 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
Aftershocks: The aftershock sequence of a
magnitude-7 earthquake will continue for months if not years in the
affected area. The frequency of events will diminish with time, but
damaging earthquakes will remain possible in the coming months. There
is also a small chance of subsequent earthquakes larger than the
initial shock. The sequence from the Port-au-Prince earthquake
continues to be very strong and active. Based on this activity and the
statistics of aftershock sequences, our estimate for aftershock
activity during a 30-day period beginning January 21, 2010, is as
- The probability of one or more earthquakes of magnitude 7 or greater is less that 3 percent.
- The probability of one or more earthquakes of magnitude 6 or greater is 25 percent.
- The probability of one or more earthquakes of magnitude 5 or greater is about 90 percent.
- Approximately 2 to 3 aftershocks of magnitude 5 or greater are expected within this time period.
These estimates will be updated as new information becomes available.
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. Anyone living
in Haiti or involved in relief work there must maintain situational
awareness with regard to their personal earthquake safety. They should
always be aware of what action they are going to take if the ground
starts to shake. Open spaces are generally safe but running through
falling debris to get to an open space may be dangerous. 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.
Near-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 this event. 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. We shall continue to study this
situation using radar, LiDAR, and photographic data taken from
satellites and aircraft. Field studies and ground observations of fault
offsets during this earthquake and past events are essential to
evaluating the potential for future earthquakes in proximity to
Long-term concerns: It is essential that the
rebuilding effort in Haiti take into account the potential for, indeed
the inevitability of, future strong earthquakes. Haiti is cut by two
major plate boundary fault zones. 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. Engineers and construction professionals know
how to design and build structures that will not collapse in strong
earthquake shaking. Seismic hazard assessments provide the basis for
the development of appropriate building codes and the identification of
regions at greatest risk. A thorough seismic hazard assessment of
Haiti, as well as of other countries in the Caribbean, will provide the
basis for establishing or improving building codes and strengthening
building resilience over the long-term. Such assessments involve
geologic investigations of faults and soil conditions, reoccupation of
geodetic measurement sites to determine strain accumulation, and
studies of recent and historic earthquakes and seismicity patterns and
statistics. These assessments usually take several years but can be
accelerated to provide results markedly better than what is currently
available. From these investigations, we can assess the likelihood and
nature of strong shaking and ground failure over various time frames.
The development of more resilient structures and infrastructure is a
long-term goal, particularly in the face of economic limitations. Over
the short-term, it is critical that the rebuilding effort be undertaken
with an awareness of the potential for subsequent damaging events
during the next months and years. It is essential that structures such
as hospitals, schools, and critical facilities be reconstructed with
greater resilience for the preservation of life and functionality.
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. The arc of islands that forms the Lesser
Antilles and Greater Antilles generally outlines the contact zone
between the Caribbean and North American plates. This entire region is
seismically active due to the relative motion between the plates and is
prone to damaging earthquakes: a small-scale “ring of fire” like 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.
You can access a current map of aftershocks at the M7.0 Haiti Earthquake and Aftershocks Web site, and you can listen to podcast interviews at the USGS CoreCast Web site.
U.S. Geological Society