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The
Earthquake and Tsunami of 2 September 1992 in Nicaragua
George
Pararas-Carayannis
The
Earthquake
 A
major tsunamigenic earthquake occurred off the Pacific coast
of Nicaragua at 18.16 local time, (00h 16m GMT) on September
2, 1992 . The initial surface wave magnitude was estimated at
7.2 and its epicenter was at 11.761° N, 87.419 West, about
120 Kms West/Southwest of the city of Managua. The magnitude
was later revised to a moment magnitude (Mw 7.6.) The main earthquake
was followed by several strong aftershocks. Tsunami waves of
up to 10 meters reached the Nicaraguan coast .
Tectonic Setting
The earthquake occurred
along a convergence zone of subduction that has created the Middle
America Trench (MAT). This is the region in the Pacific where
the Cocos tectonic plate is subducted under the western end of
the Caribbean plate. It is an active zone of stress and deformation,
characterized by major, shallow-focus, and potentially tsunamigenic,
erthquakes. Seismic wave analysis of the 1992 event indicated
that this event was much shallower than other typical subduction-zone
earthquakes.
Bathymetric Map of Central
America showing the Middle America Trench where the Cocos plate
underthrust the western end of the Caribbean plate
Magnitude
and Energy Release
Analysis of the seismological
record showed the fault motion of this earthquake to be unusually
long in duration and occurring in the top 10 km of oceanic crust
- a depth much shallower than that of typical subduction-zone
earthquakes. It was determined that the magnitude of this quake
had been underestimated because most of the seismic stations
measured only seismic waves of short period. Short period waves
had not been readily produced by this quake because ot its long
duration of faulting. Subsequent analysis of seismic records
made with long period seismometers showed that the moment magnitude
of this earthquake was larger (Mw -7.6), and the energy release
to be five times greater.
 Rupture and Crustal
Displacements
In general, oblique,
shallow subduction at a high rate of convergence which occurs
along the greater portion of the Middle America Trench, results
in northwest-trending crustal block motions that parallel the
trench. However near Nicaragua most of the crustal motions are
accomodated by "bookshelf " faulting. It is believed
that such was the block motions that were associated with the
2 September 1992 earthquake. Additionally, the block motions
were extremely shallow, occurred wihin subducted sediments and
there was a lot of shear - thus the rupture was slower in speed.
As for dimensions -
given the fact that the tsunami struck hardest along a 220 km
of the Pacific coast of Nicaragua - the fault rupture length
is estimated to be between 220 to 250 km long. The width of the
block is estimated to be about 35-40 km. Slip displacements are
estimated to be up to 3 meters. The duration of the rupture lasted
100 seconds, which was extremely slow - thus the event has been
characterized as a "slow" earthquake.
Map of Nicaragua (modified
CIA graphic)
A Slow Earthquake - The earthquake is characterized
as a slow earthquake because of its slow rupture speed.
A "Silent "Earthquake
- The
earthquake was also characterized as "silent" because
it did not generate rumbling sounds as most of the other shallow
earthquakes
"Tsunami Earthquake"
- Because the earthquake
generated a larger tsunami than what would be expected from surface
wave magnitude (Ms -7.2), the 1992 Nicaragua earthquake is also
classified as a "tsunami earthquake".
The
Tsunami of 2 September 1992 in Nicaragua
Introduction
 The
earthquake of 2 September 1992 in Nicaragua generated a greater
tsunami than expected from its surface wave magnitude, thus known
as a 'tsunami earthquake". The unusual wave heights and
greater destructiveness are attributed to the earthquake's shallow
focal depth and to the slow fault rupture within subducted sediments.
The earthquake generated
a very destructive local tsunami which struck along a 220 km
coastal section of the Pacific coast of Nicaragua with waves
of up to 10 meters in certain areas. The first of the tsunami
waves reached the coast of Nicaragua about 20 minutes after the
earthquake. The first wave was relatively small but the second
and third waves were higher and more destructive.
Tsunami
Generating Area
Based on the plate
tectonics of the convergence zone that has formed the Middle
America Trench, on aftershock distribution, and on the fact that
the tsunami struck along a 220 km of the Pacific coast of Nicaragua,
the tsunami generating area is believed to be a somewhat irregular
, broken up ellipsoid. Its major axis is estimated to be approximately
220-250 km and its minor axis to be about 35-40km. Its total
area is estimated to be anywhere from 8,000 to 10,000 Km2. Slip
displacements are estimated to be up to 3m.
It is believed that
the ellipsoid type of block movement occurred along an oblique
but very shallow subduction angle. According to geologic studies
of the offshore Nicaragua region - and in contrast to the northern
belt of convergence, crustal motions near Nicaragua are accomodated
by "bookshelf" faulting that may have an overall northwest-trending
direction that parallels the trench, but may also include northeast-striking
left-lateral faults. Additionally, most of the block motions
occurred wihin subducted sediments with a great deal of shear
which must have been the factor s responsible for the slower
rupture velocity. Such a mechanism would explain the long duration
of the rupture which lasted 100 seconds, and the sediment contribution
to tsunami excitation, that characterizes this event as a "tsunami
earthquake".
Historical
Tsunamis in the Region
The historic tsunami
record shows that, prior to 1992, two other major earthquakes
along this convergence zone generated two destructive tsunamis
along the western coast of Central America. On 5 August 1854
an earthquake with an estimated magnitude of 7.3 generated a
tsunami that struck Costa Rica. On 26 February 1902 an earthquake
of magnitude 7.0 generated a tsunami that was extremely destructive
in El Salvador and was responsible for more than 185 deaths.
Effects
of the 2 September 1992 Tsunami in the Immediate Area - Deaths
and Damages
There was major damage
to coastal towns and villages from San Juan Del Sur to Masachapa.
At least 70,000 people living in the coastal areas of 27 communities
s were affected by the tsunami.
El Transito - Tsunami runup ranged from 6
- 9.9m. The maximum runup of 9.9m, occurred in the central part
of the inundation area near El Transito. This extreme may be
partially due to reflection of the waves. The waves were particularly
damaging at El Transito, a small town of about a thousand people
located about 55 km W of Managua. The town faces the Pacific
openly without any breakwater or bank. It was the closest town
to the earthquake's epicenter. According to local reports, waves
of up to 9.9 meters destroyed most of the houses of this town
(about 200), killed 16 people (14 children and injured 151. Fortunately,
the first wave was not very large and people had time and managed
to escape the second and third waves that were larger.
Masachapa - Hard hit was the town of Masachapa
where the tsunami waves penetrated 300 to 400 meters inland.
Nine people lost their lives there
El Popoyo - The maximumum wave height at
El Popoyo was 5.6 meters. The waves caused extensive damage and
killed 15 people.
The most affected
coastal communities by the 2 September 1992 tsunami in Nicaragua
were also La Boquita, Casares and Huehuete . Other towns that
sustained loss of life and damage to property were San Martin,
Pochomil and Poneloya. Sixty percent of all houses were destroyed
in San Juan Del Sur, a small town with 13,000 people. At Corinto,
the tsunami caused extensive damage to food and fuel storage
facilities. The town was partially protected by a large seawall
. Fresh water wells were flooded by sea water. There were no
deaths reported from other Central American countries but waves
of 2 to 4 meters damaged harbors and boats in Costa Rica.
An UNDRO mission that
surveyed the area determined that the tsunami had an indirect
or indirect impact on about 40,500 people in Nicaragua. According
to the report a total of 116 people lost their lives, 63 were
missing, 489 were injured and 13,000 were left homeless. The
total death toll was later revised to 170 people. The tsunami
was responsible for the destruction of most of Nicaragua's Pacific
fishing fleet. Total property losses were estimated at around
$ 25 million.
No Warning
There was no time
to issue a timely tsunami warning for the residents of coastal
areas in Nicaragua. The tsunami source was too close and it only
took twenty minutes for the first of the destructive waves to
reach the coast. Furthermore, there had been no studies assessing
the tsunami threat for this area. The large tsunami which struck
was a complete surprise - even to scientists. It was much larger
than what would have been expected for an earthquake of that
magnitude, and it did not behave in the traditional way.
Map of the
Western Coast of Nicaragua Affected by the Tsunami of 2 September
1992 (modified IRIS graphic)
Lessons
Learned
There were many lessons
learned from this event - by the public, by government officials
of disaster agencies ,and by scientists. Specifically, the unpredictability
of this tsunami and the lack of a timely warning raised the degree
of public awareness that the tsunami hazard in the region is
very real and there is a need for programs of preparedness and
public education.
There were several
reports by coastal residents in the area affected by the tsunami
that they did not feel any strong ground motions when the earthquake
occurred, so they did not believe that there was a danger of
a tsunami and thus did not run to a higher elevation. Also survivors
claimed that they did not hear the characteristic rumbling that
a local earthquake would be expected to produce . This was in
sharp contrast to the 1902 earthquake in El Salvador which was
heralded by loud noises resembling the rumble of cannon shots.
The 1992 quake in Nicaragua was "silent". Coastal residents
only felt a minor tremor . They were totally surprised when waves
of up to 10 meters arrived.
Also , scientists
learned from this event that each earthquake, even in the same
subduction zone, can be different.
As previously
discussed, the energy release of the earthquake was five times
greater - something that was not expected. It was learned that
the quick dissipation of short period seismic waves from this
earthquake accounted for the absence of strong ground movements
and the lack of rumbling that the coastal residents reported.
Scientists also learned that unusually large tsunamis can be
generated by earthquakes, with slow fault motion occurring within
subducted sediments. In fact, several similar earthquakes which
have occurred around the world are now being called "tsunami"
earthquakes, because of their higher tsunamigenic potential.
It is now estimated that 5 to 10 percent of all tsunami generating
earthquakes may be similarly "silent" "tsunami
earthquakes" and that sediments play an important role in
both faulting rupture velocity and tsunami generation. The 1992
Nicaragua earthquake was a good learning lesson for all - even
the "experts".
Conclusions
The 2 September 1992
event indicates that the potential tsunami hazard for the Central
America region has been underestimated. Major tsunamigenic earthquakes
occur primarily near the Middle America Trench along the intra-oceanic
convergent margins of the Cocos and Caribbean plates - a zone
of active, oblique, shallow subduction. The quantity and thickness
of subducted sediments along certain sections of these margins
can alter seismic focal mechanisms and the geometry and velocity
of ruptures. Certain large earthquakes, along such zones, can
be expected to be "slow" - that is to have lower rupture
velocities and to generate seismic waves of longer periods. If
measured with short period seismic instruments, the energy release
of such major earthquakes will always be underestimated. The
greater tsunamigenic potential of such earthquakes -along such
zones of oblique shallow subduction - is not only the result
of greater energy release but of the contribution of subducted
sediments to the geometry of ocean floor deformation and volumes
of displacements. "Bookshelf" faulting of subducted
sediments, as that which characterizes the offshore area of Nicaragua
(and which probably occurred during the 1992 earthquake) can
generate the so-called "tsunami earthquakes" that can
be potentially more catastrophic.
REFERENCES
Aki K., 1992, Higher-Order Interrelations Between
Seismogenic Structures and Earthquake Processes, Tectonophysics 211: (1-4) 1-12, Sep 30.
Ihmle P.F. and T.H.
Jordan, 1994, Teleseismic
Search for Slow Precursors to Large Earthquakes, Science 266: (5190) 1547-1551, Dec 2.
Ihmle P.F., 1996,
Frequency-Dependent
Relocation of the 1992 Nicaragua Slow Earthquake: An Empirical
Green's Function Approach,
Geophysical Journal International, 127: (1) 75-85 Oct.
Ihmle P.F., 1996,
Monte Carlo Slip
Inversion in the Frequency Domain: Application to the 1992 Nicaragua
Slow Earthquake,
Geophysical Research Letters, 23: (9) 913-916, May.
Kanamori H. and E.
Hauksson, 1992, A
Slow Earthquake in the Santa-Maria Basin, California, Bull. Seis. Soc. 82: (5) 2087-2096,
Oct.
Kanamori H. Masayuki K.
, 1993. The 1992
Nicaragua earthquake: a slow tsunami earthquake associated with
subducted sediments.
Nature 361, 714 - 716
Kawasaki I., Asai
Y., Tamura Y., Sagiya T., Mikami N., Okada Y., Sakata M., and
Kasahara M., 1995, The
1992 Sanriku-Oki, Japan, Ultra-Slow Earthquake, Journal of Physics of the Earth, 43: (2) 105-116.
La Femina, Peter
C., Dixon, T.H., Strauch, W. , 2002, Bookshelf faulting in Nicaragua. Geology Volume: 30 Issue: 8 Pages: 751-754
Linde A.T., Gladwin
M.T., Johnston M.J.S., Gwyther R.L., and R.B. Bilham., 1996,
A Slow Earthquake
Sequence on the San Andreas Fault, Nature, 383: (6595) 65-68 Sep 5.
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