NICARAGUAN EARTHQUAKE 1992 What was unusual about the earthquake that caused this tsunami?


On September 2nd, 1992 at 00:16 GMT, an earthquake with a magnitude of 7.6 generated a tsunami with waves up to ten meters high. Total damage in Nicaragua was estimated at between 20 and 30 million U.S. dollars.¹ The waves left 116 dead, 68 missing and over 13,500 left homeless in Nicaragua.¹
Fig.1 Map of Nicaragua, located between Honduras and Costa Rica in Central America. Note Lake Managua and Lake Nicaragua.
Fig.1 Map of Nicaragua, located between Honduras and Costa Rica in Central America. Note Lake Managua and Lake Nicaragua.

Nicaragua is located south of the country of Honduras and north of the country of Costa Rica in Central America as illustrated by Fig.1 and Fig. 6 (at the end of the document). The country of Nicaragua is situated on the Caribbean Plate while the Pacific Ocean coast is on the Cocos tectonic plate. Because of this ocean-continent convergent plate boundary the Cocos tectonic plate is slowly subducting under the Caribbean Plate as illustrated by Fig.2. This type of plate collision in this area can cause earthquakes, volcanic activity, and tsunamis. When strain is built up between two plates it will release the energy causing an earthquake. If this earthquake is caused in ocean waters it can displace water and send large waves outwardly therefore causing a tsunami as illustrated in Fig.3. Normally, on shore you would notice the waters pull back like a rapid changing tide and then come rushing back in large tsunami waves as illustrated in Fig.4 and Fig.5.

The earthquake on September 2nd, 1992 occurred off the Pacific coast of Nicaragua. Its epicenter was at 11.761°N, 87.419 west, about 120 Kms West/Southwest of the city of Managua.² The earthquake that occurred at 6:16 pm local time was initially estimated to be a 7.2 only to be later revised to a moment magnitude of 7.6.²

So, what was unusual about the earthquake that caused this tsunami?
The reason the earthquake was so largely underestimated was because most of the seismic stations only measured the short period seismic waves. “Analysis of the seismological record showed the fault motion of this earthquake to be unusually long in duration and occurring in the top 10km of oceanic crust – a depth much shallower than that of typical subduction-zone earthquakes”² This shallow focal point and its slow fault rupture caused this earthquake to be classified as unusual. “In general, oblique, shallow subduction at 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 accommodated by ‘bookshelf’ faulting. It is believed that such was the block motions that were associated with the 2 September 1992 earthquake. Additionally, wi[t]hin subducted sediments and there was a lot of shear – thus the rupture was slower in speed.”² It was estimated that the earthquake slowly moved a 120 mile-long section of the seafloor more than three feet taking about two full minutes to displace.⁵
Fig.2 An illustration of the Cocos plate subducting under the Caribbean plate.
Fig.2 An illustration of the Cocos plate subducting under the Caribbean plate.


People in earthquake-prone coastal areas have learned to run fro higher grounds when an earthquake occurs. “Unfortunately, many people living on Nicaragua’s Pacific coast barely felt the effects of a…earthquake. But just about 45 minutes after the main shock, say survivors, a tsunami reaching 10 meters high crashed onto a 300 kilometer-long stretch of the coastline.”³
Illustrated on Fig. 5 is a comparison to an elephant and building so you can see just how high these waves were. This made this earthquake very destructive. The shallowness of the earthquake made it a slow, silent, tsunami earthquake that caught Nicaraguans by surprise.² Residents felt a very small tremor and some none at all, and assumed there was no danger. Newspapers globally told the horrific stories of survivors like the Los Angeles Times, “A major earthquake at sea caused a wall of water up to 30 feet high to sweep over much of the Nicaraguan coast.” It continued by describing how residents did not know why in seconds they were swimming in their own homes and losing children.³

The tsunami caught everyone by surprise including shore restaurant owner, Inez Ortega. “I started running, but I didn’t even get out of the restaurant when the wave hit,” she says. She along with other experienced what they described as a “blender” effect spending about half an hour swimming in a “debris-filled stew” before they managed to drag themselves out of the water.⁵ The most vulnerable were the children. Villagers tried desperately to grab sleeping children but could not grip them in the violent waves. The tsunami just sucked them out of their beds with no chance of survival. Socorro Lopez lost two of her grandchildren. In her town of Masachapa, eight of the nine confirmed dead were children.
Fig.3 An illustration of waves normally flow and then how water reacts to the impact of an earthquake thus creating a Tsunami.
Fig.3 An illustration of waves normally flow and then how water reacts to the impact of an earthquake thus creating a Tsunami.


Now, the coast was littered with debris, broken trees, splintered huts and a huge mess for poor villagers to redevelop. UN officials promised $30,000 in supplies and the US sent $25,000 in preliminary aid. Although they were holding $104 million in economic aid due to political disputes. Most of the villagers affected were poor fishermen that were barely surviving on what they caught. Many of the dead other than the children were fisherman. Leaving behind women to fend for themselves and rebuild.⁴

We now understand that this tragedy could have been prevented or may be prevented in the future. Normally, a significant earthquake in the Pacific Ocean will alert the Hawaiian warning center. Unfortunately, the network of seismological station in Nicaragua had greatly deteriorated. If modern seismometers were have been available and researchers had known the importance or the possibility of long period seismic waves there may have been time to warm Nicaraguan to run for high ground and the death toll could have been minimal if anything at all.⁵
Fig.4 An Illustration of a beach, the second shows the beach being drained by the tsunami just before it hits land and the third illustrates the tsunami hitting land.
Fig.4 An Illustration of a beach, the second shows the beach being drained by the tsunami just before it hits land and the third illustrates the tsunami hitting land.


Destructive tsunamis occur globally on an average of once a year. However, due to recent devastating events it seems like that figure should be higher. One of the main reasons for the high death tolls is because an adequate warning system was not in place or not suited for what was to come. In light of these disasters experts now know what they need to work on and learn in order to provide adequate machinery, sensors and a warning system. Eddie Bernard and Frank Gonzalez, oceanographers with NOAA’s Pacific Marine Environmental Laboratory in Seattle are now working on developing a deep-sea tsunami sensor.⁵ Currently, the team has been testing these sensor off the coast of the Aleutians, Hawaii and the Oregon-Washington border. The sensors would measure the weight of the ocean water above it. When a wave passes over the sensor on the surface, it increases the height and therefore the weight of the water above it.

“The sensor consists of a small metal tube about four inches long, which floats just about the ocean bottom; it is held in place by an anchor (usually an old railroad wheel). Partially enclosed within the metal tube is a small device- called a Bourdon tube- shaped like a comma with a very long tail. The end of the tail sticks out of the bottom of the metal tube, exposed to the ocean, and is open like a straw. The other end of the Bourdon tube is closed. When a wave passes overhead, increasing the weight of the water column, the pressure slightly straightens the Bourdon tube in the same way a paper noisemaker unfurls when you blow into it. When the trough comes by, the tube curls up again. As the Bourdon tube alternately straightens and curls, it pushes and pulls on a sensitive quartz crystal, which in turn produces an electric signal that varies along with the changes in pressure of the water above.”⁵

The sensors are placed for a 12 month period and report every 15 seconds. The sensor are not affected by storm waves or passing ships. The sensors are not tied to a warning system yet but tsunamis are easy to spot in the sensors’ records, which would mean that it could serve as an effective warning system. Records from the sensors clearly showed a change during the September 1992 Nicaraguan earthquake and tsunami.⁵
Sometimes it takes one large tragedy to prevent others in the future. In this case many had to suffer in Nicaragua but hopefully soon they will also have an early warning system that will assist with the long period waves that made this earthquake a silent tsunami. Much the destruction will be the same but at least we will save lives, especially precious innocent little lives.

how_tsunami_forms.jpg
Fig. 5 Top image shows how a tsumani forms. Bottom image gives you an idea how large/high the waves were.
























The cause of most tsunami are underwater earthquakes which displace large masses of water. These earthquakes usually originate from reverse moving subduction zone faults. Tsunami occasionally occur from normal faults, but rarely originate from strike-slip earthquakes as they don’t have much effect on water displacement. The seafloor typically will snap upwards as a result of a subduction zone earthquake. The waster is then pushed up as the seafloor drops forming a “trough”. The pushed water then rises forming a crest and gravity forces water back to an “equilibrium position”. Then lastly waves are sent in both directions. Big earthquakes which cause long period waves/earthquakes cause the largest waves.


Citations:
Works Bibliography

¹ "Historical Earthquakes: Nicaragua." US Geological Survey. 1992. USGS. 8 Nov. 2008 <
http://earthquake.usgs.gov/regional/world/events/1992_09_02.php>
² Pararas-Carayannis, George.”The Earthquake and Tsunami of 2 September 1992 in Nicaragua.” 2007. 20 Nov 2008. <http://www.drgeorgepc.com/Tsunami1992Nicaragua.html>
³ Pendick, Daniel "Slow-motion slip may drive tsunami surprise". Science News. . FindArticles.com. 20 Nov. 2008. http://findarticles.com/p/articles/mi_m1200/is_n9_v143/ai_13534241
⁴ “Nicaraguan Tidal Wave Toll Nears 100 Disaster: Thousands are homeless. Government makes an urgent appeal for international aid” :[Home Edition]. (1992, September 3). Los Angeles Times (pre-1997 Fulltext),p. 10. Retrieved November 21, 2008, from Los Angeles Times database. (Document ID: 61635635).
⁵ “Waves of Destruction. (Tsunamis)” [Cover Story]. (1994, May 01). Discover. Retrieved November 27, 2008, from Discover database.

Stills - Figures
¹ "Nicaragua." Online Map/Still. Encyclopedia Britannica Online. 26 Oct. 2008 <http://0-search.eb.com.opac.library.csupomona.edu/eb/art-52678>.
³ "Subduction Zones." Online Still. 8 Nov. 2008 <http://sio.ucsd.edu/volcano/about/subduction.html>
⁵ "Earthquakes: How a Tsunami Forms and Giant Sea Waves." Online Still. 8 Nov. 2008 http://mail.colonial.net/~hkaiter/earthquakes.html
Unknown Name. Online Map/Still. 8 Nov 2008 <http://www.lib.utexas.edu/maps/americas/nicaragua_rel_97.jpg>

Large illustration of the country of Nicaragua
Large illustration of the country of Nicaragua