Earquakes and Tremors in Nicaragua

Earquakes and Tremors in Nicaragua

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EXECUTIVE SUMMARY

Managua, the capital city of Nicaragua, is the country's political, commercial, and industrial center. However, the city is prone to frequent earthquakes and tremors due its very close proximity to the convergent plate boundary occurring only a few kilometres off the coast of Nicaragua: the Cocos plate slowly being subducted under the Caribbean plate (“Historic Earthquake”, 1972). This movement caused significant destruction to Managua in 1931, 1968 and 1972 when earthquakes of magnitudes 5.6, 4.5 and 6.2 on the Richter scale, respectively, rocked the capital city (“Historic Earthquake”, 1972).
One of most destructive earthquakes recorded above the equator in the western hemisphere was the earthquake of December 23, 1972 which struck Managua. The economic losses associated with this earthquake were very significant as 80% of the buildings in Managua were severely damaged and a shocking 10,000 deaths were declared (“Earthquake wreaks devastation in Nicaragua”, 1973). This loss was primarily attributed to several factors: strong seismic shaking due to the shallow focus of the earthquake, permanent ground displacement due to surface faulting, type of materials used to construct buildings, and other secondary factors.
Even though earthquakes and tremors occur frequently in Managua, the city was unprepared for the earthquake of 1972. No risk assessments had ever been performed for earthquake related damages even though it was known that earthquakes were frequent in Managua. In addition to this, the city did not have any warning systems in place or any practices in place to reduce the severity of the damage caused.

BACKGROUND

Managua, the capital city of Nicaragua, is located along the southern coast of Nicaragua at approximately 12°8′11″N 86°15′5″W as shown in Figure 2 (“Latitude and Longitude” 2014).
Tectonically, Managua sits on the southern boundary of the Caribbean plate, close to the boundary with the Cocos plate (“Historic Earthquake”, 1972). The convergent plate boundary movement of the Cocos plate in a north-eastwardly direction is slowly being subducted under the Caribbean plate at a rate of approximately 3 inches per year (Gunn, 2008). In addition, the Cocos and Caribbean plates are two of several plates forming the Ring of Fire: an area in the Pacific Ocean responsible for ninety percent of the total seismic and volcanic activity recorded each year (“Ring of Fire” 2012). The close proximity of Managua to the Cocos-Caribbean subduction zone along the Ring of Fire is known to be the cause of the frequent volcanic and seismic activity experienced by Managua.

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Fig 2. Map showing the location of the capital city of Nicaragua, Managua. 2

The last major earthquake to strike Managua occurred on December 23, 1972 at 12:29 AM Central Standard Time and has been reported by the U.S. Geological Survey to be the deadliest earthquake recorded above the equator in the western hemisphere (“Historic Earthquakes” 1973).
The earthquake had a surface wave magnitude of 6.2 on the Richter scale with an approximate energy release of 6.3*1014 millions of ergs (Tarbuck et al, 247). This is equivalent to the energy release of the 1946 Bikini atomic bomb test in the Marshall Archipelago (Tarbuck et al, 247). Following the initial earthquake, hundreds of aftershocks were recorded. Of these, two were major aftershocks of magnitude 5 and 5.2. They were experienced within an hour of the main shock at 1:18 am and 1:20 am, respectively (Kates, 1973).

Due to the significant damage to many poorly constructed buildings, the earthquake was given an intensity scale rating of VII on the Modified Mercalli Intensity scale (Kates, 1973). This rating can be described as “Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures”. (Tarbuck, 245)

The climate in Managua is a tropical climate with two seasons: rainy and dry. On December 23, 1972 the atmospheric conditions were hot and humid and the city experienced a dry season with less recorded rainfall; this resulted in the country experiencing a drought (De Boer, 2005). At that time, the dry season winds in Managua helped fuel blazing fires triggered from the 1972 earthquake.

EARTHQUAKE TRIGGERS

The earthquake originated along four major strike-slip faults which run parallel to the coast of Managua. These faults were surface faults which affected an area of 270 square kilometres, encompassing the entire city of Managua (De Boer, 2005).

The main earthquake of magnitude 6.2 was caused by the Tiscapa fault (see Fig. 3) that strikes N30° to 35°E and dips 80° to 90°E. This fault, extending beneath Lake Managua, began rupturing at a depth of 10 kilometres below ground level and quickly extended upward (Ward et al, 1974). The rupturing of the fault was primarily due to the built up strain within the southwestern portion of the Caribbean plate (“Historic Earthquakes” 1973).


Fig 3. Photo of a volcanic crater in Managua. The red line indicates the location of the Tiscapa fault, which had a horizontal movement during the earthquake. 3

The two major aftershocks experienced were also generated at shallow depths. This caused additional damage to already weakened structures. It was reported that aftershocks continued well into January of 1973 (De Boer, 2005).

The shallow-focus earthquake, caused from the rupturing of the Tiscapa fault, resulted in most of the energy released close to the ground surface of Managua and was the cause of intensified ground shaking to the capital city. The damage from the earthquake was cataclysmic due to the epicentre of the earthquake located within the densely populated city of Managua (Algermissen et al, 1974).


HAZARD FREQUENCY

Prior to the earthquake of 1972, the city of Managua had been struck by a series of moderate earthquakes in 1844, 1858, 1881, 1898, 1913 and 1918 due to the movement of the Cocos plate being slowly subducted beneath the Caribbean plate (“Historic Earthquakes” 1973). The most severe earthquakes occurred in 1931, 1968 and, more recently, 1972. From these records, it is clear that Managua can experience a devastating earthquake from 4 years to 40 years after the last major earthquake. Overall, this reveals a relatively short time period between severe earthquakes and this makes it even more imperative that Managua should be prepared when these types of earthquakes strike.
On March 31, 1931 an earthquake of magnitude 5.6, on the Richter scale, struck the western part of Managua. It caused a major fire and destroyed thousands of buildings and homes. The earthquake resulted in 1,000 deaths, left 410,000 people homeless and had a projected loss of USD $35 million (Gunn, 2008). Thirty seven years later, on January 4, 1968 an earthquake of magnitude 4.5 struck the eastern part of the city causing damage to several hundred homes and resulting in an approximate economic damage of USD $1 million (De Boer, 2005).
Four years after the earthquake of 1962, the city of Managua suffered another major earthquake. The surface wave magnitude reached 6.2 on the Richter scale (Kates, 1973). The earthquake was a rare, yet common natural disaster for the nation. The magnitude of the earthquake was relatively common for Managua. Magnitudes of this nature have been felt in previous earthquakes experienced by the city as illustrated in Fig 4. However, it was the unusually shallow depth of the earthquake which resulted in an overall higher intensity earthquake that was uncommon for the city of Managua.


HUMAN AND ECONOMIC LOSSES

Managua is considered the economic center of Nicaragua, accounting for a large portion of the country’s total economy. Hence, the significant destruction of the city in 1972 resulted in major economic losses for both the city and the country.

Approximately 80% of the total buildings and homes in the city were destroyed. This damage included 53,000 units of mostly low and middle income family housing, 4 hospitals and 740 classrooms. The estimated damage was a colossal USD $400 million (Kates, 1973).

In 1972, Managua had a population of 450,000 people. More than 10,000 people lost their lives in the earthquake while 250,000 people were left homeless (De Boer, 2005). This accounts for 1% of Nicaragua’s total population that lost their lives and another 25% of the country’s population that was left homeless. Overall, no less than 25% of the population was directly affected by the earthquake. Fig. 5 shows the Nicaraguan earthquake of 1972 as one of the deadliest natural disasters recorded, just below the catastrophic 2010 Haiti earthquake.


Fig. 6 shows that Nicaragua is the second deadliest disaster recorded since 1970 as a proportion of the total population affected. As mentioned previously, no less than 25% of the total population of Nicaragua was directly affected by the earthquake of 1972. This was primarily because the earthquake struck Managua: the country’s most densely populated city. A large majority of buildings in the city were very poorly constructed resulting in large scale structural damage. This is again illustrated in Fig. 6 where the damages reported as a percentage of the country’s total gross domestic product index was equal to 102%.

The major primary effects of the Nicaragua earthquake of 1972 were attributable to surface faulting, poor construction of Managua's buildings, duration of seismic shaking and unconsolidated volcanic rock fragments that many of the city’s structures were built on (“Historic Earthquakes” 1973).
One of the geological effects seen from the earthquake was surface faulting throughout the city. This caused permanent ground displacement of streets, structures and utility pole lines in many areas of the city (Algermissen et al, 1974). The displacement of roads resulted in waterline breaks on many streets as seen in Fig 6.

Most of the building destruction in Managua was caused by strong seismic ground shaking of about 10 - 15 seconds during the main shock (“Historic Earthquakes” 1973). Many building structures are not built on solid bedrock but rather unconsolidated rock fragments of volcanic material. This would have made any structure built on the volcanic material at risk for moderate to severe destruction depending on the intensity of the earthquake experienced. These rock fragments were disturbed more easily leading to the amplification felt by structures causing most to flatten and collapse.
The rock fragments and volcanic material, that the city was constructed on, was enough to cause significant damage. However, the type of material that the buildings were made of also contributed to the economic damages.
In the city of Managua, half of the population was living in poverty and it was not uncommon for homes to be built of Taquezal (“Historic Earthquake” 1973). As a result, homes constructed with this material were more susceptible to severe damage as seen in Fig. 7. Many of the wooden frame structures of the homes were also termite ridden further adding to damages experienced.

Some modern buildings such as churches, museums and hotels, seen in Fig. 8, experienced little structural damage because they were constructed from more earthquake resistant materials. In Managua, some upper income housing owners were insured; however, the coverage was only up to 50% of damage to buildings (Kates, 1973).

The main secondary effect of the earthquake resulted from fires throughout the city. Residents were unable to put out the fires due to fire-fighting equipment being destroyed during the earthquake tremors, broken water mains and no electricity for the use of electric water pumps so the fires raged for days (De Boer, 2005).

In addition to the fires, all of the four main hospitals on Managua were partially destroyed making it unserviceable to victims (“Historic Earthquakes” 1973). This contributed to the death toll as thousands of victims who were injured by the earthquake could not receive any medical treatment. Furthermore, the lack of immediate aid relief workers to the city after the earthquake left many people trapped under the rubble and unable to escape alive. When aid workers did arrive, the cracks in roads and highways slowed the relief efforts to the city considerably.

EARTHQUAKE MITIGATION AND PREVENTION

Despite its seismic history, prior to the earthquake of 1972, mitigation strategies and preparedness measures for earthquakes in Managua were almost non-existent. The only warning system in place was a radio frequency which had been set aside for emergency broadcasts as part a Central American network (Kates, 1973). The earthquake caused a power outage almost immediately so it was unlikely most residents had not heard the warning. In addition, at the time the earthquake struck at 12:30 am most residents were asleep (“Earthquake rocks Managua” 1973).
When the first tremor was felt by residents at 10:00 pm on December 22, 1972, some residents decided to take precautions and as a result chose to sleep outdoors to prevent their homes from collapsing on them in the event of an earthquake (Kates, 1973). In the case of the 1972 earthquake, these small preventative measures were not effective in preventing the damages that occurred.
Early warning signals for residents, such as radio broadcast warnings and sirens followed by immediate evacuation procedures out of danger zones can be a key factor in the initial phase of an imminent natural disaster to get residents to safety quickly. However, for residents and tourists to be made aware of evacuation procedures and warnings, the community would have needed to fund campaigns for updating residents on safety plans and practicing yearly safety drills. Immediate aid relief to individuals who were affected as well as emergency medical care for those injured would have also prevented further loss of life.

In addition to the lack of earthquake preventative measures for its citizens, Nicaragua also failed to implement codes to follow when constructing public or private buildings, including homes. At the time of the earthquake, only 6 major structures were constructed in accordance with the U.S design standards applied for structures in high seismic zones. The country had passed a law requiring major structures in Managua to adhere to the policy; however, that law had not yet been implemented at the time the earthquake struck the city (Kates, 2008).
Due to half of the Managua’s population living in poverty, many homes were made with taquezal: a local resource of Nicaragua consisting of rough wood frame with adobe, stone infilling and clay roofs (“Historic Earthquakes” 1973). Houses built from this type of construction collapsed easily during the earthquake. Strict building codes for businesses and homes in Managua would have prevented severe damage to these buildings and as a result, the inhabitants of these buildings. Buildings and homes in Managua should be constructed with additional earthquake resistant materials, such as steel rods, to give additional support in the case of strong seismic shaking. Furthermore, if building structures were constructed on bedrock instead of fragments of volcanic rocks, there would have been less amplification during seismic ground shaking, again resulting in less damage to buildings.
Since the city of Managua is prone to earthquakes, mitigation strategies for additional secondary effects such as tsunamis and landslides should also be taken into account in the event of more severe earthquakes.



CONCLUSION

Managua, the capital city of Nicaragua is located in close proximity to the Cocos-Caribbean subduction zone along the Pacific Ring of Fire. This is believed to be the cause of frequent volcanic and seismic activity experienced by Managua.
The Nicaraguan earthquake of 1972 was one of the most destructive earthquakes recorded in the Western Hemisphere. This was largely due to a rupture along the Tiscapa fault at a shallow depth focus. This resulted in an earthquake of magnitude 6.2 which caused strong seismic shaking which consequently led to over 10,000 deaths and 80% of the infrastructure being destroyed (“Earthquake wreaks devastation in Nicaragua” 1973).
Many features of the 1931 earthquake were evident in the 1972 earthquake, yet there was no significant mitigation strategies developed within the 40 year time period. Although Managua is prone to frequent earthquakes and tremors, the city had no mitigation strategies against the earthquake or its secondary effects. There were no effective warning systems or evacuation policies in place, no structural building codes and no immediate government aid available to residents. Managua received aid of several millions of pounds from more than 25 countries after the earthquake of 1972. Sadly, many victims never received most of this foreign aid (“Earthquake wreaks devastation in Nicaragua” 1973).
Natural disaster mitigation is not a major priority with the Nicaraguan government. Understandably, the reason for this low priority is the cost associated with mitigation strategies. Nicaragua is already one of the poorest countries in Central America. However, if any mitigation strategies had been in place, the loss of human and economic damage could have been significantly reduced. Putting measures in place to reduce the damage experienced during an earthquake will help the country in the long term since they would not have to worry about rebuilding the city each time a severe earthquake occurs. In addition to this, they would not have periods where their economy is not at full capacity or a large portion of their population displaced or affected in any other way.



EPILOGUE

Today, remnants of the 1972 Nicaraguan earthquake can still be seen throughout Managua. Central Managua was never fully rebuilt as the original foreign aid money was never given to the victims and many relocated to areas that were unaffected by the earthquake. At present, many small businesses, markets and parks have been set up to draw people back to the city centre.
The current Government of Nicaragua appears more focused on disaster risk management. The army’s defence unit has put a permanent plan for safety into place. However, SINAPRED, The National System for Prevention, Mitigation and Attention to Disasters in Nicaragua, conducted a study on Managua’s vulnerability and predicted that more than 30,000 people would be killed and 53,000 homes would be destroyed if the capital city were hit by an earthquake registering 6.9 on the Richter scale (Silva, 2013). This loss would be due to poverty and a lack of urban planning in the city which has led to settlements, where seismic building codes are disregarded (Silva, 2013). This shows how important it is for Nicaragua to implement and enforce strict codes that should be abided by all parties. In the long term, this will prevent natural disasters from taking a toll on the economic and human contributions to the country.
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