Managua's Seismic Vulnerability
RSJGringita's post on Earthquake-resistant construction in Managua, deserves a closer look I think. I did some small research and the results I found are worrying. In the week after the earthquake of Port au Prince, some articles that were published in LP and END expressed the same worries.
Managua has been struck by a series of moderate earthquakes in recent history: 1844, 1858, 1881, 1898, 1913, 1918, 1928, 1931, 1968 (Magnitude 4.5) and the last one on December 23, 1972 with magnitude 6.2. The epicenters of these earthquakes were all right below Managua. The Managua Fault Zone is seen as a North-South-trending, graben structure of regional importance. A graben is a depressed block of land bordered by parallel faults. These grabens and fault zones are all side effects of the big subduction zone laying 50 miles before Nicaragua's Pacific coast. Take a look at this seismic map.
The first more or less earthquake resistant building codes after the 1972 Managua earthquake were introduced 11 years later in 1983 (Including requirements for rigid floor and roof diaphragms). So a lot of rebuilding after 1972 wasn't done earthquake resistant. The latest building codes are of 2005, which you can find here. Never the less even today most new buildings are still not compliant with these building codes. This could be because of ignoring the building codes, bad design, badly trained construction workers, bad materials, bad supervision or not following the design (by example bad concrete mix or rebar implementation).
In August 2005 the World Institute for Disaster Risk Management was asked by INETER and SINAPRED to study the seismic vulnerability of Managua. The report is available here . It is over 200 pages and it is a "bit worrying" if you read it. A "positively adjusted" executive summary of the report is published on INETER's website and is leaving out all sensitive conclusions and recommendations. How remarkable! The report was financed with Worldbank money. Apparently their was no follow up project defined.
Some of the conclusions and recommendations (the list is much longer):
- In Managua (luckily) buildings of one and two levels dominate.
- Construction methods that are considered highly vulnerable, such as adobe and taquezal have fallen into disuse in Managua.
- It is noteworthy that 99% of buildings lacks a rigid horizontal roof diaphragm. A horizontal diaphragm transfers lateral forces by acting like a horizontal beam. It spans between shear walls or frames located in the story below the diaphragm. During an earthquake, the diaphragm is subjected to horizontal forces that are based on its own mass and the tributary mass of the walls attached along its edges. A rigid roof diaphragm is composed of a ring beam, wall plate and a roof truss, or a ringbeam and a solid attic floor.
- the fire department building in Zona 1 (1980) is in very bad ("unacceptable") state. It needs urgent repair. The roof will collapse in the event of an earthquake and that would disqualify this vital service . This point certainly deserves priority attention from the authorities
- something needs to be done with the old cathedral since it will not withstand an earthquake of medium intensity. The value to the city is incalculable, and its total loss will be deeply regrettable and will be a tough emotional blow to the population.
- Regulate and strengthen the supervision and quality control during construction.
- We conclude that the current conditions of Managua are much more favorable if an earthquake like the one in December 1972 will happen again. In relative terms, the losses would be lower in that event. The same applies for the number of victims, ie, although the estimated number of deaths in a similar scenario is more than 9,000, in relative terms it is substantially lower than what happened in 1972 (5000).
Back in 1972 most of the structures were of taquezal construction and at least 40 years old. Taquezal construction means walls composed of wood post and beam construction using horizontal wood strips with adobe or stone fill and plaster on both the interior and exterior face. Because of termite damage and other erosive processes, the older taquezal buildings were damaged more severely than the newer ones. Because all the fire-fighting equipment had been demolished, fires raged out of control in the downtown area for several days.
According to INETER, 70 percent of the buildings in Managua has been realized without any control of municipal authorities or any government entity. The thirty per cent that has been supervised in some way or another are shopping malls, hospitals, gas stations and new housing developments, which are required to meet certain requirements prior to construction. CIGEO is calling for enforcement of the 2005 building codes in its Tierra Journal no. 20. La prensa is referring to this journal on November 21, 2009.
"The whole city is in a micro-graben (ie, fractured and traversed by faults in preferential directions that are mainly North-South). The risk is not just in certain areas of the city. According to data of the Centro de Investigaciones Geocientíficas de la Universidad Nacional Autónoma de Nicaragua (CIGEO), the population of Managua is at grave risk, because it's sitting right on the largest seismic swarm that cuts right through the capital. CIGEO also indicated that the soil type of Managua is soft because of loose volcanic material, that is weak in itself and provides greater energy release in case of an earthquake."
This post here on NL is only about the seismic vulnerability of Managua. In case you didn't know, the city of Managua is quite vulnerable to other (natural) hazards as well: Landslides, tsunami's, inundations, floods, volcanic ash, volcanic gases, lava streams, pyroclastic flows. Risk Assessment Bureaus have even suggested in the past to "move" the capital city of Managua to a safer place. Maybe the biggest hazard is the fact that the government is located in Managua and all other vital organizations that need to act in case of a disaster are located there too.