Nigeria hedges on the quake zone

THE GUARDIAN Newspaper-Chukwuma Muanya

ASK the Haitians. An earthquake is not a phenomenon a people hears about and go to sleep without preparations. The January 12 earthquake in that country killed no fewer than 150,000.

That is why the warning of Nigerian scientists at the National Space Research and Development Agency (NARSDA), that Nigeria should be prepared for earthquake experience should set the authorities thinking and planning now. The predicted date of 20 years from 2008 of an earthquake occurrence in Nigeria is not far.

In a recent publication in a learned journal, the scientist said:

"At exactly 3:10 GMT on September 11, 2009, an earth tremor occurred in the Abeokuta environs, Ogun State. This earthquake was felt mainly in most parts of Ogun State and some parts of Lagos State. Three seismic stations of the Centre for Geodynamics and Geodesy (CGG) network in Nigeria, recorded this event. Since 2008 when CGG network started, September 11 event was the first major tremor recorded instrumentally. Although there have been many earth tremors (minor to major) in the past in this part of the country and other areas but there have

not been local instrumental records to analyse the seismicity (vulnerability to earthquakes) of the region."

The NARSDA researchers said the tremor is a sign that Nigeria is not immune from earthquake occurrence.

One of the researchers from the Geology Department of Obafemi Awolowo University (OAU), Ile Ife, Osun State, Dr. Abraham Adekunle Adepetumi told The Guardian that the country is not in the earthquake-safe region.

Predicting long-term earthquake possibility for the Southwest, he said: "The Empirical Earthquake Recurrence Model - a time-dependent model - was employed to predict the probabilistic occurrences of earthquakes in the Ijebu-Ode and environs between the year 2008 and 2028.

"This probability model takes the mean recurrence intervals and standard deviation of historic earthquake events in this area in order to determine the probability of earthquakes occurrence for the predicted years."

Meanwhile, research at the University of Liverpool, United Kingdom (UK), studying a large fault zone in the Atacama Desert in northern Chile, has produced new insight into how fluid pressure can cause earthquakes.

In the report published in Nature, the researchers found how fluids such as water, become sealed within the earth's fault planes for a long period of time. This fluid pressure makes it easier for the earth's plates to move alongside each other, eventually resulting in an earthquake.

Oxford University, UK, scientists have uncovered that very large earthquakes can trigger an increase in activity at nearby volcanoes.

An analysis of records in southern Chile has shown that up to four times as many volcanic eruptions occur during the year following very large earthquakes than in other years. This "volcanic surge" can affect volcanoes up to at least 500 km away from an earthquake's epicentre.

A report of the work was published in the journal Earth and Planetary Science Letters.

Previously, scientists had identified a few cases where volcanic eruptions follow very large earthquakes - but up until now, it had been difficult to show statistically that such earthquakes may be the cause of an increase in eruptions, rather than the events just being a coincidence.

Researchers at OAU had investigated the fault zone-case history of Ile-Ife. Their work, published in Geophysical Prospecting in 2006, investigated a fault zone of a major linear structure from aerial photographs of the OAU Campus.

Results of a multi-method geophysical survey indicate that the zone is characterized by relatively low resistivity and high magnetic effect. They are characteristic of a sheared and perhaps saturated fault-zone with magnetic mineral infillings along its plane.

The authors - M. O. Olorunfemi, V. O. Olarewaju and M. Avci - wrote: "The near symmetry of the magnetic anomalies over the fault-zone may not be indicative of a step faulting. The geoelectric sections show no indication of any significant displacement. The above supports results of a previous study that the fault is a strike-slip fault."

The result of the analysis of Adepelumi's team showed that the fault that resulted into the tremor rupture was about 10 km within the upper crust with a stress drop of about 0.265 bar and a 3.07 km radius rupture.

The researchers in a technical report submitted to the Director, CGG, Toro, Bauchi State, wrote: "Considering the areas where the tremor was felt, the rupture process propagated up to north-east and down to south-west. The majority of the fault plane solutions for the rupture process suggested a normal dip-slip mechanism.

"In the past there have been occurrences of earth tremors ranging from minor to major in the southwest Nigeria. Between the year 2000 and 2009 there have been earth tremors not less than four, having moment magnitude greater than and equal to 4.0 going by the magnitude of the September 11, 2009 earth tremor. Therefore, the faults within these areas are very active with a return period of about two years."

The result of the analysis of this tremor is titled: "Preliminary interpretation of the September 11, 2009 aarthquake in Abeokuta environs with Mw=4.2". Adepelumi said the result would not only serve as basis for future research studies but has also revealed possible seismotectonic nature of the region.

The geologist said there is probability of recurrence of earthquake of intensity V-VII in Southwestern Nigeria.

"The results of the model showed that the probability of earthquake occurrence in the study area between the year 2008 and 2028 increased from 2.8 per cent to 91.1 per cent. The result also showed that the probability of two events occurring has the highest likelihood within the predicted years," he said.

Indeed, natural hazards are assuming ever-greater economic importance, not only on a regional scale but also on a global scale. The growth of major cities in hazard prone areas, and the public anxiety associated with earthquake risks such as destruction of buildings, fire outbreak and lost of life has focused attention on the problems of insurance against natural hazards, disaster mitigation, and disaster prevention.

Meanwhile, scientists at the Carnegie Institution, United States (U.S.), have found a way to monitor the strength of geologic faults deep in the Earth. This finding, published in Nature, could prove to be a boom for earthquake prediction by pinpointing those faults that are likely to fail and produce earthquakes. Until now, scientists had no method for detecting changes in fault strength, which is not measurable at the Earth's surface.

It has been shown that fault strength is a fundamental property of seismic (caused by an earthquake) zones. Earthquakes are caused when a fault fails, either because of the build-up of stress or because of a weakening of the fault.

In geology, a fault or fault line is a planar fracture in rock in which the rock on one side of the fracture has moved with respect to the rock on the other side. Large faults within the Earth's crust are the result of differential or shear motion and active fault zones are the causal locations of most earthquakes.

Before now, changes in fault strength are much harder to measure than changes in stress, especially for faults deep in the crust. The result of the U.S. study opens up exciting possibilities for monitoring seismic risk and understanding the causes of earthquakes.

Adepelumi added: "Therefore, it is instructive as a necessary measure to henceforth incorporate seismic criteria in the site investigations for design and construction of major and sensitive structures like nuclear power plants the country is seriously considering as alternative source of energy for electricity generation, whose sudden damage can lead to disruption of major essential services, loss of lives and property and general degradation of the environment."

Adepetunmi said: "We have been working in partnership to monitor what is happening in the subsurface of Nigeria and very recently, on September 11, there was a tremor in Abeokuta, Ogun State. I was in there to monitor what happened to get the data for our stations.

"We have learnt before that Nigeria is outside the seismic region, that is to say it is safe. In the last two to three years, with what we gathered from the CCG, some parts of Nigeria especially South-west, are not safe."

He said Nigeria is now regarded to be within the seismic region and anything can happen anytime.

He warned that there is urgent need for the country to be prepared and get ready to invest in disaster monitoring equipments that will help in mitigating such disasters.

Adepelumi in a paper tilted: "Broadband earthquake monitoring at OAUIFE Station, Nigeria", said although, no active faults have been established in Nigeria, however, a careful review of seismicity record in Nigeria had showed that Nigeria may not be seismic as has hitherto been believed.

He buttressed this with recorded events over the years in the country: Ijebu-Ode (July and August, 1984); Gulf of Guinea (December, 1984); Warri (1933); Lagos (1939); Umuahia (July, 1961); Kundunnu, Bauchi (1981); Dambatta, Kano (July, 1975); Yola (December, 1984) and Gombe (June, 1985).

"Thus, monitoring and prediction of earthquake hazards could begin by a proper documentation of the pathways through which they emanate, using seismological data."

He said long-term earthquake monitoring should be embraced by Nigeria. "Expand OAUIFE network to a comprehensive and effective earthquake information system centre. Acquire through the United Nations Educational, Scientific and Cultural Organisation (UNESCO) more sensitive seismometers that will serve as an early earthquake warning system. Study the crustal and mantle structure beneath Southwestern Nigeria," Adepelumi said.

He said Guralp system was installed at OAUIFE seismic station (7Œ 31'23"N and 4Œ31'27"E) in November 2007 through the funding provided by UNESCO office in Italy. "Inverter and solar panel were installed in November 2008 for the smooth running of the seismic station," Adepelumi added.

Some of the events recorded at OAUIFE Earthquake Station include: Iceland, May 29, 2008; Mexico event - 2008; and Tonga event - 2008.

He said: "Iceland is sitting on a plate boundary where the North America and Eurasian plates are drifting apart. So earthquakes are common but large earthquakes are relatively rare. On May 29, 2008, a strong earthquake measuring 6.1 hit southern Iceland, 50 km from the capital, Reykjavik.

"The U.S. Geological Survey said the earthquake struck at 15:46 GMT at a shallow 6.2 miles (10 km). The earthquake was also picked at the same time at OAUIFE."

On the implication of the Iceland teleseismic to Nigeria, Adepelumi said it provided insights into the structure of the upper mantle beneath Southwestern Nigeria; lithospheric structure of Southwestern Nigeria from the teleseismic data; into the crustal structure beneath Southwestern Nigeria; character and kinematics of the Megashear system in the Gulf of Guinea; and implications for tectonic evolution of Southwestern Nigeria.

Paul Silver and Taka'aki Taira of the Carnegie Institution's Department of Terrestrial Magnetism, with Fenglin Niu of Rice University and Robert Nadeau of the University of California, Berkeley, U.S., used highly sensitive seismometers to detect subtle changes in earthquake waves that travel through the San Andreas Fault zone near Parkfield, California, over a period of 20 years. The changes in the waves indicate weakening of the fault and correspond to periods of increased rates of small earthquakes along the fault.

The section of the San Andreas Fault near Parkfield, sometimes called the "Earthquake Capital of the World," has been intensively studied by seismologists and is home to a sophisticated array of borehole seismometers called the High-Resolution Seismic Network and other geophysical instruments. Because the area experiences numerous repeated small earthquakes, it is a natural laboratory for studying the physics of earthquakes.

Seismograms from small earthquakes revealed that within the fault zone, there were areas of fluid-filled fractures. What caught the researchers' attention was that these areas shifted slightly from time to time. The repeating earthquakes also became smaller and more frequent during these intervals - an indication of a weakened fault.

Niu said: "Movement of the fluid in these fractures lubricates the fault zone and thereby weakens the fault. The total displacement of the fluids is only about 10 meters at a depth of about three kilometers, so it takes very sensitive seismometers to detect the changes, such as we have at Parkfield."

What causes the fluids to shift? Intriguingly, the researchers noticed that on two occasions, the shifts came after the fault zone was disturbed by seismic waves from large, distant earthquakes, such as the 2004 Sumatra-Andaman earthquake. Pressure from these waves may have been enough to cause the fluids to flow.

"So it is possible that the strength of faults and earthquake risk is affected by seismic events on the other side of the world," Niu said


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