2007 Peru Earthquake

Introduction
Earthquakes occur when there is faulting, a shift in the plates takes place and the stress is released along a fault line. There are different plate boundaries that can produce earthquakes: divergent, convergent, and transform boundaries. In a divergent boundary the plates are moving apart, they produce shallow earthquakes that are fairly small and not common. At a convergent boundary the plates move towards each other, creating deep and shallow earthquakes, greatest in size. A transform boundary is when the plates are sliding past each other creating shallow earthquakes that can be large. Earthquakes are measured by different scales. The Richter-Gutenberg Scale is based on a logarithmic calculation looking at the magnitude of the seismic energy released by the earthquake. Its effects will change according to distance, ground conditions, and construction standards, as well as other factors.

Below (Figure 1) is the Richter-Gutenberg Scale and the damage the earthquake can produce at each level. Another scale used to measure earthquakes is the Mercalli Intensity Scale, which is based on the intensity of the earthquake. The scale below (Figure 2) looks at the effects of the earthquake on the Earth's surface, humans, objects of nature, and man-made structures. This scale is based on the numbers I through XII, with I being not felt, and XII being the one that causes almost complete destruction. The numbers can change based on the distance to the earthquake, with the highest intensities being around the epicenter. This information is gathered from individuals who have experienced the quake, and an intensity value is given at their location while experiencing the quake.



Figure 1: Richter-Gutenberg Scale
external image richterscale.jpgexternal image merc_mag.gif Figure 2: Mercalli Scale


Background Information
On the evening of August 15, 2007 a devastating earthquake shook Peru just south of its capital city of Lima. This earthquake was measured at a 7.9 on the Richter scale (Figure 3) and it was reported that strong ground motion was felt for about two minutes. The earthquake was result of trust faulting at the boundary between the Nazca and the South American Plate. The Nazca plate is subducting underneath the South American plate at a rate of about 70mm / per year. The Nazca plate seems to be subducting into the trench smoothly but as it gets deeper and deeper it is breaking in to smaller pieces that become stuck which will eventually generate large earthquakes. According to the U.S Geological Survey the hypocenter was 25 miles west-northwest of Chincha Alta, Peru, and 90 miles south-southeast of Lima and the epicenter was 30.2 km deep (18.8 miles).
The Andes mountain range, which runs north-to-south along Peru's Pacific coast, has many active fault lines but no large earthquake like this had occurred in this area since at least 1746 and possibly even earlier (around 1687). After the mainshake about a dozen aftershocks took place ranging in magnitude from a 4.5 to a 6.3 which are big for aftershocks!Looking at the history of the Peru-Chile tectonic boundary it has the capability of producing large earthquakes and is very active fault releasing stress often. This earthquake also resulted in a tsunami; the most destructive tsunamis along the South American coast have been caused from large shallow earthquakes close to the trench, which is exactly happened with this earthquake. Deeper earthquakes cannot produce tsunamis.


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Figure 3- shows the location of the epicenter and aftershocks


Damages
Almost over 600 people lost their lives in this natural disaster and over 1,000 were injured. Water and electricity were out and communication was disrupted. About 75% of buildings were destroyed due to the lack of urban planning and also the softness of the soil where the buildings were constructed which resulted in landslides (Figure 4). Urban planning has only existed within the last 60 years, and it the well thought out planning of the city or buildings design and functioning. The architecture is very important so that structure failure is at a minimum or none at all. This is very important for mitigation, so that the lowest number of fatalities and structural damage will occur. If people are aware of the make up of an area they can take precaution when building, because most of the deaths and damages were due to structures collapsing.
The city of Pisco was hit the hardest by the earthquake, about 10,000 homes were destroyed or badly damaged, and struggled to receive resources for rebuilding their towns. The local aid that was set aside for disasters disappeared quickly due to the size of the devastation and lack of investment in risk prevention and emergency response. The area was declared a state of emergency and international help was brought in. The areas hit hard were difficult to reach due to the large cracks in the Panamerican highway.
This large earthquake also generated a tsunami along the central coast of Peru creating waves from 3 meters to 10 meters and the first waves reached the coast only about 15 minutes after the earthquake. Luckily most of the coastal area is unoccupied desert land and people that were in the area did evacuate which saved many lives. Further down the coast where communication was down and the villages did not receive the tsunami warning, it was very dangerous as boats where being push inland and flowing into the streets unfortunately lives were lost.

The quake had a few particularities that may have helped save many lives. There were two peaks in amplitude of the earthquake rather than one large duration quake, which you can see in figure 4. This means that the initial large shaking would have sent many people from their homes, followed about 30 seconds later by even greater shaking. As mentioned before, many buildings collapsed but thankfully many of the people fled before the second round of shaking could deliver the final blow to their homes. The separated amplitudes are the result of two phenomenon of the quake. First, the fault rupture had two zones of displacement. Second, the velocity of the quake itself was only a third of the speed compared to other similar quakes. Combined, the two phenomenon resulted in a stuttered quake, and allowing time for citizens to escape collapsing homes.

Figure.JPG
Figure 4 - Siesmic Graph of Peru Quake. Note the two large amplitudes



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Figure 5- Landslide due to faulting and liquefaction of the soft soil near the Pacific Ocean

Conclusion
There is so much to learn from events like this, unfortunately we begin to take the further steps to try and mitigate for a big event like this after a similar devastation occurs. Mitigation is the effort to reduce loss of life and property by lessening the impact of disasters. This is accomplished through risk analysis, which gives us information so we can practice activities that reduce risk, like the Shake Out. The Shake Out is the largest earthquake drill where everyone practices Drop, Cover, and Hold On so people are aware of what to do to protect themselves when an earthquake occurs.

Sources
http://vulcan.wr.usgs.gov/Glossary/Seismicity/what_causes_earthquakes.html
http://www.usc.edu/dept/tsunamis/2005/tsunamis/Peru_2007/index.html
http://content.undp.org/go/newsroom/2009/august/two-years-after-the-2007-peru-earthquake.en

http://www.seismo.unr.edu/ftp/pub/louie/class/100/mercalli.html
http://www.agu.org/pubs/crossref/2008/2008GL033494.shtml

http://www.drgeorgepc.com/Earthquake2007Peru.html