BIG EARTHQUAKES IN SOUTH AMERICA

 

            The five largest measured earthquakes in South America also happen to be some of the world’s largest. These are the big ones listed in the USGS earthquake magnitude catalogue since 1900:

9.5       Valdivia Earthquake, Chile, May 22, 1960

8.8       Ecuador coast, January 31, 1922

8.8       Concepcion, Chile, February 27, 2010

8.5       Vallenar, Chile, October 10, 1922

8.4       Southern Peru, between Acari and Camana, June 23, 2001

             Many equally, or larger, earthquakes come estimated by the reported damage from periods before seismic recording. Notable major earthquakes from near Lima are:

            LIMA

8.6       1586 July 6

8.5       1687 October 20

8.6       1746 October 28

7.9       1828 March 28

8.2       1940 May 24

8.1       1966 October 17

7.6       1974 October 3

            Whether or not you include the large 8.1 in August 15, 2007 Pisco earthquake in this list depends on the expected size of the fault surface rupture patch. The section of the subduction trench between Pisco and Ica may be more mechanically linked than the segment offshore of Lima. Looking at the interval of time between these major quakes, we have 101, 59, 82, 112, 26, and 8 years. The 1974 earthquake was in northern Lima department/south of Ancash and probably should not factor into the running clock to the next major earthquake for Lima. If correct, then the duration from the 1966 earthquake has 51 years, which is within the wait time for another possible large earthquake. Another way to look at the earthquake recurrence pattern is compare the amount of fault slip to generate a large quake, and then back calculate the time from the amount of relative plate motions between the South American continent and the Pacific plate. The current convergence rate of the plates is 6.7 cm/yr. Thus with 51 years of grinding the Pacific plate beneath the South American western edge, we have 3.42 meters of stored elastic energy, provided that the Lima fault surface is indeed locked and has not relieved the strain through aseismic slip or a series of small earthquakes. The 1906 San Francisco 7.9M earthquake had up to 7 meters for displacement across surface ruptures along the San Andreas Fault strands. For a more comprehensive look at earthquake mechanics, see the paper “New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement” by Wells and Coppersmith (1994) published in the Bulletin of the Seismological Society of America. Just reading the title of the paper should provide a sense of the many factors that collectively are involved during an earthquake.

            Looking at another section of the South America subduction trench with longer records earthquakes, the region of Santiago has the following measured and estimated events:

            SANTIAGO

            8.5       1647 May 13

            8.7       1730 July 8

            8.2       1906 August 8

            8.0       1985 March 3

            These intervals are comparable with those found at Lima, having 83, 176, and 79 years. If one accepted varying conditions from along strike of the subduction system as not being significant, then the average recurrence times of earthquakes from Lima and Santiago combined comes out to be 89 years. The many variations that come into the process of forming a major earthquake will not cooperate to make the leap that in Lima we have another 38 years before the next major event. The above listed shorter hold times between slip events clearly indicate this. What is important is to acknowledge that numerous large earthquakes will continue to shake South America.

            Chile is adapted to living with big earthquakes, having building codes and emergency plans developed through experience. The people feel them often enough that they do keep emergency supplies at hand and take seriously tsunami warnings. Steps for earthquake preparedness can be found at the USGS Earthquake Hazard site or in Spanish provided by La Oficina Nacional de Emergencia del Ministerio del Interior y Seguridad Publica (ONEMI) at Chile Preparado by ONEMI. In many events, it’s not just the initial damage that causes harm, aftershocks can tumble already damaged structures, and other serious issues result from lack of water, spread of fires, mass controls for looting, and having supplies on hand for survival in areas of extreme weather. At minimum, keeping a week’s worth of water and food, plus a stash of blankets in a backpack can make a huge difference if everyone follows this practice. What happens in a city of millions when the earthquake topples the hospitals and cuts off transportation routes and water? A humanitarian disaster, education about earthquakes and following best practices in building and preparation is our only logical recourse.

The building practices in many cases determine how devastating the earthquakes can be. The use of adobe structures in the Andes is incredible prone to collapse, setting up terrible results for much smaller magnitude earthquakes. Surface motion during an earthquake interacts with building designs, materials, and their height; buildings can have wavelengths at which they are more susceptible to damage. The practice of building with brick and cement, without rebar, well, leads to those pancake-staked collapsed floors that are just grim. An excellent effort on reinforcing and redesigning Latin American building practices was put in a 2016 Geological Society of America paper by David Greene, called “Increasing resilience to earthquakes through educating community builders: Teaching earthquake-resistant building techniques in Guatemala.” The building standards can be looked at for codes used in California and methods employed in Chile. It’s hard to visit Lima and not look out on the vast sea of brick and cement buildings and know if the Callao quake of 1746 happened now, much of the city would be leveled. The same goes for small towns in the highlands of Peru, Ecuador, and Bolivia. The Andes were made through earthquakes, the crust is crumpling up, thickening over tens of millions of years, and living on top of this deforming mass of land has its risks in addition to the incredible landscape and lofty peaks. If everyone could see South America through the eyes of a geologist the cities and towns would be different and safer.

 

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South America seems to refuse to show its inexhaustible creative force.