In the past three days, the Indonesian Meteorology, Climatology and Geophysics Agency (BMKG) has recorded at least five consecutive earthquakes in the Asian island country, including one of magnitude 5.6 on the Richter scale that last Monday shook the main island of Java, where about 145 million people live.
Although this magnitude would normally be expected to cause slight damage to surface structures and, therefore, few casualties, more than 270 deaths, hundreds injured, trapped and missing, and more than 13,000 displaced have been reported after the earthquake. , reports the BBC.
How was it possible that an earthquake considered to be of moderate magnitude —which is usually very frequent in the countries located within the Ring of Fire— has resulted in so many human losses and victims? To resolve our doubt, La República spoke with Hernando Tavera Executive President of the Geophysical Institute of Peru (IGP).
“The earthquakes that have occurred in Indonesia are of moderate magnitude, which here in Peru occur once a week. These have happened due to the reactivation of a fault very close to the surface, and for this reason they have generated high levels of ground shaking that have caused precarious houses to collapse,” said Dr. Tavera.
“earthquakes do not kill people. People only lose their lives due to the total or partial collapse of their homes, which cannot withstand the level of shaking of the ground”, points out the expert.
In this sense, the buildings that collapse are those that were poorly built, that were made of inadequate material, that have an inadequate geometry or that have been built in areas where the soils are bad or on surfaces that are not suitable (on the slopes , for instance).
Now, it is important to note that the magnitude 5.6 earthquake in Indonesia occurred 10 kilometers deep under the city of Cianjur (Java), a distance considered almost superficial.
“The closer the earthquake is, in distance and depth the shaking of the ground is greater,” says Tavera. This is because seismic waves reach the surface faster and with greater energy at shallower depths.
“In the Colca Canyon, for example, earthquakes of magnitudes close to five also occur, which cause damage in nearby urban areas because they also occur due to geological faults, because they have shallow depths and because the houses are very weak.”
Like Indonesia, Peru is also located in the Ring of Fire of the Pacific Ocean, for that reason it is a highly telluric country. However, it is impossible to know when the earthquakes of what magnitudes will occur, much less at what depth they will occur.
Although both cases are very different, according to Tavera, it could be predicted which areas would have more victims just by looking at the places with weaker infrastructures.
“For example, in Lima where there are bad soils it is Villa El Salvador, the Pantanos de Villa, in Chorrillos or Ventanilla. Now, where there are houses that are not good is in San Juan de Lurigancho, Villa El Salvador, San Juan de Miraflores, El Agustino, Puente Piedra, Carabayllo. You only have to go up to a tall building and look around and realize it”, indicated the executive president of the IGP.