Earthquakes tell the story of changing tectonic plates along the Andes
We are now in Córdoba city (capital of the province of Córdoba), which is nearly 600 km (360 miles) east of the Cordillera Principal (high Andes). To the east of Córdoba are the flat plains of the pampa; to the west are the Sierras de Córdoba that, together with the other ranges lying eastward of the Cordillera Principal, are referred to as the Sierras Pampeanas, because they rise abruptly up from the flat pampa. Although reaching an elevation of almost 2900 meters (9500 feet), these sierras seem mere hills compared to the high Andean mountains to the west.
We have traveled through about 10 degrees of latitude, from north of Jujuy (23 degrees south) to Mendoza (33 degrees south). The Sierras Pampeanas are the mountains that extend east from the Andes from Tucuman to Córdoba. The Sierras de Córdoba are the mountains west of Córdoba city; they are very young, geologically speaking (less than 2 million years).
Why do these mountains extend so far east of the Andes? The Andes are produced by the convergence between the continental plate (South America) and the oceanic plate (Nazca), which is subducting beneath South America. In the Pacific Northwest of the U.S., an oceanic plate is subducting beneath North America, and we have the usual line of volcanoes (Cascade Range), but there are not additional mountain ranges located farther east. In Argentina, there are active volcanoes north and south of the Sierras Pampeanas, but not where these ranges extend so far east. We can look at the distribution of earthquakes to understand both the lack of volcanoes and the presence of subduction-related mountains so far to the east.
Notice that in the far north of Argentina, the earthquakes occur at shallow depths (red color) near the coast and are progressively deeper farther east (blue, then green for the deepest earthquakes). This is the expected distribution because the subducting ocean plate, which remains brittle and can store stress to create earthquakes, gets deeper and deeper as it is subducted farther beneath the continent. West of Córdoba, there are shallow (red color) earthquakes extending much farther east and there are no really deep (green color) earthquakes. This is because the oceanic plate is staying flat (so-called “flat slab”) and is not descending deep within the earth as it usually does.
These images are cross sections or profiles (perfiles) that show the two types of subduction. On the top is the normal distribution, with earthquakes getting deeper to the east, reflecting the steep inclination of the subducting oceanic plate. On the bottom is the “flat slab” distribution, with earthquakes at shallow depths, even at great distances from the subduction zone. Because the “flat slab” of the oceanic plate is not sinking much into the hot aesthenosphere, it does not cause melting and volcanoes. Because it is moving right beneath the continental plate, it affects this overlying continental plate and causes uplift within the plate, far from the subduction zone, along thrust faults.
Why does this happen, you may ask? There are a variety of reasons, including age of the downgoing oceanic plate and whether or not the downgoing plate has ridges or seamounts that interfere with the sinking process. One reason the Andes have been studied so much is because we had a “flat slab” situation in the western U.S. about 65 million years ago, when mountain ranges formed farther east, but as a result of subduction along the coast, in the region that is now the Rocky Mountains. Studying mountains where “flat slabs” are happening today helps us to understand how they happened in the past.
In San Juan, we visited INPRES, El Instituto Nacional de Prevención Sísmica (http://www.inpres.gov.ar), which aims to keep people safe in a seismically active region. They focus on two goals: monitor the earthquakes that occur, and develop standards for building that will keep them from falling during earthquakes. The center is located in San Juan because they had a strong earthquake in 1944 that destroyed the whole town, where most buildings were constructed of adobe. The number of fatalities was huge and this city has focused much attention on making sure that buildings are reinforced and able to withstand shaking.
We met with a civil engineer who was passionate about the need for better building standards. The seismographs are historical relicts; today they have modern equipment and computers for earthquake monitoring.