Patricia Alvarado's Research Page

Crustal seismicity in the back-arc region of the Southern Central Andes from historic to modern times

The Andean Cordillera is the result of the compression between the Nazca and South America tectonic plates. The convergence rate between the two plates of 85 mm/yr at an azimuth of 78 generates a large number of earthquakes. In the south-central Andes, four seismogenic zones are identified: the outer-rise region located in the subducting plate seaward from the trench, the plate interface, within the down-going slab, and within the overriding South America plate. While the largest magnitude earthquakes are the interplate subduction zone events, the historically most devastating earthquakes have been the moderate-to-large magnitude crustal earthquakes in the South American plate.

The subducting slab under the South Central Andes exhibits some significant along-strike changes in the subduction geometry. One of these changes occurs around 33S where the Nazca slab subducts near horizontally north of 33S and inclines ~30 south of this latitude. The region above the flat subduction is marked by a shutoff of the active volcanic arc, and has a wider back-arc region of active deformation and earthquakes. My studies will focus on the crustal seismicity and crustal structure in the back-arc region of the Sierras Pampeanas of Argentina above the flat slab. I m studing crustal earthquakes using broadband seismic data collected during the Chile-Argentina Geophysical Experiment (CHARGE) between November 2000 and May 2002. During that period, 75 crustal earthquakes with magnitudes greater than 4.0 occurred under the main cordillera and in the backarc region. I obtained the moment tensor solutions and depths for as many of these events as possible using a regional moment tensor inversion technique.

For more details see:
Regional Moment Tensor Inversion of crustal earthquakes

In addition to providing seismotectonic information, these continental earthquakes can also be used to determine the crustal structure. Some geologic studies suggest that the Paleozoic terrane sutures, some of them later reactivated during the Mesozoic extension, have an important control on the present day Andean deformation. One of my goals is to study the crustal seismic velocity structure by utilizing raypaths that sample different geologic provinces in the region. With the available event-station paths, I will be able to compare crustal velocities from the western and eastern Pampean ranges, as well as in the Cordillera and Precordillera. The basement rocks exposed in the western Sierras Pampeanas are more mafic than the basement exposed in the eastern Sierras Pampeanas. If this is representative of the entire crust, then I should be able to map out these differences using crustal earthquake data.

For more details see:
Structure: Sensitivity to crustal models

The Sierras Pampeanas and Precordillera have a long history of devastating earthquakes in Argentina. Within the past century, at least three crustal earthquakes with magnitudes larger than 7.0 have caused approximately 5,000 deaths and major economic losses. Only the 1977 earthquake has been studied in any detail. I will study these previous large events using historic seismograms recorded at teleseismic distances, which will require collecting and digitizing historic paper seismograms, and developing techniques to constrain their source parameters. Although a time-consuming process, this will allow me to extend my study of the crustal seismicity from the present day to a 60-year-long period. The longer period of time will be very important to evaluating the seismic hazards of the region.

For more details see:

Historical Earhquakes in San Juan, Argentina

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Latest revision: January 9, 2005