Source characterization of the San Juan (Argentina) crustal earthquakes of 15 January 1944 (Mw 7.0) and 11 June 1952 (Mw 6.8)

The backarc region of the Andes in the vicinity of San Juan, Argentina, is one of the most seismically active fold and thrust belt regions in the world. Four large damaging crustal earthquakes (1894, 1944, 1952 and 1977) occurred during the last 111 yr between 30 degrees S and 32 degrees S. We have determined the source parameters for two of these important earthquakes, the 1944 and 1952 events, using historic seismic records. The earthquake on 15 January 1944 had an epicentral location between the eastern thin-skinned Precordillera fold and thrust belt and the thick-skinned Sierras Pampeanas basement-cored uplifts. The I I June 1952 earthquake occurred in the eastern Precordillera about 35 kin southwest of the 1944 epicenter location. The P-wave first motions, long-period teleseismic P waveform modeling, and SV/SH amplitude ratio indicate a thrust focal mechanism for the 1944 event (strike N45 degrees E, dip 35 degrees to the southeast, and rake 110 degrees) with M-0=3.01 x 10(19) N m and M-w=7.0. The 1952 earthquake focal mechanism solution indicates a more oblique mechanism (strike N40 degrees E, dip 75 degrees to the southeast, and rake 30 degrees) with M-0 = 2.20 x 10(19) N in and M-w= 6.8. Both the 1944 and 1952 earthquakes have focal depths < 12 km and simple source time functions with one pulse of moment release with durations of 10 s and 8 s, respectively. Both the shallow focal depth and the east-dipping fault plane in the focal mechanism solution for the 1944 earthquake are consistent with the parameters observed along the La Laja fault in the frontal part of the eastern Precordillera that generated a 6-8-km-long coseismic surface rupture. The 1952 earthquake focal mechanism solution and its shallow source depth suggest it is related to faults in the eastern Precordillera, but a particular fault association is difficult. The 1944 earthquake was clearly the most destructive event because its proximity to the most populated area in San Juan, large size and shallow focal depth. (c) 2006 Elsevier B.V. All rights reserved.