Seismic source study and tectonic implications of the historic 1958

ALVARADO, P.; BARRIENTOS, S.; SAEZ, M.; ASTROZA, M.; BECK, S.

Niza, Francia

Simposio; 7th International Symposium on Andean Geodynamics; 2008

Although Chile is recognized as the site of the largest megathrust earthquakes related to the coupling between the subducting Nazca plate and the overriding South American plate like the 1960 Valdivia earthquake, infrequent crustal earthquakes within the continental plate can be very damaging. The earthquake on 4 September 1958 that occurred in Las Melosas, Central Chile (-33.826°S and -70.140°W; Engdalh et al., 1998) represents one of the large damaging intraplate events located in the Andean cordillera crust at about 60 km away from Santiago. In this study, new estimates of fault orientation, depth and size using teleseismic bodywave modeling of the 1958 Las Melosas earthquake are presented (Fig. 1). Although global seismic catalogues (BSSA, 1959) include only one earthquake on 4 September 1958, Lomnitz (1960), Piderit (1961) and Pardo and Acevedo (1984) have reported the occurrence of more than one seismic event (at least three) separated by a few minutes and of similar sized-magnitudes. In fact, these authors assigned a 6.9 magnitude for the three subevents, but Flores et al. (1960) reported 6.9, 6.7 and 6.8, respectively. These estimations of the seismic magnitudes are mainly based on historical intensity reports (Lomnitz, 1970). Our results for the first event in the sequence of earthquakes on 4 September 1958 that occurred in Las Melosas indicate a focal mechanism solution with fault planes of right-lateral displacement on an east-west fault and left-lateral displacement on a north-south fault and a focal depth of 8 km produce the best fit to teleseismic long period P-waveforms (Fig. 1). A seismic moment M0 of 0.227 x 1019 N-m associated with a momentmagnitude Mw of 6.3 has been estimated, which is 0.4 to 0.7 units larger than the surface-wave magnitude Ms earlier reported. Although no surface rupture was reported, the displacement along east-west structures like that one suggested for one of the fault plane in our focal mechanism solution of the 1958 event seems to be an efficient mechanism to accommodate differences in shortening from north to south along the high Andean Cordillera (Alvarado et al., 2008). New findings on the 1958 Las Melosas earthquake intensities by Sepúlveda et al. (2008) allow us to compare them with our study about the seismic source of this crustal event in order to put more constraints on the seismic hazard to which this zone, and others along the western foothills of the Andes, is exposed.