Seismotectonic characterization of the 1948 (MW 6.9) Anta earthquake, Santa Bárbara System, central Andes broken foreland of northwestern Argentina

Abstract

The region of the Andean back-arc of northwestern Argentina has been struck by several magnitude ≥6 crustal earthquakes since the first historically recorded event in 1692. One of these events corresponds to the Anta earthquake on 25 August 1948, with epicenter in the Santa Bárbara System causing three deaths and severe damage in Salta and Jujuy provinces with maximum Modified Mercalli seismic intensities (MMI) of IX. We collected and digitized analog seismograms of this earthquake from worldwide seismic observatories in order to perform first-motion analysis and modeling of long-period teleseismic P-waveforms. Our results indicate a simple seismic source of M0 = 2.85 × 1019 N m consistent with a moment magnitude Mw = 6.9. We have also tested for the focal depth determining a shallow source at 8 km with a reverse focal mechanism solution with a minor dextral strike-slip component (strike 20°, dip 30°, rake 120°) from the best fit of waveforms. Using magnitude-size empirical relationships, the comparison of the obtained Mw 6.9 magnitude value and the ca. 10,000 km2 area of MMI ≥ IX from our seismic intensity map, which was obtained from newspaper and many historical reports, indicates a rupture length of 42 ± 8 km for the Anta earthquake. We show our results in a 3D geological model around the epicentral area, which integrates modern seismicity, geological data, and information of a previously studied east-west cross section located a few kilometers south of the 1948 epicenter. The integration of all available information provides evidence of the re-activation of the Pie de la Sierra del Gallo fault during the 1948 Mw 6.9 shallow earthquake; this thrust fault bounds the Santa Bárbara System along its western foothill.