Local earthquake tomography of the Altiplano and the Eastern Cordillera of northern Bolivia

Abstract

The velocity structure in the crust beneath the central Andes of northern Bolivia is obtained by the simultaneous inversion of local earthquake arrival-time data for velocity and hypocentral parameters. The data were recorded between October 1990 and February 1991 by the French “Lithoscope” network; the 34 vertical, short-period seismic stations were installed along a 320-km-long profile crossing two main morphological units of the Andean chain; the Altiplano and the Eastern Cordillera. The arrival times of seismic waves generated by earthquakes located in the subduction zone of the Nazca plate underneath the South American plate were inverted using a modified version of Thurber's 3-D iterative simultaneous inversion code. The results are consistent with the inversion of teleseismic travel-time residuals from the same network and closely correlate with the mapped surface geological features. The Altiplano Basin is characterized by a low-velocity zone down to 20 km, enclosed by the San Andres and Laurani faults. At the surface, the Cordillera Real fault zone (CRFZ) bounds to the west the high P-wave velocity anomaly lying under the axial zone of the Eastern Cordillera. The velocity discontinuity dips steeply to the southwest, to the bottom of the crust. From the velocity model, the depth of the Moho is estimated to be 66 km under the Altiplano. Moreover, the different behaviour of P- and S-waves found below the Western Cordillera and above the slab is consistent with the presence of partially melted asthenosphere that may be responsible for the magmatic activity. Finally, the seismicity defines, between 90 and 225 km, a part of the slab dipping to the north-northeast with a low angle of about 30°.