High Frequency Shear Waves Guided by a Continuous Lithosphere Descending beneath western South America

Abstract

Very prominent high frequency seismic waves arriving after the S phase from deep earthquakes (550–600 km) are interpreted as shear waves guided by the part of the suboceanic lithosphere, the Nazca plate, that has descended into the mantle beneath western South America. The large amplitudes and character of the phase suggest that the lithosphere descending beneath northern Chile and Argentina is continuous through the gap in seismicity between deep and intermediate depth earthquakes. The unusual phase is well recorded by the Peruvian stations ARE and NNA at epicentral distances of 13° to 21° from deep earthquakes located beneath northern Argentina and southern Bolivia. The phase is characterized by an apparent velocity near 4.7 km s−1, predominant frequencies near 1 Hz and an emergent beginning and long duration. At an epicentral distance of 20° the phase arrives nearly a minute after the normal S phase. The apparent velocity and travel times are in agreement with the hypothesis that the phase goes upwards through the gap in seismicity, traverses the intermediate depth seismic zone, and then travels horizontally in the uppermost mantle as the seismic phase Sn.