Propagation efficiency of long-period Lg waves in the South American continent

Abstract

We investigate the propagation-efficiency characteristics of long-period (3-12 s) Lg waves in continental South America as recorded in the last few years on stations of the GTSN, IRIS/USGS, IRIS/IDA and GEOSCOPE networks. This paper reports the results of a first look at the available Lg data for events with Ms > 5.0 from 1990 to 1995. In this period seven main seismic events and aftershocks along the length of the Andes produced a total of 29 usable paths from which preliminary results indicate that (1) long-period Lg waves are unusually prominent in seismograms of continental paths covering the shield areas of South America, and are clearly observed on broad-band instruments at epicentral distances exceeding 3500 km, (2) Lg propagation is substantially attenuated (weak or absent) for paths along the strike of the Andes and along the three active volcanic chains of Chile, Peru and Colombia, (3) away from the active volcanoes, the data show Lg propagating more efficiently across than along the strike of the Andes, and (4) in general, the ratio of the transverse to the vertical components of the Lg phase decreases with decreasing propagation efficiency. These first results also raise a number of exciting questions regarding the exploration of the crustal waveguide in South America, such as the potential for Lg-based tomography to detect a postulated fossil plume in southeastern Brazil, or how the propagation efficiency of high-frequency Lg in volcanically active areas may be used to detect magma chambers. Consequently, we intend to carry out a systematic study of Lg in the continent as part of the ongoing Surface-wave Inversion for the South American Lithosphere (SISAL) project, a collaborative effort (Vdovin et al. 1996) to determine the crustal and lithospheric structure of the South American Plate from measurements of surface-wave dispersion in the period range 2-200 s.