Propagation and Attenuation of Lg Waves in South America
Final rept. 1 Aug 1987-31 Jul 1989
OBSERVATORIO SAN CALIXTO LA PAZ (BOLIVIA)
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Characteristics of Lg waves in La Paz station LPB are analyzed. After realizing that earthquakes with oceanic path and those deep do not produce Lg, they were discarded. Remaining 486 earthquakes, occurred from 1974 to 1986, are considered, looking for Lg characteristics, according to origin region. Lg are guided waves SH type, originated in surficial and in subduction intermediate depth earthquakes. Apparent velocity is 3.57 kms independent of distance, but with some dispersion beginning often is not clear. Predominant period is 1.1 to 1.3 s. Amplitude in most cases equals P amplitude it is normalized by dividing LgP, with results similar to Baths normalized wave energy. They are transmitted efficiently through shields, poorly along cordilleran structures from Peru Lg recording is uneven, meriting a more detailed study from southern region Argentina and Chile waves are weak, but not so much as suggested by a first glance wave period longer finds lesser recording gain. Recording in several South American stations is considered. Some earthquakes were revised. They confirm previous conclusions and help to identify efficiency of different paths and type of Lg recording, since origin regions and recording stations are at the ends of wave path. Type of recording may unveil hidden cordilleran structure in Andes-plains transition.