NEAR-FIELD DEFORMATION SEEN ON DISTANT BROAD-BAND SEISMOGRAMS

Although far-field body waves and surface waves are widely recognized, the improvements in broadband networks now allow the measurement of ' 'near-field'' deformation of large earthquakes at great distances. Nea r-field motions have been recognized previously only in theory and in close-in recordings of earthquakes. We show examples from two recent e vents. For the large deep event on June 9, 1994 in Bolivia, there is a clear offset after the arrival of the P wave that has the amplitude e xpected for the near-field term. In the shallow September 2, 1992 Nica ragua earthquake, the very long-period motion observed between the P a nd S waves has roughly the amplitude expected from near-field terms. S uch near-field terms are insensitive to earth structure, but supply in formation on long-period source processes, and their observation begin s to close the gap between long-period seismology and geodesy.