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. Near-field motions have been recognized previously only in theory and in close-in recordings of earthquakes. We show examples from two recent events. 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 expected for the near-field term. In the shallow September 2, 1992 Nicaragua earthquake, the very long-period motion observed between the P and S waves has roughly the amplitude expected from near-field terms. Such near-field terms are insensitive to earth structure, but supply information on long-period source processes, and their observation begins to close the gap between long-period seismology and geodesy.