Near‐field deformation seen on distant broadband seismograms

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Abstract
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.