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Higher-mode Rayleigh waves as an aid to seismic discrimination

LAMONT-DOHERTY GEOLOGICAL OBSERVATORY COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964

Abstract

Long-period records of surface waves from earthquakes in central Asia are often complex, with rapid amplitude fluctuations and apparent multiple arrivals of energy of the same frequency. Comparison of records from closely spaced earthquakes shows that the complexity is not caused by multipath propagation, but is associated with the seismic source geometry. The multiple arrivals are higher-mode Rayleigh waves traveling with group velocities faster than the fundamental mode. A relatively high ratio of higher-mode to fundamental-mode energy can be produced when an unusual combination of source depth and focal mechanism leads to poor excitation of the fundamental-mode Rayleigh wave, in which case, the amplitude of the higher mode yields a more reliable estimate of the size of the event than the amplitude of the fundamental mode. Several anomalous events in central Asia, which are characterized by unusually low M_s : m_b ratios and hence could be suspected to be nuclear explosions, are reclassified as earthquakes when the new, higher-mode scale, M^h_s, is employed. The focal depth and mechanism of these anomalous events apparently causes poor excitation of the fundamental-mode Rayleigh wave without significantly affecting the amplitude of the higher mode. Elimination of these source-related factors from traditional M_s : m_b discriminants is an essential tool for reliable discrimination between earthquakes and nuclear underground tests.

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