A single‐force model for the 1975 Kalapana, Hawaii, Earthquake

A single force mechanism is investigated as the source of long‐period seismic radiation from the 1975 Kalapana, Hawaii, earthquake (M S = 7.1). The observed Love wave radiation pattern determined from the spectra of World‐Wide Standard Seismograph Network and High Gain Long Period records at 100 s is two‐lobed with azimuth, consistent with a near‐horizontal single force acting opposite (strike ∼330°) to the observed displacement direction of the earthquake; this pattern is inconsistent with the expected double‐couple pattern. Assuming a form of the force time history of a one‐cycle sinusoid, the total duration of the event estimated from Rayleigh waves at two International Deployment of Accelerometers stations is approximately 180 s. The peak amplitude f o of the time function is 1 × 10 15 N from amplitudes of Love and Rayleigh waves. The interpretation is that the bulk of the seismic radiation was produced by large‐scale slumping of a large area of the south flank of Kilauea volcano. The single force is a crude representation of the effect on the earth of the motion of a partially decoupled large slide mass. Using the mass estimated from the tsunami generation area (∼ 10 15 –10 16 kg), the peak acceleration of the slide block (0.1–1 m s −2 ) inferred from the seismic force is comparable with the acceleration due to gravity on a gently inclined plane. The slump model for the Kalapana earthquake is also more qualitatively consistent with the large horizontal deformation (8 m on land) and tsunami associated with the earthquake, which are difficult to explain with the conventional double‐couple source model. The single‐force source has been used previously to model the long‐period seismic waves from the landslide accompanying the eruption of Mount St. Helens volcano, and may explain other anomalous seismic events as being due to massive slumping of sediments or unconsolidated material and not to elastic dislocation.