Deep and intermediate-depth non-double couple earthquakes: interpretation of moment tensor inversions using very broadband seismic waves,

Kuge, K., and H. Kawakatsu
Geophys. J. Int., 111, 589-606, 1992.

Abstract

Analysis of moment tensor inversions using a wide part of the seismic spectrum provides clear evidences that some non-double couple moment tensors of intermediate-depth and deep earthquakes result from the superposition of different double couple sources which are aligned with the predominant strain state within the subducting slabs. Although the possibility has been proposed, such evidences as we observe in this paper for the predominantly tensional slab as well as compressional slab have not been shown. The analyses are performed for three intermediate-depth and deep earthquakes: January 1, 1984 South of Honshu (386km), April 23, 1985 Luzon (181km), and May 7, 1987 northern Sea of Japan (417km). Consistent non-double couple moment tensors obtained using different sets of seismic waves in various low frequency bands suggest that these three significant non-double couple components are not caused by unmodeled errors in the inversion procedure, because the various sets of seismic waves traverse very different ray paths. For the South of Honshu and the Luzon events, two major arrivals in the broadband P-wave displacement seismograms are observed, with varying relative amplitudes or polarities from station to station. The different double couple mechanisms which model the two phases combine to produce the significant non-double couple moment tensors obtained at long periods. The principal axes of the subevents, closest in orientation to the predominant strain states within the slabs, tend to be quite stable, whereas the other two principal axes rotate between subevents. This observation may explain the global nature of non-double couple components in relation to the strain regime within the slab. For the northern Sea of Japan event, two major phases are observed in the P-wave displacement waveforms, but the variation in mechanism of two subevents that we model is not enough to explain the large non-double couple component observed at long periods. However, since the non-double couple component is exceptionally large compared with those for other deep earthquakes, changes of focal mechanism with shorter delays than we can resolve may so far explain the overall radiation most simply.