Rupture of the large (M-w 7.8), deep earthquake of 1973 beneath the Japan Sea with implications for seismogenesis

The earthquake of 29 September 1973 beneath the western edge of Japan Sea i s the largest deep earthquake along the western Pacific (M-W 7.8, focal dep th 575 km). The source region is near the tip of the Japan Wadati-Benioff z one, where a continuous belt of seismicity clearly defines a unique, shallo w-dipping (similar to 30 degrees) slab down to depths about 600 km. This un ique geometry of slab predicts a limited vertical extent available for eart hquake rupture, in direct conflict with previous reports of a large, subver tical fault for the 1973 earthquake, Using a comprehensive data set of near ly 20 P and SH waveforms recorded at teleseismic distances, we investigate rupture associated with this event in some detail by inversion of waveforms . In particular, we use wave trains with all relevant depth phases (pP, sP, and sS) that are crucial for resolving the vertical extent. of rupture. In addition to a northward component of rupture as in previous reports, we fi nd clear evidence for a significant component of eastward rupture propagati on that cannot fit within a subvertical, north-south striking nodal plane. Meanwhile, all regions of major slip are at essentially the same depth. Thu s the true fault plane appears subhorizontal, with the entire region of sli p extending only about 15 +/- 6 km across the slab's thickness. The spatio- temporal distribution of slip is well explained by a simple, circular ruptu re front, propagating at a speed of 3-4 km/sec. A comparison among the larg est deep earthquakes shows that most rupture planes are more or less dimens ionally equal, with high aspect ratios ranging approximately from 0.5 to 1. While some events have fairly restricted source regions, others seem to ex tend more than 30 km into the thickness of the slabs.