Rupture process of the Great 1963 Kurile Islands Earthquake Sequence: Asperity interaction and multiple event rupture
Authors: Beck Susan L. , Ruff Larry J. 
Published: February 2008

The great Kurile Islands underthrusting earthquake (Mw = 8.5) of October 13, 1963, was accompanied by a large foreshock and an aftershock. This sequence allows us to investigate the rupture process and fault heterogeneities along a subduction zone. We have characterized the rupture process of the main shock event by deconvolving long‐period P wave seismograms from azimuthally well‐distributed stations to obtain source time functions. Directivity associated with the three main pulses of moment release in the source functions indicates a total source duration of 93 s, a fault length of 245 km, and a rupture direction of N40°–60° E. Three asperities along the fault are identified by the variation in moment release and located as follows: The first asperity is from 0 to 60 km NE of the epicenter and has an average rupture velocity of 2.7 km/s. The second asperity is centered at 130 km NE of the epicenter and has a length scale of 40–50 km. The final asperity is from 185 to 245 km NE of the epicenter and has a rupture velocity of 3.8 km/s. The seismic moment release of the main shock determined from the P waves is less than one‐half the surface wave moment of 70 × 1027 dyn cm. Body wave modeling of the largest foreshock (October 12, 1963; Ms = 6.7) indicates a source duration of 12 s. The foreshock rupture area is adjacent to but does not significantly overlap the main shock epicentral asperity and may have been important in loading the main shock epicenter. The largest aftershock (October 20, 1963: Ms = 7.2) occurred trenchward of the main shock epicentral asperity and produced an unusually large tsunami. Body wave modeling of the aftershock suggests that the anomalous ringing of the P waves is a result of reverberations in the oceanic layer and not the result of an unusually long source duration. Our preferred model includes a source duration of 24–28 s and at least part of the source located at shallow depths in the accretionary prism. The previous earthquake sequence along this portion of the plate boundary occurred in 1918 with a main shock on September 7 (M= 8.4) and a large aftershock on November 8 (M = 7.7) trenchward of the main shock. The 1918 event initiated approximately 200 km NE of the 1963 epicenter. Tsunami data suggest that the 1918 event ruptured the second and third asperities but not the 1963 epicentral asperity. This indicates the potential for variations in the rupture mode between successive earthquake cycles along this segment of the Kurile Islands subduction zone. The 1963 event is the only multiple asperity earthquake to have occurred along the southern Kurile arc during the most recent cycle of underthrusting earthquakes. A comparison of the asperity distribution along the Kurile arc indicates that the asperity separation along the 1963 earthquake segment is less than the separation between the other single asperity earthquakes. This suggests that the asperity distribution may be important in determining the mode of rupture. We have compared the 1963 earthquake to the great 1906 Colombia‐Ecuador earthquake (Mw = 8.8). The 1906 earthquake ruptured three asperities in a single great event similar to the 1963 earthquake. For both these multiple asperity earthquakes, the seismic moment release is larger than the sum of each individual asperity. This indicates that multiple asperity ruptures trigger larger amounts of moment release in adjacent “weaker” regions than do single asperity earthquakes.