Seismotectonics of intermediate-depth earthquakes from properties of aftershock sequences

Abstract

Aftershock sequences following two intermediate-depth earthquakes have been analyzed to determine their spatial and temporal distributions. The Kanaga Pass earthquake ( M b = 6.0) occurred at 105 km depth in the central Aleutian Islands. Data were provided by the Central Aleutians Seismic Network. The Iwaizumi Town earthquake ( m b = 6.4) occurred at 75 km depth, under northern Honshu, Japan. Data were provided by the network of Tohoku University. For both events the hypocenter distribution of the aftershocks agreed well with focal mechanism solutions derived by others, with down-dip compression for both. Both events were followed by numerous aftershocks that decayed with time initially according to the modified Omori relation, with p-value close to 1. In both cases, activity at a higher rate than that expected from the Omori relation began after several months, without a strong aftershock to trigger it. The Kanaga Pass activity is modeled by two Omori sequences, the second beginning 64 days after the mainshock. The p-values are 0.92 and 1.04, respectively. The corresponding b-values are 1.01 and 1.03. The Iwaizumi Town sequence is modeled by a single Omori sequence, p = 0.96, plus a constant rate of activity, 0.25 events/day, for 63 days, beginning 120.5 days after the mainshock. The b-value of the main sequence was 0.91 ± 0.14, up to the start of the linear surge, for which the b-value was 1.23 ± 0.47. No strong aftershock or event in the vicinity of the aftershock zone occurred to trigger the secondary activity, in either case. The cause of the secondary activity is not known. The two sequences occurred in subduction zones with similar properties, and in both cases previous seismicity shows a deficiency of strong earthquakes in the main thrust zone above the aftershock source volumes. A strong creep event, or silent earthquake, in a weakly-coupled interplate contact zone is a plausible explanation for the secondary sequences. The unusual setting of the Iwaizumi event, at the bottom of the upper plane of the double-planed Wadati-Benioff zone under Honshu, may play a role in that case.