Aftershock sequences of moderate‐sized intermediate and deep earthquakes in the Tonga Subduction Zone

Abstract

Intermediate and deep focus earthquakes are generally thought to produce few aftershocks. The two year operation of a network of broadband seismographs in the Tonga‐Fiji region provides an opportunity to monitor the aftershock activity of moderate‐sized earthquakes. In addition to the March 9, 1994 (Mw 7.6) event that shows 144 aftershocks, we found that all nine intermediate and deep earthquakes with Mw > 6.0 showed some aftershock activity, suggesting that aftershock sequences are ubiquitous. On average, deep Tonga earthquakes show an order of magnitude fewer aftershocks above a given magnitude than shallow California earthquakes of similar moment. The largest aftershocks of each sequence have an mb that is 2.0 units smaller on average than the Mw of the mainshock, whereas the California earthquakes show a mean difference of 1.1. Since b‐values of the deeper sequences appear to be similar to shallow sequences, the difference in aftershock productivity results from the larger size difference between the mainshock and the larger aftershocks. The most prolific sequences show an activity level not far below shallow earthquakes of the same moment. This is in contrast with observations of little aftershock activity for deep earthquakes in South America, Japan, and Kuriles, and suggests that the Tonga subduction zone is unusually prolific in aftershock production, perhaps due to cold slab temperatures.