Comparison of Tectonic Tremor in Southwest Japan and Cascadia

Abstract:

Tectonic tremor in southwest Japan and Cascadia have a number of similarities and differences. We analyze tremor catalogs from these two subduction zones in order to highlight some of the differences and to determine whether the differences are related to data quality and methodologies. We start with existing tremor catalogs produced by Obara in Japan and Wech in Cascadia. The raw catalogs are constructed with a number of important differences related to data quality, time window size, horizontal versus vertical component data and use of amplitudes. This allows the Japan catalog to have finer temporal resolution and to detect lower amplitude tremor. We have developed an algorithm to cluster the individual tremors into spatial and temporal swarms, which we interpret to represent slow slip events. We find that for both regions, the number of tremor swarms exceeding a given duration follows a power-law distribution. If duration is proportional to seismic moment, as has been commonly assumed, the moment inferred from these swarms follows a standard Gutenberg-Richter logarithmic frequency-magnitude relation with b-values close to 1. We find that for each catalog there is a clear region where this distribution explains the data well. In each case, as with all earthquake catalogs, there is a level below which which the catalogs are incomplete. This level is lower in Japan than Casciadia. However, there is also a clear size above which the catalogs have significantly fewer events than expected by this distribution. This occurs at a duration of about 10-15 hours for southwest Japan and about 150 hours for Cascadia. We interpret this as representing a cutoff between unbounded, small events that follow the scale-invariant Gutenberg-Richter distribution, and large slip events that fill the down-dip width of the slow-slip zone and are therefore geometically bounded. This is consistent with the much narrower spatial distribution of tremor in southwest Japan than in Cascadia. The catalogs are now of sufficient size (1500-2000 swarms) to resolve regional differences in the slow-slip statistics.