Non-volcanic tremor in Costa Rica: b-values, moment release and tidal modulation

In the Central American subduction zone non-volcanic tremor (NVT) events have been recorded on Nicoya Peninsula (N Costa Rica) since July 2006 by using an array of short period borehole seismometers. NVT hypocenters locate between 30 and 45 km depth near the interface of the subducting Cocos and overriding Caribbean plates. They form a depth continuation of the seismogenic zone. We present the results of a statistical analysis of the time series of NVT events recorded continuously over the past 5 years. Using tremor time duration as a measure of their magnitude we found that different scaling laws apply to different phases of NVT activity. Bursts (swarms) of NVT events follow a Gutenberg-Richter type of law with a b-value of 1 if the cumulative tremor duration of each burst is converted to seismic moment and earthquake magnitude by application of corresponding empirical constants found for the Cascadia subduction zone (e.g. Wech et al., 2010). However, the time length of single tremors follows a different statistics. It cannot be characterized by a Gutenberg-Richter type relation but can be represented by two pairs of a,b-values and a "corner duration time". These characteristic constants are different for the time intervals of bursts and between bursts. This difference in the statistical behavior can possibly be explained by tidal forcing of the tremor activity during burst phases: It was found that about 2/3 of the NVT events belonging to bursts coincide with tidal stress maxima whereas no such correlation could be identified for the time intervals between bursts.