Non-volcanic tremors are found at several subduction zones like in SW Japan, and Cascadia. There are indications that non-volcanic tremors are related to fluid processes. In Central America the serpentinized Cocos plate subducts below the Caribbean plate. An array of borehole stations is operated since June 2006 on Nicoya peninsula in Northwest Costa Rica. The Geosig seismometer were installed in 100 m below surface and the signals are recorded on Guralp data recorders. Since June 2006 4 major periods of non-volcanic tremor activity occurred in May 2007, August 2008, February 2009 and June 2009. They lasted between 6 and 11 days and have a minimum tremor activity of 500 minutes. Statistical analysis show that a major tremor period of minimum 7 days and 600 minutes can be expected every year. 14 minor periods show an tremor activity between 50 and 300 minutes and a duration between 2 and 6 days. We find also a strong correlation with tide. This indicates that during tremor periods the stress regime is in a critical state.