ALBERT

Description: The 2011 M w  9.0 Tohoku, Japan, earthquake triggered deep tectonic tremor and shallow microearthquakes in numerous places worldwide. Here, we conduct a systematic survey of triggered tremor in regions where ambient or triggered tremor has been previously identified. Tremor was triggered in the following regions: south-central Alaska, the Aleutian Arc, Shikoku in southwest Japan, the North Island of New Zealand, southern Oregon, the Parkfield–Cholame section of the San Andreas fault in central California, the San Jacinto fault in southern California, Taiwan, and Vancouver Island. We find no evidence of triggered tremor in the Calaveras fault in northern California. One of the most important factors in controlling the triggering potential is the amplitude of the surface waves. Data examined in this study suggest that the threshold amplitude for triggering tremor is ~0.1 cm/s, which is equivalent to a dynamic stress threshold of ~10 kilopascals. The incidence angles of the teleseismic surface waves also affect the triggering potentials of Love and Rayleigh waves. The results of this study confirm that both Love and Rayleigh waves contribute to triggering tremor in many regions. In regions where both ambient and triggered tremor are known to occur, tremor triggered by the Tohoku event generally occurred at similar locations with previously identified ambient and/or triggered tremor, further supporting the notion that although the driving forces of triggered and ambient tremor differ, they share similar mechanisms. We find a positive relationship between the amplitudes of the triggering waves and those of the triggered tremor, which is consistent with the prediction of the clock-advance model. Online Material: Table of measured parameters and other information related to triggering/nontriggering information, and figures of observed seismograms.