PageRank for Earthquakes

Abstract

Using Hi‐Net data in southwest Japan, Obara (2002) discovered a new type of seismic source that he called nonvolcanic tremor due to its similarity to volcanic tremor, but distinct origin. Tremor is characterized by weak signals, typically of deep (>30 km) origin, that are most prominent in the 1–10 Hz frequency band. At about the same time, slow‐slip events and tectonic tremor were found to be strongly correlated indicating that tremor accompanied these events (Obara, 2002; Rogers and Dragert, 2003). Tectonic tremor is challenging to locate because it is an extended and persistent signal, and lacks the clear P ‐ and S ‐wave arrivals that are typical of earthquakes. Despite this, there is clear modulation of tremor amplitudes that can be tracked between stations and used to locate the tremor source. Several envelope techniques (Obara, 2002; McCausland et al. , 2005; Wech and Creager, 2008; Aguiar et al. , 2009) were developed for this purpose. Low‐frequency earthquakes (LFEs) were discovered by Katsumata and Kamaya (2003) and have since proven to be critical to understanding the genesis of tectonic tremor. Shelly et al. (2006) showed that LFEs correlate strongly with times of tremor and that they occur on the plate interface. Shelly et al. (2007a) showed that tremor consists of swarms of LFEs, and by inference occurred on the plate interface as well. Follow‐up work (Ide, Shelly, Beroza, 2007) demonstrated that LFEs per tremor occurred as shear slip and hence were part of a family of slow earthquakes (Ide, Beroza, et al. , 2007). Initially, LFEs were found through visual inspection by enterprising network analysts in Japan (Beroza and Ide, 2011). LFEs have been identified as periods of high‐amplitude arrivals by the same visual inspection technique (Shelly, 2009). Because LFEs have proven so useful for studying tremor, other methods have been developed to …