Các phương pháp được sử dụng để xác định vị trí động đất chủ yếu phụ thuộc vào bản chất bốc đồng của giai đoạn sóng cơ thể của họ, khiến họ thay vì không hiệu quả cho vị run. | Non-volcanic Tremor 291 Fig. 4 Recordings of non-volcanic tremor in a the Cascadia subduction zone b the Nankai Trough c the Alaska subduction zone d Parkfield California on the San Andreas strike-slip fault and e the Mexican subduction zone. Records are bandpass filtered at 1-8 Hz. b is modified from Shelly et al. 2007a waveforms poses a challenge for those trying to identify it. Most use very simple methods based on envelope amplitude like those that Obara 2002 used to initially identify tremor although more complex automated methods to identify tremor are starting to be developed Kao et al. 2007a Wech and Creager 2008 Suda et al. in press . The absence of easily identified body wave arrivals also contributes to the difficulty in locating non-volcanic tremor. Methods used to locate earthquakes largely depend on the impulsive nature of their body wave phases rendering them rather ineffective for locating tremor. The issue of tremor location is more fully explored in section Locating Non-volcanic Tremor . While non-volcanic tremor usually lacks distinguishable arrivals impulsive arrivals in Japanese tremor have been observed Katsumata and Kamaya 2003 . These arrivals are typically s waves but P waves have also been found Shelly et al. 2006 . These body wave arrivals are regularly identified and cataloged by the Japanese Meteorological Agency JMA as Low Frequency Earthquakes LFEs . These observations are made primarily on the Hi-Net in Japan a nationwide network of high-sensitivity borehole seismometers Obara et al. 2005 . The unprecedented density and low noise of the instruments in the Hi-net facilitates the detection of weak signals. LFEs are only rarely identified in regions with tremor outside of Japan . Kao et al. 2006 Sweet et al. 2008 . It is unclear if this difference represents a real variation in tremor activity or simply a limitation in the observation capabilities of networks outside of Japan. At many time-scales tremor can appear to be very stable .
