A wide depth distribution of seismic tremors along the northern Cascadia margin

• Published in: Nature 2005-08, Vol.436 (7052), p.841-844
• Main Authors: Kao, Honn, Shan, Shao-Ju, Dragert, Herb, Rogers, Garry, Cassidy, John F, Ramachandran, Kumar
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Earthquakes; Earthquakes, seismology; Exact sciences and technology; Geodetic surveys; Humanities and Social Sciences; Internal geophysics; letter; multidisciplinary; Science; Science (multidisciplinary); Seismic activity; Seismology; Tectonics. Structural geology. Plate tectonics
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature03903

The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as Mw = 9 at an interval of several hundred years. The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data. However, episodic surface displacements-in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface-are observed about every 14 months with an unusual tremor-like seismic signature. Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.