Deep low‐frequency tremor that correlates with passing surface waves

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B01307, doi:10.1029/2006JB004890, 2008 Deep low-frequency tremor that correlates with passing surface waves Masatoshi Miyazawa 1 and Emily E. Brodsky 2 Received 5 December 2006; revised 23 July 2007; accepted 28 September 2007; published 24 January 2008. [ 1 ] The large surface waves from the 2004 Sumatra-Andaman earthquake dynamically perturbed the upper mantle structure in Japan and triggered periodic deep low-frequency seismic tremor in eastern and western Shikoku, western and central Tokai, and the Kii peninsula. We use the relationship between the amplitude of the triggered tremor and the stresses of the seismic waves to investigate the mechanism of deep low-frequency seismic tremor. Volumetric strain changes from the 15–30 s Rayleigh waves play an important role in the strong triggering, likely via Coulomb failure stress changes. Building on previous results that the tremor signals become increasingly strong with increasing dilatation, we observe a clear increase in the triggered tremor with an increase in the dilatation due to the Rayleigh waves at the 30 km depth source regions. We also observe a correlation with the Coulomb failure stress change resolved on an appropriate plane. There is an exponential relationship between the signal amplitude from triggered tremor and both the dilatation and the Coulomb shear stress at the source region. This combined with the shape of the tremor packets implies that the tremor amplitude is predictable based on the amplitude of the incoming waves. The amplitude variations can be explained by a distribution of sources in the tremor source region. Citation: Miyazawa, M. , and E. E. Brodsky (2008), Deep low-frequency tremor that correlates with passing surface waves, J. Geophys. Res., 113, B01307, doi:10.1029/2006JB004890. 1. Introduction [ 2 ] Deep low-frequency tremor on subducting slabs is a recently discovered and novel seismic signal [Obara, 2002]. The waves are unusually long-period for their amplitude and the signal is often continuous with little punctuation over times ranging from tens of minutes to a few days. These seismic features are often seen in volcanic regions and usually interpreted as signs of fluid-filled resonant conduits in that setting. Therefore early work on deep, slab tremor suggested a fluid source by analogy. Geochemical and petrological constraints supported the inference by suggesting that fluids are released from the subducting slab in a series of dehydration reactions at the depth of the tremor [e.g., Toriumi and Inui, 2001; Omori et al., 2004]. The coincidence of the tremor with slow slip also suggests that there is some physical connection with the large-scale slab processes at the depth of 30 –40 km. [ 3 ] More recent work has suggested that low-frequency earthquakes and tremor can be generated by simple shear failure in both the subduction and the volcanic settings. A low rupture velocity or low-stress drop can result in the low- frequency waves and overlapping earthquakes can generate the apparently continuous signal [Harrington and Brodsky, Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan. Department of Earth Sciences, University of California, Santa Cruz, California, USA. Copyright 2008 by the American Geophysical Union. 0148-0227/08/2006JB004890$09.00 2007; Shelly et al., 2006]. Still, the occurrence of the tremor and slow slip at the depth of dehydration suggests that fluids may be an important component. [ 4 ] Here we study the origin of slab tremor by using a constraint posed by special cases of slab tremor that are triggered by the seismic waves from distant earthquakes. The 2004 Sumatra-Andaman earthquake (M w 9.2) was enor- mous and the surface wave amplitudes measured in Japan were comparable to or a few times as large as those from the Denali earthquake (M w 7.9) in 2002 that contributed to the well-studied earthquake triggering around North America [e.g., Prejean et al., 2004]. Miyazawa and Mori [2006] showed that periodic triggering of deep low-frequency events in western Japan was due to the Rayleigh waves from the Sumatra earthquake, and suggested that the triggering is well correlated with the large tensile dilatation at the source regions (Figures 1 and 2). Similar triggered tremor occurred after small local earthquakes and large teleseisms [Obara, 2003], and during the surface waves of the 2003 Tokachi-oki earthquake (M w 8.1) [Miyazawa and Mori, 2005]. These initial observations were thought to distinguish the tremor from the ordinary earthquakes triggered in Alaska that were promoted by shear failure [West et al., 2005]. Recently, Rubinstein et al. [2007] found that the bursts of similar nonvolcanic tremor in Cascadia subduction zone were trig- gered by Love waves from the 2002 Denali earthquake. [ 5 ] More careful analysis of the relationship between the input strains of the seismic waves and the resultant tremor may help us untangle the mechanisms of deep-slab tremor in western Japan. B01307 1 of 17