Dual-Frequency Coherence of Seismic Tremor and Source Properties

Last fall Dorman and Schwartz reported discovery of a statistical signature (Dual-frequency coherence-DFC) which distinguishes nonvolcanic tremor from the usual microseismic noise. This was first observed in OBS data from the CRSEIZE experiment of 1999-2000 (DeShon and others, 2006), and later on land data from the Nicoya Peninsula. The tremor observed on OBSs was correlated with fluid flow through the seafloor (Brown and others, 2005). The observation of this signature on both land and OBS data indicates that it is a property of the source, rather than the path or the instrument or its environment. Possible causes include the fluid flow noise proposed by Julian (1994) and a repetitive stick-slip motion. Recent work has shown that nonvolcanic tremor accompanying slow slip in three different subduction zones, including northern Costa Rica, consists of swarms of low-frequency earthquakes that occur as stick-slip motion on the plate interface (Brown et al., 2008). Thus, both fluid flow and shear failure sources may be responsible for tremor generation. New work reveals that a Rössler attractor of the type studied by Julian produces a DFC similar to that of the tremor. Since the frequency of the oscillator is controlled by the physical properties of the source (fluid viscosity, channel dimension, flow velocity) this establishes a link between the seismically observable offset frequencies and one potential source mechanism.