The Relevance of High-frequency Analysis Artifacts to Remote Triggering

Abstract

The high-frequency content observed in teleseisms recorded by seismometers can be produced either by the nonlinear behavior of seismometers and digitizers (Delorey et al. 2008; Hellweg et al. 2008) or by the real Earth response. The latter include scattering from small-scale heterogeneities during seismic wave propagation ( e.g. , Chen and Long 2000) and high-frequency radiations from the earthquake source ( e.g. , Peng et al. 2006) or near-surface regions near the recording site (Fischer et al. 2008). Recent studies have shown that large-amplitude surface waves generated by earthquakes at regional and teleseismic distances could also trigger high-frequency seismic sources, either in the form of regular earthquakes at seismogenic depth near the recording site (Hill and Prejean 2007 and references therein) or “non-volcanic” tremor in the lower crust (Rubinstein et al. 2010; Peng and Gomberg 2010 and references therein). In teleseisms the presence of high-frequency content ( e.g. , >5 Hz) in the seismogram is inconsistent with the expected attenuation of waves from a distant source ( i.e. , >1,000 km). The lack of frequencies above 5 Hz in a teleseism makes it easy to separate the seismic signals of locally triggered events from those of the teleseism by applying a high-pass or band-pass filter to broadband continuous recordings ( e.g. , Hill and Prejean 2007; Velasco et al. 2008). Another effective way to demonstrate triggered seismicity is the spectrogram display ( i.e. , frequency-time plot) of the seismic data ( e.g. , West et al. 2005; Hill and Prejean 2007; Peng and Chao 2008; Peng et al. 2008). In such a plot, the locally triggered seismic signals typically show as narrow vertical bands rich in high-frequency energy within the low-frequency body and/or surface waves of teleseismic events. When examining high-frequency signals for evidence of remote triggering, it is important to …