Diversity in the High-Frequency Wave Radiation of Small Repeating Earthquakes
Abstract:Repeating earthquake (REQ) sequence, which is a series of earthquakes with nearly identical waveforms and locations, is thought to represent repeated ruptures of small asperities on a fault plane. For understanding the generation process of recurrent interplate earthquakes, it is important to reveal how the rupture pattern changes in small REQs.
Rupture pattern changes of REQ will make difference especially in high-frequency components of the waveforms. In this study, we performed 1 kHz sampling-rate seismograph observation at permanent borehole stations (300-500m in depth) along the Sanriku coast, Japan for the period from April to November 2011, immediately after the Tohoku-Oki M9 earthquake, to investigate rupture pattern changes of M3.1-3.5 REQs in the northeastern Japan subduction zone.
First, we make a pair of events belonging to the same REQ sequence and calculate their coherences. Although all the pairs show high coherences in lower-frequency band than their corner frequencies (6-10Hz), there are both high- and low-coherence pairs in the high frequency band. We find that pairs composed of an event (perturbed event) occurring immediately after nearby larger events and another (non-perturbed event) without such preceding events show lower coherences in high-frequency band, suggesting a stress perturbation by nearby earthquakes causes the high-frequency waveform differences.
Second, we investigate inter-event differences in group delay times of high-frequency components (30-45Hz) of the waveforms relative to low-frequency components (2-8Hz). We find that the inter-event differences are larger for the pairs of perturbed and non-perturbed events. This result indicates timings of high-frequency wave radiation with respect to centroid are different for the perturbed events.
Our results suggest rupture patterns of REQs including high-frequency wave radiation do not change at random but are controlled by the stress condition near the asperity of the REQs.