Simulation of complex tremor migration patterns

Yingdi Luo, & Jean-Paul Ampuero

Published December 2012, SCEC Contribution #1783

The discovery of slow-slip events (SSE) and non-volcanic tremors has greatly enriched the spectrum of earthquake behavior and offers a unique window into the mechanics of the deeper portion of the seismogenic zone of active faults, an uncharted region of great importance in the nucleation of large earthquakes. In Northern Cascadia, tremors show an intriguing hierarchy of migration patterns: large-scale tremor migrating along-strike at about 10 km/day, sparsely distributed swarms that propagate 10 times faster in the opposite direction ('rapid tremor reversals' or RTRs) and even 10 times faster swarms that propagate along-dip. Moreover, during the initial phase of ETS (Episodic Tremor and Slip) the tremor source amplitude shows a linear growth and up-dip propagation. We have proposed a model to reproduce these observations based on interaction of brittle asperities (frictionally unstable, velocity-weakening patches) embedded in a relatively stable fault, mediated by creep transients. We continue quantitative studies of this model through numerical simulations of heterogeneous rate-and-state faults under the Quasi-DYNamic approximation (open-source software project QDYN, hosted online at http://code.google.com/p/qdyn/). We performed both 2D and 3D simulations and successfully reproduced all the major phenomena of complex tremor migration patterns (forward migration, RTRs and along-dip swarms). We will show a complete analysis of friction properties and geometrical settings (i.e. asperity size, distance, etc.) that affects spatial-temporal distribution and migration velocity of tremors. Our study shows that by decreasing the distance between asperities or by increasing the value of (a-b)*sigma inside them, both RTR migration velocity and distance increase positively correlated, and the proportion of moment released seismically during the ETS increases, while the ratio of RTR versus forward tremor migration speed remains mostly the same. While the density of these deep brittle asperities is relatively high and with a high contrast of friction properties comparing with the background, these asperities also play important roles affecting the macroscopic behavior of the background Slow Slip Event, including its propagation velocity, process zone size, recurrence time and moment release. Our model also successfully reproduces the characteristic spatio-temporal distribution of tremors along-dip (more frequent/ shorter recurrence time of tremor bursts at deeper part of the seismic/aseismic transition zone).

Citation
Luo, Y., & Ampuero, J. (2012, 12). Simulation of complex tremor migration patterns. Oral Presentation at AGU Fall Meeting 2012.