Earthquake ruptures modulated by waves in damaged fault zones
Faults are usually surrounded by damaged zones of lower elastic moduli and seismic wave velocities than their host rocks. If the interface between the damaged rocks and host rocks is sharp enough, earthquakes happening inside the fault zone generate reflected waves and head waves, which can interact with earthquake ruptures and modulate rupture properties such as rupture speed, slip rate, and rise time. We find through 2–D dynamic rupture simulations the following: (1) Reflected waves can induce multiple slip pulses. The rise time of the primary pulse is controlled by fault zone properties, rather than by frictional properties. (2) Head waves can cause oscillations of rupture speed and, in a certain range of fault zone widths, a permanent transition to supershear rupture with speeds that would be unstable in homogeneous media. (3) Large attenuation smears the slip rate function and delays the initial acceleration of rupture speed but does not affect significantly the rise time or the period of rupture speed oscillations. (4) Fault zones cause a rotation of the background stress field and can induce plastic deformations on both extensional and compressional sides of the fault. The plastic deformations are accumulated both inside and outside the fault zone, which indicates a correlation between fault zone development and repeating ruptures. Spatially periodic patterns of plastic deformations are formed due to oscillating rupture speed, which may leave a permanent signature in the geological record. Our results indicate that damaged fault zones with sharp boundaries promote multiple slip pulses and supershear ruptures.
Additional Information©2014 American Geophysical Union. Received 26 September 2013; Accepted 26 March 2014; Accepted article online 30 March 2014; Published online 11 April 2014. This work was supported by the NSF (grants EAR-0944288 and EAR-1151926) and SCEC (funded by NSF EAR-0106924 and USGS 02HQAG0008 cooperative agreements). We thank the two reviewers and the Associate Editor for very helpful reviews. We also thank Eric Dunham for constructive comments and researchers who have done detailed observations of fault zone structures.
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