Pre-seismic, Post-seismic and Independent Slow Slip Events
Many of the large earthquakes are preceded by precursory foreshock sequences, which are essential to short-term earthquake forecasting. I are interested in the mechanisms of these foreshocks, which are subject to debates between cascade triggering of mainshock-aftershock sequence and the unloading by large-scale slow slip. Our tools to pursue this question are the matched filter analysis that detects missing small events and repeating earthquake analysis that provides in-situ measurements of local aseismic slow-slip.
In Meng et al., EPSL, 2015, our observations find that the foreshock sequence was driven by an independent large-scale aseismic slow slip event that eventually triggered and interacted with the M8.2 mainshock. In 2015 Mw7.8 Gorkha earthquake, we find extensive foreshocks undocumented in the regular earthquake catalog. We observe a significant increase in seismicity rate a few days prior to the mainshock, initiating ~6 h after a local M 5.2 earthquake. The increase of seismic activities occurs in a wide region around the Gorkha principal slip zone, indicating that the effect of delayed dynamic triggering may contribute to the large-scale unloading process prior to the Gorkha mainshock (Huang et al., Tectonophysics, 2016). We observed similar slow unlocking process of the 2015 Mw 8.4 Illapel earthquake (Huang and Meng., GRL, 2018). We obtain a new catalog of small events and repeating earthquakes in a ~4-year period before the 2015 Mw 8.4 Illapel earthquake. The last episode of aseismic-slip transients are most significant at both sides of the Illapel rupture zone, suggesting a large-scale slow unlocking process. Both seismicity and aseismic slip progressively accelerate around the epicenter, indicating the interplay of seismic and aseismic slip. Our effort also extends into the post-seismic processes, we find distinct early aftershock expansion following the Illapel earthquake: sudden southward expansion and relatively steady northward expansion. Early afterslip estimated from repeater data and geodetic data are consistent and appear to wrap around the Illapel principal coseismic slip zone (Huang et al., EPSL, 2016).
I am also interested in the physical mechanism of the episodic slow slip events. These events are widely observed in the major subduction zones at oceanic plate convergent margins and occur at the brittle-ductile transition depths (~20-50 km). The physical controls on the wide range of their recurrence intervals and slip durations remains unclear. I am involved in developing a simple mechanical model that attempts to account for the observed temporal evolution of slow slip events. In our model we assume that slow slip events occur in a viscoplastic shear zone (i.e., Bingham material) instead of a predefined fault interface, which has an upper static and a lower dynamic plastic yield strength (Yin et al., EPSL, 2018). Our model reproduces a linear increase in slip with time during the landward motion and an exponential decrease in slip magnitude during the trenchward motion. This model provides an alternative mechanism of generating deep aseismic motions and is likely to be influential in the slow-slip community. It also provides a means of extracting information on the rheology of the subduction zones around the world.