Spatio-temporal evolution of aseismic slip along the Haiyuan fault, China: Implications for fault frictional properties

摘要

We use 20 years of Synthetic Aperture Radar acquisitions by the ERS and Envisat satellites to investigate the spatial and temporal variations of strain rates along the 35-km long creeping section of the Haiyuan fault, at the north eastern boundary of the Tibetan plateau. We then use the derived displacements to infer the fault's frictional properties and discuss the relationship between creep and the seismic behavior of the fault. Located in between a millennial seismic gap and the 1920 M8 surface rupture trace, this section has an average creep rate of 5 ± 1mm/yr, about the interseismic loading rate. The comparison of average surface velocity profiles derived from SAR interferometry across the creeping section reveals a creep rate increase and/or a creep migration to shallower depth between the 1990s and the 2000s. We apply a smoothed time series analysis scheme on Envisat InSAR data to investigate the creep rate variations during the 2004-2009 time period. Our analysis reveals that the creep rate accelerated in 2007, although data resolution does not allow to better constrain the onset of creep acceleration and its amplitude. Both decadal and short term transient behaviors are coeval with the largest earthquakes (M ~ 4-5) along the fault segment in recent years. From the precise mapping of the surface fault trace, we use the fault strike variations and the Mohr circle construction to compute the along-strike distribution of the friction coefficient along the creeping segment and compare it with the observed distribution of the creep rate. We find that the creep rate scales logarithmically with the friction coefficient, in agreement with the rate-and-state friction law in a rate strengthening regime. The estimated value of δμ/δlogV ~ 2 ×~(10 -3) indicates that the earthquakes occurring along the creeping section cannot be the cause for a significant change in the overall segment's creep rate and that the recorded micro seismicity is most likely creep-driven. Finally, given the size and frictional properties of the creeping section, we estimate, based on previous models of dynamic rupture simulations, a 0-20% probability for a rupture to break through this section. Together with the geometrical configuration of the Haiyuan fault, these results suggest that the creeping segment may act as a persistent barrier to earthquake propagation.