slip dynamics and morphology of a MAJOR CREEPING FAULT STEP-OVER at the eastern end of the Tianzhu seismic gap (Haiyuan fault, China), from INSAR

Advances in geodetic monitoring of faults have revealed slow deformation transients and complex interactions between slow aseismic events and rapid seismic events. The Haiyuan Fault (northeast China) is of particular interest to decipher fault slip behavior and the associated physical mechanisms, due to its dual slip modes, similar to that of the San Andreas Fault in California, with both locked segments prone to major earthquakes and segments where aseismic slip ("creep") is observed. Here we focus on its 35 km-long creeping segment in the junction area between the western termination of the 1920 Mw7.9 earthquake and the eastern termination of a seismic gap [1], revisiting the spatial distribution and temporal evolution of creep from the joint analysis of ERS, Envisat and Sentinel-1 data.

We primarily use Sentinel-1 displacement time series over a seven-year period (2015-2022) processed by the FormaTerre FLATSIM service [2], corresponding to two tracks along ascending orbits and three tracks along descending orbits covering the creeping section of the fault. We first analyze the linear and seasonal components of the displacement time series, then decompose the linear term into fault-parallel horizontal velocity and vertical velocity fields. The creep signature and spatial extent are clearly identified in the line of sight and horizontal velocity maps. The surface creep rate distribution shows along-strike variations with peaks reaching up to 5 mm/yr for the horizontal component. Subsidence at a rate of 6,5 mm/yr is also observed in the extensional relay zone at the eastern end of the creeping section. We then invert InSAR line of sight velocity maps for the slip distribution along the seismogenic zone using the CSI software [3], using GNSS data as additional constraint. Creep distribution is compared with those derived from ERS and Envisat data to discuss the potential evolution of creep over decades.

We also investigate such potential temporal variations in the creep rate as seen from previous ENVISAT observation. We analyze the temporal evolution of the cumulative relative creep for each track independently, considering both the raw displacement time series and the decomposed horizontal and vertical components. Applying Principal and Independent Component Analysis as exploratory tools, we separate the extensional relay zone from the western part of the creeping section. Preliminary results show specific spatial patterns associated with temporal evolutions of mixed horizontal creep and subsidence, consistent on all tracks. They highlight in particular the morphology and slip partitioning in the fault step-over area, as well as multiple periods with transient events. These transients include variations in the subsidence velocity in the basin marking the extensional relay zone, as well as in the horizontal displacement velocities in the creeping section and in the uplift velocity to the west of the basin. Investigations into the tectonic and hydrological origins of these transients are ongoing.

References

[1] Jolivet,R.et al,J.Geophys.Res.Solid Earth,2012,doi:10.1029/2011JB008732

[2] Thollard,etal,Remote Sens 2021,doi.org/10.3390/rs13183734

[3] Jolivet,R.et al,Geophysical Research Letters,2020,doi.org/10.1029/2019GL085377