Geodetic Observations of Shallow Creep on the Laohushan‐Haiyuan Fault, Northeastern Tibet.

We investigated the spatial distribution of aseismic creep on the Laohushan‐Haiyuan fault using Global Positioning System (GPS) data (1999–2017) and Interferometric Synthetic Aperture Radar (InSAR) data (2003–2010). Comparisons among GPS, InSAR line‐of‐sight (LOS) rates, and leveling show that neither leveling nor GPS vertical velocities can fit the vertical signal mapped into the LOS, implying either complicated vertical crustal deformation in northeastern Tibet and/or complex error structures in the InSAR data. Thus, we combined horizontal GPS with high‐pass filtered InSAR data to produce a continuous LOS rate map crossing the fault. Our geodetic data reveal three creep sections along the fault. Both the restored LOS data and decomposed ascending and descending InSAR data highlight the fact that vertical motion can cause an overestimation of creep rate; we obtained a refined creep rate of 2.5 ± 0.4 mm/a on the Laohushan fault. We further identified a 10 km‐long, ∼3–5 mm/a creep section (∼104.2°E−104.3°E) and a 43 km‐long, ∼1–3 mm/a creep section (∼105.3°E−105.7°E) on the western and eastern Haiyuan fault respectively. Both are located on fault sections that ruptured during the 1920 M∼8 earthquake, suggesting that the 1920 earthquake was able to cross pre‐existing creep sections or that the fault shows heterogeneous relocking after large earthquakes, with creep lasting decades on some parts of the rupture. Fault coupling shows a highly variable rate of slip deficit accumulation along strike, suggesting that coupling might significantly evolve during the period between two large earthquakes. Plain Language Summary: Ample observations document slip on a fault plane is seismic and/or aseismic; the former situation refers to a fault slips suddenly and produce earthquakes, the latter means a fault slips gradually without generating seismic energy. Studying the fault‐creep activity contributes to better understand the seismic behavior of crustal faults. We focus on the Haiyuan fault system, which is located on the northeastern margin of Tibet. Several large earthquakes occurred along the fault system in the past, including the 1920 M∼8.0 Haiyuan and the 1927 M ∼8–8.3 Gulang earthquakes. Using Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) data, we identified three creep sections along the fault. Our results show vertical deformation that has mapped into the InSAR line‐of‐sight (LOS) could leads to overestimation of the creep rate. Besides, our findings imply either large earthquakes can rupture creep regions or they can generate long‐lasting afterslip/creep. Key Points: Of three creep segments on the Laohushan‐Haiyuan fault, two are located along fault sections that ruptured during the 1920 M∼8 earthquakeVertical motion leads to overestimation of the creep rate on the Laohushan fault; our refined creep rate is 2.5 ± 0.4 mm/aOur findings support that large earthquakes can rupture pre‐existing creep regions or that they can generate long‐lasting afterslip or creep [ABSTRACT FROM AUTHOR]