Spatiotemporal dominance of afterslip and viscoelastic relaxation revealed by four decades of post-1973 Luhuo earthquake observations

Abstract

Measuring surface displacements driven by afterslip and viscoelastic relaxation following large earthquakes enables inferring frictional and rheological properties of Earth's outermost layers where hazardous earthquakes occur. However, the two concurrent mechanisms generate similar and intertwined surface deformations, complicating the extraction of physical information from the measurements. Here, we quantify the spatiotemporal dominance of co-evolving afterslip and viscoelastic relaxation following the 1973 Ms 7.6 Luhuo earthquake in eastern Tibet, using 42 years (1976‒2018) of fault-crossing short-baseline observations. The finite-fault slip of this M7+ earthquake was constructed from triangulation data measured in 1961‒1975. Based on the model incorporating two postseismic relaxation mechanisms and calibrated by the measurements over four decades, we show that, in the temporal domain, afterslip may produce geodetically measurable deformations (>1.5 mm/yr) in local near-field regions even 20 years after the mainshock, with spatially broader impact within ∼10 years. Viscoelastic relaxation may last for nearly six decades, imposing a broad influence for three decades. In the spatial domain, afterslip may produce deformations within ∼2 times the seismogenic depth (SD; 20 km) from the fault, but dominantly in the near-field of ∼1 times the SD. In contrast, viscoelastic relaxation may affect a much wider region reaching 200 km. While afterslip and viscoelastic relaxation collectively affected the region ∼2 times the SD from the fault in the early postseismic period, their resulting deformations gradually separated in space with time, with afterslip-induced deformation shrinking toward the fault, and surface deformation caused by viscoelastic relaxation concentrating in the mid-field, ∼2–3 times the SD from the fault. Their spatiotemporal partitioning helps better learn fault frictional properties and lithospheric rheology based on geodetic data acquired from different domains.