Overlapped Postseismic Deformation Caused by Afterslip and Viscoelastic Relaxation Following the 2015 Mw 7.8 Gorkha (Nepal) Earthquake

Abstract

By accommodating ∼50% of the India-Eurasia plate convergence, the Main Himalayan Thrust (MHT) is considered to be a seismogenic zone with potential for generating large earthquakes (Avouac, 2003; Bilham, 2004). The recent 2015 Gorkha earthquake partly ruptured a known seismic gap on the central MHT between the 1505 M 8.5 earthquake to the west and the 1934 M 8.2 earthquake to the east (Figure 1). Seismological studies reveal that the rupture of the Gorkha earthquake initiated 15-18 km beneath the Gorkha region, then propagated eastward for ∼140 km (e.g., Avouac et al., 2015; Fan & Shearer, 2015; Lay et al., 2017). Geodetic slip models indicate that coseismic slip of the earthquake was concentrated on the subhorizontal décollement at a depth of 10-15 km, whereas the shallow portion of the fault remains unbroken (e.g., Diao et al., 2015; Elliott et al., 2016). After the event, significant postseismic surface deformation has been observed by Global Positioning System (GPS) and InSAR (Interferometric Synthetic Aperture Radar) techniques (e.g., Mencin et al., 2016; K. Wang & Fialko, 2018). Based on these postseismic observations, several previous studies have investigated two main mechanisms: afterslip and viscoelastic relaxation. Postseismic deformation in early periods is mostly attributed to afterslip at the downdip extension of the coseismic slip (

Weakness of the Indian Lower Crust Beneath the Himalaya Inferred From Postseismic Deformation of the 2015 Mw 7.8 Gorkha Earthquake

“…Our modeled afterslip by a shear zone with a viscosity of 4 × 10 18 Pa s is up to 45 cm within 5 years (Figure 5a). The distribution is generally consistent with that in Diao et al (2021), Zhang et al (2021), and B. Zhao et al (2017).…”

“…The largest aftershock with a magnitude Mw 7.3 occurred about 150 km east of the mainshock (Lindsey et al, 2015). A number of studies have investigated postseismic processes following these two events and have concluded that both afterslip and viscoelastic relaxation in the lower crust and upper mantle are required to explain geodetic observations (Diao et al, 2021;Tian et al, 2020;Wang & Fialko, 2018;Zhang et al, 2021;B. Zhao et al, 2017).…”

“…The third data set includes 12 continuous GPS sites with approximately 1.2 years of observations between the China-Nepal border and YZS that were published by Liu-Zeng et al (2020). The three data sets mentioned above, either individually or in combination, have been used to study the postseismic deformation processes of the Gorkha earthquake in previous studies (Diao et al, 2021;Hong & Liu, 2021;Liu-Zeng et al, 2020;Tian et al, 2020;Zhang et al, 2021;B. Zhao et al, 2017).…”

Depth‐Varying Friction on a Ramp‐Flat Fault Illuminated by ∼3‐Year InSAR Observations Following the 2017 Mw 7.3 Sarpol‐e Zahab Earthquake

“…However, a much smaller deep afterslip downdip of the coseismic rupture than updip (Figures 5 and 6; Figure S15 in Supporting Information S1) is required by our kinematic and mechanical afterslip models for this 2017 Sarpol‐e Zahab event; the viscoelastic response is also negligible, as the estimated viscosity should be greater than $${10}^{19}$$ Pa s, which confirms the result from Wang and Bürgmann (2020). The existence of minor downdip afterslip is physically reasonable (e.g., Diao et al., 2021; Zhao et al., 2017) because of the velocity‐strengthening frictional properties at depths below the coseismic rupture. The prominent afterslip updip of the coseismic rupture from our afterslip models coincides with the strong frictional contrast between the updip and downdip portions of the fault.…”

“…For some thrust earthquakes that share tectonic settings similar to the 2017 Sarpol‐e Zahab earthquake, such as the 1999 Chi‐chi, 2005 Kashmir and 2015 Gorkha earthquakes, the significant afterslip occurred in the downdip portion of the fault, in conjunction with possible viscoelastic relaxation (e.g., Diao et al., 2021; Hsu et al., 2002; Wang & Fialko, 2014, 2015, 2018; Zhao et al., 2017). However, a much smaller deep afterslip downdip of the coseismic rupture than updip (Figures 5 and 6; Figure S15 in Supporting Information S1) is required by our kinematic and mechanical afterslip models for this 2017 Sarpol‐e Zahab event; the viscoelastic response is also negligible, as the estimated viscosity should be greater than $${10}^{19}$$ Pa s, which confirms the result from Wang and Bürgmann (2020).…”