Himalayan strain reservoir inferred from limited afterslip following the Gorkha earthquake

Abstract

The magnitude 7.8 Gorkha earthquake in April 2015 ruptured a 150-km-long section of the Himalayan décollement terminating close to Kathmandu 1-4 . The earthquake failed to rupture the surface Himalayan frontal thrusts and raised concern that a future M w ≤ 7.3 earthquake could break the unruptured region to the south and west of Kathmandu. Here we use GPS records of surface motions to show that no aseismic slip occurred on the ruptured fault plane in the six months immediately following the earthquake. We find that although 70 mm of afterslip occurred locally north of the rupture, fewer than 25 mm of afterslip occurred in a narrow zone to the south. Rapid initial afterslip north of the rupture was largely complete in six months, releasing aseismic-moment equivalent to a M w 7.1 earthquake. Historical earthquakes in 1803, 1833, 1905 and 1947 also failed to rupture the Himalayan frontal faults, and were not followed by large earthquakes to their south. This implies that significant relict heterogeneous strain prevails throughout the Main Himalayan Thrust. The considerable slip during great Himalayan earthquakes may be due in part to great earthquakes tapping reservoirs of residual strain inherited from former partial ruptures of the Main Himalayan Thrust.The spatial distribution of surface deformation in the April M w 7.8 Nepal earthquake and its M w 7.3 aftershock ten days later was captured by several InSAR scenes and by continuously operating GPS receivers 1-8 . These data indicate that an average slip of 3.5 m occurred on a 60-km-wide × 150-km-long rupture of the northdipping décollement, the Main Himalayan Thrust (MHT), with maximum slip locally reaching 7 m approximately 15 km north of Kathmandu. Rupture propagated eastwards along the region of maximum interseismic strain near the Greater Himalaya, but terminated to the south near the Kathmandu valley (Fig. 1).The interseismic convergence rate in Nepal observed geodetically 9,10 is 18-20 mm yr −1 , similar to the rate of advance of the Himalaya over the Indian plate inferred from geological evidence 11 . The Indian plate descends beneath Tibet aseismically north of a seismically active transition zone in northern Nepal, south of which three decades of geodetic data indicate that the MHT is effectively locked 12 . The occasional rupture of this locked décollement accommodates the incremental northward passage of the Indian plate beneath the Himalaya. A several-thousand year record of this slip is emerging from the exhumation of surface ruptures of Himalayan frontal faults 13-15 (Main Frontal Thrust, MFT); however, the 2015 earthquake is one of several M ≤ 7.8Himalayan earthquakes whose southward rupture failed to rupture the MFT, and whose contribution to facilitating Himalayan moment release thereby eludes palaeoseismic investigation.InSAR data reveal that the 2015 earthquake did, however, trigger ∼5 cm of near-surface slip on a subsidiary branch of the frontal fault, the Main Dun Thrust (MDT), with no significant coseismic slip on the décollement between the...

Can the Updip Limit of Frictional Locking on Megathrusts Be Detected Geodetically? Quantifying the Effect of Stress Shadows on Near‐Trench Coupling

“…Based on thermal models of the megathrust, the fault should reach a temperature of 80°C at 4-5 km depth or 22-28 km from the frontal fault (which we consider analogous to the trench in a subduction zone; Ader et al, 2012;Herman et al, 2010). If we consider the updip limit of the Gorkha rupture as the maximum extent of the frictionally locked portion of the fault, then W F /W T = 0.4, although we note that almost no afterslip has been observed updip of the Gorkha earthquake in 2015, suggesting that this part of the fault is velocity weakening (e.g., Mencin et al, 2016;Zhao et al, 2017). If we consider the updip limit of the Gorkha rupture as the maximum extent of the frictionally locked portion of the fault, then W F /W T = 0.4, although we note that almost no afterslip has been observed updip of the Gorkha earthquake in 2015, suggesting that this part of the fault is velocity weakening (e.g., Mencin et al, 2016;Zhao et al, 2017).…”

Fault mechanics and post-seismic deformation at Bam, SE Iran

“…Deep afterslip has primarily been found to be co-located with, and share the same temporal evolution as, deep aftershock sequences (Perfettini 2004;Savage et al 2005;Hsu et al 2006;Peng & Zhao 2009;D'Agostino et al 2012;Mencin et al 2016). In addition, aftershock sequences are often localized around the edges of the co-seismic rupture, and share at least one nodal plane with the mainshock (Bodin & Horton 2004;Tatar et al 2005), indicating that some aftershocks represent failure on the same fault surface as the mainshock.…”

The 2015 Gorkha Earthquake: Earthquake Reflection Imaging of the Source Fault and Connecting Seismic Structure With Fault Slip Behavior

“…Two high-Vp zones are present along the MHT, at~55-65 and~80-90 km from the MFT. The shallower high-Vp zone shows low coseismic slip but coincides with an area of minor afterslip (Mencin et al, 2016;Figure 4e). It coincides with a large coseismic slip area (Elliott et al, 2016; Figure 4e).…”

“…Vertical cross section of seismic velocity structure along x axis. Purple area indicates afterslip area reported byMencin et al (2016). Relocated hypocenters and focal mechanisms located within ±5 km of the x axis are plotted on image.…”