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...
Abstract. We revisit the 1976 Friuli earthquake sequence by combining hypocenters relocation, long period surface wave inversion, field geology and strong motion modelling. We show that fault-related folding is the main active deformation by which the seismic energy was released during the main shock (Ms=6.5) and that some of the surface effects reported in 1976 correspond to widespread bedding planes displacements induced by flexural-slip folding. The fault evolved from blind to semi-blind along strike showing the control of the inherited structural geology on the fault surface break and rupture arrest. Our fault model produces waveforms that fit the accelerograms recorded in the area.
Abstract. We study the 1998 Bovec-Krn mountain (Slovenia) earthquake sequence by combining hypocenters relocation, strong motion inversion, digital elevation modelling and field geology. The main shock (Ms = 5.7), a 12 km right lateral strike-slip event on the Dinaric fault system, occurred on a sub-vertical fault plane. The rupture, confined between 3 and 9 km depth, with no evidence of surface faulting, propagated bilaterally within two structural barriers. The northwestern barrier is at the junction between Dinaric and Alpine structures where there is a sharp change in the geometry of faulting. The southeastern barrier is within the Dinaric system and its surface expression corresponds to the Tolminka-spring perched basin, a 1 km restraining stepover. At this site, the Bovec-Krn earthquake-fault overlaps with a 30 km strike-slip fault segment that is free of aftershocks and could be undergoing an increase of stress. This fault system represents the northern branch of the Idrija right-lateral fault.
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