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Partitioning of Oblique Convergence During Simultaneous Rupture of a Megathrust and Splay Fault: Observations From the Western Nepal Fault System

Description: 

The subaerial megathrust system of the Himalaya provides opportunities for examining subduction zone - splay fault behavior using methods that are difficult for the typical submarine subduction zone environments. Some major historic megathrust ruptures (e.g., the 1964 Good Friday earthquake in Alaska) have produced simultaneous displacement on sub-parallel splay faults, but how common is this splay-fault behavior and what role does it play in slip partitioning in subduction zones undergoing oblique convergence? In the Himalayan orogen, we examine the Western Nepal fault system, a generally NW-trending series of faults that cut obliquely across the structural grain of the Himalayan orogen and is interpreted to intersect with the Main Himalayan Thrust (MHT) at depth. Paleoseismic investigations along most of the fault system reveal youthful surface ruptures, and while several sites have limited temporal resolution, several of these sites specifically constrain the occurrence of a surface-rupturing earthquake within the past 700 years. This timing overlaps with the Great 1505 Himalayan earthquake, the only historic rupture of the MHT interpreted to have ruptured beneath the WNFS. The trace of the WNFS across topography illustrates a typically moderate to steep N to NE dip and landforms offset across the fault define long-term right-lateral and extensional motion, with relative proportions of lateral vs. extensional slip increasing to the NW along the fault system. Based on the lack of documented large historic earthquakes in the area of the WNFS, we propose that the WNFS ruptured simultaneously with the rupture of the megathrust in the 1505 earthquake, or as several smaller earthquakes that are below the current limit of detection from pre-instrumental records. The slip-partitioning across the Himalaya of western Nepal is accommodated by earthquakes occurring on faults bounding a sliver of the orogenic wedge, and the potential for simultaneous ruptures of these faults (MHT and WNFS) presents a new scenario for seismic hazard assessment of the region.

Session: Learning Across Geological, Geophysical & Model-Derived Observations to Constrain Earthquake Behavior - I

Type: Oral

Date: 5/1/2024

Presentation Time: 08:15 AM (local time)

Presenting Author: Sean

Student Presenter: No

Invited Presentation: 

Authors