Post Earthquake Aggradation Processes to Hide Surface Ruptures in Thrust Systems: The M8.3, 1934, Bihar‐Nepal Earthquake Ruptures at Charnath Khola (Eastern Nepal)

Abstract

The Charnath Khola is a large river crossing the Himalayan thrust system in the region devastated by the great M8.3 1934 Bihar‐Nepal earthquake. Fluvial terraces are abandoned along the river and at the base of a ~20‐m high cumulative thrust escarpment. A trench across the fault scarp exposed Siwalik mudstone/siltstone overthrusting Quaternary units and three colluvial wedges interfingered with fluvial sands. The 85 accelerator mass spectrometry radiocarbon dates, from detrital charcoals sampled in the trench, a river cut and river terraces, constrain the timing of the sedimentary processes following the last two major earthquakes, in 1934 and 1255 CE. Although several samples straddle the main earthquake horizon, associating it with the 1934 earthquake, based solely on radiocarbon ages, remains challenging. The 49 detrital charcoal ages found in the pre‐earthquake and postearthquake units fall between 65 and 225 BP, a period with a flat calibration curve. Many of these radiocarbon ages are suspected to include a part due to inbuilt time (i.e., age of the wood at the time of burning), transport time, and reworking processes, which are difficult to resolve. Considering these ages at their face value could lead to dates older than the actual earthquake dates. We suggest that a part of this chronological bias is also related to a local postseismic aggradation pulse of 4 to 5 m of sediments, which is documented in the trench and terraces. This fluvial sequence, hiding the most recent surface rupture, is likely related to landslide‐sediment deposition triggered by the 1934 Bihar‐Nepal earthquake.

Establishing primary surface rupture evidence and magnitude of the 1697 CE Sadiya earthquake at the Eastern Himalayan Frontal thrust, India

“…However, the face value of the confidence index percentile ages of unit 1b has been used to represent that the earthquake happened after the lower bound calendar age i.e., 1445 CE. Similar interpretations have widely been used in Himalaya by several researchers 12 , 13 , 27 , 28 . Based on the highest probability distribution of radiocarbon ages of samples combined with additional seismotectonic information discussed later in the discussion, we limit the timing of displacement any time after 1445 CE.…”

Paleoseismological Studies in India (2016-2020): Status and Prospects

“…As for the Himalaya studies, the paleoseismic excavation records are likely to miss the blind fault stands. Such questions assume greater importance in the context of the 1934 Bihar-Nepal as the debate on whether this has activated a blind thrust or it had indeed ruptured the surface (Wesnousky et al, 2018;Rizza et al, 2019). Another example is the controversy regarding the location of the fault on the northern side of the Shillong Plateau, which has been variously called as the 'Oldham' fault and the 'Brahmaputra' fault -the former is placed on the northern topographic edge of the plateau and the latter along the Brahmaputra River (Fig.…”

Structural Evolution of a Crustal‐Scale Seismogenic Fault in a Magmatic Arc: The Bolfin Fault Zone (Atacama Fault System)

“…Indeed, moderate to large in magnitude ( M > 6) earthquakes rupture faults extending for >15 km in length, but such large faults are rarely well‐exposed along their whole length at the surface due to weathering and vegetation or Quaternary cover. Major mature faults typically record a long, polyphase deformation history, which might obliterate the incipient stages of nucleation and growth (e.g., Rizza et al., 2019 ). Thus, the field geologists' challenge in studying ancient, crustal‐scale seismogenic fault systems is to find large areas which meet the following criteria: excellent preservation over kilometer‐scale exposures of the spatial arrangement of structures (e.g., joints, dykes, and faults) related to multiple deformation stages; faults exhumed from depths (i.e., 5–15 km depending on tectonic regime, rock composition, temperature gradient, etc.…”

“…Indeed, moderate to large in magnitude (M > 6) earthquakes rupture faults extending for >15 km in length, but such large faults are rarely well-exposed along their whole length at the surface due to weathering and vegetation or Quaternary cover. Major mature faults typically record a long, polyphase deformation history, which might obliterate the incipient stages of nucleation and growth (e.g., Rizza et al, 2019). Thus, the field geologists' challenge in studying ancient, crustal-scale seismogenic fault systems is to find large areas which meet the following criteria:…”