Paper summaries 2021

Abstract: The frontal sections of subduction zones are the source of a poorly understood hazard: “tsunami earthquakes,” which generate larger‐than‐expected tsunamis given their seismic shaking. Slip on frontal thrusts is considered to be the cause of increased wave heights in these earthquakes, but the impact of this mechanism has thus far not been quantified. Here, we explore how frontal thrust slip can contribute to tsunami wave generation by modeling the resulting seafloor deformation using fault‐bend folding theory. We then quantify wave heights in 2D and expected tsunami energies in 3D for both thrust splays (using fault‐bend folding) and down‐dip décollement ruptures (modeled as elastic). We present an analytical solution for the damping effect of the water column and show that, because the narrow band of seafloor uplift produced by frontal thrust slip is damped, initial tsunami heights and resulting energies are relatively low. Although the geometry of the thrust can modify seafloor deformation, water damping reduces these differences; tsunami energy is generally insensitive to thrust ramp parameters, such as fault dip, geological evolution, sedimentation, and erosion. Tsunami energy depends primarily on three features: décollement depth below the seafloor, water depth, and coseismic slip. Because frontal ruptures of subduction zones include slip on both the frontal thrust and the down‐dip décollement, we compare their tsunami energies. We find that thrust ramps generate significantly lower energies than the paired slip on the décollement. Using a case study of the 25 October 2010 Mw 7.8 Mentawai tsunami earthquake, we show that although slip on the décollement and frontal thrust together can generate the required tsunami energy, <10% was contributed by the frontal thrust. Overall, our results demonstrate that the wider, lower amplitude uplift produced by décollement slip must play a dominant role in the tsunami generation process for tsunami earthquakes.

Abstract: The largest (M8+) known earthquakes in the Himalaya have ruptured the upper locked section of the Main Himalayan Thrust zone, offsetting the ground surface along the Main Frontal Thrust at the range front. However, out-of-sequence active structures have received less attention. One of the most impressive examples of such faults is the active fault that generally follows the surface trace of the Main Boundary Thrust (MBT). This fault has generated a clear geomorphological signature of recent deformation in eastern and western Nepal, as well as further west in India. We focus on western Nepal, between the municipalities of Surkhet and Gorahi where this fault is well expressed. Although the fault system as a whole is accommodating contraction, across most of its length, this particular fault appears geomorphologically as a normal fault, indicating crustal extension in the hanging wall of the MHT. We focus this study on the reactivation of the MBT along the Surkhet-Gorahi segment of the surface trace of the newly named Reactivated Boundary Fault, which is ~ 120 km long. We first generate a high-resolution Digital Elevation Model from triplets of high-resolution Pleiades images and use this to map the fault scarp and its geomorphological lateral variation. For most of its length, normal motion slip is observed with a dip varying between 20° and 60° and a maximum cumulative vertical offset of 27 m. We then present evidence for recent normal faulting in a trench located in the village of Sukhetal. Radiocarbon dating of detrital charcoals sampled in the hanging wall of the fault, including the main colluvial wedge and overlying sedimentary layers, suggest that the last event occurred in the early sixteenth century. This period saw the devastating 1505 earthquake, which produced ~ 23 m of slip on the Main Frontal Thrust. Linked or not, the ruptures on the MFT and MBT happened within a short time period compared to the centuries of quiescence of the faults that followed. We suggest that episodic normal-sense activity of the MBT could be related to large earthquakes rupturing the MFT, given its proximity, the sense of motion, and the large distance that separates the MBT from the downdip end of the locked fault zone of the MHT fault system. We discuss these results and their implications for the frontal Himalayan thrust system.

Teaser: Following the 1999 Chi-Chi earthquake in Taiwan, buildings were found precariously balanced on a slope. New modeling shows how to better plan for such damage.

Plain Language Summary: The Earth's crust deforms through both faulting (brittle fracturing and sliding) and folding (continuous deformation). During earthquakes, we observe the signal of faulting using geophysical instrumentation, but geological observations show that this faulting is often associated with related folding. There are only scarce observations of this earthquake-related folding captured in geophysical datasets, and as a result it is not known when we should expect this process to occur: during or soon after earthquakes? or in the long time period between earthquakes? We address this issue for the case of an anticline that is, crust folded due to slip on an underlying bent fault. We develop a numerical model of folding of a layered crust evolving over geological time. The stress and strain outputs from this long-term model are used to drive short-term earthquake sequence simulations. Slip on the fault during earthquakes raises the stress level in the surrounding crust. To relax this elevated stress level, slip occurs along multiple layer interfaces within the crust; the rate of this stress relaxation is controlled by the material properties of the crust. In sedimentary rocks, these layer interfaces are the stratigraphic bedding plane contacts. Because the material properties of the rocks influence the rate of relaxation, it should be possible to use observations of relaxation and folding following earthquakes to learn more about these properties.

Teaser: Aftershocks of the multiple large-magnitude earthquakes that occurred recently in the South Sandwich Islands suggest shallow rupture to the trench.

Plain language summary: In Bangladesh, the interface between two tectonic plates has created a large, nearly flat, earthquake-producing fault called a décollement. Above the décollement, sedimentary layers are compressed to create a series of north-south trending folds and thrust faults that extend for hundreds of kilometers. This study uses 28 seismic reflection data sets (a technology similar to ultrasounds), originally collected by the oil and gas industry, and reinterprets the subsurface structure of the folds as a way to determine the décollement depth. We find that the décollement has a curved shape, with a depth of ∼9 km in northeast and southeast Bangladesh, and ∼5 km in east-central Bangladesh. We hypothesize that the greater amount of sediment deposited in north and south Bangladesh has weighed down the surface of the earth and warped the décollement. This is the first study to constrain the geometry of this décollement, and we find that it has the potential to host a magnitude 8.5+ earthquake. The fault model presented here can be incorporated into studies of the rupture patterns of earthquakes on this décollement to better understand the earthquake hazard in a region inhabited by over 160 million people.

Abstract: Most destructive tsunamis are caused by seismic slip on the shallow part of offshore megathrusts. The likelihood of this behaviour is partly determined by the interseismic slip rate deficit, which is often assumed to be low based on frictional studies of shallow fault material. Here, we present a new method for inferring the slip rate deficit from geodetic data that accounts for the stress shadow cast by frictionally locked patches, and show that this approach greatly improves our offshore resolution. We apply this technique to the Cascadia and Japan Trench megathrusts and find that, wherever locked patches are present, the shallow fault generally has a slip rate deficit between 80 and 100% of the plate convergence rate, irrespective of its frictional properties. This finding rules out areas of low kinematic coupling at the trench considered by previous studies. If these areas of the shallow fault can slip seismically, the global tsunami hazard could be higher than currently recognized. Our method identifies critical locations where seafloor observations could yield information about frictional properties of these faults so as to better understand their slip behaviour.

Highlights

• We propose a new 3D Vs model for the crust and uppermost mantle in the Myanmar region.

• The forearc mantle of Indo-Burma subduction zone is at least 19% serpentinized.

• We identify a northward reduction of water input into the Indo-Burma mantle wedge.

• Localized accretion is observed beneath the northern Indo-Burman Ranges.

• Basaltic intrusions are found in the middle/lower crust beneath the Sagaing Fault.

Key points

• The Himalayan mountain belt is a unique subaerial orogenic wedge characterized by tectonically rapid, ongoing crustal shortening and thickening, intense surface denudation and recurrent great (Mw 8+) earthquakes.

• The history of the orogen has been investigated from long (million-year) to short (seconds to days) timescales using a variety of geological and geophysical techniques.

• The magnitude 7.8 Gorkha earthquake and aftershocks were monitored by extensive local geophysical networks, providing a unique set of observations of a major Himalayan earthquake and the Himalayan seismic cycle.

• Observations across the Himalaya reveal along-strike segmentation patterns at various temporal scales, controlled by inherited tectonic complexities developed over millions of years.

• Developing a complete understanding of deformation across timescales from seconds to millions of years requires an integrated, interdisciplinary effort.

Abstract: The intermediate-depth (50–180 km) seismicity beneath Myanmar provides direct evidence of the subducting Indian slab. However, the historic lack of regional seismic observations leads to previous low-resolution models that show large variations in slab geometry beneath Myanmar. The depth extent and morphology of the slab are still poorly known. In this study, we conduct a joint inversion of regional and teleseismic P-wave traveltimes from recently installed networks to image seismic velocity structures beneath central Myanmar by adopting an eikonal equation-based traveltime tomography method. The observations contain a total of 6,069 regional first P-wave arrivals and 29,787 teleseismic P-wave differential traveltimes. We find a high P-wave velocity anomaly beneath central Myanmar, which starts from ∼50 km depth and extends continuously to the mantle transition zone (MTZ) and is interpreted as the subducting Indian slab. Below 100 km depth, the dip angle of the slab in the south is ∼15° larger than that of the slab in the north, suggesting a possible slab tearing. Based on our tomographic results and previous studies, the slab in the north is inferred to have a deep stagnant segment lying above the 660-km discontinuity in the MTZ, but whether it is connected with the shallow dipping slab cannot be confirmed. Meanwhile, the slab in the south may just stay in the upper mantle (above 410 km), but it may also have penetrated the 410-km discontinuity. Taking into account all the scenarios, we propose four possible models of the Indian Plate subduction system beneath central Myanmar.