Tsunami heights and limits in 1945 along the Makran coast estimated from testimony gathered 7 decades later in Gwadar, Pasni and Ormara

The towns of Pasni and Ormara were the most severely affected by the 1945 Makran tsunami. The water inundated land for almost 1 km at Pasni, engulfing 80 % of the huts of the town, while at Ormara the tsunami inundated land for 2.5 km, washing away 60 % of the huts. The plate boundary between the Arabian Plate and Eurasian Plate is marked by Makran subduction zone (MSZ). This Makran subduction zone in November 1945 was the source of a great earthquake (8.1 Mw) and an associated tsunami. Estimated death tolls, waves arrival times, and the extent of inundation and runup have remained vague. We summarize observations of the tsunami through newspaper items, eyewitness accounts and archival documents. The information gathered is reviewed and quantified where possible to obtain the inundation parameters specifically and understand the impact in general along the Makran coast. The quantification of runup and inundation extents is based on a field survey or old maps.

Probabilistic quantification of tsunami current hazard using statistical emulation

In this paper, statistical emulation is shown to be an essential tool for the end-to-end physical and numerical modelling of local tsunami impact, i.e. from the earthquake source to tsunami velocities and heights. In order to surmount the prohibitive computational cost of running a large number of simulations, the emulator, constructed using 300 training simulations from a validated tsunami code, yields 1 million predictions. This constitutes a record for any realistic tsunami code to date, and is a leap in tsunami science since high risk but low probability hazard thresholds can be quantified. For illustrating the efficacy of emulation, we map probabilistic representations of maximum tsunami velocities and heights at around 200 locations about Karachi port. The 1 million predictions comprehensively sweep through a range of possible future tsunamis originating from the Makran Subduction Zone (MSZ). We rigorously model each step in the tsunami life cycle: first use of the three-dimensional subduction geometry Slab2 in MSZ, most refined fault segmentation in MSZ, first sediment enhancements of seabed deformation (up to 60% locally) and bespoke unstructured meshing algorithm. Owing to the synthesis of emulation and meticulous numerical modelling, we also discover substantial local variations of currents and heights.

Tsunami heights and limits in 1945 along the Makran coast estimated from testimony gathered seven decades later in Gwadar, Pasni and Ormara

The towns of Pasni and Ormara were the most severely affected by the 1945 Makran tsuami. The water inundated almost a kilometer at Pasni, engulfing 80 % huts of the town while at Ormara tsunami inundated two and a half kilometers washing away 60 % of the huts. The plate boundary between Arabian plate and Eurasian plate is marked by Makran Subduction Zone (MSZ). This Makran subduction zone in November 1945 was the source of a great earthquake (8.1 Mw) and of an associated tsunami. Estimated death tolls, waves arrival times, extent of inundation and runup remained vague. We summarize observations of tsunami through newspaper items, eye witness accounts and archival documents. The information gathered is reviewed and quantized where possible to get the inundation parameters in specific and impact in general along the Makran coast. The quantization of runup and inundation extents is based on a field survey or on old maps.

Seismic hazard of the Western Makran subduction zone: Effect of heat flow on frictional properties combining mechanical and thermo-mechanical modelling approaches

<p>Western Makran is one of the few subduction zones left with a largely unconstrained seismogenic potential. According to the sparse GPS stations, the subduction is accumulating some strain to be released during future earthquakes. Mechanical modelling is first used to retrieve the spatial variations of the frictional properties of the megathrust, and discuss its seismogenic potential. To do so, we first build a structural map along the Iranian part of the Oman Sea and investigate three N-S seismic profiles. The profiles are characterized by a long imbricated thrust zone that takes place at the front of the wedge. A diapiric zone of shallow origin lies in between the imbricated zone and the shore. Along the eastern and western shores, active listric normal faults root down to the megathrust. Eastern and western domains have developed similar deformation, with three zones of active faulting: the normal faults on shore, thrusts ahead of the mud diapirs, and the frontal thrusts. On the contrary, no normal faults are identified along the central domain, where a seamount is entering into subduction. From mechanical modelling, we show that along the eastern and western profiles, a transition from very low to extremely low friction is required to activate the large coastal normal fault. To propagate the deformation to the front, an increase of friction along the imbricated zone is necessary. These along-dip transitions could either be related to a transition from an aseismic to seismic behavior or the brittle-viscous transition.</p><p>To decipher, we run 2-D thermo-mechanical modelling incorporating temperature evolution, with a heat flow boundary condition. Our simulations are first calibrated to reproduce the heat flow estimates based on the BSR depth. Then the effects of the illite-smectite and brittle-viscous transitions on the deformation are investigated. The decrease in heat flow landward is due to the landward deepening of the oceanic plate and thickening of sediments of the accretionary wedge. Deformation starts at the rear of the model and migrates forming in-sequence, forward verging thrust sheets. The two brittle-viscous and illite-smectite transitions affect the topographic slope and friction. A reduction of friction due to the illite-smectite transition reduces the slope by normal faulting that does not appear in the brittle-viscous transition simulations. Therefore, the presence of normal faults could permit to distinguish viscously creeping segments from segments that deform seismically. As a consequence, the normal fault is most probably related to the presence of a seismic asperity, and the difference in deformation along strike would thus reveal the existence of two different patches, one along the eastern domain and a second along the western domain. Since no large earthquake has been historically reported and given the high convergence rate, a major earthquake will strike the Makran region. We suggest that the magnitude of this event will depend on the behavior of the Central region, and the ability of the earthquake to propagate from the eastern to the western asperity or the Pakistani Makran.</p>

Slab segmentation and continental collision: reconstructing slab deformation in South-East Iran during the closure of Neotethys.

<p>As the Arabian continent converged with Eurasia during the closure of Neotethys, the subducting slab of oceanic lithosphere deformed and tore into segments. In the Makran subduction zone, ongoing rollback and extension within a convergent setting was accommodated by subduction zone bounding strike-slip systems. A 3D slab model for the subducted lithosphere in the Makran region was constructed using <em>eQuakes</em> and <em>SKUA-GOCAD</em>, based on the UU-P07 tomographic velocity model, and the available records of modern seismic activity. Seismotectonic analysis suggests remnants of subducted slab within the adjacent collision zone influence active tectonics. Slab floating and analysis of the morphology of the model suggests potential sites of slab tearing, which are evaluated against the record of regional magmatism. The slab model also allows for the examination of proposed models for subduction against the magnitude and location of reconstructed lithosphere.</p>

Overriding plate deformation and topography during slab rollback and slab rollover: insights from subduction experiments

<p>Overriding plate deformation (OPD) and topography vary at different subduction zones, with some subduction zones showing mainly overriding plate extension and low topography (e.g. Mariana, Tonga, Izu-Bonin subduction zones), while some showing mainly shortening and elevated topography (e.g. Makran, southern Manila subduction zones). Here we investigate how different subduction modes, namely trench retreat and trench advance, affect OPD and generate corresponding topography with fully dynamic analogue models of time-evolving subduction in three-dimensional space. We conduct two sets of experiments, one of which is characterized by trench retreat and slab rollback, and the other characterized by trench advance and slab rollover. We compute the mantle flow, the overriding plate strain and topography during subduction using the particle image velocimetry technique (PIV). The overriding plate in the experiments showing continuous trench retreat experiences overall extension, while in the experiments with trench advance following trench retreat it experiences overall shortening. The overriding plate in both trench retreat and trench advance subduction modes present fore-arc shortening and intra-arc extension. Our experiments indicate that the overall OPD except in the fore-arc region is mainly driven by the horizontal mantle flow at the base of the OP inducing a viscous drag force (F<sub>D</sub>), and is determined by the gradient of the horizontal mantle flow velocity (dv<sub>x</sub>/dx). Furthermore, a large-scale trenchward overriding plate tilting and an overall subsidence of the overriding plate were observed in the experiments showing continuous trench retreat, while a landward tilting and an overall uplift of the overriding plate were observed during long-term trench advance. The two types of topography during the two different subduction modes can be ascribed to the large-scale trenchward and landward mantle flow, respectively, and thus represent forms of dynamic topography. Our models showing trench advance provide a possible mechanism for OPD in the Makran subduction zone, which has experienced overall trench-normal tectonic shortening in the overriding plate, but shows extension in a local region of the coastal Makran that is spatially comparable to that in our experiments.  In addition, these models might also provide an explanation for the regional topography at the Makran subduction zone, which shows a long-wavelength topographic high in the overriding plate near the trench that decreases northward.</p>