Cascading hazards of a major Bengal basin earthquake and abrupt avulsion of the Ganges River

Domain: earth_science
Published: 2024-06-17
Editorial outcome: accept

Abstract
Earthquakes present severe hazards for people and economies and can be primary drivers of landscape change yet their impact to river-channel networks remains poorly known. Here we show evidence for an abrupt earthquake-triggered avulsion of the Ganges River at ~2.5 ka leading to relocation of the main stem channel belt in the Bengal delta. This is recorded in freshly discovered sedimentary archives of an immense relict channel and a paleo-earthquake of sufficient magnitude to cause major liquefaction and generate large, decimeter-scale sand dikes >180 km from the nearest seismogenic source region. Precise luminescence ages of channels and, channel fill, and breached and partially liquefied floodplain deposits support coeval timing of the avulsion and earthquake. Evidence for reorganization of the river-channel network in the world’s largest delta broadens the risk posed by seismic events in the region and their recognition as geomorphic agents in this and other tectonically active lowlands. The recurrence of comparable earthquake-triggered ground liquefaction and a channel avulsion would be catastrophic for any of the heavily populated, large river basins and deltas along the Himalayan arc (e.g., Indus, Ganges, Brahmaputra, Ayeyarwady). The compounding effects of climate change and human impacts heighten and extend the vulnerability of many lowlands worldwide to such cascading hazards.

Introduction
Avulsion is a fundamental delta-building process that sustains coastal landscapes by redistributing the focus of sediment deposition over time to offset basin subsidence, sea-level rise, and localized aggradation of channel belts. The processes that govern avulsion remain debated1–4, and the dominant mechanisms may vary between relatively high-gradient, coarse-sediment fan settings and relatively low-gradient, fine-grain coastal settings as well by climate and sediment supply5. In its simplest form, avulsion results when channel belts become superelevated relative to their floodplain and shift to a new course through gradual abandonment by stream capture6,7. This mechanism is largely supported by autogenic river processes, but allogenic forces such as tectonic deformation or earthquakes may contribute to the avulsion of rivers in seismically active basins8,9. In regions such as the Himalaya, ongoing collision, uplift, and erosion have produced thick sedimentary sequences in both foreland and continental-margin basins that lie adjacent to the active seismogenic zones10. Among the largest and most heavily populated of these basins are the major rivers valleys and their coastal delta systems, including the Indus, Ganges, Brahmaputra, and Ayeyarwady.

The seismic risk to such lowlands is broadly recognized11–16 but remain poorly constrained with respect to response magnitude and cascading hazards17. In part, the large size of these basins mean that they often extend >100 km from the nearest faults and may only be impacted by larger, less frequent events for which there are scant historical records. Instrumental and historical timescales of years to centuries are likely to miss large, rare events with millennial recurrence intervals18. For example, a recent satellite-based synthesis of channel avulsions found no statistical correlation with earthquakes occurring in that instrumental timescale5. However, millennial-timescale reconstructions suggest evidence of earthquake impacts on inland river channel geometries in the New Madrid seismic zone, USA19,20, a region which corresponds to channel-belt avulsion sites of the Mississippi River over the Holocene21,22. Similarly, earthquakes in India’s Rann of Kutch region have caused widespread liquefaction and flooding from fault-induced water diversions16,23–25. Paleoenvironmental reconstructions alongside archaeological sources indicate this may be a recurring event16,26. The displacement of river channels proximal to ruptured faults is previously recognized9 yet the impact of seismicity on more distal lowland channels is not known or well documented. Although often far from seismogenic sources, the potential for earthquake impacts to lowland river-channel networks is amplified by the propensity of mud-capped, saturated fine-medium sands to liquefy, the greater relative effect of land-surface deformation and water routing on low-gradient landscapes, and fracture or failure of river bank levees through lateral spreading27,28.

As a vivid and empirical example of seismic risk to lowland basins, we present evidence from the sedimentary archive of the Bengal basin (Fig. 1) showing abrupt and complete avulsion of the main stem Ganges River coincident with a major paleo-earthquake. The river-channel avulsion is evidenced by the cessation of sand transport and overbank mud deposition at ~2.5 ka along an 85-km reach of this Ganges paleochannel belt. Located within 1-km of the abandoned channel are large (30–40-cm wide) sand dikes which breach the entire ~2.6–2.5 ka floodplain sequence. Such an archive is previously undocumented in the Bengal basin. The characteristics of the sand dikes, including their orientation, fracture bifurcations, brecciated sediment clasts, and liquefied sedimentary structures, all rule out a non-seismic origin and indicate that the dikes were formed by sediment liquefaction during an earthquake. A suite of thirteen precise optically stimulated luminescence (OSL) ages obtained from quartz silt29 constrain the apparent coeval occurrence of the avulsion and seismic events. These findings support a largely unrecognized allogenic avulsion mechanism and highlight the elevated risk of cascading earthquake hazards in river deltas and other lowland basin settings.

Results
Paleochannel archive
The 150,000 km2 Bengal basin is part of the trailing-edge continental margin of the eastern Indian subcontinent. Its northern and eastern perimeters lie at convergent plate boundaries that actively deform the kilometers of Neogene and older fluvio-deltaic sediments that infill the basin30. The Holocene delta sequence is up to 90 m thick and its sediments are primarily supplied by two of Earth’s largest rivers – the Ganges and Brahmaputra31.

In the central delta plain, ~45 km south of the modern Ganges River, the scar of a 1.0–1.7 km-wide meandering paleochannel is evident in the delta’s surface morphology (Fig. 1a, b). The relative elevation of the present-day paleochannel surface is ~3 m below the adjacent levee and floodplain surfaces; the paleochannel is consequently poorly drained and used for rice cultivation (Fig. 2). The wide paleochannel is partially infilled with muds that are 3–15 m thick and abruptly overlie a well-preserved sandy channel bed. This sand surface defines the meandering paleochannel’s point-bar front and thalweg (Fig. 2a). OSL ages of the upper point-bar and channel-fill sands both place the last occurrence of active sand transport at 2.61 ± 0.11 ka with an abrupt transition to mud infilling at 2.48 ± 0.14 ka (Fig. 2a). Ages from the adjacent floodplain show coeval, rapid overbank aggradation producing a ~4 m-thick sequence dating from 2.63 ± 0.15 ka at its base to 2.49 ± 0.10 ka near the modern land surface (Fig. 3a). Combined with the paleochannel results, these data indicate that a large, aggrading river system was active here around 2.6 ka, and the cessation of sand transport and overbank deposition indicate abrupt channel abandonment around 2.5 ka. Prior to abandonment, the average overbank sedimentation rate was a rapid 1.9 ± 0.8 cm/yr, which is consistent with rates from other proximal floodplain settings in the delta32,33. The abrupt and widespread cessation of sand transport along the entire paleochannel belt is supported by a coeval age of 2.62 ± 0.16 ka for the shallow-buried sands of a preserved mid-channel bar top ~85 km downstream (Fig. S3). These shallow, contemporaneously deposited bar-top units lie along a low-elevation swath (Fig. 1a, S3) indicating the continuous absence of fluvial clastic deposition since the river course was abandoned at ~2.5 ka.

The sands at these upstream (MA IV-1, MA IV-2, MA V-1) and downstream (RD III-1 and RD III-2) locations consistently yield bulk strontium values ranging 83 ± 3 to 98 ± 3 ppm, which unambiguously fall within the range for Ganges River provenance for the central and western delta34 (Supplementary Methods 3. Geochemical and grain-size measurements). Furthermore, the scale of the paleochannel indicates that it represents the main stem pathway of the Ganges River and was not a distributary. The paleochannel is 1.0–1.7 km wide with a cross-sectional area of ~17.7 × 103 m2 based on the depth to sand from our borehole transect (Fig. 2). This cross-sectional area is within 5% of the ~16.7 × 103 m2 value for the modern Ganges channel at Hardinge Bridge, where the river is 1.5 km wide (Fig. 2c)35. In contrast, modern distributary channels of the Ganges River, including the Gorai and Bhagirath-Hooghly distributaries, are only 0.2–0.5 km wide in most places and rarely exceed 0.6 km. These distributary channels have cross-sectional areas of 1–2 × 103 m2, which are only ~10% that of the modern main stem river and the abandoned paleochannel (Fig. 2c).

Sand-dike archive
One kilometer east of the abandoned channel (Fig. 1), two large, linear sand dikes and their subordinate fractures rupture the thick, muddy floodplain sequence and reflect major liquefaction of underlying sand units (Figs. 1, 3). Terrestrial clastic dikes such as these are commonly associated with earthquakes, wherein intensive shaking can fluidize buried saturated sediments and generate sufficient pore-fluid pressure to drive injection of confined sediments into overlying deposits. Modern and ancient examples of earthquake-triggered liquefaction and sand-dike emplacement can be found worldwide28,36–39. Clastic dikes may also form through non-seismic mechanisms such as riverbank slumping, load-induced pressurization, and wind events40,41. The dikes we identified consist of moderately well-sorted, fine sands (d50 = 200–250 µm) that breach a nearly 4-m thick capping unit of moderately sorted silty muds (d50 = 35–60 µm). Both units are typical of river-channel sands and floodplain deposits in the central Bengal delta, respectively42. The size and sorting distribution of the sands fall squarely within the ideal range for liquefaction41,43,44, as do most of the shallow (<30 m depth) sands across the lower Bengal delta (Fig. 4) and elsewhere in Bangladesh45.

The two main dikes are ~15 m apart and lie parallel to one another at a roughly east-west strike (Figs. 1c, 3). The dikes are both 10–15 cm wide at their base and widen to 30–40 cm in the upper few meters before reaching ground level (Fig. 3). In this upper zone, the main dikes yield a number of subordinate dikes (5–10 cm wide) that often conjoin one another to form a connected network of structures.

The margins of the sand-dike intrusions are sharply bound and quasi-linear with some angular edges and fractures. Brecciated mud clasts (1–15 cm) are regularly incorporated within the dike sands, but they are most common at bifurcation points of the subordinate dikes (Fig. S1c), where the clasts appear to disrupt flow of the fluidized sands. This arrangement suggests that the subordinate dikes formed in succession after the main dikes and may reflect minutes of sustained ground-surface deformation during emplacement. The large dikes reported here comprise unconsolidated Holocene sands that intruded actively aggrading overbank muds.

Together, the large width (5–40 cm) and complex structure (e.g., bifurcating and intersecting fractures, brecciated clasts) of these Ganges sand dikes suggests sustained, high-energy disturbance that is consistent with a seismic origin. Fracturing and sand injection due to riverbank slumping can be ruled out because of the dikes’ orientation normal to the paleochannel (Fig. 1b, c); in contrast, bank extension would cause tensile fractures roughly parallel to the channel39. The timing of sand dike emplacement is well constrained by OSL ages of the capping floodplain muds breached by the dikes. This floodplain deposit yields burial ages of 2.63 ± 0.15 ka for muds near its base at 3.5 m depth, as well as an age of 2.58 ± 0.15 ka for the silt-to-very-fine-sand layer at ~1.0 m depth and 2.49 ± 0.10 ka for muds ~0.8 m below the present-day ground surface (Fig. 3a).

When viewed in cross-section, the scale and architecture of the dikes are similar to those associated with sand blows generated by the 1811–1812 New Madrid earthquakes near the Mississippi River28,38. Similar surface expressions of liquefaction (i.e., sand blows) were also reported across the Bengal basin following the 1897 Shillong Massif earthquake46, but to our knowledge there is no comparable record of vertical exposures like we present here.

Subordinate seismite archive
In addition to the main sand dikes, other subordinate deformation features (Fig. 5) also support a seismic origin. The first example is the disturbed bedding of a 30-cm thick unit of silt-to-very-fine sand (<125 µm), which is positioned within the capping mud unit ~1 m below the ground surface (Fig. 5a). The decimeter-scale, silt-sand unit is finely laminated and typical of small splays or overbank deposits in the proximal floodplain, but in this example the laminae are strongly convoluted from being partially fluidized after deposition. Since the fine sands of this fluidized layer also flow into the margins of the sand-dike (Fig. 5e), it is required that both sand bodies were fluidized contemporaneously. Furthermore, fluidization of the silt-sand unit suggests that it was under-consolidated and thus not very old when the sand dike breached it during the earthquake, consistent with the OSL ages showing it to have been deposited just over a century before overbank deposition ceased following the river avulsion (Fig. 3a).

Other subordinate features that help to constrain the timing and mechanisms of disturbance are mud-filled pipes and micro-fractures that crosscut the sand dikes and capping mud unit. These penecontemporaneous intrusions appear as roughly cylindrical pipes of dark brown mud (3–10 cm diameter, Fig. 5a, b) that all have arcuate, mud-filled micro-fractures radiating from their centers (Fig. S1b). We interpret these dark-colored sediments as disseminated organics and fluidized muds that were generated at the ground surface and infilled void spaces created by the sustained shaking of trees and their root systems. It is the ubiquitous presence of mud-filled microfractures radiating from these mud pipes that indicate the surrounding sediments were being actively compressed by tree-root motion. The possibility that these features were formed during a wind event can be ruled out by the mud pipes and micro-fractures that formed within one of the sand dikes (Fig. 5a); in this instance, the mud pipe and its microfractures are both deformed upward in the direction of sand injection, indicating that the mud pipes and sand dikes formed contemporaneously – or more specifically, the mud pipes probably formed during the waning phase of dike emplacement, enabling their preservation. The abundant formation of mud pipes also suggests that the earthquake occurred during the wet season, when standing surface water and saturated soils make fine surface sediments subject to fluidization.

Event reconstruction
The preservation of large, extensive, and under-filled channel scars such as those documented here (Fig. 1b, Fig. S3) is rare for the Ganges and Brahmaputra rivers, because their high-water discharge and sediment loads typically infill old channel courses with sand during the avulsion process47,48. Thus, (i) the scale and distribution of the abandoned channel reaches, (ii) the abrupt cessation of sand deposition, (iii) the muddy and only partial infilling of the paleochannel, and (iv) its enduring, topographically low surface expression all suggest that this was an abrupt avulsion involving a major reorganization of the Ganges River system. In contrast, a gradual autogenic avulsion6,7 would be inconsistent with the sharp termination of both sand transport and overbank deposition along this 85-km channel-belt reach. Furthermore, OSL ages from the cutbank floodplain (Fig. 3) indicate that deposition had been active for only a few hundred years (from ~2.7 ka to ~2.5 ka). This two-century period is considerably shorter than the ~1–2 millennia recurrence interval for autogenic avulsion49 or delta-lobe cycles50 in the Bengal delta.

Like the record of channel avulsion, the archive of clastic sand dikes allows for a partial reconstruction of the associated paleo-earthquake event. Extensive research on earthquake impacts has yielded empirical relationships between earthquake magnitude and (a) the distance at which liquefaction occurs and (b) the width of resulting sand dikes37,39,51 (Fig. 6). The Ganges floodplain sand dikes presented here are located >180 km from the two closest seismically active zones that occur around the Shillong Massif ~200–300 km to the northeast52 and along the locked portion of the Indo-Burman subduction zone ~180–280 km to the east12,53 (Fig. 1 and S8). The generation of liquefaction at these distances suggests a minimum earthquake magnitude of M 7.5–8.0 (Fig. 6a), with event scales of M > 8.0 more typical for the major liquefaction and sand-dike formation we document39 (Fig. 6a). Furthermore, the seismogenic zones at Shillong Massif and the Indo-Burman subduction regions are both capable of producing M > 8 earthquakes54. Shillong Massif was subjected to a ~M 8 earthquake in 189746,55,56 that generated widespread liquefaction in Bengal basin46,57. The Indo-Burman subduction zone along the Chittagong-Arakan coast produced a ~M 8.5 earthquake in 176258,59.

The width of sand dikes can also be used as a general proxy for earthquake magnitude, and these Ganges floodplain dikes are comparable to the largest terrestrial sand dikes reported in a recent global compilation37 (Fig. 6b). Their 30–40 cm width implies a minimum M ~6.5 event, but most similarly sized dikes are associated with much stronger M > 7 events (Fig. 6b). Taken together, both size and distance relationships suggest that the ~2.5 ka earthquake was likely in the range of M 7.0–8.0. Such an event could originate as a M 7.0 splay-fault rupture in the Indo-Burman ranges 180+ km to the east, or a larger M 8.0 megathrust rupture produced at either the Shillong or Indo-Burman fronts. Regardless of the earthquake’s specific source or magnitude, its impact was immense.

Collectively the sand dikes and abandoned paleochannel record the cascading hazards of far-reaching subsurface liquefaction and an abrupt avulsion of the main stem Ganges River, each triggered by a major (~M 7.0–8.0) earthquake that originated >180 km away. The relationship between these cascading hazards is that earthquake-generated ground roll and shallow-sediment liquefaction are plausible mechanisms for channel avulsion by rerouting surface water on the low-gradient delta plain (~10−4 to 10−5 slope), where water flow paths are susceptible to subtle elevation or topographic changes. Recently, differential subsidence hypothetically linked to major Bengal earthquakes in 1762 and possibly ~900 CE has been inferred from the rapid burial and preservation of salt kilns at coastal archaeological sites60,61 and uplifted micro-atoll corals62.

Similar observations of rapid topographical change along the banks and proximal tributaries of the Mississippi River are documented for the 1811–1812 New Madrid earthquakes, resulting in what Charles Lyell referred to as “the sunk country”63,64. Water-flow disruptions have also occurred in the Rann of Kutch region of western India, where the 1819 Allah Bund (‘Dam of God’) earthquake generated an 80-km-long fault scarp damming several local rivers16,25,65. Prior seismic activity between 712 and 1361 CE is thought to have transformed parts of the Rann of Kutch from a coastal lowland with navigable channels to the seasonal saline lake persisting today, which Lyell referred to as “neither land nor sea”16,26,66. Yet, it is important to keep in mind the limitations of historical records and archaeological correlations29. The environmental impacts of the New Madrid and Kutch earthquakes are descriptively rich, but the relative contributions of river flooding versus tectonic subsidence are difficult to discern in these older accounts.

If confirmed for the Bengal basin, rapid differential subsidence60–62 would be consistent with the potential for earthquakes to abruptly alter delta-plain elevations and river gradients. We further infer that the earthquake and avulsion occurred during the wet monsoon season when (i) sediments are saturated and more prone to failure, (ii) liquefaction potential rises with water loading and overpressure, (iii) lateral water gradients increase during bankfull river discharge, and (iv) high water discharge is more capable of channelizing a new flow path. The 1950 Assam earthquake occurred in August during the monsoon, triggering extensive land sliding and bank failures, with the vastly increased sediment load creating changes in elevation, width, and braiding along the Brahmaputra River67,68. Water loading may also be a plausible trigger for major fault rupture, as the mass of monsoon-fed water that seasonally inundates the Bengal basin is sufficiently immense to cause up to 60 mm of elastic strain in the upper lithosphere69.

Methods
Sedimentology and stratigraphy
Stratigraphy was determined up to 5.8 m depth by hand coring with an Edelman hand auger and gouge for 2 boreholes (one including the pit exposure) at the seismite site, 11 boreholes in and on the banks of the paleochannel, and one borehole in the floodplain ~85 km southeast toward the Bay of Bengal. These shallow strata were described in 10-cm intervals with attention to sediment texture and color. Three long boreholes including one at the seismite site (Fig. 3b) and two within the paleochannel (Fig. 2a) were obtained by tube-well drilling. Sediment samples from the 43-, 61-, and 91-m-long boreholes were described in 2–3 m intervals. An overview of all boreholes and exposures is given in Supplementary Data 1. The extent and character of sand dikes and other soft-sediment deformation and seismite features were surveyed on cleaned exposed walls and floor of a freshly dug and unfilled pond, and their orientation was mapped using a theodolite. Bulk major and trace element concentrations, including strontium, were measured on 20 g of dried bulk sediment using a portable XRF (Thermo-Scientific Niton XL3 Analyzer). The grain-size distributions of each luminescence sample and of sediment obtained from the BNG-MS3 borehole were measured by laser diffraction using a Malvern Mastersizer 3000 particle-size analyzer.

Chronology
Fourteen samples for OSL dating (Table S2.2) were obtained with a light-tight vertical lined sampler which collects 30-cm of undisturbed sediment within a borehole and by hammering light-tight, lined PVC tubes horizontally into exposed pond walls. All samples were analyzed for grain-size distribution and bulk strontium content (Table S2.2) which is an indicator of provenance of sand-rich deposits34. Luminescence dating was performed using the 4–11 or 4–20 micron silt fraction29 (Table S2.3) which was isolated and measured using procedures of ref. 107 and described below. A multiple-signal single-aliquot regenerativedose (MS-SAR) protocol108 was used to screen the samples for sensitivity and paleodose. Two standard SAR protocols with recuperation, recycling, and IR depletion tests were applied to purified quartz; one with a 240°C preheat and four regenerative doses for high-paleodose (>5 Gy) samples and one with a 200°C preheat and three regenerative doses for low-paleodose (<5 Gy) samples. Dose recovery ratios of 1.013 ± 0.006 (n = 41) and 1.036 ± 0.004 (n = 45), respectively, verified the measurement sequences (Fig. S7). Early background subtraction109 was used to optimize the relative contribution of the fast component to the quartz OSL signal. The paleodose was calculated as a mean and standard error of at least 3 and most often 6 multigrain 10-mm diameter (~2 mg) silt aliquots per sample. Dose rate was obtained using mainly standard procedures including measurement of radionuclide activities of dried, ashed, and ground bulk sediment on a high-resolution broad-range gamma spectrometer. A water content correction based on grain size and seasonality was applied to unsaturated samples to estimate their time-averaged water content (Fig. S4). Luminescence ages were determined as the paleodose divided by the dose rate and are reported with one sigma uncertainties. Notably, one sample yielded no measurable quartz and thus no luminescence age. The last occurrence of active sand transport in the paleochannel was determined as a weighted mean of the two point bar sand samples (Fig. 2a). Overbank aggradation rate was calculated by differencing the ages and depths for OSL samples MAI-2 and MAI-3 (Fig. 3a), which were located near the top of the floodplain unit (below the surface mixing zone) and near its base, respectively. Detailed luminescence-dating methodology and interpretation is given in the Supplementary Information and resulting data for each sample are provided in Supplementary Data 3.

Event magnitude and liquefaction risk
The Supplementary Information provides a detailed consideration of the tectonic setting of the Bengal basin and of prior Bengal Basin earthquakes on record. This information was used to support our reconstruction of the paleo-earthquake including likely origin and magnitudes that are possible for the Bengal Basin. The magnitude of the event was estimated based on general relationships with distance to origin and dike width that were established by prior studies using global datasets of known-magnitude events37,39. Dredge-fill construction sites were quantified using Google Earth Engine110 to establish the location, timing, and area of site development for five-year intervals over the last two decades (2003–2023).

Data availability
The data generated in this study are provided in the Supplementary Information and Supplementary Data files. Supplementary Data 1–3 are offered as one excel file with three sheets.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Authors present a well-supported link between seismicity and river avulsion within deltaic lowlands. They argue, convincingly in my opinion, that such a large-scale dramafic response to seismicity should be taken into account for tectonically-affected deltaic environments within a cascade of hazards. The study was executed at the highest standards in the field and the wrifing is clear and to the point.\nI do not support the claim that seismic hazards have not been recognized previously in deltas. In fact, the twin of their GB delta (i.e. the Indus) is a classical for this topic. This does not diminish the importance of their study, which is befter supported. However, I suggest the authors to review work on the Allah Bund fault and the blockage of the Nara course of the Indus in the Indus delta by authors like C. P. Rajendran and K. Rajendran as well as Marfifia Tuftle. Such work should be cited and integrated as precursors into their paper.\nOtherwise, I am looking forward to see this study in print.", "reviewer_2": "Overall, this paper represents a thorough, well-documented study showing new evidence for the abrupt avulsion of the Ganges River leading to the abandonment and relocafion of the mainstem channel as a consequence of a major paleo-earthquake. The paper is well illustrated with some excepfional documentafion of field data.\nThe study represents a good example of a modern work as the authors use a combinafion of high-quality field data supported by absolute dafing. I therefore commend the authors for their excellent, thorough and systemafic documentafion of data and their holisfic approach.\nThis is a study that presents a useful and significant contribufion to science, stressing the high risk associated with the cascading effects of earthquakes in deltas and other lowland seftings around the world. The scope and aims of the study are of interest to a wide range of scienfists, parficularly those interested in tectonically acfive seftings, paleo-earthquakes & seismites or hazard management.\nThe authors generally do a good job outlining the rafionale and aims of the project. They use suitable methods to address the aims and present high-quality field data coupled with luminescence dafing. The conclusions are largely supported by the data. The manuscript is fairly well wriften although there is certainly scope to befter organize the observafions in order to make it more concise, and befter support the interpretafions. The manuscript is therefore suitable for Nature Communicafion and would be of interest to a range of scienfists in different fields.\nImprovements\n-Please provide a general background of sand intrusions, their trigger mechanism and why they are used as palaeo-earthquake indicators.\n-Please provide more details of dikes and fracture orientafion in relafionship to the paleochannel (a stereonet could be useful) to befter support the exclusion of riverbank slumping as a trigger.\n-Ensure that you are separafing observafions and interpretafions into different secfions. A sub-secfion is needed to discuss all evidence that supports the earthquake origin of the fluidizafion and intrusions.\n-Please provide a more comprehensive background of the link between the width of the dikes and the magnitude of the earthquakes and show your interpretafions in light of current knowledge, limitafions and uncertainfies regarding that.\nOther minor suggesfions\n-The sentences are very long in places, consider breaking them into two or three separate sentences.\n-The presentafion of field data is excellent, however, you should make sure that the descripfions are as organized and signposted as possible.\nSee detailed annotated comments on the aftached PDF.", "reviewer_3": "Review of the manuscript\nCascading hazards of a major Bengal basin earthquake and abrupt 1 avulsion of the Ganges River\nby Elizabeth Chamberlain and co-authors.\nThis study is a very nice piece of work aftempfing to link very peculiar sedimentary and deposifional structures, related to a river avulsion event, to a past major, remote EQ. The careful collecfion of various excellent field observafions in combinafion with OSL dafing of various types of normal, sheeted, and intrusive deposits provides a convincing picture of the ‘cascading’ impacts the strafigraphic and sedimentary system had experienced in response to the actual local river avulsion process. The evidence is nicely supported by the figures, supplementary material, and describes a scienfifically spectacular story.\nWhat needs clarificafion is to what extent these findings are unique observafions so far, or whether comparable structures have previously been found in other geographic areas of the delta. See Lines 228 to 230 in this context: Does the comparison with impacts of the Assam EQ on the banks of the Brahmaputra River mean that comparable liquefacfion features are already known from other parts of the delta? If so, it needs to be described in detail how the sedimentary elements look like there, how they compare to the features found in this study, and what makes the new discovery unique. This effort would support the desire to publish this work in Nature.\nThe second issue appears to be related to their conclusions that a major EQ of a magnitude exceeding M 7.5 is required to trigger such a remote geomorphological response. Parficularly around Lines 156, 191, and 195 in the manuscript, the authors build on speculafions that would greatly benefit from a more geotechnical corroborafing approach: If an EQ leads to liquefacfion of certain sediment beds that are composed of relafively sorted coarse-silty/fine-sandy material, I wonder to what extent it might be appropriate to assume that those original beds were under-consolidated as well as young. I would assume ‘under-consolidafion’ is a relafive descripfion in this context? Commonly, an under-consolidafion of non-cohesive material should be relafively age-independent. Thus, I suggest – here and elsewhere across the manuscript text – that the authors aftempt to add some professional geotechnical calculafions about the fluidizafion potenfial of the deposits that parficipated in the liquefacfion event. It would support their suggesfion that the observed liquefacfion features can technically be related to an EQ magnitude of 7.5 to 8.0. How this specific EQ esfimate is currently presented is not very convincing. In Line 258, the authors refer to ‘saturated, un-consolidated (what is the difference between un- and under-consolidated then?) Holocene sands’. It is thus, neither clear which consolidafion degree the original deposits needed to have, nor how old the deposits might have been.\nSuch a geotechnical theorefical calculafion would also help to exclude other condifions that might lead to a major river avulsion. In Lines 252 to 254, the authors refer to a seemingly similar ‘cascading’ situafion in the Brahmaputra River system. They might want to describe the sedimentological/depositional expression of the major shift in river avulsion there. Are there any features known that might be comparable to the elements observed in this study? Could it be that a flood-related major avulsion process can lead to sufficient enough overloading and pore-pressure changes that lead to the formafion of similar geological structures? If even a strong monsoonal event may trigger a major avulsion event (s. a.), more substanfial informafion needs to be provided to assume a magnitude of M 7.5+.\nGiven the experfise of some of their co-authors, it would be helpful to inform the reader if there has been any EQ reported in geological archives closer to the subducfion zone or the Himalayans that dates around 2.5 ka? Is there any seismo-strafigraphic evidence for an EQ that happened 2.5 ka ago? How about the liquefacfion beds found within the offshore strafigraphic record?\nWhy might modern engineering river bank management, seeking to stabilize the river course, negafively impact the natural ‘cascading’ process that might happen during a catastrophic EQ-triggered avulsion event? The Implicafions chapter currently sounds like there is no solufion to be prepared or to offer protecfion in such a major avulsion case, may it be related to monsoonal flooding or a remote EQ. Would the authors agree with this conclusion? If so, the last chapter on anthropogenic and technical modificafion is not very needed; if not, what would be their advice?\nThe following comments are of minor nature but might help to improve the manuscript.\nLine 41 What does ‘vary between fan and coastal seftings’ mean? The apron near the mountains, the delta plain, a deep-sea fan? Please provide some term definifion/explanafion for readers from the outside.\nLine 42 What are ‘channel belts’? S. a.\nLine 177 What are ‘under-filled channel scars’? S. a.\nLine 123 Provide sedimentological informafion, specifically on the degree of sorfing and the conclusions that can be drawn from it. How about skewness as a source or transport sorfing indicator?\nLines 211 to 217 read somehow repefifive since they do not provide new informafion or context.\nLine 197 What locafion/region does ‘occurring here’ refer to? Are the observafions of various, supposedly EQ-related geological/sedimentological features a unique finding, or are there other examples in the Ganges river delta? The provided citafion refers to an example in the US. What is the link between the two systems?"}, "author_response": "We thank the three reviewers for their thoughtful comments, which have improved the\nmanuscript. Please find our response to each comment below in blue.\nReviewer #1 (Remarks to the Author):\nAuthors present a well-supported link between seismicity and river avulsion within deltaic\nlowlands. They argue, convincingly in my opinion, that such a large-scale dramatic response\nto seismicity should be taken into account for tectonically-affected deltaic environments\nwithin a cascade of hazards. The study was executed at the highest standards in the field and\nthe writing is clear and to the point.\nI do not support the claim that seismic hazards have not been recognized previously in deltas.\nIn fact, the twin of their GB delta (i.e. the Indus) is a classical for this topic. This does not\ndiminish the importance of their study, which is better supported. However, I suggest the\nauthors to review work on the Allah Bund fault and the blockage of the Nara course of the\nIndus in the Indus delta by authors like C. P. Rajendran and K. Rajendran as well as Martitia\nTuttle. Such work should be cited and integrated as precursors into their paper.\nOtherwise, I am looking forward to see this study in print.\nIt was very encouraging to read this; thank you. Thanks also for pointing us toward the\ninteresting case of the Allah Bund fault. We agree this research is relevant, and we have added\ntext and references to include it as well as referencing Tuttle’s contributions on the New\nMadrid seismites.\nTo clarify our claim, we changed the last sentence of the Implications to “…may face\nunrecognized hazards as the result of seismic risk”. We do not intend to claim that seismic\nrisk alone to deltas is not recognized; there is ample literature on seismic risk to tectonic\nbasins including Bangladesh, and we cite this literature where relevant. Rather, we\ndemonstrate that the cascade of hazards resulting from earthquakes – specifically the avulsion\nof a major river channel distal to the earthquake origin and coupled with extensive\nliquefaction – is not previously well considered for deltas.\nReviewer #2 (Remarks to the Author):\nOverall, this paper represents a thorough, well-documented study showing new evidence for\nthe abrupt avulsion of the Ganges River leading to the abandonment and relocation of the\nmainstem channel as a consequence of a major paleo-earthquake. The paper is well illustrated\nwith some exceptional documentation of field data.\nThe study represents a good example of a modern work as the authors use a combination of\nhigh-quality field data supported by absolute dating. I therefore commend the authors for their\nexcellent, thorough and systematic documentation of data and their holistic approach. This is a\nstudy that presents a useful and significant contribution to science, stressing the high risk\nassociated with the cascading effects of earthquakes in deltas and other lowland settings\naround the world. The scope and aims of the study are of interest to a wide range of scientists,\nparticularly those interested in tectonically active settings, paleo-earthquakes & seismites or\nhazard management.\nThe authors generally do a good job outlining the rationale and aims of the project. They use\nsuitable methods to address the aims and present high-quality field data coupled with\nluminescence dating. The conclusions are largely supported by the data. The manuscript is\nfairly well written although there is certainly scope to better organize the observations in\norder to make it more concise, and better support the interpretations. The manuscript is\ntherefore suitable for Nature Communication and would be of interest to a range of\nscientists in different fields.\nThank you; we are happy to read this positive overview!\nImprovements\n-Please provide a general background of sand intrusions, their trigger mechanism and why\nthey are used as palaeo-earthquake indicators.\nGood idea. We have added this to the first paragraph of the Paleo-earthquake Archive – Sand\nDikes section.\n-Please provide more details of dikes and fracture orientation in relationship to the\npaleochannel (a stereonet could be useful) to better support the exclusion of riverbank\nslumping as a trigger.\nGood idea; we now include a stereonet in the supplement (Fig. S9).\n-Ensure that you are separating observations and interpretations into different sections. A\nsubsection is needed to discuss all evidence that supports the earthquake origin of the\nfluidization and intrusions.\nSeparating data and discussions into different sections is not the typical format of Nature\nCommunications; rather we wrote the manuscript with topical sections addressing the\ndifferent aspects of our study, including both data and interpretation in each topical section.\nAs the first reviewer commended the writing and clarity, we opt not to significantly\nrestructure the manuscript.\n-Please provide a more comprehensive background of the link between the width of the dikes\nand the magnitude of the earthquakes and show your interpretations in light of current\nknowledge, limitations and uncertainties regarding that.\nWe already do this to the extent that is possible for this paleo-earthquake reconstruction, and\nwe have carefully edited our phrasing to better make clear the uncertainties.\nOther minor suggestions\n-The sentences are very long in places, consider breaking them into two or three separate\nsentences.\nThank you for the careful proofreading. We have reviewed each comment on the annotated\npdf and made changes where we felt it improved the manuscript (e.g., clarifying that these are\nnot lithified sediments, referring to other seismite features as “subordinate” rather than\n“secondary”, moving a figure from the supplement (now Fig. 4) to better detail sediment\nlithology).\n-The presentation of field data is excellent, however, you should make sure that the\ndescriptions are as organized and signposted as possible.\nAgreed, and thank you for the comments on figures and data presentation (e.g., labelling of\nFig. 1 panel c as d). We have incorporated these in our revised manuscript.\nSee detailed annotated comments on the attached PDF.\nThanks; we have reviewed these comments and incorporated your feedback.\nReviewer #3 (Remarks to the Author):\nThis study is a very nice piece of work attempting to link very peculiar sedimentary and\ndepositional structures, related to a river avulsion event, to a past major, remote EQ. The\ncareful collection of various excellent field observations in combination with OSL dating of\nvarious types of normal, sheeted, and intrusive deposits provides a convincing picture of the\n‘cascading’ impacts the stratigraphic and sedimentary system had experienced in response to\nthe actual local river avulsion process. The evidence is nicely supported by the figures,\nsupplementary material, and describes a scientifically spectacular story.\nThank you for these positive and encouraging comments.\nWhat needs clarification is to what extent these findings are unique observations so far, or\nwhether comparable structures have previously been found in other geographic areas of the\ndelta. See Lines 228 to 230 in this context: Does the comparison with impacts of the Assam\nEQ on the banks of the Brahmaputra River mean that comparable liquefaction features are\nalready known from other parts of the delta? If so, it needs to be described in detail how the\nsedimentary elements look like there, how they compare to the features found in this study,\nand what makes the new discovery unique. This effort would support the desire to publish this\nwork in Nature.\nArchives such as these are exceptionally rare for reasons we outline in the second paragraph of\nthe manuscript. To be candid, as a team that has worked 30+ years in the Bengal basin, we\nwere astounded when we discovered these dikes and even more so when we realized the\nconnection to the avulsed paleochannel. To clarify the novelty, we added “Such an archive is\npreviously undocumented in the Bengal basin” to the third paragraph of the manuscript.\nThe absence of published prior Bengal basin examples of exposed liquefaction features and\ntheir structures supports the rarity of the archives we detail. Liquefaction features associated\nwith the 1897 earthquake were previously mapped in research we cite, however, these maps\nrecorded only the surface expression (sand blows and fissures) and not the vertical profiles. As\nsuch, there are no documented liquefaction features in the Bengal basin that are comparable\nwith the ones we describe, to the best of our knowledge. We added a sentence to the Paleo-\nearthquake Archive – Sand Dikes section to make this clear.\nThe second issue appears to be related to their conclusions that a major EQ of a magnitude\nexceeding M 7.5 is required to trigger such a remote geomorphological response. Particularly\naround Lines 156, 191, and 195 in the manuscript, the authors build on speculations that\nwould greatly benefit from a more geotechnical corroborating approach: If an EQ leads to\nliquefaction of certain sediment beds that are composed of relatively sorted coarsesilty/fine-\nsandy material, I wonder to what extent it might be appropriate to assume that those original\nbeds were under-consolidated as well as young. I would assume ‘under-consolidation’ is a\nrelative description in this context? Commonly, an under-consolidation of non-cohesive\nmaterial should be relatively age-independent. Thus, I suggest – here and elsewhere across the\nmanuscript text – that the authors attempt to add some professional geotechnical calculations\nabout the fluidization potential of the deposits that participated in the liquefaction event. It\nwould support their suggestion that the observed liquefaction features can technically be\nrelated to an EQ magnitude of 7.5 to 8.0. How this specific EQ estimate is currently presented\nis not very convincing. In Line 258, the authors refer to ‘saturated, un-consolidated (what is\nthe difference between un- and under-consolidated then?) Holocene sands’. It is thus, neither\nclear which consolidation degree the original deposits needed to have, nor how old the\ndeposits might have been.\nWe carefully reviewed and edited our phrasing to make clear the uncertainties of the paleo-\nearthquake reconstruction. “Required” is a strong word from the reviewer here, and we do not\nmake the claim that an M7.5 earthquake or greater is required for the event we describe.\nRather, we provide a realistic reconstruction of the paleo-earthquake magnitude based on\nglobal databases of the relationship of dike width and distance to origin. The approach we use\nfor paleo-earthquake magnitude reconstruction is widely accepted. We acknowledge its\nlimitations and uncertainties, for example by stating “both size and distance relationships\nsuggest that the ~2.5 ka earthquake was likely in the range of M 7.0-8.0”. Finally, the\ncombination of both dike width and the site’s great distance from the seismogenic centers\nsupports a large magnitude event. Through this careful framing we attempt to avoid such\nmisunderstandings as occurred here and to be open and realistic about the uncertainty of\npaleo-reconstructions.\nWhile placing constraints on the magnitude of the paleo-earthquake is of interest, its specific\nmagnitude is of secondary importance to the study. Primary here is that, regardless of\nmagnitude, the paleo-earthquake we document generated significant and previously\nunrecognized cascading hazards including abrupt rerouting of a a very large river channel, and\nwe state this in the manuscript.\nRegarding quantitative properties of the sediments, we now provide in the main text grain-size\ndistributions of typical sand and mud deposits for the upper 30m of the Bengal basin and for\nsamples taken at the seismite location, with their associated liquefaction potential (now Fig.\n4).\nSuch a geotechnical theoretical calculation would also help to exclude other conditions that\nmight lead to a major river avulsion. In Lines 252 to 254, the authors refer to a seemingly\nsimilar ‘cascading’ situation in the Brahmaputra River system. They might want to describe\nthe sedimentological/depositional expression of the major shift in river avulsion there. Are\nthere any features known that might be comparable to the elements observed in this study?\nCould it be that a flood-related major avulsion process can lead to sufficient enough\noverloading and pore-pressure changes that lead to the formation of similar geological\nstructures? If even a strong monsoonal event may trigger a major avulsion event (s. a.), more\nsubstantial information needs to be provided to assume a magnitude of M 7.5+.\nIn the interest of minimizing speculation, we do not include geotechnical theoretical\ncalculations for reconstruction of this paleo-event. We did consider making geotechnical\nestimates when first preparing the manuscript, and we re-evaluated in light of this comment.\nAt both times, we judged it ineffective due to the large assumptions required regarding\nprincipal stresses, pore water pressure, overburden, etc. As previously stated, we do not\n“assume” an M7.5+ earthquake; rather we adopt a data-supported approach to magnitude\nreconstruction drawn from two different empirical parameters as described in our response to\nthe prior paragraph.\nGiven the expertise of some of their co-authors, it would be helpful to inform the reader if\nthere has been any EQ reported in geological archives closer to the subduction zone or the\nHimalayans that dates around 2.5 ka? Is there any seismo-stratigraphic evidence for an EQ\nthat happened 2.5 ka ago? How about the liquefaction beds found within the offshore\nstratigraphic record?\nTo the best of our knowledge there are no previously published studies that report a Himalaya\nor Bengal basin earthquake circa 2.5 ka. In fact, chronology in general is lacking for the\nHolocene deposits/archives of the basin (see Chamberlain et al., 2020 ESPL for a review on\nthis topic), and this is something our team aims to develop.\nWhy might modern engineering river bank management, seeking to stabilize the river course,\nnegatively impact the natural ‘cascading’ process that might happen during a catastrophic EQ-\ntriggered avulsion event? The Implications chapter currently sounds like there is no solution to\nbe prepared or to offer protection in such a major avulsion case, may it be related to\nmonsoonal flooding or a remote EQ. Would the authors agree with this conclusion? If so, the\nlast chapter on anthropogenic and technical modification is not very needed; if not, what\nwould be their advice? The following comments are of minor nature but might help to improve\nthe manuscript.\nIn general, embanked rivers have fewer natural outlets for water and are therefore particularly\nvulnerable to bank erosion, enhanced stream power, and thereby avulsion during high-stage\nevents. This is explained in the first two paragraphs of the Implications. Regarding flood risk,\nwe believe that the best hazard mitigation policy is one underpinned by scientific knowledge,\nand our study develops this knowledge.\nWe agree that the implications of our findings can be better illustrated. Toward this, we\nadapted content from the supplement (formerly Figs S4, S10a,c, and S11a) to the main text to\nillustrate the character and extent of modern dredge sand platforms (now Fig. 7) and their\nvulnerability to liquefaction based on sediment texture (now Fig. 4), and we updated the title\nto include sediment liquefaction. Fig. 4 also satisfies the recommendations of Reviewers 2 and\n3 to provide more information in the main text describing sediment texture. Offering a specific\nmanagement approach for Bangladesh to earthquake-triggered cascading hazards is beyond\nthe scope of this manuscript and should involve a multi-disciplinary team including\nBangladeshi policy makers and social scientists as well as physical scientists.\nLine 41 What does ‘vary between fan and coastal settings’ mean? The apron near the\nmountains, the delta plain, a deep-sea fan? Please provide some term\ndefinition/explanation for readers from the outside.\nWe improved to “between relatively high-gradient, coarse-sediment fan settings and relatively\nlow-gradient, fine-grain coastal settings”, so we now mention specific aspects of these\ngeomorphic zones that may influence avulsion.\nLine 42 What are ‘channel belts’? S. a.\nThis is common terminology in (fluvial) geomorphology and we do not feel it serves a short-\nformat manuscript to define it here.\nLine 177 What are ‘under-filled channel scars’? S. a.\nWe describe this in several sentences in the preceding section on the Paleochannel archive,\nparagraph 2, and we added reference to two figures which further illustrate the terminology.\nLine 123 Provide sedimentological information, specifically on the degree of sorting and the\nconclusions that can be drawn from it. How about skewness as a source or transport\nsorting indicator?\nWe moved the grain-size distribution figure (now Fig. 4) to the main manuscript. Additional\ninformation is provided in the supplementary files (e.g., Table S2.2). Further analysis of\nBengal basin sediments can be found in prior, referenced work, most recently Raff et al.\n(2023, Nature Communications).\nLines 211 to 217 read somehow repetitive since they do not provide new information or\ncontext.\nWe prefer to keep this important synthesis statement for clarity and to avoid misunderstanding\nby non-specialist readers.\nLine 197 What location/region does ‘occurring here’ refer to? Are the observations of various,\nsupposedly EQ-related geological/sedimentological features a unique finding, or are\nthere other examples in the Ganges river delta? The provided citation refers to an\nexample in the US. What is the link between the two systems?\nWe replaced “occurring here” with “we document” to make it clear that this interpretation is\nabout the Bengal basin sand dikes we show in this study. The reference is for a broader dataset\nrelating earthquake magnitude to distance to origin that is presented in Fig. 6 and discussed in\nthe preceding lines. To clarify we added a reference to Fig. 6a in parentheses after this\nsentence."}]

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