Tsunami earthquakes generate large tsunamis but only moderate ground shaking. The discrepancy between the magnitude of the surface wave and the resulting tsunami height makes them a serious threat to ...coastal communities and a problem for tsunami early warning. Although at least 13 tsunami earthquakes took place since 1896, there is little consent on what controls their genesis. It remains unclear if they are tied to distinct subduction-zone structures or geometries or if they can occur along all active margins. To help shed light on the genesis of these unusual earthquakes, I combine marine acoustic data from subduction-zone segments that experienced tsunami earthquakes in order to search for structural similarities in the nature of the subducting oceanic plate. The structural comparison indicates that tsunami earthquakes preferentially occur in regions where the subducting plate is characterized by excess topography which is not blanketed by trench sediment. While subducted, the topographic obstacles likely cause fracturing along their track. This contributes to the development of a thick, structurally and lithological complex, and fragmented plate-boundary shear-zone, which may be prone to fail at a low velocity during a tsunami earthquake. The results suggest that when assessing the risk of a tsunami earthquake, special focus should be placed on the combination of lower-plate topography and trench sediment thickness.
•I collated bathymetric data from active margins that experienced tsunami earthquakes.•Tsunami earthquakes occurred where subducting relief is not blanketed by sediment.•Tsunami earthquakes are linked to structurally and lithological complex megathrusts.
To date, the parameters that determine the rupture area of great subduction zone earthquakes remain contentious. On 1 April 2014, the Mw 8.1 Iquique earthquake ruptured a portion of the ...well-recognized northern Chile seismic gap but left large highly coupled areas un-ruptured. Marine seismic reflection and swath bathymetric data indicate that structural variations in the subducting Nazca Plate control regional-scale plate-coupling variations, and the limited extent of the 2014 earthquake. Several under-thrusting seamounts correlate to the southward and up-dip arrest of seismic rupture during the 2014 Iquique earthquake, thus supporting a causal link. By fracturing of the overriding plate, the subducting seamounts are likely further responsible for reduced plate-coupling in the shallow subduction zone and in a lowly coupled region around 20.5°S. Our data support that structural variations in the lower plate influence coupling and seismic rupture offshore Northern Chile, whereas the structure of the upper plate plays a minor role.
In order to investigate how submarine weathering processes may affect the water balance of sediments at convergent plate margins, six sediment cores were retrieved off Central Chile at water depth ...between ∼800 and 4000m. The sediment solid phase was analyzed for its major element composition and the pore fluids were analyzed for dissolved sulfate, sulfide, total alkalinity, major cations, chloride, bromide, iodide, hydrocarbons as well as the carbon isotopic composition of methane.
Because of negligible weathering on land, surface sediments off Central Chile are rich in reactive silicate minerals and have a bulk composition similar to volcanic rocks in the adjacent Andes. Deep-sourced fluxes of alkalinity, cations and chloride indicate that silicate minerals are subject to weathering in the forearc during burial. Comparison of deep-sourced signals with data from nearby Ocean Drilling Program Sites reveals two different types of weathering processes: In shallow (tens of meters), methanic sediments of slope basins with high organic carbon burial rates, reactive silicate minerals undergo incongruent dissolution through reaction with CO2 from methanogenesis. At greater burial depth (hundreds of meters), silicate weathering is dominated by authigenic smectite formation. This process is accompanied by uptake of water into the clay interlayers thus leading to elevated salinities in the surrounding pore water. Deep-seated smectite formation is more widespread than shallow silicate dissolution, as it is independent from the availability of CO2 from methanogenesis. Although solute transport is not focused enough to form cold seeps in the proper sense, tectonically induced, diffuse fluid flow transfers the deep-seated signal of smectite formation into the shallow sediments.
The temperature-controlled conversion of smectite to illite is considered the most important dehydration process in marine forearc environments (depth of kilometers). However, in agreement with other studies at active margins (e.g. Aleutians, Cascadia, Nankai Trough) and despite ubiquitous evidence for smectite formation, little evidence for seafloor seepage of dehydration fluids could be found off Central Chile. We argue that the circular process of pore water uptake during smectite formation and release upon illitization implies a balanced freshwater budget and therefore a rather limited potential for net pore water freshening on a margin-wide scale. According to this rationale, pore water freshening at seafloor seeps preferentially occurs at lower latitudes (Central America, Barbados, Mediterranean Ridge) where terrestrial weathering is more intense thus leading to external (i.e. detrital) smectite and thus freshwater inputs to the subduction system.
Abstract
The updip limit of seismic rupture during a megathrust earthquake exerts a major control on the size of the resulting tsunami. Offshore Northern Chile, the 2014 Mw 8.1 Iquique earthquake ...ruptured the plate boundary between 19.5° and 21°S. Rupture terminated under the mid-continental slope and did not propagate updip to the trench. Here, we use state-of-the-art seismic reflection data to investigate the tectonic setting associated with the apparent updip arrest of rupture propagation at 15 km depth during the Iquique earthquake. We document a spatial correspondence between the rupture area and the seismic reflectivity of the plate boundary. North and updip of the rupture area, a coherent, highly reflective plate boundary indicates excess fluid pressure, which may prevent the accumulation of elastic strain. In contrast, the rupture area is characterized by the absence of plate boundary reflectivity, which suggests low fluid pressure that results in stress accumulation and thus controls the extent of earthquake rupture. Generalizing these results, seismic reflection data can provide insights into the physical state of the shallow plate boundary and help to assess the potential for future shallow rupture in the absence of direct measurements of interplate deformation from most outermost forearc slopes.
The comparison of bathymetric datasets compiled before and after the Mw
=
8.8 Maule Earthquake of the 27 February 2010 offshore Central Chile proves that no new submarine landslides on a size scale ...detectable with hull-mounted bathymetric echosounders (features of a horizontal size of >
1
km) formed as a direct consequence of the ground shaking. Gravity coring around a pre-existing slide feature offshore Concepción (BioBio Slide), however, documents that (1) a number of events occurred as retrogressive failures of the BioBio Slide wall, the youngest of which is 700–1000
years old, and that (2) a very recent small scale slide structure resulted from non-destructive imbricate stacking of a thin sediment layer. Pore water geochemical data show that this event post-dates the Maule Earthquake, suggesting that it was triggered by one of the numerous aftershocks. The absence of larger failures and the presence of a small slide let us propose that in contrast to apparent logic, frequent violent earthquakes at convergent margins do not necessarily pose a particular tsunami risk by landslides. The frequent shaking might even limit the slide volume and therefore their tsunami hazard, as instead of rare and large slides, frequent smaller slides are induced.
► Changes in the bathymetry resulting from the 2010 Maule Earthquake were looked for. ► New submarine landslides >
1
km
×
1
km have formed. ► Small-scale sliding has occurred months after the main shock. ► Frequent quakes induce small-scale submarine mass wasting at the cost of large events.
Upper‐plate normal faults are a widespread structural element in erosive plate margins. Increasing coverage of marine geophysical data has proven that similar features also exist in accretionary ...margins where horizontal compression usually results in folding and thrust faulting. There is a general lack of understanding of the role and importance of normal faulting for the structural and tectonic evolution of accretionary margins. Here we use high‐resolution 2‐D and 3‐D seismic reflection data and derived seismic attributes to map and analyze upper‐plate normal faulting in the marine forearc of the accretionary Hikurangi margin, New Zealand. We document extension of the marine forearc over a wide area along the upper continental slope. The seismically imaged normal faults show low vertical displacements, high dip angles, a preference for landward dip, and often en echelon patterns. We evaluate different processes, which may cause the observed extension, including (1) stress change during the earthquake cycle, (2) regional or local uplift and decoupling of shallow strata from compression at depth, and (3) rotation of crustal blocks and resulting differential stresses at the block boundaries. The results suggest that normal faults play an important role in the structural and tectonic evolution of accretionary margins, including the northern Hikurangi forearc.
Key Points
High‐resolution 3‐D seismic data indicate widespread normal faulting on the upper slope of the northeastern Hikurangi margin
The normal faults show two major strike directions, primarily landward dip, low vertical displacements, and steep dip angles
Extension may be controlled by regional uplift or/and extensional strain due to rotation of tectonic blocks around nearby poles
Coastal and ocean island volcanoes are renowned for having unstable flanks. This can lead to flank deformation on a variety of temporal and spatial scales ranging from slow creep to catastrophic ...sector collapse. A large section of these unstable flanks is often below sea level, where information on the volcano-tectonic structure and ground deformation is limited. Consequently, kinematic models that attempt to explain measured ground deformation onshore associated with flank instability are poorly constrained in the offshore area. Here, we attempt to determine the locations and the morpho-tectonic structures of the boundaries of the submerged unstable southeastern flank of Mount Etna (Italy). The integration of new marine data (bathymetry, microbathymetry, offshore seismicity, reflection seismic lines) and published marine data (bathymetry, seafloor geodesy, reflection seismic lines) allows identifying the lineament north of Catania Canyon as the southern lateral boundary with a high level of confidence. The northern and the distal (seaward) boundaries are less clear because no microbathymetric or seafloor geodetic data are available. Hypotheses for their locations are presented. Geophysical imaging suggests that the offshore Timpe Fault System is a shallow second-order structure that likely results from extensional deformation within the moving flank. Evidence for active uplift and compression upslope of the amphitheater-shaped depression from seismic data along with subsidence of the onshore Giarre Wedge block observed in ground deformation data leads us to propose that this block is a rotational slump, which moves on top of the large-scale instability. The new shoreline-crossing structural assessment may now inform and improve kinematic models.
The physical state of the shallow plate‐boundary fault governs the updip extent of seismic rupture during powerful subduction zone earthquakes and thus on a first order impacts on the tsunamigenic ...hazard of such events. During the 2004 Mw 9.2 Aceh‐Andaman Earthquake seismic rupture extended unusually far seaward below the accretionary prism causing the disastrous Indian Ocean Tsunami. Here we show that the formation of a strong bulk sediment section and a high fluid‐pressured predécollement, that likely enabled the 2004 rupture to reach the shallow plate‐boundary, result from thermally controlled diagenetic processes in the upper oceanic basement and overlying sediments. Thickening of the sediment section to >2 km ∼160 km seaward of the subduction zone increases temperatures at the sediment basement interface and triggers mineral transformation and dehydration (e.g., smectite‐illite) prior to subduction. The liberated fluids migrate into a layer that likely host high porosity and permeability and that is unique to the 2004 rupture area where they generate a distinct overpressured predécollement. Clay mineral transformation further supports processes of semilithification, induration of sediments, and coupled with compaction dewatering all amplified by the thick sediment section together strengthens the bulk sediments. Farther south, where the 2005 Sumatra Earthquake did not include similar shallow rupture, sediment thickness on the oceanic plate is significantly smaller. Therefore, similar diagenetic processes occur later and deeper in the subduction zone. Hence, we propose that shallow seismic rupture during the 2004 earthquake is primarily controlled by the thickness and composition of oceanic plate sediments.
Key Points
Early clay dehydration shift seismic updip limit close to the trench
Shallow rupture controlled by thickness and composition of marine sediments
Likelihood of shallow rupture in subduction zones with thick input sediments
Oceanic transform faults (OTFs) are an inherent part of seafloor spreading and plate tectonics, whereas the process controlling their morphology remains enigmatic. Here, we systematically quantify ...variations in transform morphology and their dependence on spreading rate and age‐offset, based on a compilation of shipborne bathymetric data from 94 OTFs at ultraslow‐ to intermediate‐spreading ridges. In general, the length, width and depth of OTFs scale systematically better with age‐offset rather than spreading rate. This observation supports recent geodynamic models proposing that cross‐transform extension scaling with age‐offset, is a key process of transform dynamics. On the global scale, OTFs with larger age‐offsets tend to have longer, wider, and deeper valleys. However, at small age‐offsets (<5 Myr), scatters in the depth and width of OTFs increase, indicating that small age‐offset OTFs with weak lithospheric strength are easily affected by secondary tectonic processes.
Plain Language Summary
In the past 5 decades, studies on oceanic transform faults (OTFs) have revealed significant complexity in their morphology, which calls for detailed quantitative analysis to study the processes controlling the morphology of OTFs. Using the most complete and advanced compilation of bathymetric data from ultraslow‐ to intermediate‐spreading ridges, we parameterized the morphological characteristics of OTFs and extracted length, width and depth for each transform fault from the compiled bathymetric data. Moreover, correlations between these morphological parameters and related tectonic factors (e.g., spreading rate, age‐offset) were investigated in this study. We find that correlations between morphological features and spreading rate are rather weak. Comparison of correlations suggests that age‐offset scales better with the morphological parameters, along with scatters mostly at small age‐offsets, indicating small‐age‐offset OTFs are unstable due to their weak lithospheric strength. Our observation evidences extensional tectonics at OTFs.
Key Points
We compiled multibeam bathymetric data of 94 oceanic transform faults (OTFs) to quantify their morphological characteristics
Morphology of OTFs is dominated by age‐offset rather than spreading rate
Transform valleys get systematically deeper and wider with increasing age‐offset, implying extensional tectonics at OTFs
In this paper, we constrain the input and output fluxes of H
2
O, Cl and S into the southern-central Chilean subduction zone (31°S–46°S). We determine the input flux by calculating the amounts of ...water, chlorine and sulfur that are carried into the subduction zone in subducted sediments, igneous crust and hydrated lithospheric mantle. The applied models take into account that latitudinal variations in the subducting Nazca plate impact the crustal porosity and the degree of upper mantle serpentinization and thus water storage in the crust and mantle. In another step, we constrain the output fluxes of the subduction zone both to the subcontinental lithospheric mantle and to the atmosphere–geosphere–ocean by the combined use of gas flux determinations at the volcanic arc, volume calculations of volcanic rocks and the combination of mineralogical and geothermal models of the subduction zone. The calculations indicate that about 68 Tg/m/Ma of water enters the subduction zone, as averaged over its total length of 1,480 km. The volcanic output on the other hand accounts for 2 Tg/m/Ma or 3 % of that input. We presume that a large fraction of the volatiles that are captured within the subducting sediments (which accounts for roughly one-third of the input) are cycled back into the ocean through the forearc. This assumption is however questioned by the present lack of evidence for major venting systems of the submarine forearc. The largest part of the water that is carried into the subduction zone in the crust and hydrated mantle (accounting for two-thirds of the input) appears to be transported beyond the volcanic arc.
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