Sedimentary response of a structural estuary to Holocene coseismic subsidence

Stratigraphic evidence for coseismic subsidence has been documented in active-margin estuaries throughout the world. Most of these studies have been conducted in subduction zone or strike-slip settings; however, the stratigraphic response to coseismic subsidence in other tectonic settings would benefit from further study. Here we show evidence of late Holocene coseismic subsidence in a structural estuary in southern California. Below the modern marsh surface, an organic-rich mud containing marsh gastropods, foraminifera, and geochemical signatures indicative of terrestrial influence (mud facies) is sharply overlain by a blue-gray sand containing intertidal and subtidal bivalves and geochemical signatures of marine influence (gray sand facies). We use well-established criteria to interpret this contact as representing an abrupt 1.3 ± 1.1 m rise in relative sea level (RSL) generated by coseismic subsidence with some contribution from sediment compaction and/or erosion. The contact dates to 1.0 ± 0.3 ka and is the only event indicative of rapid RSL rise in the 7 k.y. sedimentary record studied. Consistent with observations made in previous coseismic subsidence studies, an acceleration in tidal-flat sedimentation followed this abrupt increase in accommodation; however, the recovery of the estuary to its pre-subsidence elevations was spatially variable and required 500−900 years, which is longer than the recovery time estimated for estuaries with larger tidal ranges and wetter climates.

Study and modelling of rapid shoreline displacement due to coseismic subsidence along the Kamchatka subduction zone

The first studies of geological evidences of coastal coseismic subsidence (associated with subduction-zone earthquakes) were carried out in Russia at the Institute of Volcanology and Seismology, in Kamchatka. We developed a special method based on tephrostratigraphy and tephrochronology, descriptions and dating of the soil-pyroclastic sequence (SPS) overlying the coastal wave-build beach ridges. Three seismic events accompanied by coastal coseismic subsidence were detected in the northern part of Avachinsky Bay during the past ~5 thousand years. We found subsidence from one of the greatest historical tsunamigenic earthquake (1952 AD) south of Petropavlovsk-Kamchatsky. We identified, that 5 events of coastal coseismic subsidence had occurred during the past ~6 thousand years at the coast of Kronotsky Bay and Shipunsky Peninsula. Amplitudes of subsidence were estimated by geological data using three different methods. Erosion of the active beach and marine accumulative terrace becomes active after coastal subsidence. We calculated the shoreline retreat process and the amount of horizontal erosion by numerical simulation using Bruun rule. In some areas, shoreline retreat was about 300 m according to the model results.

Supplemental Material: Sedimentary response of a structural estuary to Holocene coseismic subsidence

Supplemental Materials 1 (supplemental figures S1, S2, S3, S4); Supplemental Materials 2 (Table S1. Core Information); Supplemental Materials 3 (Bacon and OxCal Scripts); and Supplemental Materials 4 (Invertebrate depth references).

Supplemental Material: Sedimentary response of a structural estuary to Holocene coseismic subsidence

Supplemental Materials 1 (supplemental figures S1, S2, S3, S4); Supplemental Materials 2 (Table S1. Core Information); Supplemental Materials 3 (Bacon and OxCal Scripts); and Supplemental Materials 4 (Invertebrate depth references).

Multiple Submerged Tidal Notches: A Witness of Sequences of Coseismic Subsidence in the Aegean Sea, Greece

In some islands of the Aegean, there is evidence of the occurrence of repeated rapid subsidences during the Late Holocene. In this paper, the shape of tidal notches that may be well-preserved underwater is recalled in order to reconstruct sequences of coseismic subsidences and other relative sea-level changes, which occurred during, at least, the last few millennia. A reanalysis of the published measurements of submerged tidal notches in several islands reveals that subsidence trends in many areas of the Aegean are not continuous with gradual movement but, also, are the result of repeated coseismic vertical subsidences of some decimetres at each time. The estimated average return times are of the order of approximately some centuries to one millennium. Although the results cannot be used for short-term predictions of earthquakes, they may provide useful indications on the long-term tectonic trends that are active in the Aegean region.

Post-seismic recovery of subsided coastal northeast Japan after the 2011 Tohoku-oki earthquake

<p>During the 2011 Tohoku-oki earthquake the pacific coast of northeast Japan experienced significant subsidence, while in the years after it has undergone a continuous phase of uplift during the post-seismic period. The dense geodetic network deployed by GEONET and Tohoku university between 2011 and 2016 have captured variations in surface deformation along the coast, highlighting rapid uplift rates of ~7 cm/year on the Miyagi coast (Muto et al., 2019, Sci.Adv.) and ~3-4 cm/year on the Fukushima and Iwate coasts. Previous studies in the last decade have revealed the post-seismic deformation is due to a combination of both rapid viscoelastic flow and stress-driven afterslip, explaining the post-seismic vertical deformation pattern over northeast Japan as well as unravel its associated rheological complexity (e.g., Agata et al., 2019, Nat. Commun; Freed et al., 2017, EPSL; Hu et al., 2016, JGR; Muto et al., 2019, Sci.Adv.). Furthermore, continuous coastal uplift has had societal consequences, where the piers at the port are no longer suited to conduct many activities, particularly those for the fish industry. The large co-seismic subsidence of coastal areas caused the submersion of port piers, with rapid rebuilding to return the now submerged piers to sea-level. Nevertheless, the continuous uplift in the post-seismic period has now raised these rebuild piers above sea level and necessitates reduction in height back to sea level again (Iinuma, 2018, JDR). In this presentation, we employ forward modeling to improve estimates of future uplift and the time required for full recovery of coastal regions to their pre-event relative sea level.</p><p> </p><p>We present a numerical model using laboratory-derived constitutive laws and compare our modeled displacement with the geodetic observations (Ozawa et al., 2012, JGR; Tomita et al., 2017, Sci.Adv.; Watanabe et al., 2014, GRL). The model is constrained by terrestrial and seafloor geodetic observations in both horizontal and vertical components and incorporates a three-dimensional heterogeneous viscoelastic rheology fully coupled with stress-driven afterslip on the plate interface.</p><p> </p><p>Our model exhibits good agreement with the cumulative displacements, both in magnitude and azimuthal direction. We extend the time-series simulation for a further 20 years and estimate the recovery time to pre-event levels for the GNSS sites along the coastal areas. Our results show a recovery period of ~18 years after the mainshock for Ishinomaki site in Miyagi prefecture, which had the largest coseismic subsidence (up to ~1.2 m). We also estimate a recovery period of ~14-16 years for the coastal areas of Iwate and Fukushima prefectures, which experienced coseismic subsidence of ~0.5 m. The model adds an improvement to the previous estimates (Iinuma, 2018, JDR) by incorporating consideration of the coupling of viscoelastic relaxation and stress-driven afterslip.</p>

Impact of a high rainfall event on the water level, current velocity, and total suspended solids in tidal flats environments of the estuarine Cruces River wetland, south-central Chile

The effects of a sudden rainfall (40 mm d-1) event on the surface waters covering muddy tidal flats were studied during April 2016 at the estuarine Cruces River wetland in south-central Chile (~40ºS). The study area included flooded vestigial tree trunks, which is evidence of coseismic subsidence associated with the 1960 Valdivia earthquake as a source of environmental variability. The tidal flat with vestigial tree trunks registered the fastest and highest depth of inundation. In contrast, the tidal currents velocity and total suspended solids' concentrations were higher at the flat without trunks. Sudden rainfall events can significantly modify the characteristics of surface waters above sedimentary intertidal surfaces, where structures such as flooded trunks are present.

Timing and amount of southern Cascadia earthquake subsidence over the past 1700 years at northern Humboldt Bay, California, USA

Stratigraphic, lithologic, foraminiferal, and radiocarbon analyses indicate that at least four abrupt mud-over-peat contacts are recorded across three sites (Jacoby Creek, McDaniel Creek, and Mad River Slough) in northern Humboldt Bay, California, USA (∼44.8°N, −124.2°W). The stratigraphy records subsidence during past megathrust earthquakes at the southern Cascadia subduction zone ∼40 km north of the Mendocino Triple Junction. Maximum and minimum radiocarbon ages on plant macrofossils from above and below laterally extensive (>6 km) contacts suggest regional synchroneity of subsidence. The shallowest contact has radiocarbon ages that are consistent with the most recent great earthquake at Cascadia, which occurred at 250 cal yr B.P. (1700 CE). Using Bchron and OxCal software, we model ages for the three older contacts of ca. 875 cal yr B.P., ca. 1120 cal yr B.P., and ca. 1620 cal yr B.P. For each of the four earthquakes, we analyze foraminifera across representative mud-over-peat contacts selected from McDaniel Creek. Changes in fossil foraminiferal assemblages across all four contacts reveal sudden relative sea-level (RSL) rise (land subsidence) with submergence lasting from decades to centuries. To estimate subsidence during each earthquake, we reconstructed RSL rise across the contacts using the fossil foraminiferal assemblages in a Bayesian transfer function. The coseismic subsidence estimates are 0.85 ± 0.46 m for the 1700 CE earthquake, 0.42 ± 0.37 m for the ca. 875 cal yr B.P. earthquake, 0.79 ± 0.47 m for the ca. 1120 cal yr B.P. earthquake, and ≥0.93 m for the ca. 1620 cal yr B.P. earthquake. The subsidence estimate for the ca. 1620 cal yr B.P. earthquake is a minimum because the pre-subsidence paleoenvironment likely was above the upper limit of foraminiferal habitation. The subsidence estimate for the ca. 875 cal yr B.P. earthquake is less than (<50%) the subsidence estimates for other contacts and suggests that subsidence magnitude varied over the past four earthquake cycles in southern Cascadia.

Origen y distribución de depósitos de tsunami en la marisma de Chaihuín (40° S/73,5° O), Chile

At Chaihuín marsh, south of Valdivia (39°56’ S/73°33’ W), a sand bed was deposited during the 1960 earthquake. The aim of this study is to map the 1960 tsunami deposit in detail and to associate earlier sand layers with past tsunamis. Geologic field mapping by means of stratigraphic sections constructed using 111 cores in the marsh revealed the existence of three sand layers. The source of these sand layers was determined by a statistical comparison of their sedimentological and mineralogical signatures with modern depositional environments. The results show that tsunami waves probably transported the sand layers found in the marsh. It is inferred that these sand layers were deposited in the marsh by tsunamis that followed subsidence associated with the great historical megathrust earthquakes of 1575, 1737 or 1837, and 1960. However, the three layers are different from each other in terms of lateral distribution and source, which we interpret as either changes in the sand bar associated with human occupation or differences in coseismic slip distribution resulting in variable accommodation space provided by coseismic subsidence as well as in tsunami wave height.