Yuichi Namegaya

Reexamination of the A.D. 869 Jogan earthquake size from tsunami deposit distribution, simulated flow depth, and velocity

The rupture parameters and magnitude of the A.D. 869 Jogan earthquake, a predecessor of the 2011 Tohoku earthquake, were previously estimated by matching tsunami deposit distributions with simulated inundation areas. The tsunami inundation associated with the 2011 Tohoku earthquake, however, extended farther inland than the sandy tsunami deposits. Numerical simulation of the 2011 tsunami indicated that flow depths and velocities were approximately 1 m and 0.6 m/s, respectively, at the most inland sand deposit sites on the Ishinomaki and Sendai plains. While these values depend on the assumed bottom roughness, we used these values to compare tsunami deposits and inundation simulation of the 869 Jogan earthquake from both uniform-slip and 2011-type variable-slip fault models. The results showed that the rupture length of the 869 Jogan earthquake was at least 200 km and its minimum moment magnitude was 8.6.

Shorter intervals between great earthquakes near Sendai: Scour ponds and a sand layer attributable to A.D. 1454 overwash

A sparsely documented tsunami in 1454 may subdivide the recurrence interval between the 869 and 2011 tsunamis near Sendai, as judged from geomorphic, stratigraphic, and archival evidence. Pond-filled breaches cut across beach ridges on century-old topographic maps. The basal pond deposit in one of these breaches postdates 1454. Stratigraphy on Sendai Plain includes a sand sheet that contains marine and brackish diatoms. Radiocarbon ages suggest that the sheet dates to 1406–1615 (2σ), and written records for this interval in Tohoku mention a tsunami in 1454. The inferred inundation extended 1.0–2.5 km inland from an approximate medieval shoreline. Simulated tsunamis that best account for the sand sheet require a thrust earthquake of moment magnitude 8.4 or larger. If the sand sheet represents the 1454 tsunami, the two most recent intervals between great thrust earthquakes in Sendai region spanned 585 and 557 years.

Evidence for coseismic and aseismic uplift in the last 1000 years in the focal area of a shallow thrust earthquake on the Noto Peninsula, west‐central Japan

[1] Raised shorelines on the Noto Peninsula on the western coast of central Japan suggest a millennial history of recurrent uplift. Emergence of the coastline by up to 50 cm was recently recorded in association with a moderate earthquake (M w 6.7) on 25 March 2007. The shoreline uplift is indicated by the displacement of sessile organisms such as calcareous tubeworms, which can also be examined as indicators of past shifts in shoreline level. Three levels of pre-2007 paleoshorelines are thus identified on this stretch of the Japan Sea coast. The lowest of these, dated at AD 1720-1950, tilts northward away from the likely source of a pair of ca. M 6.4 earthquakes in 1892, which occurred in a different area to the 2007 earthquake. The middle paleo-shoreline, dated at AD 1430-1655, records a 30-40 cm emergence that may be aseismic. The high paleo-shoreline, dated at AD 1025 1235, records a coseismic uplift of 50 cm and is limited to the area raised coseismically in 2007. The most recent event preceding the 2007 earthquake thus appears to have occurred approximately 1000 years ago.

Erosion and sedimentation during the September 2015 flooding of the Kinu River, central Japan

Erosional and sedimentary features associated with flooding have been documented in both modern and past cases. However, only a few studies have demonstrated the relationship between these features and the corresponding hydraulic conditions that produced them, making it difficult to evaluate the magnitude of paleo-flooding. This study describes the characteristics associated with inundation depth and flow direction, as well as the erosional and sedimentary features resulting from the disastrous flooding of the Kinu River, central Japan, in September 2015. Water levels rose rapidly due to heavy rainfall that eventually overtopped, and subsequently breached, a levee in Joso City, causing destructive flooding on the surrounding floodplain. Distinctive erosional features are found next to the breached levee, while depositional features, such as a sandy crevasse-splay deposit are found further away from the breach. The deposit can be divided into three units based on sedimentary facies. The vertical and lateral changes of these sedimentary facies may be the result of temporal and spatial changes associated with flow during the single flooding event. These observations and quantitative data provide information that can be used to reveal the paleohydrology of flood deposits in the stratigraphic record, leading to improved mitigation of future flooding disasters.

Searching tsunami affected area by integrating numerical modeling and remote sensing

The present paper reports a preliminary result of searching tsunami-affected area using recent advances of GIS analysis and remote sensing combined with a numerical modeling of tsunami propagation/inundation and world population database. Applying the method of searching tsunami affected area to the 2009 Samoa earthquake tsunami and the 2010 Chilean earthquake tsunami, the potential tsunami affected area have been detected at some coastal cities/communities. The results are utilized to detecting tsunami impacted area for conducting disaster relief activities.

Diatom (Bacillariophyceae) assemblages in salt marshes of south‐central Chile: Relations with tidal inundation time and salinity

To reconstruct sea‐level history from changes in tidal environments using diatom assemblages, we need to better understand the relations among brackish diatom assemblages and changing environments along elevational gradients from diverse coastal sites. Our statistical analysis reveals relations between environmental variables and brackish benthic diatom assemblages in the little studied region of south‐central Chile. Along four transects across salt marshes at two sites, we identified 224 diatom taxa in 112 samples. Detrended canonical correspondence analysis showed that tidal exposure time index and salinity were appropriately regressed against the abundance of diatom species using unimodal‐based methods. Our tests of classical and inverse regressions of weighted average and weighted averaging partial least squares (WA‐PLS) showed that WA‐PLS resulted in the highest coefficient of determination and the lowest root‐mean square of the error of prediction. Our regression will be useful in reconstructing environmental variables from fossil diatom assemblages in Chile.

Mid- to late-Holocene marine inundations inferred from coastal deposits facing the Nankai Trough in Nankoku, Kochi Prefecture, southern Japan

This study investigates the Holocene sedimentary history of a small coastal lowland in Nankoku, Kochi Prefecture, on the coast of southern Japan facing the Nankai Trough. The sedimentary fill of the lowland area consists mainly of marine-brackish clay overlain by beds of freshwater clay and peat. We found four laterally extensive sand sheets, one directly underlying the freshwater deposits and the other three interbedded with them. Radiocarbon dates show that these sand sheets were deposited between 5970 and 2440 cal. BP. Although the sand sheets contained few marine-brackish diatoms, they were concentrated in the seaward part of the study site, suggesting that they were deposited by marine inundations. These sand sheets were formed as a result of tsunamis or unusually large storm surges. The apparent frequency of marine inundations during 5970–2440 cal. BP was much lower than that of megathrust earthquakes along the Nankai Trough recorded during the last 1300 years. Event deposits were absent between 2440 and 960 cal. BP, a gap that we attribute to the development of beach ridges. The new marine inundation records reported here will aid efforts to reconstruct the timing and recurrence intervals of megathrust earthquakes in the western Nankai Trough.