Izu Peninsula Research Articles

Community structure of parasitoids attacking Schizomyia sasakii (Diptera: Cecidomyiidae) and local adaptation of the gall midge in the Izu region

AbstractInsect community structures and biological interactions vary with the distance from the mainland to islands. Gall inducers are key organisms in local arthropod communities because their galls harbor diverse arthropods. We investigated the parasitoid community of a gall‐inducing cecidomyiid Schizomyia sasakii on the Izu Peninsula and the Izu Islands, Japan. We examined relationships between parasitism by Inostemma sp. and Torymus hirtipennis, and gall characteristics, and analyzed the directional selection on gall characteristics induced by S. sasakii in each locality. The species richness of parasitoids on the Izu Islands decreased with the distance from the Izu Peninsula to each island, and area of respective islands. Inostemma sp. preferably attacked large galls on Ohshima Island, and T. hirtipennis tended to attack relatively small galls in the Izu Peninsula. Directional selection on gall characteristics of S. sasakii favored the induction of larger galls and thicker tissues by S. sasakii on Ohshima Island. In contrast, no directional selection was detected in the gall characteristics on Hachijojima Island. The number of alternative host species of parasitoid may affect variation in parasitoid richness of S. sasakii on distant and small islands, because the species richness of gall midges is necessary to maintain local parasitoid populations. Our study supported the ovipositor limitation hypothesis for the torymid species, related to gall size. The directional selection of parasitoid attacks on the Izu Islands may act to favor the induction of large and hypertrophic galls by S. sasakii to avoid the parasitoids.

Submarine landslides and tsunami genesis in Sagami Bay, Japan, caused by the 1923 Great Kanto earthquake

The 1923 Great Kanto earthquake occurred on September 1, in Japan, and caused severe damage mainly in the Kanto region. Tsunamis were observed over wide regions from the east coast of the Izu Peninsula to the west coast of the Boso Peninsula, and particularly, the damage in Atami was devastating. Many earthquake fault models including those of Kanamori (1971) and Ando (1971) were proposed based on the records of land deformation. However, such fault models cannot sufficiently explain the tsunami elevation and its initial sea-level motion on the coasts. Hence, the detailed mechanisms remain elusive. This study examines the possibility that a leading mechanism of the tsunami in the 1923 Great Kanto earthquake was a large-scale submarine landslide that occurred at Sagami Bay and at the mouth of Tokyo Bay based on the records of depth data measured by the Imperial Japanese Navy (1924) before and after the earthquake. We first show that the tsunami calculated by each fault model was inconsistent with the waveform at Yokosuka, the coastal tsunami elevations, and initial sea-level motion. Then, based on statistical analysis of the depth changes at the 1923 Great Kanto earthquake, we found that the seafloor bathymetric changes represented large-scale submarine landslides that may correspond to long-runout submarine liquefied sediment flows. The seafloor gradient over a 40 km flow-out distance was equal to or less than 0.4∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document}. Through the identification of the submarine landslide source by tsunami backpropagation analysis and utilizing an analytical solution of a high-density gravity flow and a sensitivity analysis, we conducted a range of numerical simulations of the 1923 Great Kanto earthquake tsunamis using a fault model and a submarine landslide tsunami source model due to a high-density liquefied gravity flow. The results quantitatively accounted for the discrepancy between the observed tsunami records with maximum tsunami elevations over 12 m and the fault-model–based simulations with maximum tsunami elevations of 2 to 5 m and explained consistently the maximum tsunami elevation distributions as well as the time-series tsunami waveforms. These results may thus facilitate and deepen our understanding of the earthquake-induced submarine landslide tsunami risk as cascading multi-geohazards.

Distribution pattern of Armadillidae species (Crustacea: Isopoda: Oniscidea) on the Izu Islands, Japan

Faunal studies of oceanic islands provide unique opportunities to study ecology and evolution. Four Armadillidae taxa were recorded from the Izu Islands, a group of oceanic islands off the coast of Japan. The distributions of these taxa and a phylogenetic tree are based on molecular data for 249 specimens from 88 sites. The Spherillo russoi (Arcangeli, 1927) species complex is distributed widely throughout these islands, and dispersal may have occurred via oceanic currents. On the nearby Izuoshima and Toshima Islands, Spherillo dorsalis (Iwamoto, 1943) may have recently migrated from the Izu Peninsula either via currents or been transported by human activity. In the middle islands, Shikinejima, Kozushima, Miyakejima, and Mikurajima, Venezillo hasegawai (Nunomura, 1991) was found, which may be derived from Spherillo obscurus (Budde-Lund, 1885) distributed in the Kanto region on the mainland. On the most southern and distant Hachijojima Island, Spherillo punctatus Nunomura, 2007 was found, and it does not differ genetically from specimens from Izenajima and Tokunoshima Islands in the Nansei Island Group, despite an approximate 1,000-km distance between them; this species may have been transferred by human activity. Our study showed that the Armadillidae assemblages in the Izu Islands were formed by several migration routes.

Tourism industry under the COVID-19 pandemic in the Izu Peninsula

Tourism industry under the COVID-19 pandemic in the Izu Peninsula

Re-Estimating a Source Model for the 1923 Kanto Earthquake by Joint Inversion of Tsunami Waveforms and Coseismic Deformation Data

ABSTRACT The 1923 Kanto earthquake generated not only strong motions that caused a devastating disaster around the metropolitan Tokyo area but also a large tsunami. Although the slip distribution of the 1923 Kanto earthquake has been estimated by several researchers, tsunami waveforms observed at tide gauges near the source have never been used. In this study, the slip distribution of the 1923 Kanto earthquake was estimated using joint inversion of tsunami waveforms and vertical crustal deformations reported in historical documents. The estimated slip distribution was generally consistent with those estimated in the previous studies except for a large slip of 9 m along the western portion of the plate interface, up-dip near the Sagami trough. The east coast of the Izu Peninsula was inundated by the tsunami and surveyed after the tsunami to determine tsunami heights in the inundation areas. The tsunami inundation computed from the estimated slip distribution explained the tsunami heights, and the large slip played an important role in large inundation. These results indicate that the large slip west of the Sagami trough is essential for explaining the observed tsunami caused by the 1923 Kanto earthquake.

Late Holocene tectonics inferred from emerged shoreline features in Higashi-Izu monogenetic volcano field, Central Japan

Emerged shoreline features such as morphology, stratigraphy, and fossil assemblages are useful to reconstruct the history of vertical crustal movement in coastal areas. In particular, resolving the history of individual tectonic events can help understand regional seismo- and volcano-tectonic processes. In the eastern coast of the Izu Peninsula, central Japan, multiple levels of emerged shoreline features formed as a result of repeated uplift caused by the activity of Higashi-Izu monogenetic volcano field or related seismo-tectonic processes during the Late Holocene. To clarify the history of uplift events and the mechanism of regional tectonics, we surveyed these features with a focus on fossil sessile assemblages. Based on an analysis of their height and age, three zones associated with intermittent emergence events, denoted Zone 1 (1.05-m uplift in 595–715 CE), Zone 2 (1.33-m uplift in 1356–1666 CE), and Zone 3 (0.82-m uplift after 1830 CE), were identified. The interval between events was 400–800 years. Zone 3 can be regarded to have emerged via a stepwise but gradual uplift that lasted 130 years. This movement was caused by geodetically observed volcanic crustal deformation with earthquake swarms. We infer that zones 1 and 2 were uplifted by similar intervals of volcanic deformation, but cannot discard coseismic contributions from offshore faults as a possible explanation. The timing of these emergence events coincides with periods of both increased volcanic and seismic activities in the vicinity, suggesting that they are interrelated. Based on the height and age of the additional older assemblage denoted Zone 0 overlying Zone 1 and those of the post-glacial transgressive deposits in the adjacent areas, we suggest that the crustal uplift of the Izu Peninsula was initiated around 3000 years ago, which coincides with the largest eruption of Kawagodaira volcano. Therefore, volcanic activity is an important factor governing coastal uplift in this region.

Ocean Bottom Seismometers in Suruga Bay reveal a shear zone inside the Philippine Sea plate slab

Suruga Bay lies along the Tokai segment of the Nankai-trough, which last ruptured in the 1854 M8.4 Ansei-Tokai Earthquake, making it a potential locus for the next major earthquake on the Nankai-trough. Moderate size earthquakes have occurred in the Suruga Trough axial region (M6.5 in 2009 and M6.2 in 2011) that resulted in a prominent excitation of seismicity. Unfortunately, our ability to accurately locate these events is hampered by a sparse routine seismic observation network around the bay. In order to improve location accuracy, a total of 2 sets of 7 OBSs were deployed inside Suruga Bay, 6 from the spring of 2017, and an additional OBS from spring 2019. By combining the data from our OBS stations with that of existing land-based stations surrounding Suruga Bay, we were able to accurately relocate the observed seismic activity inside the Philippine Sea plate (PHS). Our combined processing revealed a 25° northwest-dipping 2–5 km wide zone on which the regional seismic activity occurs. Overall, seismic activity is observed to be 5–10 km shallower than previously determined from routine network data alone. A striking feature of the analyzed activity is the presence of a prominent strike-slip component in the focal mechanisms, which exhibit P-axes almost exclusively oriented towards the north. Furthermore, taking into consideration the apparent difference in the direction of the P-axis of the stress field to the calculated sense of movement of Izu peninsula by GNSS data, and the ongoing internal deformation observed by geodetic and geological data, we propose the existence of a shear zone inside the subducting PHS starting from the southern tip of Izu Peninsula and extending across the bay under Shizuoka city. This zone of strain partitioning acts as a buffer between the northward-colliding Izu Peninsula and the northwest-directed subduction of the PHS observed by GNSS.

Cover

On the Cover: Centre: Pinus cembra L. is an enigmatic closed-cone pine of the timberline ecotone in the European Alps and the Carpathian Mountains; photograph by Felix Gugerli; pp. 1049–1062. Top right: Feeding aggregation of coney (Cephalopholis fulva, Epinephelidae) in tropical rocky reefs of Fernando de Noronha Archipelago, northeast Brazil; photograph by João Paulo Krajewski; pp. 1163–1176. Top left: Euhadra peliomphala simodae (Jay, 1856) inhabit the southern part of the Izu Peninsula and the Izu Islands in Japan; their great diversity of shell colour is attributed to differences in island elevation; photograph by Shun Ito; pp. 1103–1116. Bottom left: The biodiversity of an aquatic ecosystem is encompassed in the transient traces of environmental DNA left behind by the inhabitants; illustration by Laura L. Hauck; pp. 1151–1162. Bottom right: This circular logo graced the cover of the first issue (March 1974) and initial 14 volumes of the Journal of Biogeography; the golden text celebrates the journal's five decades and 50th anniversary.

Relationship Between Crustal Structure and Plate Convergence Around the Izu Collision Zone in Central Japan

AbstractWe conducted a receiver function (RF) analysis to understand the convergence process in and around the Izu collision zone. We calculated the P‐ and S‐wave RFs and converted them to depth using a three‐dimensional (3D) regional tomography model and regionally estimated near‐surface structure. We detected the Moho of the Philippine Sea Plate and determined its 3D geometry in and around the collision zone. In addition, we considered the 3D seismic velocity model, which was previously determined using seismic tomography, and we complemented a part of the Moho that we did not detect using the RFs. Based on the geometry of the upper surface of the Philippine Sea Plate determined in previous studies, the crust is >35 km thick beneath the Izu Peninsula and collision zone and becomes thinner (<20 km) moving east of the collision zone. Thus, the thick crust along the Izu‐Bonin arc collides, and the thin forearc crust subducts. A stable slip surface was previously assumed to exist at a 15–20 km depth beneath the Izu Peninsula and the collision zone. This study indicates that the surface should exist within the crust of the Philippine Sea Plate.

Evolutionary history of inshore oceanic island land snails diversified in shell colour

AbstractAimOceanic islands provide an excellent opportunity to study the mode and tempo of phenotypic evolution of terrestrial organisms. Many studies have focused on oceanic islands far from the mainland. Oceanic islands near the mainland may provide distinct perspectives on phenotypic evolution, but a comprehensive understanding is still lacking. To address this gap, this study aimed to reveal when a land snail species inhabiting a volcanic archipelago within 40 km of the mainland diverged and how their shell colours evolved.LocationSouthern Izu Peninsula and five Izu Islands (Oshima, Toshima, Niijima, Shikine and Kozu), Japan.Taxon Euhadra peliomphala simodae. MethodsDouble‐digest restriction site‐associated DNA sequencing (ddRAD‐seq) was conducted using 117 individual snails. Molecular phylogenetic and population genomic analyses were performed, including approximation Bayesian computation (ABC). We then examined whether the island area, elevation, distance from the surrounding landmasses and historical event (bottleneck) explain the shell colour diversity on each island using a phylogenetic generalised linear mixed model (PGLMM).ResultsSnails could be genetically categorised according to the island they inhabited, and on Niijima, their genetic structure was further divided within the island. The divergence times among the extant populations of the snail dated back to 2.1 million years ago (Ma), which is older than that of other animals occurring in this region. Additionally, island elevation positively affected shell colour diversity, and populations with similar shell colour profiles were phylogenetically different.Main conclusionsIsland land snails diversified early in the ecosystem of the Izu Islands. This suggests that proximity to the mainland, immobility and the physiological tolerance of passive dispersal in land snails were the main causes of early diversification. Moreover, our study proposes that environments covarying with elevation would determine shell colour diversity on each island through natural selection.

Holocene right-lateral slip-rate revealed by sediment core analyses and ground penetrating radar profiling, the Sagiriko fault, Izu Peninsula, Japan, at the northern tip of the Izu–Bonin–Mariana arc

On the Izu Peninsula of Japan, the Sagiriko fault group is located to the south of the Kita-Izu fault zone, which ruptured during the 1930 Kita-Izu earthquake (Mjma = 7.3). A NS-trending strand of the Sagiriko fault group was ruptured during the 1930 earthquake, but conjugate EW-trending strands were not ruptured, and no Holocene activity on them has been documented. We surveyed the near-surface structure across the Sagiriko-b fault, one of the EW-trending strands, by ground penetrating radar profiling and radiocarbon dating of sediments obtained from two drilling cores. The results revealed a right-lateral displacement of 5.5 m of fluvial terraces around the survey site that formed after about 10 ka. The right-lateral slip-rate was calculated to be more than 0.55 mm/yr. This rate, which is significantly smaller than the crustal contraction rate in this area, suggests that the crustal contraction was probably spread across several EW-trending active faults parallel to the Sagiriko-b fault.

Geological Controls on Marine Cavernous Landforms along Japanese Pacific-side Rocky Coasts

Marine cavernous landforms, including notches, caves, arches and tunnels, characterize the rocky coast landscape and are strongly influenced by the geology (e.g., rock strength and structure) of sea cliffs. Geological controls on the development and shape of marine cavernous landforms are evaluated based on field investigations along the Pacific coastlines of Honshu Island, Japan. Morphological parameters, width (w), depth (d), and height (h), of cavernous landforms were measured directly or through images taken with an unmanned aerial vehicle, together with geological factors, such as rock types, strength (Schmidt hammer rebound values: R), dips and strikes of bedding, major joints, and fault planes. In total, 76 caves are investigated in six coastal areas: Sanriku Coast, Joban Coast, Boso Peninsula, Miura Peninsula, Izu Peninsula, and Kii Peninsula. According to shape index, d/w, and presence of the open end, cavernous landforms are classified into notches (d/w < 1, closed end), caves (d/w ≥ 1, closed end), arches (d/w < 1, open end), and tunnels (d/w ≥ 1, open end). An analysis shows that the major geological controls differ between notches and the other three forms. Low rock strength (R < 40) and sub-horizontal bedding are, respectively, the primary and secondary controls on the formation of notches, whereas weak rock structures (joints, faults and bedding) with a dip steeper than 30° (vertical or steeply inclined structures) and a strike trending perpendicular to the cliff face are the major controls on the formation of caves, arches and tunnels. Rock strength also affects planar forms defined by the shape index, promoting a deepening of cavernous forms, particularly when the cliff has a medium rock strength (R = 30-50) that provides an optimal balance between erosion force and resisting force. In general, vertical or steep structures contribute to the deepening of holes, whereas horizontal or gentle structures favor widening.

A Diving Accident Checklist in Izu Peninsula can be Associated with Some Pitfalls

Objective We retrospectively investigated the degree of completion of the checklist during or immediately after diving accident, who were transported by a physician-staffed helicopter emergency medical service (HEMS).

Outcomes of patients with decompression illness transported by physician-staffed helicopter according to Japanese Registry data (2015-2020).

No nationwide reports have focused on patients with decompression illness (DCI) transported by doctor helicopter (DH) in Japan. We performed this retrospective study to examine this population using data from the Japan DH registry system (JDRS). Patients were initially selected from the JDRS database. They were divided into two groups: those transported by the Eastern Shizuoka DH (ES-DH) and those transported by other DHs. Variables were compared between the two groups. There were 44 patients who had DCI out of 41,592 patients in the JDRS. The majority of cases (70%) were transported by the ES-DH. In the ES-DH group, age, rate of request type using key words, and rate of instrumental intervention to secure an airway were significantly greater, and the median Glasgow Coma Scale score was significantly lower than that of the other DH group. However, there was no statistically significant difference in the rate of cases with fatal outcomes between the two groups. This is the first report regarding the current status of patients with DCI transported by DH in Japan. Most patients were transported by ES-DH to the Izu Peninsula. In addition, the patients transported by ES-DH due to decompression illness tended to be severely ill; however, the outcomes of the ES-DH and other DH groups did not differ to a statistically significant extent and therefore effective recompression therapy could be successfully performed at suitable hospitals owing to timely transportation.

Review: Vegetation history of Cryptomeria japonica in Japan since the last interglacial period

AbstractThe natural distribution of Japanese cedar (Cryptomeria japonica) has been disturbed by human activities, owing mainly to its value as timber; thus, its position in the vegetation zone of Japan is not clear. Therefore, it is important to determine the history of C. japonica forests before human influence. We reviewed the transitions in distribution and abundance of C. japonica in the Japanese Archipelago since the last interglacial period by using published pollen data. During the last interglacial, temperate deciduous broadleaf forests developed on northeastern Honshu Island, whereas on southwestern Honshu Island C. japonica populations increased, accompanied by warm‐temperate evergreen broadleaf trees. During the first half of the last glacial period, C. japonica forests developed throughout the Japanese Archipelago except on Hokkaido. Thereafter, 60,000–30,000 years ago, C. japonica covered the area around Lake Biwa and the Izu Peninsula. During the Last Glacial Maximum, C. japonica was most abundant on the Izu Peninsula, and small refugial populations were located along the coast of the Sea of Japan from southwestern to northeastern Honshu, and along the Pacific coast of Shikoku Island. During the Middle Holocene, C. japonica populations increased rapidly in southwestern Honshu. Two thousand years ago, before the intensification of human activities, C. japonica was widespread in warm‐ to cool‐temperate zones on the Sea of Japan side of Honshu, the Izu Peninsula, the Pacific coast of Shikoku Island, and Yakushima Island. This evidence indicates that C. japonica is one of the major tree species in the Japanese vegetation zone.

Preface: Functional biogeography in Japanese cedar

Preface: Functional biogeography in Japanese cedar

Long-term survey of longhorn beetles revealed changes in faunal features in Ito on the Izu peninsula.

Long-term biodiversity monitoring is essential for unveiling the impact of environmental changes on local fauna. Although private local records can contribute significantly to biodiversity evaluation, they are seldom published in scientific journals. In this study, a retired scientist recorded the longhorn beetles (Distiniidae and Cerambycidae) present in Ito on the Izu peninsula, Japan, for 12 years. The records showed the dynamical changes in longhorn beetles, which indicated the environmental changes around the survey site over 12 years. We also compared the longhorn beetle composition in the Ito study site to those in the survey records in 13 other locations in Kanto, Japan. We found that the species composition in Ito was stable throughout the 12 years, while the general composition in Ito reflected the land-use pattern of urban areas and the collecting methods. The species composition in the Ito study site differed from that in some of the other satoyama locations (human-influenced natural environment), but this was possibly due to methodological differences. Long-term backyard biodiversity surveys, especially those conducted by retired professionals, can play important roles in future investigations of insect groups, such as longhorn beetles, even if they are not agricultural pests nor endangered species.

New Species of Lower-Shelf to Upper-Slope Ctenostome Bryozoans from Pacific Japan, with a Family Range Extension.

Ctenostome bryozoans were collected from depths of 150-300 m in Suruga Bay, the Kumano Sea, and off Sendai Bay on the Pacific coast of Japan. Among these samples were five new species, three of which were epibiotic on other animals. Alcyonidium clavum n. sp., found encrusting cirri of the stalked crinoid Metacrinus rotundus, was previously reported in Japan as Alcyonidium mamillatum, but differs from the latter in lacking rings on the peristome. Triticella parvacrista n. sp. and Triticella cunabula n. sp. were epibiotic on the isopod Bathynomus doederleini and the pycnogonid Ascorhynchus japonicum, respectively. This is the first record of the genus Triticella from Japanese waters. These species differ from previously described species in autozooidal morphology, particularly the dimensions of the autozooidal dilatation, in the presence or absence of the frenaculum, and in the septate junction between the dilatation and pedicel. Bockiella arcatumida n. sp., found as erect colonies on pebbles and hydroids collected off Sendai Bay, differs from previously known species in the size and arrangement of the kenozoids in the branch. Metalcyonidium morum n. sp., collected southwest of the Izu Peninsula, occurred as small colonies having a peduncle of short kenozooids. This is the first record from the northwestern Pacific for both the genus Metalcyonidium and the family Clavoporidae. Metalcyonidium morum n. sp differs from Metalcyonidium gautieri, the only other species known in the genus, in having the capitulum divided into two parts, and in the length of the kenozooids comprising the peduncle.

Spawning season, recruitment of juveniles and growth of the freshwater shrimp, Caridina typus H. Milne Edwards, 1837 (Crustacea: Decapoda: Atyidae) in the Heda-ohkawa River, Izu Peninsula, Shizuoka Prefecture, central Japan

Spawning season, recruitment of juveniles and growth of the freshwater shrimp, Caridina typus H. Milne Edwards, 1837 (Crustacea: Decapoda: Atyidae) in the Heda-ohkawa River, Izu Peninsula, Shizuoka Prefecture, central Japan

Strontium isotopic age and the origin of including basalt pebbles of the Sashida Limestone in south Izu region, Japan

In the previous paper, the geological ages of the Late Cretaceous, Paleocene, late Middle to Late Eocene, Oligocene and early Early Miocene were confirmed at the first time, based on the occurrences of both the age diagnostic larger foraminifera and associated planktonic foraminifera, from the Sashida Limestone included basalt pebbles, Sashida, Kamo-Gun, south Izu Peninsula. Therefore, the Sashida Limestone was made up of clastic carbonate rocks of various ages in essence as well as in outward form. In this paper, the age determination by the microfossil of larger and planktonic foraminifera was re-investigated by the Sr isotopic age determination from the same microfossil sample materials of the Sashida Limestone in order to get the age security. As a result, the ages from 69.3 to 71.8 Ma（Late Cretaceous; Maastrichtian）and 28.8 to 33.3 Ma（Early Oligocene；Rupelian）were determined and supported the ages of foraminifera partial. While, analysis of chemical composition of the basalt pebbles was investigated in order to estimate the original generation tectonics. As a result, the basalt pebbles showed a unique composition of high magnesium and high alkali, similar to basalts of the lower Yugashima Group and were assigned to the alkali basalt series, which were thought to have emerged in the very early stage of the Shikoku Basin spreading. The antecedent of the Izu Peninsula was defined as the Izu landmass from the previous paper, which was inferred the lateral mobility toward east from the spreading, and the last, bordered on the north by the Honshu.