Yuichi Nishimura

Tsunami deposits from the 1993 Southwest Hokkaido earthquake and the 1640 Hokkaido Komagatake eruption, northern Japan

The southwest Hokkaido tsunami of July 12th, 1993, left continuous onshore sand deposits along the west coast of Oshima Peninsuka, Hokkaido, northern Japan. We investigated spatial distribution and lithofacies of the new tsunami deposits for its identification of ancient tsunami deposits. An eyewitness acount and bent plants helped our interpretation of the onshore tsunami behavior. We regard the following properties as typical of the coastal tsunami sand deposits: (1) The deposits cover the surface almost continuously on gentle topography. (2) Deposit thicknesses and mean grain sizes descrease with distance from the sea. (3) Deposit thicknesses and lithofacies vary greatly across local surface undulation. (4) Graded bedding reflecting tsunami runup and backwash is present in thick deposits. (5) The deposits are widely distributed along the coast and extend inland several tens of meters to 100 m. We examined a candidate for the paleo-tsunami deposits associated with the 1640 Komagatake eruption, and confirmed that the similar patterns are typical of ancient tsunami deposits.

Sedimentary deposits of the 26 December 2004 tsunami on the northwest coast of Aceh, Indonesia

The 2004 Sumatra-Andaman tsunami flooded coastal northern Sumatra to a depth of over 20 m, deposited a discontinuous sheet of sand up to 80 cm thick, and left mud up to 5 km inland. In most places the sand sheet is normally graded, and in some it contains complex internal stratigraphy. Structures within the sand sheet may record the passage of up to 3 individual waves. We studied the 2004 tsunami deposits in detail along a flow-parallel transect about 400 m long, 16 km southwest of Banda Aceh. Near the shore along this transect, the deposit is thin or absent. Between 50 and 400 m inland it ranges in thickness from 5 to 20 cm. The main trend in thickness is a tendency to thicken by filling low spots, most dramatically at pre-existing stream channels. Deposition generally attended inundation—along the transect, the tsunami deposited sand to within about 40 m of the inundation limit. Although the tsunami deposit contains primarily material indistinguishable from material found on the beach one month after the event, it also contains grain sizes and compositions unavailable on the current beach. Along the transect we studied, these grains become increasingly dominant both landward and upward in the deposit; possibly some landward source of sediment was exposed and exploited by the passage of the waves. The deposit also contains the unabraded shells of subtidal marine organisms, suggesting that at least part of the deposit came from offshore. Grain sizes within the deposit tend to fine upward and landward, although individual units within the deposit appear massive, or show reverse grading. Sorting becomes better landward, although the most landward sites generally become poorly sorted from the inclusion of soil clasts. These sites commonly show interlayering of sandy units and soil clast units. Deposits from the 2004 tsunami in Sumatra demonstrate the complex nature of the deposits of large tsunamis. Unlike the deposits of smaller tsunamis, internal stratigraphy is complex, and will require some effort to understand. The Sumatra deposits also show the contribution of multiple sediment sources, each of which has its own composition and grain size. Such complexity may allow more accurate modeling of flow depth and flow velocity for paleotsunamis, if an understanding of how tsunami hydraulics affect sedimentation can be established.

Rupture process of the 2004 great Sumatra-Andaman earthquake estimated from tsunami waveforms

Rupture process of the 2004 Sumatra-Andaman earthquake is estimated using tsunami waveforms observed at tide gauges and the coseismic vertical deformation observed along the coast. The average rupture speed of the 2004 Sumatra-Andaman earthquake is estimated to be 1.7 km/s from tsunami waveform analysis. The rupture extends about 1200 km toward north-northwest along the Andaman trough. The largest slip of 23 m is estimated on the plate interface off the northwest coast in the Aceh province in Sumatra. Another large slip of 21 m is also estimated on the plate interface beneath the north of Simeulue Island in Indonesia. The other large slip of 10–15 m is estimated on the plate interface near Little Andaman and Car Nicobar Inlands. The total seismic moment is calculated to be 7.2 × 1022 Nm (Mw 9.2) which is similar to the other studies using seismic waves (Park et al., 2005; Ammon et al., 2005).

A possible Caledonide arm through the Barents Sea imaged by OBS data

Abstract The assembly of the crystalline basement of the western Barents Sea is related to the Caledonian orogeny during the Silurian. However, the development southeast of Svalbard is not well understood, as conventional seismic reflection data does not provide reliable mapping below the Permian sequence. A wide-angle seismic survey from 1998, conducted with ocean bottom seismometers in the northwestern Barents Sea, provides data that enables the identification and mapping of the depths to crystalline basement and Moho by ray tracing and inversion. The four profiles modeled show pre-Permian basins and highs with a configuration distinct from later Mesozoic structural elements. Several strong reflections from within the crystalline crust indicate an inhomogeneous basement terrain. Refractions from the top of the basement together with reflections from the Moho constrain the basement velocity to increase from 6.3 km s−1 at the top to 6.6 km s−1 at the base of the crust. On two profiles, the Moho deepens locally into root structures, which are associated with high top mantle velocities of 8.5 km s−1. Combined P- and S-wave data indicate a mixed sand/clay/carbonate lithology for the sedimentary section, and a predominantly felsic to intermediate crystalline crust. In general, the top basement and Moho surfaces exhibit poor correlation with the observed gravity field, and the gravity models required high-density bodies in the basement and upper mantle to account for the positive gravity anomalies in the area. Comparisons with the Ural suture zone suggest that the Barents Sea data may be interpreted in terms of a proto-Caledonian subduction zone dipping to the southeast, with a crustal root representing remnant of the continental collision, and high mantle velocities and densities representing eclogitized oceanic crust. High-density bodies within the crystalline crust may be accreted island arc or oceanic terrain. The mapped trend of the suture resembles a previously published model of the Caledonian orogeny. This model postulates a separate branch extending into central parts of the Barents Sea coupled with the northerly trending Svalbard Caledonides, and a microcontinent consisting of Svalbard and northern parts of the Barents Sea independent of Laurentia and Baltica at the time. Later, compressional faulting within the suture zone apparently formed the Sentralbanken High.

Crustal structure and transform margin development south of Svalbard based on ocean bottom seismometer data

Abstract The Barents Sea is located in the northwestern corner of the Eurasian continent, where the crustal terrain was assembled in the Caledonian orogeny during Late Ordovician and Silurian times. The western Barents Sea margin developed primarily as a transform margin during the early Tertiary. In the northwestern part south of Svalbard, multichannel reflection seismic lines have poor resolution below the Permian sequence, and the early post-orogenic development is not well known here. In 1998, an ocean bottom seismometer (OBS) survey was collected southwest to southeast of the Svalbard archipelago. One profile was shot across the continental transform margin south of Svalbard, which is presented here. P-wave modeling of the OBS profile indicates a Caledonian suture in the continental basement south of Svalbard, also proposed previously based on a deep seismic reflection line coincident with the OBS profile. The suture zone is associated with a small crustal root and westward dipping mantle reflectivity, and it marks a boundary between two different crystalline basement terrains. The western terrain has low (6.2–6.45 km s −1 ) P-wave velocities, while the eastern has higher (6.3–6.9 km s −1 ) velocities. Gravity modeling agrees with this, as an increased density is needed in the eastern block. The S-wave data predict a quartz-rich lithology compatible with felsic gneiss to granite within and west of the suture zone, and an intermediate lithological composition to the east. A geological model assuming westward dipping Caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust, as well as the observed structuring. Due to the transform margin setting, the tectonic thinning of the continental block during opening of the Norwegian-Greenland Sea is restricted to the outer 35 km of the continental block, and the continent–ocean boundary (COB) can be located to within 5 km in our data. Distinct from the outer high commonly observed on transform margins, the upper part of the continental crust at the margin is dominated by two large, rotated down-faulted blocks with throws of 2–3 km on each fault, apparently formed during the transform margin development. Analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile, and that the downfaulting dominates the whole margin segment from Spitsbergen to Bjornoya. South of Bjornoya, the faulting leaves the continental margin to terminate as a graben 75 km south of the island. Adjacent to the continental margin, there is no clear oceanic layer 2 seismic signature. However, the top basement velocity of 6.55 km s −1 is significantly lower than the high (7 km s −1 ) velocity reported earlier from expanding spread profiles (ESPs), and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7–8-km-thick sedimentary overburden.

Kinking of the subducting slab by escalator normal faulting beneath the North Island of New Zealand

Seismic reflection imaging shows a marked shallow kink at ∼12 km depth in the Pacific plate beneath the central North Island, New Zealand, that coincides with (1) a decrease in the amplitude of the plate boundary reflection, (2) the locus of prominent landward-dipping splay thrust faults in the overlying plate, and (3) the onset of seismogenesis on the subduction interface and within the subducted plate. We propose that the sharp change in the dip of the plate interface is indicative of the downdip transition from stable to unstable slip regimes. Earthquake focal mechanisms suggest the kinking is accomplished through simple shear on reactivated normal faults in the crust of the subducted plate, akin to the down-stepping motion of an escalator. The geological record of uplift in the overlying plate indicates the escalator has been operating for the last 7 m.y.

Tsunami run-up heights of the 2003 Tokachi-oki earthquake

Tsunami height survey was conducted immediately after the 2003 Tokachi-oki earthquake. Results of the survey show that the largest tsunami height was 4 m to the east of Cape Erimo, around Bansei-onsen, and locally at Mabiro. The results also show that the tsunami height distribution of the 2003 Tokachi-oki earthquake is clearly different from that of the 1952 Tokachi-oki earthquake, suggesting the different source areas of the 1952 and 2003 Tokachioki earthquakes. Numerical simulation of tsunami is carried out using the slip distribution estimated by Yamanaka and Kikuchi (2003). The overall pattern of the observed tsunami height distribution along the coast is explained by the computed ones although the observed tsunami heights are slightly smaller. Large later phase observed at the tide gauge in Urakawa is the edge wave propagating from Cape Erimo along the west coast of the Hidaka area.

Rabaul volcano, Papua New Guinea: seismic tomographic imaging of an active caldera

Abstract The recently active Rabaul volcano on the island of New Britain in eastern Papua New Guinea is associated with a subduction zone located near the triple junction formed by the Pacific, South Bismarck and Solomon lithospheric plates. Analysis of our 1997 seismic tomography survey of the Rabaul caldera reveals the P-wave velocity structure to a depth of about 12 km using both explosive and earthquake seismic sources. The Rabaul volcanic complex is formed by a series of caldera collapse structures and a group of basalt–andesite volcanic centres with two currently active dacitic intra-caldera cones, Tavurvur and Vulcan. The 1994 eruption of these intra-caldera cones caused major infrastructure damage and required the evacuation of Rabaul township. Ongoing minor activity of Tavurvur continues to the present. The 1997 three-dimensional seismic tomography imaging identifies a 30–35 km 3 low-velocity region (?magma reservoir) at 3–6 km depth beneath the central Rabaul caldera, and gives an insight into the geological features of the caldera ‘plumbing’ system on scales of a few kilometres. The seismic survey highlights the heterogeneity in P-wave velocity both laterally and vertically within the Rabaul caldera, indicating significant complexity within quite a small area. The structural complexity is consistent with that observed in outcrop of Palaeozoic eroded calderas in other parts of the world. The low-velocity region at Rabaul ( 6.0 km/s) rock units around the caldera rims that are interpreted to indicate large volumes of mafic intrusive rock at shallow (

Seismic attenuation at Rabaul volcano, Papua New Guinea

Abstract The attenuation structure around Rabaul volcano, New Britain, Papua New Guinea, is studied using broadband records from a pair of sites, inside and outside of the Rabaul caldera complex, using regional earthquakes. Estimates of attenuation for P and S waves between 0.5 and 5 Hz indicate that near-surface rocks within the caldera complex are significantly more attenuative than outside the caldera. The average strength of this anomaly is defined in terms of t * S ∼0.2 s over and above the region outside the caldera ( t * S is the time integral of the inverse quality factor of attenuation, in this case along the path of a shear wave). This attenuation anomaly appears to be strongest at depth and to the south of the Kaivuna site in the northern part of the caldera complex. The ratio of attenuation estimates for P and S waves is δ t * S /δ t * P ≈2.7, indicating that intrinsic attenuation contributes significantly to the process of attenuation. No frequency dependence of the quality factor is resolved within this frequency band. Path-averaged lithospheric attenuation is estimated from S/P spectral ratios at both stations, yielding quality factors on average Q β ∼180, but possibly significantly lower around and below the volcano. Shear waves are observed along paths passing underneath the central southern caldera at depths greater than about 5 km, although they are strongly attenuated. This indicates that they have not passed through a large, extensively molten body.

Impact of Tsunami Inundation on Soil Salinisation: Up to One Year After the 2011 Tohoku-Oki Tsunami

The long-term effect of tsunami inundation on soil salinisation was assessed following the 2011 Tohoku-oki tsunami in two areas on the Sendai Plain, near Sendai airport in the Miyagi Prefecture and Matsukawa-ura near Soma in the Fukushima Prefecture. Data gathered over four sampling seasons 2, 5, 9 and 11 months after the tsunami near Sendai airport show that the salt content generally decreased with time. Concentrations were nevertheless higher in February 2012 than in October 2011, probably due to capillary action and evaporation following long periods with little precipitation in the winter, while the lower concentrations in October were attributed to dilution due to intense rainfall prior to the sampling period. In February 2012, the area with chloride concentrations over the guidelines for the establishment of rice seedlings still extended for nearly 1 km between 2.45 and 3.33 km inland. Chloride concentrations also reached the guideline values at the land surface 1.71 km inland. This corresponded to the limit of the area deemed not suitable for rice production by local rice farmers. However, recent observations revealed that rice crops were not only halted in 2011 but also in 2012, probably due to high salinisation of soil and/or surface and groundwater. Our study shows that soil salinisation was still recorded to nearly 15 cm depth in areas with fine-grained organic-rich soil ~2.5 km from the shoreline 11 months after the tsunami, and that water-leachable ions were preferentially retained in organic-rich muddy sediment and soil, reflecting the long-term impact of tsunami inundation. In Matsukawa-ura, salt crusts still covered the area flooded by the tsunami in February 2012 and both the soil and muddy tsunami deposit were characterised by high chloride and sulphate concentrations. The latter might also lead to sulphide toxicity. Remediation measures have been implemented in certain areas, but further research needs to be carried out to test the effectiveness of the measures being used to allow rice production to resume.