Tsunami deposits associated with the 7.3 ka caldera-forming eruption of the Kikai Caldera, insights for tsunami generation during submarine caldera-forming eruptions

Accepted Manuscript Tsunami deposits associated with the 7.3ka caldera-forming eruption of the Kikai Caldera, insights for tsunami generation during submarine caldera-forming eruptions

Nobuo Geshi, Fukashi Maeno, Shojiro Nakagawa, Hideto Naruo, Tetsuo Kobayashi PII: DOI: Reference:

S0377-0273(17)30293-7 doi:10.1016/j.jvolgeores.2017.09.015 VOLGEO 6198

To appear in:

Journal of Volcanology and Geothermal Research

Received date: Revised date: Accepted date:

15 May 2017 15 September 2017 17 September 2017

Please cite this article as: Nobuo Geshi, Fukashi Maeno, Shojiro Nakagawa, Hideto Naruo, Tetsuo Kobayashi , Tsunami deposits associated with the 7.3ka caldera-forming eruption of the Kikai Caldera, insights for tsunami generation during submarine caldera-forming eruptions. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Volgeo(2017), doi:10.1016/j.jvolgeores.2017.09.015

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Tsunami deposits associated with the 7.3 ka caldera-forming eruption of the Kikai Caldera, insights for tsunami generation during submarine caldera-forming eruptions 1*

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Nobuo Geshi , Fukashi Maeno , Shojiro Nakagawa , Hideto Naruo , Tetsuo Kobayashi 1

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: Geological Survey of Japan, AIST

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: Earthquake Research Institute, Tokyo University

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: Yakushima Chigaku Club

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: Ijuin high school

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: Kagoshima University

*: corresponding author

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Address: AIST Site 7, 1-1-1 Higashi, Tsukuba Ibaraki 305-8567 Japan

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E-mail address: [email protected]

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Abstract Timing and mechanism of volcanic tsunamis will be a key to understand the dynamics of large-scale submarine explosive volcanism. Tsunami deposits associated with the VEI 7 eruption of the Kikai Caldera at 7.3 ka are found in the Yakushima and Kuchinoerabujima Islands, ~40 km south -southeast of the caldera rim. The tsunami deposits distribute along the rivers in their

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northern coast up to ~4.5 km from the river exit and up to 50 m above the present sea level. The tsunami deposits in the Yakushima area consist of pumice-bearing gravels in the lower part of the

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section (Unit I) and pumiceous conglomerate in the upper part (Unit II). The presence of rounded pebbles of sedimentary rocks, which characterize the beach deposit, indicates a run-up current

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from the coastal area. The rip-up clasts of the underlying paleosol in Unit I show strong erosion during the invasion of tsunami. Compositional similarity between the pumices in the tsunami deposit and the juvenile materials erupted in the early phase of the Akahoya eruption indicates

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the formation of tsunami deposit during the early phase of the eruption, which produced the initial Plinian pumice fall and the lower half of the Koya pyroclastic flow. Presence of the dense volcanic

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components (obsidians and lava fragments) besides pumices in the tsunami deposit supports that they were carried by the Koya pyroclastic flow, and not the pumices floating on the sea surface. Sequential relationship between the Koya pyroclastic flow and the tsunami suggests that the

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mechanism of the tsunami.

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emplacement of the pyroclastic flow into the sea surrounding the caldera is the most probable

Key words: caldera-forming eruption; submarine eruption; volcanic tsunami; Kikai caldera; Koya

Highlights

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pyroclastic flow deposit; Yakushima

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The tsunami deposit associated with a submarine eruption of the Kikai Caldera is found. The tsunami occurred during the eruption of the main pyroclastic flow of the caldera-forming eruption.

The tsunami deposits are distributed along the rivers up to ~4.5 km from the coast and 50 m above the present sea level at ~40 km south of the caldera.

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1. Introduction An explosive eruption in water area can cause volcanic tsunami with a variety of mechanisms, including pyroclastic flows, underwater explosions, and caldera collapse (Paris, 2015 and references therein). However, the timing and mechanism of the tsunami generation during caldera-forming eruptions are still in debate due to the limited examples of submarine caldera-forming eruptions and the difficulties in accessing submarine calderas.

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Volcanic tsunamis may cause serious disasters in coastal areas facing the submarine volcanoes (e.g., Latter, 1981; Cas and Wright, 1991;). The Krakatau eruption in 1883 caused

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more than 36,000 fatalities and the destruction of 165 coastal villages along the Sunda Strait (Self and Rampino, 1981; Nomanbhoy and Satake, 1995; Carey et al., 2001). Archeological

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investigations reveal that the tsunami generated by the eruption of the Santorini caldera in the Bronze Age also caused a severe impact on the Eastern Mediterranean shores (Dominey-Howes, 2004; Bruins et al., 2008; Goodman-Tchernov et al., 2009). Several other examples of volcanic

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tsunamis associated with caldera-forming eruptions have been reported, mainly from geological investigations of their deposits (e.g., Waythomas and Neal, 1998; Ulvrova et al., 2016). However,

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almost no attention has been paid to the potential risk of volcanic tsunamis originating from submarine volcanoes.

In this paper, we present an example of a volcanic tsunami associated with a VEI=7

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caldera-forming eruption of the Kikai Caldera in the southern off of the Kyushu Island, Japan. Though the occurrence of tsunami associated with this eruption was theoretically suggested by

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Maeno et al. (2006), no robust field evidence has been found. Only sporadic deposits of pumice-bearing conglomerate and the erosion structures have been suggested as the products of tsunami during the eruption (Maeno et al., 2006; Geshi, 2009; Fujiwara et al., 2010; Kobayashi,

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2015). Thus, the size, timing and mechanism of the tsunami potentially associated with the Akahoya eruption are still unclear. We found tsunami deposits along the northern coast of Yakushima and Kuchinoerabujima area, which is located ~30-40 km south of the Kikai Caldera

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(Fig. 1). Based on the stratigraphic relationship of the tsunami deposits and the comparison of the chemical composition of the juvenile magmatic materials in the deposit, we investigated the timing and trigger mechanism of the tsunami generation during the caldera-forming eruption.

2. The Kikai Caldera and its 7.3 ka Akahoya eruption The Kikai Caldera is a submarine collapse caldera in the Osumi Strait in the southern off of Kyushu Island. The topography of caldera is 20 by 17 km in horizontal across and ~500 m in depth (Matsumoto, 1943; Ono et al., 1982; Maeno and Taniguchi, 2007). Entire area of the Kikai Caldera is covered by sea except for two islands (Iwojima and Takeshma) on the northern caldera rim.

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The outline of the Kikai Caldera was shaped by the VEI 7 Akahoya eruption at around 7.3 ka. 3

The Akahoya eruption produced 70 – 80 km in dense rock equivalent (DRE) of magmas (Maeno and Taniguchi, 2007). The Akahoya eruption consists of an initial Plinian phase (Phase 1: Maeno and Taniguchi, 2007), which produced the Koya (Funakura) pumice fall deposit. A part of the Plinian column collapsed to form the Funakura pyroclastic flow deposit (PFD) as intra-Plinian pyroclastic flow (Phase 2: Maeno and Taniguchi, 2007). The total volume of the Koya pumice fall 3

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is 8 km in DRE (Maeno and Taniguchi, 2007). The Plinian phase was followed by a main ignimbrite phase (Phase 3: Maeno and Taniguchi, 2007), which produced the Koya PFD and the

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Akahoya ash fall deposit as co-ignimbrite ash (Ui, 1973; Machida and Arai, 1978; Ono et al., 1982; Maeno and Taniguchi, 2007). The volumes of the Koya PFD and Akahoya ash fall deposit 3

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are 15-25 km and 50 km in DRE, respectively (Maeno and Taniguchi, 2007), though these estimations have large uncertainties due to the submarine distribution of the tephra. The Koya pumice fall deposit (Ui, 1973) is distributed northeastward from the Kikai Caldera

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(Fig. 1) and does not cover the Yakushima area, where we surveyed the tsunami deposits (Fig. 1). The Koya PFD covers an area within a ~60-70 km radius from the caldera rim beyond the Osumi

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Strait (Fig. 1). On the caldera wall, the Koya PFD can be subdivided into two minor ignimbrite units (Unit C1 and C2) covered by a main ignimbrite unit (C3) (Maeno and Taniguchi, 2007). The typical Koya PFD in distal area consists of a single flow unit consisted of a ground layer overlain

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by the main pumice flow deposits. The distal unit of Koya PFD corresponds to the Unit C3. The Akahoya ash fall deposit is a pisolite-rich vitric ash fall deposit which covers the Koya PFD.

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Distribution of the Akahoya ash fall deposit indicates that the ash fall deposit is a co-ignimbrite ash fall from the Koya PFD (Machida and Arai, 1978). The main magmatic component of the Akahoya eruption is pyroxene rhyolite with ~71 wt.% of

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whole-rock SiO2 content (Maeno and Taniguchi, 2007). The rhyolitic magma of the Koya pumice fall deposit and the lower half of the Koya PFD contains the groundmass glasses with ~75 wt.% of SiO2 concentration. The erupted magmas in the later stage of the eruption contain an andesitic

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component with ~58 wt.% of whole-rock SiO2 content (Maeno and Taniguchi, 2007). Mingling of two end-members formed banded pumices which typically occur in the uppermost part of the Koya PFD. In the uppermost part of the Koya PFD, the glass composition exhibits a bimodal distribution consisting of glasses with SiO2~75 and ~65 wt.% due to the mingling of mafic magma at the end of the eruption (Fujihara and Suzuki-Kamata, 2013).

3. Geological setting of Yakushima Island 3.1 Deposits of the Akahoya eruption in Yakushima and Kuchinoerabujima The Koya PFD and the Akahoya ash fall deposit are distributed as the products of the Akahoya eruption in the Yakushima area. The Koya PFD is distributed across almost the entire area of

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Yakushima and Kuchinoerabujima, from the coast to the summit ~2 km a.s.l., except for the southernmost part of the Yakushima Island. The Koya PFD in the Yakushima and Kuchinoerabujima consists of a ground layer less than 5 cm in thickness overlain by the main pumice flow deposits. Total thickness is typically less than 1 m in the Yakushima area. The main part of the Koya PFD consists of a pumiceous lapilli tuff – pyroclastic breccia without remarkable bedding structure. The diameter of pumice clasts in the Koya PFD is typically less than 5 cm,

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though small amount of pumice blocks reaches >10 cm in diameter. The pumice clasts have angular shape. The Koya PFD also contains small amount of angular obsidian less than 2 cm in

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diameter. The phenocryst assemblage and groundmass glass composition of these obsidian indicates that they are also juvenile material. Small amount of fragments of porphyritic lava less

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than 2 cm in diameter are also found in the Koya PFD. Matrix part of the Koya PFD consists of the pumiceous ash.

The Akahoya ash fall deposit covers the Koya PFD. The Akahoya ash fall deposit exhibits

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remarkable normal-grading, from pisolite and pumice grains >1 cm in diameter at the base to very-fine vitric ash at the top. The typical thickness of the Akahoya ash fall deposit in Yakushima

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area is 30-40 cm. Most part of the Akahoya ash fall deposit and the uppermost part of the Koya PFD has been eroded and not preserved in Yakushima Island. In Kuchinoerabujima Island, the Akahoya ash fall deposit is relatively preserved by the overlying tephra from the

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Kuchinoerabujima volcano.

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3.2 Basement geology of Yakushima Island Yakushima is a non-volcanic island that consists of Paleogene marine sedimentary rocks of the Hyuga Group and the Miocene Yakushima Granite (Saito et al., 2007). Sedimentary rocks of

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the Hyuga Group are distributed in the coastal part of the island and comprise of alternating sandstones and shales. The central portion of the island consists of a pluton of the Yakushima Granite that intruded into the Hyuga Group. No Quaternary volcanic deposit distributes in

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Yakushima, except for some minor distribution of the previous PFD from the Kikai caldera. Along the Miyanoura River in the northern part of the Yakushima, the sedimentary rocks of the Hyuga Group are distributed in an area within ~1 km from the coast (Fig. 2). The sedimentary rocks became hornfels in the area between 1 and 3 km. The basement rock consists of granite in the upstream area of the Miyanoura River beyond the point of ~3 km from the coast. The high topography of Yakushima indicates the rapid uplift of the basement. Analysis of the age of marine terraces along the Yakushima coast reveals that the sea level during the Akahoya eruption corresponds to the present level of 8.5 m a.s.l., owing to the ~6 m of uplift of the basement and the 2~3 m subsidence of sea level (Nakagawa et al., 2017). Kuchinoerabujima is an active volcanic island, located ~14 km west of Yakushima. There is no

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clear evidence for uplift or subsidence of the basement of Kuchinoerabujima since the Akahoya eruption.

4. Tsunami deposit in the Yakushima Area 4.1 Distribution The pumice-bearing gravel deposits were found in the northern coastal area of Yakushima and

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Kuchinoerabujima Islands (Fig. 2A). Presence of pumice and obsidian fragments in the deposits suggests that these deposits are associated with the 7.3 ka Akahoya eruption of the Kikai caldera.

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The radiocarbon ages around 7.3 ka obtained from the tree fossils in the deposits (Okuno et al., 2013) also support the idea that these deposits are associated with the Akahoya eruption.

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The gravel deposits develop on the river terraces along several rivers in the northern coast of Yakushima (Fig. 2B). The gravel deposits overlie on the conglomerate of previous river terrace and paleosol. The top of the deposit in all outcrops we observed is covered by debris flow and/or

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fluvial terrace deposits of the post-Akahoya period. The gravel deposits can be traced up to ~ 4.5 km from the present river outlet along the Miyanoura River, ~2.9 km along the Isso River, and 1.5

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km along the Nagata River (Fig. 2B). The distribution of these deposits is limited in the northern coastal area less than 50 m in altitude. Small distributions of the pumice-bearing gravel deposit are also found on the marine terrace along the Koseda coast in the eastern part of Yakushima.

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In the Kuchinoerabujima Island, pumice-bearing gravel deposits were found on the slope along the northern coast (Fig. 2A). The deposits can be traced up to ~ 500 m from the coast and

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up to 45 m a.s.l.

4.2 Stratigraphy and lithofacies

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We examined the stratification, sedimentary structures, and grain components of the deposits to clarify the nature of the tsunami induced by the Akahoya eruption. We investigated the deposits along the northern coast of Yakushima and Kuchinoerabujima (Figs. 2B and 3). Particularly, we

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focus on the deposits along the Miyanoura River because of the well preservation of the deposit. The stratigraphy and lithofacies of the deposit along the Miyanoura River are described as the type locality.

The deposits along the Miyanoura River are divided into three major sedimentary units: Unit I, II and III in ascending order, based on the sedimentary structure and grain components (Fig. 3).

Unit I Unit I is a sand and gravel layer at the lower part of the deposit. Unit I overlie unconformably the fluvial deposits or paleosol on the fluvial terrace. The maximum total thickness of Unit I is more than 3 m in the outcrop MD along the Miyanoura River. The thickness of Unit Ia decreases toward

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the upstream; the thickness in the outcrop MB is 1.0 m and in the outcrop MA is less than 0.6 m. In the outcrop Is along the Isso River, the thickness of Unit Ia is less than 1 m. Based on its composition and size of clasts, Unit I can be subdivided into Unit Ia in the lower part, which is overlain by Unit Ib, and then by Unit Ic. The boundaries between the subunits are gradual. Unit Ia is poorly-consolidated pumice-bearing gravels. The thickness of Unit Ia ranges from

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<10 cm to >3 m reflecting the topography of the basement. At least three normally-graded units are recognized in the outcrop MD. In outcrops MA and MB, Unit Ia consists of single

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normally-graded unit. Some boulders concentrate at the base of each unit. The maximum grain size of Unit Ia decreases in the upstream direction along the Miyanoura River. In outcrop MD, ~2

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km from the coast along the Miyanoura River, maximum clast diameter is ~35 cm. In outcrop MB ~3.5 km from the coast, the maximum clast diameter is ~12 cm. In outcrop MA, which is the most distant outcrop from the coast, the maximum clast diameter is ~8 cm.

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The clasts more than ~1 cm across in Unit Ia are comprised of granitic and sedimentary rocks (such as hornfels) from the basement (Fig. 4A). A small amount of volcanic materials such as

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pumice, obsidian, and lava are also present. The size of these volcanic grains is typically less than 1 cm across. Unit Ia also contains rounded blocks of unconsolidated mud up to ~5 cm across (Fig. 4B).

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Unit Ib is poorly-consolidated pumiceous conglomerate that overlies Unit Ia. The boundary between Unit Ia and Ib is gradual. Unit Ib is characterized by a higher abundance of pumiceous

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clasts in comparison to Unit Ia (Fig. 4C). The large clasts more than ~1 cm across in Unit Ia consist mainly of pumice. The size of clasts in Unit Ib is smaller than that in the Unit Ia. The pumiceous clasts exhibit subangular-subrounded shapes. The matrix of Unit Ib is a mixture of

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sand grains derived from the weathered granite, like that of Unit Ia, and pumice fragments. Obscure lamina formed by the difference of grain size and concentration of pumice are recognized particularly in the upper part of Unit Ib.

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Unit Ic is a fine-grained tuffaceous layer in the uppermost part of Unit I (Fig. 4D). Thickness of the Unit Ic is less than 0.5 m. Unit Ic consists of a pile of normally-graded beds. Cross-bedding is commonly found in Unit Ic. In some outcrops, bedding structures of Unit Ic were deformed by the load of the overlying Unit II. The Unit Ic consists of vitric ash, whereas, quartz, feldspar, and biotite grains derived from the Yakushima Granite are also contained.

Unit II Unit II is a pumiceous conglomerate that directly overlies Unit I (Fig. 4E). The total thickness of Unit II is ~1.5 m at the outcrop MB where the top part of Unit II is covered directly by the Koya PFD and the original thickness is preserved. The thickness of Unit II exceeds 2 m in the outcrops Is and

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N in which the top of the Unit II was eroded by the overlying post-Akahoya unit (Fig. 3). The clasts in Unit II consist mainly of well-vesiculated pumice. The maximum size of clast at the base of Unit II exceeds 20 cm in the outcrops MB, Is and N. Cobble-sized pumices are concentrated at the basal part (Fig. 4E). The size of pumice clasts decreases upward. Most of the pumice clasts exhibit subangular–subrounded outlines (Fig. 4F). Alignment of the pumices are recognized particularly in the upper part of Unit II (Fig. 4G).

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The matrix of Unit II consists of a mixture of volcanic sand, fragments of the weathered granite and also rounded granules of shale. Many uncarbonized – partially carbonized tree trunks and

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molds are found in Unit II (Fig. 4G). Judging from the blanching pattern, most trees are falling toward the downstream side.

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Several clastic dikes injected into the pumiceous conglomerate of Unit II in the outcrop N. The dikes consist of pumice-bearing tuffaceous sand – silt. The maximum thickness of the dike

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exceeds 30 cm. The orientation of dike is vertical.

Unit III

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Unit III is a pumiceous sand and gravel layer with remarkable cross-bedding (Fig. 4H). Unit III covers the underlying Unit II and/or the Koya PFD. The topmost part of Unit III in all outcrops we observed was covered by debris flow and/or fluvial terrace deposits of the post-Akahoya period.

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The main component of Unit III is crystal fragments and pumice grains derived from the Koya

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PFD, though small amounts of granitic and sedimentary fragments are also found.

5. Deposit in Kuchinoerabujima

The pumice-bearing deposits associated with the Akahoya eruption are widely found on the

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slope along the northern coast of Yakushima and Kuchinoerabujima, in the south of the Kikai caldera (Geshi 2009; Kobayashi 2015). Distribution of the pumice-bearing gravel is limited in an area below ~50 m a.s.l.. Presence of pumices with petrological characteristics similar to that of

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the Akahoya eruption suggests that these deposits were formed during the Akahoya eruption. Fig. 5A shows the block-rich deposit of the outcrop Ko, ~150 m from the coast and ~28 m a.s.l. in the northern coast of the Kuchinoerabujima Island (Fig. 2A). The pumice-bearing deposit unconformably overlies on the pile of the subaerial ash-fall deposit of the Kuchinoerabujima volcano (Fig. 5A). The deposit is comprised of subrounded pebbles – boulders of the Kuchinoerabujima volcano. Small amount of the pumice and obsidian clasts are also found in the deposit. Some unconsolidated mud blocks are also found (Fig. 5A). Fig. 5B shows the layer of subrounded boulders covered by the pumice-bearing ash flow deposit in the outcrop Ki, ~ 200 m from the coast and ~45 m a.s.l. in the western part of Kuchinoerabujima. The pumice-bearing boulder layer unconformably covers the paleosol. Some

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unconsolidated mud blocks are also found in the deposit. The top of the boulder layer is covered by the pumice-bearing ash deposit. At the base of the deposit in the outcrop Ki ~30 m a.s.l., the cast of plant rods in the pumiceous gravel align perpendicular to the shore line (Fig. 5C).

6. Description of the deposit 6.1 Analytical methods

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The clast fabric in Unit Ia and II was investigated. Inclinations of the long axis of clasts in the vertical outcrop were measured using an image analysis method. The photo images of the areas

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with approximately 50 by 50 cm. The outlines of the clasts are traced manually and then processed with ImageJ software to extract the orientation of their major axis. We selected ~100

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grains with long axis more than 1 cm.

The composition of the grains in Unit Ia and II was investigated. We collected ~50 clasts larger than ~1 cm from the Unit Ia and II at outcrops MA and MB. The samples were classified by their

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lithic type as granite, sedimentary rock, or unconsolidated mud block. The grains in the terrace deposit beneath Unit 1a were also examined as the reference. The matrix parts of Unit Ia and II

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were also collected from the outcrop MB for grain composition analysis. The samples were washed and sieved into 9 classes from 0.5 mm to >16 mm to identify the rock-type the clast. The grains were classified as granite, sedimentary rocks (including hornfels), pumice, obsidian, lava,

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or unconsolidated mud blocks (Table 1). The roundness of the granitic and sedimentary grains in the matrix part of Unit Ia was investigated, based on the index of the roundness of Krumbein

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(1941).

The chemical compositions of the volcanic glasses in the deposit were examined. 20–40 grains of glass fragments were selected manually from each sample sieved between 0.5 and 1.0

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mm across. The fragments were fixed in epoxy resin and polished. The surfaces of the glass fragments with carbon-coating were examined by Energy Dispersive X-ray Spectrometry (EDS; Oxford Instrumental, X-Max 20). The measurements were done with 15 kV of acceleration voltage,

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~1 nA of the probe current, and the area of the analysis is more than 100 square micrometers on a polished surface.

6.2 Clast fabric 6.2 Clast fabric Preferred orientations of the clasts in Unit Ia and II are examined. In the outcrop MB, the clasts in Unit Ia exhibit an imbrication pattern which indicates the climbing flow toward the upstream direction (Fig. 6). The granitic clasts in the bottom of Unit Ia The flat clasts of shale in the upper part of Unit Ia show clear oblique alignment tilting downstream side (Fig. 4A). The Orientation of the clasts in Unit II exhibit imbrication pattern suggesting the down-flow

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along the river in the outcrop Is and N (Fig. 6). The tree trunks in Unit II also show preferred alignment: many tree trunks are aligned sub-parallel to the river (Fig. 4G). The blanching patterns of the tree trunks indicate that the tree-top point downstream of the river.

6.3 Composition of clasts Unit I

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The clasts larger than ~1 cm in Unit Ia in the outcrop MA consist of granitic rocks (54 %), and sedimentary rocks (42 %). The blocks of unconsolidated mud represent 4 % of the sample. In the

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outcrop in outcrop MB, granitic clasts and sedimentary clasts in Unit Ia occupy 15 and 83 %, respectively. The blocks of unconsolidated mud occupy 4 %. No volcanic rock (pumice, obsidian

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and lava) is found among the clasts larger than 1 cm in both outcrops.

The ratio of the sedimentary rock decreases with the decrease of clast size below 1 cm (Fig. 7A). The sedimentary rocks occupy ~82 % of the clasts more than 8 mm in diameter, whereas

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more than 90 % of the grains less than 2.0 mm in diameter are dominated by the granitic component. Volcanic components (pumice, obsidian and lava) occupy less than 2 % of the grains

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less than 8 mm in diameter, though the volume of the fragile pumice can be underestimated. Very-fine component (<0.1 mm) consists of the fragments of volcanic glass and crystals. No

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microfossil was found in the very-fine component.

Unit II

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All clasts larger than 1 cm in Unit II consist of pumice in the outcrops MA and MB. No sedimentary rock, granite, obsidians and lava fragments larger than 1 cm is found in Unit II at the outcrop MB. The ratio of the pumice decreases with the decrease of clast size (Fig. 7B), though

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the volume of the fragile pumice can be underestimated particularly in the fine-grain class. More than 80 % of the grains larger than 4 mm are pumice, whereas the volcanic component (pumice, obsidian and lava) occupy less than 10 % of the grains smaller than 2.8 mm. Instead, more than

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60 % the grains smaller than 2.8 mm are occupied by the granitic rock. The sedimentary rocks occupy ~5-10 % of the clasts less than 8 mm in diameter.

6.4 Roundness of clasts Roundness and flatness of the sedimentary clasts in Unit Ia are higher than that of the granitic clasts in each outcrop (Fig. 8). Most of the sedimentary clasts exhibit subangular-subrounded shapes, whereas the granitic clasts typically show angular-subangular shapes, though small amount of the granitic clasts shows subrounded shape. The clasts of obsidian and lava in Unit Ia show angular shape. The pumice clasts generally exhibit surrounded shape, though the shape of the fragile pumice is difficult to examine.

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The pumice clasts in Unit II exhibit subrounded shape (Fig. 4E, F). The granitic clasts having poorly-rounded shape predominant in the grains less than 4 mm across. The sedimentary-rock clasts in Unit II also exhibit more rounded shape compare to the granitic ones. The clasts of obsidian and lava in Unit Ia show angular shape.

6.5 Lithology and major element composition of the pumices and obsidians

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The pumices in the deposit exhibit pale yellow - orange in color owing to the weathering. They typically exhibit a fibrous texture formed by extensively-elongated vesiculated glasses.

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Plagioclase, orthopyroxene, clinopyroxene, and magnetite crystals are found in the pumice as phenocrysts. These petrological characteristics of the volcanic components in the deposit

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coincide with that of the pumices of the Akahoya eruption.

The chemical compositions of these volcanic glasses in Unit I - III are enclosed in the compositional variation of the volcanic glasses of the Akahoya eruption (Fig. 9). The glass

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compositions of the pumices is clearly distinguished from the other Pleistocene pumiceous deposits (AT tephra from the Aira Caldera, Tozurahara tephra from the Kikai Caldera, and Koseda

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pyroclastic flow deposit (Machida and Arai, 2003)) found in the Yakushima area. The SiO2 contents in the groundmass glass of the pumices and obsidians of Units I and II concentrate in the range of 73.6-75.1 wt.% (Fig. 10). The range of SiO2 contents in these glasses

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coincides with that in the Koya pumice fall deposit and lower – middle part of the Koya PFD. Though SiO2 content of the glasses in Unit III concentrated between 73 and 75 wt.%, a few

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glass fragments have SiO2 contents as low as 65 wt.%. Presence of glasses with lower-SiO2 content are typically found in the uppermost part of the Koya PFD and the Akahoya ash fall

7. Discussions

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deposit erupted in the later stage of the Akahoya eruption.

7.1 Evidence for tsunami

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Based on the analysis of the sedimentary structure and component, the pumice-bearing gravel deposits in the northern coast of Yakushima and Kuchinoerabujima are considered as tsunami deposits formed during the caldera-forming Akahoya eruption of the Kikai Caldera. The sedimentary structures and their components indicate that Unit Ia emplaced under a strong water current. This water current eroded the substratum and formed “rip-up clasts” of the unconsolidated mud block. The concentration of cobbles and boulders at the base of Unit Ia also indicates a strong current at the beginning of the emplacement of the deposit. The upward-fining grading of the lithic fragments and increase of the abundance of pumiceous fragments suggest the drop of current velocity from Unit Ia to Ib. Very-slow flow or ponding at the end of the settlement of Unit I is suggested due to the very-fine component of Unit Ic.

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Presence of the rounded pebbles of sedimentary basement rocks in Unit Ia indicates the transportation of these sedimentary-rock clasts from the downstream area by a strong water current. These sedimentary pebbles cannot be supplied from the upstream area because all the basement rocks in the upstream area beyond the granite-sedimentary boundary, consist of granitic rocks (Fig. 2). Absence of the sedimentary pebbles in the terrace deposits below Unit I also contradicts the supply of the sedimentary pebbles from the upstream area by normal river

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flow. The difference of the roundness of the clasts between the granitic and sedimentary rocks in the outcrop at outcrop MB (Fig. 8) may indicate a different origin of these clasts; poorly-rounded

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clasts of granitic rocks were derived from a mountain stream and slope, whereas the rounded sedimentary clasts may come from the downstream area, including the beach environment.

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The lithofacies of Unit II indicates that Unit II was also formed by the tsunami during the Akahoya eruption. The imbrication of pumiceous clasts and the orientation of the tree trunks in Unit II suggests the emplacement of Unit II within a down flow. The localized erosion observed at

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the topmost part of Unit I (Fig. 4E) could be formed by the channelized down-flow of tsunami. The concentration of the pumice blocks and lack of large lithic clasts in Unit II show the sorting and

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accumulation of the drifting pumice in the water current. Rounded shape of the pumice clasts (Fig. 4F) can be caused by the transportation by turbulent tsunami flow. The presence of the tree trunks in Unit II also suggests the accumulation of the drifting trees brought by the tsunami. The

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accumulation of lower-density materials such as pumice and tree trunks pointing their tree-tops toward the downstream in Unit II suggests the emplacement of Unit II with the drop of the water

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level during the down-flow of the tsunami.

Unit III, which consists of many layers sorted by density and size of particles, is also an aqueous deposit related to the Akahoya eruption since it contains a large amount of pumices of

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the Akahoya eruption. The sorting by size and density of clasts observed in the Unit III indicates the deposition in the water current. The trough cross-bedding in Unit III indicates the channelized downstream flows during the setting of Unit III. These lines of evidence suggest that the Unit III

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was also formed in the water stream. There are both the possibilities that Unit III is also a (a) part of the tsunami deposit and was formed by the latest stage of the down flow, or (b) Unit III is the post-tsunami deposit formed by the lahar. The pumice-bearing deposits found in the coastal area of Yakushima and Kuchinoerabujima (Figs. 2, 5) also can be the trace of tsunami, though no clear evidence for run-up current from the sea was found. The distribution of these deposits limited in the northern coast of the islands supports that they were formed by the tsunami from the Kikai caldera. The stratigraphic relationship and the presence of pumices in the deposit also support that they are associated with the Akahoya eruption.

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7.2 Distribution The tsunami deposit can be traced up to ~ 4.5 km from the present river outlet along the Miyanoura River, ~2.9 km along the Isso River, and 1.5 km along the Nagata River. The highest point of the distribution of the deposit in the Miyanoura River area (outcrop MA) is ~ 50 m above the present sea level. In Kuchinoerabujima, the potential deposit can be traced up to ~45 m above the present sea level. Taking account of the sea level change from the age of the Akahoya

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eruption to present (~8.5 m subsidence of sea level, including the 2~3 m subsidence of ocean level and ~6 m of uplift of the basement in the northern coast of Yakushima), the height of the

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tsunami invasion was ~ 40 m above the sea level in the Yakushima and Kuchinoerabujima area. Distribution of the tsunami in the northern and eastern sides of the caldera is still undisclosed

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owing to the poor geological evidences in the southern coastal area of Kyushu Island and the western coast of Tanegashima Island (Fig. 1). Maeno et al. (2006) reported the evidence for the erosion at the base of the Koya PFD in the southern coast of the Satsuma Peninsula and they

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suggested the probability of the tsunami invasion up to ~30 m between the deposition of the Koya

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pumice fall deposit and the arrival of the Koya PFD.

7.3 Timing

The sequence and components of the tsunami deposit suggest the arrival of tsunami during

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the settlement of the Koya PFD. Petrological characteristics and the glass composition of the pumices and obsidians in the deposit indicate that these volcanic components were derived from

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the products of the Akahoya eruption. The radiocarbon ages obtained from the buried trees trunks in Unit II (Okuno et al., 2013) also coincide with the age of the Akahoya eruption. The compositional variation of the volcanic glasses in the tsunami deposit also constrains

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timing of the tsunami during the eruption. Chemical composition of the erupted magmas shifted from rhyolite with ~75 wt.% of SiO2 in their groundmass glass in the early phase to the hybrid magma with wider SiO2 variation down to 65 wt.% in the later phase of the eruption. The hybrid

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magmas occurred in the uppermost unit of the Koya PFD and the Akahoya ash fall deposit as shown by Fujihara and Suzuki-Kamata (2014). The chemical composition of the volcanic glasses of pumices and obsidians in units I and II are similar to the rhyolite of the early-stage of the eruption that produced the Koya pumice fall deposit the lower - middle part of Koya PFD (Fig. 10). In contrast, the wider variation and lower silica concentration of the glass composition in the upper part of the Koya PFD and Akahoya ash fall deposit is clearly distinguishable from that of the glasses in Units I and II (Fig. 10). Units I and II at outcrops MA and MB are covered by the Koya PFD, which is characterized by volcanic glasses having a high-silica concentration (Fig. 3). This indicates that the emplacement of the tsunami deposits in Yakushima lasted before the arrival of the upper part of the Koya PFD with

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lower-SiO2 glasses. This sequential relationship is consistent with the observation in the coast of Kyushu (Maeno et al., 2006; Fujiwara et al., 2010). In the southern coast of Kyushu, the arrival of tsunami followed the emplacement of the Koya pumice fall deposit from the initial Plinian eruption and before the end of the Koya PFD (Maeno et al., 2006). The flooding of tsunami lasted before the emplacement of the co-ignimbrite Akahoya ash fall deposit in the Yokoo archeological site at the northeastern coast of Kyushu (Fujiwara et al., 2010), though the migration times of tsunami

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wave and the co-ignimbrite ash cloud from the caldera are poorly constrained.

The Koya pumice fall deposit is distributed only in the northeast of the caldera and does not

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cover Yakushima area (Fig. 1). Therefore, the tsunami deposits with pumices and obsidians from the Akahoya eruption can be formed after the arrival of the Koya pyroclastic flow that brought the

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pumice and obsidians to Yakushima. Another possibility of the origin of the pumice clasts is that the pumice clasts were drifted from the pumice-fall area to Yakushima and later stranded by the first major tsunami which formed Unit I. However, the presence of the dense volcanic clasts such

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as obsidian and lava fragments, besides pumices, in Unit I is inconsistent with the drifting of these fragments before the entrainment in the tsunami. Partially-carbonized tree trunks found in Unit II

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also support the tsunami after the arrival of the Koya pyroclastic flows. It is still unclear whether Unit I and II represent individual tsunami event with time interval or was formed during a single tsunami event. Development of Unit Ic indicates the presence of a

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time gap to deposit the fine ash within a static water to form Unit Ic between the emplacement of Unit Ia-Ib and Unit II under strong water current. However, the glass compositions of the pumices

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in Unit I and II constrains the formation of Unit I and II within the emplacement of the Koya PFD with higher SiO2 concentration, even if there is a certain time gap between the formations of Unit I and II.

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The timing of Unit III is still not clear. The presence of the volcanic glasses with lower SiO2 content in Unit III (Fig. 9) suggests that Unit III could be formed during/after the settlement of the

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upper part of the Koya PFD and Akahoya ash fall deposit.

7.4 Cause of the tsunami In general, tsunamis associated with volcanic eruption can be triggered by 1) entering of pyroclastic flow into the sea or lake, 2) caldera collapse, 3) volcanic earthquake, 4) slope instability, 5) underwater explosion, and 6) shock waves (Paris, 2015). The rapid emplacement of voluminous pyroclastic flow into the sea can generate tsunami (e.g., Waythomas and Neal, 1998; Hard et al., 2006; Nomikou et al., 2016). Simultaneity of the arrival of the tsunami and the emplacement of the Koya PFD in Yakushima is consistent with the generation of tsunami by a rapid emplacement of the pyroclastic flow into the sea surrounding the caldera. Presence of a submarine ignimbrite sheet of the Koya PFD with ~100 m thickness (Ono

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et al., 1982) indicates the emplacement of massive pyroclastic flow into the sea area surrounding the caldera. The rapid invasion of the thick Koya PFD might push up the seawater to produce tsunami waves. Subsidence of submarine caldera floor by the evacuation of magma from a shallow chamber can produce tsunami due to the large displacement of the submarine caldera floor. The total 3

volume of ejected materials during the Akahoya eruption (70-80km in DRE; Maeno and

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Taniguchi, (2007)) is enough to induce a caldera collapse (Geshi et al., 2014). The numerical simulation by Maeno et al. (2006) shows the caldera collapse could form a >30 m-high tsunami at

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the southernmost coast of Kyushu Island. Their model shows that the negative wave is generated during the collapse and the positive wave followed the end of collapse. This means that the run-up

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wave of tsunami in the surrounding area arrived after the end of caldera collapse. However, the sequential relationship (Fig. 3) and glass compositions (Fig. 9) of the tsunami deposits in Yakushima suggest that the run-up wave of the tsunami might have arrived simultaneously with

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the arrival of the Koya pyroclastic flow, which is the main phase of the Akahoya eruption. Tsunami generation by collapse is difficult to explain these sequential relationships between the tsunami

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deposit and the Koya PFD. Recent numerical experiments of tsunami generation show that to generate a tsunami wave the subsidence of caldera floor has to last within shorter period than that is expected from the mechanism of caldera collapse (e.g., Paris, 2015). More complex collapse

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processes such as multiple collapses may be, therefore, required to explain the occurrence of tsunami in Yakushima.

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Submarine faulting also can produce tsunami. Based on the sequential relationship between the clastic dikes and the tephra of the Akahoya eruption, Naruo and Kohayashi (2002) concluded that two strong earthquakes occurred during the eruption. Distribution of the clastic dikes

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indicates that the first one occurred around the Kikai Caldera at shortly prior to the settlement of the Koya PFD in the surrounding islands, and the second one occurred at the final stage of the eruption during the deposition of the Akahoya ash fall (Naruo and Kohayashi, 2002). These two

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earthquakes could be the cause of the tsunami. However, the timing of the first earthquake (shortly prior to the settlement of the Koya PFD) is earlier than that inferred from the sequential relationship of the pumice-bearing tsunami deposits at Yakushima, which indicates that the tsunami inundation occurred after the arrival of the Koya PFD. The timing of the second earthquake during the deposition of the Akahoya ash fall deposit is clearly later than the timing of the formation of the tsunami deposit in Yakushima. More detailed investigation about the location, scale and focal mechanism of the epicentral area of these earthquake is needed to clarify the causal relation between these volcano-tectonic earthquakes and the tsunami. Slope instability, underwater explosion, and shock waves are also possible triggers of tsunami, though there is no clear evidence for these events during the Akahoya eruption. The

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sequence of the tsunami deposits discussed in 7.3 suggests that the emplacement of massive pyroclastic flow into the sea can be the direct trigger for the tsunami during the Akahoya eruption.

Conclusions We found tsunami deposits associated with the ~7.3 ka Akahoya eruption of the Kikai Caldera along the northern coast of Yakushima and Kuchinoerabujima, ~40 km away from the

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caldera rim. The tsunami deposits indicate that the tsunami reached maximum ~50 m above the present sea level and invaded ~4.5 km inland from the coast along some rivers in these islands.

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The rip-up clasts and rounded pebbles in the deposit indicate the strong run-up current of the tsunami from the coastal area along these rivers. The concentration of the pumice clasts and the

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tree trunks in the upper part of the deposit suggests the hydraulic sorting during the tsunami invasion.

The petrological characteristics of the pumices in the deposit coincide with that of the early

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phase of the eruption, including the initial Plinian pumice fall and the lower part of the Koya pyroclastic flow deposit. The tsunami deposit contains only the pumices with high-SiO2 glass,

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which mark the lower - middle part of the Koya pyroclastic flow deposit. The absence of the pumices with lower-SiO2 glass, which typically occurs in the uppermost part of the Koya pyroclastic flow deposit, in the tsunami deposits suggests that the deposits were formed during

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the emplacement of the Koya pyroclastic flow deposit, prior to the arrival of the later part of the Koya pyroclastic flow.

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The simultaneous occurrence of the tsunami with the main pyroclastic flow suggests the generation of the tsunami by a rapid emplacement of the pyroclastic flow into the sea.

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Acknowledgments

The government of Yakushima Town and National Forest Office of Yakushima supported the field survey. We appreciate the comments and suggestions of Futoshi Nanayama, Keiko Suzuki

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and Mitsuhiro Kuwahata during the work. The review comments by Raffael Paris and an anonymous referee improved the original manuscript.

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References Bruins, H.J., MacGillivray, J.A., Synolakis, C.E., Benjamini, C., Keller, J., Kisch, H.J., Klugel, A., van der Plicht, J., (2008) Geoarchaeological tsunami deposits at Palaikastro (Crete) and the Late Minoan IA eruption of Santorini. Jour. Archaeol. Sci., 35, 191-212. doi: 10.1016/j/jas.2007.08.017 Carey S., Morelli D., Sigurdsson H., Bronto S., 2001. Tsunami deposits from major explosive

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Bull. Volcanol. 53, 357-380. doi: 10.1007/BF00280227

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Cas, R.A.F., Wright, J.V., 1991. Subaqueous pyroclastic flows and ignimbrites: an assessment.

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Dominey-Howes, D., 2004. A re-analysis of the Late Bronze Age eruption and tsunami of Santorini, Greece, and the implications for the volcano-tsunami hazard. Jour. Volcanol. Geoth. Res. 130, 107-132. doi:10.1016/S0377-0273(03)00284-1

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Fujihara, M., Suzuki-Kamata, K., 2013. Glass composition and emplacement mode of Koya pyroclastic flow deposit and its proximal equivalent. Bull. Volcanol. Soc. Japan 58, 489-498.

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Fujiwara, O., Machida, H., Shiochi, J., 2010. Tsunami deposit from the 7,300 cal BP Akahoya eruption preserved in the Yokoo midden, north Kyushu, West Japan. Quaternary Res. 49,

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23-33. doi: org/10.4116/jaqua.49.23 (in Japanese with English abstract) Geshi, N., 2009. Distribution and Flow Mechanisms of the 7.3 ka Koya Pyroclastic Flow Deposits

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Geshi N., Ruch, J., Acocella, V., 2014. Evaluating volumes for magma chambers and magma

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withdrawn for caldera collapse. Earth Planet. Sci. Lett. 396, 107-115. doi: org/10.1016/j.epsl.2014.03.059 Goodman-Tchernov, B.N., Dey, H.W., Reinhardt, E.G., McCoy, F., Mart, Y., 2009. Tsunami waves

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generated by the Santorini eruption reached eastern Mediterranean shores. Geology 37, 943-946. doi: 10.1130/G25704A.1 Herd, R.A., Edmonds, M., Bass, V., 2006. Catastrophic lava dome failure at Soufrière Hills Volcano, Montserrat 12–13 July 2003. Jour. Volcanol. Geoth. Res. 148, 234–252. doi: org/10.1016/j.jvolgeores.2005.05.003 Kobayashi, T., 2015. Volcanic tsunamis. Chikyu Monthly, 428, 169-180. (in Japanese) Krumbein,W.C., 1941. Measurement and geologic significance of shape and roundness of sedimentary particles. Jour. Sed. Petrol. 11, 64-72 Latter, J. H., 1981. Tsunamis of volcanic origin: summary of causes, with particular reference to Krakatoa, 1883, Bull. Volcanol. 44, 467–490

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Machida, H., Arai, F., 1978. Akahoya Ash—A Holocene widespread tephra erupted from the Kikai caldera, south Kyusyu, Japan. Quaternary Res. 17, 143–163. (in Japanese with English abstract) doi: org/10.4116/jaqua.17.143 Machida, H., Arai, F., 2003. Atlas of tephra in and around Japan, revised version. University of Tokyo Press. ISBN 4-13-060745-6. pp.58-63. Maeno, F., Imamura, F., Taniguchi, H., 2006. Numerical simulation of tsunamis generated by

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caldera collapse during the 7.3 ka Kikai eruption, Kyushu, Japan. Earth Planet Space 58, 1013–1024. doi: 10.1186/BF03352606

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Maeno, F., Imamura, F., 2007. Numerical investigations of tsunamis generated by pyroclastic flows from the Kikai caldera, Japan. Geophys. Res. Lett. 34, L23303,

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Maeno, F., Taniguchi, H., 2007. Spatiotemporal evolution of a marine caldera-forming eruption, generating a low-aspect ratio pyroclastic flow, 7.3 ka, Kikai caldera, Japan: Implication from

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near-vent eruptive deposits. Jour. Volcanol. Geotherm. Res. 167, 212–238. doi:org/10.1016/j.jvolgeores.2007.05.003

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Matsumoto, T., 1943. The four gigantic caldera volcanoes of Kyushu. Japan Jour. Geol. Geogr., 19, 1-57.

Nakagawa, S., Nanayama, F., Sasaki, H., Omote, M., Geshi, N., Watanabe, K., Kisimoto, K.,

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Naruo, H., Maeno, F., Okuno, M., Kobayashi, T. 2017. Origin of the early modern event gravel beds on the Holocene wave-cut bench around Koseda Coast, northeastern

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Yakushima Island, south Kyushu: preliminary report. Fukuoka Univ. Sci. Rep. 47, 15-32. (in Japanese with English abstract)

Naruo, H., Kobayashi, T., 2002. Two large-scale earthquakes triggered by a 6.5ka BP eruption

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from Kikai Caldera, Southern Kyushu, Japan. Quaternary Research 41, 287-299. (in Japanese with English abstract) doi: org/10.4116/jaqua.41.287 Nomanbhoy, S., Satake, K. 1995. Generation mechanism of tsunamis from the 1883 Krakatau

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eruption. Geophys. Res. Lett. 22, 509-512. doi: 10.1029/94GL03219 Nomikou, P., Druitt, T.H., Hübscher, C., Mather, T.A., Paulatto, M., Kalnins, L.M., Kelfoun, K., Papanikolaou, D., Bejelou, K., Lampridou, D., Pyle, D.M., Carey, S., Watts, A.B., Weiß, B., Parks, M.M., 2016. Post-eruptive flooding of Santorini caldera and implications for tsunami generation. Nat. Commun. 7, 13332. doi: 10.1038/ncomms13332. Okuno, M., Nakamura, T., Geshi, N., Kimura, K., Saito-Kokubu, Y., Kobayashi, T., 2013. AMS radiocarbon dating of wood trunks in the pumiceous deposits of the Kikai–Akahoya eruption in Yakushima Island, SW Japan. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 294, 602-605. doi: org/10.1016/j.nimb.2012.05.015

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Ono, K., Soya, T., Hosono, T., 1982. Geology of the Satsuma-Io-Jima district. Quadrangle series Scale 1:50,000. Geological Survey of Japan. (in Japanese with English abstract) Paris, R., 2015. Source mechanism of volcanic tsunamis. Phil. Trans. Royal Soc. A, 373, 20140380. doi: 10.1098/rsta.2014.0380 Saito, M., Ogasawara, M., Nagamori, H., Geshi, N., Komazawa, M., 2007. Geological Map of Japan 1:200,000, Yaku Shima. Geological Survey of Japan, AIST. NH52-3 9. (in Japanese

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with English abstract)

Self S., Rampino M. R., 1981. The 1883 eruption of Krakatau. Nature 294, 699–704.

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Ui, T., 1973. Exceptionally Far-reaching, Thin Pyroclastic Flow in Southern Kyushu, Japan. Bull.

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Volcanol. Soc. Japan 18, 153-168. (in Japanese with English abstract) Ulvrova, M., Paris, R., Nomikou, P., Kelfoun, K., Leibrandt, S., Tappin, D.R., McCoy, F.W., 2016. Source of the tsunami generated by the 1650 AD eruption of Kolumbo submarine volcano

org/10.1016/j.jvolgeores.2016.04.034

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(Aegean Sea, Greece). Jour. Volcanol. Geoth. Res. 321, 125-139. doi:

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Waythomas, C.F., Neal, C.A., 1998. Tsunami generation by pyroclastic flow during the 3500-year B.P. caldera-forming eruption of Aniakchak Volcano, Alaska. Bull. Volcanol. 60, 110-124.

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doi: 10.1007/s004450050220

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Appendix We analyzed the volcanic glasses in the Koya pumice fall deposit, the Koya PFD and Akahoya ash fall deposit as references of the compositional variations of the volcanic glasses throughout the eruption sequence. The samples of the Koya pumice fall deposit were collected from an outcrop at the southernmost part of the Osumi Peninsula (outcrop Fa in Fig. 1). The total thickness of the pumice

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fall deposit in the outcrop is 0.8m. The top of the pumice-fall deposit is covered by the Koya PDF. Four samples were collected with ~0.2 m interval from the bottom to the top of the pumice fall

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deposit.

The samples of the Koya PFD and the Akahoya ash fall deposit were collected at

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Kuchinoerabujima because in Yakushima, the upper part of the Koya PFD is poorly preserved owing to the heavy rain precipitation, the steep topography and lack of the younger aerial deposit. We collected the samples of the Koya PFD from an outcrop where the complete sequence of the

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Koya PFD – Akahoya ash fall deposit is preserved by the overlying tephra layers. The total thickness of the Koya PFD in the outcrop Fl is ~25 cm. The Koya PFD in the outcrop consists of

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three flow units. The lowest unit consists of a basal layer of well-sorted coarse volcanic ash (~3 cm in thickness) overlain by main pyroclastic deposits (~10 cm). The middle unit consists of a non-contiguous layer of well-sorted volcanic sand (max 3 cm in thickness) overlain by the main

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pyroclastic deposits (~10 cm). The uppermost unit is a ~3-cm-thick uniform ash layer. Four samples were collected from the Koya PFD.

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The Akahoya ash fall deposit is a 30-cm-thick upward-fining ash layer in the outcrop Fl. The presence of the Akahoya ash deposit on the Koya PFD means that the whole sequence of the Koya PFD is preserved in this outcrop. One sample was collected from the base of the Akahoya

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ash fall deposit.

The samples were sieved between 0.5 and 1.0 mm. 20–40 grains of glass fragments were selected manually from each sample. The fragments were fixed in epoxy resin and polished. The

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surfaces of the glass fragments with carbon-coating were examined by Energy Dispersive X-ray Spectrometry (EDS; Oxford Instrumental, X-Max 20). The measurements were done with 15 kV of acceleration voltage, ~1 nA of the probe current, and the area of the analysis is more than 100 square micrometers on a polished surface. The analysis of glass compositions from the Akahoya eruption shows the change of glass composition during the Akahoya eruption as pointed by Fujihara and Suzuki (2013). The glasses in Koya pumice fall deposit and the lower and middle part of the Koya PFD have narrow and silicic composition (SiO2 =73-75 wt.%), whereas the uppermost part of the Koya PFD and Akahoya ash fall deposit have wider and less silicic compositional distribution (SiO2= 65-75 wt.%). This result shows that the later unit of the Koya pyroclastic flow with hybrid magma reached the Yakushima

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area.

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Table 1 Component of the clasts in Unit Ia and II of the outcrop MB Unit Ia

sedimen

gran

pum

obsid

alter

tary

ite

ice

ian

ed lava

83

15

>8 mm

82

18

0

5.6 - 8.0

65

32

0

30

61

0

14

82

0

4

91

1

4

90

1

2

94

0

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>1 cm*

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rock

unconsolidated mud block

4.0 - 5.6 mm

0

mm

mm 1.4 - 2.0 mm

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mm 0.71 - 1.0 mm

II

9

4

4

0

1

4

0

0

3

95

0

0

0

3

2

95

0

0

0

2

sedimen

gran

pum

obsid

alter

tary

ite

ice

ian

ed

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Unit

3

1

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0.50 0.71 mm

1

0

D

1.0 - 1.4

0

MA

2.0 - 2.8

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2.8 - 4.0

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mm

2

rock

unconsolidated mud block

lava

>1 cm*

100

>8 mm

10

60

10

20

5.6 - 8.0

6

0

59

6

29

0

6

38

27

0

19

10

14

74

5

mm 4.0 - 5.6 mm 2.8 - 4.0

6

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mm 2.0 - 2.8

9

66

13

1

9

3

6

66

7

2

9

10

5

76

3

1

6

10

4

67

11

2

5

2

67

22

0

4

mm 1.4 - 2.0 mm 1.0 - 1.4

0.71 - 1.0

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mm

0.50 -

5

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0.71 mm

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mm

11

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D

MA

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weight per cent (wt.%). Grains larger than 1cm* are counted in the outcrop

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Figures and their captions

Figure 1

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Sketch map of the Kikai Caldera and Yakushima area, south of Kyushu Island, Japan, showing the distributions of the Koya pumice fall deposit and Koya pyroclastic flow. Open diamond symbols indicate the sampling localities of the Koya pumice fall deposit (Fa), and the Koya

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pyroclastic flow deposit (Fl).

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Figure 2

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Map showing the distribution of the tsunami deposit and the lithology of the basement. Filled squares indicate the representative outcrops of the tsunami deposit. Solid line shows the intrusive boundary of Yakushima Granit against the sedimentary-rock-origin hornfels of the Hyuga Group. Broken line shows the approximate location of the outer boundary of the hornfels. These geological boundaries are after Saito et al. (2007).

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Figure 3 Stratigraphic sections along the Miyanoura, Isso and Nagata Rivers. The outcrop IDs are shown at the bottom of each columnar section. Locations of the outcrops are shown in Figure 2B. The present distances from the outlet of river and the elevation of the base of the tsunami deposits in each outcrop are also given. Levels a–d in the columnar section MB indicate the sampling points for the glass composition given in Figs. 9 and 10.

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Figure 4

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Closer views of the outcrops of Unit I. A: Lower portion of Unit Ia at outcrop MB. Arrows indicate the flat shale pebbles showing oblique alignment. Right is the upstream direction. B: unconsolidated mud clasts (arrows) in Unit Ia at outcrop MB. C: Unit Ib. Arrows indicate pumice

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clasts. Right is the upstream direction. D: Unit Ic showing parallel-cross lamination at outcrop MC.

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Left is the upstream direction.

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Figure 4 (continue)

E: Unit II overlying Unit Ib at outcrop MB. Right is the upstream direction. F: sub-rounded pumice clasts in Unit II. The outcrop Is along the Isso River. Right is the upstream direction. G: pumiceous conglomerate of Unit II exhibiting bedding structure. The outcrop N along the Nagata River. Tree molds are indicated by arrows. Top of the outcrop is covered by the boulders of the modern stream. Right is the upstream direction. H: cross-bedded sand bed of Unit III at outcrop MD along the Miyanoura River.

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Figure 5

Closer views of the outcrops of the pumice-bearing deposits in the Kuchinoerabujima Island. The

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locations of the outcrops are shown in Fig. 2. White broken lines indicate the base of the deposits. A: block-rich despite at in the outcrop Ko, ~150 m from the coast and ~28 m a.s.l. An unconsolidated mud block is indicated by arrow. B: the layer of subrounded boulders covered by

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the pumice-bearing ash flow deposit in the outcrop Ki, ~ 200 m from the coast and ~45 m a.s.l. C: a plane view of the base of the tsunami deposit in the outcrop Ki. The casts of plant rods showing

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the preferred alignment are indicated by the white arrow.

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Figure 6

Distribution of the tilt angle of the long axis of the clasts in Units Ia and II. The orientations of the

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clasts were measured along the vertical side of the outcrop parallel to the river. The upstream direction is right.

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Figure 7

The grain composition of Unit Ia and II at outcrop MB. Grains >1 cm are collected manually from

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the outcrop. Grains less than 16 mm are sieved and counted at the laboratory.

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Figure 8

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after Krumbein (1941).

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The distribution of the roundness of the clasts in Unit Ia at outcrop MB. The roundness index is

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Figure 9

Major elements composition of the volcanic glasses of the pumices found in the tsunami deposits. Horizontal axis shows the weight per cent of the SiO 2 content. Circles show the compositions of

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the pumices in Units I, II, and III at outcrop MB. The compositions of Koya pyroclastic fall deposit, lower and middle part of the Koya pyroclastic flow are shown by yellow triangle, orange square, and yellow square, respectively. The upper unit of the Koya pyroclastic flow deposit is shown by

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open diamonds. Black crosses show the glass composition of the Akahoya ash fall deposit in the Yakushima area. The glass compositions of the Aira Tn tephra (AT), Kikai-Tozurahara tephra (KTz) and Koseda pyroclastic flow deposit (Ksd) is after Machida and Arai (2003).

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Figure 10

Distribution of the SiO2 contents in the volcanic glasses of the Akahoya eruption (A) and the tsunami deposit in the outcrop at outcrop MB (B).

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Appendix Figure 1

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The close-up view of the outcrops of the Koya pumice fall deposit in the Osumi Peninsula (left) and the Koya PDF and the Akahoya ash fall deposit in the Kuchinoerabujima Island (middle). The

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distributions of the SiO2 contents in the volcanic glasses in each unit are shown in right.