The 11-kilometer-wide Irosin caldera, which is located at the southeastern tip of Luzon Island, was formed by the eruption of 41 cal kBP Irosin ignimbrite. Bulusan (1565 m), a post-caldera volcano, is an active volcano, which generated phreatic eruptions during historical times. The northern rim of the caldera is covered with post-caldera volcanoes, such as Bulusan and Sharp Peak. Lahar and fluvial deposits filled and built up the southern part of the caldera floor, which has been widely used for rice farming. (Photograph and explanation: Mitsuru OKUNO and Masayuki TORII)
In recent years, the impact of tourism on socioeconomic structures has attracted the attention of the Japanese government. To plan and implement effective tourism policies and strategies, it is important to understand the basic characteristics of tourism resources in a given region. Tourism resources have been regarded as one of the most important indices for evaluating the tourism potential of a region and analyzing tourism phenomena. However, previous studies on tourism regions in Japan were based on limited resources and analyses have been largely qualitative. Therefore, this study focuses on a statistical analysis of substantial data, such as the distribution of tourism resources, to clarify the regional characteristics of tourism resources in Japan. First, the number of tourism resources, which are assigned a weighted value on the basis of an evaluation ranking determined by the Japan Travel Bureau Foundation, is calculated for each prefecture. The results of the statistical dispersion show that, among prefectures, there are great differences in the quantities of each type of tourism resource. Next, the prefectures are classified into four regional clusters based on similarities of location quotients. The first cluster is abundant in natural landforms and is mainly located in eastern Japan. The second cluster predominantly consists of elements concerning surface water such as lake water and river water and is composed of prefectures at various locations around the country. The third cluster, composed of prefectures in the metropolitan areas of both eastern and western Japan, has extensive urban cultural resources. The fourth cluster primarily contains important natural and cultural components, which are closely located along the Seto Inland Sea and in south-west Japan, including Okinawa. Finally, to analyze the regional attractiveness of each prefecture, the coefficient of specialization and the weighted number of the resources are compared to ranking data on tourist arrivals and projected destinations of domestic tours. The results show that popular tourist destinations have high values in the dimension of the number of tourism resources, and these regions are divided into two types with high or low specialization. In conclusion, the distribution of tourism resources determines the characteristics of each prefecture as a tourism region and makes a difference in the regional attractiveness of prefectures. The comprehensive analysis of a wide variety of tourism resources is effective for identifying the regional differences of tourism resources and reinforcing the findings of classifications of Japan's tourism regions provided by prior studies.
A consolidated and almost horizontally stratified sand layer was exposed east and north of the Hirota site, southeastern Tanegashima after Typhoon 14 struck on September 5-6, 2005. According to previous criteria given to the above-mentioned sediments, they are identified as eolianite. Calibrated 14C age of the sediments indicated 1,670 cal BP with -5.2‰ of δ13C. Judging from the figure for δ13C, the origin of calcium carbonate that consolidated the sand bed seems to have derived from underground water percolating through dune sand. Peat and outer layer of the trunk sample were also collected from downstream along the Hirota River, and their calibrated ages were 530-550 cal BP and 2,070 cal BP, respectively.
The distribution of weather divides in Japanese winters was identified using 30-year data of the Automated Meteorological Data Acquisition System (AMeDAS) operated by the Japan Meteorological Agency. Two kinds of weather divide were defined, one is a cloudy weather divide (CWD) determined by the high-frequency grids of large gradients in the sunshine duration distribution, and the other is a precipitation area border (PAB) where the edge of daily precipitation areas frequently appeared. The CWD appeared continuously in eastern Japan along the Pacific backbone ranges, but it was discontinuous in the central mountain ranges and western Japan. The CWD also appeared in Pacific coastal areas, such as east of Kamikouchi, south of the Kii Peninsula, and southeast of Shikoku Sanchi. The PAB overlapped with the CWD distribution in eastern Japan, and it was enhanced throughout the Sekigahara-Tamba Kochi and Chugoku Sanchi areas, but the CWD in pacific coastal areas was not associated with the PAB. Most of the weather divides were caused by the winter monsoon pressure pattern, and some PABs in northwestern Tohoku and Hokkaido areas occurred with passing pacific coastal extratropical cyclones. The distribution of the weather divides in cold-winter years was dependent on the dominance of Satoyuki/Yamayuki weather patterns, and weather divides became unclear in warm winters.
The 2004 Mid-Niigata Prefecture earthquake induced many landslides in the Higashiyama Hills, central Japan. The epicenter of the earthquake was near the axis of an active fold called the Higashiyama anticlinorium, but the density of earthquake-triggered landslides was relatively low around the anticlinorium's axis. In contrast, landslide density was high on both limbs of the anticlinorium. This study analyzes the factors responsible for this difference in earthquake-induced landslide density from the viewpoints of landform development and microgeomorphology. The folding of the Higashiyama anticlinorium since the latest early Pleistocene has been uplifting the Higashiyama Hills; the 2004 earthquake was one event of this crustal movement. Microgeomorphological classification using a 2-m-mesh DEM around the middle fork of the Imo River indicates that landslide density is high on the steep slopes of the river valley, which are dissected by down-cutting of the Imo River and its tributaries. In contrast, the area around the axis of the Higashiyama anticlinorium is characterized by deep-seated landslide geomorphology, without dissecting valleys, and relatively low relief. As a result, earthquake-induced landslides are sparse there. These differences in slope geomorphology reflect geomorphic processes in actively uplifting hills and mountains with active folding. This viewpoint is applicable to estimating the potential for co-seismic landsliding.
Based on a questionnaire survey, this study investigates how local residents regard the strong local wind Matsubori-kaze, which occurs at Mt. Aso in Kumamoto prefecture. We distributed questionnaire sheets to students of Ozu-Higashi Elementary School, where Matsubori-kaze frequently occurs, and received 25 responses from their families. The collection rate was 71%. Application of quantification theory type III to overall replies to six questionnaires reveals that recognition of Matsubori-kaze by local residents was explained primarily according to their experiences with this strong local wind. However, duration of residence in this region was not related directly to the overall responses. The recognition of Matsubori-kaze differed between duration of residence of 31-45 years and that of 61-75 years. Analyses of key words used in responses from the respective groups reveals that the former (duration of residence of 31-45 years) had a strong adverse perception of Matsubori-kaze in the outdoors, whereas the latter (duration of residence of 61-75 years), which was engaged mainly in agriculture, had a fatalistic acceptance of this strong local wind. Application of quantification theory type III to 56 key words that appeared in multiple questionnaire sheets reveals that these key words can be summarized as “strength and mode of Matsubori-kaze,” “agricultural damage in areas with strong winds,” and “surrendering to Matsubori-kaze.”
The Irosin caldera located at the southeastern tip of Luzon Island in the Philippines was formed by the eruption of 41cal kBP Irosin ignimbrite. Bulusan, a post-caldera volcano, has repeated phreatic eruptions during historical times. The special issue on “Geology and Recent Eruptions of Irosin Caldera and Bulusan Volcano, Southern Luzon, Philippines (Part I)” provides various data and discussion mainly on the formation of Irosin caldera. First based on interpretations of volcanic landforms, the evolution of 84 volcanoes in the Philippines is outlined (Moriya, 2014). Fifty-six stratovolcanoes, three caldera volcanoes accompanying four post-caldera volcanoes, three lava domes, four scoria cones including two maars, four lava fields, and ten shield volcanoes are identified. The sequence of caldera-forming eruption at Irosin consists of a precursory fine ash eruption (Malobago lava dome), plinian pumice fallout, and intra-plinian flow deposits (Kobayashi et al., 2014a). Evidence of ground shaking during the plinian phase was also found. The total DRE volume of erupted tephra is estimated to be 30 km3 (VEI = 6). A gravity survey in February 1996 revealed a semi-circular feature with a steep gravity gradient in the Bouguer anomalies, which corresponds clearly to the southern rim of the Irosin caldera (Komazawa et al., 2014). The funnel-shaped depression structure of the gravity basement, which is significantly smaller than that of the topographic depression, was recognized from a three-dimensional analysis of residual gravity anomalies. The mass deficiency was estimated to be 1.1 × 1010 tons, corresponding to 40 km3 of DRE volume. Four thermoluminescence (TL) ages (36 ± 8 ka, 38 ± 10 ka, 33 ± 8 ka and 45 ± 10 ka) are obtained from the matrix and lithic fragments of the ignimbrite and co-ignimbrite ash-falls, respectively (Takashima and Kobayashi, 2014). Of these, the first two ages are in good agreement with a radiocarbon age of 41 cal kBP. A pictorial of representative outcrops is offered to provide an understanding of the geology in and around the caldera (Kobayashi et al., 2014b). Recent activity at the Bulusan volcano is described and discussed in the next issue (Part II).
The evolution of 84 volcanoes in the Philippines is outlined geomorphologically on the basis of interpretations of volcanic landforms using aerial photographs, topographic maps, satellite images, and geomorphological and geological field surveys. Fifty-six stratovolcanoes, three caldera, three lava domes, four scoria cones, four lava fields, and 10 shield volcanoes are identified. Large basaltic volcanoes such as lava fields and shield volcanoes were found unexpectedly in subduction zones. No lava field or shield volcano has been discovered on the Japanese Islands. Among the 34 subduction zones in the world, 19 do not have lava fields or shield volcanoes. Two oblique subduction zones form the Philippine Fault Zone. The fault zone mostly coincides with the eastern volcanic zone. At the southwestern part of Mindanao Island, a volcanic chain, consisting of Bulibu, Basilan, and Cagayan Sulu lava fields, Balatukan, Mangabon, Katangrad, Kalatungan, Makaturing, Bacolod, and Pagayawan shield volcanoes, and Pagadian lava domes (monogenetic volcanoes) trends northeast-southwest, in parallel with the trenches and main arcs. The association of the volcanoes with the trenches and arcs suggests that the volcanic chain is a marginal sea ridge between Sulu Sea and Celebes Sea and that the back-arc basin ridge might have extended under Mindanao Island from Moro Gulf to the northern part of Mindanao Island.
A large-scale pyroclastic eruption occurred 41 cal kBP that resulted in the formation of the 11 × 11-km Irosin caldera located at the southern end of Sorsogon Province, Luzon, Philippines. The eruption consisted of two distinctive events, namely a precursory eruption and a large-scale caldera-forming eruption. During the precursory eruption, fine ash was ejected, and a small lava dome, Malobago, was extruded on the southeastern slopes of the present caldera outline. Other similar lava extrusion may have occurred within the caldera. After a geologically short pause (likely within 10 years), the main caldera-forming eruption, which consisted of three distinct eruption phases, started. The first phase involved a plinian eruption, likely within the present caldera, resulting in the northward emplacement of pumiceous tephra with bulk volume of 20 km3. Partial collapse of the plinian column repeatedly occurred during the early stage of the plinian phase. These formed intra-plinian flow deposits, which are generally thin, fine grained, and associated with fallout pumice layers. A strong ground shaking occurred in the Sorsogon area during the waning stage of the plinian phase, resulting in the formation of disturbed structures both at the base and in the upper horizon of the plinian deposit. This was followed by the generation of the lower Irosin ignimbrite with associated ground layers. The final eruption was catastrophic, forming the present caldera topography and emplacing the upper Irosin ignimbrite. The upper ignimbrite is generally thick and coarse, with a coarse lithic concentration zone at the base, whose thickness is about 2 m at north of Bulusan town (Loc. D). The total bulk volume of the Irosin ignimbrite is estimated to be 25 km3. A 1-m-thick co-ignimbrite ash deposit was found at the summit of the Inascan scoria cone on the western slopes of Mayon volcano. The deposit generally consists of fine-grained glass shards with small amounts of phenocrysts. The coarse lowermost layer may be the distal facies of the plinian pumice fallout. The total bulk volume of the erupted tephra from the Irosin caldera could be as much as 70 km3.
A gravity survey was carried out in and around Bulusan volcano in February 1996. The gravity stations totaled to 225, which were restricted to roads. The value of density used for both terrain and Bouguer corrections was 2,300 kg/m3 (2.3 g/cm3), the value commonly used in volcanic terrains and the surface layer density is considered to be geologically low. A semi-circular feature with steep gravity gradient was recognized in the Bouguer anomalies from the east through the south and west of Bulusan volcano. This semi-circular feature corresponds clearly to the southern rim of the Irosin caldera. However, the northern caldera rim is not clear from gravity anomalies. In contrast, the result of three-dimensional analysis of residual gravity anomalies indicates that, the gravity basement has a circular structure, with its diameter significantly smaller than that of topographic depression. It is important to note that this circular depression is similar to funnel-shaped (or inverted cone) caldera rather than to a piston-cylinder type of caldera. The mass deficiency of the Irosin caldera was estimated to be 1.1 × 1010 tons by applying the Gaussian theorem gravity anomalies.
Irosin ignimbrite (41 cal kBP) and its related deposits were the first widespread tephra recognized in the Philippines. This paper describes petrographic properties such as the particle composition and refractive index of the volcanic glass, in order to understand the nature of the eruption that emplaced these deposits. Green hornblende, cummingtonite, and biotite were observed in most of the samples analyzed, but green hornblende was not observed in some pumice clasts from the ignimbrite. However, it occurred in the matrix of the ignimbrite, co-ignimbrite ash, and plinian pumice samples. These results are probably due to small amounts of green hornblende phenocryst in the pumice fragments. The coexistence of cummingtonite and biotite in pumice indicates differences in crystallization conditions within the magma reservoir. A basal ash from the Malobago lava dome that preceded the ignimbrite eruption contains isolated crystals of orthopyroxene and irregular glass shards. The pumiceous deposit within the caldera shows slightly different petrographic properties (e.g. wider range of refractive index) from the other samples, which supports the interpretation of Irosin ignimbrite being reworked within the caldera.
Four samples collected from the Irosin ignimbrite and co-ignimbrite ash fall deposit in southern Luzon, Philippines, were subjected to Thermoluminescence (TL) age-dating to further refine the timing of the ignimbrite eruption. IrTL-01 and -02 are the matrix part of the ignimbrite, IrTL-03 is a 10 cm-diameter lithic fragment from the ignimbrite, and IrTL-04 is co-ignimbrite ash-fall deposit collected at the summit crater of the Inascan cone, 80 km north of the eruption source. The ages obtained are 36 ± 8 ka for IrTL-01, 38 ± 10 ka for IrTL-02, 33 ± 8 ka for IrTL-03 and 45 ± 10 ka for IrTL-04. TL ages of IrTL-01 and -02 are in good agreement with a previously reported AMS 14C age of ca. 41 cal kBP. On the other hand, TL ages for IrTL-03 and -04 are not consistent with the 14C age. IrTL-03 (lithic fragment) exhibits a TL signal that is very clear and provides good data, however the fading TL signal may possibly account for the younger than expected TL age. The grain size and water content of IrTL-04 (ash-fall) may possibly account for an older than expected TL age. Further experiments are needed for more precise TL ages for IrTL-03 and -04.