High-resolution reconstruction of the Hoei eruption (AD 1707) of Fuji volcano, Japan

Journal of Volcanology and Geothermal Research 207 (2011) 113–129

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Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s

High-resolution reconstruction of the Hoei eruption (AD 1707) of Fuji volcano, Japan Naomichi Miyaji a, Ayumi Kan'no a, Tatsuo Kanamaru a, Kazutaka Mannen b,⁎ a b

Department of Geosystem Sciences, College of Humanities and Sciences, Nihon University, 3-25-40 Sakurajosui, Setagaya, Tokyo 156-8550, Japan Hot Springs Research Institute of Kanagawa Prefecture, 586 Iriuda, Odawara, Kanagawa 250-0031, Japan

a r t i c l e

i n f o

Article history: Received 27 December 2010 Accepted 26 June 2011 Available online 7 July 2011

a b s t r a c t The Hoei eruption of Mount Fuji in 1707 caused the worst ashfall disaster in Japanese history. Despite the availability of numerous historical documents describing the eruption, the detailed eruption sequence has not been verified because the correlation between these descriptions and geological sequences remains unclear. In this study, we reconstruct the sequential change in the column height using newly established stratigraphy and a detailed timeline obtained from historical documents. The eruptive deposit was subdivided into 17 units on the basis of their facies, with the mass of each unit established using isopach maps. The eruption column height is a function of the magma discharge rate; hence, the column height of each unit was estimated from its erupted mass and duration, which were inferred from the historical documents. The correlation of each unit with the actual time was based on the tephra color and grain size, an unconformity caused by rainfall, and ashfall distribution. While reconstructing the unit boundaries, we found that not all of them represented a hiatus or relenting phase of ashfall. We detected only six obvious quiet intervals from the historical documents. Therefore, many of the unit boundaries may represent a hiatus or relenting phase that was too subtle to have been recorded in the historical documents. We define an eruptive pulse as a period of continuous ashfall followed by an obvious quiet interval. We divided the 17 units of the Hoei eruption into 6 pulses, and into 3 stages on the basis of the patterns of the eruptive pulses. The characteristics of the three stages are described as follows. Stage 1 had two energetic eruptive pulses (pulses 1 and 2; at least 20 km high column), each showing an intense initial outburst, followed by a decrease in intensity. The eruption sequence indicates ruptures of highly overpressured dacite and andesite magma chambers. The initial silicic eruption was followed by basaltic magma withdrawal from a deep and voluminous magma chamber. Stage II consisted of discrete subplinian pulses of relatively degassed basaltic magma. Although the eruption rate throughout the stage decreased, the magma supply from depth appears to have been sustained because extensive intrusion near the surface created the Mt. Hoei cryptodome near the vent. Stage III was principally characterized by sustained column activity without a clear repose time. During this stage, the column height appears to have been more than 13 km, and we recognized at least two distinct periods of increased activity in which the column height is presumed to have exceeded 16 km. The Cu-rich vesicular scoria and continuous eruption in stage III indicate a stable supply of volatile-rich magma from depth. No significant decay was observed in the magma discharge rate; hence, the eruption could have been halted by a sudden process such as conduit collapse, rather than decompression of the magma chamber. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Fuji volcano, the symbolic mountain of Japan, is also one of the most dangerous volcanoes in the country. Since the last eruption at the summit crater more than two thousand years ago, the volcano has erupted from the flank (Miyaji, 1988). Most of the flank eruptions have been relatively mild; however, two major historical eruptions producing lava flows (AD 864–866) and a Plinian event (AD 1707)

⁎ Corresponding author. Tel.: + 81 465233588; fax: + 81 465233589. E-mail address: [email protected] (K. Mannen). 0377-0273/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2011.06.013

were also the largest in the entire history of the volcano (Miyaji, 2007). In particular, the AD 1707 Hoei eruption, named after the era in which it occurred, caused severe damage in the proximal downwind areas, and the ash fall reached distal areas such as Tokyo and Yokohama approximately 100 km east of the volcano. The eruption lasted sixteen days from 16 December 1707 to 1 January 1708 (Tsuya, 1955). Although no death toll from the eruption itself is known, crop fields in proximal areas are still far from recovery even now since thick ash fall and rampant vegetation inhibit reclamation (Sumiya et al., 2002). Even though the ash fall was limited, downstream granaries and inhabited areas of the watershed were frequently devastated by lahars until the end of the 19th century (Inoue, 2007).

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assumed Plinian eruption [Miyaji (1984) using method of Walker (1973)]. We re-estimate the erupted masses of each eruption unit based on newly created isopach maps. We also describe the detailed eruption chronology based on historical documents in a recent extensive compilation (Koyama, 2009) to estimate eruption column height that is a function of mass discharge rate. From the sequential change in mass discharge rate and components of erupted materials, a schematic model of the magma plumbing system and its evolution is proposed.

The impacts in Edo (the name for Tokyo until 1868) from the Hoei eruption were limited to respiratory problems suffered by many residents, probably caused by inhalation of fine ash (Koyama, 2009). However, the present densely populated and modernized cities in the Greater Tokyo Area would be rather vulnerable to a similar event. To plan mitigation measures for a future eruption of this type, detailed reconstruction of the Hoei eruption is critical to delineate the worstcase scenario for eruptions of Fuji volcano. As a response to the deep low-frequency earthquake activities beneath the volcano from 2000 to 2001 (Ukawa, 2005), the Japanese government conducted its first comprehensive research to estimate damage in the case of another Hoei class eruption, and estimated that worst-case scenario would cause an economic loss of up to 2.5 trillion yen (Review Committee of Volcanic Hazard Mitigation of Mt. Fuji, 2004; hereafter referred as RC, 2004). This governmental study includes the estimation of sequential changes in column height of the Hoei eruption based on geological and paleographical analyses. However, the estimation of the maximum height of the eruption is sub-plinian size (15 km; RC, 2004) and not consistent with the previous estimation that

2. Background 2.1. Previous studies on stratigraphy and chronology The Hoei tephra is the latest eruption deposit of Fuji volcano and is preserved near the surface in the southern Kanto area, including Tokyo, Kanagawa and Chiba prefectures (Fig. 1). The earliest modern scientific studies broadly revealed the distribution and stratigraphy of the tephra (Tsuya, 1955; Machida, 1964). Miyaji (1984) subdivided

E139°

140°

141°

Ibaraki aki Iba N36°

Saitama

50km

S N 1.3

Tok Tokyo okyo

Yamanashi amanashi

E

4

8

Kanagawa Kanag 24

Sb Sy

21

O G

Od

15

Chiba 24

H

10.5

Y

36

45

16 9

8

Sn Shizuoka

4 2

35°

1

b)

a) N40°

Mt. Fuji

c)

Pacific Ocean

Yamanashi amanashi 15 5

N35.4°

42 Kanag Kanagawa 48 64 81 37 20 0 108 75 1 33 35 39 26 64 46 121 111 46 129 39 75 87 28 128 Sb241 135 60 45 39 105 115 149 101 256375 300 47 89 52 94 46 84 75 146 93 O 503 45 57 42 71 142 80 108 395 385 119 244 135 105 105 48 51 137 490+ 39 15 39 90 G 44 50 18 60 54 32 85 60 30 40 38 27 15 25 60 15 54 60 16 15 18 0

35.2°

+

Shizuoka

9

Sn

10km

E140°

Od

12

Sy

500 km

8

6

4 2

+

1

E138.8°

139.2°

Fig. 1. Index and Isopach maps of the Hoei tephra. a) Location of Fuji Volcano. b) Isopach map of the total Hoei tephra in southern Kanto area. c) Isopach map of the proximal area. The isopach maps are created from both geological investigation and historical documents newly obtained and from previous studies (Miyaji, 1993; RC, 2004). Solid dots are outcrops and stars indicate locations where thicknesses were recorded in historical documents. Figures show thickness in cm. Squares indicates cities or locations; N, Narita; S, Sawara; E, Edo or present Tokyo; Y, Yokohama, H; Hiratsuka; Od, Odawara; O, Oyama; G, Gotemba; Sb, Subashiri; Sn, Susono; Sy, Suyama.

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

the tephra stratigraphy into four groups (Ho-I to IV) and described the distribution of each group. The early studies also focused on correlating the deposit stratigraphy with the eruptive chronology in historic documents in Edo and Fuji's flank areas (Tsuya, 1955; Shimozuru, 1983). These studies concluded that the eruption started with the most intense explosion before noon on 16 December and ceased at the end of the year. 2.2. Source vents The source of the Hoei eruption is not a single vent but three craters located on the southeastern flank of the volcano (3000– 2100 m above sea level), aligned NW-SE for a length of 2.2 km (rim to rim) or 1.2 km (distance between vent centers). The craters are called the Hoei craters and numbered from the first to the third from higher altitude (Tsuya, 1955; Fig. 2). The second and third craters were deformed on their eastern margins by an edifice named Mt. Hoei (or Hoei-san), which formed during the eruption (Tsuya, 1955). Although the original shapes of these craters were deformed by the formation of Mt. Hoei, the present longest diameter is approximately 600 m. The northern part of Mt. Hoei is intersected by the rim of the first crater. Investigations of historical documents and the geology reveal that Mt. Hoei is composed of older volcanic deposits and is presumed to have been up-thrusted by magma intrusion beneath the edifice. It seems Mt. Hoei was created during the eruption and the source of the eruption is assumed to have migrated from the lower craters to the upper first crater. The dimension of the first crater is 1.3 × 1.1 km, which is larger than the summit crater and the largest of Fuji volcano. No scoria cone or lava flow was deposited around the vents; however, a small spatter cone (150 m in diameter), of which the northern half is lost, remains in the bottom of the first crater (Tsuya, 1955) and may have formed during the last phase of the main eruption (27 to 31 December). The northern half may have been destroyed by the explosion on 1st January, 1778 (Miyaji and Koyama, 2007). Since the Hoei craters formed on a ~17° slope, vigorous erosion in spring by avalanches removed the Hoei deposit. In fact, the present surface of the crater rim consists of old volcanic deposits. In the crater area, the Hoei deposit crops out only on the rim between the first and second craters, with a thickness of over 3 m. 2.3. Erupted magma Fuji volcano has issued basalt through its whole history (~100 ka) and has formed the largest volcanic edifice in Japan (estimated

N

First Crater

115

approximately 500 km3). The magma is characterized by high FeO/ MgO (N1.6), which implies significant differentiation without Si enrichment (Fujii, 2007). This chemical composition and the continued massive production of basalt magma are the prominent features of the volcano when compared with other volcanoes on the Izu-arc such as Izu-Oshima and Miyakejima. From phase analysis, Fujii (2007) attributed this peculiar chemical composition to pyroxene fractionation in a deep magma chamber (20 km) beneath the thick dioritic midcrust in the plate collision zone, which prevents further ascent of denser basaltic magma. The magma chamber depth has been estimated with seismic tomography (Lees and Ukawa, 1992; Nakamichi et al., 2005) and a magnetotelluric survey (Aizawa et al., 2004). The erupted magma of the Hoei eruption shows significant petrological variation from dacite in the earliest stage (Ho-I; SiO2 b70 wt.%) to andesite (Ho-II) and through to basalt in the latter half of the eruption (Ho-III and IV; SiO2 = 51-53 wt.%; Table 1). Although Ho-III and IV issued chemically homogeneous basalt, in the early stages of the eruption (Ho-I and II) the products contained banded pumices, which implies magma mixing during and/or prior to the eruption. Yoshimoto et al. (2004) carefully selected homogeneous particles and proposed that the magma that contributed to the Hoei eruption was composed of three end members — dacite, andesite and basalt magmas. The dacite and the andesite magmas could have been generated by crystal fractionation of Fuji basalt; however, not that of the Hoei eruption (Yoshimoto et al., 2004; Watanabe et al., 2006). These authors found no evidence showing mixing of the basalt magma with other end members. From these observations, it is proposed that the basalt magma of the Hoei eruption was stored in a separate reservoir and that interaction with the other end members was limited during the eruption. Sato et al. (1999) reported that the plagioclase phenocrysts of the Hoei basalt show a Ca-rich core (An = 85–94) surrounded by a Capoor rim (An = 80–86) with a sharp boundary between. Based on experiments using an internally-heated pressure vessel that showed a positive linear relationship between An and water contents, these authors proposed that the phenocryst cores were formed at depth having 3–4 wt.% dissolved water in the magma and that the rim grew during magma ascent and decompressional dehydration. The water content determined by Sato et al. (1999) was later confirmed by melt inclusion analysis (Iida et al., 2004). Sato et al. (1999) also focused on the difference in eruption style between the Hoei and another historical eruption of Fuji volcano, the Jyogan eruption (AD 864–865), which was a mild lava producing eruption but issued comparable amount of magma to the Hoei eruption. Sato et al. (1999) found that the Jyogan lava is rich in phenocrysts (20–30%), while the Hoei tephra is aphyric, and that most of the Ca-poor plagioclase phenocrysts were inferred to have been formed during the course of degassing. From these observations, Sato et al. (1999) attributed the explosivity of the Hoei eruption to the rapid ascent and less degassing of magma from the source chamber. A similar degassing model to explain the difference in eruption styles was also proposed by Iida et al. (2004). 3. Hoei tephra fall deposit

Spatter cone Second Crater Mt. Hoei Third Crater 1km Fig. 2. Aerial photo of the source vents of the Hoei eruption and Mount Hoei (aerial photograph of Geographical Survey Institute of Japan, CCB-75-17).

The tephra sequence of the Hoei eruption is composed of numerous beds presumably resulting from interruptions and fluctuations of the eruption column. In previous studies, based on the appearance of essential and accidental, the Hoei deposit was subdivided into four groups named Ho-I to IV in ascending order (Miyaji, 1984). This subdivision is useful since the groups are traceable even the deposit is thin; however it is too rough to obtain sequential changes in eruption intensity that can be correlated with historical records. We subdivided the sequence into 17 units based on their facies and designate them from A to Q in ascending order (Fig. 3; digital

116

Group unit SiO2 TiO2 Al2O3 tFeO MnO MgO CaO Na2O K2O P2O5 Sc V Cr Co Ni Cu Zn Ga Rb Y Sr Zr Nb Ba Pb La Ce

I

II

III

IV

A

B

C

C

D

D

E

E

H

H

H

J

J

J

L

L

L

N

N

N

O

O

O

P

P

64.1 0.9 15.9 5.9 0.1 1.9 5.2 3.6 2.1 0.3 17 130 10 15 6 66 77 17 43 33 316 159 4 443 10 12 37

62.0 1.0 16.2 6.7 0.1 2.4 5.9 3.6 1.8 0.3 18 163 10 18 9 68 84 19 37 33 348 148 4 398 10 15 36

57.7 1.2 16.8 8.4 0.2 3.3 7.1 3.7 1.4 0.3 25 204 16 22 7.8 68 99 19 26 33 401 133 2.5 338 9.0 11 33

57.7 1.2 16.8 8.4 0.2 3.3 7.1 3.6 1.3 0.3 24 205 16 23 9.2 77 101 20 27 33 402 133 3.8 361 10 11 25

57.2 1.2 16.7 8.7 0.2 3.4 7.2 3.6 1.3 0.3 24 209 19 23 11 72 100 20 25 33 400 129 3.4 337 8.1 13 31

57.3 1.2 16.8 8.5 0.2 3.4 7.2 3.6 1.3 0.3 24 208 15 23 7.2 80 101 20 26 33 401 129 3.6 341 7.9 6.5 28

56.7 1.3 16.8 8.9 0.2 3.5 7.3 3.6 1.3 0.3 25 219 17 25 10 78 102 21 24 33 405 127 4.2 324 8.0 10 32

56.5 1.3 16.9 8.9 0.2 3.7 7.5 3.5 1.2 0.3 25 225 20 25 10 82 102 19 22 34 404 124 3.3 323 8.3 9.4 32

52.3 1.3 17.0 10.7 0.2 5.1 9.5 2.9 0.8 0.2 37 368 37 34 19 137 100 20 14 25 397 83 2.8 222 6.7 7.0 12

52.3 1.3 17.0 10.7 0.2 5.1 9.6 2.8 0.8 0.2 35 370 37 34 19 153 99 20 14 25 396 81 3.1 214 6.1 6.0 14

52.0 1.4 17.1 10.8 0.2 5.2 9.6 2.7 0.8 0.2 36 377 40 34 20 170 100 21 14 25 396 81 3.2 220 5.7 – 4.4

52.1 1.3 17.0 10.8 0.2 5.2 9.6 2.8 0.8 0.2 34 367 42 35 22 142 100 21 13 26 395 81 2.8 230 5.2 4.8 12

51.7 1.4 17.1 10.8 0.2 5.3 9.8 2.7 0.8 0.2 33 373 43 35 22 153 100 20 12 25 398 79 2.0 211 6.6 7.4 13

51.8 1.3 17.1 10.8 0.2 5.3 9.7 2.7 0.7 0.2 35 378 39 34 21 169 99 20 12 25 398 79 3.0 217 6.4 13 15

51.7 1.3 17.1 10.9 0.2 5.3 9.7 2.8 0.7 0.2 36 378 41 37 20 139 100 20 13 24 402 78 3.6 215 6.0 5.7 10

51.5 1.4 17.0 11.0 0.2 5.4 9.8 2.7 0.7 0.2 34 383 47 35 24 149 100 19 12 25 397 78 2.5 212 6.0 6.0 12

51.6 1.4 17.1 10.9 0.2 5.4 9.8 2.7 0.7 0.2 36 380 41 35 21 162 100 19 13 24 401 79 2.2 214 6.3 6.6 7.4

51.5 1.4 17.1 11.0 0.2 5.4 9.8 2.7 0.7 0.2 35 383 40 36 21 153 101 19 12 25 398 79 2.7 207 5.9 7.4 10

51.6 1.4 17.2 10.9 0.2 5.3 9.8 2.7 0.7 0.2 34 382 38 36 21 162 100 20 13 23 399 78 3.4 212 5.7 2.8 12

51.6 1.4 17.2 10.9 0.2 5.3 9.8 2.7 0.7 0.2 35 380 40 36 19 177 101 21 13 24 399 79 2.7 204 5.5 13 13

51.5 1.4 17.1 11.0 0.2 5.3 9.8 2.7 0.7 0.2 36 381 40 34 19 137 101 21 13 24 403 79 2.9 199 5.5 12 6.9

51.5 1.4 17.2 11.0 0.2 5.3 9.7 2.7 0.7 0.2 37 384 33 35 20 171 102 21 12 25 400 80 3.4 213 6.3 6.2 16

51.5 1.4 17.2 11.0 0.2 5.3 9.7 2.7 0.7 0.3 36 382 40 37 21 180 102 20 13 25 400 80 2.7 208 5.7 3.7 8

51.5 1.4 17.1 11.0 0.2 5.3 9.8 2.7 0.7 0.3 35 387 40 37 21 157 101 21 13 25 402 79 3.1 221 5.3 5.1 12

51.4 1.4 17.2 11.1 0.2 5.3 9.7 2.7 0.7 0.3 35 380 38 36 21 176 103 20 13 25 401 80 3.2 214 5.4 0.4 14

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

Table 1 Whole rock analyses of the Hoei eruption products. Major element composition (wt.%) is normalized to 100%; trace element concentration is expressed as ppm.

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

photograph is available as the web Supplementary materials (1)). These units are recognized up to a distance of approximately 30 km from source. Isopach and isopleth maps are generated based on geological investigations and data in historical records. Here we describe each unit only briefly. We also used historical documents that describe the eruption (Koyama, 2009). From these documents, we reconstructed a chronology of the eruption that includes such information as the start/stop times of tephra fall, repose times, and intensity changes. The chronology is then correlated with the tephra sequence based on color of tephra, grain size, thickness and boundaries. Here we describe the reconstructed chronological and geological sequences. The chronology is summarized in Table 2. All isopleths are available as the web Supplementary materials (2). 3.1. Ho-I The basal part of the Hoei deposit is quite distinctive since it is composed of white pumice that contrasts with the dominantly of dark scoriaceous lapilli and ash. Ho-I is also remarkable for abundant accidental lithic fragments including gabbro and basaltic lava. The deposit distributions are shown in Fig. 4. The unit A is composed of white pumice, and is correlated with the initial and most energetic phase of the eruption. Many historical documents describe the eruption beginning at around 10:00 on 16 December. The ash cloud was recorded to be the highest of the Hoei eruption and an analysis of eyewitness documents in distal areas and line-of-sight analysis concluded that the column was at least 20 km high (Koyama and Maejima, 2009). Unit B is composed of grayish to brownish pumice. Proximal to the vent (b2 km), the unit contains large accidental blocks, possibly the result of vent widening and/or opening and could have been related to the formation of the second and/or the third crater(s). Although the eruption could have been violent, the erupted mass and intensity of the eruption seems to have been limited since this layer thins quickly with distance. The Ho-I stage may have ended at approximately 15:30

(m) 5

Unit Group

50

SiO2 (wt.%) 60

70

117

on the 16th. The transition time from unit A to unit B is not obvious; however, a historical document in Edo describes a color change in the ash approximately 1.5 hours after the start of the eruption. Since the Ho-I tephra is mainly composed of vesicular pumice, the average particle density is 0.5 to 0.7 g/cm 3 (Tables 3). 3.2. Ho-II Ho-II is principally composed of coarse, angular, dense scoria colored dark gray and can be subdivided into four units proximally. Isopach maps of each unit are shown in Fig. 5. This group also contains fragments of basalt lavas and reddish scoriaceous lapilli. Ho-II deposition is correlated to the eruption that restarted after a short repose, at about 17:00 on December 16th, whose deadly firing of incandescent bombs torched the town of Suyama (13 km SE of the source vent). This initial phase is correlated to unit C at the base of Ho-II, which is a thick layer of coarse scoriaceous lapilli with abundant lithic materials. Following the eruption of bombs, documents describe an ash shower. Although the ash fall seems to have relented, some areas became darkened temporarily or for the entire day of 17 December. This continuous ash fall is correlated to units D through F. Unit D is characterized by alternation of finer scoriaceous lapilli layers. Historical documents record ash fall to the south of the source at around 04:00 on 17 December, the time of unit D emplacement, which is the only ash fall event to have traveled southward during the eruption. Although this southern ash fall deposit is not preserved, we assume a second, southerly lobe in the distribution of Unit D as shown in Fig. 5. Unit E resembles unit D but the scoriaceous lapill are larger. Unit F forms the uppermost part of Ho-II and contains coarse, round scoriaceous lapilli. Ho-II is rich in dense scoria, and the average particle density of the group is 1.3 to 1.7 g/cm 3. 3.3. Ho-III Ho-III is composed of alternating blackish-gray scoriaceous lapilli mixed with relatively abundant reddish scoriaceous accidental lapilli,

Grain size (Mdφ) 0 2

4

Clast density (g/cm3) 1 2

Q P

4

O

IV

N

3 M L

2

1

0

K J I H G F E D C B A

III

II I

White pumice

Scoriaceous lapilli (well vesiculated)

Accesory fragment

Alternation

Brown pumice

Scoriaceous lapilli (moderately vesiculated)

Scoriaceous ash

Soil

Scoriaceous lapilli (poorly vesiculated)

Fig. 3. Schematic columnar section of the Hoei tephra and changes in SiO2 content (after Yoshimoto et al., 2004), median grain size and clast density throughout the sequence. Clast density in here represents average of 10 to 20 particles of − 2 phi size class colleced from the outcrop of Dainichido (7.5 km east of the source vent).

118

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

138.7o

138.8o

138.9o

Unit A (proximal ~ medial) 0 0 0 1

35.4o

0

139.1o

0 1

1

1

2

2 3 5

2 2

25

139o

4 5 4 4 13 Sb 12 15 11 9 12 33 7 10 25 15 10 12 O 35 12 16 14 18 14 9 32 40 17 16 8 10 15 45 8 5 10 8 60 6 88 5 3 50 2 10 1 G 30 5 35 0 0 1 1 1 3

25

35.3o

4 4

4

4

2

2

1 1

1

0

1

4

10km

32

16

8

4

A (distal) G

4

2 2

4

H

3

2

+ +

1

35.4 35.3

Od

20km

139.5

139 35.4o

2

Unit B 3 10

254 64 100

340 75

2 7

18 25 32

45

35.3o 138.7o

20

0 5

138.8o

15

7 16

1 3 7 6 6 5 6 9 3 8 7 3 7

2

3

2 5

8

4 2

3

1 1

138.9o

139o

139.1o

Fig. 4. Isopach map of units A and B (Ho-I). Numbers indicate values of thickness in cm.

lithic fragments and free crystals. The vesicularity of the essential scoria within the group varies widely; however as a whole rich in vesicular scoria. Since the amount of denser scoria, which forms major component of Ho-II, decreases upward through Ho-III, average particle density decreases from 1.2 to 0.5 g/cm 3. Based on thin ash layers within the Group and the grain size of each alternating bed, Ho-III is subdivided into 7 units named G through M. The isopach maps of each unit of the Ho-III group are shown in Fig. 6. The onset of Ho-III deposition is inferred to have occurred late on the night of 17 December, a period of decreasing ash fall. The relatively mild ash fall phase lasted until early afternoon on 18 December and is correlated with unit G, which is composed of fine, less-vesicular ash. Unit H also contains alternating layers of medium sized scoriaceous lapilli with occasional pumiceous fragments presumed to be reworked from Unit A at the source region. Scoria layers in unit I are medium to moderately coarse and in unit J they are medium to moderately fine grained. Unit K consists of alternating medium sized scoriaceous lapilli and ash, and often has an unconformity contact with the underlying Unit J. This unconformity could have been formed during a rainfall on 23 December, which is described in historical documents. Unit L is alternating medium to moderately fine sized scoriaceous lapilli. Unit M is formed by alternating ash and is rich in coarse grained free crystals, presumably from accidental gabbroic rocks at depth. From the facies, it is reasonable to assume Unit M was a weak eruption. We thus correlated Unit M to the period with no ash-fall in Edo. The timing of onsets of Unit H to L are correlated with ash fall events in Edo, and the rainfall that removed the upper part of Unit K. However, the number of ash falls recorded in Edo before the rainfall exceeds the number of units by one. In this study we assume Unit J is a deposit of

two ash fall events that are geologically not distinct, because fine scoria layer of Unit J can obscure the boundary of different ash fall events. Since Unit J contains fine grains more than other units, we consider that Unit J may have been formed during two ash fall events. In this study, the masses of each event of Unit J are calculated on a pro-rata basis of eruption durations. 3.4. Ho-IV Ho-IV is composed of black, less dense scoria, some of which show a characteristically spiky surface and larger bubbles. Since the amount of these scoria did not change much vertically, the average particle density remains at around 0.9 to 0.7 g/cm 3. The amount of accidental material is lower (b5%) than in other groups (typically 5–15%). Absence of reddish scoriaceous lapilli is a key feature to distinguish Ho-IV from Ho-III. We subdivided the Ho-IV into four units (N to Q) based on grain size and abundance of ash. Isopach maps of each unit of the Ho-IV group are shown in Fig. 7. Unit N forms alternating thick layers of coarse scoria, and characteristically contains tabular shaped scoria. Unit O shows alternating medium to fine scoria and Unit P is alternating coarse to medium scoria and scoriaceous ash. Unit Q forms alternating layers composed of medium to fine scoria and also contains tabular scoria. Units N and P, composed of coarser scoria, may have been formed from higher columns, since they are composed of relatively coarse scoria, and we correlate these with timings of ash fall in Edo and Sawara (170 km ENE of the vent; Table 2). After the end of the Ho-IV ash fall, no significant ash falls have been described; however, it is evident that some explosions followed until the early morning of 1 January 1778.

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

119

Table 2 Summary of the units and their deposition times. Geology

Time

Key descriptions

Unit

Type

dd

hh

mm

Near the volcano

Edo and Sawara

A

Very coarse pumice single layer

16

10

00

10:00 “Large earthquake. Soon after, astonishing rumbling from Mt. Fuji. Then black cloud rose and,,, ” (TIJ)

B

Very coarse pumice single layer

16

11

30

10:00 “Door and window patters without wind” (ISD) 12:30 “Gray ash-like sands fell from sky” (ISD) 14:00* “ash colored like buckwheat flour fell and increased in thickness”(ORK) 15:30* “ash and black sand fell as mixture” (ORK) 17:30* “black sand fell as mixture” (ORK)

Repose C

Very coarse single layer

16 16

15 17

30 00

D

Fine thin alteration

17

01

00

EF

Coarse thick single layer, lithic-rich

17

06

00

G

Fine thin alteration

17

21

30

H

Fine to medium thick alteration

18

13

00

I

Medium thick alteration

19

06

30

19 20

22 15

30 30

21 21

07 21

00 30

21 22

22 20

30 00

23 23

04 19

30 30

repose J repose J repose K repose L

Fine to medium thick alteration Fine to medium thick alteration Medium thick alteration

Medium to fine thick alteration

repose M

Fine thin single layer

24 25

04 12

30 30

N O

Medium thick alteration Fine thick alteration

25 26

20 01

30 00

P

Medium thick alteration

27

10

30

Q

Fine thick alteration

27

22

00

30

07

00

End

15:30 “Finally fall of fire stone ceased.” (TIJ) evening (description of incandescent bomb fountain in many documents including TIJ) midnight “fire stone fell and torched Subashiri village. No building remained.” (FYS) before dawn “midnight, near-deafening sound of thunder. Rumblings as if mountain falls. Doors and windows patter strongly” (TIJ) 6:00 “it dawned around sunrise but soon turned into darkness” (TIJ) 17th “ash fall of peach-plum size” (FZK) 17th dawn “ash fall did not cease” (TIJ) 18th morning “slightly lighten by sunrise continuing with ash falls”(TGD) 18th “ash fall did not cease” (TIJ) 18th “ash fall of bean-wheat size containing peach-plum size” (FZK) 19th “fine ash fall with bean-wheat sized particles” (FZK) 20th morning “sand fall ceased and see clear sky” (TIJ) 15:30 “ash fall again. Didn't cease even at midnight” “had fallen until morning” (TIJ) morning “clear sky” (TIJ) 21st to 24th “no ash fall during daytime” (TIJ)

25th “sand fall continued day and night until the end. Strong thunder and earthquakes. Rumblings were strong and continued too.” (TIJ)

evening “at nightfall, black sand as seen in river bed fell” (ISD)

17th morning, First observation of Mt. Fuji held from Edo since the eruption started (many) midnight “ashfall” (ORK)

morning “see the sun” (ORK) 15:30 “darkness as if night time” (ORK/ISD) 9:00 “black sand of millet size larger than yesterdays sand, fell on the roof and make sound like rain fall” (ISD) 0:00 of 19th “saw stars a little bit” (IKD). 15:30 “ash fall again” (ORK) 19:00 “ash fall ceased” (ORK) night “ash fall” (KHD/ORK)

22:30 to 4:30 of 23rd “small amount of ash fall” 23rd early morning “ash fall” (KHD) morning to noon, intermittent rain fall (ISD) 22:00 “mild ash fall began” (ISD) 23:00 “ash fall ceased” (ISD) 2:00 “dark black cloud appeared, many sand (fall?)” (ISD) sunrise “ash fall ceased” (ISD) morning “no black cloud appeared” (ISD) 15:00 “see black cloud stretching south west to east” (ISD) 23:30 to 3:30 of 26th “ash fall little by little” (ISD) morning “saw dark cloud from south west to south east,,” (ISD) 13:00 “sand fall, didn't permit to open eyes and mouths ” (KHD) 0:00 of 28th “weak ash fall. Then sky cleared. See stars.” (ISD) 29th & 30th “No ash fall” (AHD)

“ash fall ceased before the morning” (TGD)

Times marked by * indicate two hours late when inferred with other documents (see Appendix A). Three letter code after the description indicates source document; AHD, Arai Hakuseki Nikki (Diary of Arai Hakuseki in Edo); FYS, compilation from Fuji-Yoshida-Shishi of documents from the eastern flank area (City of Fuji-Yoshida, 1994); FZK (Furizuna-ki in Oyama); IKD, Ino Kageyu Nikki (Diary of Ino Kageyu in Sawara); ISD, Ito Shimano-kami Nikki (Diary of Ito Suketaka in Edo); KHD, Kashima Han Nikki (Daybook of Kashima Domain); ORK, Ohmu-Rouchu-ki (Diary of Asahi Jyoemon Sigeaki in Edo); TGD Takiguchi-ke Monjyo (Documents of Takiguchi family in Gotemba); TIJ Tsuchiya Idayu Funka Jijyou-gaki (Memorandum on the eruption of Mt. Fuji by Tsuchiya Idayu in Suyama).

4. Mass estimation Pyle (1989) proposed that the thickness (T) decays logarithmically as a function of square root of the area enclosed by the isopach of the thickness (A 1/2). Since this fitting has been widely accepted after the evaluation by Fierstein and Nathenson (1992), we adopted this fitting. The application to the total Hoei deposit is shown in Fig. 8, which also can be referred as a notation of parameters used in the following. In the plot, the log T – A 1/2 relationship seems to fit with two straight lines. The intersection of the distal and proximal lines appears to be at A 1/2 = 1.9 × 10 4 m. We thus integrate both proximal and distal

Table 3 Summary of pumice densities for each size class (after Miyaji, 1984). Size class (phi)

Density (g/cm3)

− 4.0 − 3.5 − 3.0 − 2.5 − 2.0 − 1.5

0.45 0.48 0.52 0.51 0.55 0.60

to to to to to to

− 4.5 − 4.0 − 3.5 − 3.0 − 2.5 − 2.0

120

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

Table 4 Volumes of each unit of the Hoei tephra calculated to fit log (mass loading) vs square root area relation. Parameters for the calculation are also shown. Proximal

I II

III

IV

A B C D E F G H I J K L M N O P Q

Interception

Distal

Volume (×107 m3)

T0 (m)

k0 (×10− 4)

1/2 Aip (×104)

Tip

T1

k1 (×10− 4)

Proximal

Distal

Total

+ 0.11 0.65− 0.09 + 0.39 0.84− 0.27 + 0.08 0.58− 0.07 0.25±0.02 + 0.12 0.54− 0.10 0.10* 0.05* + 0.08 0.29− 0.06 0.37±0.00 + 0.07 0.59− 0.06 + 0.10 0.40− 0.08 0.84±0.06 + 0.07 0.32− 0.06 0.61±0.01 0.53* 0.46* 1.21*

2.1±0.2 2.6±0.4 0.12±0.11 0.13±0.07 0.16±0.24 0.11* 0.12* 0.11±0.20 0.17±0.01 0.17±0.13 0.18±0.20 0.16±0.07 0.19±0.19 0.18±0.02 0.15* 0.15* 0.18*

1.3

0.04±0.00 0.01±0.00 0.06±0.00 0.02±0.00 0.03±0.01 0.01* 0.01* 0.03±0.00 0.01±0.00 0.03±0.00 0.01±0.00 0.04±0.00 0.01±0.00 0.02±0.00 0.03* 0.02* 0.04*

0.09±0.01 0.01±0.00 0.11±0.02 0.04±0.01 0.05±0.01 0.02±0.01 0.01±0.00 0.06±0.01 0.02±0.01 0.05±0.01 0.02±0.01 0.07±0.01 0.01±0.00 0.04±0.01 0.05±0.02 0.04±0.02 0.06±0.03

0.64±0.05

2.3 ± 0.01 +0.2 2.4− 0.4 5.5±0.1 2.1±0.0 + 0.1 3.5− 0.0 1.1* 0.5* + 0.2 3.0− 0.1 2.1±0.0 3.5±0.0 2.1±0.1 5.2±0.1 1.5±0.1 3.3±0.0 3.6* 3.1* 6.1*

3.5 ± 0.2 2.0 ± 0.0 +0 21− 1 + 0.4 6.9− 0.8 9.2±0.5 + 0.1 4.4− 0.5 + 0.1 1.7− 0.2 +1 12− 0 + 0.8 4.5− 1.2 + 0.4 8.4− 1.0 + 0.4 4.1− 0.5 +1 13− 2 + 0.3 2.6− 0.4 + 1.2 6.8− 1.8 +1.3 9.8− 2.2 + 1.1 8.2− 1.9 +3 12− 4

5.7 ± 0.3 + 0.3 4.4− 0.4 +0 26− 1 + 0.4 8.9− 0.8 13±1 + 0.1 5.5− 0.5 + 0.1 2.1− 0.2 14±1 + 0.8 6.6− 1.2 +0 12− 1 + 0.5 6.2− 0.6 18±2 + 0.4 4.1− 0.5 +1 10− 2 +1 13− 2 +1 11− 2 +3 18− 4 + 13 179− 7

1.9

0.31±0.03

Total *Only insufficient number of isopachs was obtained to deduce errors from least aquare fitting.

volumes across the intersection. The total volume of the Hoei tephra is thus calculated as 1.6 km 3. The log T – A 1/2 plots for each unit of the Hoei tephra are shown in the web Supplementary material (3). Since all units except unit A lack the distal data, we have determined their distal volumes using Aip and k1 that are given by bulk-tephra distribution (Fig. 8). The fitting parameters and obtained volumes are shown in Table 4 with errors deduced from least squares fitting. Summing all units gives a total erupted volume of the Hoei eruption as 1.8 km 3. We here assume the deposit density of the Hoei tephra to be 1000 kg/m 2 based on our measurements in the field. We thus estimate the total mass of the Hoei tephra to be 1.8 × 10 12 kg. This estimation is fairly consistent with the previous estimation (1.7 km 3; RC, 2004), which used methodology of log A – log T relationship (Suzuki, 1981, 1983). 5. Column height estimation 5.1. Calculation of column height There have been several methods proposed to estimate column height. Theoretical backgrounds of these methods vary and for Table 5 Summary of mass discharge rates and column heights. Group I II

III

IV

Unit A B C D E F G H I J K L M N O P Q

Mass (×1010kg) 5.7±0.3 + 0.3 4.4− 0.4 +0 26− 1 + 0.4 8.9− 0.8 13±1 + 0.1 5.5− 0.5 + 0.1 2.1− 0.2 14±1 + 0.8 6.6− 1.2 +0 12− 1 + 0.5 6.2− 0.6 18±2 + 0.4 4.1− 0.5 +1 10− 2 +1 13− 2 +1 11− 2 +3 18− 4

Duration (h)

historical eruption that has good constraints on erupted mass and eruption duration, method using theoretical relationship between heat flux of magma (Q) and eruption column height (Ht) will be applicable. In previous studies (RC, 2004), this approach was taken and estimated as 15 km using the equation proposed by Wilson et al. (1978); 1

Ht = 8:2Q 4 :

ð1Þ

Although the equation gives broadly reasonable estimates, in this study we re-calculate the relation based on numerical calculation by Woods (1988) that can include probable parameters for certain eruption such as atmospheric structure and initial gas mass fraction. We assumed initial magma temperature as (1000 K), initial gas mass fraction (0.03; Iida et al., 2004), an arbitrarily selected initial upward velocity (140 m/s) and a twenty years average atmospheric structure (1991–2010) observed at Tateno, where is the closest aerological observatory (145 km NWW of Mt. Fuji). The calculation gave the following approximate equations of Ht and Hb (neutral buoyancy height) as functions of magma mass discharge rate (Qm) for the range of Ht = 10–26 km; 0:17

ð2Þ

0:19

ð3Þ

Hb = 1014Qm

Ht = 1009Qm :

Flux (× 107kg/s)

Hb (km)

Ht (km)

1.5 4.0 8.0 5.0

+ 1.2 1.1− 0.4 + 0.34 0.30− 0.12 + 0.92 0.91− 0.32 + 0.54 0.50− 0.20

+3 17− 2 +4 14− 3 +3 17− 2 +4 15− 3

+5 23− 3 +4 18− 3 +4 22− 3 +5 19− 3

15.5

+ 0.35 0.33− 0.12

+4 14− 3

+4 18− 3

15.5 17.5 16 16.5 8.5 9.0 8.0 4.5 33.5 11.5 57

+ 0.04 0.04− 0.02 + 0.26 0.23− 0.08 + 0.14 0.11− 0.05 + 0.22 0.20− 0.08 + 0.24 0.20− 0.08 + 0.64 0.55− 0.23 + 0.17 0.14− 0.06 + 0.77 0.62− 0.28 + 0.13 0.11− 0.05 + 0.33 0.27− 0.12 + 0.11 0.09− 0.04

+3 10− 2 +4 13− 3 +4 12− 3 +4 13− 3 +4 13− 3 +4 15− 3 +4 12− 3 +4 16− 3 +5 11− 3 +4 13− 3 +4 11− 3

+4 12− 3 +4 17− 3 +4 15− 3 +4 16− 3 +4 16− 3 +5 20− 3 +4 15− 3 +5 20− 4 +4 15− 3 +5 17− 3 +4 14− 3

The mass and eruption duration gave the mean mass discharge rate for each unit and column heights are deduced as shown in Table 5. The column height is affected by lapse rate. We also calculated for the atmospheric models that represent the lowest and highest lapse rates observed in the twenty years and obtained maximum and minimum column heights. The result showed that the error of column height due to the selection of atmospheric structure is less than ±2 km. In Table 5 errors of column heights that can be caused by uncertainties of mass estimation, atmospheric structure and eruption duration (assumed to be up to ±50%) are also shown. 5.2. Cross checkings of column height For unit A, the column height deduced above was checked by other two methods. One is the method proposed by Carey and Sparks (1986), which gives column height of the unit A using the isopleth map

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

of the maximum lithic (see Supplementary material (4)). The plots deduced from the isopleth map show the eruption column height to be around 22 km (Fig. 9), which is also close to the estimations shown above (Ht = 23+5 - 3 km). The other one is the method proposed by Mannen (2006), which estimates column height from mass loading decay as a function of isopleth area based on the model of Bursik et al. (1992). Applying the method, isopleth maps of 32 to 0.5 mm give the column height of 28 ± 3 km (Ht), which is also consistent with the other two estimations shown above. The isopleth maps used in this procedure are available from the web Supplementary material (5). From these calculation and recent line-of-sight analysis (Koyama and Maejima, 2009), we conclude that the eruption column of unit A eruption exceeded at least 20 km and could reach up to 28 km high. The lower column height estimated in a previous study (RC, 2004) is an artifact of longer duration estimated at the time of the research. From the method of Carey and Sparks (1986), we have some insight into wind structure at the time of the eruption. The implied wind velocity increase from 10 m/s to far more than 30 m/s as clast size decreases. Since the height reached by clast increases as clast size decrease, this wind velocity change would reflect wind velocity that is much higher in higher altitude. This result is consistent with general wind structure of winter Japan, which is characterized by stronger wind (often over 100 m/s) in higher altitude. The average column height can be also given by break-in-slope of thickness thinning rate, which is resulted by changes in the Reynolds number for fallen particles as a function of distance (Bonadonna et al., 1998). The relationship between column height and area where break-in-slope appears (Abs) is proposed as: Ht ≈12 + 0:3

pffiffiffiffiffiffiffi Abs :

ð4Þ

Using the bulk tephra data and assuming Abs is identical to the Aip obtained in this study (A1/2 ip = 19 km; Fig. 8), we obtain an average column height Ht = 18 km, which is close to the average column height estimated by mass discharge rate (17 ± 3 km).

138.8o

121

5.3. Column height and the reliability Based on the time sequence and column heights obtained from the mass discharge rates, we reconstruct sequential changes in column height throughout the Hoei eruption (Fig. 10). The eruption started with a Plinian eruption (Ht N 20 km) but the subsequent eruption remained sub-plinian (Ht b 20 km). The timing of tephra falls in remote Edo, where many observations of the eruptive sequence were documented, is also shown in Fig. 10. The tephra falls in Edo seem to have taken place about two hours later than rise of eruptions columns higher than 15 km. This relationship indicates that only high columns could provide ash to Edo. Because of such coincident observations and because of inter-method consistency as shown in the previous section, we consider that the quantitative height and sequential fluctuation of the Hoei eruption is reasonably reconstructed.

6. Change in eruption products 6.1. Component analysis As mentioned previously, each group of the Hoei eruption can be discriminated by their components. Since changes in components will reflect the style of degassing and vent erosion, the component analysis is critical to reconstruct eruption processes. We conducted a component analysis for an outcrop located on the distribution axis of the Hoei tephra (Dainichido, 7.5 km east of source). At this site, we collected samples from 48 horizons, and for certain particle sizes (− 2.0 to − 2.5 phi). We classified the particles into 7 classes; white pumice, brown pumice, dense scoria (no visible bubbles to the naked eyes), vesiculated scoria with small bubbles (largest bubble less than 0.25 mm in diameter), vesiculated scoria with large bubbles (largest bubble greater than 0.25 mm in diameter), spiky scoria (highly vesiculated to form a highly irregular surface) and lithic + free crystals. The results are averaged for each unit and shown in Fig. 11. The Ho-I deposit is principally composed of pumices, with scoria absent or very rare. Ho-II deposit consists of dense andesitic scoria.

139.0o

Unit C 8

6

35.4o

11

15 14

7

Sb

30

42

53

50 30

19

32

35.4o 1

O

16

1

10

3

1 0

1

2

33

32

10

0

35.3o

G

10

22

+ 23

12

16

0

+

10 13

8

7

35.3o

+

Sy 16 9

35.4o 4

Unit F 2

3

3

4

3

2

3

2

8 3

4 5 6

4

5 3

6

5

+

3

5

4

35.2o 6

Sn

5km

4 4

35.3o

O

7 3

15

22 11

7

4 5

2

8 9

10

7

4

4

2

0

9 5

9

16 Sb 10

40 2 3

Unit E

8 4

0

8

G

35.3o 35.4o

4

12

28 15 23 20 35 37 31 27 30 20 23 20

25

7

Unit D

11 16

4

4 3

2

138.8o

139.0o

138.8o

139.0o

35.1o

Fig. 5. Isopach map of units C to F (Ho-II). Stars indicate locations where thicknesses were recorded in historical documents. Numbers indicate values of thickness in cm. The plus (+) by the star indicates description of ashfall without measured thickness.

122

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

Unit G

Unit H

1 2 1

9

2

7

2

1

2

2

O

4

3

13

37

2

4 13 11

24

16

4

8

9

Unit L

16

4

8

21 5 22 38 32 3 17 22 14 5 32 25 1610 7 8 5 12 4

8

2

22

23

5 14 3

60 1

80

62

1

2

2

1 6

2 2 3

3 10 4

5

9

8 8

4

5

12

10

12

5

4

6

4

2

2

5km

35.3o 138.7o

4 15

8

25

27

4

5 1 12 19

11

20

32

9

9

18

30

2 1

3 9

14

10

4

10

8

3

3

8 11

6

16

9

13 11

8

10

4

5

10 17

3

11

Unit J

12

2

35.4o

16

4

16 8

4 10

5

Unit M

20

9

7

G

Unit I

35.3o

45

1

1

Unit K

13

9

1 0

6 1 2

Thickness (cm)

5

4

2

Sb

7

4

1 1

138.8o

138.9o

139.0o

Fig. 6. Isopach map of units G to M (Ho-III). Numbers indicate values of thickness in cm.

The lithic fragment + free crystals represent accidental materials that were derived from the conduit wall. The free crystals may be fragments of gabbroic body dissected by the conduit system, which is inferred from xenoliths in the deposit (Yasui et al., 1998). The lithic

Vesiculated scoria forms the principal component of the basaltic groups (Ho-III and IV) but a more vesiculated type dominates Ho-IV. In addition to the highly vesiculated type, spiky scoria forms a substantial part of the Ho-IV deposit.

Unit N

Unit O

10

8 9

10

7 62 15

32

2024 17 24 Sb 18 25 16 5

35.3o

10 3

7

9

O

27

13

55

22

48

8

16

3

10

8

G

Unit P

Unit Q 8 16

10

31

16

25

59 13

8

63

32

16

5km

35.3o 138.7o

3

16

4

20

35.4o

138.8o

138.9o

8

21

139.0o

Fig. 7. Isopach map of units N to Q (Ho-IV). Numbers indicate values of thickness in cm.

6

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

2

15

6.4 cm lithic

43km m

20

10

0

y = 1.85 - 0.00015x R= 0.99

/s

m

30

10

y = -0.478 - 3.09e-05x R= 0.99

-2

m

35.6km

ln T1 ln Tip 28.3km

5 21.0km 13.8km

8

10

12

14

A1/2 ip Fig. 8. Thickness decay as a function of isopach area for the total Hoei tephra. Notations used in the text and tables are indicated. Data points with upward arrows indicate the isopachs that have an area uncertainty because of poor constraints in the distal regions, and are omitted for the fitting calculation.

fragment + free crystals contribute to more than a few percent in the earlier and the middle parts of the sequence; however, the content diminishes in the latest part indicating very limited conduit erosion during the upper part of the eruption.

15

20

/s

3.2 cm lithic

0

20 m /s

6

10

m

25

/s

20

/s 43km

m

30

m

15 35.6km

10

28.3km

5

21.0km 13.8km 6.8km

0 0 30

5

10

15

20

25

30

/s

43.0km

1.6 cm lithic /s

0

m

/s

/s

4

Square root of isopach area (× 104 m)

5

m

2

Maximum cross wind range (km)

0

0

10

0

-4

20

10

ln of thickness (m)

/s

0

/s

m /s

ln T0

123

m

30

m

20

6.2. Number density of bubbles and microphenocrysts To have an insight into the generation process of each type of particle, we measured the number densities of bubbles and the microphenocrysts of the particles. The bubble number density (BND) and the microphenocryst number density (MND) are based on numbers in a unit area (NA) measured in the photomicrographs. First, we measured the combined area of all the bubbles and microcrystals in an area of 15 mm 2 using the image measurement software ImageJ. We calculated the diameter of an equal area circle (D) for each bubble and microlites and obtained a size frequency over 0.02 mm intervals for each sample. BND and MND are represented by the number density (Nv), which is deduced by summing NA/D for the total size range. Particle features of the erupted product seem to be classified into two groups on the BND vs MND plot (Fig. 12). One group is the high BND and low MND populations, characteristic for Ho-I and II particles (type 1). The other group forms a trend from low BND and MND to high BND and MND, which is characteristic of Ho-III and IV particles (type 2). Generally, a higher magma decompression rate forms higher BND and it is known that the product of higher intensity eruptions show higher BND (e.g. Toramaru, 2006). The higher BND of type 1 is thus considered to be the result of a higher magma ascent rate. On the other hand, as inferred from the trend, bubble nucleation of type 2 seems to be correlated with advance of crystallization. Since there was no significant change in eruption intensity during Ho-III and IV, intensity cannot have caused the observed BND variation. The parallel increases in both BND and MND for Ho-III and IV particles rather suggests secondary vesiculation due to volatile concentration in melt and/or increase of bubble nucleation site due to advance of microphencryst crystallization. For Ho-III and IV, the higher BND and MND thus indicates more advanced degassing than lower BND and MND. It is complicated but as shown in Fig. 12, less vesicular scoria such as dense scoria and small bubble scoria shows higher BND and MND. Thus the less vesicular scoria is considered to be more degassed. This will be checked in the next section.

35.6km

10

0

28.3km

13.8km 6.8km

0

10

20

21.6km

30

40

50

Maximum downwind range (km) Fig. 9. Estimation of column height and downwind velocity deduced from ranges of down and cross-wind isopleth widths (after Carey and Sparks, 1986). Solid circles indicate isopleths contour widths of the Hoei eruption. The coutour data are available from the web Supplementary Material (4).

6.3. Cu content as volatile proxy Volatile content prior to the eruption can be inferred from the Cu content. For Ho-III and IV, the Cu contents of more vesicular scoria are systematically higher than those of dense scoria (Fig. 13a) and the increase of Cu content seems not to be caused by fractional crystallization (Fig. 13b). Cu shows semi-volatile behavior in certain environment since it acts as incompatible elements and forms compounds with chloride and hydroxyl species in water-rich magmatic fluid and gasses (Collins et al., 2009). The lower Cu content of dense scoria thus indicates degassing process in presumably shallower part of magma plumbing system where water-rich fluid or gas coexists in the magma. The degassing of dense scoria prior to the eruption is consistent to the implication from BND/MND relationship shown in the previous section. 7. Discussion 7.1. Mode of eruption styles Since geologists tend to assume that each depositional unit represents some specific event, one might expect that each unit of the sequence represents an explosion or a culminating phase, possibly

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

30

Ho-I (dacite)

Ho-III (basalt)

Ho-II (andesite)

Ho-IV (basalt)

Bubble number density (x106 number/cm3)

124

Column Height (Ht; km)

25 20 15 10 5 0

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 date of December, 1707

35 30 25 II-C

separated from the adjacent units by a change in column height or by a hiatus during the eruption. In the course of the Hoei eruption, however, we have only detected six obvious quiet intervals from historical documents in the downwind area (Fig. 10). The six quiet intervals are only recognized from documents describing observations from the eastern flank of the mountain and/or from Edo. Thus, we cannot deny that subtle eruption could have occurred even during quiet intervals. In addition, since our chronologic and stratigraphic resolutions are still low, short quiescent periods may have existed in addition to the six intervals identified. However, to describe the intermittency or continuity of the eruption column during the sequence, we define pulses to be the periods of continuous tephra fall that are divided by the six obvious quiet intervals. The introduction of eruption pulses enables quantitative reinterpretation of historical documents from the view of physical volcanology, and articulates the eruptive sequence better than previously. Based on the patterns of eruptive pulses, we may subdivide the whole eruption into three stages (Fig. 10). Stage 1 was characterized by quick eruption of two energetic pulses (pulse 1 [units A and B] and 2 [unit C to I]), each of them

III-G

20 II-F

15

III-L IV-N

10

III-K

III-K

IV-Q III-K III-L

III-G

III-G

IV-N

5

III-L IV-N

IV-Q

0

Fig. 10. Change in column height throughout the Hoei eruption. The periods of ash fall in Edo and rainfall, deduced from document analysis, are also shown.

Brown pumice Dense scoria Vesicular scoria (small bubble) Vesicular scoria (large bubble) Spiky scoria

I-B

0

1

2

3

4

5

6

20

showing intensive initial outbursts followed by a decrease in intensity. Stage 2 was composed of discrete bursts that formed subplinian eruption columns (pulses 3 to 6 or units J to L). Stage 3 was principally characterized by sustained column activity without any obvious repose times (pulse 7 or units M to Q). During stage 3, column height seems to have remained higher than 14 km and at least two distinctive higher activity periods were recognized in which the column was presumed to exceed 16 km. Cumulative plot depicts magma supply probably from deep that omits fluctuation formed by pulses (Fig. 14). In the early part of stage 1, the magma discharge was very high (3.3 × 10 11 kg/day) but in later part of stage 1, the magma discharge rates are almost same as that of the whole eruption (1.2 × 10 11 kg/day). This change is coincident with switch of erupted magma from silicic to basalt. Although stage 2, which has several interruptions, shows slightly lower magma discharge rate (0.8 × 10 11 kg/day), basaltic phases in

40

60

80

100 18

Q

III

II

P O N M L K J I H G F E D

vesiculated scoria (large bubble)

spiky scoria

10

vesiculated scoria (small bubble)

dense scoria

cumulative erupted mass (×1011kg)

IV

C I

B A

0

white pumice pumice white

8

Fig. 12. Bubble number density and microphenocryst number density of juvenile material from the Hoei eruption. Numbers and letters beside the symbols indicate group and unit; for example, I-B indicates unit B of Ho-I.

component wt% 0

7

Crystal number density (x106 number/cm3)

brown pumice

lithic / free crystal

Fig. 11. Sequential changes in the components of erupted materials. The height of each bar represents the erupted mass of each unit.

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

2.0

200

a

stage 1

cumulative erupted mass (× 1012 kg)

Ho-III & IV basalt

180

Cu (ppm)

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Ho-II andesite Ho-I dacite

60 0.4

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1.0

1.2

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density (g/cm3) 200

b

1.0

0.5

0.0 5

10

15

Fig. 14. Cumulative erupted mass during the course of the Hoei eruption.

Brown pumice Dense scoria

Vesicular scoria (large bubble)

160

Spiky scoria

Cu (ppm)

stage 3

day after December 16th

Vesicular scoria (small bubble)

140

Ho-III & IV basalt

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Ho-II andesite Ho-I dacite

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60 1.8

stage 2

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0

White pumice

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120

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2.0

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2.4

2.6

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FeO* / MgO Fig. 13. Cu content as a function of a) clast density and b) FeO/MgO ratio. Clast density in here represents average of 10 to 20 particles of − 2 phi.

the eruption show almost same magma discharge rate and significant decay was not detected even in the very last phase of the eruption. The stable magma supply of the Hoei eruption would be characteristic of the eruption because many basaltic eruptions, including basaltic Plinian eruption, change their eruption style even in the course of a single eruptive event (Walker et al., 1984; Houghton and Wilson, 1998; Houghton and Gonnermann, 2008; Costantini et al., 2009, 2010). Since the average magma discharge rate (1.2 × 10 11 kg/day) also can be seen during sustained basaltic phase of the eruption, this rate can be thought as magma supply rate from depth. 7.2. Narrative model of plumbing system and its evolution 7.2.1. Magma chambers and magma ascent before the eruption In this section, we attempt to establish a working model of the magma plumbing system of the Hoei eruption (Fig. 15) based on the discussion and contexts of previous petrological and geophysical studies. First, we distribute the magma chambers referring to previous

studies (Fig. 15a). Then we discuss magma plumbing system and change in magma ascent mode based on magma discharge rate and component of erupted material. A basaltic magma chamber is supposed to accommodate at about 20 km depth and the dimension is large enough to be detected by geophysical analysis(Lees and Ukawa, 1992; Nakamichi et al., 2005). Although there is no implication from petrological and geophysical analysis, for less dense silicic magmas, it is natural to assume a shallower and smaller chamber(s). We assume separate chambers for dacite and andesite magmas to make the model simple (Yoshimoto et al., 2004), although the storage can be single storing both magmas (Watanabe et al., 2006). Prior to the eruption, basaltic magma in the deep chamber started to ascend as a dyke. The dyke formation apparently caused earthquakes and rumblings in early December or perhaps earlier as recorded in historical documents (Tsuya, 1955; Koyama, 2009), although these earthquakes are difficult to distinguish from the aftershocks of the Hoei Tokai Earthquake (28 October 1707; M = 8.4; e.g. Rikitake, 1999), which occurred at the subduction zone near the volcano and could be a trigger of the Hoei eruption. The basalt dyke reached the andesite magma chamber and the injection of basalt magma and consequent vesiculation of andesite magma increased the internal pressure of the chamber and andesite magma then ascended to intersect the dacite chamber (Yoshimoto et al., 2004; Watanabe et al., 2006; Fujii, 2007). The introduction of andesite lava also increased the internal pressure of the dacite chamber and triggered the dacite magma ascend (Fig. 15b). 7.2.2. Eruption sequence The eruption pulses of stage 1 seem to begin with maximum intensity followed by waning. This sequence was not that of intensive plinian eruptions, which is characterized by slow waxing stage (Scandone, 1996) but rather similar to that of basaltic eruptions, which is explained by release of overpressure from the magma chamber and elastic response of the chamber wall to the decompression (Wadge, 1981). The sequencial change of the initial two plinian eruptions are thus interpreted as tappings of the overpressured shallow chambers; first dacite then the second andesite (Fig. 15b, c). As the eruption continued, the overpressure of the shallow chambers lowered and the eruption intensity decreased gradually, and then, the less viscous basaltic magma rose past silicic magmas

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a) November or

b) Stage 1: Unit A-B

c) Stage 1: Unit C-F

Plinian eruption of silicic magma

Plinian eruption of silicic magma

early December ~ Magma ascent and frequent earthquakes in depth

Second crater Third crater

SE slope 0km Old magma chamber (gabbroic cumulate)

10km

Degassing

Degassing Degassed dacite magma

Extraction of gabbroic xenolith Mixing of silicic magmas

Dacite magma

Mixed silicic magma

Vesiculation in silicic chambers Andesite magma

20km

Degassing

Magma ascent via dyke formation (from November?)

d) Stage 1: Unit G-I

e) Stage 2: Unit J-M

f) Stage 3: Unit N-Q

Sub-plinian eruption of microcrystal-rich denser scoria (=degassed & cooled)

Intermittent sub-plinian eruptions and intrusion of degassed,cooled basaltic magma

Sustained sub-plinian eruption of volatile rich magma from depth

Dyke intrusion of degassed magma Degassed andesite magma

First crater

Formation of Mt. Hoei

Flank erosion of Mt Hoei

Degassing Less degassing due to wall insulation of precedent magma

Fig. 15. Schematic model of the magma plumbing system and its evolution throughout the Hoei eruption. White circles within magma indicate volatile content within magma, not necessarily bubbles.

and reached the surface (Fig. 15d). The basalt erupted during stage 1 suffered degassing and might have been cooled on its course to the surface as implied from its poor vesiculation and microlite rich

texture (Figs. 11 and 12). Stage 1 seems to have ended suddenly without a clear decline in intensity, which could be caused by conduit collapse.

N. Miyaji et al. / Journal of Volcanology and Geothermal Research 207 (2011) 113–129

Stage 2 was characterized by a succession of intermittent eruptions that could have reflected clearing of a clogged plumbing system. During the stage, the magma supply rate lowered slightly (0.8 × 10 11 kg/day); however, erupted vesicular scoria indicates that magma supply from deep chamber still continued in this stage. On the other hand, wide variety of scoria's BND and MND (Figs. 11 and 12) suggests that the residence time of basalt magma within the plumbing system was varied in this stage. In stage 2, some of degassed magma may have been injected laterally to form dykes. The amount of the lateral injection can be deduced as 0.4 × 10 11 kg/day from the difference between magma supply rate from deep chamber (1.2 × 10 11 kg/day; inferred from average magma supply rate of the whole eruption) and eruption rate during the stage. Since the stage continued for five days, the injected mass is calculated to be 2 × 10 11 kg or 8 × 10 7 m 3 DRE, which is close to the volume of Mt. Hoei (0.05 km 3). We thus propose that Mt. Hoei was formed by intrusion and updoming in this stage. In stage 3, the magma provided from depth rose smoothly and the eruption rate (1.1 × 10 11 kg/day) was close to the average of the Hoei eruption. The conduit wall covered by early magmas would have inhibited degassing and cooling of the newly derived magma, and would have allowed it to rise smoothly. The smooth rise of magma created the deposit that is rich in less degassed spiky scoria and poor in accidental material. The continuous supply of magma from depth enabled enduring eruption of magma during basaltic phase of stage1 and stage 3. However, some fluctuation of intensity, as seen in the spikes of mass flux (unit L, N, and P) and vertical oscillations in grainsize, may have been caused by hydrodynamic instability of viscous two-phase flow (e.g. Houghton and Gonnermann, 2008) and/or disparity between magma influx and outflux within the shallow conduit (Scandone and Malone, 1985; Bursik, 1993). The deep and voluminous chamber in our model can explain the stability of the magma supply rate with a steady pressure difference between the chamber and the surface as the driving force. 7.3. Some caveats to hazard mitigation plans Referring to the detailed Hoei eruption sequence, one might expect there will be several quiet periods throughout future intensive eruption such as seen in stage 2, in which emergency operations, such as unloading of ash from roofs and evacuation from isolated areas, can take place. However, our reconstruction of the eruption sequence indicates that the magma supply from depth could continue at almost the same rate throughout an eruption. The quiet periods within the Hoei eruption sequence seem to have depended on degassing mechanisms in relatively shallow areas and it is therefore difficult to give a feasible physical constraint to the modeling (Jaupart and Allegre, 1991; Woods and Koyaguchi, 1994). In future studies, reconstruction and modeling of shallow degassing processes (e.g. Suzuki and Fujii, 2010) will be critically important. Also important to this study is the timing of the end of the eruption. Our newly reconstructed sequence of mass discharge rate did not show any clear evidence of a downward tendency before the eruption ended. The eruption thus did not terminate by a loss of chamber pressure but rather by a conduit process, such as sudden wall collapse. If the basaltic lavas of Fuji volcano originated from deep, large magma chambers, as petrological and geophysical data suggest, we have to consider that future eruption volumes could be larger than that of the Hoei eruption, as withdrawal of such voluminous magma seems not to affect the supply rate from depth. This proposal may cause some disagreement with a common expectation that assumes the rareness of Hoei-class eruptions as known from volcano-historical statistics. Here, we do not intend to argue that the probability of such a large eruption is high. However, we would like to emphasize that it is inappropriate to expect a

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moderate eruption as the most possible next eruption of Fuji volcano, even if we have already had the largest eruptions twice in our historical era. 8. Conclusions (1) The mass discharge rate and the sequential change throughout the Hoei eruption of Fuji volcano in 1707, the largest ash fall disaster in modern Japanese history and the history of the volcano itself, was reconstructed based on geological investigations and examination of old documents. (2) The sequence of the eruption is stratigraphically divided into 4 groups and 17 units, and the mass of each unit was obtained from isopach analysis. For each of the unit boundaries, time is determined by correlation with historical documents, therefore allowing the durations of each unit to be established. (3) The mass discharge rates of each unit are obtained from unit masses and the length of time in which they were deposited, and column heights for each unit were calculated. These results show that the intensity of the Hoei eruption changed from Plinian in the initial silicic phases to subplinian in later mafic phases. (4) For unit A, we were also able to estimate column height by alternative methods. These results were consistent with those deduced using the mass discharge rate. (5) Based on the quiet intervals that are obvious from historical documents, we defined seven independent sequences of continuous ash fall, which we termed as pulses. (6) Based on the pattern of pulses, we subdivide the Hoei eruptive sequence into three stages. We interpret the three stages as follows: a) Tapping of shallow silicic magma chamber(s), which was overpressured by injection of basaltic magma from a deep massive chamber (stage 1) b) Discrete explosive eruption and lateral intrusion of degassed basaltic magma, which formed Mt. Hoei (stage 2). c) Continuous outflow of less viscous, volatile rich basaltic magma from depth through a stable conduit (stage 3) (7) Even though the mass discharge rates varied throughout the pulses and stages, the overall mass discharge rate remained constant at about 1.2 × 10 11 kg/day throughout the eruption. This implies that the large quantity of erupted material did not significantly affect eruption capability possessed by the magma chamber. In addition, the observation indicates that the mass discharge rate to the surface was principally controlled by shallow processes such as degassing, lateral crustal injection and conduit collapse. (8) The shallow processes such as degassing and lateral injection of magma contribute to make eruption characteristics difficult to predict. We thus have no reason to expect the next eruption to be milder even if we have had the largest eruptions twice in our historical era. Supplementary materials related to this article can be found online at doi:10.1016/j.jvolgeores.2011.06.013. These data include Google maps of the most important areas described in this article. Acknowledgment We thank Drs. Hans–Urlich Schmincke, Grant Heiken and Christina Magill for helpful comments and review on the manuscript. Through review by Professor David Pyle and an anonymous reviewer helped to improve this paper. Several figures in this paper were refined by Dr. Mari Sumita. NM thanks Professor Toshitsugu Fujii for fruitful discussions.

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Appendix A Reconstruction of eruption sequence based on historical documents Based on historical documents, the timing of each fall unit is reconstructed (Table 2). Here we give some notes on the source documents and methodology used to interpret timing. (6) Source documents There are many documents describing the Hoei eruption; however, the quality and accuracy vary. Thus meticulous examinations for each historical document were critical to reconstruct the time sequence of the eruption. The most complete listing and analysis of the historical documents are available in Koyama (2009). The documents we used here to give times for each unit boundary also appear in Koyama (2009). Since the historical documents were not based on scientific observation and include author's memory or hearsay information from others, the descriptions in the documents sometimes show significant contradiction. Although we selected reliable or reasonable descriptions on timing within the eruption based on a comparative review of documents and geological investigations, we admit that our time sequence contains some uncertainty. We thus acknowledge that the sequence presented in this paper is subject to future revision based on newly found documents or geological techniques. However, we believe that our estimations such as column height will not be significantly modified. To show the basis of our sequence reconstruction, the key descriptions used are shown in Table 2. Hereafter, we give some explanation for each document to allow critical review in future studies and help readers who are not familiar with Japanese language and history, to understand the valuable information in Koyama (2009). (1) Arai Hakuseki Nikki (AHD; Diary by Arai Hakuseki in Edo) The author of the diary, Arai Hakuseki (1657-1725), was a leading scholar of Edo middle period and promoted to a highlevel bureaucrat of Tokugawa Shogunate government after the eruption. The absence of ashfall in Edo on 29th and 30th is clear from the description in the diary. He attributed the reason to the strong north wind in those days. (2) Compilation from Fuji-Yoshida-Shishi (FYS; City of Fuji-Yoshida, 1994) This document is a facsimile of “Fuji-san Yakedashi no setsu no koto” (On the burn out of Mt. Fuji). The author is Fushimi Chubei and it had been held by the Yamaguchi family in FujiYoshida. Since Fuji-Yoshida is not in the area of ash fall, the description is assumed to come from someone's experience on the eastern flank of the volcano during the eruption. (3) Furizuna-ki (FZK; Memorandum of ash fall in Oyama) Furizuna-ki seems to be written in February 1716 by Tomihigashi Ittou Ou. The author's name is clearly a penname, which means “bald headed old living east of Mt. Fuji”. This document includes changes in grain size, which gives a good constraint on the early part of the eruption when compared to geological observation. (4) Ino Kageyu Nikki (IKD; Diary by Ino Kagetoshi in Sawara) Ino Kagetoshi (1668–1726), the author of the diary, is the head of a wealthy merchant family in the Sawara region, which prospered as a hub for river transportation on the Tone River. The diary began in 1698 and covered 20 years. The diary has two versions named ‘main’ and ‘summary’ but ‘summary’ is more descriptive for the beginning stage of the eruption. Ino Kagetoshi was also a collector of rocks and his collection includes the volcanic ash of the eruption, which still exists. (5) Ito Shimano-kami Nikki (ISD; Diary of Ito Suketaka in Edo) Ito Shimano-kami Sukekata is a direct retainer of the Tokugawa

(7)

(8)

(9)

Shogunate and his diary is the most detailed and reliable document of the eruption in Edo. His description is highly continuous throughout the eruption, including night times. The description is not only on ash fall but also atmospheric vibration, color of the volcanic cloud and its direction of movement, wind direction, color of ash, thunder and rainfall. Kashima-Han Nikki (KHD; Daybook of the Kashima Domain in Edo) This daybook is an assemblage of journals of everyday events in the residential office of Kashima Domain in Edo. From the assemblage, Koyama (2009) picked up “Abstract from the daybook of the Kashima Domain” and “Daybook of his Highness Naokata in Edo”, which depict the eruption from 16 to 28 of December 1707. We use the summary, which appears in Koyama (2009), to give constraint to eruption times of some events that occur in the latter half of the eruption. Ohmu-Rouchu-ki (ORK; Diary of Asahi Jyoemon Sigeaki in Edo) The author of this diary, Asahi Jyoemon Shigeaki, was a retainer of the lord of the Owari Domain. The diary is the second most descriptive document after ISD, and depicts situations of the residence office of Owari Domain in Edo and Nagoya, capital of the Owari Domain. Although the description is detailed, we concluded that the time shown in the description of the first day is behind about two hours, judging from the commencement time of the eruption. In this study, the duration of unit A is known based on the description of ash color change in this document. The name of the diary means “diary of a parrot in a cage”, which may be some irony of his life as a retainer samurai. Takiguchi-ke Monjyo (TGD; Documents of the Takiguchi family in Gotemba) The Takiguchi family is an old family in Gotemba that has kept a large number of documents including memorandum of the eruption used in this study. We adopted the time of the end of the eruption in the memorandum since this is the only clear description of ash fall termination. Tsuchiya Idayu Funka Jijyou-gaki (TIJ; Memorandum on the eruption of Mt. Fuji by Tsuchiya Idayu in Suyama) The author of the diary, Tsuchiya Idayu, was a mountain guide of Mt. Fuji who lived in Suyama, and helped people climb up to worship on the mountain. From this diary, intermittency of the middle part of the eruption (stage 2) is known.

Hour used in Edo era Since clocks were not common for ordinary people at that time, one may be skeptical about the reliability of time presented in this study. Although we cannot show the accuracy of time quantitatively, some explanation on the time counting method used at that time will be helpful to examine the eruption sequence proposed in this study. Before introduction of the Gregorian calendar in 1873, another type of counting for hour was used in Japan. The counting method, referred to as temporal hour, is based on the timing of sunset and sunrise. In this scheme, the daytime and nighttime are evenly divided into six units called “tsu” or “tsu-doki”. So durations of each unit differ between day and night, and season to season. When the eruption took place (December 16, 1707), the sunset and sunrise were 6:12 and 17:13 of present Japan Standard Time and the duration of “tsu” in day and night were approximately 110 min and 130 min respectively (Koyama, 2009). It is confusing but “tsu” starts at midnight and midday, where “tsu” is equal to 9, and decreases with time down to 4. People knew the time by the peals of bells installed in temples or time stations, since the number of peals meant the number of “tsu” plus 3. In the documents, the oriental zodiac was also used to indicate hours. The oriental zodiac is composed of 12 units and starts at midnight. The duration and boundary of an oriental zodiac is the same as “tsu”.

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This was the standard time system at that time; however, the rule shown above was not thoroughly used even in Edo and slightly different versions existed at that time. In addition, the system to broadcast the time in local areas at that time is unknown. So the time description shown in this study could include errors up to a few hours, and this likely to be the most significant source of error in the study. However, we also know that the time in historical document have some accuracy. For example, the commencement time of the eruption is recognized in Susono and Edo at the same time (4th tsu or the time of snake in the oriental zodiac). Thus, in this study, we did not attempt to reinterpretation time in the documents except in the case of ORK. As shown in Table 2, ash fall in Edo seems to occur approximately 2.5 hours after the sound of an explosion or ash fall in the near vent area. Thus we assume eruption time at the source from ash fall time in Edo, even if there is no document describing ash fall near the vent area. This is particularly true for units L and M, where conditions had been getting worse close to the volcano and the eruption was not as well recorded. References Aizawa, K., Yoshimura, R., Oshima, N., 2004. Splitting of the Philippine Sea Plate and a magma chamber beneath Mt. Fuji. Geophys. Res. Lett. 31, L09603. Bonadonna, C., Ernst, G.G.J., Sparks, R.S.J., 1998. Thickness variations and volume estimates of tephra fall deposits: the importance of particle Raynolds number. J. Volcanol. Geotherm. Res. 81, 173–187. Bursik, M.I., 1993. Subplinian eruption mechanisms inferred from volatile and clast dispersal data. J. Volcanol. Geotherm. Res. 57, 57–70. Bursik, M.I., Sparks, R.S.J., Gilbert, J.S., Carey, S.N., 1992. Sedimentation of tephra by volcanic plumes: I. Theory and its comparison with a study of the Fogo A Plinian deposit, Sao Miguel (Azores). Bull. Volcanol. 54, 329–344. Carey, S., Sparks, R.S.J., 1986. Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns. Bull. Volcanol. 48, 109–125. Collins, S.J., Pyle, D.M., Maclennan, J., 2009. Melt inclusions track pre-eruption storage and dehydration of magmas at Etna. Geology 37, 571–574. Costantini, L., Bonadonna, C., Houghton, B.F., Wehrmann, H., 2009. New physical characterization of the Fontana Lapilli basaltic Plinian eruption, Nicaragua. Bull. Volcanol. 71, 337–355. Costantini, L., Houghton, B.F., Bonadonna, C., 2010. Constraints on eruption dynamics of basaltic explosive activity derived from chemical and microtextural study: the example of the Fontana Lapilli Plinian eruption, Nicaragua. J. Volcanol. Geotherm. Res. 189, 207–224. Fierstein, J., Nathenson, M., 1992. Another look at the calculation of fallout tephra volumes. Bull. Volcanol. 54, 156–167. Fujii, T., 2007. Magmatology of Fuji Volcano. In: Volcanological Society ofJapan (Ed.), Fuji Volcano. Yamanashi Institute of Environmental Sciences, Fuji-Yoshida, pp. 233–244 (in Japanese with English abstract). Houghton, B.F., Gonnermann, H.M., 2008. Basaltic explosive volcanism: constraints from deposits and models. Chem. Erde 68, 117–140. Houghton, B.F., Wilson, C.J.N., 1998. Fire and water: physical roles of water in large eruptions at Taupo and Okataina. Water-Rock Interact. 1998, 25–30. Iida, A., Fujii, T., Yasuda, A., 2004. Melt inclusion study on two contrasting eruptions, the Jyogan and Hoei Eruptions at Mt. Fuji. Chikyu Monthly Extra Ed. 48, 131–138 (in Japanese). Inoue, K., 2007. Distribution of sediment-related disasters after the Hoei eruption of Fuji Volcano in 1707, based on historical documents. In: Volcanological Society of Japan (Ed.), Fuji Volcano. Yamanashi Institute of Environmental Sciences, Fuji-Yoshida, pp. 427–439 (in Japanese with English abstract). Jaupart, C., Allegre, C.J., 1991. Gas content, eruption rate and instabilities of eruption regime in silicic volcanoes. Earth Planet. Sci. Lett. 102, 413–429. Koyama, M., 2009. Eruption of Fuji Volcano and Hazard Map. Kokon Shoin, Tokyo. (in Japanese). Koyama, M., Maejima, Y., 2009. Estimate of the height of eruption column of the 1707 Hoei eruption of Fuji Volcano, Japan, based on historical documents and CG scenery reconstruction. Programme and Abstracts the Volcanological Society of Japan 2009, p. A8 (in Japanese). Lees, J.M., Ukawa, M., 1992. The south Fossa Magna, Japan, revealed by high-resolution P and S wave travel time tomography. Tectonophysics 207, 377–396. Machida, H., 1964. Tephrochronological study of volcano Fuji and adjacent areas. J. Geogr. (Tokyo) 73, 293–308 (in Japanese with English Abstr.). Mannen, K., 2006. Total grain size distribution of a mafic subplinian tephra, TB-2, from the 1986 Izu-Oshima eruption, Japan: an estimation based on a theoretical model of tephra dispersal. J. Volcanol. Geotherm. Res. 155, 1–17.

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