4-4. Research Project on Fuji Volcano

 

Mt. Fuji has been dormant since 1707, and some regarded it was a non-volcanic mountain. Recently, however, Mt. Fuji has been attracting attention as an active volcano. It is because a series of low-frequency earthquake was recorded at the depth of around 15 km just beneath Mt. Fuji from November 2000 through May 2001(Fig.1). In the 20 years of earthquake monitoring around Mt. Fuji, this was the largest and longest activity.  These twenty years, low-frequency earthquake was observed only a few times a month; however, the number exceeded 100 times a month in May 2001. Although the activity decreased after 2002, and now we can observe low-frequency earthquake only a few times a month again, it became obvious for many people Mt. Fuji is an active volcano. After this event, Japanese government and domestic counties around Mt. Fuji decided to issue hazard map on Fuji volcano. JMA is also planning to increase seismic monitoring stations within Fuji area.

One of the purposes of this research project is to establish seismic observation network designed to study the mechanism of low-frequency earthquakes and to determine the precise location where the earthquakes are generated. Another purpose is to elucidate the volcanic history of Mt. Fuji and to explore the possible style of next eruption. This research started at the end of 2001 as a three-year project.

Fig. 1. Seismic activities beneath Mt Fuji in the period of 1999-2001. The earthquakes  with magnitude less than 2 are low-frequency earthquakes and are concentrated in the depth around 15 km beneath the area around a few kilometers northeast of the summit.

 

4-4-1. Construction of seismic observation network

 

The main portion of the project is to bore holes in three locations, with an average horizontal separation of 2 km. The boring location is on the northeast slope of Mt. Fuji (Fig.2). Each hole will have a broadband seismometer installed at the bottom, and the deepest hole, more than 650 meters deep, will have a seismometer installed at a mid-way point as well. These four seismometers will form a three-dimensional array for monitoring earthquakes (Fig.3). With this set-up, it is expected to detect the direction from which the low-frequency earthquakes are being generated with little interference from undesired noise. At present, two of the borehole drillings have been completed, and seismometers have been installed at their bottoms (Fig.4).  The obtained data at these stations are sent to the station located at the northern foot of Mt Fuji by radio, and are sent to ERI by cable.

Fig. 2. Locations of boring sites.  Three boreholes, FJ-1 to FJ-3, are designed to form an underground array of seismometers.

 

Fig.3. Schematic view of the three-dimensional seismometer array. Broadband  seismometers placed at four points within the bore holes are designed to form tetrahedron, allowing to detect precisely the source area of low frequency earthquake.

 

Fig. 4. A seismic station constructed at the site FJ-2.

 

4-4-2. Study of volcanic history

 

The core samples extracted during boring will be analyzed to understand the past volcanic succession. As Mt. Fuji is a young volcano and it is only 300 years from the last eruption, it is not dissected deeply enough to study the older geologic section of the volcano. By studying the core samples, detailed eruption sequence before human recorded history will become available. In addition to the three boreholes mentioned above, another hole was bored near the Lake Yamanaka to examine the tephra layers deposited east of Mt. Fuji. It will provide reference samples because most of tepha have been deposited eastern area of the mountain carried by the west wind which blows almost through the year. Within a borehole of 75 meters in depth, a tephra sequence of these 25000 years has been recovered.

In Fig. 5, a part of the borehole samples are described. This boring site is located at the elevation of 1400 m above sea level. At a relatively shallow portion, we can detect the unconformity between the Older Fuji (Ko-Fuji) and the Younger Fuji (Shin-Fuji). Another interesting feature within this section is the presence of pyroclastic flow deposit. On the charcoal recovered from this deposit,  14C age was determined to be around 3500 yrs BP. Until recently, several pyroclastic flow deposits of around 3000 yBP have been recognized only in the western area. The finding of the pyroclastic flow deposit within the borehole indicates that the pyroclastic flow could be rather common at basaltic volcano with steep edifice like Mt. Fuji.

Another new finding is the wide compositional variation in the ejecta of Komitake volcano, which preceded the activity of Mt. Fuji and is buried in the edifice of Fuji volcano. After 350 m of boring, andesite lava and pyroclastic rocks were recovered. Their compositions are totally different from any rocks of Ko-Fuji or Shi-Fuji (Fig.6), and they contain hornblende as a phenocryst. It has been argued that Komitake is composed mainly of andesite lava with little compositional variation; however, poor exposure of Komitake have prevented us to investigate the compositional variation. The recovery of the hornblende-bearing andesite just beneath the Ko-Fuji sequence yields chance to study the magma activity prior to the Fuji volcano.

Fig. 5. Schematic description of recovered borehole samples at the site FJ-3. Pyroclastic flow deposit recovered at the depth of 8 m indicates the 14C age of 3500 BP.

 

Fig.6. Schematic columnar section at the site FJ-3 and the chemical compositions of representative samples. The lower portion of the core is composed of andesitic rocks with hornblende phenocryst, and is estimated to belong to the Komitake volcano which preceded the activity of Fuji volcano.

 

4-4-3. Study of volcano structure

 

The present project also includes the study of seismic structure beneath Mt. Fuji by accumulating the seismic data passing through the Mt Fuji. For this purpose, more than 70 seismic stations are incorporated (Fig.7). Almost half of them are newly deployed by this project. Installation of such many stations within short time was not possible without the cooperation with other national universities under the National Research Project for Prediction of Volcanic Eruptions. Waveforms recorded at several stations for one low frequency earthquake are shown in Fig. 8.

Fig. 7. Dense seismic network around Fuji volcano.  Twenty stations including two borehole stations have been operated since June 2002. Additional twelve stations are planed to be deployed around the volcano in 2003.

 

Fig. 8. Waveforms of a low-frequency earthquake recorded at several stations deployed by this project.  The magnitude is 1.6 and the focal depth is 16 km beneath Mt. Fuji.