109

Bulletin of Glaciological Research ,. (,**1) +*3ῌ+,+ῌJapanese Society of Snow and Ice

Dynamic observations focused on dry avalanche and occurrence conditions of largescale dry slab surface avalanche
Kazuya AKIYAMA+, Toshiya TAKESHI,, Masaaki HANAOKA- and Shinji IKEDA.
+ Niigata Experimental Laboratory, Public Works Research Institute, Arai, 3..ῌ**/+, Japan [Now in Research Center for Disaster
Risk Management, National Institute for Land and Infrastructure Management, Tsukuba, -*/ῌ*2*. Japan], Niigata Experimental Laboratory, Public Works Research Institute, Arai, 3..ῌ**/+, Japan [Now in Department of Water Induced
Disaster Prevention, Pulchowk, Lalitpur, Kathmandu, +-+*/ Nepal]- Snow Avalanche and Landslide Research Center, Public Works Research Institute, Myoko, 3..ῌ**/+, Japan. Argos Co., Ltd., Myoko, 3..ῌ***3, Japan(Received October -, ,**0; Revised manuscript accepted November ,1, ,**0)

Abstract
In order to clarify the release of avalanches every winter, observation site has been established incold snowy regions where the air temperature is below *ῌ almost all winter to observe the meteorological elements and avalanche occurrences using video camera and seismometers. These observations obtained records of ++/ dry surface avalanches, ,0 wet surface avalanches and 2- wet full-depth
avalanches. The form of large-scale avalanches is a dry slab surface avalanche and they werereleased at two slopes on .* degrees between elevations of +0/* and +2/* m a.s.l. The large-scaleavalanches with horizontal run-out distance longer than +*** m were almost all dry surface avalanches.
A study of calculated the snow stability index (SI) shows that SI was low and conditions for theavalanche release from inside snow cover accumulated during heavy snowfall were satisﬁed except
for two large-scale avalanches occurred under little snowfall when large-scale avalanches occurred.
The snow pit measurements near the release area of large-scale dry slab surface avalanches reveal thepresence of weak layers such as solid-type depth hoar or depth hoar and stability index of the weaklayers are low. Moreover, it is under the condition that solid-type depth hoar layers are apt to beformed around the avalanche release area by regularly measuring snow pits.
Therefore, the conditions for large-scale dry slab surface avalanches occurred in the newly
deposited snow layer are satisﬁed when snowfall is heavy, the release of large-scale avalanches ispresumably related to the existence of a weak layer such as solid type depth hoar or depth hoar.

+.

Introduction

The state of avalanche occurrences during winterseason are now determined mainly by surveys performed after avalanche disasters occur instead of bycontinuous monitoring at points or over wide areas.
When large-scale dry surface avalanches that cause
particularly severe disasters have occurred, the available information concerning such disasters is usuallyobtained after their occurrence. So the information
about air temperature and snow cover at the time ofavalanche occurrences near the release area and about
the characteristics of the avalanche itself can rarely
be obtained. Moreover, there is very little information about the state of avalanche occurrences during asingle winter in avalanche occurrence region and time-history information that shows changes in thenumber of occurrences in di#erent winters.
For that reason the acquisition of avalanche datais necessary, but it is di$cult to observe avalanche in
full-scale conditions owing to the experimental di$culties, dangerousness, cost, social condition, etc.
In Japan, several avalanche observations havebeen measured using visual observation and measuring avalanche tremors with other measuring instruments. The movement observation has been carriedout systematically on the purpose of researching especially powder dry surface avalanche named ‘Hou’ by
using many measuring instruments containing thevisual and avalanche tremor observations at Shiaidani in Kurobe, Toyama Prefecture (Kurobe Avalanche Measurement Group, +323). This research also
carried the artiﬁcial avalanche observation. Many
precious knowledge were gained especially in avalanche dynamics. A video camera and three seis-

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Bulletin of Glaciological Researchmometers were installed at Kanisawa-shinden in Shiozawa, Niigata Prefecture in +33* (Muramatsu, +33-).
Some wet full-depth and surface avalanches were detected. This research referred the detection level ofavalanche tremor compared with micro-tremor andartiﬁcial ground vibration. Dry snow avalanches observation has been carried out by using visual observation and avalanche tremors at Makunosawa in
Myoko, Niigata Prefecture (Takeuchi et al., ,**+).
These avalanche release data were compared withsnow stability index and snow pit wall measurements.
The avalanche release point and avalanche mass were
estimated by observing dry full-depth avalanches
measured in avalanche scale using , video cameras
and . seismometers in ,**+ at Toikanbetsu, Hokkaido(Imanishi et al., ,**.).
It is true a few observation cases of avalanches
are carried out even at present but the accumulationof avalanche data is needed to make clear the phenomenon of avalanche. It is best to observe avalanches themselves for the research, but the condition
that an artiﬁcial avalanche cannot be carried out because of dangerous and di$culties makes hard to
obtain avalanche data. Therefore, it is di$cult toobserve the avalanche occurrences and natural avalanches are the main target for avalanche research inJapan.
The authors have performed avalanche observations by the visual observation using video camera at++ points where were proved that an avalanche disaster broke out in the past or avalanche often occurred(Akiyama and Takeshi, ,**.). In these observation
points, the south slope of Happone in Hakuba Village,
Nagano Prefecture had been thought of large-scaleavalanche prone area. A large-scale avalanche broke
the wing of sabo dam on Kuzure-sawa River in thesouth slope of Happone in December +32+, and theaerial photograph could take the trace of large-scalesurface avalanche with run-out distance of ,-** malong Kuzure-sawa River in March +330. Only a visual observation of avalanches was started using video
camera installed at the point with a view of entire thesouth slope of Happone every winter from December+330. The scope of avalanche observation reaches at
an elevation of almost ,*** m a.s.l. on the south slopeof Happone that is a cold region with heavy snowfall
and where the winter temperature is usually below*ῌ. The video images have conﬁrmed that many
dry surface avalanches including large-scale dry slab
surface avalanches occurred (Akiyama and Takeshi,,**.). This observation point has three merits for the
research of natural avalanche.
+) Many scale natural avalanches occur.
,) It is possible to view all the avalanche movement
using one video camera from avalanche release area
to deposit area.
-) It is possible to arrive at a location close to ava-

lanche release areas throughout the winter using the
ski resort facilities, so the data of meteorological dataand snow condition near the avalanche release area
can be measured and commercial power source can beused for observation instruments.
Accordingly, a variety of ﬁeld measurements
newly performed along with video observation nearavalanche release and deposit areas began in December ,**,. This paper reports on the studies of thecharacteristics of avalanche occurrences at this siteand of the release mechanisms of dry surface avalanches up to March ,**/.

,.

Contents of the surveys and research

Outline of the avalanche observation area and contents of the observations
The avalanche observation site is a slope containing the drainage basin of Kuzure-sawa valley that ison the south slope of Happone, its elevation ranges
from 21* to +31* m a.s.l., its maximum width is ,./ km,
and its maximum slope length is ,.1 km (Fig. +).
The observation locations and items are shown in
Fig. ,. Visual observation of avalanches using video
camera from a point (location + in Fig. ,; inside
Hakuba .1 Ski Resort) with a distant view of theslopes began in December +330. The observation periods varied according to snow cover conditions each
winter, but extended from December to April centeredon January to March. The video observation of avalanches is able to monitor slopes from S+ to S- (Fig. ,)on a single screen. Many avalanches were releasedon slope S+; A and B are large-scale surface avalanche
release areas, and they sometimes reach to Hirakawa
River. On the slopes of S, and S-, large-scale avalanches have not broken out and avalanche occurrences were fewer than in area S+. The video images
help to guide understanding of the occurrences andform of avalanches. The video observations at thesite were based on recording one image every *./ to +second using time-lapse video that is capable of longterm recording, and beginning December ,**,, digitalrecording equipment was also used, making possibleof monitoring slopes where avalanches are released
throughout the winter without replacing recording
media for the long time.
In addition, it is possible to reach a location close
to avalanche release areas throughout the winter using the ski resort facilities, beginning in December,**,, meteorological observations, measurements ofsnow pit walls, and measurements of ground vibration caused by avalanches (avalanche tremor) were
performed directly in a higher place of the avalanche
release area (location , in Fig. ,). At the same time,measurements of avalanche tremors and snow pit
walls were performed in the vicinity of avalanche
pass or deposit area (location - and . in Fig. ,).
,.+Akiyama et al.

Two seismometers, Tokyo Sokushin VSE-+/D(range: *.,ῌ+**Hz, natural frequency: +Hz) and Mark
Products L-,,D (range: ,ῌ: .*Hz, natural frequency: ,Hz) were installed for measuring avalanche tremors at
location , and -, respectively. Tremors data were
measured on the vertical motion of ground vibration
every *.*+ second (+**Hz) recording at all times near
the avalanche release area (location ,) , and recording
only triggered when ground vibration exceeded thestandard value near the avalanche pass (location -).
These seismometers may catch all the ground vibration data including noises. The ground shock suchas mass snow falling from trees or movement of animals is able to be distinguished easily because thesewaveforms have the characteristics of short durationand instant vibration. If avalanches occur and ﬂow,

either or both seismographs sometimes measure avalanche movement. Earthquakes can also be detected
but two seismometers can not always catch thembecause of the di#erence in the ground conditions
where installed, characteristics of seismometers andrecording method. The Hakuba Earthquake Observation Station (E.HKB) of the Earthquake Observation Center of Earthquake Research Institute in the
University of Tokyo measured only earthquake records, so these records were used in order to extract
avalanche tremors exactly.
Snow pit walls were regularly measured in theinterval for +* to +/ days. Snow depth, temperature,grain shape, density, hardness and shear strength of
weak layer were measured.
Furthermore, meteorological records from automated meteorological data acquisition system of Japan Meteorological Agency (AMeDAS Hakuba: located elevation 1*- m a.s.l.) and meteorological data
observed Hakuba .1 Ski Resort (observed elevation++0* m a.s.l.) were used.
Analysis of the measured avalanche data
When the video recording media were replayedand video images for the release of avalanches until
their deposition were obtained, +) time of release, ,)forms, -) scale and location information, and .) motionof avalanches were revealed. Some image had only
the trace after the avalanche occurrence. In this case,+) and .) are not clear but it was possible to obtainrecords of ,) and -) as avalanche information.
The forms of the avalanche were classiﬁed using
those factors such as form of release (slab or point,.,

Fig. +. Avalanche observation site (south side slope ofHappone in Hakuba Village).

Fig. ,.

111

Location map of the avalanche observation points.

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Bulletin of Glaciological Researchstarting), water content of the snow (dry or wet) andposition of the slip (surface or full-depth). When theform of release is a small-scale surface avalanche, it
may be di$cult to perform a classiﬁcation. It can beassumed that it is dry snow when a surface avalanchethat is clearly accompanied by snow cloud, however,
it is di$cult to classify as dry or wet based only on theimage when it is a ﬂowing avalanche without a snow
cloud. In this case, the judgment is made based on
snowfall and changes in air temperature shown inmeteorological records obtained before the avalanche
occurrence.
The speciﬁcations such as release location andrun-out distance were calculated by setting the elevation of the release and deposition using +/+**** topographical maps. In addition, /* m mesh DEM (digital
elevation model) was used to obtain the slope inclination for each mesh to perform the inclination classiﬁcation of the entire slope that was observed.
Video observation often fails to watch slopes
when snow is falling or at night. So when an investigation was done to determine if it is or is not possible
to see an avalanche release slope in one hour units innine winters from December +330 to March ,**/, it
was possible to monitor the slope for between ,2ῌand /0ῌ of the time (an average of approximately.*ῌ) during a single winter. Surface avalanches
di#er from full-depth avalanches in that the ground is
not exposed where they breaks out. If small-scaleavalanches break out, surface avalanches are hard to
be recognized rather than full-depth avalanches because it is di$cult to see judging from the color tones.
Therefore, it is impossible to record all avalanches byvisual observations. So the avalanche occurrences
are detected by observing avalanche tremors using
seismometer that can be detected when observation
by video is impossible. Using video and seismometer
together for avalanche detection is an e#ective method and various analyses were done by combining
both data (Sabot et al., +332; Imanishi et al., ,**.).

-.

Fig. -. Inclination of the observed slopes (elevation of
3** m a.s.l. or higher).

Fig. ..

State of avalanche occurrences by forms.

Characteristics of detected avalanches

Figure - shows the results of classifying the inclines in / degrees classes of those slopes higher than
3** m where avalanches occurred. Observing slopes
were classiﬁed as slope S+ where many avalanches
were released and the other slopes S, and S-, but thecharacteristics of the inclination frequency distribution are similar, the commonest inclinations are between -/ and .* degrees, and almost none are morethan /* degrees. The inclination on slope S+, A andB, where large-scale surface avalanches occurred released is approximately .* degrees.
The results of the observations clariﬁed the speciﬁcations of avalanches in ,,. cases (+.+ surface avalanches, 2- full-depth avalanches) until March ,**/.

Fig. /. Relation between horizontal run-out distanceand vertical di#erence in elevation of avalanches.

Figure . shows the state of avalanche occurrences
every month by form of these avalanches. At thissite, during the observation period, the most common
form of avalanche was dry surface avalanche andmostly occurred in February, with none occurring in
April. Many full-depth avalanches were broken out
since March.
Figure / shows the relationship of the horizontal
run-out distance by form of avalanche with the elevation di#erence from the avalanche release area to theAkiyama et al.

deposit area. In the case of a large-scale avalanche,
the run-out distance is lengthened along its course.
The run-out distance of almost all avalanches wasapproximately up to 1** m, and the wet full-depth
avalanches are all in this range. Large-scale avalanches with run-out distance exceeding + km were
dry surface avalanches excluding one wet surface
avalanche. In case of dry surface avalanches, run-out
distance exceeding ,/** m also occurred.
Figure 0 shows the relationship with horizontal

run-out distance and release area elevation of avalanches. The avalanche release points of almost all
surface avalanches are between elevations from +-**
to +3** m a.s.l., but large-scale cases occurred on slopesat A (approximate elevation of +2/* m a.s.l.) and B(approximate elevation of +0/* m a.s.l.) in Fig. ,. Wet
full-depth avalanches occurred at elevations between+*** and +0** m a.s.l., but not above +0** m, and many
avalanches broke out on parts other than the slopes of
A and B. The wet surface avalanches show characteristics intermediate between dry surface avalanches
and wet full-depth avalanches.

..

Fig. 0. Relation between horizontal run-out distanceand release elevation of avalanches.

Fig. 1.

113

Examples of the large-scale avalanche occurrence

Table + shows the large avalanches in the detected data that occurred during each of the past six
winters. It was possible to obtain video records fromthe release to the deposition of only three large-scaleavalanches that occurred in February ,*** and in
March ,**-; the rest were judged based on video images of avalanche trace, ﬁeld surveys during measurements of snow pit walls, and avalanche tremor observations. During the winter period from December

Avalanche tremors of large-scale dry slab surface avalanches released from slope A.

Table +.

Examples of Large-scale Avalanche.

114
Bulletin of Glaciological ResearchAkiyama et al.

,*** to April ,**+, the largest avalanche was a wet
slab surface avalanche, but the others were dry slab
surface avalanches.
On January ,., ,**. and January +,, ,**/, recordsof avalanche tremor of large-scale surface avalanches
were obtained. There are no video records of avalanches when they occurred because snow was falling,
but visible images after the avalanche occurrences
and ﬁeld surveys have shown that two dry slab surface avalanches broke out from the slope A. Figure 1
shows avalanche tremor records at those times. The
wave form in the avalanche pass area (location -)shows characteristic in spindle shape, but near the
release area (location ,), there is no distinctive shape,
it is unclear, and even the wave forms of the micro
tremors are disrupted. This is a result of the fact thatthe avalanche tremors were observed in a higher
place of the slope B, it is estimated that vibration
declines because of a long distance from release areain the slope A. On December ,*, ,**-, large-scalesurface avalanche occurred from slope B and seismometer in a higher place of the avalanche release
area caught the avalanche tremor. Unfortunately,
another seismometer was o# line so it is not clear if
two seismometers could catch the avalanche tremor
simultaneously.

/.

Comparison of large-scale avalanche occurrences and snow stability index

Snow stability index
Snow stability index (SI) in the snow on the slope
is represented as follows based on the shear stress t(N mῐ,) and the shear strength SFI (N mῐ,) inside thesnow layer (Roch, +300).
/.+

SIῒSFIῌt῍
The shear stress t is the force component in the direction of incline of the snow load per unit of horizontal
area of the snow on a slope, and if the inclination isrepresented by q;tῒWῌcosqῌsinq῍Danger of avalanche occurrence is high when SI becomes smaller than . (Roch, +300).
Shear strength SFI was measured using the ,/*cm, shear frame (Sommerfeld, +32.) and comparing
many avalanche occurrences and SI using the datameasured by the ,/* cm, shear frame; danger of surface avalanche occurrence was critical when SI became smaller than +./ (Schleiss and Schleiss, +31*;
Perla, +311).
Regarding that snowfall changes from new snow
to lightly compacted snow, then to compacted snow
under the condition of equi-temperature metamorphism in snow, stability of snow on the slope is able tobe calculated. Shear strength of each snow layer is115

able to be estimated based on the concept of changing
snow density according to viscous compression theory of snow and the relation between shear strengthand density of snow (Endo, +33-). The calculation
method of SI was described in detail mainly reviewed
based on Endo, +33-, what is more, it was added to theideas of the coe$cient changed by snow temperature
between compressive viscosity and snow density up
to -**kg mῐ- , calculating snow temperature for judging dry or wet in a snow layer and calculating initialsnow density using snow temperature (Suizu, ,**,).
In case of calculating SI at Happone, air and snow
temperature were usually below *ΐ and snow density
sometimes became more than -**kg mῐ-. Accordingly, only a dry snow is thought and another relationship between the coe$cient of compressive viscosity and snow density (Kojima, +301) is used whencalculated density exceeds -**kg mῐ-. In accordancewith this method, stability index (SI) of each snow
layer in the snow cover was calculated at intervals of
six hours from December to March of each winter andthese values were compared with the occurrences ofavalanche. The initial snow density was assumed tobe 0*kg mῐ- and snow layer inclination was adopted
to .* degrees which equals to the inclination of slopes
A and B where large-scale surface avalanches werereleased. The rainfall was set by a correction basedon wind speed (Ohno et al., +332) as snowfall in a caseof air temperature of ,ΐ or less. The snow depth andsnowfall obtained by calculating were estimated values at the elevation of +1** m a.s.l. in the avalanche
release areas. Therefore, records of the snow depth
and snowfall (measured twice a day) at the Hakuba .1
Ski Resort (++0* m a.s.l.) near the avalanche release
area were compared to assess the suitability of thecalculation.
As meteorological and snow conditions for the
calculation for SI, records of observation at location ,in Fig. , were used as meteorological informationfrom December ,**,. Data from AMeDAS Hakuba
was used up to that time, so for air temperature andrainfall were estimated using correlation between location , and AMeDAS Hakuba meteorological datafrom December ,**, to March ,**/. Data of air temperature (ΐ) and rainfall (mm) were calculated asfollows.
Air temp῍ ῎location ,῏ ῒ*.3ῑAir temp῍ ῎AMeDAS Hakuba῏ ῐ..0ῌ
Rainfall῎location ,῏ῒ+.,0ῑRainfall῎AMeDAS Hakuba῏The snow stability index was calculated by obtaining
the SI of the bottom surface of each layer at intervalsof six hours, then obtaining the minimum SI value ofall SI values within the snow layers as the snow
stability index of the snowpack.
Next part explains meteorological conditions,snow and avalanche release conditions, and change ofthe snow stability index from December to March116

Bulletin of Glaciological Researchduring the winters, +333 to ,***, ,**+ to ,**,, ,**- to,**., and ,**. to ,**/, when large dry slab avalanches
exceeding +*** m in run-out distance occurred.
Results for the winter of +333 to ,***
During the winter of +333 to ,***, only video
observation was made and the meteorological conditions were represented by corrected values from AMeDAS Hakuba. Video observation found +0 dry surface avalanches and 0 wet full-depth avalanches (Fig.
2). On February / and ,-, large-scale dry slab surface
avalanches were released (No. + and , in Table +).
Snow stability index at release time was ..0 on
February / and /., on February ,-, and concentrated
snowfall did not occur immediately before these avalanches. During this winter, snow pit walls were notmeasured, so it was not clear if there were any weaklayers.
/.,Results for the winter of ,**+ to ,**,
During the winter of ,**+ to ,**,, only video
observation was done again and the meteorologicalconditions were set using corrected AMeDAS Hakuba
data. The video observation found 3 dry surface avalanches and 0 wet full-depth avalanches (Fig. 3). It
was determined that on January 0, a large-scale surface avalanche (No. . in Table +) was released at theslope at B in Fig. ,, but because of continuous snowfall that started +/: ** on January ., it was impossible
to obtain video image of this avalanche, so the precise
time of its release was not known. The series ofsnowfall that ended this period of snowfall increased
the snow depth by ., cm (added up di#erence perhour of 0* cm and precipitation of 0- mm) according tovalues measured by AMeDAS Hakuba. Snow stability index from January . to 0 was within a range from+.* to ,.3, so it can be estimated that the period had
been under the condition of avalanche danger. The/.-

Fig. 2. Meteorological conditions, variation of stability index and avalanche detections (Dec.
+333ῌMar. ,***)(a) Air temperature and rainfall at the avalanche release area (+1** m a.s.l.) estimated by
AMeDAS Hakuba meteorological data.
(b) Height of snow observed at the meteorological station (++0* m a.s.l.) in Hakuba .1 Ski
Resort.
(c) Height of snow on the avalanche release area (calculated).
(d) Snow stability index (calculated).
(e) Avalanche detection by the visual observation.

Akiyama et al.

117

Fig. 3. Meteorological conditions, variation of stability index and avalanche detections (Dec.
,**+ῌMar. ,**,).
(a) Air temperature and rainfall at the avalanche release area (+1** m a.s.l.) estimated by
AMeDAS Hakuba meteorological data.
(b) Height of snow observed at the meteorological station (++0* m a.s.l.) in Hakuba .1 Ski
Resort.
(c) Estimated height of snow on the avalanche release area (calculated).
(d) Snow Stability index (calculated).
(e) Avalanche detection by the visual observation.

snow pit wall measurements performed near the large-scale avalanche release area on January 0 revealthat there were solid-type depth hoar layers with total
thickness of 3+ cm below +/* cm from the snow surface.
Results for the winter of ,**- to ,**.
The video observations conﬁrmed +/ avalanches
by April -, but almost all were small-scale full-depth
avalanches (Fig. +*). Avalanche tremors were observed .+ times above the avalanche release areas(location ,) ,0/ times in deposit areas (location -) and+1 times in both areas, but judging from a ﬁeld survey
and avalanche tremor results, two large surface avalanches occurred on December ,* and January ,.,when video observations could not record images (No.
0 and 1 in Table +).
The series of snowfall ending on December ,*/..

when the avalanche occurred increased the snow
depth prior to the avalanche release by .- cm according to AMeDAS Hakuba measurement values (added
up di#erence per hour .0 cm, precipitation of -3 mm).
The measured precipitation based on meteorologicalobservations at location , was 30 mm. Snow stability
index at the time of the avalanche occurrence was *.3that is less than +.*.
The series of snowfall prior to the avalanche release on January ,. increased the snow depth prior to
the avalanche release by /0 cm according to AMeDAS
Hakuba measurement values (added up di#erence perhour /3 cm, precipitation of .- mm), and the measured
precipitation at location , was -/ mm. Snow stability
index at the time of the avalanche release was +.+. In
both cases, it can be estimated that the snowfall period had been under the condition of avalanche danger.

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Bulletin of Glaciological Research

Fig. +*. Meteorological conditions, variation of stability index and avalanche detections(Dec. ,**-ῌMar. ,**.).
(a) Air temperature and rainfall at location , (+1** m a.s.l.).
(b) Height of snow observed at the meteorological station (++0* m a.s.l.) in Hakuba .1 Ski
Resort.
(c) Height of snow on the avalanche release area (calculated).
(d) Snow stability index (calculated).
(e) Avalanche detection by the visual observation.
(f) Avalanche detection by the seismometers.

The snow pit wall measurements were not donenear the avalanche release area after the occurrencesof large-scale avalanche, each four times near the
release area (location ,) and near the deposit area
(location .) in Fig. ,. The stratigraphies of snow near
the release area were formed of mostly new snow andcompacted snow with a weak layer of solid-type depth
hoar. However, the snow structure near the deposit
area was mostly formed compacted snow and granular snow. There were cases of snow temperature of*ῌ and no weak layer like that found near the release
area was detected.

During this winter, video images revealed a wet
surface avalanche and four wet full-depth avalanches
triggered by the e#ects of a rise of the air temperatureand rainfall caused by low atmospheric pressure fromFebruary ,* to ,,. These avalanches broke out atelevations lower than +./* m a.s.l. Many avalanche
tremors recorded during this period were detected in
the low elevation deposit area (location -) from February ,+ to ,-.
/./Results for the winter of ,**. to ,**/
During this winter, no avalanche occurrence wasAkiyama et al.

119

Fig. ++. Meteorological conditions, variation of stability index and avalanche detections(Dec. ,**.ῌMar. ,**/).
(a) Air temperature and rainfall at location , (+1** m a.s.l.).
(b) Height of snow observed at the meteorological station (++0* m a.s.l.) in Hakuba .1 Ski
Resort.
(c) Height of snow on the avalanche release area (calculated).
(d) Snow stability index (calculated).
(e) Avalanche detection by the seismometers.

detected during the period when video observation
was possible, so the number of detections by avalanche tremors were compared (Fig. ++). Avalanche
tremor observations were not performed after February 2. The large-sale surface avalanche occurred on
January +, (No. 2 in Table +). The continuous snowfall up to the release time increased the snow depth by
1* cm (added up di#erence per hour of 1- cm, precipitation of 0/ mm) according to values measured by
AMeDAS Hakuba, and +,+ cm (added up di#erence perhour of +,. cm, precipitation of 12 mm) according tovalues measured at location , and the SI was +... On
January +,, Avalanche tremors were detected ﬁvetimes near the release and deposit area (location , and-) at the same time including the large-scale surface
avalanche, so it can be estimated that the period had
been under the condition of avalanche danger.
Measurements of snow pit walls were performed

each four times near the release area (location ,) andthe deposit area (location .) on and later than January+3 after the large-scale avalanche. In the former case,there was a minus snow temperature and forming
solid-type depth hoar layers. In the latter case, thesnow temperature was also minus but granular snow
and compacted snow were dominant and solid-typedepth hoar was not formed. On January +3, anothersnow pit wall measurements were performed near thearea of large-sale surface avalanche release. Its measurements reveal that there was solid-type depth hoar
and depth hoar layers with a total thickness of .3 cm
on the ground surface.

0.

Snow stability index and snow pit wall measurements
The snow wall pit measurements near the large-

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Bulletin of Glaciological Research

Fig. +,. Snow stability index calculated by the meteorological data, snow pit walls measurement
after the release of large-scale dry slab surface avalanches and snow stability index
measured by shear frame.

scale dry slab surface avalanche release area could bedone in January ,**, (No. . in Table +) and in January,**/ (No. 2 in Table +). Fig. +, shows the snow stability index at the time of large-scale avalanche occurrences and the stratigraphies of snow measured bysnow pit wall performed near the release area of largescale avalanches after its occurrences.
The trace of large-scale avalanche No. . was dis
covered in the morning on January 0 but it was impossible to obtain video image because of snowfall.
Snow pit wall near the release area was measured on
January 0, ,**,. Proﬁles of SI and stratigraphies ofsnow related avalanche No. . were shown in the left
ﬁgure. Two proﬁles of SI are shown when the maximum height of snow was appeared (+// *:**) andsnow pit wall was measured (+/0 +,:**). The value of
SI was low at the upper parts of the snow and thisperiod had been under the possibility of avalanche
occurrence.
The lower parts of the snowpack were almost all
consisted of solid-type depth hoar by measuring snow
pit. The weak layer was found about +0+ cm under
the surface of snow cover by strength test of weaklayer. Stability index of the weak layer was *.3 bymeasuring shear strength SFI using the ,/* cm, shear
frame and snow load on the spot.
The large-scale avalanche No. 2 was detected byseismometer January +,, ,**/, but it was impossible toobtain video image because of snowfall. Snow pit
wall near the avalanche release area was measured on
January +3. Proﬁles of SI and stratigraphies of snow
related avalanche No. 2 were shown in the right ﬁgure. Two proﬁles of SI are shown when the avalanche occurred (+/+, 0:**) and snow pit wall wasmeasured (+/+3 +,:**). The value of SI when thelarge-scale avalanche occurred is low at the upperparts of the snow and the new snow snowpack had
the possibility of avalanche occurrence.
The lower parts of snowpack were consisted ofsolid-type depth hoar and depth hoar by measuringsnow pit. The weak layer was found in the depthhoar layer about +*2 cm under the surface of snow
cover by strength test of weak layer. Stability index
of the weak layer was +.2 by measuring shear strength
SFI using the ,/* cm, shear frame and snow load on
the spot.
A study of the snow stability index calculated bymeteorological data shows that the stability index
inside snow layers was low and conditions for the
release of an avalanche from inside snow cover accumulated during heavy snowfall were satisﬁed otherthan two large avalanches that occurred in February,*** (No. + and , in Table +). However, the snow pit
wall measurements near the large-scale avalanche release area reveal the existence of solid-type depthhoar or depth hoar that is the weak layer of theavalanche, and it is assumed that a rise of the load
produced by the heavy snow caused the avalanchewith this layer as the slip layer. Moreover, manysolid-type depth hoar layers were found in the regularly snow pit wall measurements at location ,.
These layers had +ῌ,, cm of thickness and the weaklayers detected strength test of weak layer had stability index from +.0 to /.,. Although the avalanche
observation area is the south slope of Happone in thisstudy, solid-type depth hoar or depth hoar were conﬁrmed on the north slope of Happone by the investigation as to the avalanche accident (Ikeda, ,**,). For
these reasons, it is under the condition that weaklayer such as solid-type depth hoar or depth hoar isAkiyama et al.

apt to be formed around the large-scale dry slab surface avalanches release area.
If cases where large-scale avalanches occur evenwhen little snow has fallen and the snow stability
index is high are included, large-scale dry slab surface
avalanches are presumably caused mainly by weaklayers that are solid-type depth hoar or depth hoar.
However, it can be concluded that conditions for theoccurrence of large-scale dry slab surface avalanches
are able to be caused only by heavy snowfall even
without existing a weak layer.

121

Acknowledgments
The authors thank the establishment of observation sites for Hakuba .1 Ski Resort and Happone Ski
Resort, and thank using observation data for Hakuba.1 Ski Resort and Earthquake Observation Center ofEarthquake Research Institute in the University ofTokyo. They also thank for ﬁeld survey assistanceto S.Sato, T.Uchida and Y.Naito.

References
1.

Conclusions

Although the slopes observed were wide and theobservation items were not necessarily a uniform observation system because it was periodically improved, the state of avalanche release and the characteristics of large-scale avalanches were clariﬁed. The
main conclusions from this study are as follows.
ῌThe avalanche observations obtained records of ,,.
avalanches and ++/ (/+῍) of them were dry surface
avalanches. Many dry surface avalanches occurredin February.
ῌThe large-scale avalanches with horizontal run-out
distance longer than +*** m were all dry surface avalanches excluding one wet surface avalanche.
ῌThe large-scale dry slab surface avalanches occurred on two slopes at elevations between +0/* and+2/* m a.s.l. The wet full-depth avalanches did notoccur above an elevation of +0** m a.s.l.
ῌThe occurrences of large-scale dry slab surface avalanche include cases that can be explained as theresult of a decline of snow stability index inside snow
layers accumulated by heavy snowfall. However,there are also cases when avalanche occurred without
heavy snowfall and when the minimum snow stability index inside the snowpack was high. These cases
are di$cult to explain as those released only by falling snow.
ῌRegular measurements of snow pit show that the
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ῌThe results of snow pit measurements near the
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depth hoar layer and stability index of weak layer islow. So the conditions for large-scale dry avalanches
occurred in the newly deposited snow layer are satisﬁed when snowfall is heavy, the release of large-scale
dry slab surface avalanches is presumably related tothe existence of a weak layer such as solid type depthhoar or depth hoar.

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