Avalanches induced by earthquake in North Tochigi prefecture on 25 February 2013

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1 Interntionl Snow Science Workshop Grenole Chmonix Mont-Blnc - 3 Avlnches induced y erthquke in North Tochigi prefecture on 5 Ferury 3 Hiroki Mtsushit, *, Shinji Iked, Ysuhiko Ito, Msru Mtsuzw nd Hiroshi Nkmur Civil Engineering Reserch Institute for Cold Region, PWRI, Spporo, Jpn Snow Avlnche nd Lnd Slide Reserch Center, PWRI, Myoko, Jpn ABSTRACT: We hve investigted chrcteristics of vlnches induced y the North Tochigi Prefecture erthquke with mgnitude of 6.3 on Ferury in Jpn nd oserved snowpck structure t vlnche strting zone. The results indictes tht vlnches occurred in entire re long the rod with length of out 8 km locted on the seismic center nd the numer of vlnches including smll scle collpse of snow on slope ws or more. Surfce vlnches mostly occurred nd hd one or two ed surfces. These ed surfces corresponded to wek lyers which consisted of fceted crystls with low hrdness. We lso evluted stility of snowpck on slope under the erthquke nd suggested tht the vlnches will e induced y decrese in the stility with increse in stress to snowpck due to ccelertion cused y the erthquke. KEYWORDS: Erthquke-induced vlnches, Snow pit oservtion, Stility index Corresponding uthor ddress: Hiroki Mtsushit, Civil Engineering Reserch Institute for Cold Region, PWRI, Spporo, Jpn; tel: ; fx: ; emil: hmtsu@ceri.go.jp INTRODUCTION Erthquke with mgnitude of 6.3 occurred in the northern prt of Tochigi prefecture, Jpn, t 63 JST on 5 Ferury 3. The erthquke induced mny snow vlnches nd lndslides round Okukinu-Onsen sp ner the seismic center (36.9 o N, 39.4 o E). In prticulr, rod to the sp ws filled due to much snow deris of vlnches, so tht 5 persons styed in the Okukinu-Onsen sp were isolted with no ssistnce during 4 hours until much snow ccumulted on rod hd een removed from the rod. The erthquke-induced snow vlnches hve een reported in Alsk on 7 Mrch 964 during the Gret Alsk Erthquke with mgnitude of 9.; the Western Himly, Indi; the Skhlin Islnds, Russi; Jpn nd so on. Using the erthquke-induced vlnche events, Podolskiy et l. () investigted the reltionship etween the occurrence of the erthquke-induced snow vlnches nd the mgnitude of erthquke, the distnce from the seismic center. Kmiishi et l. () conducted snow pit oservtions t strting zone of vlnches induced y the North Ngno prefecture erthquke with mgnitude of 6.7, occurred on Mrch in Jpn. They lso nlyzed the stility of snowpck on slope sed on the results of the snow pit oservtions y considering the horizontl ccelertion of seismic ground motion. In ddition, there re previous ttempts to evlute the effects of seismic ground motion on the snowpck stility concerning the erthquke-induced vlnche occurrences y theoreticl exmintions (Ogur et l., ; Mtsuzw et l., 7) nd lortory experiments using shking-tle (Podolskiy et l., ). However, these evlutions for the erthqukeinduced vlnche occurrences hve not een verified enough ecuse snow profiles otined y snow pit oservtion t the strting zone of erthquke-induced vlnches re very few (Higshiur et l., 979; Ogur et l., ; Kmiishi et l., ). In this pper, we investigted chrcteristics of vlnches induced y the North Tochigi Prefecture erthquke on Ferury nd oserved snowpck structure t strting zone of vlnche. We lso considered n effect of seismic ground motion on snowpck stility concerning the occurrence of erthqukeinduced vlnche sed on the result of snow pit oservtion. CHARACTERISTICS OF AVALANCHES INDUCED BY THIS EARTHQUAKE. Investigtion on vlnche chrcteristics Investigtion on the chrcteristics of vlnche occurrence induced y the North Tochigi prefecture erthquke ws conducted long the rod with length of out 8 km in the Okukinu- Onsen sp ( 3 m.s.l.) on 8 Ferury 3, three dys fter the erthquke (Figure ). We ttended to the points t ed surfce in vlnche strting zone nd much deris snow on rod. In the investigtion, snowpck condition on the level ground ws lso oserved.

2 Interntionl Snow Science Workshop Grenole Chmonix Mont-Blnc - 3 N Snow pit oservtion (36 o N, 39 o E) 4 o N Seismic center of the North Tochigi prefecture erthquke (36.9 o N, 39.4 o E) 4 o N 38 o N Seismic center rod X X 36 o N 34 o N 5m 3 o N 34 o E 36 o E 4 o E 44 o E Copy right; The Geosptil Informtion Authority of Jpn (GSI) Figure. Investigtion re nd site of snow pit oservtion.. Chrcteristics of vlnches The vlnches induced y this erthquke were oserved in the entire re long the rod. The numer of vlnches including smll scle collpse of snow on slope ws or more. Figure. Exmple of deris snow ccumulted on rod due to vlnche (). Deris snow ccumulted on the rod hd the height of m or more (Figure ) nd the length long the rod of pproximtely 6 m (Figure 3) t the res such s pth of vlley (Figure 4). Surfce vlnches minly occurred nd hd one or two ed surfces (Figures 5 nd 6). Crown surfces of the snow vlnches were stepped nd irregulr in form. Snowpck filure t its se (like smll full-depth vlnche) nd the collpse of lock-like snowpck were lso confirmed on slope covered with low moo ushes nd grss (Figures 7 nd 8). Crcks were formed t rndom in snowpck on not only slope (Figure 9), ut lso level ground (Figure ). According to Kmiishi et l. (), the chrcteristics of snow vlnches nd collpse of snowpck mentioned ove hd een lso oserved s the North Ngno prefecture erthquke occurred on Mrch. Relesing mny types of snow vlnches nd snow collpses re the chrcteristics when the erthquke-induced vlnche occurs. Figure 3. Exmple of deris snow ccumulted on rod due to vlnche (). Figure 4. Exmple of deris snow tht pssed down on vlley. 3

3 Interntionl Snow Science Workshop Grenole Chmonix Mont-Blnc - 3 Figure 5. A cse of surfce vlnche with single ed surfce. Figure 8. Snowpck filures t its se on the ground surfce nd collpses of lock-like snowpck on slope covered with low moo ushes nd grss. Figure 6. A cse of surfce vlnche with two ed surfces. Snow pit oservtion ws crried out on this slope. Figure 9. An exmple of crcks formed within snowpck t rndom on slope. Figure 7. Collpses of lock-like snowpck. Figure. An exmple of crcks formed within snowpck t rndom on level ground. 4

4 Interntionl Snow Science Workshop Grenole Chmonix Mont-Blnc SNOW PROFILE AT STARTING ZONE OF AVALANCHE 3. Snow pit oservtion We crried out snow pit oservtion on the slope t strting zone of vlnche (36 o N, 39 o E) on 8 Ferury 3 (Figures nd 6). Slope incline is 4 o nd spect of slope is the north. In the snow pit oservtion, verticl profiles of temperture, density nd hrdness of snowpck were mesured nd snow grin s shrp nd size were oserved. Snow temperture ws mesured using thermistor thermometer nd snow density ws otined y using rectngulr smpler with cpcity of cm 3. Snow hrdness ws given from mesurement of resistnce s circulr ttchment of force guge with dimeter of 5 mm insert into snowpck (Tkeuchi et l., 998; Höller nd Fromm, ). Hnd test ws lso crried out to judge the snow hrdness for ech snow lyer. Shpe nd size of snow grin were oserved y using loupe with mgnifiction of. In ddition, sher frme index (SFI) (Perl et l., 98) ws mesured for the ed surfce of vlnche using the sher frme with re of.5 m. Men of SFIs mesured in 5 or 6 times were used. 6 4 Grin size (mm) Symol nd Clss - Precipittion Prticles 3 mm WL.-.3 / WL i i Decomposing nd Frgmented precipittion prticles Rounded Grins Fceted Crystls Melt Forms Ice Formtions i Grin shpe 3 mm Figure. Snow structure t the strting zone of vlnche on 8 Ferury 3. WL nd WL men the wek lyers nd, respectively. Figure 3. Photogrphs of snow grins in snow lyers t heights of () 84 cm nd () 39 cm from the ground c Snow surfce Bed surfce Bed surfce d WL WL Temperture ( o C) Density (kg m -3 ) Hrdness (kn m - ) F 4F F P K I Hnd hrdness (code) Figure. Verticl profiles of () temperture, () density, (c) hrdness nd (d) hnd hrdness of snowpck t the strting zone of vlnche on 8 Ferury 3. Broken lines indicte the heights of ed surfces. 5

5 Interntionl Snow Science Workshop Grenole Chmonix Mont-Blnc Snowpck profile nd structure On slope in where snow pit oservtion ws crried out, there were two ed surfces nd crown surfce ws formed like step (Figure 6). Figure shows snowpck structure nd Figure indictes verticl profiles of temperture, density nd hrdness of snow. Snow depth ws 39 cm. Snow lyers, which correspond to two ed surfces, were locted t heights of 39 cm nd 84 cm from the ground nd consisted of fceted crystls (Figure 3). Hrdness of two snow lyers, existed on the ed surfces, were lower thn tht of lyers ove nd elow (Figures c nd d). In ddition, Snow density hs chnged significntly t two ed surfces (Figure ). Snow lyers elow the ed surfces hd lrge hrdness nd consisted of snow grins of melt forms with lrge size. Therefore, two snow lyers which consisted of fceted crystls would ct s wek lyers for relesing surfce vlnches. We will cll the upper nd lower wek lyers WL nd WL. The mesured SFIs for WL nd WL were.5 nd.89 kn m -. The thicknesses H of snow lyers ove the WL nd WL were.55 nd. m, respectively. 4 SNOWPACK STABILITY AND AVALANCHE INDUCED BY EARTHQUAKE Using the dt otined from the snow pit oservtion in the strting zone of vlnche induced y the erthquke, we ttempt to evlute stility of snowpck on slope nd will consider the effects of seismic ground motion on the vlnche occurrence. 4. Method of estimting snowpck stility including the seismic ground motion Snowpck stility on slope in nturl condition cn e simply expressed s the rtio of sher stress to sher strength of snow (Figure 4), so clled stility index SI given s SI s. () n sin Where s is the sher strength of wek snow lyer (N m - ) nd is commonly replced y the SFI ( s = SFI). n is the norml stress to snow lyer per unit re (N m - ) nd cn e clculted from density (kg m -3 ) nd thickness D (m) of snow lyer ove the wek lyer (i.e., n = gd; g is the grvittionl ccelertion (m s - )). is the slope incline ( o ) nd n sin is the sher stress cting on wek lyer. Seismic ground motion will give rise to stress within snowpck on slope. According to D H L n n cos - sin Snow surfce Wek leyer Snow surfce Wek leyer Figure 4. Schemtics of () stress due to snow weight on wek lyer without seismic ground motion, sher strength of wek lyer nd tensile strength of snowpck ove wek lyer nd () sher nd norml components of horizontl ccelertion ginst sher plne. Podolskiy et l. () nd Kmiishi et l. (), the stility index SI expressed in Eqution () cn e rewritten to the SI E given in Eqution () y considering only horizontl ccelertion cused y seismic ground motion (Figure 4). SI E s () sin cos n Where is the degree of horizontl ccelertion, which is defined s the rtio of the horizontl ccelertion cused y seismic ground motion to the grvittionl ccelertion g. In ddition to the Eqution (), Mtsuzw et l. (7) proposed snow sfety fctor under erthquke y referring sfety fctor for slope of soil nd the lnce of stresses within snowpck on slope under erthquke indicted y Ogur et l. (). We rewrote the sfety fctor proposed y Mtsuzw et l. (7) to stility index under the erthquke SI E expressed s CL nl cos sin tn td SI E. (3) L sin cos n Where C is the cohesion fctor etween snow grins (N m - ), tn is the internl friction fctor 6

6 Interntionl Snow Science Workshop Grenole Chmonix Mont-Blnc - 3 of snow grin, t is the tensile strength of snow ove the wek lyer (N m - ) nd L is the length of snow lyer ove the wek lyer (m) shown in Figure 4. The tensile strength t cn e otined from the reltion with the snow density provided y Wtne (977), for rounded grins, given s t. (4) In Eqution (3), the sher strength of snow s is expressed in form of the Mohr Coulom s expression shown s C tn. (5) s n In this pper, the cohesion fctor C is replced y the SFI (C = SFI). The internl friction fctor tn should e considered for precipittion prticles (PP), decomposing nd frgmented precipittion prticles (DF) nd fceted crystls (FC), ut cn e neglected for other grin shpes (Jmieson nd Johnston, 998; Ymnoi nd Endo, ; Zeidler nd Jmieson, 6; Mtsushit et l., ). If t = nd tn =, Eqution (3) is the sme s Eqution () (i.e., SI E = SI E ). 4. Effect of seismic ground motion on vlnche occurrence under erthquke The length of snow lyer L ove oth the WL nd WL is 9.5 m, the slope incline is 4 o. From the results of the snow pit oservtion shown in Figure, the men density nd the thickness D of snow lyer ccumulted on the WL re 85 kg m -3 nd.4 m (= Hcos =.55 m cos4 o ). Therefore, the norml stress n cting on WL is.74 kn m -. In the sme wy, the men density nd the thickness D of snow lyer ccumulted on the WL re clculted s 4 kg m -3 nd.74 m (=. m cos4 o ), nd the norml stress n cting on WL is.6 kn m -. The tensile strengths t of snow lyers ove WL nd WL re given from Eqution (4) using the men snow density of snow lyers ove ech wek lyer. The mesured SFIs for Stility index SI E ' WL SI E ' = =. =.85 (E-W) =.5 (N-S) Lyer thickness D (m) Stility index SI E ' WL SI E ' = =. =.85 (E-W) =.5 (N-S) Lyer thickness D (m) Stility index SI E ' WL.9.5 =. =.85 (E-W) =.5 (N-S) SI E ' = Lyer thickness D (m) Stility index SI E ' WL =. SI E ' =.9.4. =.85 (E-W) =.5 (N-S) Lyer thickness D (m) Figure 5. Stility indexes SI E clculted y using Eqution (3) for () the wek lyer ; WL nd () the wek lyer ; WL. The is the degree of horizontl ccelertion. Figure 6. The sme s Figure 5 except for the condition fter crck formtion within snowpck ( t = ). 7

7 Interntionl Snow Science Workshop Grenole Chmonix Mont-Blnc - 3 WL nd WL re used s the cohesion fctors C. The internl friction fctor tn is chosen to e. for fceted crystls found y Zeidler nd Jmieson (6). Accelertion of ground motion cused y the erthquke hve een recorded t Kuriymnishi (36.88 o N, o E) of the KIK-net of the Ntionl Reserch Institute for Erth Science nd Disster Prevention (NIED), locted t distnce of out 4 km from the seismic center. The mximum vlues of horizontl ccelertions recorded t this site re.4 m s - ( =.5) in the direction of N-S nd 8.35 m s - ( =.85) in the direction of W-E. Sustituting these vlues mentioned ove for Eqution (3) gives the stility index of snowpck on slope under the erthquke SI E (Figure 5). As seen in Figure 5, in the condition of no seismic ground motion ( = ), the stility indexes SI E for WL nd WL re the sme s 3.9. However, in the mximum vlue of the degree of horizontl ccelertion oserved ( =.5), oth the stility indexes SI E for WL nd WL decrese to.5. Therefore, the horizontl ccelertion cused y the erthquke dd the sher stress to snowpck on slope nd the vlnches will e induced due to the decrese in the stility. The stility index SI E fter crck formtion within snowpck is lso clculted (i.e., t = in Eqution (3)) nd is shown in Figure 6. In the cse of the degree of horizontl ccelertion =.5, the stility indexes SI E for WL nd WL re.3 nd., respectively. If the crck formed within snowpck (it my occur due to the verticl ccelertion cused y the erthquke), the snowpck on slope would e more unstle under the erthquke. Kmiishi et l. () hs estimted tht the stility index SI E for the wek lyer which consisted of wet grnulr grins of melt forms under the North Ngno prefecture erthquke on ws.5 or less, using Eqution () (i.e, Eqution (3) with tn = nd t = ). Consequently, the stility index cn e used for evluting the effect of seismic ground motion on vlnche occurrence. 5 CONCLUSION We hve investigted the chrcteristics of vlnches induced y the North Tochigi Prefecture erthquke on Ferury nd oserved snowpck structure t vlnche strting zone. The results indicted tht vlnches occurred in entire re long the rod with length of out 8 km locted on the seismic center nd the numer of vlnches including smll scle collpse of snow on slope ws or more. Surfce vlnches mostly occurred nd hd one or two ed surfces. These ed surfces corresponded to the wek lyer consisted of fceted crystls with low hrdness. We lso evluted the stility of snowpck on slope nd suggested tht the horizontl ccelertion cused y the erthquke dd the sher stress to snowpck on slope nd the vlnches will e induced due to the decrese in the stility of snowpck on slope. However, the verticl motion cused y the erthquke will lso ffect the filure of snowpck on slope (Podolskiy et l., ; Mtsushit et l., 3). To etter understnding of mechnism of snowpck filure under the erthquke, we will conduct n experiment using snow-lock on virtion tle, which cn e control the ccelertion nd period of the virtion, nd will otin the dt on the response chrcteristics of snowpck to the ground motion nd on the snowpck filure (e.g., Mtsushit et l., 3). ACKNOWLEDGMENTS We thnk the Nikko So Office of the Ministry of Lnd, Infrstructure, Trnsport nd Tourism (MLIT) nd the Nikko City for coopertion to our field investigtion. We lso cknowledge Emi Nkmur, the Snow Avlnche nd Lnd Slide Reserch Center, for support during the field investigtion, nd Dr. Iso Kmiishi, the Ntionl Reserch Institute for Erth Science nd Disster Prevention (NIED), for providing the informtion on the erthquke-induced snow vlnches. REFERENCES Higshiur, M., Nkmur, T., Nkmur, H. nd Ae, O., 979. An vlnche cused y n erthquke. Rep. Nt. Res. Cent. Disster Prev., : 3-. [In Jpnese with English strct] Höller, P. nd Fromm, R.,. Quntifiction of the hnd hrdness test. Ann. Glciol., 5: Jmieson, J.B. nd Johnston, C.D., 998. Refinements to the stility index for skier-triggered drysl vlnches. Ann. Glciol., 6: Kmiishi, I., Motoyoshi, H., Ishizk, M. nd Sto, T.,. Avlnches induced y North Ngno Prefecture Erthquke on Mrch. J. Jpn. Soc. Snow Ice (Seppyo), 74: [In Jpnese with English strct] Mtsushit, H., Mtsuzw, M. nd Ae, O.,. The influences of temperture nd norml lod on the sher strength of snow consisting of precipittion prticles. Ann. Glciol., 6: Mtsushit, H., Hrd, Y., Ksmur, S., Mtsuzw, M. nd Nkmur, H., 3. Experiment on response chrcteristics of snowpck on slope to erthquke motion. Proc. JSSI & JSSE Joint Conference 3 in Kitmi: (sumitted). [In Jpnese] 8

8 Interntionl Snow Science Workshop Grenole Chmonix Mont-Blnc - 3 Mtsuzw, M., Kjiy, Y. nd Ito, Y., 7. Assessment of snow sfety fctor under erthquke. Snow Ice Hokkido (Hokkido no Seppyo), 6: [In Jpnese] Ogur, Y., Izumi, K., Miyzki, N. nd Koyshi, S.,. An vlnche cused y n erthquke t Nkzto villge, Niigt Prefecture, on Jnury 4th. Ann. Rep. Sigi-ken, Niigt Univ., 3: 9-5. [In Jpnese with English strct] Perl, R., Beck, T.M.H. nd Cheng, T.T., 98: The sher strength index of lpine snow. Cold Reg. Sci. Technol., 6: -. Podolskiy, E.A., Nishimur, K., Ae, O. nd Chernous, P.A.,. Erthquke-induced snow vlnches: I. Historicl cse studies. J. Glciol., 56: Podolskiy, E.A., Nishimur, K., Ae, O. nd Chernous, P.A., : Erthquke-induced snow vlnches: II. Experimentl study. J. Glciol., 56: Tkeuchi, Y., Nohguchi, Y., Kwshim, K. nd Izumi, K., 998. Mesurement of snow-hrdness distriution. Ann. Glciol., 6: 7-3. Wtne, Z., 977. The influence of snow qulity on the reking strength. Sci. Rep. Fukushim Univ., 7: Ymnoi, K. nd Endo, Y.,. Dependence of sher strength of snow cover on density nd wter content. J. Jpn. Soc. Snow Ice (Seppyo), 64: [In Jpnese with English strct] Zeidler, A. nd Jmieson, B., 6. Refinements of empiricl models to forecst the sher strength of persistent wek snow lyers: Prt A: Lyers of fceted crystls. Cold Reg. Sci. Technol., 44: