Seismic anisotropy of the upper crust and response to dynamic

stress around Mount Fuji, Japan

K.  Run  Araragi1*Martha  Savage2 Ohminato  Takao1 Yosuke  Aoki  and  Florent Brenguier3  

1:Earthquake Research Institute, University of Tokyo, Japan2:Victoria University of Wellington, NZ3: Université Joseph Fourier, France

mainly from KR Araragi, MK Savage, T Ohminato and Y Aoki, Seismic anisotropy of the upper crust around Mount Fuji, Japan., Journal of Geophysical Research: Solid Earth 120 (4), 2739-2751

Contents  (Araragi  et  al.,  2015  +  α)• Introduction

Background  of    Mt.  Fuji• Shear  wave  splitting(SWS)                                                                                                        &  MFAST  [Savage  et  al.,  2010]

• Design  of  seismic  networks  &  used  events• Results

Horizontal  distribution  of  SWSSWS  in  regional  scale

• Stress  estimation• Summary• Subtle  seismic  velocity  change

Using  seismic  anisotropy  around  Mt.  Fuji  for  interpretation  of  the  regional  and  local  geologic  structure  and/or  processes  of  the  area.  

Purpose  of  research:

Introduction• No  eruption  (-­‐ AD  1707)

• The  triple  junction  of  

PHS,  NAM,  and  EUR.

• Radial  pattern  of  dyke  

near  the  summit  [e.g.  

Takada,  2007]

• Regional  NW-­‐SE  

compressive  stress  field  

[e.g.  Nakamura,  1977].

Modified   from  Bird[2003]

Calculated  by    Hardebeck et  al.  [2004]

Takada  et  al.  [2007]

Why  do  we  study  Mt.  Fuji  with  SWS  ?

Seismic  anisotropy/velocity  change  around  Mt.  Fuji  may  constrain  the  geologic  structure  and/or  stress-­‐related  processes  of  the  area.  

• Dike  distribution  and  the  formation  processes  of  the  mountain  edifice.

• Regional  stress  field(NW-­‐SE)

• Increase  of  dilatation  strain  [Harada  et  al.,  2010]

• Change  of  regional  stress  field  by  the  2011  Tohoku-­‐Oki  earthquake  

• Mw5.9  event  on  15,  March,  2011

Triggering  factors  for  the  change  of  geologic  processesGeologic  background

What  is  shear  wave  splitting  (SWS)  ?

In anisotropic media, seismic wave is projected in fast direction(ø) and slow direction. In slow direction, seismic wave cause delay time (δt).

Fast polarization (ø): Direction in which shear wave propagates faster than other directions

Delay time (∆t) : Difference of waves along the fast direction and along the slow direction.

http://en.wikipedia.org/wiki/Shear_wave_splitting

delay  time(δt) Direction  of  faults  or  

compressive  stresses  (ø)

SWS  in  volcanic  region

SWS can be used for detection of stress-related events. (e.g. Gerst and Savage, 2004)

Structural or stress-related anisotropy?

Anisotropy

Maximum compression

What  are  the  problems  of  SWS  measurements?Fast direction (Φ)

Slow direction (δt)

Φ

I  think the  shapes of  waves  are  similar.  Shear  wave  splitting  occurs!

I  hate Φ =  20˚.  I  need  Φ=  45˚  and  δt =  0.05!

The results may be subjective.We use automated methodology”MFAST”(Savage et al., 2011)

event 110402.042644.HSO.2.0−10.fb3− − − − − − − − − − − − − − − − − − − − − − − − − − −

depth: 10.752 km

distance: 8.75424 km

magnitude: 1.7

− − − − − − − − − − − − − − − − − − − − − − − − − − −

splitting windows (relative to S−Pick at 5.53 s):

wbeg: −0.60 − −0.20 (5)

wend: 0.13 − 0.31 (23)

selected: 5.125 − 5.8343, length: 0.7093 s

results: GRADE ACl− − − − − − − − − − − − − − − − − − − − − − − − − − −

fast: 6.0 +/− 6.2 (°)

�t = 0.037 +/− 0.004 (s)

spol: 117.5 +/− 3.2 (°)

fast (abc): 6.0 +/− 6.2 (°)

/Users/kohtaroararagi/Seismo_Lab/Fuji/data/fuji.dat

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Select  data

SNR>3?

Determine  measurement  window  based  on  period  and  S-­‐pick

Splitting  measurement  by  clustering  method  (Teanby et  al.,  2004)

Selection  criteria(quality  of  cluster,  polarity  of  

waveforms,  consistency  of  results,  and  delay<0.8  maximum)

1010

0.005

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0.02

Frequency (Hz)

Ampl

itude

Apply  multiple  filters  /  choose  three

Procedure  of  MFAST  (Savage  et  al.,  2010)

Select  data

SNR>3?

Determine  measurement  window  based  on  period  and  S-­‐pick

Splitting  measurement  by  clustering  method  (Teanby et  al.,  2004)

Selection  criteria(quality  of  cluster,  polarity  of  

waveforms,  consistency  of  results,  and  delay<0.8  maximum)

Apply  multiple  filters  /  choose  three

Procedure  of  MFAST  (Savage  et  al.,  2010)

event 110327.015135.N.FJY2.3.0−20.fb2− − − − − − − − − − − − − − − − − − − − − − − − − − −

depth: 14.088 kmdistance: 2.03233 kmmagnitude: 1.2

− − − − − − − − − − − − − − − − − − − − − − − − − − − splitting windows (relative to S−Pick at 5.46 s):

wbeg: −0.60 − −0.20 (5)wend: 0.09 − 0.20 (15)selected: 4.864 − 5.6649, length: 0.8009 s

results: GRADE BCl− − − − − − − − − − − − − − − − − − − − − − − − − − −

fast: 177.0 +/− 7.2 (°)�t = 0.154 +/− 0.002 (s)spol: 44.8 +/− 4.3 (°)fast (abc): −3.0 +/− 7.2 (°)

/Volumes/MinasTirith/Projects/SWS/FUJI/data/fuji.da

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110327.015135.N.FJY2.3.0−20.fb2

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Corrected p

Corrected p�

event 110402.042644.HSO.2.0−10.fb3− − − − − − − − − − − − − − − − − − − − − − − − − − −

depth: 10.752 km

distance: 8.75424 km

magnitude: 1.7

− − − − − − − − − − − − − − − − − − − − − − − − − − −

splitting windows (relative to S−Pick at 5.53 s):

wbeg: −0.60 − −0.20 (5)

wend: 0.13 − 0.31 (23)

selected: 5.125 − 5.8343, length: 0.7093 s

results: GRADE ACl− − − − − − − − − − − − − − − − − − − − − − − − − − −

fast: 6.0 +/− 6.2 (°)

�t = 0.037 +/− 0.004 (s)

spol: 117.5 +/− 3.2 (°)

fast (abc): 6.0 +/− 6.2 (°)

/Users/kohtaroararagi/Seismo_Lab/Fuji/data/fuji.dat

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110402.042644.HSO.2.0−10.fb3

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Seconds

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Corrected p

Corrected p�

Station  locations  (JMA,  ERI  and  NIED)

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FJ6V

FJHV

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N.TR2HN.KKKH

N.SSNH

N.SMBH

N.TU2H

N.YM2H

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15 k

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5 km

16  stations8  stations  (Hi-­‐net)Seismic  stations  were  installed  by  Japan  Metrological  Agency,  Earthquake  Research  Institute,  and  the  National  Research  Institute  for  Earth  Science  and  Disaster  Prevention..

01 2009 08 2009 03 2010 09 2010 04 2011 10 20110

20

40

60

80

100

120

140

160

180number of events blue:all eq magenta:deep low frequency results

Events  used  for  SWS  measurements

2009.1.1-­‐2012.10.31Depths:  0-­‐20  km

Maximum  distance  to  stations:  25  km

Fuji regional map

138˚25' 138˚30' 138˚35' 138˚40' 138˚45' 138˚50' 138˚55' 139˚00' 139˚05' 139˚10'35˚10'

35˚15'

35˚20'

35˚25'

35˚30'

35˚35'

400

400

400

1200

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138˚25' 138˚30' 138˚35' 138˚40' 138˚45' 138˚50' 138˚55' 139˚00' 139˚05' 139˚10'

0

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25

30

160

120

80

40

Jan.  2009

11  &  15  Mar.  2011

Dec.  2011

Gray  circles  :  Deep  Low  FQ  events

Measured  SWS• Dominant  frequency:  ~ 7  

Hz  or  less  than  5  Hz  • Delay  times  :  0.04-­‐0.09  [s]• Consistent  fast  and  delay  

at  each  station• Temporal  consistency  

Example  of  station  FJHV

04 2011 07 2011 10 2011 02 2012 05 2012 08 20120

0.05

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dela

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e [s

]

MMS Delay times Depths:0.1[km]−20[km]

04 2011 07 2011 10 2011 02 2012 05 2012 08 20120

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FJ5V Delay times Depths:0.1[km]−20[km]

04 2011 07 2011 10 2011 02 2012 05 2012 08 20120

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04 2011 07 2011 10 2011 02 2012 05 2012 08 20120

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]

FJHV Delay times Depths:0.1[km]−20[km]

event 110320.081830.N.FJHV.3.0−20.fb1− − − − − − − − − − − − − − − − − − − − − − − − − − −

depth: 12.707 kmdistance: 11.105 kmmagnitude: 1

− − − − − − − − − − − − − − − − − − − − − − − − − − − splitting windows (relative to S−Pick at 5.64 s):

wbeg: −0.60 − −0.20 (5)wend: 0.09 − 0.20 (15)selected: 5.143 − 5.8439, length: 0.7009 s

results: GRADE ACl− − − − − − − − − − − − − − − − − − − − − − − − − − −

fast: 92.0 +/− 9.0 (°)�t = 0.014 +/− 0.001 (s)spol: 46.8 +/− 6.3 (°)fast (abc): −88.0 +/− 9.0 (°)

/Volumes/MinasTirith/Projects/SWS/FUJI/data/fuji.da

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110320.081830.N.FJHV.3.0−20.fb1

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Seconds

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Sp

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Corrected p�

Spatial  distribution  of  SWS

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

Fuji, FJO

2

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

Fuji, FJO

2

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

Fuji, FJO

2

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

Fuji, FJO

2

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

Fuji, FJO

2

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

Fuji, FJO

2

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

Fuji, FJO

2

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

Fuji, FJO

2

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

FJO

3

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

FJO

3

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

FJO

3

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

FJO

3

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

FJO

3

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

FJO

3

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

FJO

3

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

FJO

3

FUJ2

2

OIS

94

FJZ

4

FUJ

68

HSO

14

MMS

15

MTS

16

FJY2

66

FJ5V

114

OSWA

62

FJ6V

81

SBSR

48

NHOW

14

FY1

2

FJHV

53

FJNV

9

FJO

3

FUJ2

2

33.3km

24.1km

Group  1  Group  2  

Group  3  

Group  4  Directions  of  bins  :  fast  polarization  directionsLengths :  number  of  

measured  events

SWS  in  regional  scale

15  km10  km

5  km

Honda  &  Yukutake,  2014

Depths  of  anisotropy

Interpretation  of  radial  pattern

The  gravitational  stresses  due  to  the  mass of  the  volcanic  edifice  [e.g.  Acocella and  Neri,  2009]

Fast  polarization  directionsSWS  close  to  the  summit  

Takada  et  al.  (2007)

the  spatial  distribution  of  dyke  structure.

Regional  stressesSWS  far  from  the  summit  

Quantitative  analysis  of  stress  fields

We  assumed  a  point  load  beneath  the  summit  of  Mt.  Fuji.    

Boussinesq’s problem.    Gravitational  effectsTectonic  stresses

We  estimated  stresses  from  lithostaticpressure(σ2)  at  depths.    

The  factor(A)  was  determined  by  trial  and  error.

R =σ1 −σ 2( )σ1 −σ 3( )

Generating  stress  tensors  &  their  eigenvectors  

σ1 = Aσ 2

Stress  fields  and  SWS

Summary

• Radial  pattern  of  fast  directions  is  consistent  with  the  directions  of  the  strikes  of  dikes  around  Mt.  Fuji.

• The  regional  NW-­‐SE  trends  of  fast  directions,  and  radial  anisotropy  around  Mt.  Fuji  are  consistent  with  stress  fields  in  the  region.

• Stress  modeling  supports  the  interaction  of  stresses  and  anisotropic  structure  in  the  Mt.  Fuji  volcanic  regions.  

Our  SWS  measurement  shows  that  the  anisotropy  around  Mt.  Fuji  has  clear  spatial  variations.  We  interpreted  them  and  made  the  following  conclusions:

Thank you for your kind attention!