Seismic anisotropy of the upper crust and response to dynamic stress around Mount Fuji, Japan

Transcription

1 Seismic anisotropy of the upper crust and response to dynamic stress around Mount Fuji, Japan 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 1 (), K. Run Araragi1* Martha Savage Ohminato Takao1 Yosuke Aoki and Florent Brenguier 1:Earthquake Research Institute, University of Tokyo, Japan :Victoria University of Wellington, NZ : Université Joseph Fourier, France

2 Contents (Araragi et al., + α) Introduction Background of Mt. Fuji Shear wave splitting(sws) & MFAST [Savage et al., 1] Design of seismic networks & used events Results Horizontal distribution of SWS SWS in regional scale Stress estimation Summary Subtle seismic velocity change Purpose of research: Using seismic anisotropy around Mt. Fuji for interpretation of the regional and local geologic structure and/or processes of the area.

3 No eruption (- AD 177) The triple junction of Introduction PHS, NAM, and EUR. Radial pattern of dyke Modified from Bird[] near the summit [e.g. Takada, 7] Takada et al. [7] Regional NW- SE compressive stress field [e.g. Nakamura, 177]. Calculated by Hardebeck et al. []

4 Why do we study Mt. Fuji with SWS? Geologic background Dike distribution and the formation processes of the mountain edifice. Regional stress field (NW- SE) Triggering factors for the change of geologic processes Increase of dilatation strain [Harada et al., 1] Change of regional stress field by the 11 Tohoku- Oki earthquake Mw5. event on, March, 11 Seismic anisotropy/velocity change around Mt. Fuji may constrain the geologic structure and/or stress- related processes of the area.

5 What is shear wave splitting (SWS)? delay time(δt) Direction of faults or compressive stresses (ø) 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. In anisotropic media, seismic wave is projected in fast direction (ø) and slow direction. In slow direction, seismic wave cause delay time (δt).

6 SWS in volcanic region Anisotropy Maximum compression SWS can be used for detection of stress-related events. (e.g. Gerst and Savage, ) Structural or stressrelated anisotropy?

7 What are the problems of SWS measurements? Slow direction (δt) Φ Fast direction (Φ) I hate Φ =. I need Φ = 5 and δt =.5! I think the shapes of waves are similar. Shear wave splitting occurs! The results may be subjective. We use automated methodology MFAST (Savage et al., 11)

8 Procedure of MFAST (Savage et al., 1) Select data S Apply multiple filters / choose three SNR>? Determine measurement window based on period and S- pick Splitting measurement by clustering method (Teanby et al., ) Selection criteria (quality of cluster, polarity of waveforms, consistency of results, and delay<.8 maximum) Amplitude Frequency (Hz) 1 1

9 fb Procedure of MFAST (Savage et al., 1) e n Select data z Apply multiple filters / choose three SNR>? Seconds ( ) S Determine measurement window based on period and S- pick ( ) t Window number Splitting measurement by clustering method (Teanby et al., ) 6 Selection criteria (quality of cluster, polarity of waveforms, consistency of results, and delay<.8 maximum) t (s)

10 Station locations (JMA, ERI and NIED) N.KKKH N.TRH N.TUH N.SMBH FJY 5 km 1 km N.SSNH FJSV km FJO N.YMH N.ASGH 16 stations 8 stations (Hi- net) N.NMZH Seismic stations were installed by Japan Metrological Agency, Earthquake Research Institute, and the National Research Institute for Earth Science and Disaster Prevention..

11 Events used for SWS measurements 5 5' 5 ' & Mar ' 5 ' ' 5 1' 18 5' 18 ' 18 5' 18 ' 18 5' 18 5' 18 55' 1 ' 1 5' 1 1' Jan. Dec Gray circles : Deep Low FQ events Depths: - km Maximum distance to stations: 5 km

12 (. Dominant frequency: ~ 7 Hz or less than 5 Hz.1 Delay times :.-. [s].5 Consistent fast and delay. at each station 6 Window number Temporal consistency t 6. Measured SWS delay time [s] delay time [s] Delay times Depths:.1[km] [km] date. Delay times Depths:.1[km] [km]..1.5 ( ) t (s) S S delay time [s] date FJY Delay times Depths:.1[km] [km] Fast Direction ( ) date Delay times Depths:.1[km] [km] Example of station delay time [s] t (s) date

13 Spatial distribution of SWS Fuji, FJO FJY Fuji, FJO FJY Fuji, FJO FJY Fuji, FJO FJY Fuji, FJO FJY 1 8 FJO FJY FJO FJY FJO FJY FJO FJY 8 FJO FJY FJO FJY FJO FJY 1 8 FJO FJY FJO.km.1km Group 1 Group Group Group Directions of bins : fast polarization directions Lengths : number of measured events

14 SWS in regional scale km 5 km 1 km Honda & Yukutake,

15 Depths of anisotropy

16 Interpretation of radial pattern SWS close to the summit Fast polarization directions the spatial distribution of dyke structure. The gravitational stresses due to the mass of the volcanic edifice [e.g. Acocella and Neri, ] SWS far from the summit Regional stresses Takada et al. (7)

17 Quantitative analysis of stress fields Tectonic stresses We estimated stresses from lithostatic pressure(σ ) at depths. ( R = σ σ ) 1 ( σ 1 σ ) Gravitational effects Boussinesq s problem. We assumed a point load beneath the summit of Mt. Fuji. σ 1 = Aσ The factor(a) was determined by trial and error. Generating stress tensors & their eigenvectors

18 Stress fields and SWS

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

20 Thank you for your kind attention!