Using Microtremor Array Measurements and Their Applications - PowerPoint PPT Presentation

Estimations of Shallow S-Wave Velocity Structures Using Microtremor Array Measurements and Their Applications Huey-Chu Huang, Cheng-Feng Wu and Ying-Chi Chen Department of Earth and Environmental Sciences National Chung Cheng University, Chia-Yi, Taiwan August 16, 2016 1 1

Motivations Motiva tions  Factors for affecting ground motions include source effect, path effect and site effect.  Shallow velocity structure is very important! According to damage patterns of famous large earthquakes (e.g. 1985 Mexico EQ, 1995 Kobe EQ, 1999 Chi-Chi EQ, etc.), surficial geology could affect ground motion seriously and cause heavy damage.  Reliable shallow V S structure is still lacking.  Purpose: site effect study & ground motion simulation. shallow V S structure: microtremor array measurement Ground motion simulation 2

Contents Motivation Validity of V S structure: TCDP drilling site Applications Shallow V S structures of Taipei basin Site-effect estimations of Taipei basin Shallow V S structures of the Chia-Yi area Ground motion simulation of the Chia-Yi Earthquake on Oct. 22, 1999 Detection of fracture zones of Chelungpu fault 3 3

S-wave velocity structure of the Taiwan Chelungpu Fault Drilling Project (TCDP) site using microtremor array measurements Wu, C.F. and H.C. Huang* (2015). Pure Appl. Geophys., 172, 2545-2556. 4

Geological map Lithostratigraphy of TCDP-A: 0 lower Cholan Fm. 1013 Chinshui Shale 1300 Kueichulin Fm. 1707 upper Cholan Fm. 2003 (Lin et al ., 2007) (m) TCDP-A: • located at Dakeng. • depths 500-1900 m • 2.4 km east of surface rupture. • meets Chelungpu fault and Sanyi fault. 5 (modified from Lo et al ., 1999; Ho and Chen, 2000)

Geometries of the S- and L-arrays  Ten stations are in the form of three different aperture triangles around the center station at each array. sampling rate ： 200 Hz S array (radii: 50-200 m) gain ： 100 co-stations for S- and L-arrays data length ： 582 sec L array (radii: 200-800 m) recording length ： 1.5-2 hr 6 TCDP-A

An Analys ysis is Methods ods F-K spectral analysis The power spectrum at frequency f and vector wavenumber k for an array of N sensors by Maximum Likelihood Method (Capon, 1969) is given by ：  1        N         1   P f , k f exp i k r ij ij    i , j 1  n : number of sensors.  ψ lm : cross-power spectrum between the i th and the j th sensors at frequency f.         : and are position vectors of the i th and the j th sensors. r r r r r ij j i i j Inversion of velocity structure • The equation, joining the dispersion curve and velocity model parameters, can be written as follows (Hwang and Yu, 2005):  N C T ( )      j ( ) ( ) C T   j i  i 1 i   : difference between observed and predicted phase velocity derived from C ( T ) j initial velocity model at the j th period ( T j ).  N : number of layers.     : partial derivative of phase-velocity of the j th period with respect C ( T ) / j i to V S of the i th layer.    : resulting difference in V S of the i th layer between adjacent inversions. 7 i • Using surface wave inversion method – program SURF (Herrmann, 1991).

Dispersion Curves • The results of the L-array are stable at lower frequencies, whereas those of the S-array are stable at higher frequencies. • The observed phase velocities at these two sites results are similar. 8

Inversion results at DAK The initial model is a half-space structure with V S =C max /0.92. The velocity structure from the surface to a depth of 3500 m can be roughly divided into 12 layers. 9

Comparisons of V S Structures • Geophysical logging of TCDP-A was conducted for depths between 500 and 1,900 m. • The inverted V S gradually increases from 1.52 to 2.22 km/s at depths between 585-1710 m, and the averaged V S is 1.899 km/s. • Our results are similar to those from velocity logs (1.4-2.98 km/s between 597-1705 m) and the averaged V S is 1.860 km/s (Wu et al ., 2007). • Our inversion results approximate to the regression result by Wang et al . (2009).  0 . 27 V S ( z ) 0 . 29 z 10

Comparisons of V S Structures km CS CLPF SYF Chinshui Shale Chelungpu fault Sanyi fault Formations (CS) (CLPF) (SYF) Methods (depth: m) (depth: m) (depth: m) seismic reflection 900-1200 1100 1800 method (Wang et al ., 2007) microtremor array 855-1440 1125 1755 measurements 11

Comparison of Structures between Different Methods Formations Chinshui Shale Chelungpu fault Sanyi fault Methods (depth: m) (depth: m) (depth: m) DAK 855-1440 1125 1755 microtremor array measurement TCD 900-1395 1125 1755 seismic reflection method 900-1200 1100 1800 (Wang et al. , 2007) lithostratigraphy 1013-1300 1111 1707 (Lin et al. , 2007) physical properties 1013-1300 1111 1712 (Hung et al ., 2007) lithology and stratigraphy 1029-1303 1111, 1153 1712 (Song et al ., 2007) The stochastic inversion results are comparable to those from the geophysical methods. 12

CASE 1A: S-wave velocity structures of the Taipei basin, Taiwan, using microtremor array measurements Huang, H.C., C.F. Wu, F.M. Lee and R.D. Hwang (2015). J. Asian Earth Sci., 101, 1-13. 13

 Taipei Basin is triangular in shape with an area of about 20 km  20 km.  The basin is formed by alluvial deposits from the Tanshui River and its three tributaries, namely Hsindian Creek, Dean Creek, and Keelung River.  Taipei Basin is bordered by Western Foothills, Linkou Tableland and Tatun Volcanoes. 14

The Quaternary sediments overlie the half-graben-shaped Teriary basement. Kanchiao fault forms a boundary which separates the deep NW and the shallow SE parts of the basin. Quaternary stratigraphy:  Sungshan Formation plays an important role on site amplification  Chingmei Formation  Wuku Formation  Banchiao Formation SE NW NW SE half-graben-shaped Kanchiao fault Teriary basement (at most 700 m deep) (adopted from Wang et al. , 2004) 15

Microtremor array measurements are conducted at 15 sites. The two used well-logging sites (WK-1E and PC-2) are also showed here. 16

90 m 612 m 1000 m/s 1000 m/s Estimated V S structures by differential inversion technique at all sites. the V S of the shallower depths (about 0-800 m) at sites REA and WUK are lower than those at other sites. If we assume that the averaged V S of the Tertiary Basement in the Taipei Basin is about 1,000 m/s (Wang and Sung, 1999, Wang et al ., 2004 and Chen, 2004), the 17 depths of the Quaternary sediments are between 90 m (LEL) and 612 m (WUK).

Lower velocities appear at the northwest part (WUK) and the northeast part (XIS) of the basin while the higher velocities are evident at the southwest part (LEL) and the southeast 18 part (NTU) of the basin.

Lower velocities appear at the northwest part (WUK) and northern part (GUD) of the basin while higher velocities prevail at the central part (SAC) of the basin. 19

CASE 1B: Site-Effect Estimations for Taipei Basin Based on the Shallow V S Structures Chen, Y.C., H.C. Huang* and C.F. Wu (2016). J. Asian Earth Sci., 117, 135-145. 20

1D Haskell method ( Haskell, 1960 ) Purpose : to simulate ground motions of the horizontally layered structure at different depths. (suppose it consists of n homogeneous layers) The m th layered propagating matrix i s      1 Free Surface cos Q i sin Q    m m m m  a    a , , , d   m 1  i sin Q cos Q  1 1 1 1     m m m m a , , , d 2 2 2 2 2 θ : incident angle of plane SH wave . . (in this study, θ =0 ° ) . β m : shear wave velocity of m th layer    a , , , d m m m m m d m : thickness of m th layer . μ m : shear modulus .   . Q kd  m m m    a , , , d    c   2 1 / 2  [( / ) 1 ] n 2     n 2 n 2 n 2 n 2 m m    c    a , , , d / sin      n 1 n 1 n 1 n 1 n 1 m      a , , , d k / c n n n n n  The transfer function is Plane SH wave [A] =a n -1 a n -2 ∙∙∙ a 2 a 1 ※ Considering the vertically incident plane SH wave ※ Ignoring attenuation parameter Q ※ Using the ground motion at the surface to simulate those at different depths 21

V S assumption in Taipei Basin: Bottom of Sungshan Formation: V S = 350 m/s 0.78 Hz Chingmei Formation: V S = 450 m/s Wuku Formation: V S = 700 m/s 0.70 Hz Banchiao Formation: V S = 880 m/s Tertiary Basement: 0.58 Hz V S = 1,000 m/s At WUK array site, the depths of these five formations are about 92, 119, 209, 484 and 616 m, respectively. 0.40 Hz The predominant frequencies at these five depths are about 0.78, 0.70, 0.58, 0.40 and 0.34 Hz, respectively. 0.34 Hz 22

Estimations of Site Characteristics Sungshan Formation (V S = 350 m/s) Depth: 0 m (edge) ~ 92 m (REA and WUK) Predominant frequency: 0.6 ~ 3.8 Hz Northwestern part has deeper sediments and smaller predominant frequency while the sites at the southwestern and southeastern parts have opposite results. 23