Inverse Correlation between Surface Slip and Strong Ground Motion Hongjun Si, Kazuki Koketsu, Hiroe Miyake, and Rami Ibrahim

Earthquake Research Institute, University of Tokyo (E-mail: shj@eri.u-tokyo.ac.jp)

Ground motions of the 1999 Chi-Chi earthquake were densely observed by the strong motion networks all over Taiwan. Especially at over 10 observation stations very close to the surface rupture, high-quality strong motions recordings are derived. This provides us valuable opportunity to get the insight into the characteristics of strong ground motion in near-surface-rupture area. So far, It is found that the short period strong motion generally decrease from southern to northern part of the surface rupture, though the seismic moment release on fault plane is found in the opposite tendency. However, This feature on ground motion in near-surface-rupture area still remains not well explained. In the analysis, We found that the short-period ground motion inversely correlated with the slips of surface rupture. This inverse correlation implies that the dissipative friction energy may reduced the short period ground motion.

Ground motions of the 1999 Chi-Chi earthquake were densely observed by the strong motion networks all over Taiwan. Especially at over 10 observation stations very close to the surface rupture, high-quality strong motions recordings are derived. This provides us valuable opportunity to look into the characteristics of strong ground motion in not just near-source, but near-surface-rupture area. So far, based on these recordings, It is found that, the high-frequency ground motion observed at the stations in southern part are generally larger than those in northern part [e.g., Wang et al., 2002], while the moment release on the fault plane have the opposite tendency. However, This feature on ground motion in near-surface-rupture area still remains not well explained. In order to solve this problem, in this study, paying attention to the feature of the distribution of the slip of surface rupture is also increasing from southern to northern along the Chelungpu fault, we focused on the influence of the slip of surface rupture on the ground motion. In the study, we discuss the correlation between the slip of surface rupture and the ground motion at the observation stations close to the surface rupture. Since the decay of high frequency motion in displacement spectra shows the same tendency [Huang and Wang, 2002], we also discuss the correlation between the decay and the slip of surface rupture. Based on the results, we discuss the physics behind the correlation relationship.

We showed the short-period ground motion inversely correlated with the slips of surface rupture. The inverse correlation implies that the decreasing of short-period ground motion may be caused by the friction loss due to the surface faulting. The results imply that, the strong motion in the vicinity of the surface faulting may be easier to be affected by the friction of the surface faulting. Generally our results can be thought to be consistent with the relative low ground motion near surface rupture reported so far, such as Brune (2003, the near-fault precarious rock).

In order to explain the inverse correlation between the surface slip and the short-period ground motion, cross sections at two adjacent stations with different surface slips [e.g., TCU065 and TCU075] are considered in the analysis [Figure 6]. This mean that the fault model (a) and (b) can be assumed to be horizontally homogenous ones, so they are comparable. The energy budget is considered based on the slip weakening model as shown in Figure 6. Here, the total energy change consists of the radiated energy, the fracture energy, and the friction energy (e.g., Kanamori and Heaton, 2000). From the figure, the significant difference between the two models is the existence of the dissipative energy (mainly friction energy) near the surface in model (b). This imply that the decreasing of short-period ground motion may be caused by the friction loss due to the surface faulting. Though the influence on seismic wave by friction has been experimentally investigated in rock science (e.g., Walsh, 1966; Gordon and Davis, 1968; Winkler et al., 1979; Duffaut et al., 2010), further investigation is needed.

S21B-2473

Inactive area

Slip

of s

urfa

ce ru

ptur

e (m

)

0.0

5.0

10.0

15.0

Distance in northern direction from epicenter (km)

Sei

smic

mom

ent p

er 5

km

-40 -20 0 20 40 60 801e+25

1e+26

1e+27

GM

Rot

I50

at 0

.02s

(cm

/s2 )

0.2

0.4

0.6

0.8

1.0

ObservationStation

Latitude LongitudeDistance to

the surface slip(km)

Site no. of surface slip in Lee et al. (2003)

Latitude Longitude Net slip(m)

Average slip(m)

TCU068 24.2772 120.7658 0.46 A16 24.2787 120.7700 1.00 4.63

0.53 A18 24.2804 120.7619 3.65

0.96 A20 24.2844 120.7605 11.65

0.97 A15 24.2796 120.7750 4.25

1.10 A19 24.2858 120.7604 1.00

1.28 A14 24.2800 120.7780 3.95

1.41 A24 24.2790 120.7520 1.90

1.43 A21 24.2822 120.7528 7.50

1.43 A22 24.2822 120.7528 6.80

TCU065 24.0588 120.6912 1.52 C3 24.0451 120.6901 2.95 2.05

1.96 C5 24.0411 120.6898 1.15

TCU067 24.0912 120.7200 0.45 B12 24.0883 120.7169 2.15 1.40

0.45 B13 24.0883 120.7231 1.10

0.89 B14 24.0880 120.7280 0.85

1.40 B19 24.0800 120.7136 1.50

TCU052 24.1980 120.7393 2.84 B8 24.1750 120.7268 5.10 5.87

2.90 B9 24.1749 120.7258 6.00

2.82 B10 24.1759 120.7254 6.50

TCU075 23.9827 120.6778 0.45 C16 23.9816 120.6821 4.30 4.30

TCU102 24.2493 120.7208 1.82 B5 24.2560 120.7372 8.30 8.27

1.48 B6 24.2379 120.7284 7.45

1.46 B7 24.2370 120.7260 9.05

TCU129 23.8783 120.6843 2.72 C23 23.8978 120.7005 0.60 1.20

2.71 C24 23.8977 120.7005 0.65

2.64 C26 23.8956 120.7021 1.70

2.29 C27 23.8760 120.7067 1.85

WNT 23.8783 120.6843 2.72 C23 23.8978 120.7005 0.60 1.20

2.71 C24 23.8977 120.7005 0.65

2.64 C26 23.8956 120.7021 1.70

2.29 C27 23.8760 120.7067 1.85

TCU101 24.2420 120.7092 2.00 B6 24.2379 120.7284 7.45 8.25

1.79 B7 24.2370 120.7260 9.05

TCU076 23.9077 120.6757 1.38 D9 23.9095 120.6891 4.55 4.55

TCU049 24.1790 120.6902 3.74 B8 24.1750 120.7268 5.10 5.87

3.64 B9 24.1749 120.7258 6.00

3.59 B10 24.1759 120.7254 6.50

CHY028 23.6320 120.6052 4.93 E5 23.6517 120.6486 1.00 1.10

4.88 E6 23.6515 120.6481 1.20

Introduction

Feature of ground motion near surface rupture of the Chi-Chi earthquake

Data used in this study

Inverse correlation between surface slip and strong motion

Physics behind the inverse correlation

Discussion and Conclusions

The strong motion data are adopted from the database developed by the NGA project[Chiou et al., 2008]. Decay of displacement spectra is adopted from Huang and Wang (2002). Since it is indicated that the behavior of station TCU129 seems to be abnormal [Wen et al., 2001], we excluded the data from TCU129 and co-located WNT. The surface slip for each station is decided as shown in Tables and Figures.

Figure 1 shows the location of the surface rupture, and the location of stations very close to the surface rupture. At these stations, high-quality strong ground motions recordings are derived. Figure 2 shows that the high-frequency ground motion observed at the stations in southern part are generally larger than those in northern part, while the moment release on the fault plane have the opposite tendency.

Figures 4 and 5 show the relationship between surface slip and ground motion. From the figures, we found that (1) the short-period ground motions with a period up to about 0.8s are in inverse correlation with the slip of the surface rupture. the correlation coefficients are close to 0.8; (2) at periods longer than 2.5s, positive correlations are observed, and the correlation coefficient increasing with the periods; (3) at periods of 10s, ground motions at 2 stations located on hanging-wall area are clearly larger than the others.

Abstract

Brune (2003) PEPI, 137(1-4):229-239; Duffaut et al. (2010) Geophysics 75(3),E143-E152; Gordon and Davis (1968) J. Geophys. Res. 73, 3917-3935; Huang and Wang(2002) Geophys. Res. Lett., 29, 47:1-4; Kanamori, H., and T. Heaton (2000), Geophysical Monograph 20, 127-141; Lee et al. (2003) J. Struct. Geol., 25, 1917-1931; Ma et al.(2001), Bull. Seismol. Soc. Am. 91, 1069-1087; Walsh (1966) J. Geophys. Res. 71, 2591-2599; Wang et al.(2002) J. Chin. Inst. Engin. Series A, 25(5), 507-519; Wen et al.(2001), Bull. Seismol. Soc. Am. 91, 1255-1266; Winkler et al. (1979), Nature, 277,528-531; Chiou et al. (2008), Earthq. Spectra 24, 23–44.

References

The authors thank Professor K.-F. Ma for providing us the digital data of the source model for the Chi-Chi earthquake. We acknowledge financial support of this research from the Research Funding for Earthquake Insurance in Japan operated by NLIRO and the Grants-in-Aid for Scientific Research “Construction of Next Generation of Ground Motion Prediction Equation (Project Number: 23241054, Principal Investigator: K. Koketsu)”.

Acknowledgement

Figure 3. Detailed location for strong motion observation stations and the surface break in the northern (upper) and southern part (lower) and in Figure 1.

Table 1 Slip data around each observation stations used in the analysis

Figure 5. [Upper panel] Correlation coefficients for the ratio of response spectra and the slip of surface rupture (red circles and left axis), the coefficient for slip (blue circles and right axis), and [lower panel] the dispersion parameter (red circles and left axis), and AIC (blue circles and right axis), with respect to periods, respectively. The same results for the decay ratio of Beta are shown in reverse triangles in the right hand of the same panels (label as β).

Figure 4. Plots of ratio of response spectra [Panels (a)], and the decay of displacement spectra [Panel (b)] relative to the slip of the surface rupture.

Figure 2. Plots of the response spectra at a period of 0.02s [upper panel], the surface slips and their average [middle panel], the moment release along the fault length [lowest panel], respectively. Zero show the location of the epicenter.

Figure 1. Slip distributions of surface rupture and the strong motion observation stations along the surface rupture. Color bar shows the value of the slip.

Figure 6. Schematic explanation of the physics for the influence of surface slip on ground motion. Model (a) and (b) show the cross section of two almost the same model but significant difference in with or without surface slip in the near-surface parts [shown as region (1)]. Energy changes occurred during the earthquake faulting is schematically shown in the figure.

(a) (b)

Period (s)

Cor

rela

tion

Coe

ffici

ent

β

Coefficient for S

lip

β

10-1 100 101-1.0

-0.5

0.0

0.5

1.0 -0.4

-0.2

0.0

0.2

0.4

Period (s)

Dis

pers

ion

para

met

er β

β

AIC

10-1 100 1010.0

1.0

2.0

3.0

4.0

5.0 0.0

30.0

60.0

∆σ>0

∆σ→0, SE=f(z)Non-slip

(a) Non surface slip model

∆σ>0

∆σ→0, SE=f(z)∆σ→0, SE→0

(b) Surface slip model

(1)(2)

(3)

(1)(2)

(3)

Fracture energyFriction energy

Radiated energy