USE OF V S30 TO REPRESENT LOCAL SITE CONDITIONS

Transcription

1 USE OF V S30 TO REPRESENT LOCAL SITE CONDITIONS Presented by I. M. Idriss, Professor Emeritus University of California at Davis imidriss@aol.com Presented at the Session 6 on The V s30 Dialog How We Can & Cannot use V s30 in Site Response Estimation? 4 th IASPEI / IAEE International Symposium Effects OF Surface Geology on Strong Ground Motion Santa Barbara, California August 26, 2011

2 Wave propagation theory suggests that ground motion amplitude should depend on the density and shear wave velocity, V s, of the near surface material (e.g., Bullen, 1965; Aki & Richards, 1980). Density does not vary very much with depth; hence V s becomes the logical choice for representing site conditions. The essential issue then becomes in how to express this dependence.

3 Two methods have been proposed over the past 30 years.

4 Method No The "average" velocity over the depth range corresponding to ¼ wavelength of the period of interest (Joyner et al, 1981).

5 Average Vs = 1100 m/s Average Vs = 800 m/s Average Vs = 400 m/s 100 Average Vs = 200 m/s 1/4 Wave Length (m) Frequency (Hz)

6 Method No The use of V s30 (Borcherdt, 1994), who recommended the use of Vs30 as a means for classifying Site Categories for building codes.

7 * * n V V f5 PGA 1100,VS30 a10ln bln PGA1100 c bln 1100 for V V VLIN V LN I * S30 S30 ( ) = ( + ) + PGA + cln S30 < L * * V S30 f5( PGA 1100,VS30) = ( a10 + bn) Ln for VS30 V VLIN LIN IN V = V * S30 S30 for V < V S30 1 V = V * S30 1 for V S30 V 1 V 1 = 1500 m/s for T 0.50 sec ( 1 ) V1 = exp Ln T 0.2 for 0.50 sec < T 1 sec V1 = exp Ln ( T ) for 1 sec < T < 2 sec V = 700 m/s for T 2 sec 1 From Abrahamson & Silva (2008)

8 More recently, four of the five NGA relationships used V s30 as an independent parameter to explicitly represent local site conditions. Amplification [w.r.t. V S30 = 1100 m/s] V S30 (m/s) Linear PGA1100 = 0.1 g PGA1100 = 0.3 g PGA1100 = 0.5 g V 1 Example of the V S30 scaling for T = 0.2 sec From Abrahamson & Silva (2008)

9 1.4 PGA for V s30 / PGA for V s30 = 1100 m/s PGA Abrahamson & Silva (2008) Boore & Atkinson (2008) 0.2 Campbell & Bozorgnia (2008) Chiou & Youngs (2008) Average Shear wave velocity, V s30 (m/s)

10 PSA for V s30 / PSA for V s30 = 1100 m/s PSA for T = 1sec Abrahamson & Silva (2008) Boore & Atkinson (2008) Campbell & Bozorgnia (2008) Chiou & Youngs (2008) Average Shear wave velocity, V s30 (m/s)

11 NGA DATA 2011 FLATFILE

12 From Flatfile 8 Earthquake magnitude, M Closest distance, R (km)

13 From Flatfile 8 Earthquake magnitude, M V s30 (m/s)

14 From Flatfile 1 Best estimate for R 160 km 0.1 PGA (g) 0.01 Best estimate for R > 160 km Tottori; M = 6.6; Mech 0 Dist, R 160 km Dist, R > 160 km PGA values adjusted to V s30 = 580 m/s Closest distance, R (km)

15 Total No. of Entries 8163 Non-FF 426 Mag not listed 201 Mechanism not listed 139 Distance not listed 92 Vs30 not listed 17 PSA not listed 63 Mag < Distance > 160 km 1890 Others 136 Total No. of FF Entries to be used 5026

16 FF Records; Dist 160 km; M Earthquake magnitude, M Closest distance, R (km)

17 FF Records; Dist 160 km; M Earthquake magnitude, M V s30 (m/s)

18 FF Records; Dist 160 km; M Earthquake magnitude, M PGA (g)

19 FF Records; Dist 160 km; M Earthquake magnitude, M PGV (cm/s)

20 DATA SET Stations having V s m/s and R 160 km Total Number of records used = 1288 [147 recordings (Vs m/s) from the Chi-Chi main shock were excluded] 4.5 M km R 160 km 450 m/s V s m/s [only 10 recordings at site with V s30 > 1200 m/s]

21 ( ) ( ) ( ) ( ) g x( - 45 ) Ln y = a + a M - b + b M Ln R R + V 0 + ff s30 M > 6.75 Period (sec) α 1 α 2 β 1 β 2 γ φ 0.01 (PGA) ξ M 6.75 Period (sec) α 1 α 2 β 1 β 2 γ φ ξ 0.01 (PGA) Range of M ==> 4.5 to 8 Range of R ==> 0 to 160 km Range of Vs30 ==> 450 to 2100 m/s IMI_2011

22 Individual recording Overall trend Discrete averages over M, R, or V s30 bins Residual Earthquake magnitude, M

23 Residual Closest distance, R (km)

24 Residual V s30 (m/s)

25 Residual V s30 (m/s)

26 1 PGA (g) 0.1 M = relationship V s30 = 450 m/s 2011 relationship V s30 = 900 m/s 2008 relationship 450 m/s V s m/s M = Closest distance, R (km)

27 1.4 PGA for V s30 / PGA for V s30 = 1100 m/s Based on IMI_ PGA Abrahamson & Silva (2008) Boore & Atkinson (2008) 0.2 Campbell & Bozorgnia (2008) Chiou & Youngs (2008) Average Shear wave velocity, V s30 (m/s)

28 PSA for V s30 / PSA for V s30 = 1100 m/s PSA for T = 1sec Abrahamson & Silva (2008) Boore & Atkinson (2008) Campbell & Bozorgnia (2008) Chiou & Youngs (2008) Average Based on IMI_ Shear wave velocity, V s30 (m/s)

29 WHAT DO THE NGA DATA SHOW?

30 Examination in terms of the ratio of: PGV/V s30 which may be considered as a "proxy" for shear strain induced by the earthquake ground motions Plotting PGA versus this ratio can then be examined as if it were a "stress- strain" relationship

31 1 V s30 based on measured V s 0.1 PGA (g) 0.01 Range & number of V s30 values to 900 m/s; to 1000 m/s; to 1200 m/s; to 1400 m/s; to 2000 m/s; Shear strain (%)

32 1 V s30 based on measured V s 0.1 PGA (g) recordings Range & number of V s30 values to 500 m/s; to 550 m/s; to 600 m/s; to 700 m/s; to 800 m/s; Shear strain (%)

33 1 0.1 PGA (g) recordings 16 recordings 22 recordings V s30 based on measured V s Shear strain (%) Range & number of V s30 values 450 to 500 m/s; to 550 m/s; to 600 m/s; to 700 m/s; to 800 m/s; 131

34 G/G max Comp rock Weathered rock Clay; PI = Shear strain (%)

35 1 0.1 PGA (g) 0.01 Range & number of V s30 values to 250 m/s; to 300 m/s; to 350 m/s; to 400 m/s; 144 V s30 based on measured V s 400 to 450 m/s; Shear strain (%)

36 G/G max Sand layers over depth range of 500 ft 120 to 250 ft 50 to 120 ft 20 to 50 ft 20 ft Shear strain (%)

37 1 0.1 PGA (g) V s30 based on measured V s Shear strain (%) Range & number of V s30 values 100 to 140 m/s; to 180 m/s; to 200 m/s; to 210 m/s; 74

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39 Examination in terms of : Variations of PGA and PSA, at T = 1 sec, with Distance For various bins of V s30 Chi-Chi main shock data

40 1 Peak horizontal acceleration (g) pga V 30 (m/s) = Chi Chi main shock M = 7.6; Mechanism Closest distance to rupture surface (km)

41 1 Peak horizontal acceleration (g) pga Regression curve for V s30 (m/s) = V s30 (m/s) = V s30 (m/s) = V s30 (m/s) = Chi Chi main shock M = 7.6; Mechanism Closest distance to rupture surface (km)

42 PGA (g) PGA calculated at a Distance = 3 km Distance = 30 km Distance = 100 km V S30 (m/s)

43 PSA (T = 1 s) for V s30 PSA (T = 1 s) for V s30 = 450 m/s Dist = 1 km Dist = 3 km Dist = 10 km Dist = 30 km Dist = 100 km V s30 (m/s) Based on Chi-Chi Recordings

44 Based on recorded data from other than the Chi-Chi earthquake

45 Range of V s30 Landers earthquake, M = to 450 m/s (10) 350 to 400 m/s (8) 300 to 350 m/s (34) 250 to 300 m/s (13) 200 to 250 m/s (5) PGA (g) Range of V s30 Sierra el Mayor earthquake, M = to 450 m/s (27) 350 to 400 m/s (32) 300 to 350 m/s (46) 250 to 300 m/s (56) 200 to 250 m/s (22) 0.01 Range of V s30 Hector Mine earthquake, M = to 450 m/s (5) 350 to 400 m/s (34) 300 to 350 m/s (32) 250 to 300 m/s (33) 200 to 250 m/s (6) Closest distance (km)

46 Recorded values at sites with range of V s30 = 400 to 450 m/s Regressed curve 0.1 PGA (g) 0.01 Landers earthquake, M = 7.28 Sierra el Mayor earthquake, M = 7.2 Hector Mine earthquake, M = Closest distance (km)

47 Recorded values at sites with range of V s30 = 350 to 400 m/s Regressed curve 0.1 PGA (g) 0.01 Landers earthquake, M = 7.28 Sierra el Mayor earthquake, M = 7.2 Hector Mine earthquake, M = Closest distance (km)

48 Recorded values at sites with range of V s30 = 300 to 350 m/s Regressed curve 0.1 PGA (g) 0.01 Landers earthquake, M = 7.28 Sierra el Mayor earthquake, M = 7.2 Hector Mine earthquake, M = Closest distance (km)

49 Recorded values at sites with range of V s30 = 250 to 300 m/s Regressed curve 0.1 PGA (g) 0.01 Landers earthquake, M = 7.28 Sierra el Mayor earthquake, M = 7.2 Hector Mine earthquake, M = Closest distance (km)

50 Recorded values at sites with range of V s30 = 200 to 250 m/s Regressed curve 0.1 PGA (g) 0.01 Landers earthquake, M = 7.28 Sierra el Mayor earthquake, M = 7.2 Hector Mine earthquake, M = Closest distance (km)

51 Regressed curves Range of V s30 = 400 to 450 m/s Range of V s30 = 350 to 400 m/s Range of V s30 = 300 to 350 m/s Range of V s30 = 250 to 300 m/s Range of V s30 = 200 to 250 m/s 0.1 PGA (g) 0.01 Landers earthquake, M = 7.28 Sierra el Mayor earthquake, M = 7.2 Hector Mine earthquake, M = Closest distance (km)

52 Recorded values at sites with range of V s30 = 300 to 350 m/s Regressed curve 0.1 PGA (g) 0.01 Loma Prieta earthquake, M = 6.9 Iwate earthquake, M = 6.9 Chuetsu-Oki earthquake, M = 6.8 Kobe earthquake, M = Closest distance (km)

53 Regressed curves Range of V s30 = 400 to 450 m/s Range of V s30 = 350 to 400 m/s Range of V s30 = 300 to 350 m/s Range of V s30 = 250 to 300 m/s Range of V s30 = 200 to 250 m/s 0.1 PGA (g) 0.01 Loma Prieta earthquake, M = 6.9 Iwate earthquake, M = 6.9 Chuetsu-Oki earthquake, M = 6.8 Kobe earthquake, M = Closest distance (km)

54 WHAT DO THE NGA DATA SHOW? The data generally do not fully support the "published" trends.

55 Site Response Studies

56 Shear wave velocity, V s (m) 0 Layer Depth below ground surface (m) Layer 2 Layer 3 Layer 4 Layer 5 Layer Profile abc V s30 = 220 m/s 120 Halfspace V s = 1100 m/s 120 Profile abc

57 Shear wave velocity, V s (m) Depth below ground surface (m) Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 Layer Profile cba Vs30 = 220 m/s 120 Halfspace V s = 1100 m/s 120

58 CONCLUDING REMARKS ü V s30 is not a fundamental geotechnical parameter ü V s30 is not a unique geotechnical parameter ü Since G/G max is material-dependent ü V s30 is neither necessary nor sufficient to describe or accommodate nonlinear effects

59 CONCLUDING REMARKS ü The contributions of the soil profile below 30 m to site response, cannot be accommodated with the use of V s30 with or without including the depth to V s = 1 km/s or to V s = 2.5 km/s (even if these depths are known for a sufficient number of sites). ü Sites with "identical" V s30, but differing layering, can have significantly different response

60 CONCLUDING REMARKS ü Additional work is needed before V s30 should be used as a continuous independent parameter in earthquake ground motions attenuation relationships. ü The use of a range of V s30 to describe a generalized "site category" for building code purposes, is reasonable.

61 CONCLUDING REMARKS ü Site-specific response calculations (with at least 7 rock outcrop input motions) are preferable for assessing local site effects this requires high quality site response calculations. ü Distance dependence can be critical further studies are in progress, incorporating V s30 in estimating the slope of the attenuation relationship.

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