Geo-Marine Letters Volume 36, 2016, electronic supplementary material

Transcription

1 1 Geo-Marine Letters Volume 36, 016, electronic supplementary material Submarine landslides offshore Vancouver Island along the northern Cascadia margin, British Columbia: why preconditioning is likely required to trigger slope failure Nastasja A. Scholz 1, Michael Riedel,3+, Morelia Urlaub 3, George D. Spence 1, Roy D. Hyndman,1 1 School of Earth and Ocean Science, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8P 5C, Canada Natural Resources Canada, Geological Survey of Canada-Pacific 9860 West Saanich Road, Sidney, BC, V8L 4B, Canada 3 GEOMAR, Helmholtz Center for Ocean Research Kiel, Wischhofstrasse 1-3, 4148 Kiel, Germany + Corresponding Author: mriedel@geomar.de Phone: +49 (0) Fax: +49 (0431)

2 1 Atkinson (005) Atkinson (005) developed a ground motion prediction equation (GMPE) for the horizontal acceleration due to crustal and offshore earthquakes along the Cascadia margin using a hybrid-empirical approach. Due to the apparent similarity to Cascadia events, Atkinson (005) multiplied Californian relations with frequency-dependent factors to account for regional differences in crustal amplification. The resulting GMPE equation can be written as: Y 5 a1 a( M 6.0) a3( M 6.0) a4 log R a R (A.1) where Y is the site response and M the moment magnitude. Offshore events are calculated with the same equation and coefficients but the term (M-6.0) is replaced by (M-6.5). In equation A1 the distance parameter R is calculated via R D h, where D is the closest distance to the fault (i.e. hypocenter in the case of small earthquakes) and h is calculated via log h M. Table A.1 summarizes the coefficients a 1 to a 5 resulting from Atkinson s (005) regression. Boore and Atkinson (008) Boore and Atkinson (008) developed an empirical GMPE for average horizontal ground accelerations for shaking periods between 0.01 s and 10 s. According to the authors the equations are valid for magnitudes between 5.0 and 8.0, for distances R JB < 00 km and V S30 of m/s. R JB (Joyner-Boore distance) is the closest distance to the surface projection of the fault plane whereas V S30 is the average S-wave velocity within the uppermost 30 m of the sediment column. In contrast to the other GMPEs used in this study, the GMPE of Boore and Atkinson (008) also includes the fault type. The final form of the equation of Boore and Atkinson (008) is: Y = F M (M) + F D (R JB, M) + F S (V S30, R JB, M) (A.) where F M, F D, and F S stand for magnitude scaling, distance function, and site amplification, respectively. The distance function can be written as R FD ( RJB, M ) c1 c( M M Ref ) c3( R RRef ) R (A.a) Ref

3 3 With R R h (A.b) JB with M Ref = 4.5 and R Ref = 1km. The magnitude scaling term is calculated via: a) M M h F M ( M) eu 1 ess e3ns e4rs e5 ( M M h) e6 ( M M h) (A.c) b) M > M h F M( h M) eu e SS e NS e RS e ( M M ) (A.d) where U, SS, NS, and RS stand for unspecified, strike-slip, normal-slip, and reverse-slip faults, having a value of One for the respective fault type. M h is the hinge magnitude for the shape of the magnitude scaling. The site amplification term is given by F S = F lin + F where F lin is the linear and F the non-linear part. The linear term can be written as V S30 F lin blin (A.e) V Ref b lin is a period dependent coefficient, and V Ref is a reference velocity here set to 760 m/s (Boore and Atkinson, 008). The value for the non-linear term depends on the value for the peak ground acceleration (PGA) at V=V Ref as well as on a few threshold levels for linear and non-linear amplification. a) pga4nl a 1 : pga low F b (A.f) 0.1 b) a 1 < pga4nl a : pga pga4nl pga4nl low F b c 0.1 d (A.g) a1 a1 c) a < pga4nl: pga4nl F b (A.h) 0.1 3

4 4 with a 1 = 0.03g, a = 0.09g, and pga low = 0.06g. pga4nl is a reference PGA value for an average layer velocity of 760 m/s. The coefficients c and d are calculated via: 3y bx c (A.i) x and y bx d (A.j) 3 x with x a, and a a 1 y b (A.k) pga low b, representing a non-linear slope, depends on both the shaking period and the average shear velocity of the uppermost 30 m below ground level: a) V S30 V 1 b = b 1 (A.l) b) V 1 <V S30 V b V S30 V Ref b1 b b (A.m) V 1 V c) V <V S30 V Ref V S30 VRef b b (A.n) V VRef d) V Ref V S30 b = 0 (A.o) with V 1 = 180 m/s and V = 360 m/s. Tables A. to A.4 summarize the coefficients resulting from the regression for the respective terms in equation A..

5 5 3 Gregor et al. (00) Gregor et al. (00) used a stochastic finite-fault calculation to develop a GMPE for megathrust earthquakes to avoid the uneven sampling of site and source geometries that come with empirical relationships. Empirical calculations also do not account for effects such as rupture propagation, directivity, and source-site geometry in a systematic way and uncertainties in source, path, and site parameters were included via parametric variations. The resulting PGA and the 5% damped peak spectral acceleration (PSA) for earthquake magnitudes of 8.0, 8.5, and 9.0 can be calculated using: c c M logr exp c c M 3 Y c (A.3) 1 cm Here, Y is again the peak ground motion parameter, R is the closest distance to the rupture plane, M is the magnitude, and c 1 to c 6 are coefficients fit to rock and soil conditions given in Tables A.5 and A.6. The model holds for earthquakes of M w =8.0 and higher and accounts for rock- and soil conditions and is based on extrapolating relationships up to M w =9.0, the magnitude of the Cascadia megathrust earthquake in References Atkinson GM (005) Ground motions for earthquakes in southwestern British Columbia and northwestern Washington: crustal, in-slab, and offshore events. Bulletin of the Seismological Society of America, 95(3), Boore DM, Atkinson GM (008) Ground-motion prediction equations for the average horizontal component of PGC, PGV, and 5%-damped PSA at spectral periods between 0.01s and 10.0s. Earthquake Spectra, 4(1), Gregor NJ, Silva NJ, Wong IG, Youngs RR (00) Ground-Motion Attenuation Relationships for Cascadia Subduction Zone Megathrust Earthquakes Based on a Stochastic Finite-Fault Model, Bulletin of the Seismological Society of America, 9(5),

6 6 Table A.1: Regression coefficients for equation (A.1) taken from Atkinson (005) f(hz) a 1 a a 3 a 4 a 5 PGA

7 7 Table A.: Regression coefficients for distance for equation (A.) taken from Boore and Atkinson (008) f(hz) c 1 c c 3 h PGA

8 8 Table A.3: Regression coefficients for magnitude for equation (A.) taken from Boore and Atkinson (008) f (Hz) e 1 e e 3 e 4 e 5 e 6 e 7 M h PGA

9 9 Table A.4: Regression coefficients for site effects for equation (A.) taken from Boore and Atkinson (008) f (Hz) b lin b 1 b PGA

10 10 Table A.5: Regression coefficients for rock sites taken from Gregor et al. (00) f(hz) c 1 c c 3 c 4 c 5 c 6 PGA

11 11 Table A.6: Regression coefficients for soil sites taken from Gregor et al. (00) f(hz) c 1 c c 3 c 4 c 5 c 6 PGA