Nonlinear Horizontal Site Amplification for Constraining the NGA-West2 GMPEs

Abstract

The nonlinear soil amplification models developed by Walling et al. (2008) are revisited for three main reasons: (a) the simulation database on which the models were developed has been updated and extended, (b) two alternatives for the input shaking parameter—(PGA and Sa( T))—are explored, and (c) a constraint on the nonlinearity at long periods is removed. The model is based on site amplification factors, relative to a V S30 = 1;180 m/s site. Simulations included a wide range of soil profiles, shaking amplitudes and soil properties, from which only a subset was used herein. Finally, four models for the nonlinear site amplification are developed using two nonlinear material property models (peninsular range and EPRI) and two input-shaking parameters ( PGA1180 and Sa1180( T)). These results are intended for use by the NGA-West2 developers to constrain the nonlinear scaling of the site response for the horizontal ground motion models.

A Site Amplification Model for Crustal Earthquakes

“…The first one is the linear site response which is currently modeled with a continuous function of V S30 (time-based average of shear wave velocity of top 30 m soil media) after Boore et al [8]. The site amplification is assumed to decrease linearly with increasing natural logarithm of V S30 [6][7][8][9][10][11][12][13]. Some researchers uses a period-independent fixed reference V S30 for linear scaling (e.g., 760 m/s in [12]) or period-dependent reference V S30 (e.g., [13]).…”

“…The site amplification is assumed to decrease linearly with increasing natural logarithm of V S30 [6][7][8][9][10][11][12][13]. Some researchers uses a period-independent fixed reference V S30 for linear scaling (e.g., 760 m/s in [12]) or period-dependent reference V S30 (e.g., [13]). Using either period-independent or dependent values does not affect the slope of linear site scaling [14].…”

“…The period independent constraining value can be used as well (e.g., V S30 = 1000 m/s in [7] or 1130 m/s in [18]). The second behavior in site amplification is soil nonlinearity, which is again modeled using V S30 , which represents soil stiffness and input rock motion [6,7,[9][10][11][12][13][15][16][17][18][19][20][21]. The level of soil nonlinearity decreases as soil stiffness increases or input rock motion decreases.…”

Using ambient vibration measurements for risk assessment at an urban scale: from numerical proof of concept to Beirut case study (Lebanon)

“…We thus had to associate a friction angle and a cohesion to each layer for the NOAH input files: This was done using specific "rules" applied to the shear wave velocity profiles. We simply followed the approach used by PEER ("Pacific Earthquake Engineering Research Center") for the incorporation of a NL term in the recent revision of Ground Motion Prediction Equation for Western United States (NGAW2 project, Kamai et al 2014): They used two sets of generic, depthdependent NL degradation curves depending on the soil cohesion: The "EPRI" curves are assigned to cohesionless soils, while "IV" (Imperial Valley) curves are assigned to cohesive soils. Following the PEER approach, all soil profiles having a V S30 value lower than 190 m/s were considered as cohesive ("IV"), while sites with V S30 higher than 190 m/s were considered as cohesionless, in both cases from the surface down to the bedrock.…”

Squeezing Kappa (κ) Out of the Transportable Array: A Strategy for Using Bandlimited Data in Regions of Sparse Seismicity

“…These individual profiles were grouped into two classes, one stiff soil (V S30 ≤ 670 m=s, with site Z14A chosen as representative) and one firm rock (V S30 > 670 m=s, with site 113A chosen as representative). These two near-surface profiles were appended onto a generic profile appropriate for the western United States (WUS; Kamai et al, 2013) and extrapolated to 10 km depth (where the source region is assumed) with β 3:5 km=s (Fig. 3b).…”