An Empirical Model to Account for Spectral Amplification of Pulse-Like Ground Motion Records

Abstract

Near-source effects can amplify seismic ground motion, causing large demand to structures and thus their identification and characterization is fundamental for engineering applications. Among the most relevant features, forward-directivity effects may generate near-fault records characterized by a large velocity pulse and unusual response spectral shape amplified in a narrow frequency-band. In this paper, we explore the main statistical features of acceleration and displacement response spectra of a suite of 230 pulse-like signals (impulsive waveforms) contained in the NESS1 (NEar Source Strong-motion) flat-file. These collected pulse-like signals are analyzed in terms of pulse period and pulse azimuthal orientation. We highlight the most relevant differences of the pulse-like spectra compared to the ordinary (i.e., no-pulse) ones, and quantify the contribution of the pulse through a corrective factor of the spectral ordinates. Results show that the proposed empirical factors are able to capture the amplification effect induced by near-fault directivity, and thus they could be usefully included in the framework of probabilistic seismic hazard analysis to adjust ground-motion model (GMM) predictions.

Near-Source Magnitude Scaling of Spectral Accelerations: Analysis and Update of Kotha et al. (2020) Model

“…= 1, 2 here, residuals from pulse-like recordings are preferentially above zero baseline, indicating that these ground-motions are systematically stronger than the GMM predictions. Without dwelling further, we refer to a similar finding by Sgobba et al (2021a); wherein, pulse-like recordings are characterised by a higher than average long-period and lower than average short-period .…”

“…1, around 30% of NESS recordings show pulse-like characteristics, which are often characterised by higher long-period and lower short-period than average, depending on the event-to-site distance and azimuth. Instead of biasing the (isotropic) GMM median (at short distances and long periods) with pulse-like effects, we chose to reserve NESS datasets for investigation of complex near-fault phenomenon, as done by Sgobba et al (2021a).…”

“…If the recording stations were concentrated on this side of the fault trace, then it is likely that the finite-fault effects and anisotropic radiation (along with site-effects) might have aggravated their recorded ground-motions (e.g. Spudich 2013, Donahue and Abrahamson 2014, Dujardin et al 2018, Kotha et al 2019, Sgobba et al 2021a Table .1 lists the events whose between-event random-effect is often larger than 0 of the Kotha et al…”

Fling-Step Recovering from Near-Source Waveforms Database

“…(2) develop new PD attenuation models, which also take normal fault scenarios into account; and (3) provide empirical coefficients to adjust displacement response spectral ordinates from existing prediction models (e.g., [29,30]), similarly to [31], who provide empirical correction factors for pulse-like ground motion records. In particular, further developments of the long-period data processing scheme will allow for the detection of near-source waveforms affected by one-side velocity pulses before applying the baseline-correction.…”

Empirical Evidence of Frequency‐Dependent Directivity Effects From Small‐To‐Moderate Normal Fault Earthquakes in Central Italy

“…In the context of empirical ground motion models (GMMs), directivity was clearly seen in the residuals of most earthquakes collected worldwide, such as in NGA‐West (Rowshandel, 2010) or other recent near‐source databases (Pacor et al., 2018; Sgobba, Felicetta, et al., 2021). Many researchers in the past have empirically modeled directivity effects by fitting the azimuthal variation of within‐event residuals to get unbiased prediction (Sgobba, Lanzano, et al., 2021; Shahi & Baker, 2011; Somerville et al., 1997).…”