Empirical nonergodic shaking scenarios based on spatial correlation models: An application to central Italy

Abstract

This paper provides a new methodological framework to generate empirical ground shaking scenarios, designed for engineering applications and civil protection planning. The methodology is useful both to reconstruct the ground motion pattern of past events and to generate future shaking scenarios, in regions where strong-motion datasets from multiple events and multiple stations are available. The proposed methodology combines (1) an ad-hoc nonergodic ground motion model (GMM) with (2) a spatial correlation model for the source region-, site-, and path-systematic residual terms, and (3) a model of the remaining aleatory error to take into account for directivity effects. The associated variability is a function of the type of scenario generated (bedrock or site, past or future event) and it is minimal for source areas where several events have occurred and for sites where recordings are available. In order to develop the region-specific fully nonergodic GMM and to compute robust estimation of the residual terms, the approach is calibrated on a highly dense dataset compiled for the area of central Italy. Example tests demonstrate the validity of the approach, which allows to simulate acceleration response spectra at unsampled sites, as well as to capture peculiar physical features of ground motion patterns in the region. The proposed approach could be usefully adopted for data-driven simulations of ground shaking maps, as alternative or complementary tool to physic-based and stochasticbased approaches.

Spatial correlation of broadband earthquake ground motion in Norcia (Central Italy) from physics-based simulations

“…At the same time, the analysis of possible spatial anisotropy patterns reveals that the ground motion field has a strong azimuthal dependence due to rupture propagation effects along the strike direction (FP). This result is consistent with the tectonic setting of the region, as also demonstrated by Sgobba et al (2020). The availability of different rupture scenarios generated for the same causative fault allowed the investigation of several aspects of the spatial correlation, such as the dependence on the magnitude, slip distribution and hypocentral location.…”

“…This result is noteworthy, as it is common practice to work with normalized residuals and therefore the sill is not estimated, being equal to 1. Similar outcomes are provided in Sgobba et al (2019Sgobba et al ( , 2020, who estimated the range and sill of the source-, path-and site-corrective terms of the ground motion model specific for the Po Plain and central Italy regions, respectively. Secondly, the FN sill and range values tend to be larger than the FP ones over a broad range of periods, in agreement with Infantino et al (2021), who investigated the FN/FP ratios for 6 different 3D PBSs.…”

“…While the FN direction is mainly affected by the faulting mechanism, rupture directionality effects have a strong impact on the FP direction, leading to more coherent (larger range) and more variable (higher sill) ground motions. Such an observation is also made by Sgobba et al (2020). While applying their novel methodology to produce non-ergodic shaking scenarios for the Mw6.5 Norcia case-study, the authors identify a strong azimuthal dependence due to rupture directivity along the strike direction.…”

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

“…The data set is the same as used by Sgobba et al. (2021) and consists of high‐quality accelerometric and velocimetric waveforms related to stations and earthquakes located in Central Italy since 2008. The tectonic setting of this region is complex in terms of mechanical discontinuities and rheological properties (Carafa & Barba, 2011; Chiarabba et al., 2018), featuring mainly normal faults that caused several seismic sequences in the last 20 years (i.e., 1997–1998, Umbria‐Marche Mw 6.0; 2009, L’Aquila Mw 6.1; 2016–2017, Amatrice‐Visso‐Norcia Mw 6.5; 2018, Muccia Mw 4.6).…”

“…With the increasing dissemination of nonergodic approaches for ground‐motion modeling in highly sampled areas (e.g., Baltay et al., 2017; Kuehn et al., 2019; Lavrentiadis et al., 2021; Sgobba, Lanzano, & Pacor, 2021), thanks to the use of repeated observations, it is now possible to decompose further the residuals into systematic effects, also known as random‐terms, via a mixed‐effect regression approach (Stafford, 2014), permitting better isolation of different sources of the model variability. Yet, in nonergodic models, directivity rupture effects are not repeatable in the sense that directivity from any earthquake does not allow predicting that of another event (Sahakian et al., 2019).…”