What Is Fling Step? Its Theory, Simulation Method, and Applications to Strong Ground Motion near Surface Fault Ruptures

ABSTRACT We present the theory of the fling step and a theoretical method for simulating accurately the near-fault strong motions, and apply it to reproduce various strong-motion records near surface faults. Theoretically, the fling step is the contribution of the static Green’s function in the representation theorem (Hisada and Bielak, 2003), and we show that this theory holds for any seismic velocity structure. We first demonstrate the validity of this theory using theoretical solutions of a circular fault model in a homogeneous full-space. Next, we apply the theory to layered half-spaces, present a theoretical method based on the wavenumber integration method, and introduce various techniques to simulate the near-fault ground motions including fling steps with high accuracy. Finally, we demonstrate the effectiveness of the method by reproducing various strong-motion records near surface fault ruptures and discuss the characteristics of near-fault strong motions including the fling step and the forward directivity pulse. We made all of the software and data used in this article available on the internet.

Impulsive Signals Produced by Earthquakes in Italy and Their Potential Relation with Site Effects and Structural Damage

Near fault seismic records may contain impulsive motions in velocity-time history. The seismic records can be identified as impulsive and non-impulsive depending on the features that their waveforms have. These motions can be an indicator of directivity or fling step effect, and they may cause dangerous effects on structures; for this reason, there is increasing attention on this subject in the last years. In this study, we collect the major earthquakes in Italy, with a magnitude large or equal to Mw 5.0, and identify the impulsive motions recorded by seismic stations. We correlate impulsive motions with directivity and fling step effects. We find that most earthquakes produced impulsive signals due to the directivity effect, though those at close stations to the 30 October 2016 Amatrice earthquake might be generated by the fling step effect. Starting from the analyzed impulses, we discuss on the potential influence of site effects on impulsive signals and suggest a characterization based on the main displacement directions of the impulsive horizontal displacements. Finally, we discuss on the damage of three churches in Emilia, which were subject to impulsive ground motion, underlying in a qualitative way, how the characteristics of the pulses may have had influences the structural response of the façades.

Evolution law of overall damage of concrete gravity dam under near-fault ground motion

Strong earthquake cases of concrete gravity dams show that the foundation damage has an important influence on the seismic response and damage characteristics of the dam body. Compared with non-pulse ground motions, pulse-like near-fault ground motions have a wider response spectrum sensitive zone, which will cause more modes of the structure to respond, resulting in more serious damage to the structure. In order to study the real dynamic damage characteristics of concrete gravity dams under the action of near-fault ground motions, this paper takes Koyna gravity dam as the object and establishes a multi-coupling simulation model that can reasonably reflect the dynamic damage evolution process of dam concrete and foundation rock mass. A total of 12 near-fault ground motion records with three types of rupture directivity pulse, fling-step pulse and non-pulse are selected, deep research on the overall damage evolution law of concrete gravity dams. Considering the additional influence of different earthquake mechanisms, different site types and other factors on the study, the selected ground motion records are from the same seismic events (Chi-Chi), the same direction but different stations. The results show that the foundation of the concretes gravity dam often get damaged before the dam body under the action of strong earthquakes. Compared with the near-fault non-pulse ground motion, the structural damage of the gravity dam under the action of the near-fault directivity pulse ground motion is significantly increased, and causes greater damage and displacement response to the dam body. The near-fault fling-step pulse ground motion has the least impact on the dynamic response of the gravity dam structure.

Fling-Step Recovering from Near-Source Waveforms Database

We present an upgraded processing scheme (eBASCO, extended BASeline COrrection) to remove the baseline of strong-motion records by means of a piece-wise linear detrending of the velocity time history. Differently from standard processing schemes, eBASCO does not apply any filtering to remove the low-frequency content of the signal. This approach preserves both the long-period near-source ground-motion, featured by one-side pulse in the velocity trace, and the offset at the end of the displacement trace (fling-step). The software is suitable for a rapid identification of fling-containing waveforms within large strong-motion datasets. The ground displacement of about 600 three-component near-source waveforms has been recovered with the aim of (1) extensively testing the eBASCO capability to capture the long-period content of near-source records, and (2) compiling a qualified strong-motion flat-file useful to calibrate attenuation models for peak ground displacement (PGD), 5% damped displacement response spectra (DS), and permanent displacement amplitude (PD). The results provide a more accurate estimate of ground motions that can be adopted for different engineering purposes, such as performance-based seismic design of structures.

Accuracy of Near-Fault Fling-Step Displacements Estimated Using the Discrete Wavenumber Method

ABSTRACT Near-fault fling steps might cause severe damage to near-fault structures such as bridges or base-isolated buildings. Therefore, the accurate simulation of ground displacements including fling steps based on fault models is an important issue not only for seismological but also for engineering purposes. The discrete wavenumber (DWN) method (e.g., Bouchon, 2003) has been established as a method to calculate complete elastic wavefield, including permanent displacement for a homogeneous or a layered half-space. However, the accuracy of the permanent displacements calculated by the DWN method is influenced by the selection of parameters, such as the imaginary part of the complex frequency and the subfault size in the case of extended sources. The objective of this study is to clarify the requirement for these parameters for the accurate simulation of fling-step displacements to further enhance the use of the DWN method. Honda and Yomogida (2003) also addressed the issue of calculating fling-step displacements using the DWN method; however, their study was focused on cases in which a large amount of seismic moment is released at depth. This study was focused on fling-step displacements due to rather shallow slip, in which the fault distance was as small as several meters in an extreme case, motivated by recent damaging earthquakes such as the 2016 Kumamoto, Japan, earthquakes. The ground displacements including fling steps were calculated by the DWN method and compared with the analytical solutions for the static displacements (Okada, 1985, 1992), both for point sources and extended sources in a homogeneous half-space. According to the results, following recommendations were made. For the imaginary part of the complex frequency, ωc=ω−λi, λ=ξπ/Tw with ξ≥2.0 can be recommended, with the understanding that the waveforms are effective only within the range of [0,Tw/ξ]. For extended sources, the subfault size should be as small as 0.5 times the fault distance to accurately simulate fling steps.