Multi-pulse characteristics of near-fault ground motions

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Highlights

  • This procedure localizes all pulses in the time domain independently and discontinuously.

  • Multi-pulse ground motions are more likely to be recorded in relatively limited areas.

  • TPE is almost identical to the period of the first pulse in the time domain (TP1).

  • Pulse periods are related not only to magnitudes but also to fault-types.

  • PGVs of inherent pulses can be expressed by PGVE in a linear attenuation relationship.

Abstract

Multi-pulse characteristics of near-fault ground motions, such as the number of inherent pulses, pulse periods, and amplitudes, have notable influences on the response of structures. To investigate these important parameters, an automatic detection procedure, which is conducted on the rough pulse signal that is extracted by the HHT method, is proposed in this work. This procedure can localize all inherent pulses in the time domain independently and discontinuously. Important parameters can be automatically obtained at the same time. Then, statistical relationships between these multi-pulse parameters and earthquake parameters, including moment magnitudes, site conditions (Vs30), rupture distances and types of faults, are investigated comprehensively. With an increasing number of pulses, the multi-pulse ground motions are more likely to be recorded in relatively limited areas. All pulse periods in a velocity record are similar to each other and can be represented by the period of the pulse with the largest energy (TPE). TPEs are almost identical to periods of the first pulse in the time domain (TP1). They are related not only to magnitudes but also to fault-types and site conditions. New empirical models are proposed in this work according to fault-types that can predict most TPEs across all magnitudes (from 5.7 to 7.6). For amplitudes, all PGVs of inherent pulses can be expressed by the PGV of the pulse with the largest energy (PGVE) in a linear attenuation relationship. PGVEs are related to rupture distances, site conditions, and fault-types. New empirical models are also developed in this work.

Introduction

The impulsive characteristics of near-fault ground motions have been studied by many researchers in recent decades. The large-energy and long-period inherent velocity pulses can result in large displacement cycles in structural responses, which can impose severe ductility or strength demands on the structures subjected to this kind of ground motions [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]]. Many previous studies focused on the effect of a single inherent pulse on structural responses, however, in many cases, there are multiple pulses hidden in the velocity records, which may result in different structural behavior [12,13].

To better understand the multi-pulse characteristics of near-fault ground motions, Alavi and Krawinkler [14] attempted to describe the acceleration history by square waves and then investigated the effect of repeated pulses on response parameters. Makris and Black [15,16] also characterized multiple inherent pulses by harmonic functions. Menun and Fu [17] represented these pulses by a composite function as well. These models are based on certain critical parameters, such as pulse periods and amplitudes, which should be determined in advance, and the number of pulses needs to be determined artificially. Some of these studies treated the multiple pulses as simple repeats of a single pulse, but the pulses occur in order based on time; additionally, the amplitude of the rear pulse may be smaller than that of the front pulse because of the dissipation of energy. This characteristic will be verified in this paper.

With the development of wavelet theory, Mavroeidis and Papageorgiou [18] created the M&P wavelet to represent inherent pulses. Based on the M&P wavelet, Vassiliou and Makris [19] proposed an extended wavelet method, but it is also possible to determine several parameters and identify the number of pulses artificially. The selection of the mother wavelet has an effect on the results as well. Baker proposed a signal-processing method, but it can only classify the records with one pulse [20]. Lu and Panagiotou [21] introduced another signal-processing method, referred to as the cumulative pulse extraction method, to extract multiple pulses based on the M&P wavelet. This method has a problem of overlap in the time domain following multiple extractions. Although Lu et al. provided a technique for addressing this overlap, the extracted pulses were still not precisely identified in the time domain for all available records and each pulse could not be clearly defined. Zhai et al. [22] proposed an energy-based method that can only classify multi-pulse ground motions, but cannot extract the inherent pulses nor provide the pulse periods.

The authors proposed a concise signal-processing method in previous work, termed the HHT method, for identifying inherent pulse signals in near-fault ground motions [23]. In the HHT method, the complete pulse signal that carries all the inherent pulses is extracted based on only two physical parameters (PGV/PGA ratio and energy contribution), and all useful information, such as pulse periods, is obtained by a single extraction process without any assumptions or modifications. In this paper, we applied the Rain-Flow counting method and an energy criterion to the extracted pulse signal to automatically locate every individual pulse in the time domain. The number of inherent pulses can also be obtained without any manual operation that was necessary in previous methods at the same time. Pulse periods are obtained by calculating the instantaneous frequency of pulse peaks. All ideal pulses that are localized in the time domain are independent and correspond to the time that they occur, which means that inherent pulses can be discontinuous in the time domain. This concept is different from previous work and may be closer to the reality of ground motion records. Structural responses dictated by interval or continuous multi-pulse ground motions may be different due to the low-cycle fatigue of structures subjected to a long-duration earthquake. It is should be noted that elastic structural responses and elastoplastic displacement responses of these localized pulses are similar to those of the original extracted rough-pulse signal. Finally, all the inherent pulses of multi-pulse records corresponding to important parameters of engineering interest, such as the period, amplitude, and timing of each pulse, and the number of inherent pulses, are obtained automatically and completely in a signal-processing approach.

The relationships between these parameters and certain earthquake parameters, such as magnitudes, site conditions, rupture distances, and fault types, are studied to provide insight into the factors that influence the multi-pulse characteristics of near-fault ground motions. The number of inherent pulses is related to rupture distances and site conditions. With an increase in the number of pulses, the area in which multi-pulse ground motions are more likely to be recorded are concentrated to a limited region that is centered at a point over a distance of 20 km and a shear wave velocity of 350 m/s. Pulse periods of inherent pulses in one record are not significantly different from each other, and these periods are not only related to moment magnitude but also to fault-types. However, the effect of fault-type was not considered in previous models. For peak ground velocity, all PGVs of inherent pulses can be expressed by the PGV of the pulse with the largest energy (PGVE) in a linear attenuation relationship. A group of empirical models is also proposed that consider the effects of magnitudes, fault-types, site conditions, and rupture distances.

Section snippets

Extraction of PULSE signals

The database used in this paper is based on 91 records which were classified as pulse-like ground motions by Baker in 2007 [20]. Among these records, five were excluded because they were not classified as pulse-like ground motions by the HHT method [23]. The other 86 records are used as the database in this work.

The pulse signals are extracted from these records by the HHT method which is a signal-processing procedure that is based on the identification of important low-frequency components in

Automatic detection of inherent pulses in time domain

For multi-pulse cases, pulse signals extracted by the HHT method represent a relatively rough signal that carries all the inherent pulses. In order to locate every velocity pulse in the time domain and acquire the number of inherent pulses automatically and rationally, an additional process is needed.

Relationships between number of inherent pulses and earthquake parameters

The number of inherent pulses that are determined according to the definition of a single inherent pulse, are related to rupture distances; and there is not a clear relationship between the number of inherent pulses and magnitudes, as shown in Fig. 5 (a). Ground motions with two or three inherent pulses are more likely to occur at greater rupture distances, and records with only one pulse were all obtained within a distance of 20 km, as shown in Fig. 5(b). The number of inherent velocity pulses

Conclusions

Multi-pulse characteristics of near-fault ground motions result in different structural behaviors. The damage to structures is also related to the number of inherent pulses, the timing of each pulse in a record, and the periods and amplitudes of these pulses.

The HHT method can extract all inherent pulses as a rough pulse signal from the original record. In order to automatically detect every inherent pulse in the time domain, the rough pulse signal is subjected to a procedure that is based on

Author statement

The manuscript entitled “MULTI-PULSE CHARACTERISTICS OF NEAR-FAULT GROUND MOTIONS” is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere. All the authors listed have approved the manuscript that is enclosed.

Declaration of competing interest

No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere. All the authors listed have approved the manuscript that is enclosed.

Acknowledgements

All the authors would like to express their sincere gratitude to Jack W. Baker of the Stanford University for his comment in the review of the work introduced in reference [20], this is of benefit to the work proposed in this paper. This research is supported by the National Natural Science Foundation of China under Grant (NNSFC51778206).

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