Soil Dynamics and Earthquake Engineering 25 (2005) 93–115 www.elsevier.com/locate/soildyn Processing of strong-motion accelerograms: needs, options and consequences David M. Boorea, Julian J. Bommerb,* aUS Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA bCivil and Environmental Engineering, Imperial College London, London SW7 2AZ, UK Accepted 25 October 2004 Abstract Recordings from strong-motion accelerographs are of fundamental importance in earthquake engineering, forming the basis for all characterizations of ground shaking employed for seismic design. The recordings, particularly those from analog instruments, invariably contain noise that can mask and distort the ground-motion signal at both high and low frequencies. For any application of recorded accelerograms in engineering seismology or earthquake engineering, it is important to identify the presence of this noise in the digitized time- history and its influence on the parameters that are to be derived from the records. If the parameters of interest are affected by noise then appropriate processing needs to be applied to the records, although it must be accepted from the outset that it is generally not possible to recover the actual ground motion over a wide range of frequencies. There are many schemes available for processing strong-motion data and it is important to be aware of the merits and pitfalls associated with each option. Equally important is to appreciate the effects of the procedures on the records in order to avoid errors in the interpretation and use of the results. Options for processing strong-motion accelerograms are presented, discussed and evaluated from the perspective of engineering application. q 2004 Elsevier Ltd. All rights reserved. Keywords: Strong-motion accelerograms; Signal-to-noise ratios; Instrument corrections; Baseline adjustments; Filters 1. Introduction 1933, has been of primordial importance to the development of earthquake engineering. Seismic design is primarily concerned with the balance Although strong-motion accelerograms have provided between the potential of ground shaking to cause damage seismologists and engineers with valuable insight into the (demand) and the ability of structures to resist damage nature of earthquake ground shaking close to the earthquake (capacity). The seismic capacity of engineered structures source—where damage can be expected—the information can be assessed from experimentation, analytical modeling that can be retrieved from the recordings is limited: it can and field observations following earthquakes, and indeed never be claimed that a complete and accurate description of from the interaction of these three channels of investigation. the ground shaking can be obtained from accelerograms. The characterization of seismic demand, on the other hand, For engineering uses of strong-motion data it is important to has been developed primarily from recordings obtained be able to estimate the level of noise present in each from strong-motion accelerographs. The global databank of accelerogram and the degree to which this may affect strong-motion accelerographs that has been accumulated different parameters that are derived from the records. The since the first records were obtained in Long Beach, CA, in main parameters of interest for engineering application are the ordinates of response spectra, both of acceleration and displacement. The peak ground acceleration (PGA), although of limited significance from both geophysical * Corresponding author. Tel.: C44 20 7594 5984; fax: C44 20 7225 and engineering perspectives, is also a widely used 2716. parameter in engineering. The peaks of velocity (PGV) E-mail address: [email protected] (J.J. Bommer). and displacement (PGD), measured from the time-histories 0267-7261/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.soildyn.2004.10.007 94 D.M. Boore, J.J. Bommer / Soil Dynamics and Earthquake Engineering 25 (2005) 93–115 obtained by integration of the acceleration, are also important long-period cut-off—are then presented, followed important parameters. The focus of this paper is on the by a discussion of the implications of these parameters for effects of noise in accelerograms, and the effects of the usable period range of response spectral ordinates. The ‘correction’ procedures, on the peak ground-motion ampli- question of whether the same filter parameters should be tudes and on the ordinates of acceleration and displacement applied to the three components of triaxial accelerograms is response spectra. then briefly addressed. The section closes with a discussion The objective of the paper is to provide engineering of the combined use of filters and baseline adjustments. seismologists and earthquake engineers who are not specialized in strong motion with an overview of reasons for which record processing is performed, the options 2. Noise characteristics of strong-motion data available for carrying out this processing, and the consequences of applying each of the available approaches. The purpose of recording strong earthquake shaking is to The paper also aims to highlight the fact that there is no obtain detailed information regarding the ground motion of panacea that can be prescribed for record processing and engineering interest, which can be referred to as the signal. that a degree of subjectivity is involved. Generally it is not For a variety of reasons, explained below, digitized possible to identify the ‘best’ processing for an individual accelerograms also contain extraneous ‘motions’ that are record: assumptions always need to be made and the optimal referred to as noise. From the outset it is important for users procedure for a given record will depend on the application. of strong-motion data to appreciate that digitized accel- The limitations of the data and the processing routines need erograms are never pure and complete reproductions of the to be appreciated by the end users. seismic signal. The purpose of processing accelerograms, Following this introduction, the paper begins with an which is the topic of this paper, is to optimize the balance overview of the sources and nature of noise in accelero- between acceptable signal-to-noise ratios and the infor- grams, making the important distinction between analog and mation required for a particular application, both of which digital recordings, whilst highlighting the fact that digital depend on period or frequency. recordings are by no means entirely free of noise. The distinction is also made between the standard types of noise, 2.1. Analog and digital accelerographs for which the routine processing techniques discussed in the main body of the paper are appropriate, and non-standard The first accelerographs were developed in the US in errors that should be removed prior to the application of any 1932, almost four decades after the first seismographs were general processing. Section 3 of the paper deals with high- installed, the delay being due to the difficulty in producing frequency noise and distortion due to the dynamic instruments sufficiently sensitive to produce detailed characteristics of the instrument, discussing procedures recordings of the ground motion whilst being robust enough that can be applied to compensate for these effects. to remain operational when subjected to strong shaking. The Throughout the paper, the procedures are qualified by the first strong-motion accelerograms were obtained in the adjective ‘adjustment’ rather than ‘correction’, since the Long Beach (California) earthquake of March 1933. Over boundary conditions are nearly always unknown and hence the 70 years since the Long Beach earthquake, thousands of users should be aware that the true ground motion, across accelerographs have been installed around the world and the the full range of periods that may be of engineering interest, global databank of strong-motion records—which already cannot be unambiguously determined. The fourth section of numbers tens of thousands—continues to grow at an ever the paper discusses baseline adjustments, both for the effects increasing rate. of reference baseline shifts (introduced at the recording or The first accelerographs were optical-mechanical digitizing stages) and as a technique to remove long-period devices, generally referred to as analog recorders. These noise. This section closes with a discussion of the special instruments produced traces of the ground acceleration group of baseline fitting procedures that do not impose the against time on film or paper; the most widely used and best condition of zero displacement at the end of the motion. known analog accelerograph was the Kinemetrics SMA-1. Section 5 of the paper deals with the use of filters as a tool Analog accelerographs present three important disadvan- for the reduction of long-period noise in the accelerograms, tages, the first being that in order not to waste vast quantities probably the most important issue for engineering appli- of the recording medium, they operate on standby, triggered cations of strong-motion data as well as the area in which by a specified threshold of acceleration, which means that there is the greatest confusion. This section begins with the the first motions are often not recorded. The second issue of choosing a filtering technique, the key issues being disadvantage is related to their dynamic characteristics: whether the filter is causal or acausal. This is followed by a for the displacement response of a simple pendulum to be discussion of