Evolution of accelerographs, data processing, strong motion arrays and amplitude and spatial resolution in recording strong earthquake motion q M.D. Trifunac * , M.I. Todorovska University of Southern California, Department of Civil and Environmental Engineering, Los Angeles, CA 90089-2531, USA Accepted 11 February 2001 Abstract This paper presents a review of the advances in strong motion recording since the early 1930s, based mostly on the experiences in the United States. A particular emphasis is placed on the amplitude and spatial resolution of recording, which both must be `adequate' to capture the nature of strong earthquake ground motion and response of structures. The ®rst strong motion accelerographs had optical recording system, dynamic range of about 50 dB and useful life longer than 30 years. Digital strong motion accelerographs started to become available in the late 1970s. Their dynamic range has been increasing progressively, and at present is about 135 dB. Most models have had useful life shorter than 5±10 years. One bene®t from a high dynamic range is early trigger and anticipated ability to compute permanent displacements. Another bene®t is higher sensitivity and hence a possibility to record smaller amplitude motions (aftershocks, smaller local earthquakes and distant large earthquakes), which would augment signi®cantly the strong motion databases. The present trend of upgrading existing and adding new stations with high dynamic range accelerographs has lead to deployment of relatively small number of new stations (the new high dynamic range digital instruments are 2±3 times more expensive than the old analog instruments or new digital instruments with dynamic range of 60 dB or less). Consequently, the spatial resolution of recording, both of ground motion and structural response, has increased only slowly during the past 20 years, by at most a factor of two. A major (and necessary) future increase in the spatial resolution of recording will require orders of magnitude larger funding, for purchase of new instruments, their maintenance, and for data retrieval, processing, manage- ment and dissemination. This will become possible only with an order of magnitude cheaper and `maintenance-free' strong motion accelerographs. In view of the rapid growth of computer technology this does not seem to be (and should not be) out of our reach. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Strong motion recording; Strong motion arrays; Accelerographs; Data processing; Strong motion databases; Damage detection; Structural health monitoring; Permanent displacement 1. Introduction 1.1. Recording strong motionÐthe early beginnings In the chapter on `Ground Motion Measurements' of the ®rst book on Earthquake Engineering in the United States, Hudson [1] introduced the subject by stating that any full- scale experimental study of `earthquake engineering that is to have a sound scienti®c foundation must be based on accurate knowledge of the motions of the ground during destructive earthquakes. Such knowledge can be obtained only by actual measurements in the epicentral regions of strong earthquakes'. He continued by stating `typical seis- mological observations with their sensitive seismographs are not intended to make measurements in the epicentral regions of strong earthquakes and cannot be adapted to do so effectively. Fundamentally different objectives of the engineer will require a basically different instrumentation than that needed for seismological studies. Such instrumen- tation must be designed, developed, installed and operated by earthquake engineers, who will be thoroughly familiar with the ultimate practical objectives of earthquake resistant design'. Today these statements are still timely and relevant. In the epicentral regions of strong and moderate earth- quakes, damage to structures is caused by the permanent fault displacement, as well as by strong ground shaking triggered landslides, large scale soil settling, liquefaction, Soil Dynamics and Earthquake Engineering 21 (2001) 537±555 0267-7261/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0267-7261(01)00013-6 www.elsevier.com/locate/soildyn * Corresponding author. Tel.: 11-213-740-0570; fax: 11-213-744-1426. E-mail address: trifunac@usc.edu (M.D. Trifunac). q We dedicate this paper to Donald E. Hudson (1916±1999), a pioneer in the ®eld of Earthquake Engineering, and our teacher and mentor. His contributions to academic research and development of earthquake instru- mentation are without parallel. With a rare ability to attract, motivate and support young scientists, he created a long and impressive list of PhD graduates who are now professors, researchers and leaders in Earthquake Engineering.