Some Milestones In Strong Motion Monitoring (Abstract)
by W. D. Iwan
W. D. Iwan (2008). Some Milestones In Strong Motion Monitoring (Abstract). SMIP08 Seminar on Utilization of Strong-Motion Data, p. 57 - 58.
This presentation describes some significant milestones in the development of strong motion monitoring of earthquakes as judged by the author. Strong motion earthquake monitoring was motivated by the Great Tokyo earthquake of 1923 and was strongly influenced by Prof. Romeo Martel in the US and Prof. Kyoji Suyehiro in Japan. Also greatly influential in the development of a strong motion instrument was John R. Freeman who became interested in earthquakes at age 70. The first strong motion instrument was constructed by the US Coast and Geodetic Survey in 1932 and the first significant strong motion record was obtained during the Long Beach earthquake of 1933. This presentation traces the development of strong motion instruments and the analysis of strong motion data from the era of the Wood-Anderson Seismograph to more recent digital recorders.
Early strong motion instruments were analog and data was recorded on photographic film. There were many challenges in getting the recorded data into a form that was useful to engineers and others studying strong earthquake motions. Initially, photographic records were examined visually to determine notable features of the motion including peak acceleration, duration of shaking, and the nature of the envelope of the time history of motion. But in 1934 Prof. Hugo Benioff of Caltech introduced the Response Spectrum of an earthquake. This concept was later refined for engineering use by his colleague Prof. George Housner. The Response Spectrum provided earthquake engineers with an easily applied tool that could be used to estimate the response of a structure to earthquake excitation. Computation of Response Spectra from early film records was no easy task and relied heavily on the use of analog computers. However, in spite of these difficulties, sufficient data were analyzed so that the first “Design” Response Spectrum was published by Prof. Housner in 1959. Later, Prof. Newmark and Prof. Hall of the University of Illinois produced a further refined Design Response Spectrum that was widely distributed in a 1982 monograph by EERI.
Due to a landmark program instituted by the City of Los Angeles which mandated the installation of strong motion instruments, a treasure trove of approximately 400 strong motion records was obtained during the San Fernando earthquake of 1971. It was also significant that this earthquake occurred at the time when analog computation was giving way to digital computation in many fields of engineering. Capitalizing on the convergence of these two events, the NSF funded a project at Caltech to digitize and distribute the time history and Response Spectra data for all of the San Fernando records as well as other key historical records. The process of digitization revealed certain base-line problems with the data and band-pass filtering algorithms were developed to eliminate drift in the integrated acceleration data. New digital programs were also developed to compute Response Spectra.
In 1976, the Great Tangshan earthquake occurred in China killing hundreds of thousands of people. The following year, at the 6WCEE in New Delhi, India, a new international committee was formed on strong motion instrumentation. At that time, there were about 5,000 strong motion instruments deployed worldwide, 3,000 of which were in the US. In 1978 an International Workshop on Strong-Motion Instrument Arrays was held in Hawaii. The participants of that workshop concluded that understanding strong ground motion was critical to earthquake safety, that there was a scarcity of engineering data near the source of destructive earthquakes, and that countries needed to make a concerted effort to deploy instrument arrays capable of resolving the nature of the source mechanism, wave propagation, and local site effects associated with earthquakes. As a result of this workshop, a number of digital strong motion arrays were deployed worldwide including in Taiwan (SMART-1) and China.
The digital strong motion array deployed in China was in the aftershock region of the Great Tangshan earthquake. This array recorded more than 1050 near-field accelerograms from more than 400 events. On October 19, 1982, nine digital instruments recorded the ML=5.7 Lulong event with the closest instrument being only 5 km from the epicenter. After overcoming some processing challenges, this record showed an interesting new type of “pulse-like” ground motion that had not been previously reported. After some initial dispute over the validity of this record, it was gradually accepted as indication a real phenomenon. This result was further validated by the 1992 ML=7.5 Landers earthquake. An analog instrument installed by the Southern California Edison Company was located within 2 km of the fault trace of that event. The instrument was retrieved from the field and subjected to extensive testing at Caltech. The integration algorithms developed for the Lulong record were then applied to the Landers record. What was revealed was a clear indication of the pulse-like near-field ground motions. The same techniques were applied to recorded data from the 1992 ML=6.7 Erzincan earthquake and the results were very similar. By this time, there was no disputing the existence of near-field pulse-like ground motions.
The 1995 Hyogoken-Nambu (Kobe) earthquake in Japan triggered a significant expansion of strong motion networks in Japan as well as in other Asian countries. The 1999 Chi-Chi earthquake in Taiwan yielded accelerograms from more than 600 instruments. The State of California presently has over 2,000 strong motion instruments deployed on the ground and in buildings.
As strong motion networks have gone from analog instruments to digital instruments, data retrieval and processing has also changed. It is now possible to retrieve and process data in near or true real-time. This opens up many exciting opportunities for enhanced decision making using string motion data. The presentation gives an example of a currently operating real-time monitoring system in the Millikan Library Building on the Caltech campus. This system is capable of providing real-time inter-story hysteresis diagrams to assist in damage assessment. Other possible applications of decision making based on real-time string motion monitoring are also given. These applications address the needs of a broad spectrum of stakeholders throughout society including public officials, building owners, building occupants, and individual citizens.
Strong motion monitoring has reached a level of maturity where we no longer celebrate the good fortune of obtaining one additional ground motion record from a distant earthquake. Therefore, it is the position of the author that our efforts need to be refocused from the capture of isolated records to obtaining integrated real-time information that can be used for rapid decision making.