SMIP 2001 Seminar

In this study, various ground shaking, response and damage parameters are examined for post-earthquake applications. Peak ground motion values, elastic response spectra, spectrum intensity, drift spectrum, inelastic spectra, and hysteretic energy spectrum are examined. Two improved damage spectra are also examined. The improved damage spectra will be zero if the response remains elastic, and will be unity when the displacement capacity under monotonic deformation is reached. Furthermore, the proposed damage spectra can be reduced to the special cases of normalized hysteretic energy and displacement ductility spectra. The proposed damage spectra are promising for various seismic vulnerability studies and post-earthquake applications.

A disastrous earthquake took place in central Taiwan at 01:47 of September 21, 1999 (Taiwan local time). The Seismology Center of the Central Weather Bureau (CWB) located the epicenter near the town of Chichi, Nantou County . The magnitude of this earthquake was ML 7.3 (CWB) and MW 7.6 (NEIC). The earthquake has caused heavy casualties and building damages: 2,539 people killed or missing, 11,306 people injured; 51,751 household units totally collapsed, 54,406 household units partially collapsed. In addition, there were widespread destruction and disruption of lifelines, including roads and bridges, water supply, gas supply, communication and electricity. The Chi-Chi, Taiwan earthquake produced a rich set of 441 strong ground motion recordings. In this paper we present some results from analysis of these recordings.

First, we found that the overall level of the observed horizontal peak ground acceleration (PGA) values was relatively low (about 50% less) when compared with what would be predicted for an earthquake of the same magnitude by existing attenuation models based on worldwide data. High horizontal PGA values at sites on the hanging wall and within 20 km of the surface fault ruptures are notable exceptions. The horizontal PGA values are indistinguishable among the four different site classes. However, the horizontal PGA values in Taipei Basin, Ilan Plain, and Hwalien areas are significantly higher than the average at similar distances. Unlike the horizontal PGA, the observed horizontal peak ground velocity (PGV) values are relatively high (at least 100% higher) when compared with what would be predicted for an earthquake of the same magnitude by an existing PGV attenuation model based on worldwide data. Thus, as far as peak ground motion parameters are concerned, the Chi-Chi earthquake may be called a high-PGV, low-PGA earthquake.

Next, we analyzed the 5% damped acceleration response spectrum shapes for the four different site classes B, C, D, and E, in order to study possible dependence of the response spectrum shape on local site conditions. It is found that the peak spectral amplification factor ranges between 2.3 and 2.5 for all four classes of site conditions. In general, the response spectrum shape for Class B sites on soft rocks older than the Pliocene age has spectral amplification for periods up to about 1.5 seconds. The spectral amplification of Classes C and D sites on stiff soils occurs over periods up to about 2.0 seconds. The spectral amplification of Class E sites on soft soils occurs over periods up to about 3.0 seconds. The response spectrum shapes for Taipei Basin and Ilan Plain are quite similar to Class E sites, whereas Hwalien area is similar to Class C or D sites.

Finally, we analyzed the observed characteristics of acceleration response spectra from 44 near-fault sites. For the eight sites within 2 km from the surface fault ruptures, the median horizontal PGA value is about 0.5 g. The corresponding spectral peak is about 1.0 g. The median-plus-one-standard-deviation horizontal PGA value is about 0.7 g and the corresponding spectral peak is about 1.8 g. Thus, for sites within 2 km from the surface fault ruptures, application of a scaling factor of 1.5 to the current seismic design spectrum anchored at a PGA value of 0.33 g for Zone 1A appears to be appropriate. For the 18 sites located at 2 to 10 km from the surface fault ruptures, the median horizontal PGA value is about 0.25 g. The corresponding peak spectral value is about 0.6 g. The median-plus-one-standard-deviation horizontal PGA value is about 0.4 g and the corresponding peak spectral value is about 0.8 g. Thus, for sites between 2 and 10 km from the surface fault ruptures, the current seismic design spectrum anchored at a PGA value of 0.33 g for Zone 1A appears to be adequate. For the 33 sites at 10 to 20 km from the surface fault ruptures, the median horizontal PGA value is about 0.18 g and the corresponding peak spectral value is about 0.45 g. The median-plus-one-standard-deviation horizontal PGA value is about 0.3 g and the corresponding peak spectral value is about 0.7 g. Thus, for sites located between 10 and 20 km from the surface fault ruptures, the current seismic design spectrum anchored at a PGA value of 0.33 g for Zone 1A is more than adequate.

The development of the TriNet/CISN Engineering Data Center is in progress. As one part of the Engineering Data Center, the California Division of Mines and Geology’s Strong Motion Instrumentation Program has developed a prototype Internet Quick Report (IQR) that uses Internet technology to distribute processed strong-motion data immediately after an earthquake. The IQR is dynamic and cumulative, containing an up-to-date table listing recorded peak ground and structural accelerations arranged in either epicentral distance or alphabetical order. Also, users will have direct access to processed strong-motion data and detailed information on instrumented stations from the IQR. The Engineering Data Center is in the process of compiling detailed structural information on instrumented stations to assist users in utilizing the strong-motion data and to allow users to search for data from specific structure types.

Approximately 100 strong-motion digital accelerographs recorded ground motions throughout the Pacific Northwest during the M 6.8 Nisqually earthquake, which occurred near Olympia on the subducted Juan de Fuca plate in the same general vicinity of the M7.1 1949 and M6.5 1965 events. Although many ground-motion records were obtained, only two buildings (DNR in Olympia and the Crowne Plaza Hotel in Seattle) recorded the shaking.

The peak ground acceleration (PGA) data from the Nisqually earthquake exhibited a higher rate of attenuation with distance than predicted by representative attenuation equations, an observation attributed mainly to the historical processing of older, strong motion paper and film records above a certain PGA threshold. Nevertheless, PGA values from the few records from the 1949 and 1965 earthquakes are within the band of PGA values from the Nisqually earthquake. In fact, records from common or nearby sites (within 100m) during these three earthquakes are similar when allowances are made for differences in magnitude and size of the recording stations.

The above observations, which pertain to stiff soil motions, suggest the Nisqually earthquake was a typical Puget Sound intraplate subduction event. However, this conclusion may not be valid in the softer soil deposits of the Duwamish River Valley in the industrial area of South Seattle where widespread liquefaction was observed during all three events. Strong motions were recorded at several of these soft soil sites that liquefied during the Nisqually earthquake with PGA’s ranging from 0.25 to 0.30 g, the highest values generally recorded in the region from this event. The anomaly is the 1949 Seattle accelerogram, also recorded in this area (Army District site) on soft soil but with acceleration levels around a factor of 4 less than the soft soil Nisqually motions. This site, within 500m of the Duwamish River, showed no apparent signs of liquefaction during the 1949, 1965, and 2001 earthquakes, whereas, the historical evidence indicates that many of the same strong motion sites that liquefied during the Nisqually earthquake also liquefied during the 1949 and 1965 events. Although these observations suggest that the Army District site may have anomalously low site response, it is difficult to imagine that the actual ground motion at this site during the Nisqually earthquake was significantly less (by factors of 5 to 10 in spectral acceleration within some period bands) than the motions at the other strong motion sites that did liquefy during this event. A resolution of this issue is important because the question posed by structural engineers engaged in post-Nisqually seismic retrofit of buildings in South Seattle is whether consistently strong motions have been and will continue to be observed on soft soil sites in this area during future intraplate events, which are a dominate contributor to the seismic hazard in Puget Sound. A continually operating station at the Army District site would have helped address the question.

Another interesting observation from the Nisqually earthquake was the site response in South Seattle. The soft soil and nearly soft rock records from this area indicated relative site amplification factors of 2 to 3 in response spectra across a wide oscillator period band from 0 to 5 sec. Estimates of soft soil motions from the SHAKE code were in fair to good agreement with observed motions provided both liquefaction and non-liquefaction cases were modeled. The latter case represents the soil profile prior to the onset of liquefaction, which appears to have occurred several seconds after the first S-wave arrival at one of the strong motion stations.

The ATC-54 Report, Guidelines for Using Strong-Motion Data for Postearthquake Response and Postearthquake Structural Evaluation, under preparation by the Applied Technology Council for the California Division of Mines and Geology, provides guidance on (1) the use of near-real-time computer-generated ground-motion maps in emergency response, and (2) the use and interpretation of strong-motion data to evaluate the earthquake response of buildings, bridges, and dams in the immediate postearthquake aftermath. Guidance is also provided on the collection of data describing the characteristics and performance of structures in which, or near which, strong-motion data have been recorded.

This paper describes portions of the document, Guidelines for Using Strong-Motion Data for Postearthquake Response and Postearthquake Structural Evaluation, currently being developed by the Applied Technology Council (ATC) for the California Division of Mines and Geology’s (CDMG) Strong Motion Instrumentation Program (SMIP) 2000 Data Interpretation Project. The focus of this paper is on the use of computer-generated ground-motion maps, i.e., TriNet ShakeMaps, for emergency response applications. Two companion papers presented at the SMIP01 Seminar, by C. Rojahn et al. and by A.G. Brady and C. Rojahn, focus, respectively, on the overall description of the Guidelines and on the use of strong-motion data for structural evaluation. The procedures outlined in this paper are a summary of the information contained in the current draft of the document, which addresses ShakeMap applications for ten areas of emergency response. The general framework is given here, with illustration for one application – damaged buildings and safety inspections.

This paper describes portions of the document, Guidelines for Using Strong-Motion Data for Postearthquake Response and Postearthquake Structural Evaluation, currently being developed by the Applied Technology Council for the California Division of Mines and Geology’s Strong Motion Instrumentation Program 2000 Data Interpretation Project. The focus of the paper is on guidance for using strong-motion data to evaluate the performance of structures in the immediate earthquake aftermath. Topics addressed include strong-motion data sources and processing, damage indicators that can often be observed in recorded data, and methods for rapid interpretation of strong-motion data to evaluate structural performance.

Development of COSMOS is continuing with a number of activities and initiatives. A workshop on “Instrumental Systems for Diagnostics of Seismic Response of Bridges and Dams” was successfully held and “Recommended Guidelines” were published. The COSMOS Strong Motion Programs Board completed the guideline “Guidelines for Advanced National Seismic System Strong Motion Station Installation” with funding provided by the U. S. Geological Survey. Development of the COSMOS Strong Motion Virtual Data Center (VDC) is continuing on four fronts: improvement of the user interface to make data more accessible for users; the COSMOS strong motion data format has been approved as a standard; data being held directly in the VDC continue to be expanded; and work is being initiated to develop a mirror sites. Two workshops are in planning: “Archiving and Web Dissemination of Geotechnical Data” is scheduled to be held on October 4 and 5, 2001; and, “Strong Motion Instrumentation of Buildings” is scheduled to be held on November 14 and 15, 2001. In an effort to expand strong motion recording in earthquake prone areas of the world that have few or no current strong motion stations, COSMOS and the World Seismic Safety Initiative (WSSI) have entered into an agreement to facilitate deployment of surplus instruments.