CSMIP Near-Fault Instrumentation Projects and Accuracy of Relative Displacements Computed from Strong-Motion Records (Abstract)
by Anthony Shakal
Anthony Shakal (2008). CSMIP Near-Fault Instrumentation Projects and Accuracy of Relative Displacements Computed from Strong-Motion Records (Abstract). SMIP08 Seminar on Utilization of Strong-Motion Data, p. 3 - 4.
In the early 1980s a strong motion array was deployed near Parkfield in central California because of the expectation of an earthquake in the area. Important data had been obtained during the 1966 Parkfield, and given the prediction of a repeat earthquake in the 1980s, special arrays were deployed by both CGS and USGS. The CSMIP array included 45 strong motion stations, a significant investment at the time. The Parkfield earthquake eventually did occur on September 28, 2004 (magnitude 6.0 Mw). The resource investment was well warranted as very important data was recovered including some of the strongest accelerations ever recorded from a moderate earthquake. Beyond that, because of the design and scale of the array, one of the most dense set of near-fault recordings was obtained. These recordings showed near-fault motion that was highly variable, spatially, with accelerations well over 1 g within 1 or 2 km of accelerations near 0.1g – a factor of 10. The earthquake showed that near fault motion could be quite variable, and that more observations would be important to understand this result and its generality.
Currently, the two areas with the highest likelihood of a moderate or larger earthquake in California, according to the Working Group on California Earthquake Probabilities (WGCEP), are the Hayward fault in the San Francisco Bay area and the Coachella segment of the San Andreas fault in southern California. Considering the Hayward area, given the experience of the Parkfield ground motion, it was clear that if the ground motion were to be as variable as in the Parkfield event, the station density was far too limited to capture the variability. The CSMIP Strong Motion Instrumentation Advisory Committee (SMIAC) recommended that the Hayward area instrumentation be substantially increased. This represents a departure from the urban ground motion instrumentation focus of recent years, aimed at improving ShakeMap for response, to a focus of improving the learning from an event.
Significant progress has been made in the past two years on the Hayward area, partly because the USGS has also been working to expand the instrumentation. With the completion of stations planned by CSMIP, and those being considered by the USGS, station density should become comparable to Parkfield. Special purpose arrays are also important. For example, the USGS deployed a special subarray of 14 stations with spacing of a km or less, called UPSAR. Important questions, such as tracking the rupture process, can be addressed with such arrays. Although not directly equivalent, some special USGS arrays in the San Jose area will provide important close-spacing ground motion data.
In contrast with the Hayward East Bay area, the Coachella segment of the Southern San Andreas has relatively few stations. The expected event is larger, near a magnitude 7 according to the Working Group. The number of records from the close-in region of M>7 earthquakes is very limited. For input ground motion in designing for large earthquakes, artificially generated strong motion records are sometimes needed. To address this data paucity, a relatively large number of instruments are needed within the near fault zone of large events. Since the focus of these instruments, especially those located in lightly populated areas, is not for immediate usage in ShakeMap, but to obtain data to guide future design assumptions, some of the features necessary for ShakeMap-caliber instruments can be relaxed and simpler, more economical instruments can be deployed. For example, these instruments do not need communication capability and certain other features that drive up cost and power usage. Over the next two years, a significant number, perhaps over 100, of these simple low-cost instruments are planned to be deployed in the near-fault zone of the expected Coachella earthquake. They will of course be complemented by conventional seismic instruments of the USGS and CGS arrays.
Relative Displacement Accuracy
The accuracy of displacement computed from strong-motion records is important in accessing structural response. The inter-story drift, or relative horizontal displacement of adjacent floors, is a major factor in the seismic response of a building. High inter-story drifts are likely associated with incipient damage in the structure.
For early strong motion accelerographs, displacements could only be obtained after analog film records were laboriously digitized and processed. Because of the high noise intrinsic to this procedure, the displacements obtained from doubly integrating the digitized acceleration generally had high noise. Thus, relative displacements, obtained after differencing records from nearby sensors, often had high noise, especially at long period.
Error in computed displacements increases with period. At short period (e.g., 1 second or less), the error amplitude is small (a fraction of a cm). At periods of 5 to 10 seconds, the error in displacement can be significantly greater (several cm). In comparison with early instruments, modern 18-bit digital accelerographs have very low noise. Because of this, serious consideration can be given to utilizing the inter-story drift obtained through differencing accelerations obtained at nearby floors. Some tests were recently conducted as payload instrumentation on a NEES 3-story test structure at the UCSD shake table. Accelerometers were attached to the structure as well as on a nearby stationary tower, and a relative displacement sensor recorded the motion between them. The tests indicate that except for permanent displacements, the inter-story displacements obtained from a nearby pair of modern, low-noise accelerometers is quite accurate even at periods of several seconds.