SMIP 2005 Seminar

The M6.0 Parkfield earthquake of September 28, 2004 that occurred on the San Andreas fault near the town of Parkfield in central California produced the most extensive and dense set of near-fault strong motion recordings ever obtained in California. As a result of a widely accepted likelihood of an earthquake in the area, a large number of strong motion stations had been deployed in the area. The arrays and the resultant strong-motion measurements of the earthquake, as well as preliminary observations are described here. The data includes very high variability in the near fault motion and accelerations as high as 2.5g.

An animation tool for visualizing ground shaking amplitude, oscillations, and duration using existing strong-motion datasets was developed to help interpretation and understanding of strong-motion propagation and attenuation. The system uses readily available strong-motion datasets, seismic velocities, and the ShakeMap model to interpolate ground motions by time-shifting and amplitude-scaling proximal records across a study area. The animation system essentially adds a temporal dimension to the ShakeMap model. Five significant historical California earthquake animations were developed with this system (1999 Hector Mine, 1992 Landers, 1989 Loma Prieta, 1994 Northridge, and 2004 Parkfield earthquakes).

Efforts underway to quantify uncertainties associated with ShakeMap ground motions through efforts by the California Integrated Seismic Network (CISN) ShakeMap Working Group are discussed. There are multiple sources of uncertainty in producing a ShakeMap, including sparse ground motion measurements, approximate representation of fault finiteness and directivity, empirical ground motion predictions, numerical interpolation, and site corrections. These ground motion uncertainty measures are critical for evaluating the range of possible losses and allow users to associate appropriate levels of confidence when using rapidly produced ShakeMaps as part of their post-earthquake critical decision making process. We quantify the uncertainties of the maps on a point-by-point basis, by combining the separate, but related, contributions of uncertainty for each ShakeMap parameter as a function of location on the map. Finally, we show examples of results of estimates of uncertainty for ShakeMap for earthquakes in California with/without defined fault traces. We discuss future developments and plans for integration of these uncertainty measures, both quantitative and qualitative, into the online system and user interfaces of ShakeMap.

This paper discusses modifications to the existing bridge visualization program, previously developed by the author, to properly include nonlinear behavior of the 5/14 North Connector bridge expected from future, severe, design-level earthquake motions. Such modifications recognize that nonlinear behavior will develop at predetermined column locations, based on current state-of-the-art seismic design practice. For single-column-bent bridges, plastic hinges are expected to develop at the base of the column in transverse bending and at both ends of the column under longitudinal loading. The bridge can now be viewed in full 3-D animation, developing plastic hinges at all critical locations and showing color-coded damage or ductility levels for transverse and longitudinal behavior for each bridge column. Spline functions were modified from cubic equations representing elastic member response to a combination of plastic and elastic responses.​

A set of methodologies for automated post earthquake damage assessment of instrumented buildings are presented. These methods can be used immediately after an earthquake to assess the probability of various damage states in the N-S and E-W directions and throughout the height of each building. The methods have been applied to more than 40 CSMIP instrumented buildings which have recordings from more than one earthquake. The results indicate that the proposed methods, when used in combination, can provide very useful information regarding the status of a building immediately after an earthquake by simple and rapid analysis of sensor data and prior to any building inspections.

A data-driven approach for post-earthquake posting of buildings is presented. The approach is based on the analysis of residuals obtained by subtracting the measured responses from reference signals that reflect the behavior of the healthy system. The residuals are used to compute two indices from which the impact of the motion on the structure is assessed. One index measures the extent of nonlinearity and the other looks at changes in structural characteristics after the strong motion part of the record is over. Results obtained for a number of buildings taken from the CSMIP database suggest the approach may be suitable for automated posting.

The Sumatra earthquakes of December 26, 2004 (Mw 9.15) and March 28, 2005 (Mw 8.7) are the largest subduction earthquakes that have ever been recorded on modern digital instruments. Both earthquakes were caused by the subduction of the India–Australia plate beneath the Eurasian Plate. Although these earthquakes were not recorded on scale at close distances, they were recorded at regional distances. These regional recordings shown strong spatial variations in amplitude and duration that are consistent with rupture directivity effects. The duration of ground motion of the December event to the north in Thailand was about 600 seconds, while the duration in other directions, including Sumatra, was about 1,000 seconds.

The construction for the seismic rehabilitation of the Los Angeles City Hall was completed in 2001. An integral part of the project is the installation of seismic instrumentation throughout the building. The instrumentation program was a collaborative effort involving the Los Angeles City Department of Public Works Bureau of Engineering, California Geologic Services Strong Motion Instrumentation Program (CGS-SMIP) and the Engineering Design team. The future data recorded by these sensors will provide valuable insight on the actual response of the building during an earthquake. This information will aid the structural engineering community to better understand the behavior of base isolated structures with supplemental damping.

The building was originally constructed in 1926 and was the first building to exceed the 150 feet height limitation for all privately constructed buildings in Los Angeles. It is 32 stories (460 feet) tall. The original building was designed prior to the enactment of explicit seismic design requirements and therefore, was not specifically designed to resist earthquake generated forces.

Over the past 75 years, regional earthquakes have caused damage to the building. Terra cotta cladding has been cracked, broken or destroyed in portions of the building's exterior. With every significant earthquake, unanchored masonry debris has been scattered about the building's interior. Large cracks in the masonry walls appeared at the 24th floor after the 1971 Sylmar Earthquake, the 1987 Whittier Earthquake and the 1994 Northridge Earthquake.

The building has been seismically rehabilitated in order to preserve life safety, mitigate damage, maintain the integrity of the building's exterior facade, and protect the historic interior fabric from damage. Base isolation with supplemental damping was used to enhance its seismic performance. This approach was determined to be the most effective strengthening scheme based on performance and cost.

This paper presents an overview of the rehabilitation project including the development of seismic performance goals, identification of inherent seismic deficiencies of the original building, description of the final seismic strengthening and instrumentation program.