MCEER HIGHWAY PROJECT
FHWA CONTRACT DTFH61-98-C-00094

Seismic Vulnerability of the Highway System

Task C2-1: Long Period Ground Motions and Spatial Variation Review

Subject Area: Special Bridges - Ground Motions & Geotechnical Studies
Research Year 2

Principal Investigator(s) and Institution(s)

Apostalos S. Papageorgiou, University at Buffalo

Objective

The specific objectives of this task are to provide a comprehensive review and study of the factors that influence long period ground motions and their spatial variability that are important for long span bridges, provide guidelines for incorporating these factors explicitly in the estimation of seismic design ground motions, and to complement the deficient data base of ground motion recordings close to large earthquakes with synthetic time histories.

These objectives will be accomplished by an exhaustive survey of available pertinent empirical data (i.e., strong motion recordings near the source of large earthquakes and motions recorded on deep sedimentary deposits) and with the implementation of effective, accurate and efficient simulation techniques.

Approach

As we have described in detail in our original proposal for this two-year Task, there are two effects that have strong influence on long period motions and their spatial variability:

  1. near-fault effects (forward rupture directivity), and
  2. basin response effects and topography effects.

Long period ground motions (T 1 sec) and their spatial variability may be modeled deterministically, unlike high frequency motions for which this kind of modeling would be impractical. High frequency ground motions and their spatial variability, are stochastic in nature.

In the comprehensive review of all factors that influence long-period ground motions and their spatial variability, we will address both effects listed above. However, for reasons that we presented in our original proposal, we will focus and study in detail, using numerical simulations, only near-source effects.

Near-fault effects (forward rupture directivity) - Near-source motions are characterized by the presence of a relatively long period pulse on the displacement and velocity records. This source "fling" (or "killer pulse") is the result of forward rupture directivity. Directivity, in turn, is the inevitable result of rupture propagation velocities that are close to the shear-wave of the propagation medium. Such "killer pulses" may be detrimental for high-rise buildings, base isolated structures and long span bridges.

Near-source ground motions may also be associated with significant torsional and rocking motions/rotations. These may also be detrimental for the types of structures mentioned above, including long span bridges. There are many major long span bridges that have been built in the vicinity (or over) major active faults, such as the Golden Gate bridge (near San Andreas fault), the Akashi Kaikyo bridge (at the junction of the Nojima and Suma faults where the epicenter of the 1995 Hyogo-ken Nanby (Kobe) earthquake was located), and the Bosporus bridge (near the northern strand of the Anatolian fault).

Near-source ground motions are not adequately represented in modern codes, because these codes historically were developed based on the experience of recorded motions not sufficiently close to the causative fault. At present, there is little empirical guidance to select the shape, aptitude, and duration of the "killer pulses" mentioned above. Attempts to modify the amplitudes of empirical response spectra to account for near-source effects, although pointing in the right direction are not sufficient to describe the nature of impulsive near-source ground motion, simply because synthetic motions generated by a spectral matching technique cannot build a rupture directivity pulse where none is present to begin with (remember that a signal, such as a seismogram consists of both amplitude and phase). Furthermore, the database of recordings close to large earthquakes is deficient and therefore any extrapolation of existing regression equations into such regions of the Magnitude-Distance space is not controlled by data. (It should be mentioned parenthetically that prior to the recent destructive earthquakes in Turkey and Taiwan, there were only 8 ground motion recordings worldwide for earthquakes greater than magnitude 7 at a distance of less than 20 kilometers from the fault. The 1999 Marmara, Turkey earthquake generated an additional 5 recordings, while the 1999 Chi-Chi, Taiwan earthquake generated the unprecedented number of 65 recordings). Finally, ground motion displacement is also a quantity poorly determined from the available processed accelerometer data recorded with analog instruments. Thus, there is a clear need to complement empirical data by theoretical models.

Although "killer pulses" may be characteristic of near-source motions of strike-slip faults, they are not a universal feature of near-source ground motions, as has been demonstrated with the Caleta de Campos record of the 1985 Michoacan earthquake. Near-source motions, being influenced primarily by source effects, may display a multitude of characteristics reflecting the complexity of the source process.

Based on the observations and facts described above, the following research has been proposed and is conducted under this Task:

Subtask 1: During the first year of this effort we initiated a comprehensive review of all the factors that influence long period ground motions and their spatial variability. This will result in a report that will provide a comprehensive review of the state of knowledge (and state of practice) with regard to the selection of long period ground motions for seismic design of long-span bridges. One important aspect of such a report will be the thorough documentation of the existing near-source recordings of moderate and large events. The task of collecting and processing these data and presenting them is visually informative form relative to the causative faults is almost complete (we have successfully overcome some difficulties that we reported in Progress Report #3). Other items that we have incorporated in the report so far relate to the limitations of ground displacements and velocities determined by processing the accelerometer data characterization of the wave field near rupturing faults (including apparent velocities that are necessary to describe the spatial variability of the motion as well as torsional and rocking motions), basin induced surface waves and their apparent velocities, topography induced variability of ground motion etc. Our ambition is to make this report the definitive document that summarizes the most important relevant information and thus helps identify gaps in our present knowledge regarding long period ground motions. We anticipate to complete the bulk of this report by the end of Summer 2000.

Subtask 2: In Year 2 of the present Task we will proceed with the synthesis of the near-source wavefield for all those events for which the slip-function has been determined by inversion of strong motion, teleseismic and/or geodetic data (there are 18 such events for which the slip function has been inferred by inversion, covering a very wide range of magnitudes). Stating this in layman's terms, displacement and velocity time histories as well as torsional and rocking motions for a dense grid of points surrounding the rupturing faults will be generated. Thus, the motion at sites where no recording instrument existed during the event can be observed. These simulations will be performed using the Kinematic Modeling Approach based on the Elastodynamic Representation Theorem. The source wavefield will be expressed using the Discrete Wavenumber Method because this is the most efficient method for ground motion computations over an extended area.

A computer code that implements the above method has already been developed and success-fully used by the Principal Investigator. We are about to start work for incorporating one more feature to the code, and that is the capability of calculating stresses on the fault plane. The results of these simulations will permit a more thorough investigation of the character of long period near-source ground motions. Also, with these simulations, it is anticipated that an improved understanding of the spatial variability of long period near-source ground motions vis--vis the spatial distribution of slip and stress on the fault plane, will be developed. It is our vision ultimately to represent near source ground motions and their spatial variability using simple kinematic models (such as a Haskell dislocation model or a circular crack model) the values of the key parameters of which can be selected by an appropriate scaling low. Hopefully, these results will form the basis for specific guidelines in selecting such motions.

It should be mentioned at the outset that the motions synthesized by the simulations described above will include frequencies up to 1 Hz. Synthesis of broadband motions (i.e., motions containing both long period as well as high frequencies) will be part of a future project task.

Technical Challenges

Successful completion of this task requires substantial computational effort. the most challenging objective though is the synthesis of the results of the simulations so as to provide simple yet reliable guidelines for selecting near-source ground motions for engineering design of long-span bridges.

8/14/01

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