Seismic Vulnerability of the Highway System

Task C2-1: Long Period Ground Motions and Spatial Variation Review
Subject Area: Long Span Bridges — Ground Motion and Geotechnical Studies

Principal Investigator(s) and Institution(s)
Apostolos Papageorgiou, University at Buffalo

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.

It is widely recognized that there are two effects that have strong influence on long period motions: (1) near-fault effects (forward rupture directivity), and (2) basin response effects. Long period ground motions (T w 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 order to obtain reliable estimates of the spatial incoherence of ground notion at a particular site, it may be necessary to use accelerograms of a small earthquake or explosion recorded on a small array at the site. For the purposes of this task, although the spatial variability of both long period and high frequency ground motions will be reviewed, the focus in simulating such motions will be long periods, in view of the fact that this range of periods is expected to affect the response of long span bridges more significantly. Modeling/simulation of high frequency motions, and their spatial variability, will be part of a future task.

Near-fault effects (forward rupture directivity) - The strong motion records that had by far the most decisive effect on the analysis, modeling, and prediction of strong motion near a rupturing fault are the Station No. 2 record obtained from the 1966 Parkfield earthquake and the Pacoima Dam record obtained from the 1971 San Fernando earthquake. Both records exhibit strong displacement velocity pulses of relatively long periods (~2 to 3 seconds). With their seminal study of damage of the Olive View Hospital, Bertero and others were the first engineers to recognize and analyze the destructive potential of such pulses on flexible structures. Such pulses may be detrimental for high rise buildings, base isolated structures and long span bridges. The presence of relatively long period, high amplitude pulses on the displacement and velocity records at near-source sites was confirmed by the 1979 Imperial Valley earthquake. More recently, the occurrence of well recorded major events such as the 1994 Northridge and the 1995 Hyogyo-Ken Nanbu (Kobe) earthquake in the immediate vicinity of large metropolitan centers, highlighted the tremendous destructive potential of near-source strong motions, and provided the necessary data basis to study the effects of such motions on buildings.

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, amplitude, and duration of these "killer pulses." 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. Furthermore, the data base 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. Finally, ground 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.

The source "fling" (or "killer pulse") that we mentioned above 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 velocity of the propagation medium.

The question that naturally arises is the following: are these near-source pulses a universal phenomenon? The answer to this question is provided by the recording at station Caleta de Campos obtained during the 1985 Michoacan, Mexico earthquake. This record displays a simple ramp-function displacement which cannot be explained by dislocation models but requires crack-like slip functions, in contrast to the Station No. 2 record obtained from the 1966 Parkfield earthquake. Furthermore, the velocities recorded by the Caleta de Campos station display a high frequency (~3 sec) component overriding on the longer frequency (~10 sec) motion. The high frequency ripples were successfully interpreted by the kinematics of the rupture front (i.e., by a series of changes of the rupture velocity). Changes of the rupture velocity will result in emission of high-frequency waves that have a very strong directivity.

Although "killer pulses" may be typical characteristics 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 Michioacan earthquake. Near-source motions, being influenced primarily by source effects, may display a multitude of characteristics reflecting the complexity of the source process.

Basin response effects - Sedimentary deposits in the form of basins, and in particular deep basins, have a profound influence on long period ground motions. The strongest effects occur when the earthquake occurs at the basin edge, outside the basin, because entering body waves are trapped by the dipping edge of the basin. Such entrapment of energy results in prolongation of the duration of long period ground motions and in amplification of the amplitudes of velocities and displacements.

Basin effects at a given site strongly depend on the location of the earthquake source, because such effects are controlled by the geometry of the basin between the site and the earthquake source. Therefore simple modifications of existing attenuation relations may not be possible and the only viable approach to the problem may be 2-D or 3-D simulations of each specific case.

Based on the observations and facts described above, the following research will be conducted under this task:

Subtask 1 - A comprehensive review and study of all the factors that influence long-period ground motions and their spatial variability. This will result in a report which 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. Although various collections of such data have been published by various authors in the form of tables and lists of recordings, no one has presented these data in a clear visual form, so that the position of the recording station relative to the rupturing faults is clearly evident (such information is currently scattered in the published literature).

Subtask 2 - 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 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 of seismologists, which is based on the elasto-dynamic representation theorem. Consistent with the inversion studies that provided the slip-functions of the above events, the model of each earthquake event will consist of a fault plane embedded in a layered half-space. 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 region.
A computer code that implements the above method has already been developed and successfully used by the task Principle Investigator. 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 distributions of slip on the fault plane, will be developed. 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 1Hz. Synthesis of broadband motions (i.e., motions containing both long-periods as well as high frequencies) will be part of a future project task.

Specific simulations for basin models will not be conducted, even though codes for simulating 2-D and 2.5-D basin response effects have been developed. Clearly, such effects are very important for areas such as the Los Angeles basin region. However, a major effort is under way by Southern California Earthquake Center (SCEC) researchers to study the site amplification in the Los Angeles basin from 3-D modeling of ground motion, and thus research on this could result in an unnecessary duplication of effort.

Anticipated Start Date and Duration
January 1, 1999 - 24 months