Characterizing Near Fault Ground Motion For The Design And Evaluation Of Bridges

Near-fault ground motions are different from ordinary ground motions in that they often contain strong coherent dynamic long period pulses and permanent ground displacements. The dynamic motions are dominated by a large long period pulse of motion that occurs on the horizontal component perpendicular to the strike of the fault, caused by rupture directivity effects. Near fault recordings from recent earthquakes indicate that this pulse is a narrow band pulse whose period increases with magnitude, as expected from theory. This magnitude dependence of the pulse period causes the response spectrum to have a peak whose period increases with magnitude, such that the near-fault ground motions from moderate magnitude earthquakes may exceed those of larger earthquakes at intermediate periods (around 1 second). The static ground displacements in near-fault ground motions are caused by the relative movement of the two sides of the fault on which the earthquake occurs. These displacements are discontinuous across a fault having surface rupture, and can subject a bridge crossing a fault to significant differential displacements. The static ground displacements occur at about the same time as the large dynamic motions, indicating that the static and dynamic displacements need to be treated as coincident loads. At the FHWA/NCEER Workshop on the National Representation of Seismic Ground Motion for New and Existing Highway Facilities held in San Francisco on May 29-30, 1997, a consensus was reached that the response spectrum alone is not an adequate representation of near-fault ground motion characteristics, because it does not adequately represent the demand for a high rate of energy absorption presented by near-fault pulses. This is especially true for high ground motion levels that drive structures into the non-linear range, invalidating the linear elastic assumption on which the elastic response spectrum is based. To fully portray the response of structures to near-fault ground motions, nonlinear time history analysis may be required. Fortunately, near fault ground motions containing forward rupture directivity may be simple enough to be represented by simple time domain pulses, thus simplifying the specification of ground motion time histories for use in structural response analyses. Preliminary equations relating the period of the pulse to the earthquake magnitude, and the effective velocity of the pulse to the earthquake magnitude and distance, have been developed. The directivity pulse can be combined with the static fault displacement to provide a complete description on near-fault ground motions. The effect of the simultaneous dynamic and static ground motions on the response of a bridge should be analyzed using time histories that include both types of motion. The probabilistic approach to seismic hazard analysis has an important advantage over the deterministic approach in that it takes into account the degree of activity of the faults that contribute to the hazard, providing explicit estimates of the likelihood of occurrence (or return period) of the hazard level that is specified in the design ground motions. Paul Somerville, Principal Seismologist, URS Corp., 566 El Dorado St., Pasadena, CA 91101