S. K. Sarma

Hazard-consistent earthquake scenarios

Earthquake-resistant design and seismic analysis often require the earthquake action to be represented in the form of acceleration time-histories. Real accelerograms can be selected based on matching an earthquake scenario, defined by magnitude and distance, and scaled if necessary. The scaled accelerograms should reflect the hazard in terms of the parameters that characterise the inelastic demand on structures, including response spectral ordinates, duration and energy content. In order to maintain realistic ground motions, the scaling factors should not differ greatly from unity. It is found that in many cases, where the hazard is influenced by more than one seismic source, it is impossible to define a single earthquake scenario that is compatible with the results of probabilistic seismic hazard assessment. Even if a hazard-consistent scenario can be defined, there are difficulties encountered in using the results to select and scale real accelerograms.

A SIMPLIFIED METHOD FOR PREDICTION OF KINEMATIC SOIL–FOUNDATION INTERACTION EFFECTS ON PEAK HORIZONTAL ACCELERATION OF A RIGID FOUNDATION

The kinematic soil-foundation interaction changes the free field ground motion to a different motion at the foundation of a structure. This interaction effect may be expressed by the ratio of the peak horizontal acceleration of a rigid and relatively lightweight foundation to the peak horizontal acceleration at the ground surface in the free field. It is found that the interaction effect can be defined by a simple function of the ratio of the peak horizontal ground velocity and ground acceleration in the free field, the length of the foundation and the shear wave velocity in the soil. Predictive equations for the kinematic soil foundation effect are derived using 350 strong motion records generated by 114 earthquakes world-wide. At the same time, an attenuation relationship is derived for the ratio of the peak horizontal ground velocity and acceleration from the same set of data. Ten case histories are studied; the interaction effects are calculated by using the predictive equations and then compared with measured field values. The results of the comparison illustrate the degree of predictive capability of the method when the foundation mass and the inertial soil-foundation interaction are not considered.

Energy flux of strong earthquakes

Abstract In this paper, the energy flux of strong earthquakes at a station is determined considering the progressive rupture of a fault as the source of earthquakes. It is found that the motion of the source and the relative position of the station with respect to the fault are important in determining the energy density, the energy flux and the duration of the earthquake at this station. There is a “sphere of influence” beyond which the source may be assumed to be stationary. The analytical results are in good agreement with those of the 5 strong motion records obtained very near the fault from the Parkfield event of 27th June, 1966. 21 strong motion records are studied for energy densities at the stations from which a magnitude-energy relationship is obtained which agrees closely with other existing relationships.

Correction for Geometry Changes during Motion of Sliding-Block Seismic Displacement

The sliding-block model is often used for the prediction of permanent coseismic displacements of natural slopes and earth structures. This model assumes motion in an inclined plane but does not consider the decrease in inclination of the sliding soil mass as a result of its downward motion, which is the usual condition in the field. The paper studies the above effect and proposes an empirical equation correcting the predictions of the sliding-block model. The investigation is performed by using a recently developed multiblock model. The equation correcting the predictions of the sliding-block model depends on the slip length, the difference in inclinations of the upper and lower part of the slip surface, the seismic displacement predicted by the sliding-block model and the maximum value of the applied horizontal acceleration.

A UNIFORM ESTIMATION OF SOME BASIC GROUND MOTION PARAMETERS

A dataset of 690 strong motion component records of horizontal accelerations are analysed to derive predictive relationships for Arias Intensity, duration and number of pulses as functions of acceleration level as well as their dependence on the magnitude and distance of the source. The relationships show that the records should follow the patterns set by such relationships and cannot entirely be random. The use of some of the parameters to generate artificial time histories of acceleration is shown merely as an example.

THE ASSESSMENT OF TOTAL SEISMIC MOMENT

In this paper we address two problems: (a) the estimation of the total moment release for the calculation of average strain rates and (b) the assessment of the contribution to the total moment of earthquakes smaller than those available in regional or global catalogues. We also examine the bias associated with the different methods currently in use to estimate earthquake recurrence and total rate of seismic moment release, which are important considerations in engineering seismology and tectonics.

New inclined boundary conditions in seismic soil-structure interaction problems

Two new inclined boundary conditions are developed which constrain a homogeneous elastic isotropic half-plane into a limited space in the near-field of a structure. The solution to the two-dimensional boundary-value problem of the phenomenon of seismic wave propagation in the elastic medium is obtained by converting the boundary conditions into a finite difference form and then applying the finite element method for the core region in the near-field. The two mutually perpendicular boundaries inclined to the horizontal free surface are capable of limiting the reflections from these boundaries. Both transient and steady-state situations are considered. The resulting boundary conditions are applicable in the time domain, and therefore can be applied to non-linear analysis.

Static and Seismic Slope Safety Displacement-Based Criterion for Seismic Analysis

Methods of slope stability analyses for static and seismic conditions are discussed. The ideas of factor of safety, critical acceleration and critical slip surface are examined. The idea of displacement of slopes during earthquakes is also discussed; it is emphasised that displacements are a better criterion for seismic design of slopes. Post-seismic large displacement of slopes is examined in a test case.