The reflection of plane waves in a poroelastic half-space saturated with inviscid fluid

This paper discusses surface displacements, surface strain, rocking, and energy partitioning during reflection-of-plane waves in a fluid-saturated poroelastic half-space. The medium is modeled by Biot's theory, and is assumed to be saturated with inviscid fluid. A linear porosity-modulus relation based on experimental data on sandstones is used to determine the material parameters for Biot's model. Numerical results in terms of angle of incident waves and Poisson's ratio are illustrated for various porosities and degrees of solid frame stiffness. The results show that the amount of solid frame stiffness controls the response of a fluid-saturated porous system. A poroelastic medium with essentially dry-frame stiffness behaves like an elastic medium, and the influence of pore fluid increases as dry-frame stiffness is reduced. The effects of a second P-wave become noticeable in poroelastic media with low dry-frame stiffness.     

Rocking strong earthquake accelerations

In this paper, the method presented by Lee and Trifunac (1985) for generating synthetic torsional accelerograms has been extended to the estimation of synthetic rocking accelerograms and of their response spectra. Results from our previous regression analyses for the characterization of strong shaking in terms of (1) earthquake magnitude and epicentral distance, or (2) Modified Mercalli Intensity at the site are utilized here again. The effects of geologic environment, in terms of site parameters or the representative depth of sediments, which influence amplification, and the dispersive properties of ground motion are also included. The synthetic rocking accelerogram is then constructed from the horizontal and vertical acceleration components.     

Spectral amplitudes of strong earthquake accelerations recorded in buildings

Fourier spectrum amplitudes of horizontal and vertical earthquake accelerations recorded at the foundation levels of 57 buildings in the Los Angeles metropolitan area have been used to study the dependence of spectral amplitudes on the building foundation sizes. Comparison of these amplitudes with those predicted by empirical models for scaling ‘free field’ Fourier amplitude spectra does not indicate any significant dependence of the spectral amplitudes on the size of the foundation. Third degree polynomials have been employed to smooth the spectra of the accelerations recorded inside the buildings and their coefficients have been examined as functions of the foundation plan dimensions. These results also indicate no significant dependence of the spectral amplitudes on the foundation dimensions. A qualitative analysis of the spectral amplitudes for possible effects caused by the phenomena associated with soil-structure interaction indicates that the Fourier spectra of the recorded accelerations may experience some amplification as the relative ‘density’ of the foundation-structure system increases.     

Transient pressures in hydrotechnical tunnels during earthquakes

Transient pressures generated by earthquake shaking in hydrotechnical tunnels are evaluated by the discrete Fourier transform technique. The effects of the horizontal ground motion accelerating the closed downstream tunnel gate, as well as the upstream dam face, and the influence of the vertical motion of the reservoir floor are considered in this analysis. An example of a typical bottom outlet is analysed by subjecting it to several computed accelerograms. It is shown that high hydrodynamic pressures can be developed, several times larger than the hydrostatic pressure.     

Fourier amplitude spectra of strong motion acceleration: Extension to high and low frequencies

It is shown how the empirical equations for scaling the Fourier amplitude spectra in the frequency band from ～0.1 to 25 Hz can be extended to describe the strong motion amplitudes in a much broader frequency range. At long periods, the proposed equations are in excellent agreement with (1) the seismological and field estimates of permanent ground displacement (near field) and (2) the independent estimates of seismic moment (far field). At high frequencies, f ≥ 25 Hz, the spectral amplitudes can be described by exp (? πkf), where k ranges from 0·02 (near source) to about 0·06 at an epicentral distance of about 200 km. It is also shown how amplification by local soil and geological site conditions can be defined to apply in the same broad frequency range.     

Pseudo relative velocity spectra of earthquake ground motion at long periods

This paper analyses the processes which govern structural response, and uses observations of strong earthquake ground motion to propose quantitative extrapolation of pseudo relative velocity spectral amplitudes to long (100 > T > 1 s) periods. This will eliminate the current difficulties with rough estimation of long period spectral amplitudes and will open new possibilities by enabling the strong motion hazard calculations to be extended to the same long period band. So far, the scaling equations of response spectrum amplitudes have been valid only up to periods less than several seconds. The design of long structures and of structures on multiple distant supports requires knowledge and specification of design ground motions well beyond 1–10 s periods. With the results presented in this paper it will be possible to compute site-specific uniform hazard spectra and associated synthetic accelograms for essentially all long period response problems.     

A note on the useable dynamic range of accelerographs recording translation

Since the late 1970s, the dynamic range and resolution of strong motion digital recorders have leaped from 65 to 135 dB, opening new possibilities for advanced data processing and interpretation. One of these new possibilities is the calculation of permanent displacement of the ground or of structures, associated with faulting or with non-linear response. Proposals on how permanent displacements could be recovered from recorded strong motion have been published since 1976. The analysis in this paper concludes that permanent displacements of the ground and of structures in the near-field can be calculated provided all six components of strong motion (three translations and three rotations) have been recorded, and the records are corrected for transducer rotation, misalignment and cross-axis sensitivity.     

Evolution of accelerographs, data processing, strong motion arrays and amplitude and spatial resolution in recording strong earthquake motion

This paper presents a review of the advances in strong motion recording since the early 1930s, based mostly on the experiences in the United States. A particular emphasis is placed on the amplitude and spatial resolution of recording, which both must be ‘adequate’ to capture the nature of strong earthquake ground motion and response of structures. The first strong motion accelerographs had optical recording system, dynamic range of about 50 dB and useful life longer than 30 years. Digital strong motion accelerographs started to become available in the late 1970s. Their dynamic range has been increasing progressively, and at present is about 135 dB. Most models have had useful life shorter than 5–10 years. One benefit from a high dynamic range is early trigger and anticipated ability to compute permanent displacements. Another benefit is higher sensitivity and hence a possibility to record smaller amplitude motions (aftershocks, smaller local earthquakes and distant large earthquakes), which would augment significantly the strong motion databases. The present trend of upgrading existing and adding new stations with high dynamic range accelerographs has lead to deployment of relatively small number of new stations (the new high dynamic range digital instruments are 2–3 times more expensive than the old analog instruments or new digital instruments with dynamic range of 60 dB or less). Consequently, the spatial resolution of recording, both of ground motion and structural response, has increased only slowly during the past 20 years, by at most a factor of two. A major (and necessary) future increase in the spatial resolution of recording will require orders of magnitude larger funding, for purchase of new instruments, their maintenance, and for data retrieval, processing, management and dissemination. This will become possible only with an order of magnitude cheaper and ‘maintenance-free’ strong motion accelerographs. In view of the rapid growth of computer technology this does not seem to be (and should not be) out of our reach.