Strong earthquake ground motion must be recorded for the purpose of seismic engineering. The ground motion is usually recorded with strong motion accelerographs placed at various locations.
Earthquake ground motion
Strong earthquake ground motion must be recorded for the purpose of seismic engineering. The ground motion is usually recorded with strong motion accelerographs placed at various locations. The acceleration record of a strong earthquake usually consists of two horizontal components and one
vertical component. Generally, the two horizontal components are of equal magnitude and the vertical component is somewhat smaller. The accelerograph record frequently includes instrumentation errors, owing to frequency characteristics of the accelerograph and other inherent features that must be corrected by filtering and other procedures. The corrected accelerogram is then integrated to obtain the velocity and displacement histories of ground motion. The records of Northridge, Helena and El Centro earthquakes are shown in Fig. 16.11. The Northridge accelerogram is extremely irregular and complex and is a typical earthquake accelerogram recorded on firm ground. On the other hand, on the surface of the soft strata the earthquake ground
motion assumes an almost harmonic nature, resulting from filtering of the seismic waves as they travel through soft strata.
Earthquake accelerograms are thus complex and can vary considerably from one another. They are significantly affected by local site conditions, distance from the causative fault, and the transmission path of the seismic waves. Newmark and Rossenblueth classified earthquake ground motion into four groups in accordance with their surface ground motion characteristics:
1. Single shock type. This occurs only at close proximity with epicentre on firm strata and for shallow earthquakes. Port Hueneme earthquake is an example for this.
2. A moderately long, extremely irregular motion. This is associated with an intermediate focal depth and occurs only on firm ground. It is typical of earthquakes originating in the circum-Pacific belt. The NS component of 1940 E1 Centro earthquake is indicative of this type.
A long ground motion exhibiting pronounced prevailing periods of vibration. Motions of the type are recorded at layers of soft strata, through which seismic waves have been filtered and subjected to multiple reflections at the layer boundaries. The 1964 Mexico City earthquake exemplifies this behaviour.
4. A ground motion involving large-scale permanent deformation of the ground. These types of earthquake may entail landslides or soil liquefaction. The Alaska and Niigata earthquakes of 1964 characterize this type of earthquake.
From the examinations of the ground motions shown in Fig 16.11 three characteristics of ground motions are important: (1) peak of maximum ground motion; (2) duration of ground motion; and (3) the frequency content. The structural response is affected by each of these factors. Peak ground motion, primarily peak ground acceleration (PGA), influences the vibration amplitude, and has been employed to scale earthquake design spectra and acceleration time forces. The severity of ground shaking is significantly influenced by the duration of ground motion. For example, an earthquake with high peak acceleration poses a high hazard potential, but if it is sustained for only a short period of time it is unlikely to inflict significant damage to many types of structures. Conversely an earthquake with moderate peak acceleration and a long duration can build up damaging motion in certain types of structure. Finally, ground motion amplification to a structure is more likely to occur and the frequency content of ground motion is in close proximity to the natural frequency of the structure.
A correlation equation for peak ground acceleration can be given in terms of Richter magnitude M as
Log10 PGA = –2.1 + 0.81M–0.027M2 ----- 16.16
Table 16.9 shows peak ground acceleration and time duration for various Richter magnitudes.
Equation 16.16 is site dependent. Although PGA decreases with distance from the causative fault, the rate of decrease is relatively small, over a distance comparable to the vertical dimensions of the shipped fault. The values given in Table 16.9 are conservatively high, and most actual earthquakes exhibit somewhat small values of PGA.