EARTHQUAKE ENGINEERING

Earthquake Engineering:

An earthquake is caused by a sudden grinding slippage between two parts of the earths crust, which propagates motions in the surrounding ground. These ground motions, which occur in all directions, shake buildings and can lead to collapse or cause building components to fall, either of which can be life threatening. Whether or not a building survives an earthquake depends primarily on how it behaves when subjected to the ground motions generated by the earthquake.

Seismic hazards are calculated considering earthquake rates based on earthquake history and fault slip rates, combined with site soil types. The result is expressed as the level of ground shaking (as a percentage of gravity).

Seismic Probability Evaluation helps to determine which buildings need an engineering evaluation and rank them with respect to their need for attention. The method is based on a rapid inspection (approximately an hour) of each building or its drawings. The inspector uses a form to obtain a score for each building based on the following seismic risk factors:

• seismicity
• ground conditions
• type and age of construction (both of which influence integrity, strength and ductility)
• building irregularities
• use (e.g., hospital or office)
• presence of heavy or dangerous nonstructural building components, which may fall, or building services lines and equipment, which may fail.

Response spectra are very useful tools for analyzing the performance of structures and equipment in earthquakes, since many behave principally as simple oscillators (also known as single degree of freedom systems. Thus, if you can find out the natural frequency of the structure, then the peak response of the building can be estimated by reading the value from the ground response spectrum for the appropriate frequency. In most building codes in seismic regions, this value forms the basis for calculating the forces that a structure must be designed to resist (seismic analysis).

Seismic shear wave velocity for comprehensive earthquake preparedness requires methods to rapidly assess the possibility of unusually strong shaking at a large number of sites. Estimating seismic shear velocity structure can be an important component of site-response estimates of possible shaking.

A need for the rapid and inexpensive assessment of earthquake hazard at large numbers of sites has led to the development of several geophysical testing methods that do not require drilling. The well-known Spectral Analysis of Surface Waves (SASW) and microtremor array techniques both use surface-wave phase information to interpret shear-velocity or rigidity profiles.