Foundation Repair News Earthquake North Texas

For Structural and Bridge Engineers

A new method has been developed for measuring the force of ground shaking during earthquakes at particular locations. The proposed Earthquake Shaking Force (EqSF) rating is based on the maximum vector sum of the recorded ground accelerations in the three main directions scaled with the strong ground motion duration. An equation for calculating the Earthquake Force values is proposed. This new method has been used to analyze and compare more than 220 ground station recordings from 48 earthquakes in the United States and around the world. The results show that the new method can provide objective ranking of the ground shaking forces, and can help engineers in designing seismically-resistant structures.

Today geologists and engineers use Magnitude scales and the Modified Mercalli Intensity scale to measure and rank earthquakes. For a century the Modified Mercalli Scale has been used for evaluating the intensity of local ground shaking. Different versions of this scale are still used, with its twelve-point range from I to XII, around the world. The Modified Mercalli Scale is based on the feelings and reactions of individuals and on observed damage to structures and underground facilities. This makes the scale subjective and inaccurate, because of the different individual interpretations (due to different sensitivity and reactions), the specific construction conditions of the country, the year of construction, or the level of building development.

Some improvement was achieved in California by modifying the Mercalli Intensity scale again, using Instrumental Intensity, in an attempt to correlate the intensity scale values with the peak ground accelerations and velocities. Instrumental Intensity is used by the TriNet system to produce ground shaking maps showing the peak ground accelerations and velocities by ground stations for a specific earthquake.

Today, California and many other states and countries have developed large nets of ground motion recording stations. During an earthquake, all necessary ground motion data are recorded and the Mercalli Intensity Scale is no longer providing reliable information. The new Earthquake Shaking Force method is a step forward in providing objective measurement of the ground shaking based on recorded ground motions during an earthquake.