Read more: “Instant Expert: Earthquakes“
Understanding the shaking caused by earthquakes is crucial if we are to prepare for these events – but the impact of an earthquake on people and cities depends on more than magnitude alone. The Earth’s crust can amplify or dampen the severity of shaking
Shake, rattle and roll
Seismic waves cause perceptible ground motion if they are strong enough. For seismic hazard assessment, the study of ground motion is where the rubber meets the road. If we understand the shaking, we can design structures and infrastructures to withstand it.
The severity of earthquake shaking is fundamentally controlled by three factors: earthquake magnitude, the attenuation of energy as waves move through the crust and the modification of shaking due to the local geological structure.
Bigger earthquakes generally create stronger shaking, but not all earthquakes of a given magnitude are created equal. Shaking can depend significantly on factors such as the depth of the earthquake, the orientation of a fault, whether or not the fault break reaches the surface and whether the earthquake rupture is relatively faster or slower than average.
Attenuation of seismic waves varies considerably in different regions. In a place like California or Turkey, where the crust is highly fractured and relatively hot, waves dissipate – or attenuate – quickly. Following the 1906 San Francisco earthquake, pioneering geologist G. K. Gilbert observed: “At a distance of twenty miles [from the fault] only an occasional chimney was overturned…and not all sleepers were wakened.” In regions that are far from active plate boundaries, such as peninsular India or the central and eastern US, waves …