Occasionally earthquakes may occur within plates rather than at the edges. This was the case with a series of strong earthquakes in New Madrid, along the Mississippi River in Missouri in 1811-1812. Earthquakes are also caused by volcanic eruptions, underground explosions, or similar man-made events.
Buildings are shaken by ground waves, but their inertia tends to resists the movement which causes lateral forces. The building mass (dead weight) and acceleration effects these forces. In response, structure height and stiffness, as well as soil type effect the response of buildings to the acceleration. For example, seismic forces for concrete shear walls (which are very stiff) are considered twice that of more flexible moment frames. As an analogy, the resilience of grass blades will prevent them from breaking in an earthquake; but when frozen in winter they would break because of increased stiffness.
The cyclical nature of earthquakes causes dynamic forces that are best determined by dynamic analysis. However, given the complexity of dynamic analysis, many buildings of regular shape and height limits, as defined by codes, may be analyzed by a static force method, adapted from Newton’s law F= ma (Force = mass x acceleration).
1 Seismic wave propagation and fault rupture
2 Lateral slip fault
3 Thrust fault
4 Building overturn
5 Building shear
6 Bending of building (first mode)
7 Bending of building (higher mode)