Understanding Earthquake Mechanics, Plate Tectonics, and Building Seismic Behavior

Posted on Oct 7, 2026 in Geology

1. A. Elastic Rebound Theory of an Earthquakes

Continuous Plate Movement: Tectonic plates are continuously moving due to forces acting inside the Earth.
Stress Accumulation: Rocks along a fault may be locked due to friction, while the plates continue to move. This causes stress to accumulate.
Elastic Deformation: The rocks slowly bend and deform because of the increasing stress and store elastic strain energy.
Breaking Point: When the accumulated stress becomes greater than the strength and frictional resistance of the rocks, the rocks can no longer remain locked.
Sudden Fault Movement: The rocks suddenly slip or move along the fault and partially return towards their original shape. This is called elastic rebound.
Release of Energy: The stored elastic energy is suddenly released in the form of seismic waves, which cause ground shaking.
Repetition: After the earthquake, tectonic movement continues and stress starts accumulating again, making the elastic rebound process a repeated cycle.

Basic Concept: The plate tectonics theory states that the Earth’s outer rigid layer, called the lithosphere, is divided into several large and small plates.
Movement of Plates: These plates move slowly over the semi-fluid asthenosphere below them due to heat and convection currents inside the Earth.
Plate Boundaries: The movement of plates occurs mainly at their boundaries. Depending on their movement, boundaries are classified as divergent, convergent, and transform boundaries.
Divergent Movement: At divergent boundaries, two plates move away from each other. Magma rises from the mantle and forms new crust, mainly along mid-oceanic ridges.
Convergent Movement: At convergent boundaries, two plates move towards each other. One plate may subduct below the other, producing earthquakes, volcanoes, and mountain ranges.
Transform Movement: At transform boundaries, two plates slide horizontally past each other. Friction and sudden movement along these faults can produce strong earthquakes.
Major Plates: The major tectonic plates of the Earth are the Pacific, North American, South American, African, Eurasian, Indo-Australian, and Antarctic plates. Smaller plates also exist, such as the Nazca, Arabian, Philippine, and Caribbean plates.

Building Shape: Regular and symmetrical buildings perform better during earthquakes, while irregular shapes may develop uneven forces and twisting.
Plan Irregularity: L-shaped, T-shaped, or U-shaped plans can cause torsional effects and uneven distribution of earthquake forces.
Vertical Irregularity: Sudden changes in building height, stiffness, or mass can concentrate earthquake forces at particular floors and increase damage.
Soft Storey: An open ground floor with fewer walls or columns has lower stiffness. It may undergo large deformation and can become a weak point during an earthquake.
Heavy Structures: Heavy roofs, walls, and other structural components increase the seismic mass of the building, resulting in higher earthquake forces.
Openings in Walls: Large or improperly placed doors and windows reduce the strength and stiffness of walls and may lead to cracking or failure during an earthquake.
Symmetry and Proper Layout: Properly distributed walls, columns, and other structural elements provide a balanced load path and reduce torsion and excessive deformation, improving seismic performance.

Plan Irregularity: L-shaped, T-shaped, and U-shaped buildings have uneven mass and stiffness distribution, causing torsion during earthquakes.
Vertical Irregularity: Sudden changes in storey height, stiffness, or strength cause a concentration of earthquake forces at particular levels.
Soft Storey: A storey with much less stiffness than the storeys above, such as an open ground floor, may undergo excessive deformation.
Weak Storey: A storey having lower lateral strength than the storeys above may suffer severe damage during strong ground shaking.
Mass Irregularity: A sudden increase in mass at a particular floor increases the seismic force acting on that floor.
Setback Irregularity: A sudden reduction in building width at upper floors creates discontinuity and a concentration of stresses.
Floating Columns: Columns that do not continue directly to the foundation create an irregular load path and may cause serious damage during earthquakes.