How Earthquakes Shake a Building

An earthquake is a sudden release of energy in the earth's crust - usually rock slipping along a fault - that sends vibrations racing outward in all directions. At a building site, the net effect is that the ground beneath the foundation moves, mostly back and forth horizontally, accelerating first one way and then the other, many times, over seconds that feel much longer. The building is along for the ride, but not willingly. Newton's first law is the whole story: a mass at rest wants to stay at rest. When the ground darts sideways, the building's heavy upper floors lag behind, and the structure linking them to the moving base must drag them along. The force needed to accelerate that mass is the inertia force - and it is an earthquake's real weapon.

Crucially, this force is horizontal, while buildings are mostly designed to carry vertical gravity loads straight down their columns and walls. A building is usually very good at holding its own weight up; it can be surprisingly poor at resisting a large sideways push it was never shaped for. The inertia force is, in effect, gravity turned on its side and applied in pulses. Its size follows a famous relationship - force equals mass times acceleration - so it grows with how heavy the building is and with how hard the ground accelerates. Double the mass and you roughly double the force the structure must carry and the foundations must resist.

This is the first, liberating design insight of seismic work: because the demand scales with mass, a lighter building attracts a smaller earthquake force. Heavy stone roofs, thick unreinforced masonry and needless mass high up all increase the inertia force and the damage it can do; lighter roofs, sensible weights and mass kept low all reduce it. Long before any calculation, the architect's choices about materials and weight are already setting how hard the earthquake will hit. The engineer will quantify the force to IS 1893; the designer decides how much mass there is to shake in the first place.

The energy from a fault travels as seismic waves, and it helps to know the cast, because they arrive in order and do different things. First come the P waves (primary, or pressure waves) - the fastest, pushing and pulling the ground in the direction they travel, like sound through rock. They are the early jolt that rattles windows and wakes people; they rarely cause the worst damage. Behind them come the slower S waves (secondary, or shear waves), which shake the ground side to side, perpendicular to their travel. Because buildings are weakest against horizontal motion, S waves are far more dangerous - this transverse shaking is much of what the structure has to survive.

Last, and often most destructive to buildings, come the surface waves - energy trapped near the ground surface that rolls and sways in long, slower cycles. Two kinds matter: Rayleigh waves, which move the ground in a rolling, elliptical motion like an ocean swell, and Love waves, which whip it horizontally from side to side. Surface waves carry a large share of the energy, shake with long period and large amplitude, and can set tall or flexible buildings swaying heavily. The sequence - sharp P-wave jolt, strong S-wave shaking, then long surface-wave rolling - is why shaking feels like it builds and changes character rather than arriving all at once.

For a designer, the practical lesson is not to memorise wave physics but to grasp two things. First, the motion that threatens a building is overwhelmingly horizontal and cyclic - back and forth, not a single shove - which is why fatigue of connections and progressive loosening matter, not just peak strength. Second, different waves carry different periods of shaking, and a building is most endangered by the waves whose rhythm matches its own. That idea - matching rhythms - is resonance, and it is where we turn next. The exact ground motion expected at a site is captured, for design, in the response spectra and zone factors of IS 1893, derived by seismologists and applied by your engineer.

Give any building a push and let go, and it will sway back and forth at a rhythm all its own before settling - its natural period, the time for one complete sway. A low, stiff building (a squat two-storey masonry house) has a short natural period - it flicks back and forth quickly. A tall, flexible building (a slender multi-storey frame) has a long natural period - it sways slowly and grandly. Period is set mainly by two things: mass (more mass lengthens the period) and stiffness (more stiffness shortens it). This is not trivia; it decides how hard a given earthquake will hit a given building.

The reason is resonance. Earthquake ground motion is not a single frequency but a mixture, and it has its own dominant rhythms. When the building's natural period happens to match the period at which the ground is shaking hardest, each push arrives in step with the building's own sway and feeds it - exactly like pushing a child's swing at just the right moment. The swaying builds far beyond what the raw ground motion would suggest, and forces can multiply several times over. A mismatch is protective; a match can be catastrophic. Two buildings of different height, side by side in the same earthquake, can fare utterly differently because one was in tune with the shaking and the other was not.

This is why period, and the whole shape of how a building responds across a range of periods, sits at the heart of seismic engineering - captured in the response spectrum that IS 1893 uses to set design forces. For the designer, the intuition is what counts: a building has a rhythm; the ground has a rhythm; danger peaks when they coincide. You influence your building's rhythm through its height, mass and stiffness - and, as the next section shows, the soil can shift the ground's rhythm too, sometimes straight into resonance with common buildings. The engineer computes the period and checks the spectrum; the designer should know the swing is there, and that weight and stiffness tune it.

The same earthquake does not shake every site equally. Soft, loose soil amplifies shaking. Bedrock transmits sharp, short-period motion, but when those waves pass up into deep, soft sediments - old riverbeds, reclaimed land, soft clay, loose fill - the soil behaves like jelly on a plate: it slows the waves, stretches them into longer periods, and can greatly magnify their amplitude. Ground that would be mild on rock can become violent on soft soil, and - worse - the soil often amplifies exactly the longer-period shaking that matches ordinary multi-storey buildings, nudging them toward resonance. Mexico City in 1985 is the textbook case: buildings on an old lake bed suffered while similar ones on firmer ground nearby did not. This is why a site-specific geotechnical investigation is non-negotiable, and why the soil type directly changes the design forces in IS 1893.

Bring the threads together and you arrive at the idea engineers call base shear: the total horizontal earthquake force the structure must transfer, through its columns and walls, down to the foundations and into the ground. Conceptually, base shear grows with the building's weight (more mass, more inertia force), with the intensity of shaking expected at the site (its seismic zone), with soft soil (amplification), and with resonance (how the building's period meets the ground's). It is reduced by lighter construction and by a structure detailed to bend without breaking - ductility, the subject of Module 3 - which lets a building ride out the shaking by flexing and absorbing energy rather than shattering.

Treat base shear as a concept here, not a number. The actual value - with its zone factor, importance factor, response-reduction factor and spectral coefficient - comes from IS 1893 and your structural engineer, using your site's soil data. What the designer must carry away is the chain of cause and effect: shaking ground, lagging mass, inertia force, tuned by period and resonance, amplified by soft soil, summed as base shear, and survived through lightness, sound configuration and ductility. Understand that chain and every seismic decision in the rest of this course will feel less like rules to obey and more like physics you can reason with.