Earthquakes and What Engineers are Responding To

WHAT IS AN EARTHQUAKE?

An earthquake occurs when stress builds up along a fault, and that stress is suddenly released in the form of seismic waves. The rupture starts underground at the hypocenter and is felt at the surface near the epicenter.

Foreshocks

Smaller earthquakes that happen in the same place as the larger earthquake that comes before the mainshock.

Mainshocks

The mainshock is the primary earthquake, characterized by the greatest magnitude.

Afterschocks

Smaller earthquakes that happen in the same place as the larger earthquake that comes after the mainshock.

WHY DO EARTHQUAKES HAPPEN?

The Earth’s crust is composed of pieces that fit together like a jigsaw puzzle. These pieces are called tectonic plates. The boundaries of the plates are made up of many faults. Whenever a plate moves, and a part of the edge gets stuck on another plate, the other plate keeps trying to move, and once it gets unstuck, an earthquake occurs.

SEISMIC WAVES

Seismic waves are the vibrations that are caused when one plate boundary gets unstuck from another one. The seismic waves cause ground motion on the surface of the Earth.

P-Waves

Primary waves (P-Waves) arrive first and involve back-and-forth particle motion in the direction of travel, making them less damaging.

S-Waves

Secondary waves (S-waves) arrive later and involve motion perpendicular to the direction of travel, making them more damaging.

Surface Waves

Surface waves arrive after P and S waves, travel along the Earth’s surface, and cause the most significant damage.

The time difference between the P and S waves arriving at a seismograph (an instrument that records ground-shaking) helps determine the radius and the general distance to the epicenter. To specifically pinpoint the epicenter, you need two more seismographs (a total of three), and where their radii intersect is where the epicenter is.

GROUND MOTION

Ground motion is the shaking of the Earth’s surface caused by seismic waves. This is the characteristic of earthquakes that engineers design for.

Characteristics of Ground Motion

Amplitude

Amplitude is the intensity of ground movement measured by PGA (Peak Ground Acceleration).

Frequency

Frequency is how fast the ground moves back and forth.

Duration

Duration is the length of strong shaking.

Factors that Influence Ground Motion

Earthquake Source

Large-magnitude, shallow earthquakes shake wider areas more strongly, particularly during surface-breaking ruptures.

Path

Shaking decreases with distance from the epicenter. Certain types of rocks absorb waves more quickly than others.

Site

Soft soils, deep sediments, and sedimentary basins amplify seismic waves; hills and ridges intensify shaking at peaks.

SOIL EFFECTS

All soils have a natural period, a time interval at which a portion of the soil will naturally vibrate when subjected to ground motion. A lower period value means that the soil is stiffer (takes less time to vibrate), and a larger period value means that the soil is softer (takes more time to vibrate). Softer soils amplify ground motion, in a process called soil amplification.

HOW BUILDINGS RESPOND TO EARTHQUAKES?

There are a few important building properties that influence how they respond to earthquakes.

Ground motion generates internal forces within the building. The greater the mass of a building, the greater the internal force generated. More internal forces mean greater damage to the building.

Buildings also have a natural period. The natural period of the building is proportional to the height of the building. If this natural period matches the period of the seismic waves, the building will resonate, and its vibration will amplify.

Torsion is when the building twists. This happens when the center of mass (where the building’s weight is concentrated) doesn’t line up with the geometric center of the building. When the ground shakes, for buildings that have this mismatch (typically irregularly shaped buildings), they undergo torsion, which is unpredictable.

Damping is the process of dissipating vibrations by converting them into heat to reduce swaying. The more damping systems a building has, the less shaking it goes through.

Ductility is the ability of a material to bend or deform without breaking suddenly. The more ductility present in a structure, the better, since it gives a warning before collapse.

The strength of a material is how much force it can resist before failing. Stiffness is how much a building resists sway/drift. You want the building to be strong and stiff, but too stiff can be bad.

In a regular configuration, where you have symmetrical, uniform floors, a low height-to-base ratio, short spans, and direct load paths, you achieve balanced stiffness, low torsion, and predictable behavior. For an irregular configuration, the situation is unpredictable.

WHY DO TWO BUILDINGS PERFORM DIFFERENTLY IN THE SAME EARTHQUAKE?

Two examples are the Christ Church Cathedral and the CTV Building after the 2011 Christchurch Earthquake in New Zealand. The Christ Church Cathedral, built in 1904, while suffering significant damage, was still standing. Whereas the CTV Building, built in 1968, mostly collapsed. The two buildings performed drastically differently due to the following factors: soil properties, building period, configuration, and materials. Another key point to consider is that the buildings were built in different “seismic code eras” that prioritized different design properties.

HOW DO ENGINEERS DESIGN?

Taking all the characteristics of ground motion, earthquakes, and buildings into consideration, there are some key tools that seismic design codes identify to define how buildings should be designed for earthquakes.

Response Spectra

These curves link peak shaking to natural period, guiding resonance avoidance and building height.

Site-Specific Data

Maps for soils and how they’ll interact with seismic forces are also provided in seismic codes.

Probabilistic Seismic Hazard Analysis (PSHA)

PSHA estimates the probability of specific ground shaking levels at a given location over time.