35.1. Definition of Peak Ground Acceleration (PGA)

Peak Ground Acceleration (PGA) is defined as the maximum absolute value of horizontal acceleration recorded at a particular location during an earthquake.

Peak Ground Acceleration (PGA) is a critical measure in earthquake engineering. It quantifies the maximum force that the ground can exert during an earthquake, represented as the highest acceleration experienced at a specific location on the ground. This value is crucial for understanding seismic forces that buildings and structures must withstand. PGA is measured in units such as 'g' (acceleration due to gravity) or meters per second squared (m/s²).

Think of PGA as a roller coaster ride that suddenly drops at a steep angle. The sharp pull of your body towards the ground as the coaster accelerates downwards can be likened to how ground acceleration increases during an earthquake. Just as you would feel a strong force pushing you down, buildings feel the same forces from the shaking ground.

Mathematically, if a(t) is the ground acceleration time history, then: PGA=max∨a(t)∨¿

In mathematical terms, Peak Ground Acceleration is represented by the maximum of the ground acceleration function, denoted as a(t). This means that during an earthquake, various acceleration values are recorded over time, and PGA is simply the largest of these values. This mathematical representation helps engineers and seismologists quantify and analyze the forces acting on structures during seismic events.

Imagine watching a video of a bouncing ball. As the ball hits the ground, it accelerates quickly. If you were to measure how high it bounces each time, the highest bounce you observe would be like the PGA — the peak acceleration of the ball during its contact with the ground reflects the maximum force experienced.

It is typically measured in g (acceleration due to gravity) or m/s².

PGA can be expressed in two primary units: 'g', which represents the acceleration due to gravity (approximately 9.81 m/s²), or in metric units as meters per second squared (m/s²). Using 'g' allows for easier comparisons of earthquake forces to the gravitational force that we experience daily. For example, a PGA of 1g means the ground shakes with acceleration equal to the force of gravity.

Consider the sensation of being in an elevator that suddenly drops. For a brief moment, you might feel weightless as the elevator descends. If the acceleration of the elevator were to match that of gravity (1g), you would feel a similar force exerted on you as during an earthquake when PGA is 1g, showing how intense the shaking can be.

PGA does not provide information about duration or frequency content but gives a direct indication of the force exerted on structures at the base.

While PGA is a valuable metric for understanding the maximum force during an earthquake, it has limitations. It does not account for how long these forces act on the ground (duration) or the varying frequencies of the shaking. Both duration and frequency can significantly impact how structures respond to seismic forces. Therefore, while understanding PGA is crucial, it cannot give a complete picture of the potential damage during an earthquake.

Think of a drumming session. A quick, strong beat (analogous to high PGA) may create a loud sound and a strong force, but if it only lasts a moment, it may not shake the room as much as a slower, persistently strong beat that lasts longer — even if each individual hit is not as forceful as the quick beat. In earthquake terms, this means that PGA might show a strong shake, but without considering how long the shaking lasted, we can't fully understand the potential damage.

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