Peak Ground Acceleration (PGA): Measuring Earthquake Shaking Intensity
When an earthquake strikes, the impact on a specific location is determined not just by the size of the quake, but by how much the ground actually moves. This measurement of ground movement is known as Peak Ground Acceleration (PGA). While many people focus on the magnitude of an earthquake, engineers and geologists rely on PGA to understand the actual intensity of shaking at a given site.
PGA represents the maximum acceleration that occurs during earthquake shaking at a specific location. It is recorded as the amplitude of the largest absolute acceleration found on an accelerogram—a record of ground motion—during a particular seismic event. Because earthquake energy travels in multiple directions, PGA is typically analyzed in three components: two horizontal directions and one vertical direction.
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Key Facts
- PGA vs. Magnitude: Unlike the Richter scale, PGA measures local ground shaking rather than the total energy released by the earthquake.
- Engineering Importance: PGA is a critical parameter used in seismic building codes and the definition of the Design Basis Earthquake Ground Motion (DBEGM).
- Horizontal vs. Vertical: Horizontal PGA is generally larger than vertical PGA, though this can change during large, nearby earthquakes.
- Measurement Units: PGA is commonly expressed in fractions of g (Earth's gravity), meters per second squared (m/s²), or in Gal (where 1 Gal = 0.01 m/s²).
- Damage Correlation: For moderate earthquakes, PGA is a strong predictor of damage; for severe quakes, Peak Ground Velocity (PGV) often becomes a more significant factor.
The Geophysics of Ground Motion
Earthquake energy disperses in waves from the hypocentre (the point within the Earth where the rupture begins), causing ground movement in all directions. To fully describe this movement, scientists look at three distinct values:
- Peak Ground Acceleration (PGA): The rate of change of speed of the ground movement.
- Peak Ground Velocity (PGV): The maximum speed reached by the ground.
- Peak Displacement: The actual distance the ground moves.
Several factors influence these values at any given site. These include the length of the fault, the magnitude and depth of the quake, the distance from the epicentre, the duration of the shaking, and the local geology (the subsurface composition). Notably, shallow-focused earthquakes tend to generate stronger acceleration because the energy is released closer to the surface.
Ground type plays a massive role in how shaking is felt. Because different soils and rocks respond differently to seismic waves, PGA values can vary wildly even over distances of just a few kilometers. This variability is why seismologists create shake maps to visualize the distribution of intensity across a region.
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Seismic Risk and Engineering Applications
Seismic engineers use historical data and geographic assessments to determine seismic risk. This information is compiled into seismic hazard maps, which show the likely PGA values a region might experience with a specific probability of exceedance (PE). These maps are essential for government planning and for designing critical infrastructure—such as hospitals, bridges, and power plants—to survive a Maximum Considered Earthquake (MCE).
In structural design, engineers often use the Effective Peak Acceleration (EPA), which represents the maximum acceleration to which a building actually responds. The EPA is typically estimated to be between 2/3 and 3/4 of the total PGA.
Comparing Instrumental and Felt Intensity
There are two primary ways to describe earthquake intensity: instrumental intensity and felt intensity. Instrumental intensity is measured objectively by devices like accelerographs. Felt intensity, such as the Mercalli scale, relies on human observation and reports of damage.
While there is a correlation between what instruments record and what people feel, they do not always agree perfectly. Factors like the quality of local building construction can cause significant damage even when instrumental intensity is moderate, or conversely, allow well-engineered buildings to survive high acceleration.
Instrumental Intensity Correlation Table
| Instrumental Intensity | Acceleration (g) | Velocity (cm/s) | Perceived Shaking | Potential Damage |
|---|---|---|---|---|
| I | < 0.000464 | < 0.0215 | Not felt | None |
| II–III | 0.000464 – 0.00297 | 0.135 – 1.41 | Weak | None |
| IV | 0.00297 – 0.0276 | 1.41 – 4.65 | Light | None |
| V | 0.0276 – 0.115 | 4.65 – 9.64 | Moderate | Very light |
| VI | 0.115 – 0.215 | 9.64 – 20 | Strong | Light |
| VII | 0.215 – 0.401 | 20 – 41.4 | Very strong | Moderate |
| VIII | 0.401 – 0.747 | 41.4 – 85.8 | Severe | Moderate to heavy |
| IX | 0.747 – 1.39 | 85.8 – 178 | Violent | Heavy |
| X+ | > 1.39 | > 178 | Extreme | Very heavy |
Historical PGA Records
The following table highlights notable earthquakes and the peak ground acceleration recorded during these events, demonstrating how even moderate magnitude quakes can produce high acceleration.
| Earthquake | Magnitude (Mw) | Depth (km) | PGA (Single Direction, g) | Fatalities |
|---|---|---|---|---|
| 2008 Iwate–Miyagi Nairiku | 6.9 | 8 | 4.36 | 12 |
| 2016 Kaikoura | 7.8 | 15 | 3.23 | 2 |
| 1960 Valdivia | 9.5 | 33 | 2.93 | 1,000–6,000 |
| 2024 Noto | 7.5 | 16 | 2.88 | 260 |
| 2011 Tohoku | 9.1 | 30 | 2.7 | 19,759 |
| 1994 Northridge | 6.7 | 18 | 1.82 | 57 |
| 2023 Turkey–Syria | 7.8 | 40 | 1.62 | 62,013 |
| 2010 Haiti | 7.0 | 13 | 0.5 | 100,000–316,000 |
Frequently Asked Questions
What is the difference between earthquake magnitude and PGA?
Magnitude measures the total energy released at the source of the earthquake. PGA measures the intensity of the shaking at a specific geographic point. A large magnitude earthquake might result in low PGA if the epicenter is very far away, while a smaller earthquake could produce high PGA if it occurs directly beneath a location.
Why is horizontal acceleration more important in engineering?
Most buildings and infrastructure are designed to withstand lateral (side-to-side) forces. Because horizontal PGA is typically larger than vertical PGA and is the primary driver of structural stress, it is the most commonly used parameter in seismic building codes.
How does ground type affect PGA?
The geology of the subsurface significantly influences acceleration. Different types of soil and rock can amplify or dampen seismic waves, meaning two locations just a few kilometers apart can experience vastly different PGA values during the same earthquake.
What determines the amount of damage caused by an earthquake?
Damage is related to the ground motion (measured by PGA), the peak ground velocity (PGV), and the duration of the shaking. The longer high-level shaking persists, the more likely it is that structures will sustain significant damage.
How is PGA measured?
PGA is measured using scientific instruments called accelerographs, which record the rate of change of ground speed during seismic activity.