Seismic Intensity Scales: Measuring the Impact of Ground Shaking
When an earthquake strikes, the experience varies wildly depending on where you are standing. While scientists often discuss the "magnitude" of a quake, that number only tells part of the story. To understand the actual impact on people and infrastructure, experts use seismic intensity scales. These scales categorize the severity of ground shaking at specific locations, focusing on observed effects rather than the energy released at the source.
Unlike magnitude scales, which provide a single value for the overall strength of an earthquake, intensity scales describe the perceptible shaking and the resulting damage. By analyzing the maximum intensity observed and the area affected, seismologists can estimate the location and magnitude of historical earthquakes that occurred before the invention of modern recording instruments.
Key Facts
- Intensity vs. Magnitude: Magnitude measures the total energy of the earthquake; intensity measures the local shaking and damage.
- Observation-Based: Intensity is determined by human perception, animal behavior, and structural damage.
- Site Response: Local geology, such as soft soil or sedimentary basins, can amplify shaking by up to ten times.
- Isoseismal Maps: These maps plot areas of equal shaking intensity to help locate epicenters and predict future risks.
- Global Variation: Different regions use different scales, such as the Modified Mercalli scale in the US and the JMA scale in Japan.
The Mechanics of Ground Shaking
While earthquakes—the rupturing of the Earth's crust—are the most common cause of damaging shaking, other events like volcanic tremors, large explosions, or avalanches can also cause the ground to move. The actual intensity felt at any given point depends on several critical variables:
- The overall strength (magnitude) of the source event.
- The depth of the event and the distance from the source.
- The type and orientation of the seismic waves generated.
- Site response, which refers to how local geology affects the motion.
Site response is particularly vital. Unconsolidated sediments in a basin can act as an amplifier, significantly increasing the violence of the shaking. For example, thick layers of soft fill can amplify waves even far from the epicenter. Sedimentary basins may also resonate, extending the duration of the shaking.
A clear example of this occurred during the 1989 Loma Prieta earthquake. The Marina district of San Francisco suffered extreme damage despite being nearly 100 kilometers (60 mi) from the epicenter. This was caused by soft soil amplification and geological structures that reflected seismic waves off the Earth's crust, channeling them toward San Francisco and Oakland.
The Evolution of Intensity Measurement
The effort to categorize earthquake severity began in the 1780s with Domenico Pignataro. By 1828, Peter Caspar Nikolaus Egen developed the first recognizable intensity scale in a modern sense. However, the first scientific mapping of intensity was conducted by Robert Mallet, an Irish engineer, following the 1857 Basilicata earthquake.
The late 19th century saw the introduction of the 10-grade Rossi–Forel scale, which was later succeeded in 1902 by Giuseppe Mercalli's 12-grade scale. In the 1950s, Charles Francis Richter introduced critical quantitative improvements. He established a correlation between intensity and Peak Ground Acceleration (PGA)—the maximum acceleration of the ground during shaking—and categorized buildings by their structural strength. This evolution resulted in the Modified Mercalli Intensity Scale (MMS), which provides more reliable evaluations by accounting for building vulnerability.
Global Intensity Scales
Different countries have adopted various scales to suit their specific geological needs and building standards.
| Country/Region | Seismic Intensity Scale Used |
|---|---|
| United States, Hong Kong, Indonesia | Modified Mercalli scale (MM) |
| Japan | JMA Seismic Intensity Scale |
| Russia, Kazakhstan, Israel | Medvedev–Sponheuer–Karnik scale (MSK-64) |
| Europe | European macroseismic scale (EMS-98) |
| China | Liedu scale (GB/T 17742–1999) |
| Philippines | PHIVOLCS earthquake intensity scale (PEIS) |
| Taiwan | Central Weather Administration seismic intensity scale |
| India | Medvedev–Sponheuer–Karnik scale |
Frequently Asked Questions
What is the difference between earthquake magnitude and intensity?
Magnitude measures the total energy released at the earthquake's source and is a single value for the entire event. Intensity measures the observed effects and severity of shaking at a specific location, meaning one earthquake has one magnitude but many different intensities.
How does local soil affect how an earthquake feels?
Local geology, or site response, can drastically change the experience. Soft soils and unconsolidated sediments can amplify seismic waves, making the shaking much stronger and longer-lasting than it would be on solid rock.
What is an isoseismal map?
An isoseismal map is a tool that plots areas experiencing the same level of shaking intensity. These maps are essential for estimating the epicenter and magnitude of historical earthquakes that were not recorded by seismographs.
What is Peak Ground Acceleration (PGA)?
Peak Ground Acceleration is a measure of the maximum acceleration the ground undergoes during an earthquake. It provides a quantitative link between the physical movement of the earth and the resulting intensity of damage to structures.
Why is the Modified Mercalli Scale more reliable than the original?
The Modified Mercalli Scale incorporates a quantitative understanding of building types and their specific vulnerabilities, allowing experts to evaluate intensity based on the degree of damage relative to the strength of the structure.