Seismic intensity scales

Seismic intensity scales are tools used to categorize the severity of ground shaking at specific locations. It is important to distinguish these from seismic magnitude scales. Magnitude scales measure the overall strength of an earthquake event itself. An earthquake might have a high magnitude but may not cause much perceptible shaking in a certain area. In contrast, intensity scales focus on the observed effects of the shaking. These effects include how alarmed people or animals feel during the event. They also measure the extent and severity of damage to buildings and natural features.

Several factors determine how intense the shaking will be at a particular site. First, the size or strength of the source event is a primary factor. The depth of the event and the distance from the source also play major roles. Additionally, the type and orientation of the seismic waves generated will change the impact. One of the most critical factors is the site response caused by local geology. Certain conditions, such as unconsolidated sediments in a basin, can amplify ground motions. In some cases, these local conditions can amplify the shaking by as much as ten times.

Soil conditions can significantly alter how seismic waves travel through the ground. For example, thick layers of soft soil, such as fill, can amplify waves even at great distances from the source. Sedimentary basins can also cause resonance, which increases the total duration of the shaking. We saw this during the 1989 Loma Prieta earthquake. The Marina district of San Francisco suffered heavy damage because of these conditions, even though it was not near the epicenter. Geological structures can also act as channels. In that same earthquake, waves passing under the south end of San Francisco Bay reflected off the Earth's crust. This effect channeled the waves toward San Francisco and Oakland.

Scientists have worked for centuries to refine how we measure these effects. Domenico Pignataro devised the first simple classification of earthquake intensity in the 1780s. Later, in 1828, the German mathematician Peter Caspar Nikolaus Egen created the first recognizable modern intensity scale. A major milestone occurred in 1857 when an Irish engineer named Robert Mallet was sent to research an earthquake. He was sent by Imperial College, London, to study the Basilicata earthquake, also called the Great Neapolitan Earthquake. Mallet produced the first modern mapping of earthquake intensity. Following this, the 10-grade Rossi–Forel scale became widely adopted in the late 19th century.

In 1902, the Italian seismologist Giuseppe Mercalli introduced his own 12-grade scale. However, the scale underwent significant improvements during the 1950s thanks to Charles Francis Richter. Richter discovered a correlation between seismic intensity and Peak Ground Acceleration, or PGA. PGA is a measurement of how much the ground accelerates at a specific site. Richter also developed a way to group buildings by their strength and type. By evaluating damage based on these specific building types, the scale became much more quantitative. This evolved into the Modified Mercalli intensity scale, or MMS, which provides more reliable evaluations today.

Today, different regions around the world utilize various specialized intensity scales. In Japan, experts use the JMA Seismic Intensity Scale to monitor ground motion. The European macroseismic scale, known as EMS-98, is used across Europe. Other notable systems include the Medvedev–Sponheuer–Karnik scale, also called MSK-64, which is used in several regions. China uses the Liedu scale (GB/T 17742–1999), while the Philippines uses the PHIVOLCS earthquake intensity scale (PEIS). The Central Weather Administration also employs a specific scale for its needs. These diverse systems allow scientists to tailor their observations to local requirements.

Understanding these scales is vital for both historical research and future safety. When seismographs were not available to record older earthquakes, scientists used isoseismal maps to help. An isoseismal map shows the areas where different levels of shaking were felt. By looking at the maximal intensity and the area affected, researchers can estimate the location and magnitude of historical quakes. These maps also help us predict the likely level of damage from future earthquakes of similar magnitude. In Japan, this information is used immediately after an earthquake occurs. It helps officials anticipate the severity of damage expected in different areas to better prepare for recovery.