Question

Differentiate among the Mercalli, moment magnitude, and amplitude (Richter) scales. How are these used to describe an earthquake? Why has the Richter scale been updated and modified?

Short answer

The Mercalli scale describes an earthquake's effects observed on people and buildings, the moment magnitude scale measures the total energy released by an earthquake, and the Richter scale measures the amplitude of the seismic waves produced by the earthquake. The Richter scale has been updated to the moment magnitude scale to provide a more consistent and accurate measure of large, distant or deep earthquakes.

Step-by-step solution

Understanding the Mercalli Scale

The Mercalli scale measures an earthquake based on its observed effects. It does not have a mathematical basis, but instead relies on descriptive accounts of what people in the area experienced, and the observable damage caused to buildings, natural formations, and the earth's surface.

Understanding the Moment Magnitude Scale

The moment magnitude scale is an updated version of the Richter scale, and was designed to provide a more accurate measure of an earthquake's total energy release. It is considered more accurate and reliable for larger, distant, or deep earthquakes.

Understanding the Amplitude (Richter) Scale

The Richter scale measures an earthquake's magnitude by detecting the amplitude of the seismic waves produced by the earthquake. It uses a logarithmic scale, meaning that each whole number increase on the scale represents a tenfold increase in measured amplitude and roughly 31.6 times more energy release.

Why the Richter Scale has been updated and modified

The Richter scale was modified to the moment magnitude scale because the Richter scale was found to be imprecise for very large or deep earthquakes. In these cases, the Richter scale could underestimate the true magnitude of the earthquake. The moment magnitude scale was hence introduced to provide a more accurate and consistent measure across all sizes, distances and depths of earthquakes.

Key concepts

Mercalli Scale

When earthquakes strike, the Mercalli scale is one of the tools used to describe their impact. Unlike mathematical scales, the Mercalli scale is qualitative. It uses observations of the earthquake's effects on people, buildings, and the natural environment to assign an intensity level. These levels, which range from I to XII, tell a story about the earthquake: from being not felt by anyone (I) to complete and total destruction (XII). For example, a rating of IV might be noted for shaking that is felt indoors and makes hanging objects swing, while a grade of IX could indicate that buildings have shifted off their foundations.

• Descriptive and qualitative
• Intensity levels from I to XII
• Based on observed effects

Moment Magnitude Scale

The moment magnitude scale is the current standard for measuring the size of earthquakes in terms of energy release. This scale, denoted as M_w, addresses some limitations of its predecessor, the Richter scale. It estimates the seismic moment of an earthquake, which is a product of the distance a fault moved and the force required to move it. This provides a more accurate reading of an earthquake’s true size, especially for very large or remote events. Scientists use various types of seismic data to calculate this magnitude, making it a robust scale for all earthquakes regardless of depth or distance from observers.

• Current standard for earthquake measurement
• Calculates seismic moment
• Accurate for all sizes and distances

Richter Scale

Developed in 1935 by Charles F. Richter, the Richter scale measures the magnitude of an earthquake by the amplitude of seismic waves it produces. The scale is logarithmic, so each whole number increase represents a tenfold increase in amplitude. That’s why a magnitude 5 earthquake is not just slightly stronger than a magnitude 4 – it's about 10 times as intense in terms of its wave amplitude. However, due to its limitations in assessing larger or deeper earthquakes, it led to the creation of the moment magnitude scale, which offers a broader and more precise gauge for such events. Despite this, the Richter scale is still widely used in popular references due to its historical significance and familiarity.

• Logarithmic scale measures amplitude
• Historically significant
• Replaced by moment magnitude for accuracy

Seismic Waves

Earthquakes generate seismic waves that travel through the Earth's interior and along its surface. These waves are the energy that we feel during an earthquake. There are multiple types of seismic waves, including:

• Primary (P) waves: These are compression waves that are the fastest and travel through solids and liquids.
• Secondary (S) waves: They are shear waves that only move through solids and are slower than P waves.
• Surface waves: These travel along the Earth's surface and tend to cause the most damage due to their amplitude and frequency.

Measuring the amplitude of these waves is fundamental in scales like the Richter scale, and understanding their behavior helps seismologists determine the characteristics of the earthquake, like its origin and intensity.

Energy Release

The energy released during an earthquake is a crucial concept that ultimately defines how much the ground shakes and the potential for damage. It’s important to understand that a small increase in magnitude translates into a significant increase in energy release. Mathematically, for each whole number jump in magnitude, there is approximately 31.6 times more energy released. For instance, a magnitude 6 event releases about 31.6 times more energy than a magnitude 5 event. The study of energy release helps in understanding the destructive power of earthquakes and is fundamental in the engineering of structures to withstand seismic forces.

• Key in determining earthquake damage
• Exponential increase in energy by magnitude
• Vital for engineering and safety preparations