28. Magnitude and Intensity of Earthquakes

In the domain of Earthquake Engineering, understanding the strength and impact of earthquakes is crucial for designing earthquake-resistant structures. Two fundamental parameters used to describe an earthquake are magnitude and intensity. While these terms are often used interchangeably by the public, they have distinct scientific meanings. Magnitude measures the energy released at the source of the earthquake, whereas intensity refers to the effects or shaking produced at specific locations. This chapter explores various scales used to quantify these parameters, their relevance in structural engineering, and how these measurements influence seismic design.

This introduction sets the scene for understanding earthquakes in engineering. Magnitude is a scientific term that quantifies how much energy is released during an earthquake. This is generally measured using instruments like seismographs. On the other hand, intensity deals with the shaking and damage felt or observed at a particular location, as experienced by people and infrastructure. It's like having two different ways to look at a storm: one focuses on how powerful it is, while the other examines how much it affects the people and buildings around it.

Imagine a balloon filled with air. When it pops (representing an earthquake's source), the energy released from the balloon can be thought of as the magnitude. But when the air rushes out and causes nearby objects to shake and move, that shaking represents the intensity of impact felt at those locations.

Aspect Magnitude Intensity Definition Measure of energy released at the earthquake's source Measure of shaking felt at a location Measured by Seismographs Human observation and instrumental data Units/Scales Richter Scale, Moment Magnitude Scale Modified Mercalli Intensity (MMI) Scale Value uniqueness Single value per earthquake Varies from place to place

This table format breaks down the differences between magnitude and intensity. Magnitude refers to a single numerical value that represents how strong the earthquake is based on seismograph readings. It typically uses scales like the Richter or Moment Magnitude Scale (Mw). In contrast, intensity is assessed through human observation of physical symptoms from the quake's effects, which can vary from one place to another depending on conditions, such as building structures and the local geology. So, while an earthquake might have a specific magnitude, the intensity can be different in nearby towns depending on various factors.

Think of a concert that you attend. The overall loudness of the band can be thought of as the 'magnitude'—it's the same for everyone, regardless of where they sit. However, the volume can be experienced differently depending on whether you're front row (high intensity) or way at the back (low intensity).

Magnitude is assessed using various scales:

• Richter Magnitude Scale (ML): Developed in 1935 by Charles F. Richter, this scale is based on the amplitude of seismic waves recorded on a Wood-Anderson seismograph. It's a logarithmic scale where each unit increase represents a tenfold increase in wave amplitude and approximately 32 times more energy release. Suitable for small to medium earthquakes (typically < 6.8).

• Moment Magnitude Scale (Mw): This is currently the most widely used magnitude scale, based on the seismic moment (Mo), which incorporates the area of the fault that slipped, how far it slipped, and the type of rocks involved. It provides consistent values across a wider range of earthquake sizes and does not saturate like the Richter scale for large earthquakes.

Magnitude is measured by different scales. The Richter Scale, once the standard, estimates how much energy an earthquake releases based on recorded wave amplitudes. It works well for smaller quakes. However, as earthquakes grow larger, the Moment Magnitude Scale has become preferred, as it measures the total energy released more accurately without becoming less effective for larger events. This scale incorporates various factors like how much the earth has shifted and the area affected, allowing for more comprehensive assessments especially important for large earthquakes.

Consider a pan of water on a stove. Smaller flames may reach a certain height (like a small earthquake's energy measurements), but as the flames grow and cover more area, the ‘energy’ or heat produced is better measured with an instrument that records overall heat across the entire pan (like the Moment Magnitude Scale), as it gives a more accurate picture.

Intensity is assessed through scales such as:

• Modified Mercalli Intensity Scale (MMI): Developed by Giuseppe Mercalli and modified over time, this qualitative scale ranges from I (not felt) to XII (total destruction) and is based on observed effects on people and buildings rather than instrumental readings.

• MSK Intensity Scale: Used in India and Europe, similar to MMI but emphasizes building responses. Intensity values range from I to XII and include accounts for structural vulnerability.

Intensity is about how an earthquake affects its surroundings. The Modified Mercalli Intensity Scale categorizes the impact from not felt at all (I) to total destruction (XII) based on observed damage, like fallen buildings or ground cracks. In contrast, the MSK Scale places an even stronger emphasis on how buildings withstand shocks, taking into account the type of construction. This helps engineers and planners to understand how different areas might respond based on their structures.

If you’ve ever felt a truck drive by while sitting in your home, you know the shaking can feel different based on whether you’re in an apartment building versus a sturdy, single-family home. Some buildings may sway without major damage, while others might shake excessively and show signs of distress. The scales track this experience to help predict and prepare for future quakes.

There is no direct formula connecting magnitude and intensity due to factors like local geology and building design. However, empirical relationships depict that typically, a magnitude 6.0 earthquake could produce intensity VI-VIII near the epicenter.

Although magnitude gives an idea of the earthquake’s energy, intensity can vary widely from one area to another due to various conditions. For example, an earthquake that measures a magnitude of 6.0 can shake the ground to different extents depending on the geology, materials used in buildings, and how close a location is to the epicenter. This correlation helps keep track of expected severity and damage even if there's no simple mathematical relationship.

Think of a firework show. The launch point (the epicenter) will be much louder to those close by (high intensity), while those further away will hear it more faintly (lower intensity), even though all the fireworks are launched at the same strength (magnitude). This is similar to how the energy of an earthquake dissipates as it moves through different soils and structures.

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