Earthquake Magnitude, Energy, and Intensity in an Image

 Since experiencing a 7.9 quake (thankfully from 65 miles away) as a young teenager, I've been fascinated with earthquakes.

Not enough to center a career around them, but enough to make some effort to learn about them, their hazards, and what I can do to better prepare my home and family.

Here I want to bring some of my earlier posts together to help give more meaning to an image that depicts the magnitude, energy, intensity, and frequency of earthquakes. 

The magnitude is the size of an earthquake. Originally this was measured on the Richter Scale which basically measures how big the shaking is. But this scale becomes less accurate for measuring quakes from further away, and it's not very good for measuring big quakes. A full step for example from 5.0 to 6.0) is an increase of 10 times.

In the 1970s the Moment Magnitude Scale was developed. Where the Richter Scale was more about size, Moment Magnitude measures how much energy is released. It's another type of big, only going from one step to another (like the 5.0 to 6.0) is not 10 times but 31.6 times larger. Each 0.1 increase is 1.4 times more energy released.

A magnitude 6.0 earthquake is 10 times "bigger" than a 5.0, but the amount of energy released is 31.6 (most sites round this up to 32) more.

These logarithmic scales get more pronounced with more steps. A 7.0 quake is 100 times bigger than the 5,  but the amount of energy released is 1000 times greater.

The intensity of an earthquake is harder to pin down because so many factors affect how the quake feels, the shaking, damage, and effects. You can have a smaller quake with a higher intensity than a larger magnitude earthquake.

Intensity is affected by multiple factors, with the biggest being how far away one is from the fault rupture, specifically the hypocenter which is the location on the fault where the rupture occurs. The epicenter is the surface location above the hypocenter. I think fault rupture sounds better than hypocenter.

Depth is a big factor, and this is one of the distances from the fault rupture. The deeper the rupture the less intense it feels on the surface. Conversely, the more shallow the quake is the higher the intensity it will be.

Horizontal distance from the fault rupture is the other distance factor.

Other things that can affect a quake's intensity include the ground composition and geology of the area, including the shape, density, and materials in the earth below the surface. Depending on this factors seismic waves may be amplified or dissipated. Mud and soft soils shake the most, with bedrock shaking much less. Living over a bell-shaped geology can have an amplifying effect with the seismic waves and end up causing more damage.

In the attempt to measure an earthquake's intensity, the Mercalli Intensity Scale was created

The 1985 earthquake I experienced (referenced as the 1985 Algarrobo earthquake) had a magnitude of 7.9 to 8.0 (depending on the reference)...instead of being conservative about it maybe I should say I felt an 8. The depth was 20.5 miles (33km). Near the fault rupture the maximum intensity reached level VIII (Severe) but 65 miles away, where we lived the intensity was less. I haven't found an intensity map but I was told it felt like a 6.9, but that was probably a subjective measurement.

Here's the earthquake magnitude, energy release and shaking intensity image. 

EQ Magnitude, Energy Release, and Shaking Intensity
From USGS webpage, https://www.usgs.gov/natural-hazards/earthquake-hazards/science/earthquake-magnitude-energy-release-and-shaking-intensity?qt-science_center_objects=0#qt-science_center_objects, from the Incorporated Research Institutes for Seismology, IRIS, credit Gavin Hayes

I use this image in my earthquake presentations. It does a great job in capturing the magnitudes of earthquakes with historic earthquakes and energy equivalents. The curved graph shows the increase in size with the energy released on the side. 

Up the middle is the approximate worldwide frequency of earthquakes, with lower magnitude tremors being much more frequent than the larger quakes. The frequency is interesting as it follows a somewhat close reverse logarithmic Richter Scale. In the Richter Scale a magnitude 2 is 10 times bigger than a 1, and a M3 100 times larger, etc. The graph shows that magnitude 8 quakes are (actually more than) 10 times more frequent than 7s, and M6 quakes are about 100 times more frequent than 8s, and M5 tremors are around 1000 times more frequent.

The referenced USGS page has a good quake comparison between the Northridge California 6.7 quake and the Nisqually, Washington magnitude 6.8 tremor. The Northridge earthquake was far more damaging
Northridge vs. Nisqually earthquake intensities


While there were other factors, the single biggest factor that determined the huge difference in intensity was the depth of the ruptures. The Northridge earthquake ruptured within 3-11 miles of the surface. By contrast the Nisqually's fault rupture was in the 30-36 mile depth. By the time the seismic waves in the Nisqually quake reached the surface they had already dissipated considerably resulting in less shaking and damage.


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