8.3 Measuring and Locating Earthquakes

Measuring Earthquake Intensity

There are three types of earthquakes that you might have heard of associated with a single rupture event that occurs along a fault. There are the foreshocks, which are smaller seismic events that occur before the main event. There is the mainshock, which is often the largest seismic event to occur along the fault line. Finally, there are aftershocks, which occur after the main seismic event has happened.

The intensity of any earthquake- whether it is a foreshock, aftershock or mainshock, can be measured using an agreed-upon scale. There are two generally used scales for categorizing earthquakes. There is the more famous Richter Scale, which rates an earthquake a number of 0-10 based on its intensity. The Richter Scale is logarithmic, which means each ascending number has a ten times greater intensity of the earthquake than the last (i.e., a 4.0 earthquake is 10 times more intense than a 3.0 quake, and a 5.0 earthquake is 100 times more intense than a 3.0 earthquake, etc.). The number on the Richter Scale is determined by observing the amplitude of waves recorded by a seismograph.

Another method of determining earthquake intensity that is gaining popularity is the Modified Mercalli Intensity Scale. This scale uses reports from the public and officials to assess the intensity of an earthquake and the amount of damage it has done to the surrounding area to determine its intensity. Learn more about this scale in the exercise below:

Locating Earthquakes

Earthquakes and Plate Boundaries

Earthquakes tend to happen at faults, and nothing causes faults better than an active plate boundary. But what sort of earthquakes are produced at each boundary? There are differences in the depth, intensity, and distance of the earthquakes that occur from each tectonic plate boundary, depending on the type. Let’s review:

Subduction zones (Convergent Boundaries): These are zones where cold slabs of rock are descending towards the mantle. Along the way, they rupture at depth, creating some of the most powerful earthquakes on the planet! These earthquakes occur some distance away from the actual boundary since the slab descends at an angle, but you can bet they are deep.

Transform Boundaries: These are places, such as in California, where a lot of faults are being produced due to the opposite horizontal movement of two plates. The earthquakes that happen when the rocks rupture are typically shallow, and near the boundary or faults associated with the boundary. These can be some minor to moderate earthquakes, although the powerful ones are fairly rare.

Divergent Boundaries: These are regions where new crust is born and is rapidly cooling down. Sometimes rock will rupture right at these boundaries, but the earthquakes are fairly minor to moderate and are shallow.