Several
hundred small earthquakes occur every day and the vast majority are too small
for us to notice. Major earthquakes (magnitude 7+) occur roughly once a month
and superquakes (magnitude 8+) occur about once a year. Why do earthquakes
happen and what do the magnitude numbers mean?
Elastic
rocks
Professor
Henry Fielding Reid noticed that the ground surface had moved after the 1906
earthquake. He concluded that the earthquake had released previously stored
elastic stress. You may have experienced a sudden elastic rebound if you stretched
a rubber band and it broke. Subsequent research has supported Reid’s theory,
that most earthquakes are the result of the sudden rebound of stored energy.
Geologists
monitor the deformation of large areas of rock using GPS monuments and
observing their movement over time. If monuments are getting closer together,
the rock is being compressed. If monuments are moving apart, the rock is being
stretched. Either of these forces could eventually result in a sudden movement
along a fault – an earthquake.
Forces within the Earth gradually bend rocks which then
snap when they can no longer hold more energy. (from Tompkins and Watkins (2016)
Exploring Earth and Environmental Science: Year 12.)
Video:
- Elastic Rebound in a Subduction Zone
(IRIS) - What GPS can tell us about future earthquakes in Cascadia
(IRIS)
Measuring
earthquakes
The Mercalli
Scale was invented in 1902 to express how people experience an earthquake. The
Modified Mercalli Scale developed in 1931 is still used to describe the effect
on people. Intensity is expressed in Roman numerals and reflects the damage or
experience. For example, magnitude II is weak, felt only by a few people at
rest; whereas magnitude VII is very strong, causing considerable damage to
poorly built structures. Although structural engineers can rate higher levels
on the Modified Mercalli Scale, lower levels are a matter of opinion and there
is often great disagreement between people in the same area.
A Mercalli level VII earthquake causes slight to moderate
damage in well-built ordinary buildings and leads to widespread fright. (E
Wardoyo, Government of Indonesia, public domain)
The Richter
Scale was developed in 1934 and is based on the amplitude (height) of the
largest wave recorded on a seismometer and the distance between the earthquake
and the seismometer. It was developed for southern Californian earthquakes.
Other scales had to be calibrated with the Richter scale, taking into account
local rock properties. The Richter Scale is logarithmic – a one unit increase
in magnitude represents a 10x increase in the amplitude of waves. The Richter
scale is now referred to as the Local Magnitude and is not accurate for very
large earthquakes (8+).
The amplitude recorded on a seismograph varies with the underlying
geology. This means that each seismograph reading must be calibrated to reflect
local geology for the Richter Scale. (Loma Prieta earthquake readings, USGS,
public domain)
The Moment
Magnitude Scale is used by seismologists because it measures the amount of
energy released in an earthquake, based on the area of rupture, distance of
movement and properties of the rock involved. The Moment Magnitude Scale was
created to match the Richter scale for smaller events. An increase of one unit
on the scale represents 32x the energy released. This means that there is a
great difference between a 7.1 and a 7.2 earthquake, even though the numbers
are very similar.
An earthquake with a Moment Magnitude of 10 would require
a fault 14,000 km long, as shown in black on the map. (USGS, public domain)
Video:
- Moment Magnitude Explained – What Happened to the Richter Scale?
(IRIS) - How Does the Richter Scale Work?
(Seeker)
Website:
Explore
earthquakes
·
Create your own elastic earthquakes as shown in this video.
·
Learn about earthquakes with Shaky Science.
·
Explore Earthquake Liquefaction
with reading and experiments.
·
You can also see the experiments in this video.
·
Learn what to do in case
of an earthquake.
·
Discover Australian Earthquakes & Faults.
·
Find out about S & P waves
produced by earthquakes.
·
Locate an earthquake with this activity.
·
Try an Earthquake Engineering STEM Challenge.