The main goal of this video is to explain the difference between two measures of earthquake size
as represented by earthquake magnitude and earthquake intensity.
When we discuss earthquake magnitude, we're referring to the physical shaking recorded by
instruments like this seismograph. Shaking is caused by the seismic waves released by an earthquake.
In contrast, earthquake intensity is all about the damage that results from the earthquake
and how many people experience the event.
We'll consider these two scales separately beginning with magnitude.
Charles Richter developed one of the first consistent ways to measure earthquakes in California
by analyzing the amplitude and time interval between the arrival of p and s waves.
Geologists applying Richter's methods would use a chart like this
to plot amplitude and distance values to determine earthquake magnitude.
Amplitude is plotted on a log scale
so that each step in magnitude is equivalent to a 10-fold increase in amplitude.
This method gives an accurate magnitude value for smaller local earthquakes
that occurred at shallow depths.
However as more data were collected it became clear that it didn't
accurately measure the size of larger deeper earthquakes occurring on longer faults.
The second magnitude calculation we'll discuss takes fault size into account.
This is a map of active US faults, most of them are associated with two different types of plate boundary
and we are particularly interested in the Cascade and San Andreas fault zones
as they have the potential for very large dangerous earthquakes.
Like many major fault systems around the world, these two fault zones are a thousand kilometers long or more.
These faults break in segments
and the length of the segment that ruptures determines the magnitude of the earthquake.
For example, the 7.9 magnitude San Francisco earthquake ruptured a segment of the San Andreas
that was 10 times longer than the break caused by the 6.9 magnitude Loma Prieta quake.
To get an accurate measure of earthquake magnitude on these larger fault segments
we must be able to constrain both the area of the fault that ruptures
and the amount of fault displacement
This graph plots the rupture area of several recent earthquakes
of magnitude 7 or greater against earthquake magnitude.
If all else were the same rupture area alone might be sufficient to allow us to estimate magnitude.
However, additional factors such as the amount of fault displacement or the rigidity of the surrounding rocks
can also be significant in determining earthquake magnitude.
For example, two mega earthquakes of similar size both occurred on subduction zones
and resulted in widespread devastation.
The northern Sumatra earthquake featured a large rupture surface and a significant amount of false slip.
In contrast the Tohoku earthquake compensated for a smaller rupture area by generating
more false slip than had ever been recorded for any other earthquake
The rupture area, displacement, and rigidity contribute to a value known as the seismic moment.
Geologists, or really super fast computers, plug this seismic moment value into an equation
to determine the moment magnitude and that's the value that gets reported
when you rewrite big earthquakes in the news.
Not only does the recording on the seismograph increase in size by 10 for each step of magnitude
but the energy released by the earthquake increases 32 times.
For example, let's compare the 9.1 magnitude earthquake in Japan
with a 7.9 magnitude San Francisco earthquake.
The earthquake in Japan would have been nearly 16 times larger on the seismograph record
and would have released 63 times more energy.
Since 1950 there have been just five earthquakes of magnitude 9 or greater recorded around the world.
Each occurred on a subduction zone along a convergent plate boundary
The biggest of these was a magnitude 9.5quake in Chile.
Could a magnitude 10 earthquake be in our future?
Well, no. To get a fault big enough to generate a magnitude 10 earthquake
you'd need something like this; a 14 000 kilometer long boundary that would link
together multiple fault systems along the margins of the Nazca, Cocos and Pacific plates,
and that's probably just not realistic.
The energy produced by an earthquake is a pretty abstract concept but the damages of the result are real
and can be readily observed and experienced by millions of human beings each year.
We label this the earthquake intensity and it's measured using the Modified Mercalli scale.
which focuses on the effect an earthquake has on people and the damage that might occur.
The Mercalli scale has 12 categories represented by roman numerals
we've grouped the last three categories together as their characteristics often overlap.
Each category has a brief description to help the application of the scale.
Don't expect to remember all these details, just be aware of the general trends.
For example the first two categories represent relatively minor damage
and have relatively little or almost no effect on people.
Things start to get more interesting around intensity five
You'd certainly know that you were in an earthquake and if you're the Smithsonian Museum,
your collection of preserved specimens might suffer some damage.
At intensity six, unsecured heavy furniture gets moved around; maybe those bookcases fall over.
At seven, unless you're living in a building that's engineered to deal with earthquakes,
chances are you'll start to see some evidence of damage, often represented by collapsed chimneys.
By category 8, we're seeing substantial damage to many buildings, some of which may partially collapse.
Damage gets even more significant and wide-ranging as we continue to higher levels on the scale
and we see whole buildings and other structures collapsing,
potentially resulting in multiple deaths and injuries.
Because the damage varies with distance from the earthquake and with a variety of local factors,
there will be multiple intensity values for any given earthquake.
Intensity values will be higher and more wide-ranging for larger earthquakes.
For example, the San Francisco earthquake was felt much further afield
than the smaller Loma Prieta earthquake.
Given two earthquakes of similar size, all else being equal, the shallower earthquake
will cause more damage and produce higher intensity values.
Areas underlain by weaker ground materials will experience more shaking,
greater damage and higher intensity.
For example, the Cypress Viaduct in Oakland collapsed on those sections located over soft muds
that serve to exaggerate earthquake shaking during the Loma Prieta earthquake.
Lastly, earthquake-prone regions often have building code regulations in an effort to minimize damage.
Unfortunately, the enforcement of these regulations may not be consistent,
resulting in greater damages and higher intensity values in some regions.
So, to wrap up, we have two different ways to measure earthquakes, each has its benefits
and together they allow us to examine both modern and historical earthquakes and to plan for future events.
We have just one learning objective for today's lesson,
how confident are you that you could successfully respond to the objective?