Earth Science
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Sep16_Earthquakes_seismology1.pptEarthquakes
Earthquake Definition and Terms
- An earthquake is a trembling of the Earth caused by a sudden release of energy stored in subsurface rock units - plate don’t move smoothly, they can “lurch suddenly”
- Earthquake activity is also referred to as seismic activity (not necessarily the same as volcanic activity; both occur at convergent and divergent plate boundaries, but volcanoes don’t occur at transform boundaries there are no volcanoes in California!).
- The energy is generally released along fault lines (cracks in the earth) as sections of ground move.
- Movement of plates is a key cause of earthquakes- plates don’t always move smoothly!
Where do earth quakes occur
Mostly (80%) around the Pacific Ocean on convergent plate boundaries +
transform boundary (San Andreas)
Interior of plates (Virginia) and divergent boundary are about 5%
Faults
Faults are cracks in the earth that may not be part of a plate
boundary. There are 3 types; we will study them in a couple of weeks.
but this is what caused the Virginia earthquake of 2011- it wasn’t a
plate boundary.
As it happens, the San Andreas Fault in California is part of a plate
boundary.
This orchard in CA originally had all the trees in perfect rows
Earthquake Terms
- The point of energy release is called the focus. The point on the Earth directly above the focus is called the epicenter.
- Surface waves move along the surface of the Earth while body waves penetrate the Earth’s interior. Surface waves cause most of the structural damage. Body waves arrive first- can give you a warning and tell you about where the earthquake is. We will focus on body waves
Waves
- The types and sizes of waves generated by an energy release can provide information about the location of the quake and the magnitude.
- Earthquakes are recorded by an instrument known as a seismograph.
Types of Waves- see video of slinkys
- Primary or P-waves – cause compression and dilation (expansion) along axis of propagation
https://www.youtube.com/watch?v=BxtiKodKq_E
Ways to remember P waves
Pulsed
ComPression
Primary (Spanish: Primero)- first to arrive
Types of Waves
- Shear or S-waves – cause vertical motion across the axis of propagation
Ways to remember S waves
Side to Side
Shear wave
Slower
Secondary
Through Solids only
Stronger (they cause more destruction)
Remember the Layers of the Earth?
This is how we know
Prince George’s Community College PSC 1210 Barbara A. Gage
If earthquake
is here
And seismograph
Is here
Outer
S waves don’t travel through liquid- blocked by the Outer Core
S: Solids only
P wave only!
Seismographs
- The time between the arrival of the P wave and S-wave is the S-P time interval. It is related to the distance of the detecting device from the epicenter. Note how the S waves are larger than the P waves key to earthquake warning systems!
Time along this axis
first
2nd
Effect of distance on S and P arrivals
The further you are, the more the P wave outraces the S wave.
Here (green line) P wave arrives at 6 seconds, S at 16 seconds S-P interval = 10,
distance is 60 km
But at 100 km, the P wave would come at 13 seconds and S wouldn’t show up
until 28 seconds. Now S-P interval =15 seconds.
Sinex, 2010
S-P Interval
If you know the S-P interval you can determine the distance from the recording station to the epicenter.
For your homework, a formula is provided
To locate an epicenter- we need several distance measurements and we find where the circles intersect
Triangulation (not part of assign #3)
Magnitude
- To determine the magnitude of an earthquake you need the distance to the epicenter and the strength (amplitude) of the S-wave from the seismogram.
Note: some times the S-wave zig zag is not the same for positive and negative.
To get the amplitude, pick the biggest value.
Practice
What is the S-P interval?
What is the S wave amplitude?
Practice
What is the S-P interval? Clock starts when P wave hits. S waves arrive
at 48 seconds. S-P interval = 49
What is the S wave amplitude? About 280 or 285 mm
P
S
Magnitude/Nomogram
For any seismic station, the distance to the epicenter and the amplitude of the S-wave will estimate the magnitude.
Using the nomogram shown here, draw a line connecting the distance and amplitude for a the station. Where the line crosses the magnitude scale is the approximate magnitude.
For a distance of 220 km and an amplitude of 50 mm, what magnitude do you get?
Magnitude/Nomogram
For any seismic station, the distance to the epicenter and the amplitude of the S-wave will estimate the magnitude.
Using the nomogram shown here, draw a line connecting the distance and amplitude for a the station. Where the line crosses the magnitude scale is the approximate magnitude.
For a distance of 220 miles and an amplitude of 50 mm, the magnitude is 5.3.
Our example
From slide 19
Feeling an earthquake
Intrinsic strength and distance both matter. Do you understand why someone
Might feel earthquake A (5.5 richter) to be much stronger than B (6.0 richter)?
A
B
Magnitude and the Richter Scale
More description of Richter Scale
(remember the effects depend upon how far away you are)
