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Look for recent earthquakes
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Northern California: : Map and list
Nevada: : Map and list
Utah: : Map and list
Alaska: : Map and list
Pacific Northwest: : Map and list
Hawaii: : Map and list World: : Map and list
Earthquakes release different amounts of enery at the start. Some are like a small pebble falling into a pond while others are more like ...
Significant earthquakes
Earthquakes release different amounts of enery at the start. Some are like a small pebble falling into a pond while others are more like ...
Choice of P-wave and S-wave velocities
We assumed constant values for P- and S-waves, and that only P- and S-wave appear on a seismogram, so that students in middle school would have the possibility of understand the entire line of logic that allows S-P lag times to deduce the location of epicenters.
This is obviously an oversimplification because:
1. There is illuminating variability in the speed of these waves through different parts of the crust, although that variability is less than the variability through the entire thickness of the Earth's interior.
2. Depending on the distance, the first arrivals of some earthquakes travel through layers of the Earth below the crust where they are moving at different velocities.
3. Surface waves can also appear on a seismogram. We assume there are no surface waves on our seismograms.
4. Actual seismograms can be complex and require special expertise to analyze.
How we respond to questions that arise:
1. Yes, there can be errors if we approximate the velocity and that approximation isn't the actual velocity. However, that error is limited because we know the variation of our approximation with the actual numbers is limited. For example, we know that cars travel at about 65 miles an hour on a freeway when traffic is moving at full speed. Using that assumption of velocity, we can anticipate arrivel times and deduce distances traveled with some degree of accuracy. In this case, using an approximation, anyone can spend a few days learning the principles of the method and come up with a decent result. Getting that last bit of accuracy is important when we need to know the exact locations of epicenters when we want to avoid building a hospital or your home on an earthquake-prone fault, but that last bit of accuracy isn't a good investment of time for most people. That last bit of accuracy that we really need requires the skill and knowledge of someone who is going to be a seismologist and it is going to cost years of college study to acquire. Yhat makes a seismologist valuable.
2. If our seismometer is on the other side of the world instead of close to an epicenter, the earthquake waves will have traveled at faster speeds through parts of the Earth below the crust. We didn't take that into account and it will make a differnce if it happened.
3. Since surface waves are not used in this method, we decided against using seismograms that included these waves. It makes the data too complex to use as a beginning example.
Numbers that we used and why we selected those values:
Velocity of the P-wave: 6.5 km/s
Velocity of the S-wave: 3.5 km/s
Dziewonski and Anderson, 1981 give preliminary numbers that represent typical values ...
1. Dziewonski and Anderson, 1981
Preliminary Earth Reference Model
Physics of the Earth and Planetary Interiors, 25 (1981) 297� 356 297
http://eps.mcgill.ca/~courses/c320/PREM_paper-DziewonskiandAnderson.pdf
http://geophysics.ou.edu/solid_earth/prem.html
2. Thus you will see some variation between activities if you also use ...
We can visualize ... and see pattern. We also have the ability to detect pattern in sounnd.
Why and how constant speeds were assumed for P and S waves in this activity.
We can visualize ... and see pattern. We also have the ability to detect pattern in sounnd.
Why surface waves were not considered in this activity
We can visualize ... and see pattern. We also have the ability to detect pattern in sounnd.
Why we assume the source and epicenter of an earthquake synonomous for this activity.
We can visualize ... and see pattern. We also have the ability to detect pattern in sounnd.
How the 14 earthquake epicenters were selected for this activity
We can visualize ... and see pattern. We also have the ability to detect pattern in sounnd.
How the seismograms in this activity were snthesized
We can visualize ... and see pattern. We also have the ability to detect pattern in sounnd.
How to substitute locally significant earthquakes to produce a customized activity
We can visualize ... and see pattern. We also have the ability to detect pattern in sounnd.