Explanations of the activity's features are given below. You can access
them using the clickable map for each screen, or simply by scrolling down.
When you first load the activity page, the Action Frame will
contain a list of links, and nothing else. This is the
earthquake list, described below.
When you click on an entry in the list, the
Action Frame will reload (we'll
call it Screen 2 now), and the first four fields of data in the
Data Frame will be filled in with information
about the earthquake you selected. Your next step is to
analyze the seismogram traces that fill
the Action Frame in Screen 2.
This frame also contains a menu bar of options
you can use to switch between screens, pull up this help page, or start
over with a new event.
As with the event number and the
date, the number of traces will not change
until a New Event is chosen.
The four options are: New Event, Check Traces, View Map, and Help. In Screen 1, the only one of these you should need to click is "Help". None of the others will function. They are discussed under the "Menu Bar" entries for Screen 2 and Screen 3.
The "Help" option on the menu bar will, at any time, open
this explanation page (but with a black background) in the
Action Frame. You can use this to
check on the function of a feature without exiting the activity
or opening a new window. To return to Screen 1, just click
the "New Event" option on the menu bar.
Each trace should start out roughly flat at the left end, and then deflect significantly at some point toward the right, becoming a seismic waveform. Elapsed time runs from left to right on these seismograms; the time at which the left side begins is given in the Time field of the Data Frame when Screen 2 is first brought up (by choosing an event from the earthquake list).
The three-letter station name on each seismogram is used for identification of each record; it represents the seismometer used to record that trace. This information will be more useful in Screen 3.
The red arrows on the seismograms allow you to become a seismic data analyst; with them, you are to pick the arrival time, on each trace, of the first P-wave arrival. This process is explained more fully under the P-wave arrival arrow section of this page.
You can bring up another arrow on a seismogram by clicking on the "Pick an S Arrival" button. This feature is described in more detail later.
Some technical information about the seismograms used in this activity:
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BAR DAN DGR EDW GPO GSC HEC ISA OSI PAS |
Barrett Dam Danby Domenigoni Valley Edwards AFB Geothermal Program Office Goldstone Hector Isabella Osito Adit Pasadena |
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PFO PHL PLM PLS RPV RVR SBC SOT SVD VTV |
Pinyon Flat Observatory Parkhill Palomar Pleasants Peak Rancho Palos Verdes Riverside Santa Barbara Solemint Seven Oaks Dam Victorville |
When the seismograms are first loaded (after an earthquake has been chosen from the list in Screen 1), all the P-wave arrival arrows are in a default position on the seismogram. The arrows can be moved, however -- indeed, it is an integral part of this activity that, for every trace, you click on and drag this arrow so that it aligns with the arrival time of the P wave.
How do you find the P-wave arrival? That's actually fairly simple, though some arrivals are more subtle than others. The P wave is the very first seismic wave to reach any given point after an earthquake. Consequently, it produces the first ground motion. So instead of thinking that you're looking for a certain wave arrival, it may help just to think that you're looking for the point in time when the earthquake -- the ground motion that produces the oscillations in a seismogram's trace -- first begins. In other words, look for the first noticeable deflection of the trace from its position of rest; this is the P-wave arrival. The at-rest position for each trace can be found at the far left edge of each seismogram. It is generally a flat, or very nearly flat, line. The record of ground motion should be obvious, in contrast. However, sometimes the first P arrival is weak, so look carefully to determine the exact point at which the trace behavior deviates from its normal at-rest behavior. Then drag the arrow so that the point (and the faint vertical line) aligns squarely with this initial trace deflection.
Depending on your system and browser, you may have trouble dragging the arrows. If you can't seem to get them to drag, try this: instead of clicking on the arrows, try clicking off to either side of them. There are invisible "handles" there that should let you drag the arrows normally. This may take some getting used to. If you find you cannot "let go" of an arrow, try clicking and releasing on the arrow or the invisible "handle." This should "drop" the arrow.
When you have picked the arrival times for the P wave on every trace,
you still have one more arrival to pick before you can locate the
earthquake: an S-wave arrival. How to pick
this arrival is outlined in the entries below.
Once this button has been clicked and the blue arrow appears, the button should then read "Remove the S Arrival." Clicking on this same button again will cause the blue arrow to vanish. The arrow can be brought back and removed as many times as you like. Since there can only be one blue S-wave arrival arrow visible in the Action Frame at any time, you will need to remove an S-wave arrival arrow from one seismogram before you can reveal the arrow on a different one.
If any S-wave arrival arrow is visible (i.e.
this button has been clicked accordingly), the button labelled
"Find Origin Time and Epicenter"
will also be visible at the bottom of the page, below the last trace.
If you remove this S-wave arrival arrow, that
button will no longer be visible, because without an S-wave arrival
time chosen, the computer cannot calculate the origin time of the
earthquake.
To locate the earthquake you've chosen, you will need to pick an accurate S-wave arrival time. This can be very difficult to do, even for those with experience analyzing seismic data. This is the reason only one S arrival can be picked per event; asking you to pick the S-wave arrival on every trace would be asking too much. If you only need to pick one arrival, you can afford to be more selective, and try to pick the most obvious S-wave arrival in the bunch.
Recall that the arrival of the S wave on a waveform is most
obviously noted by a sudden increase in the amplitude
(deflection) of the trace oscillations. There may also be an
associated increase in the wavelength of the oscillations on
the waveform. To choose an S-wave arrival, look for the trace
on which these clues appear most evident, click the
"Pick an S Arrival" button there, and
then drag and place the blue S-wave arrow that appears. It
should drag in the same manner as the red P-wave arrows.
If you have trouble dragging it, remember the tips
we suggested to help.
This button will cause the Action Frame to
reload, bringing up Screen 3 -- a map with
travel-time circles superimposed on it.
In Screen 3, you will have the chance to mark
the location of the earthquake's epicenter on this map.
While Screen 3 is loading, the Time field
in the Data Frame will be updated. Whereas
before it had shown the time at which the left-hand edge of the
seismogram records began, it will now display the
origin time of the earthquake in Universal Time (UTC).
This origin time will be calculated automatically be the computer
based upon the trace on which you chose both P-wave and an S-wave
arrival times. Once calculated, this origin time is used to
compute the travel-time circles you will
see in Screen 3.
Once the initial values are placed in the data fields, only one will change during the course of your work on Screen 2. That value, the time, will change not only in a numerical sense, but in terms of its definition. When Screen 2 first loads, "Time" stands for the time at which the traces in the Action Frame begin -- the value of the time at their left-hand cut-off. This lets you know you're looking at records made across southern California starting at that time.
When you are done picking wave arrivals, and you click on the
button labelled "Find Origin Time and Epicenter,"
however, both the value and the definition of the
Time field will change. The field will
then give you the calculated origin time of the earthquake,
which may actually be earlier than the start time of the
seismograms. Once an initial origin time is calculated, it
doesn't matter how many times you switch back and forth between
Screens 2 and 3; the Time field will represent
origin time from then on... until you clear all the fields
by choosing a New Event with the menu bar.
The event number will not change while you are working with or
when you leave Screen 2, unless you choose a
New Event.
The date will not change while you are working with or when you leave
The number of traces will not change while you are working
with or when you leave Screen 2, unless you choose a
New Event.
When Screen 2 first loads, "Time" stands for the time at which the traces in the Action Frame begin -- the value of the time at their left-hand cut-off. This lets you know you're looking at records made by seismometers across southern California, starting at that arbitrarily chosen time (the instruments run continuously, but you can choose when to sample the data they gather).
Once you are satisfied with the wave-arrival picks you have made, and
click on the "Find Origin Time and Epicenter" button,
the computer will calculate an origin time for the earthquake, and
place this value into the Time field in the Data Frame.
From that point on, until a New Event is
chosen, the Time field will contain the most recently calculated
value for the earthquake's origin time.
One new option is functional once Screen 2 has first been loaded: "New Event." Though it is unlikely you will want to immediately clear the data from the earthquake you just selected from the list in Screen 1, you may do just that by clicking "New Event" on the menu bar. This will erase all the data fields and bring Screen 1 back into the Action Frame, so that you may choose a new earthquake from the list there.
If you have come back to Screen 2 from Screen 3, the "View Map" option is also a valid choice. This will take you to a background map of southern California overlain with travel-time circles with which you can assess the location of the earthquake's epicenter. This can only be done once the computer has calculated these circles, however, so until you have clicked on "Find Origin Time and Epicenter" at least once, "View Map" is not an option.
Ideally, all the circles on the map should intersect at a single point, if you chose the wave-arrival times correctly. In reality, you can expect a single-point intersection for three circles; if there are more than three circles on the map, they aren't likely to ever intersect in exactly one point, no matter how well you do in picking the P-wave and S-wave arrivals on the seismograms.
If the circles on the map don't intersect very closely,
or don't intersect at all, you should probably go back and
check the traces in Screen 2.
Revise your picks if necessary, and then click the
"Find Origin Time and Epicenter" button
to have the computer calculate new travel-time circles (as
well as a new origin time.
If you would like to reconsider the location you selected, just click on the "Unlock Epicenter" button to release the crosshairs, so that you can drag them again. This will also erase the entry in the Epicenter field of the Data Frame.
Should you decide to review your arrival picks in Screen 2
by using the "Check Traces" option on the menu bar,
the old epicentral location will be marked on the
background map when you return to Screen 3,
but it will be marked with a set of "shadow" crosshairs.
They will appear yellowish and cannot be dragged, as can the
real crosshairs. This "shadow" crosshairs is put there to
remind you of exactly where you last located the epicenter,
before the arrival time revisions. Locking the real crosshairs
will cause the "shadow" set to disappear; a new
epicentral location
will appear in the Data Frame.
Should you exit Screen 3 with the crosshairs locked,
a set of "shadow" crosshairs will appear on the
map when you return. If this button is
in unlocked mode (i.e. it reads ""Lock Epicenter"), however,
no such record of the previous location of the crosshairs
will be kept. Clicking this button to lock the active set of
crosshairs will make the "shadow" set
disappear.
You can clear this field by unlocking the crosshairs with
the "Unlock Epicenter" button. If you
switch back to the Screen 2 to "Check Frames"
without the epicenter locked, however, the
"shadow" crosshairs will not exist on
the map when you return.
"Help" still functions as it did in Screen 1 and Screen 2. To return to Screen 3 from the Help page, click the "View Map" option on the menu bar.
"Check Traces" allows you to go back to Screen 2 to reassess, and if necessary revise, your wave-arrival picks. If you have locked the epicenter before you leave Screen 3 to check the traces, that location will remain in the Epicenter field of the Data Frame, and a set of "shadow" crosshairs will show up on the background map when you return to Screen 3. Otherwise, no data about the epicenter will be retained when you go back to Screen 2 to "Check Traces."
If you are satisfied with the location of the epicenter as you have it marked and locked on the map, you should write down the time and location you found for that earthquake, and, if you'd like to locate another earthquake, click on the "New Event" option on the menu bar. This will erase all the fields in the Data Frame, and bring up Screen 1, with the earthquake list in the Action Frame. You can then start the locating process all over again with a new data set.
If instead you've located your last earthquake for the time being,
simply click on the Quit button to go back
and answer the review questions at the end of the
Activity #3 introduction.