EXPLANATION for the interactive "Richter Scale" nomograph

This page should help you familiarize yourself with the purpose and operation of the interactive, online section of this activity. This online Richter-magnitude calculator uses frames and layers to allow the user to make determinations of the Richter magnitudes of southern California earthquakes from seismograms. If your browser supports neither frames nor layers, you will not be able to use this calculator. (There is an alternate page with a printable seismogram and nomograph available.)

When you start the activity, you will see four frames created in your browser window, though two of these function essentially as one. The image below shows you the main features of each frame, discussed in detail in the text that follows:


Title Frame

This frame contains the clickable earthquake list, explained below. Also, clicking on the title will open this help page in a new window, should you wish to review the features of the activity.

Earthquake List

All the earthquakes for which seismograms are available are listed in the Title Frame. You must click on one of these to start the activity. Each earthquake has its own event number. Also given here is the date each earthquake occurred, and the vertical scale used on the seismogram (shown in the Action Frame below). All the seismograms used have the same horizontal (time) scale: 10 pixels per second.

When you click on an entry in the earthquake list, the appropriate seismogram is loaded into the Action Frame, an the event data is loaded into the Status Frame.

Status Frame

To the right of the Title Frame is the Status Frame. This frame contains the event data and a Quit button. These are explained below.

Quit Button

At any point during the activity, you may click the button labelled "Quit!" to exit the online portion of this activity and return to the introduction page.

Event Data

There are three fields in the Status Frame: event number, date, and Richter magnitude. These fields will be blank when you first load up the page. Clicking on an entry in the earthquake list will place the information for the event number and date in the appropriate fields. This lets you know with which event you are working. The Richter magnitude field will only be filled in when you have locked the three arrows in the Action Frame and subsequently clicked on the "Compute Magnitude" button. The entries in the Status Frame will be wiped clean when you click the "Reset Window" button.

Action Frame

As its name implies, the Action Frame is where most of the actions in this activity take place. This frame is actually composed of two frames, but the two look and function essentially as one. The main feature of the upper sub-frame is the seismogram; the lower frame is dominated by a nomograph. These and the other features of the Action Frame are explained below.

Seismogram

This white rectangle starts out blank, with the message "Choose an earthquake from the list above," referring to the earthquake list in the Title Frame. Once you do select an earthquake, the seismogram representing that seismic event appears in the rectangle, as do three arrows: a red one labelled "P", a blue one labelled "S", and a green one labelled "A". (In addition, two buttons labelled "Lock Arrows" and "Reset Window" appear to the left of the seismogram.)

The three arrows are crucial to the operation of this activity. They are draggable, and with them you will select the P-wave arrival, the S-wave arrival, and the maximum amplitude on the seismogram. These selections give the computer the data it needs to calculate the Richter magnitude for you. The placement of the arrows is discussed below.

P, S & Amplitude Arrows

Three arrows -- a red one labelled "P", a blue one labelled "S", and a green one labelled "A" -- are superimposed atop the seismogram. The red and blue arrows run vertically, and are to be used to pick the arrivals of the P wave and the S wave on the seismogram. The green arrow runs horizontally, and should be used to select the point of maximum trace deflection -- the amplitude -- on the seismogram.

If you've read through the text of Section 3, you've seen how to recognize the P and S arrivals on a seismogram. Remember that the arrival of the P wave is marked by the first deflection, reading from left to right (the direction of elapsing time), away from the "resting" position of the seismogram's trace. The arrival of the S wave is generally marked by an obvious increase in the amplitude and/or the wavelength of the oscillations within the waveform. For this activity you will need to mark these arrivals with the appropriate arrows by dragging those arrows to the correct spots on the seismogram.

Choosing the maximum amplitude should be comparatively easy. First, locate the "level" of the trace when it is at rest (on the far left side of the seismogram, before the P-wave arrival). Then find the part of the waveform with the greatest trace deflection -- the most distance between the peak or crest of an oscillation and that initial "quiet" level of the trace.

This job is made easier by the green amplitude marker (labelled "AMP") on the right-hand scale of the nomograph below. This marker will move as you move the amplitude arrow over the seismogram. This marker goes to zero when you align the arrow with the initial level of the trace at rest. The amplitude reading is always positive; it doesn't matter if the arrow lies above or below the initial trace level. Note, however, that while the pixel-per-millimeter scale on every seismogram is linear, the right-hand scale on the nomograph is not; it is logarithmic. Thus, the amount of motion the amplitude marker experiences will not mirror the amount of motion through which you drag the amplitude arrow.

A similar marker will move along the left-hand scale of the nomograph as you adjust the positions of the P and S arrows.

Lock/Unlock Arrows Button

Two buttons will appear to the left of the seismogram when an earthquake is chosen from the earthquake list. The upper button is red and labelled "Lock Arrows" when it first appears. Click this button when you are satisfied that you have correctly identified the P arrival, the S arrival, and the amplitude using the three draggable arrows superimposed upon the seismogram. When you do, the arrows will cease to be draggable. Labels will appear atop the seismogram identifying the S-P distance in seconds and the amplitude in millimeters. (These values should be reflected by the positions of the S-P and amplitude markers on the nomograph below.)

The buttons to the left of the seismogram will also change -- the "Lock Arrows" button will turn green and read "Unlock Arrows," and the "Reset Window" button will turn gold and read "Compute Magnitude".

Clicking on "Unlock Arrows" at this point will make the labels on the seismogram disappear, and allow you to drag the arrows once again. The two buttons to the left of the seismogram will be restored to the way they looked before "Lock Arrows" was clicked. The section below will explain what would happen if you were to click "Compute Magnitude" instead.

Reset Window/Compute Magnitude/New Event Button

The lower of the two buttons that appear to the left of the seismogram is blue when it first appears, and labelled "Reset Window." Clicking "Reset Window" at any point while it is visible will clear the current entry from memory, wipe the seismogram clean, and cause the two buttons to its left to vanish. Essentially, it will make the Action Frame appear as though the activity had just been loaded.

This button changes, however, when the Lock Arrows button is clicked. "Reset Window" is then replaced with the gold-colored "Compute Magnitude" button. Click on this button when you are ready to have the computer draw a line across the nomograph to connect the S-P marker with the amplitude marker. Doing this will also cause the Richter magnitude to be displayed numerically atop the nomograph, and fill in the "Richter magnitude" field of the event data in the Status Frame. It will also replace this button with a purple button labelled "New Event". Similar to the "Reset Window" button in function, clicking on "New Event" will clear the frames of data from the previous event, and ask you to choose a new earthquake from the list in the Title Frame.

Nomograph

Below the seismogram is the interactive nomograph. A nomograph is a sort of graphical calculator used to find a third value based upon two known quantities. Nomographs generally consist of three scale bars; the two outer scales represent the known quantities and the center scale is used to read off the desired third value. In our case, the outer scales are represented by the S-P time (proportional to the distance from the seismometer to the source) on the left, and the amplitude on the right. The center scale is a linear scale off of which we will read the Richter magnitude.

Sliding along each of the outer scales is a marker. Each of these markers, covered below, moves in response to your manipulation of the P, S & amplitude arrows on the seismogram above.

When the "Compute Magnitude" button is clicked, the computer will draw a line connecting the two markers. The point where this line crosses the center scale will be marked, and a Richter magnitude read off from this intersection.

S-P Marker

The left-hand scale of the nomograph represents the difference in time between the P-wave and S-wave arrivals, abbreviated as "S-P", in seconds. A scale showing the equivalent distance in kilometers is given on the same bar. Note that the values on this scale bar do not represent a simple linear function. Instead, the scale represents the original graph determined empirically by Charles Richter from data acquired on earthquakes in southern California during January 1932.

The sliding marker on this scale is purple in color, to represent a combination of the red P arrow and the blue S arrow on the seismogram. The difference between the two arrows is calculated as an absolute value, so the arrows can be switched, and this scale will still yield the correct value for their difference in time. Note how this sliding marker moves as you move either the P or S arrow.

This marker will be one of the two endpoints of the line drawn to find the Richter magnitude of the earthquake you choose from the list in the Title Frame.

Amplitude Marker

The green amplitude marker will slide up and down the right-hand scale of the nomograph as you move the amplitude arrow over the seismogram. The amplitude scale is logarithmic, and the amount of motion along it will vary from seismogram to seismogram, as the different seismograms are displayed with differing (linear) vertical scales. Check the earthquake list in the Title Frame for these scales.

This marker will always move to "zero" (actually, its lowest possible position, since there is no zero on the logarithmic scale as shown) when the amplitude arrow matches the level of the trace at rest. Keep in mind that amplitude cannot be negative, regardless of whether the amplitude arrow falls above or below the center line.

The amplitude marker will anchor the right end of the line drawn by the computer across the nomograph in its calculation of the Richter magnitude.

If you've read through the instructions above, you should have a good idea of how to work this interactive "Richter Scale" nomograph. Click here to load this online activity now!

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