Section 2: The Distribution of Earthquakes

Activity #5: SEISMICITY RATES

Materials:

• Recent Earthquakes in California maps
  
• Monthly Seismicity animations
  
• Annual Seismicity animations
  
• Animation of M 4.5 and greater earthquakes in southern California, 1932 - 1997
  
• SCEC Data Center catalog search
  
• a map of your area with latitude and longitude markers
  
• paper for recording notes
  
• paper and/or a calculator for making calculations
  

Procedure:

This activity consists of two exercises, outlined below. Each has its own set of instructions and review questions. Work through each as directed within the exercise itself.

The first exercise will acquaint you with the concept of a seismicity rate, and more specifically, with the rates of seismicity you will typically find in southern California. You'll be asked to watch several animations of seismicity, and answer questions regarding what you saw. You'll also take a look at graphs of earthquake activity and a map of the latest earthquakes to have occurred in southern California.

In the second exercise, you'll get to use the SCEC Data Center earthquake catalog search to define a seismicity rate for an area and a time of your choosing.

As each exercise is self-explanatory, you may begin when ready.

Exercise 1 A Seismicity-Rate Tour of Southern California

In this exercise, you will take a tour of several online resources that the SCEC Data Center provides, each of which can teach you something about seismicity rates is southern California. At each "stop" on this tour, you should follow the brief directions, observe carefully, and then use your browser's "Back" button to return to this page and work through the few questions that conclude the "stop". By the time your tour is done, you should have a basic knowledge about how to define a seismicity rate, and about the kinds of seismicity rates you can expect to see in southern California.

Stop 1

The first stop on our tour will be the Recent Earthquakes in California index map. This map shows epicenters (as colored boxes) of all the earthquakes that have been recorded across the state of California in the past week. The magnitude and color scales used are shown on the right-hand side of the map.

1. Were there any earthquakes on the map of California that had occurred in just the past hour? If so, how many?

2. Looking at the entire state of California, does there seem to be more frequent seismicity in the northern or the southern half of the state?

3. Access the list of all earthquakes visible on these maps. Make a rough count of the number of earthquakes on this list. Divide this by 7 (for the days in a week) and then by 2 (to estimate the rate for only the southern half of the state). About how many earthquakes per day would you expect southern California to experience, according to these numbers? Are you surprised?

4. Go now to the list of earthquakes of magnitude 3.0 or greater. How many earthquakes are listed here? These are the ones that may be large enough to be felt, especially if they occur near densely populated areas. In other words, these are the ones most people consider to be "real" earthquakes. If the total number of earthquakes per day surprised you by being much higher than you expected, that may be why -- tiny earthquakes draw little attention, though they far outnumber the larger, more noticeable ones. There is actually a scientific term for earthquakes smaller than magnitude 3.0: microseismicity.
   

Stop 2

Moving on, we come to the second stop on our tour, the Monthly Seismicity animations. Though the animations span the period from last month all the way back to January 1996, it is only after January 1998 that we will focus, because those months feature a graph of events per day and a frame-by-frame counter in addition to the day-per-frame animation and the still-image overview map. It will probably be most convenient for you to view the animations using the Monthly Seismicity Viewer, which will allow you to play the animation at a variety of speeds, pause, back up, and even view the map or graph of the month without switching pages!

You can begin by choosing any month from January 1998 to the present, excluding March 1998 (we're saving it for the finale). Watch the animation, check out the still-image overview map, and study the graph of events per day, in whatever order you wish. (Note that the color of epicenters in these animations refers to magnitude, not "age".)

1. What month did you choose? What was its average daily seismicity rate? How does this compare to the rough estimate you calculated using the Recent Earthquakes list?

   The average daily seismicity rate you got from the graph of events per day was much more accurate than the estimate you made from the list. Indeed, it was constrained well not only in time, but in geographic area.

2. What were the geographic boundaries of the seismicity rate you got from the graph? And how, exactly, were the intervals (days) divided?

3. Looking at the graph, did the number of events per day change much during the course of the month? Browse the monthly summary paragraphs, noting the total number of earthquakes per month. How does this value change over time? Are the totals in 1996 generally higher than, lower than, or the same as those in 1998?

4. OK, at last go to March 1998. Watch the animation first, and try and keep an eye on the counter. Did you see what happened, starting on the 5th of the month? Now consult the graph. What does it show? Is this what you noticed during the animation?
   

Notice that the vertical scale of the graph had to be revised to accomodate the numbers of earthquakes that occurred per day in this month. Most of those earthquakes were very small, and followed in the wake of two magnitude 5 earthquakes -- one on the 5th, and one on the 6th. This sudden cluster of earthquakes following a moderate-sized earthquake in the same immediate area is known as an aftershock sequence. We will address this concept in more detail later. For now, just note how much of an impact an aftershock sequence can have on a local seismicity rate, and how it causes a very sudden increase in the seismicity rate, followed by a much more gradual decrease.

5. Watch the animation again, and consider this: if you calculated the daily seismicity rate for everything on this map except for the area around the pair of magnitude 5 earthquakes, would it change much over the course of the month?
   

Stop 3

Our third stop will be the Annual Seismicity animations. These are similar to the Monthly Seismicity animations, but somewhat more primitive. On the other hand, they provide a view of southern California seismicity that is more long-term than a single month can provide.

While you are welcome to browse the archives for as long as you like, make sure you start by watching the 1992 animation. Each frame of this animation represents a whole week of seismicity in southern California. When you've finished at least one viewing of the animation of seismicity from 1992 (you may watch it several times, if you like, and time permits), come back to answer the questions below.

1. How many earthquakes greater than magnitude 6 (highlighted and named) were there in the animation? Were they all quite separate, or seemingly connected?

2. How many earthquakes larger than magnitude 6 did you see noted while you were browsing the monthly summary paragraphs for 1996 through 1999? What does this imply about the rate of large earthquakes in 1992, relative to the average rate?

3. The magnitude 7.3 Landers earthquake on June 28th, set off an enormous aftershock sequence. Does it look like the sequence was over by year's end? Why or why not?

4. Unfortunately, there's no counter on this animation, but would you expect that the average seismicity rate in southern California over the latter half of 1992 was higher than, lower than, or about the same as the average rate you saw in the monthly animation you chose on your own? How do you think it would compare to the rate for March 1998?
   

The aftershock sequence of the Landers earthquake was truly an amazing event. In fact, it generated over 20,000 aftershocks in just the first six months after it occurred; roughly 10,000 of those struck in the 30 days immediately following the Landers earthquake!

Stop 4

The last stop on the tour is one more animation, but this particular one covers an extremely long time, relatively speaking. You'll be watching the Animation of magnitude 4.5 and greater earthquakes, 1932 - 1997. This animation plays at a rate of six months per frame, and in essence, it spans the entire history of instrumentally recorded seismicity in southern California. Because that data set would be too huge to show in a single animation, this one is limited to only those earthquakes above magnitude 4.5, just at the threshold of being large enough to cause damage. But this lower limit on magnitude, eliminating all but the earthquakes with which most people are concerned, will allow us to see just how common such earthquakes have been in the past decades, and this may give us an idea of what to expect in years to come.

Watch the animation now, and then come back here to finish off the tour with some points to ponder.

1. Are earthquakes capable of causing damage (magnitude greater than 4.5) very common? About how many of them per year, on average, would you expect to strike southern California?

2. Did you see clusters of earthquakes in this animation? Are these likely the same clusters we've been noting in other animations?

3. Judging by the relative sizes of the largest clusters of earthquakes you can see in this animation, in what years did the two largest earthquakes in southern California (since instrumental records have been kept) take place?

4. Were the clusters of earthquakes the only influence upon the seismicity rate, or did the sporadic earthquakes vary in rate noticeably? Which group seems to exert a greater influence upon the overall seismicity rate, the sporadic earthquakes, or the large clusters?
   

Exercise 2 Defining Your Own Seismicity Rate

In this exercise, you will learn how to use the Southern California Earthquake Center (SCEC) Data Center's catalog search to retrieve information about earthquake times and locations, with the ultimate goal of finding a seismicity rate for a place and time of your choosing (within the limits of southern California).

Since you already know the basics of what is required to define a seismicity rate, go straight to the catalog search of the SCEC Data Center Earthquake Hypocenter and Phase Database -- either by opening a new browser window, or to get a first impression of what's there, before coming back to this page to read the specific directions below.

How to Use the SCEC Data Center Catalog Search

The catalog search will allow you to obtain a list of all the earthquakes in the SCEC Data Center Database that meet certain parameters you can define. Those parameters include:
• Catalog and Format: For this exercise, we will use only the default values for these parameters.


• Start and End Dates: To narrow your search to a range between two specific dates. The database extends back only as far as August 1, 1983, and usually is complete up to as recent as five days before today's date. The date format is: YYYYMMDDhhmmss, where YYYY=year (all four digits), MM=month, DD=day, hh=hour, mm=minute, and ss=second. You should only bother with the first eight digits (leaving the last six as zero) unless you're doing a very specific search.


• Maximum and Minimum Magnitude: To select only earthquakes within a specific "size" range. A maximum magnitude of 9.0 will ensure no upper "cut-off". A minimum of 0.0 is as low as you will need to go.


• Maximum and Minimum Depth (km): To select only earthquakes within a defined range of depths, in kilometers. Generally, there's no need to change these defaults.


• Southern and Northern Latitude: You will only be able to search within a rectangular area -- these let you define its top (north) and bottom (south) edges. You may wish to consult a map to define these more precisely.


• East and West Longitude: As above, but for the left (west) and right (east) edges of the rectangle.


• Type of Event: L, the default value, stands for "local". In this activity, you should always use this default value.



When the parameters are set the way you want them, click the button labelled "Submit request" to send your search request to the database. Then click the button marked "Continue submission" in the pop-up alert window that should appear when you make your request. Depending on the speed of your connection and the breadth of your search, a list of hypocenters matching your specified parameters will appear within several seconds. For the most lengthy searches, it may take a minute or more.

There is a maximum output length cut-off of 10,000 lines. Should you exceed this, or even come anywhere close to it, it is recommended that you narrow your search to make your list of output more manageable.

To get acquainted with the catalog search, try a very basic sample search to start with: leave all the parameters at their default value (use the "Reset" button if you need to), except for "Minimum magnitude". Change this to "5.0", and submit the request. The list you'll see represents potentially damaging earthquakes that have occurred in southern California since August 1983.

Now that you've seen how the search works, it's time to use it toward the goal of this exercise -- calculating a custom seismicity rate!

Second, you must decide on a time period, and the size of the intervals within that time period. To make things easier on you, don't choose a period longer than a year unless the area you're covering is very small, and don't make your intervals too long or too small. You don't need to stick to earthquakes per day, but earthquakes per minute is a little extreme, with the possible exception of during a major aftershock sequence! You may need to experiment to find a good balance. Also, don't cover fewer than 20 intervals, or more than about 50 -- go easy on yourself!

Third, decide on a magnitude range. In most cases, 0.0 to 9.0 should work just fine, but you may wish to be more selective.

Fourth, decide on whether you want to limit the depth range of your search. Most earthquakes in southern California happen between 3 and 15 km depth. Using the default settings here is probably easiest.

Once you have in mind the parameters you want for your search, go to the catalog search page and enter these values. The trickiest part will be deciding whether to break up your search into several smaller searches. This will depend entirely upon how many earthquakes happened in your search area over the time period you specify. If you have an average rate of 2 earthquakes per interval, then you could easily perform just one search, and make all your counts from there. If it's more like 100 per interval, you may want to check only a single interval (or two) per search.

Whatever you choose, make a table on a piece of paper to tally the number of earthquakes in each interval of your study. In addition to making counts for each interval, you'll also need to find the mean seismicity rate over the entire period you define.

When you have completed your seismicity rate study, work through the questions below.

1. 
   
   1. What area did you choose to cover for your study?
   
   
   2. What time period (date and year) does it span?
   
   
   3. What interval units did you choose? (i.e. Did you note earthquakes per 3 hours, per day, per 2 weeks,...?)
   
   
   4. What magnitude range did you choose?
   
   
   5. If you limited your search to earthquakes of a certain depth, what was that depth range?
   
   

2. 
   
   1. What average seismicity rate did you find for the area and time you selected to study?
   
   
   2. How do the minimum and maximum counts per interval compare to this average value?
   
   

3. Express your average seismicity rate value in terms of earthquakes per day, or if you chose to count earthquakes per day, convert to a different unit: earthquakes per hour, if your average count was more than 20 per day, or earthquakes per week if it was less than that.

4. Did your rate vary much from interval to interval? If so, were there any systematic variations in your rate, or was the change from one interval to the next fairly random? (Compare with others, if possible.)

5. If others nearby are working on the same activity, compare the average rates you each came up with. You will need to convert one to match the other to do this. For example, if one study covers a 1° x 2° box, and the other a box that is only 1° x 1°, adjust for the relative sizes of the areas covered by dividing the first rate in half, or by multiplying the second rate by 2. Likewise, make sure your units match (i.e. convert days to hours, etc.). Once you have done this:
   1. Match each rate against the other.
      
   2. Which area (if the two rates cover the same time period), or which time period (if the two cover the same area), is characterized by a higher seismicity rate? If the two rates have neither the area nor the time period in common, is this comparison at all useful?
      

6. [Optional!] If a person owned a square mile of land in the region you selected during the time period covered by your seismicity rate study, approximately how long could he or she expect to wait before an earthquake within your specified parameters occurred beneath his or her land? (Assume 4000 square miles per 1° x 1° rectangle.)
   

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