ArcView Project 2
How long do aftershocks persist?
On December 26, 2004 the largest earthquake since 1964 struck the northwest coast of Sumatra. About 250,000 died, mostly killed by the tsunami that swept the Indian Ocean.
On February 27, 2010, a magnitude 8.8 earthquake struck west-central Chile.
On March 11, 2011, a magnitude 9.0 earthquake struck just off the coast of Japan.
Today you will use ArcView (along with other applications) to (1) investigate if aftershocks from a big earthquake have decreased in frequency, returning that segment of fault to normal background seismicity, and, if not, (2) extrapolate the decreasing aftershock frequency into the future and estimate when aftershocks will pretty much be over. I will demonstrate these steps beginning at 1 PM sharp!
Today you will learn how to:
1. Add data to an ArcView project, data derived from another
source.
2. Use a graphic to select data based on spatial
relationships.
3. Use theme-based queries to select data based on a spatial
relationship.
Step 1: Open the earthquake table you worked with several weeks ago in Access.
Step 1-A: There have been additional aftershocks since 3/27/2011, the last date on your database. This link opens a text file you can append to your Earthquakes database, bringing the catalog up to 4/18/2011.
Step 1-B: Export the table as text file, with field headers, to a folder on your flash drive. The computer may take a few minutes to accomplish this task. Once the table has been exported close Access and open ArcView with the world map used last week. Note: Make certain to keep the Field Labels! Watch carefully as I navigate the dialog boxes in Access.
Step 2: Add the table you just exported from Access to your Arcview project. This is done using the same menu you need to open a layout. Click tables - add table - the specify text file as format.
Step 3: Back to your view, in the View menu, add event theme. Select your table and specify which fields represent the (X,Y) coordinates (Longitude, Latitude).
Step 4: You should now have on your map epicenters of many earthquakes. Activate this theme and, under the theme menu Convert to Shapefile. Save the shapefile to your flash drive under a descriptive name, and add it to your view. You may now select and delete the 'event theme' (Edit menu) from your project. Once you have added the new shapefile and deleted the event theme, this would be a good time to save your project.
Assignment 1: classify by size. Magnitude 4 earthquakes are really insignificant compared to magnitude 7 (each magnitude unit represents a 10-fold difference in ground motion - in other words, a M = 5.0 makes a wiggle on the seismograph 10 times the amplitude of a M = 4.0 in the same location - a 6.0 makes a wiggle 100 times bigger than a 4.0). Select magnitude and classify by equal intervals 4.0 - 4.4, 4.5 - 4.9, 5.0 - 5.4 (etc.) with really tiny dots (2 pt?) for the low end and really big circles (24 point?) for magnitude 9 and up.
- OR -
Query the earthquake catalog for all earthquakes greater than magnitude 5.9. Convert the selected set to shapefile and add to the view. Then classify that set of earthquakes by size, as in the previous paragraph, with the M = 6 to 6.4 as really small (etc.).
Set up a layout that shows global distribution of earthquakes, with size of dot reflecting magnitude of earthquake. Export the image and send it to me. Do not forget to include a caption telling me what I am looking at.
Assignment 2: classify by depth, again with equal (100 km) intervals, but this time use colors to display earthquake depth patterns. Note that you cannot classify by both depth and color, or by 2 different fields simultaneously.
- OR -
Query the earthquake catalog for earthquakes from 100 to 200 km, 200 to 300 km, 300 to 400 km (etc.), convert each set to a new shapefile and add to the view. Now you can classify each depth range by size (big circles for big earthquakes) and set each depth range shapefile as a different color.
Set up a layout, export as an image and send it to me along with a brief interpretation.
You may perform the following analysis on either the Sumatra earthquake of 2004, the Haitian earthquake 1/12/2010, or the Chilean earthquake of 2/27/2010. I will demonstrate the 'select by graphic' tool. To define the aftershock zone of the Chilean earthquake, I would query the catalog for earthquakes occurring since February 28, 2010, and then zoom into view the Chilean cluster and use that to draw my polygon. If you do Haiti, the same approach (different dates) should work.
Select by graphic: This map shows the aftershock zone of the 2004 great earthquake.
Make the earthquakes theme active. Use the polygon drawing tool to surround the aftershock zone and the subduction zone to the southeast. When the polygon has been completed, click the Select by Graphic button. The earthquake symbols in the polygon should turn yellow. In the Theme menu, Convert to Shapefile. Add to your view, then make the global earthquakes theme (and other shapefiles) invisible.
Classify these earthquakes by magnitude, zoom to an appropriate perspective, prepare a layout and send me the image.
What was the average seismicity in this region from 1/1/1973 through 12/25/2004 (or, if Chile, 2/26/2010? or, if Japan, 3/11/2011)? Use the Query Builder to get a count and calculate how that can be expressed in terms of earthquakes per month or earthquakes per day. Then use the Query Builder to get a count of: earthquakes between 12/1/26/2004 and 1/1/2005, and, for every time span since.
Pre-earthquake seismicity might be characterized by performing a query for each year, than normalizing in terms of events per month or events per day.
Has seismicity dropped to the long-term average? I would show this by opening a table in Excel and entering the count for each month or day as I performed the queries, then graphing the result (earthquakes/month as a function of time in months following the earthquake after multiplying the number of aftershocks hitting during the final week of 12/2004 by 4 to get a 'per month' rate and calling that the first month).
For the truly skilled (extra credit): Assuming aftershocks delineate the rupture surface, measure the area of the rupture (in square kilometers). Note that these are subduction zone earthquakes - are aftershocks on one side of the aftershock zone deeper than those on the opposite side? Can you show this and measure the dip of the subduction zone? If the fault is dipping, is the surface larger or smaller than the area of the polygon you drew to surround the aftershocks? Calculate the rupture area and show your work.
All assignments due by noon Tuesday, April 26, at noon.
Next week you will be assigned your Final Project. It will deal with spatial and temporal analysis of - EARTHQUAKES!