LAB 6: LOOP STATEMENTS

Transcription

1 NAME: LAB 6: LOOP STATEMENTS This laboratory exercise will involve you writing your first loop statements in MATLAB functions and scripts. Your final code will be handed in via a single zip file following the instructions at the end of the lab. Make sure to comment your code and include the proper MATLAB header information. Because this lab focuses on loops, you should use loops to solve the following tasks, and any vectors/matrices that change size on each iteration should be pre-allocated, if possible. You should not use any vectorized code except when loading the data file(s) and plotting data at the end of your script/function. Part I: Converting Temperatures A common quantitative task in Earth sciences is to convert data from one unit to another. On the course website, I have provided an ASCII file with a temperature time series from the temperature gauges in the Boone area. The data are arranged in columns: Column 1: Time (decimal days) Column 2: Watauga Hospital (air temp, F) Column 3: Winkler s Creek (water temp, F) Column 4: Boone Creek (water temp, F) Unfortunately, the data are unusable in their current form. Your task is to: 1) Create a new MATLAB script called processtemps.m. 2) Load the data file streamtemp.dat (provided on the course website) into MATLAB and store it in a matrix called tempdat. 3) Using a single for loop, convert all of the temperatures into C and make it so the time series starts at time=0 days. Make sure that your code will work for any data file with this layout, so your code will have to figure out what the first time is automatically. 4) Make a plot of all three of the converted temperature time series on one plot (x=time y=temp) with labeled axes, and a useful legend. Plot the data with lines (no symbols) and make the Watauga Hospital data blue, Winkler Creek red, and Boone Creek green. Page 1 of 5

2 Part II: A Three-Dimensional Grid Generator Because the world around us is three-dimensional many equations that represent geologic processes are three dimensional (i.e. they are functions of x, y, and z). To evaluate a three-dimensional equation, scientists commonly use grids, or uniformly spaced data points. Your task is to: 1) Create a new MATLAB function called make3dgrid.m. 2) Your function should accept 9 arguments in the following order: xmin, xmax, numx, ymin, ymax, numy, zmin, zmax, and numz. These will represent the min/max ranges of the extent of your volumetric grid of data points. The num* variables will assign the total number of points in each coordinate direction. You should create the grid using nested for loops. Be sure to pre-allocate! Hint: you will probably want to drive your loops with linspace instead of the colon operator. 3) Your function should return a variable called gridxyz, which contains an [n x 3] matrix of the x, y, and z locations of grid points. After all, there would be no point in making a grid of data points unless you actually return the data to the user. 4) Your function should make a three-dimensional plot of your volumetric grid (use the plot3 command). Plot the data points as red-filled circles with black outlines. You do not need to label the axes since this is a generic grid generator and we do not know what the axes are. To improve rendering, turn on the rotation tool, and make sure there is no vertical or horizontal exaggeration, add the following lines of code after your plot call. Be sure to rotate around your plot and admire the coolness of 3D plotting in MATLAB! grid on; axis(gca,'vis3d'); axis equal; rotate3d on; Page 2 of 5

3 Part III: The 1994 M6.7 Northridge Earthquake: Aftershocks (and Foreshocks?) On January 17 th, 1994, a Magnitude 6.7 earthquake struck just north of downtown Los Angeles, California. At the time, it was the most expensive natural disaster the United States had ever experienced. A detailed seismic catalog of earthquakes in southern California was recently published in the Bulletin of the Seismological Society of America. The data is available in ASCII format at Because the catalog has a few columns with non-numerical data, I have provided a version of the data file that has only numeric data, but is otherwise identical. I encourage you to visit the SCEC website linked above, but please use the data file provided on the course website. Your task for this dataset is to identify which earthquakes are potential aftershocks based on parameters that I provide, and to determine if there was any unusual seismic activity before the event that should have alerted us (i.e. were there foreshocks?). 1) Create a new MATLAB script called processeqs.m. 2) Download the earthquake catalog file from the course website and load it into MATLAB and store the numeric data into a matrix called data. Note that the file is VERY large, so depending on your computer s hardware, it may take ten seconds to one minute just to load the data. Because this dataset is very large, you should not split up your data matrix into separate column vectors. You should know how to refer to a single column of data in MATLAB when plotting. 3) Download the California coastline ASCII file from the course website and load this into a matrix called coast. Also, create a 1x2 matrix called northridge, and store the epicenter location ( ) of the main shock. 4) Publications on the Northridge event suggest that the postseismic deformation associated with this event ceased after 100 days from the event. Let s assume that any earthquake within 100 days of the event (including the day of the event) are potentially aftershocks. This means that all events between Jan 17 th 1994 through and including April 27 th 1994 are potential candidates. Also, for an event to be considered an aftershock, it must be spatially correlated with the mainshock. Let s make a simple assumption and say that an event is within the aftershock zone if it is has a Lat/Lon range that falls within: 33.8 Latitude Longitude In other words, your script will need to search the entire catalog for any and all events that happened within 100 days of the mainshock and fall within the aftershock location window provided above. This is conceptually simple, but you should definitely write out a basic algorithm of what your code needs to do before you start coding. You should store your aftershock data in a matrix called ashock. Hints: This will require a single loop with several nested if-elseif-else statements. It is probably easiest if you filter by lon/lat first. Page 3 of 5

4 5) To determine if there was any unusual behavior before the event that one could claim is foreshock activity, you will need to do the same kind of search as in question 5, but look for any and all events in the 100 days before the Northridge event (i.e. October 9 th 1993). Do not include the mainshock day in your search, just the 100 previous days. Because this is a similar type of search as what you did in question 5 above, but with different conditions, you should do this in a separate loop. So, question 5 should have a single dedicated loop and question 6 should have a single separate, but similar loop. Store the potential foreshock data in a matrix called fshock. 6) Your script should print out the following useful messages to the command window: Loaded x Total Earthquakes Found x aftershocks Found x potential foreshocks Elapsed time is x seconds. Make sure that the first three entries line up neatly. The first three entries should all be printed as integers, and the last entry can be directly printed simply by using the toc command. 7) To visualize your results, you need to make two plots in two separate figure windows. The plots are both identical except that one shows the entire earthquake catalog region: 30.0 Latitude Longitude while the second plot shows a zoomed in view of the Northridge region: 33.8 Latitude Longitude ) Each plot should have the following things plotted: a) The raw/entire earthquake catalog should be plotted with red points/pixels. Use the axis( equal ) command to make sure your plots are not distorted. b) The earthquakes that you identified as aftershocks should be plotted with blue points/pixels c) The events that you determined to be potential foreshocks should be plotted with greenfilled circles (size=5) with black outlines. d) The Northridge mainshock event should be plotted as a yellow-filled pentagram (size=12) with a black outline. The epicenter location was provided in part 3. e) The California coastline should be plotted with a black line with a thickness of 2. Page 4 of 5

5 Part IV: What to Hand in? Like the previous lab, you should zip up all of your files and them to me as a single.zip file. Use the appropriate MATLAB command to zip everything into one file. Call your file lab6_lastname.zip. For your convenience, I have provided a list of the required files below. You should not include the data files. This would make your zip file too large to . I will have the data files available in the same directory as your programs and in the same format and with the same filenames as the files on the course website. Script Files: Function Files: processtemps.m, processeqs.m make3dgrid.m Page 5 of 5