Visit the U.S. Geological Survey website on earthquakes (https://earthquake.usgs.gov), and click on “Significant Earthquakes, Past 30 Days.” Answer the following questions:
1. How many significant earthquakes occurred worldwide in the past 30 days?
2. The list that appears should have several rows, each including a blue hyperlink naming the earthquake, the moment magnitude (M) to the left, and the depth to the right. Arrange the moment magnitudes of these earthquakes into a single list, from smallest to largest (one M value per earthquake). What are the minimum, maximum, and median M values in your list? Note that the median is the middle value of a set of ranked numbers. For example, if there were nine significant earthquakes, the median would be the fifth highest M value in your list. If you had eight M values, the median would be the average of the fourth and fifth highest values. For seven M values, the median is the fourth highest, and so on.
3. What are the minimum, maximum, and median depths of the significant earthquakes? Repeat (2) above, using depth instead of M.
4. Looking at your list, do larger magnitude earthquakes seem to occur at shallower or deeper depths?
5. Test your answer to (4) by calculating a correlation coefficient (rs, also called R) between earthquake magnitude and depth. This statistic measures the direction (positive or negative) and strength of association between two variables. Type the magnitudes into the x column and the corresponding depths into the y column at this statistics website (Links to an external site.).
Then click “Calculate R.” If the output rs is positive, the association is positive, meaning magnitude increases as depth increases. The closer rs is to +1.0, the stronger the positive association. In contrast, if rs is negative, the association is negative, meaning magnitude decreases as depth increases. The closer rs is to -1.0, the stronger the negative association. Generally, rs values between 0.7 and 1.0 (or -0.7 and -1.0) are strong; values between 0.3 and 0.7 (or -0.3 and -0.7) are moderate; and values between 0 and 0.3 (or 0 and -0.3) are weak.
What is the rs value?
Is the association positive or negative?
Is it a strong or weak association?
Is it statistically significant? (Note: It says on the website, below the rs calculation, whether or not it would normally be considered significant.)
Based on what you read in the textbook and information on this website, would you generally expect larger magnitude earthquakes at shallower or deeper depths? Briefly explain. Hint: Geology is complicated, and there is no one perfect answer to this question. Provide reasonable justification for whatever you decide. Do not say that shallower earthquakes are stronger because they are closer to the land surface and cause more damage. Remember, strength (energy) and damage (intensity) have different meanings when it comes to earthquakes.
6. Click any one of the significant earthquakes that was felt by people. Then click “Did You Feel It?” Next, click “Intensity Vs. Distance.” Looking at the graph, provide the following information:
Name of earthquake
Farthest distance from hypocenter it was felt
Highest intensity observed
How intensity changed (increased, decreased, or remained constant) moving away from hypocenter