Lab 3 Earthquake Frequency

Geol 116, Spring 2023 Lab 3: Earthquake Recurrence and Evaluating Seismic Hazards One of the ways that seismologists can assess the likelihood of future earthquakes is by investigating the frequency of historical earthquakes in a region of interest. Numerous studies have shown that the number of earthquakes in an area decreases logarithmically with increasing earthquake magnitude. In this lab exercise you will explore the rate of earthquake occurrence for areas of Earth that you choose. Data is accessed through the simple interface of the IRIS Earthquake Browser. After compiling your data for various sized earthquakes in the region, you will calculate recurrence intervals for each magnitude and plot the data on a semi-log graph for interpretation. Discussion of the collected data touches on strengths and limitations of the data set, possible societal implications (e.g. exposure, vulnerability, mitigation), as well as concepts related to earthquake prediction and forecasting. The Exercise We’ll be using the IRIS Earthquake Browser to compile earthquake data in some region. Your first step is to familiarize yourself with the tool. Navigate to it on your computer. You might watch the tutorial, but if you feel brave (or don’t like to be given instructions), you can navigate from this site directly to the app. Then we’ll use the app to compile how many earthquakes have occurred in a region within distinct magnitude ranges. Example of using the app. Using the tools on the left (select new region) I outline an area that to study. When you do this, choose a region that is neither too small (not very many earthquakes) nor too large (too many earthquakes to plot); you’re best choosing an area that includes a plate boundary. For this example, I randomly chose a zone across central Africa. The information panel on the right indicates that there are 3460 earthquakes recorded in this area and reported in the earthquake catalog; 1000 are shown.

If you click on Magnitude Range, you can (and should) limit your investigation to a range of magnitudes of 4 and above. When I did this, I still had 3323 earthquakes. We want to examine how the number of earthquakes in this region changes with increasing earthquake magnitude. The easiest approach is to figure out how many earthquakes of magnitude 4 or greater, 5 or greater, 6 or greater, etc. have occurred in the region and divide by number of years of record (here, 1970-2022). I’ve this in the example extra half magnitude (4+, 4.5+, 5+, etc.). I have the app tell me how many earthquakes of a particular magnitude or larger by changing the magnitude range (e.g. 4-10, then 4.5-10, 4- 10, etc.) in the right-hand box. Inserting the data into the spreadsheet on Brightspace yields: # of events of Mag or greater Years of Record Number/Yr Minimum Magnitude 3323 53 62.70 4.0 2170 53 40.94 4.5 668 53 12.60 5.0 130 53 2.45 5.5 34 53 0.64 6.0 11 53 0.21 6.5 2 53 0.04 7.0 0 53 0 7.5 I plot the number/yr for a given magnitude or greater on a logarithmic scale against the magnitude to obtain this graph, which is known as a Gutenberg-Richter diagram: 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 1.00 10.00 100.00 Magnitude Number of Events/yr Ideally, the decline in number of events per year (i.e. frequency) will be linear in this space (as it is between 4 and at least 6.5 in this case) So we can estimate recurrence (1/frequency) of events either