# SSBW - Network Tutorial 4: Earthquake Location Principles

Initial Publication Date: August 18, 2021

#### Summary

In this interactive tutorial built for a learning management system, students 1) conduct a quality check on waveforms and identify first arrivals in seismograms, 2) practice locating an earthquake using a simple geometric approach called the perpendicular bisector method, 3) practice locating an earthquake using a more modern derivative approach.

## Context

#### Audience

The IRIS Seismology Skill Building Workshop (SSBW), which is a free, online, open-access, large-enrollment, 12-week summer workshop for upper level undergraduates.

#### Skills and concepts that students must have mastered

This assignment builds on prior understanding of seismic wave propagation, and familiarity with basic seismogram interpretation (identification of first arrivals).

#### How the activity is situated in the course

This is assignment number 23 of 35 in the Seismology Skill Building Workshop (SSBW).

## Goals

#### Content/concepts goals for this activity

Understand what a seismogram graphically represents, explain the relationship between P-wave arrival time and distance to the epicenter of an event, two methods to locate the epicenter of a seismic event (perpendicular bisectors and a derivative approach), and understand that an earthquake's location is often a best fitting solution.

#### Higher order thinking skills goals for this activity

Understanding how the real world can be described through mathematical models, with special attention paid to comparisons between modeled and observed data to refine the model.

## Description and Teaching Materials

Students will:
1. Load an event using the IRIS Earthquake Location App and select seismic stations to include in the parallel bisector location process
2. Conduct a visual quality check on the data from the selected stations
3. Pick the first arrivals for each seismogram
4. Select pairs of stations and compare their first arrivals to determine at which station the seismic energy arrived at first.
5. Define the location region and compare to the USGS solution
6. Load a second event and select seismic stations to include in the derivative location process
7. Conduct a visual quality check on data from the selected stations and pick the first arrivals
8. Based on the residuals calculated from the initial location, select a new location and recalculate the residuals, repeating until the smallest residual possible has been achieved.
9. Modify the model to account for depth of the earthquake to further reduce the residuals
10. Compare the solution to the USGS solution

## Teaching Notes and Tips

This assignment was constructed in the Moodle learning management system, and has been exported in the GIFT format. More information about the syntax of this format can be found here: https://docs.moodle.org/en/GIFT_format

## Assessment

This assignment is automatically graded by the learning management system. The number of questions of each type used are:
20 multiple choice questions