Using GPS Data to Measure the Earth
Introduction
In the previous activity, we determined that there are places on Earth that are in slow but constant motion. However, a study of other places on the Earth shows us different results.
Activity
1. Using the data given you for Victoria, British Columbia, make a single graph of the North and East displacements during from 1/1/1997 to 12/31/2000.
2. Determine the overall linear trend for the entire four-year timespan. Include the vector direction of the velocity with North being 0°.
Velocity / mm/day / mm/yrNorth-South / 0.012 / 4.4
East-West / 0.018 / 6.5
Total Vector
(dir = ____°) / 55.9 / 55.9
3. There should also be hiccups in the data at periodic intervals. Find the velocity of the site only during one of those times. Include the velocity vector’s direction with North being 0°.
Velocity / mm/day / mm/yrNorth-South / -0.058 / -21.1
East-West / -0.094 / -34.4
Total Vector
(dir = ____°) / 238.5 / 238.5
4. Compare the overall trend velocity with the “hiccup” velocity. Which one is bigger? By how much (how many times bigger)? How do their directions compare?
ETS periods are faster by a factor of 5 and almost exactly in the opposite direction (difference of compass bearings is almost exactly 180 degrees)
Reflection
1. During one of the hiccups, what was the total displacement of the site?
21.12+34.42=40.4mmyr
31 days×40.4 mmyr×1 yr365 days=3.43 mm
2. How often do these hiccups occur? Does the occurrence seem to be regular?
Approximately once every 12 months
3. What could cause such drastic hiccups? Explain how it accounts for the specific behavior of the site overall (the normal velocity plus the hiccups).
4. Read the Wikipedia article on Episodic Tremor and Slip (which is what the hiccup is called) and compare you answer in (3) to the explanation given. What did you get right? What did you miss?