1. Variations in earthquake frequency and seismicity
In the last chapter, you studied both the magnitude and frequency of earthquakes in the Parkfield region.
2. Movements along faults cause earthquakes
The earthquakes there are due to stress release as the tectonic plates move relative to each other along the San Andreas fault.
But the frequency of earthquakes and seismicity can change due to other geological events.
3. Mt. St. Helens
Mt. St. Helens is a great example of this. Regular earthquakes occur around Mt. St. Helens because of motion of tectonic plates on nearby faults, but when Mt. St. Helens erupts, as it famously did in this photo taken in May of 1980, earthquakes are far more frequent, and the b-value also changes.
4. Mt. St. Helens earthquakes
Here, I have plotted the magnitude of earthquakes around Mt. St. Helens versus time. There are two periods of increased frequency of earthquakes, corresponding to the massive 1980 eruption and the more gradual 2004 to 2008 eruption.
These increased frequencies in earthquakes are due to movement of magma under and around the volcano.
Natural causes are not the only means by which a region's seismic character may change.
5. Wastewater injection in Oklahoma
Around 2010, hydraulic fracturing, known as fracking, saw a rapid rise in the US state of Oklahoma. Fracking itself does not seem to change seismic behavior, but wastewater from fracking is injected at high pressure deep into the earth, and this may cause earthquakes. Shown in pink on this map of Oklahoma are the locations of wastewater injection wells.
6. Seismic Oklahoma
On top of this, we can overlay in blue the locations of the epicenters of Oklahoma's earthquakes with magnitude of at least three that have occurred since 2010.
These seems to be a correlation between injection wells and earthquakes.
In the next set of exercises, you will perform a quantitative analysis of earthquake frequencies in Oklahoma to see how they changed when fracking became widespread in 2010.
7. Let's practice!
Happy hacking!