January 7, 2016
by Shelley Chestler
Did you know that there is a type of earthquake that happens so slowly that we can’t feel it? Every 12-15 months these slow earthquakes occur in the Cascadia Subduction Zone beneath southern Vancouver Island and northern Washington. One of these events is happening right now and is possibly connected to the magnitude 4.8 earthquake NE of Victoria, Canada that occurred on December 29.
I like to call slow earthquakes the banana slugs of earthquakes. This is because while slow earthquakes can release the same amount of energy as a magnitude 6.5-6.8 normal earthquake, they release this energy more slowly. For example, many of you who have been living in the Pacific Northwest for a long time probably remember the magnitude 6.8 Nisqually earthquake in 2001. The shaking during this earthquake lasted about 15 seconds. A slow earthquake of the same magnitude lasts for weeks; the magnitude 6.8 slow earthquake in 2010 lasted from August 8 to September 8.
The current slow earthquake began on December 21 beneath Vancouver Island, Canada. Activity continued beneath Vancouver Island for ~8 days and now the slow earthquake is migrating southward beneath the Straight of Juan de Fuca.
If we can’t feel these earthquakes, how do we track them? Like normal earthquakes, slow earthquakes shake the ground. But, the ground motions are so small only sensitive instruments called seismometers can record them. These ground motion recordings, or seismograms, look like a squiggly lines. If the ground moves up, you get an upward squiggle and if the ground moves down you get a downward squiggle.
The seismograms produced by slow earthquakes look different than those produced by normal earthquakes. In the figure below, the top seismogram is from the Nisqually earthquake. There are well-defined peaks that indicate the arrivals of the seismic waves that shake the ground. The bottom recording shows the ground motion from a slow earthquake, which is often referred to as tremor. Unlike the recording from a normal earthquake, tremor looks like a disorganized jumble of seismic wave arrivals without distinct peaks. In addition, the seismic waves from a large, normal earthquake like the Nisqually earthquake are much larger than the seismic waves that make up tremor. That is why you can feel a normal earthquake but not a slow earthquake.
We track slow earthquakes by looking at where and when tremor is occuring. You can look at tremor using this Interactive Tremor Map. The example below shows tremor during the current slow earthquake from December 21, 2015 to January 4, 2016. Each dot on the map, color coded by time, represents 5 minutes of tremor activity. Unlike normal earthquakes, where a dot represents a single earthquake, all the tremor dots are part of the ongoing slow earthquake.
How is the current slow earthquake related to the earthquake near Victoria? While the Victoria earthquake occurred on the southeastern edge of where tremor, and hence the slow earthquake was occurring, the Victoria earthquake was deeper than the tremor. The Victoria earthquake occurred within the subducting Juan de Fuca plate while the slow earthquake is occurring on the boundary between the Juan de Fuca and North American plates (see figure below). Despite the difference in location between the Victoria earthquake and the slow earthquake, the two events could still be related. Changes in the distribution of built up energy due to the slow earthquake could have triggered the Victoria earthquake.
Slow earthquakes happen almost every year and are thought to be part of the normal cycle of energy release in the subduction zone. This event is in no way alarming! If you want more information about slow earthquakes make sure to read the previous PNSN blog post by Aaron Wech or watch this video of a talk I gave at Town Hall Seattle as part of the 2014 UW Science Now lecture series.