It was a Monday morning. And the day was just getting started. As I was looking through my morning emails I noticed a strange sensation. A very light rumble. Almost like someone was running on the roof. Or maybe like a very faint rumble of thunder. It didn’t last very long. But it was an odd sensation. Something I hadn’t felt before.
I quickly opened my coffee cup to make sure someone didn’t put something extra in it. Maybe I just imagined it? About a half hour later I seen it. It was a text from my wife. “We just had an earthquake! It shook the doors on the cabinets”. Now I know I’m dreaming. South Dakota doesn’t get earthquakes. Right? As I scrambled to look it up, there it was. A small earthquake had just occurred to the northeast of Tyndall, South Dakota. So I wasn’t imagining things.
But for myself. This brought up more questions then answers. Is this common or is 2020 making a comeback? Are there even fault lines in South Dakota? Why have I lived here most of my life and there has never been an earthquake until now? As I searched for answers I found fewer. I couldn’t find hardly any information on South Dakota earthquakes. Other than another one happening outside of Bowdle a month ago. “What? Another earthquake?” Is something going on here? Is another one coming? Is now the time to by earthquake insurance?
To answer my questions, I reached out to the SDGS. These are the people that monitor earthquakes in the South Dakota. If anyone could help shed a bit of light on the subject it would be them. And I was happy to have Tim Cowman reach out and respond to my questions.
What is your position in the SDGS and how long have you been there?
I am the Director of the South Dakota Geological Survey, which is part of the Dept. of Environment and Natural Resources. I have been the Director for 2.5 years and worked for SDGS for a total of 35 years.
How do you (the SDGS) find out information about earthquakes in South Dakota?
When an earthquake occurs, it is detected through a large network of seismic sensors maintained by the USGS. The USGS uses readings from these sensors to determine the size and location of the earthquake. That information is posted on the USGS Earthquake Hazards website. I also receive an automated email notification from the website when large earthquakes occur. Information about earthquakes is made available very soon after the earthquake happens.
Is it common to have any earthquakes big or small in South Dakota?
There have been 95 earthquakes documented in South Dakota since 1872. Of those, 61 earthquakes had their magnitudes determined. The magnitudes range from 2.3 to 5.1, and the average was 3.5. We generally don’t start seeing damage until a magnitude of 5 is reached. All of South Dakota’s earthquakes have been 4.5 or less, except for the 5.1 magnitude earthquake. That event occurred in a rural area near the southern Bennett County border in 1964.
Were the two earthquakes outside of Tyndall and Bowdle considered big for the area?
The recent earthquakes near Tyndall and Bowdle had magnitudes of 3.1 and 3.2, respectively. These are typical size earthquakes for South Dakota and are not considered threatening to life or property.
Does South Dakota have fault lines and is this the cause of the recent earthquakes in the last month?
South Dakota does not have any active faults at shallow depth that could cause a major earthquake. There are faults in the basement rocks miles below the surface that are generally considered inactive, but could have some movement that causes the minor earthquakes we experience. A likely cause of the earthquakes in eastern South Dakota is isostatic or glacial rebound. When massive ice sheets covered the northern part of the continent, including the eastern half of South Dakota, during the last ice age they likely compressed the earth’s crust with their massive weight. That weight has been removed and the crust is slowly rebounding. The movements created by the rebounding action can trigger minor earthquakes.
With there being 2 earthquakes in South Dakota in the last month, is there any cause for concern?
It is not uncommon to experience one or two earthquakes in South Dakota per year. Some years there are no earthquakes, some years there are more than two. In 1924 there were four earthquakes on the same day along the Custer – Fall River county line. More recently, in 1990, there were three earthquakes in one year.
Is it possible South Dakota could have bigger earthquakes then the last 2?
The magnitude of earthquakes in South Dakota ranges from 2.3 to 5.1, and the average is 3.5. So yes, South Dakota can and has experienced earthquakes larger than the last two, but they have also been minor in strength.
Are there ways to predict in South Dakota if another quake may come?
Although earthquake prediction techniques have been implemented in areas prone to frequent and large earthquakes, such as the Pacific coast, there has not been any attempt to predict earthquakes occurrences in South Dakota. Earthquake prediction in our state would be difficult due to the absence of major faults at shallow depths. The small nature of our earthquakes does not warrant efforts to predict earthquake occurrence.
If people have more questions, where could they go for more information?
The South Dakota Geological Survey has information about earthquakes in South Dakota. It can be found at http://www.sdgs.usd.edu/earthquakes. The US Geological Survey has a website dedicated to information and education about earthquakes world-wide. It can be found at https://www.usgs.gov/natural-hazards/earthquake-hazards/earthquakes.
A huge thank you to Tim for answering some of my questions. It was very much appreciated.
It was good to find out that I wasn’t going crazy. And earthquakes are possible. And odds are, it will happen again. But I’m also grateful that it doesn’t sound like earthquake insurance probably isn’t necessary just yet.
Until next time.