February 18, 2014
The USGS’s crowd-sourced Community Internet Intensity Maps, popularly known as “Did-You-Feel-It” maps, have been collecting online surveys of seismic shaking intensity since 1997. There’ve been plenty of quakes in that time, and in 2012 researchers put together all the results into these compilation maps that show all the data together.
Have a look, and see how strong ground shaking has been in your neck of the woods. These particular maps only show the highest intensity of shaking experienced during the given timeframe; they don’t show the number of times shaking was experienced, which is a different metric of how susceptible a place is to earthquakes. There are annual maps to break it down a bit.
Notable earthquakes are highlighted with stars, revealing a lot that you’ve probably forgotten scattered across the country. Unsurprisingly, California, Washington, and Alaska take the cake for violent earthquakes. CA alone had magnitude 6+ earthquakes in 1992, 1994, 1999, 2003, 2004, and 2010. But Virginia almost got there in 2011, as did Oklahoma, and the Gulf Coast felt a magnitude 6 from the middle of the Gulf in 2006, the same year Hawaii was rocked by a 6.5. The apparent unevenness and gaps in coverage out west are most likely artifacts of a sparse population, an effect you can glean the gist of from a population map.
It’s also worth noting the growing number of responses with time as more people become aware of the USGS form. You can see that even in places with steady annual seismicity rates, like California, where the number of zip codes that are filled in with responses grows over the years. The total number of responses per event has also risen dramatically since the site’s founding. Take for example the total number of responses in the following major earthquakes in 1999, 2008, 2010, and 2011.
Some of the gaps in the national summary maps have been filled in since 2012, as for example central South Carolina and Georgia were earlier this week.
Of course many more of the gaps would be filled in if these maps covered more than just two decades of U.S. seismic history. Other beautiful maps of U.S. seismicity exist, though they don’t include data about felt intensities.
With so much data no amassed, compilations of seismic intensity data can start to guide assessment of seismic hazard and estimates of losses from earthquakes. Compiled over sufficiently long time spans (we’re nowhere close yet) the map of strongest experienced shaking should look like seismic hazard maps that forecast the strongest level of shaking expected with some probability over a certain timespan. However, because of the vagaries of earthquake production in the crust, and the probabilistic nature of hazard maps, the datasets won’t be apples-to-apples comparisons for several millennia.
The bottom line is: if you want to feel earthquake tremors, your best bet is coastal CA or southern Alaska, where the ground consistently trembles every year. If you’re looking to escape quakes entirely, your best bet is probably North Dakota. So, you know, weigh your options.