Category Archives: Earthquake

Exploding Ice and Rock - Booms Heard a Result of "Cryoseisms"

Ice Hanging From Rock

UPDATE 1/13/14: Frost-quake creates 100ft long crack here.

Over the past few days (starting around Christmas eve), there have been reports of large booming sounds associated with minor ground shaking across the northern states, as well as in Canada.  The Toronto events have a nice string of tweets that are associated with them as well.  Are these really explosions? Earthquakes? Sonic booms? The truth, as it turns out, is a rare event that produces what are known as "cryoseisms".  Oddly enough, these "frostquakes", as they are commonly known, have been discussed in the literature since about 1818!  Having a background in both meteorology and geophysics, cryoseisms are just one example of how closely related to two fields are.

So, what happens to produce such loud and potentially startling events? It's all about ice.  Cryoseisms occur when there are seasonal frost conditions, no insulating blanket of snow, lots of rain/thaw to saturate the ground, and a sharp drop in temperature.

Surface water penetrates into sufficiently permeable soil/rocks, but then is rapidly frozen with a fast drop in surface temperature.  Normally temperature drops slowly enough that the ice gradually freezes, giving the surrounding soil/rock time to adjust.  When really fast temperature drops occur and freezing is rapid, the surrounding areas are stressed by the expanding force of the ice.

The freezing process is actually a very powerful mechanism, and is one of the geologist's favorite ways to explain physical weathering of large boulders.  Freeze/thaw cycling has even been used as a quarrying technique in granite!

Expansion during this rapid freezing of infiltrated ground water stores energy in the surrounding rock/soil, like a spring, until..... BAM! Failure occurs in much the same way faults fail.  Here the driving force isn't tectonic though.

Cryoseisms can do light damage to structures in the immediate vicinity, but their intensity falls off very quickly with distance.  For the seismology buffs out there, the zero focal depth produces lots of surface waves, but these events are generally not recorded on seismic networks.

Want to know more about cryoseisms? The literature isn't too robust, but check out Barosh (2000), Nikonov (2010), and Voss & Herrmann (1980) for some starting points!

*Cryoseism is also used to refer to earthquakes at the base of glaciers as well.  That's a whole other story for another day!


April 2013 Oklahoma Earthquakes

This morning Oklahoma experienced another small sequence of earthquakes.  (There was also a large earthquake with an estimated magnitude of 7.8 in Iran over night.)  While I'm preparing for a conference very soon I'm a bit crunched for time, but thought a short post would be in order.  I would like to write a few posts concerning what magnitude is, how we calculate it, and other common questions I get asked at some point in the near future.

Okay, here's the synopsis of the most recent events. Early this morning at 01:56:29.875 CDT a magnitude 4.7 earthquake occurred centered northeast of the Oklahoma City metro area.  There have been a few significant aftershocks at magnitude 3.0, 3.6, and 4.6.  It is notable that there was a higher number of seismic events (though all small) beginning yesterday.  All these numbers are from the Oklahoma Geological Survey, the USGS estimates are lower with the largest events at 4.3, 4.2, and 3.3.  These magnitudes are computed on slightly different scales, but either way the largest earthquake released over 10 times LESS energy than the earthquakes last year.

The USGS did you feel it program has already collected around 1600 responses and the shaking reported matches very well with what was expected, probably due to the DYFI scale being pretty accurately calibrated after the large earthquake sequence last year.  It was striking that the vast majority of the responses came within 90 minutes of the quakes indicating the people actually got up and reported as soon as the event was over.  These responses really help the folks at the national earthquake information center (NEIC) and if you felt the earthquake but didn't go fill one out you should!

The moment tensor solution of the earthquake shows a strike slip solution meaning that the rock moved laterally past each other, not up and down.  This is shown by the "beachball" below with the colored regions indicating areas of compression.  There isn't enough information from one earthquake alone to tell if the fault runs SW to NE or SE to NW, but based upon the distribution of the large aftershocks it would be an okay initial guess that the fault trends to the NW.  Also notice the solution isn't perfectly strike-slip.  There is a small amount of oblique motion with a thrusting sense.  
After inspecting the infrasound instrument I have in my office I didn't see the earthquake, but the ground motion wasn't really detectable on the seismic station in Standing Stone, PA either.  It looks like the infrasound may have recorded the Iran earthquake, but I need to move it to a less noisy location.
Just for fun I've thrown in a seismograph below from a station in the Wichita Mountains in SW Oklahoma.  It would be fun to calculate the different arrivals and plot, but that's more fun for another time! I've made the trimmed .SAC file is available here in case you want to download it and try.

Oklahoma Earthquake - I felt it!

Those of us in Oklahoma this morning had quite a nice surprise when at about 9:06 AM local time the ground shook and many experienced an earthquake centered just outside of Norman, OK.  I live on the bottom floor of a 2-story apartment building and was standing when the quake hit.  I head a loud noise like a vehicle roll over, then felt the shake.  My blinds shook and a few plates rattled together.  After immediately realizing what it was I estimated the duration as about 15 seconds, but some of that could have been remnant swinging of objects in my house.  
The initial rating was 4.5 from the USGS (United States Geological Survey), and 5.1 from the OGS (Oklahoma Geological Survey).  These estimates have been revised many times over the day, as well as the location and depth of the quake.  The estimates seem to be settling around a 4.3-4.5 magnitude with a center just west of lake Thunderbird in Norman, OK.  Below is a google map (image: J. Leeman) plotting the OGS estimate of the center with the error as the shaded region.  This uncertainty is about 1.24mi in N/S and 1.12mi in E/W.  The USGS estimate is currently much less constrained, but subject to revision.  

 The earthquake was widely felt with reports from surrounding states.  If you felt the quake you should fill out the 'Did you Feel it?' question form available on the USGS website.  Many thousand reports have been submitted so far and data gathered from over 40 stations. The next image is courtesy of Bill Wilburn, planetarium director at the Science Museum of Oklahoma.  Following that is the plot of arrival times at different stations from Steve Piltz, Tulsa NWS.

 We are also fortunate to currently have the earthscope array stationed in Oklahoma.  The next figure shows current seismic stations on the OGS page.  The yellow stations are earthscope.  Those stations appear to have been saturated, but it could be a plotting issue.  I will not know until I can get ahold of the data.

The final two images are the Carlsbad, NM East Tower seismogram and a focal mechanism plot.  The Carlsbad plot just shows that the earthquake was still very detectable in NM and makes it easy to see why it was recorded by so many stations!  The focal mechanism plot (or moment tensor solution) plots the first movement (up/down) of the ground at the stations to determine the type of earthquake/fault.  Here we see evidence for a strike-slip fault along a SW-NE or SE-NW line.  Simply put this means the ground sheared on a horizontal plane, not shearing along a slanted/vertical face as in normal or transverse faults.  
I'll post more in a future post if we learn anything else significant from/about this quake.  Maybe also some neat arrival plots and a discussion of wave types.  As a note the largest earthquake recorded that originated in Oklahoma was in the El Reno area on April 9, 1952 with a magnitude of 5.5.  The USGS has the following to describe that quake:
This earthquake caused moderate damage at El Reno, Oklahoma City, and Ponca City, including toppled chimneys and smokestacks, cracked and loosened bricks on buildings, and broken windows and dishes. One crack in the State Capitol at Oklahoma City was 15 meters long. Slight damage was reported from many other towns in Oklahoma and from some towns in Kansas and Texas. The earthquake was caused by slippage along the Nemaha fault. Felt over most of Oklahoma and in Arkansas, Iowa, Kansas, Missouri, Nebraska, and Texas.

Earthquakes in NW Arkansas? Yep!

Those of you who live in the northwest Arkansas area were recently 'rocked' by two earthquakes.  These were both small being under 3.0 in magnitude.  I hesitate to give out exact magnitudes because these are determined by looking at at data from several seismic stations (as is the location and depth).  You can imagine that there are relatively few earthquakes in this part of the state, so the seismic network is lacking.

So, why did these earthquakes occur? Have they happened before? Well, in short we don't know and not in recorded history.  Yes there have been movements on faults that have been felt in Benton county before, but these two events are the only earthquakes originating in the county we know of.  The first map shows recorded earthquakes since 1974 to the present (not including these two).  You will observe almost none within 200km of the county.

The initial hypothesis by a University of Arkansas professor, given the shallow depths initially calculated, was that these were the results of cave collapses.  Karst topography (what we have in NWA) consists of layers of caves carved into the subsurface.  It is not uncommon for these to collapse and the UofA professor cited the loud noises heard as support for the collapse theory.  Though the theory is nice, but my initial thoughts were 'where are the sinkholes'?  None have been discovered, not that they would be like the recent sinkhole in Guatemala (picture from Guatemalan Govt.).  I had settled on the personal hypothesis of these being slips of old faults.  There are many of these faults throughout the area, but they have been inactive for a very long time (even in geologic time).  This slipping can be stress built up over long periods of time, introduction of fluids, or loading of the land.  I even thought about a fault running under the railroad bridge in Decatur which does not break, but creeps slowly.  That bridge has been rebuilt several times due to  fault movement.

I heard that people were rushing to buy earthquake insurance and couldn't help but to (almost literally) fall out of my chair laughing.  These are not indications of further earthquakes.  We don't know what exactly is happening, but that is mostly because the area is poorly mapped and we have little geophysical data available about it.  Could more earthquakes happen? Absolutely.  Will they? We can't say.

Next we must discuss how earthquakes are rated.  We use the Richter scale, which is a logarithmic scale.  This means that each step is an order of magnitude more energy.  A 3.0 is ten times larger than a 2.0, so a 4.0 is 100 times larger than a 2.0.  News media commonly misses this and says it was 'twice' as large which is simple ignorance.  We also know it takes a 4.0+ to start breaking glass and doing serious damage.  I have only head reports of a few cracked driveways and see no reason to expect anymore.  Finally there is no 'trend' with only two data points to support earthquakes getting stronger.