We cannot predict precisely when or where an earthquake will strike but we know pretty well where they are most likely to occur and can make some sensible preparations to mitigate their worst effects. Mostly this involves governments setting suitable building codes to ensure that buildings don't fall down during an earthquake. However individuals and organisations can do a lot to help themsleves... it is all about understanding what the risks are, making preparations to reduce some of the hazards before the earthquake happens and knowing what to do during and after an earthquake.
The Great shakeout is a nation-wide earthquake drill that involves over 20million people in the USA doing just that... preparing for an earthquake and then holding an earthquake drill at 10:20 on 20th Oct.
A quick check on the data this morning shows another nice record from a M3.6 event in Cherokee, OK. However looking at our comparison seismic station at Wichita Mountains we notice that station also picked up an event in Mexico.
Two earthquakes this morning, A M3.6 in Oklahoma and a M5.1 in Mexico
This poses a couple of questions.
1) why do both earthquakes register the same size on the Wichita Mountains station ?
2) why do we only see the local event on Ben's geophone ?
Answers
1) the larger event is further away, the "ripples" from the earthquake have to spread out over a much larger radius so become smaller. This is best explained by Father Teds small or far away clip
2) Harder to explain... it is all to do with frequency and attenuation. Earthquakes generate seismic waves with a range of different frequencies (ie different numbers of vibrations per second), the larger the earthquake the lower frequency signals it can generate. A small earthquake (M3) can only generate frequencies of vibrations above a few Hertz (cycles per second). A large earthquake (M5) can generate much lower frequencies (maybe a tenth of a hertz or so) . Really large earthquakes (M7+) can generate vibrations that last for minutes between each cycle. It is all to do with how long it takes the actual earthquake rupture to happen, samll earthquakes happen quickly, large earthquakes take longer.
So a large earthquake like the one in Mexico creates seismic waves with a range of frequencies from a few tenths of Hertz to a few tens of Hertz. If you are close to the event you will record all of these vibrations but as you go further away you start to see the higher frequencies disappearing and just the lower frequencies getting through. This is because the earth itself acts like a filter, attenuating (fancy word for reducing) higher frequencies more than lower frequencies. We are all familiar with this effect with sound, if you are listening to a loud concert from a long way away all you hear is the thud-thud-thud of the bass beat, all the high frequency treble signals are lost. The same thing happens with seismic waves in the earth.
Local M3.6 quake (top) has high frequencies (wiggles close together), distant M5.1 quake (bottom) has lower frequencies (wiggles spaced further apart), amplitudes are similar.
A spectrogram plots energy at each frequency as a colour against time, here most energy is at very low frequencies (less than 1Hz)
For the local quake the spectrogram shows most energy is at high frequencies (2-10Hz)
Seismic Sensors come in different sizes and are sensitive to different ranges of frequencies. Ben's geophone is sensitive to seismic signals from 4Hz-100Hz, the sensor at Wichita Mountain is sensitive to signals from 0.01Hz to 100Hz (we call these broadband seismometers)
Is OKlahoma fracking in oil and gas wells causing all the earthquakes ? The sudden increase in seismicity since 2009 has certainly coincided with an increase in oil and gas activities in the state.
What is fracking?
This is the process of pumping high pressure fluids into otherwise impermeable rock layers in order to induce small cracks (fractures) which are then held open by sand and allow fluids(gas, oil and water) to flow. So by definition fracking is causing microfractures deep underground, these micro-earthquakes are very very small (generally they have negative magnitudes) and are only detectable by highly specialised sensors located deep underground near the injection sites.
However the M3+ earthquakes happening in Oklahoma are releasing orders of magnitude more energy than is being put into the ground by the injection processes, these earthquakes are fractures along pre-existing faults and are releasing existing stresses in the crust. Scientists call these induced earthquakes.
So are the OK induced earthquakes caused by fracking ?
Well again it is complicated, some probably are, however in the oil and gas industries several different techniques are used that involve injecting fluids deep underground.
Different oil and gas injection operations
After careful analysis of the data in Oklahoma and comparison with known wellsite activities scientists at the USGS have concluded that the vast majority of the induced earthquakes in Oklahoma are related to "wastewater disposal" rather than "fracking" . In Oklahoma many of the producing oil wells produce a mixture of oil and (mostly) water. The water (brine) is very salty and it would not be environmentally safe to dispose of into existing watercourses without treating first. It is much cheaper to return this "formation fluid" to the subsurface than to treat it at the surface. The wastewater disposal wells usually drill into a deeper formation that is highly permeable (i.e. fluids can easily flow into it). Sometimes this water is pumped into the formation, sometimes it just naturally flows in under its own pressure. However since 2009 the volumes of this waster water injection have increased massively and are associated with the increased seismicity. In 2015 the authorities in Oklahoma started to restrict the volumes of wasterwater that could be injected. Following the M5.8 Pawnee earthquake in September this year a number of wastewater injection wells were closed, however it might take months or even years before the effects of these restrictions are seen in a reduction in earthquakes.
A couple of wobbles over the weekend
M3.1 on 8/10/2016 at 11:10 UTC near Fairview
M3.8 on 9/10/2016 at 12:23 UTC near Pawnee
We caught the M3.8 quite well ... some technical gremlins earlier on at theweekend so not quite sure about the M3.1
It looks as thoiugh our trusty comparison site MAOK has disappeared from the net so using WMOK instead (Wichita Mountains)
Looking forward to seeing if tonight's Presidential debate gets picked up seismically as people leap to their feet in admiration of their preferred candidate (or throw heavy objects at their TV's)
