Italian terremoto

A busy week in Italy with lots of earthquakes... the biggest was this morning at M6.5-6.6... early indications are that although many buildings were damaged most people had already evacuated last week because of smaller earthquakes

Italian M6.5 earthquake recorded in Rutland (top) on MAOK school sensor in Oklahoma (middle) and USGS professional site WMOK (bottom) also a M3.8OK earthquake this morning

The WMOK recording is very interesting because it shows big and very low frequency surface waves arriving about 30 minutes after the first shock.  If you look carefully from 07:20 to 07:35 you can see the frequency content of the signal changing(the wiggles get closer together).   This is because lower frequency seismic waves travel faster (they have longer wavelengths so travel deeper in the earth where pressures are greater)

Soundified

 https://drive.google.com/file/d/0B0ITK4QvKwjuZUN3c1BXVlBrOTRmZFJfbDRHV3J6LUYzTC04/view?usp=sharing  

if we speed up the seismogram recorded at WMOK and play it as an audio file you can hear these changes in frequency content as changes in pitch.

Procure shakeout

Earthquake drill for patients and families  at the Procure centre Oklahoma 

I think this lot will be pretty safe from earthquakes now !

Shakeout 2016

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.



dare to prepare public service film 

Some ideas for shakeout actvities

http://www.shakeout.org/centralus/resources/

A tale of two quakes

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)

Inside a modern "broadband" seismic sensor 

Inside a geophone 

 Note the geophone costs less than $100 and the broadband seismometer costs over $10,000 (one for the Christmas list maybe ?) 

Is it OK fracking ?

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.  

Scientific American Article   https://www.scientificamerican.com/article/drilling-for-earthquakes/

Lots more information on induced sesmicity is available at the USGS website

Frequently asked questions   https://www2.usgs.gov/faq/taxonomy/term/9833   
Myths about induced earthquakes https://earthquake.usgs.gov/research/induced/myths.php

A technical paper on the Oklahoma earthquakes (Science journal)

Sharp increase in central Oklahoma seismicity since 2008 induced by massive wastewater injection

 

Weekend wobbles

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)

New Madrid

In 1811-1812 a series of large intra-plate earthquakes struck near New Madrid in the Mississippi valley, the largest were estimated to be between M7 and M8  (no instruments were around at the time so magnitude estimates are based on contemporary felt effect reports).   The largest of these were probably bigger than the famous 1906 San Francisco earthquake.    Since this time the New Madrid Seismic Zone has been quietening down but still has a much higher level of seismicity than most places in the USA (until 2009 the New Madrid seismic zone was the most seismically active area of the USA east of the Rockies, since then it has been overtaken by Oklahoma)   Scientists estimate that such large earthquakes probably occur in this region every 500 years or so.

 Some cool facts about the New Madrid earthquake zone 

But where are the plates ?

Most people in the UK would struggle to place Oklahoma on a map of the USA, however cultural references from cowboys to Steinbeck would ensure that most placed it somewhere in the middle, a long way from the west coast and California.

 That causes a problem for geoscientists since we have spent the last 40 years or so trying to teach schoolboys like Ben that earthquakes are caused by plates moving, and that earthquakes occur wheer these plates meet.

Where are all the plates ? 

In the USA there is a clear tectonic plate boundary running all the way down the west coast, where eth North American plate meets the Pacific plate.   This explains earthquakes in California and volcanoes like Mount St-Helens.    In fact Plate Tectonics  is such a good theory that over 90% of the worlds can be explained with a relatively simple model of how the planet works.

Here is great video by IRIS about plate tectonics


However the world is a much more complicated place than any simple model can explain and we are left with a problem of trying to explain all the other earthquakes in the world.   The UK, like Oklahoma is a thousand km from the nearest tectonic plate boundary,  up until 2009 the UK and Oklahoma had very similar earthquake patterns, a few small earthquakes up to M3 each year and the occasional larger one.   In the UK the largest one we know about is M6.1 in 1931  .    Seismologists call these earthquakes intra-plate earthquakes (meaning they happen in the middle of plate rather than at the edge).   The only explanation that we can give for these earthquakes is that the tectonic plates are not uniform rigid blocks but a complicated mix of older rocks all squashed together by past tectonic plate motions.   This results in plates containing lots of old cracks and fractures some of which can be reactiviated by the present day stresses of eth plates moving.

Tuesdays tremors

A flurry of small tremors to start the week

6 small ones

We caught two of these , the smallest we detected was 2.2 in Edmond... quite close but also coincided with a quiet time in the apartment (01:09 UTC)

And a much bigger M3.5 at 09:10UTC (further away at Pawnee again) 

The big 6

Another M3 earthquake last night  (M3.0 05:45UTC 180km NW
of OK city),  a quick review of what our tiny geophone has picked up in the past couple of weeks.

Looking at some of these all on the same scale we start to see some patterns  ... most of these earthquakes are happeneing 100-200km away, at these distances seismic signals start to have a distinct pattern with P and S waves arriving a different times ... the delay being because P waves travel faster than S waves ... at 150km this gives a delay of about 20 seconds ...  

California rocks again

The last week or two have seen a sudden increase in earthquakes in California.   For the first time in several years California is once more having more eathquakes than Oklahoma.

13 OK earthquakes >M2.5 in past 7 days

48 CA earthquakes >M2.5 in past 7 days

While this is  worrying for people living in California it is somehow reassuring for a geoscientist.   Earthquakes are supposed to happen in California, that is what we have been teaching kids for the past 40 years or so since plate tectonics was discovered.

If we look in a bit more detail (USGS Earthquakes website is great for doing this )  we see that the vast majority of these Californian earthquakes have happened in a place called Bombay beach and are associated with a NE-SW trending fault under the lake.

Most earthquakes happening at Bombay beach on a NE-SW fault

This fault is related to the great San-Andreas fault system which runs through Calfornia, so people are inevitably asking ... is this the precursor to "THE BIG ONE", which in California means an earthquake >M7  which is likely to happen every 10 years or so (that Gutenberg-Richter relatioinship again) .   Unfortunately such swarms of small earthquakes are just as likely to not be followed by a big earthquake as to be the precursor, this particlur fault at has a history of producing small swarms with no big one following but that does not mean this will be the case now.  Seismologists in California  try to adjust their short term risk forecasts to take account of such swarms maybe being a precursor but the overall risk is still low (less than 1% ) so not  a usable piece of information to help people in their day to day lives.

Why worry more about earthquakes in California than Oklahoma ?

It is all to do with maximum likely earthquake magnitude and fault lengths.    Earthquakes start to become dangerous once their magnitude goes over M5, once they reach M7 they can be devastating.   California, sitting as it does on a tectonic plate boundary has a massive fault system running through it (the San Andreas) which is thousands of km long.   It has a history of M7+ earthquakes (6 in the last 40 years) and there is a possibility that they could reach M8 if the whole San Andreas fault slips at once.  

Fault length is the reason.  The thing that really decides how big an earthquake is is the total size of  the fault that ruptures (well really fault area x slip length)  this leads to another useful rule of thumb (seismologists seem to have a lot of thumbs !)

Empirical (fancy name for rule of thumb) relationship between earthquake magnitude and rupture length

      So a M4 earthquake ruptures a fault less than 1km long  whereas a M7 earthquake ruptures a fault about 100km long.   Or from a geoscientists point of view you need to have a pre-existing fault system at least 100km long to enable a M7 earthquake to happen.   There are plenty of these in California but not so many in Oklahoma (or they are much older and well buried).   So in California we are pretty much guaranteed to have an M7+ earthquake in the next twenty years or so,   maybe the next one will be preceded by a swarm of smaller earthquakes or maybe it will happen out of the blue.  No wonder seismologists are starting to pay lots of attention to California at the moment.   

What are the chances ?

Ben has been monitoring earthquakes in Oklahoma for about 10 days now ... in that time there have been about 8 earthquakes > M3   and about half of these have been registered on our trusty geophone.

(seismologists look away ! our window-sill sensor on the third floor !)

  But how long will Ben have to wait until he feels an earthquake ?   Chatting to people in the hotel it seems that  M5+ events even 160+ km away cause shaking strong enough to be easily felt.

It turns out that there is a pretty good rule of thumb that seismologists can use to estimate how frequently earthquakes occur in a given location (seismologists call it the Gutenberg-Richter Law ) It seems that for most places for most of the time earthquake size and frequency of  occurence follow a simple power of 10 rule (roughly).   For every decrease in magnitude of one unit, the number of earthquakes increases by a factor of 10.  So if there have been 8 earthquakes >M3 in the last 10 days there were probably 80 earthquakes >M2  (unfortunately we cannot detect earthquakes this small with our geophone and even the Oklahoma Geological Survey only guarantee measuring earthquakes  >M2.5 )

Looking back over the past year there have been 567 earthquakes >M3 in Oklahoma  37 >M4 and 3 >M5

The past year's earthquakes >M3 in Oklahoma 

(You can have a play at making your own maps at http://ds.iris.edu/ieb/ )

so you would expect there to be an earthquake >M5 every 100 days or so,

But Ben is only planning on being in Oklahoma for 70 days so does this mean he won't feel one ?   Well not necessarily ... earthquakes tend to occur at fairly random times so we need to start thinking about probabilities ...  Time to do some MATHS !!! 

Let's start by thinking about the probability of a M5+ earthquake occuring somehwere in Oklahoma state today as 1 in 100 (or 0.01) .... this means that the probability of an earthquake not occurring to day must be 99 in 100 (or 0.99) .  (It turns out that measuring the probability of an earthquake not occurring is a more useful way at solving this problem) 

Now if we consider the probability that two consecutive days won't have a M5+ earthquake we have to multiply the probability of this happening on each day so the probability is 0.99 x 0.99 = 0.9801 

So the probability that an earthquake not happeneing for 70 consecutive days should be 

0.99 x 0.99 x 0.99 .........   (70 times)   which is 0.99 ^70   = 0.494   i.e. roughly 50%   or 1 in 2 

So if there is a    50 % probability that an earthquake >M5 will not happen for 70 consecutive days then there will also be a 50% probability that there will be an earthquake >M5 occuring in a consecutive 70 day period.   

Midnight tremors!

How a geophone works?

Mind your P's and S's

A pretty impressive M4.3 earthquake hit Medford Oklahoma at about 05:00 UTC today (160km N of Oklahoma City).   It shows up as a massive spike on the MAOK station

M4.3 event about 160km N of Oklahoma City recorded on 

station MAOK (upper) and our geophone site KEYWB(lower) 

On first glance it looks as though this event was missed my our geophone... however the MAOK station clock is running 5 minutes fast so we need to look at the wiggles occurring at 05:00UTC ...just when our geophone  trace skips from one line to the next (in jamaseis, click and drag the bit you want and click on Extract Selection) 

Zooming in on the time around 05:00 our geophone 

shows an impressive double-peaked signal 

The double peak is because earthquakes create two types of waves ... Primary P waves which are longitudinal waves (like sound waves in air) and Secondary S waves which are transverse waves (with a side-side particle motion).  The P waves travel a bit faster than the S waves so there is a time delay between the two sets of waves arriving

Because we know how fast P and S waves travel through the earth we can estimate how far away the event is by lining up P and S wave arrial times with the calculated travel times for P and S waves on a graph (in jamaseis you can do this by clicking the calculate distance tab in the zoomed data window) 

Graph of calculated P and S wave travel times vs distance with our seismogram overlaid, the seismogram is slid around on teh graph until the P and S waves line up with the curves and the distannce can be read off the Y axis.

 This gives us an estimated distance to the event of about 1.5 degrees.  (as every good sailor will tell you 1 degree = 60 nautical miles = approx 111 km (the circumference of the earth/360).)  So about 160km away !  

Reflections on Oklahoma 5+ quake

David: 'was not scared' ....'thought it was his dog'

 

 

Harry: 'picture shaking ...pool sloshing...I didn't know what was going on'

oops missed

Our intermittent 'system down' issue coincided with a shake , this one was before the one Paul reports we did get (post below this)...
Here the media update that alerted us :

 

And on our geophone trace - bottom half of screen - you can see the gap when it was not working is at the same time as the top section, which is the MAOK station, that does capture the quake!

Never mind - but thought we should be honest - we did miss one :-)

Another one hits

Over the weekend the trusty geophone sensor was moved to a new location.   Fortunately another small earthquake came along a few hours later just to reassure us that things were still working.

M2.9 earthquake about 50km from Oklahoma city

 This one was too small to be felt and quite a bit further away.   However a quick check with the Oklahoma Geological Survey seismic station in Oklahoma City, BCOK, shows a clear signal

Professional seismic station (lower figure) indicates where to look to find the seismic wiggle on our geophone (upper figure) 

Having a couple of seismic traces to compare side by side gives us the confidence to be sure that this small wiggle is due to the latest quake (scientists are not keen on the coincidence explanation)

Zooming in to the wiggle and filtering some of the noise away confirms that the geophone is still working in its new location (our geophone above, OGS professional sensor below) 

Got One! Major County gets a 3.5

With a bit of help from Paul and the media we have this one. It showed up ok on our station but not really at all on MAOK.

You might spot that our recording trace line was broken only a couple of hours earlier ...not sure why but we just restarted the system and it all came back on line.  So the problem is not fully diagnosed I guess. Generally its all holding steady. On the trace you can see when we are busy in the room. Tomorrow this station will be relocating to a different windowsill when we move room.

Waiting for a quake!

Like waiting for the wind when you want to go sailing sometimes you have to be patient...
 Here Ben fills some time with the cube and some origami!

 

And then a breakfast discussion with David from the Staybridge Hotel who recalls the last quake they felt and its epicentre orientation versus us! - See the 'Reflections' blog post further up for video from David and others regarding their experiences

 

Online at Last

After some splendid gadgeteering by Steve the roving schoolboy seismologist is now online, streaming live seismic data to the entire interweb.

Live seismic data from Oklahoma.  Top is from amateur site Oklahoma-Shakes, bottom is from Ben's geophone, each line shows 1hour of data wrapping around to the next hour on the next line (8 hours total) time is UTC (or GMT for the oldies)

If you want to track seismic data from Oklahoma from here in the UK these data streams can be viewed online by installing the software jAmaSeis from https://www.iris.edu/hq/inclass/software-web-app/jamaseis

This free software runs on any computer and lets you monitor live data from remote stations.    Choose remote station KEYWB  for Ben's seismic station or MAOK for another station in Oklahoma  in the source configuration wizard (MAOK is a bit quieter than Ben's as it located away from people on a ground-floor location)

HINT you can adjust the amplitude of traces by clicking on the SCALE button (bottom right) for each trace 

The first earthquake !

Well that was unexpected... within a couple of hours of setting up the geophone in Ben's room there was a magnitude 3.2 earthquake in Edmond (a suburb of Oklahoma city about 20km away)

Fortunately we were able to view the signal from this earthquake on a couple of professional seismic stations run by the Oklahoma geological survey in Oklahoma city.

M3.2 earthquake recorded on professional stations in Oklahoma city

the same quake captured on Ben's geophone

It might look small but the time and shape of this wiggle confirm it really is our first earthquake 

Given that the geophone was stuck to a windowsill with bluetack on the 4th floor I think that this counts as a remarkable success !

Day One: setting up the sensor

After arriving in Oklahoma and checking into their accommodation Ben and his father, Steve were keen to start seismology.    They had some basic equipment on loan from the UK school seismology project.   A laptop, running jAmaseis (free datalogging and analysis software designed for use in schools), an old geophone (a sensor which converts ground vibrations into voltages) an old digitser (to convert the voltages into computer readable numbers) and a whole mess of cables.

 

During a  late night skype call to Paul Denton, (leader of the school seismology project at the BGS and also a resident of Oakham) Ben gets stuck into the technical details of setting up his own monitoring station.    Lessons learned 

1) cable spaghetti not only looks messy but often stops things working.

2) if things are not working for no apparent reason, switch the computer off and on again... it really works ! 

 

Finally success... a jump test confirms the sensor is working.

 

All we need to do now is wait for an earthquake !