Another Stoke-on-Trent earthquake, magnitude 1.8 occurred yesterday (January 19, 2012) provisionally located near the village of Bagnall, similar to the earthquake on January 11. Above is the recording from Keele University.

If the location of these two events remain this far east, I may have to revise my initial assessment that they are coal mining related since this area is outside the Coal Measures subcrop.

Recent Stoke-on-Trent Earthquakes

View Stoke Quakes in a larger map

I think that the official location, near Light Oaks, is a bit too far east as it is almost certainly a former coal mining induced event and the main coal measure sequence is further west. It will be interesting to find out where the felt reports come from.

We haven’t had earthquakes in the north of the city since the Smallthorne sequence of 1988-1990.

It is interesting that we have coal mining induced events of this size and nobody turns a hair, however, if it had been shale gas extraction related, the world would be about to end.

Just back from the field to find that a large earthquake has hit near Van, Turkey. Above is the record from Keele University, UK

More details from the USGS

After a large earthquake of the other side of the world, this time an earthquake a little closer to home with a magnitude 5.3 tremor occurring in the Western Mediterranean Sea between France and Corsica. There seems to be not much information on this ‘quake. The EMSC indicate that it has a thrust fault mechanism with a northeast-southwest strike. It also appears to be in a region of little recent seismicity.

This event needed a bit of high-pass filtering to get it to stand out from the background noise but the result is clear enough to identify the P-, S-, and Rayleigh wave arrivals and obtain an epicentral distance of about 11-12° from the travel-time curves. Events closer than about 30° tend to have complex waveforms as they are travelling mostly through heterogeneous crust rather than homogeneous mantle.

More information:
USGS
EMSC
Earthquake-Report

This is the recording of the large Kermadec Islands earthquake of June 6, 2011, recorded on our schools seismology SEP seismometer here at Keele University in the UK.

Being at an epicentral distance of 155.3° from the epicentre, the first arrival is the PP phase as the direct P-wave is obstructed by the core.

The event is somewhat unusual as one would normally expect an earthquake of this magnitude along a subduction system to be a low-angle thrust event along the plate boundary between the Pacific and Australian Plates dipping gently to the west. However, the CMT focal mechanism determined is one of a normal fault. This event is right on the trench so it was possibly caused as the subducting Pacific Plate is forced to flex in order to subduct beneath the Australian Plate. The historical data show a magnitude 6.6 event from 2008, also on the position of the trench with a similar mechanism.

The ‘quake seems to have caused a ~1m tsunami wave locally but a more regional tsunami alert was cancelled.

Further details from the USGS.

A small M1.7 earthquake this morning, felt in Stoke-on-Trent, recorded here at Keele University.

We also had a Guralp 6TD recording and its record is here (with thanks to Sam Toon).

Information from the British Geological Survey.

Magnitude 6.8 earthquake in Myanmar (Burma) recorded at Keele UK.

More details from the USGS.

Keele’s seismometer is part of the KAP-SEIS network, and part of the UK Schools Seismology project.

Here is the seismogram of the M8.9 Japan (Near East Coast Honshu) earthquake that occurred at 05:46:23 UTC today 11/03/2011 recorded at Keele University, Staffordshire, UK. Note that even at a quarter of the way round the world from the epicentre our seismogram clips during the surface wave train.

More details from the USGS here.

I’m going to go back nearly thirty years for this one, to when I was looking for a PhD topic. In the UK in those days geology departments produced lists of potential topics which were submitted to the Natural Environment Research Council (NERC) for approval. NERC would then provide a list of approved topics and a number of grants that they were willing to support, which was fewer than the number of topics approved. As a undergraduate student, one would apply for a particular topic, not knowing if it was actually going to be funded.

I studied for a degree in geology at Imperial College specialising in geophysics in my final year. In the list of approved PhD topics for University College, Cardiff (now Cardiff University) there was a project on “Geophysical Investigation of Small Scale Geological Problems”. This was ideal for me and I applied, only to receive a reply stating that this was one of the projects that they would probably not fund, but would I be interested in applying for a project on “Deep Earthquakes” instead. To be honest, I wasn’t that keen as I was more interested in doing practical, hands on, field geophysics but in order to keep my options open I replied saying yes. In retrospect, I should have gone for Cardiff’s other funded project which involved collecting and analysing palaeomagnetic samples from New Zealand, a place I’d still love to visit. However, after being rejected by Cambridge and Durham for other potential projects I travelled down to Cardiff for interview and to my surprise was awarded the PhD grant. I now had to find out about how deep earthquakes happened, something I had not heard about, certainly surprised me, and still baffles me somewhat today.

Shallow earthquakes are relatively straight forward. Stress builds up in a block of rock containing a fracture, whose two sides are held together by friction. Eventually the stress overcomes the friction and the two sides either side of the planar fracture move past each other, releasing the energy that had been stored previously as elastic deformation in the rock mass neighbouring the fault. The system of forces acting at the source is well known and described as a “double couple”. This gives (amongst other things) the classic quadrifoliate pattern of compressional and dilatational P-waves which can be used to work out the orientation of the fault causing the earthquake.

The model is fine for shallow situations where the rocks are brittle. However, the temperature increases by about 30° C for every kilometre you go down. In areas like California where heat flow is moderately high, by the time you get to about 15 kilometres down the rocks are too soft to deform in a brittle fashion and instead flow plastically. In intraplate areas like the UK where the heat flow is less, the brittle-ductile transition is just over 20 km.

In the shallow part of subduction zones, cold lithosphere is subducted and so cold, brittle material can be found at larger depths and earthquakes within the upper parts of the Wadati-Benioff are still explainable. Earthquakes in subduction zones tend to tail off around 200km down but in some start to reappear around 400 km and continue to the bottom of the upper mantle at 660km. It is these deep earthquakes that are particularly problematic as even in the heart of the subducted slab temperature will be well above the brittle-ductile transition. Brittle shear earthquakes simply can’t occur at these depths.

The depths at which the earthquakes switch off, on and off again (220, 410, and 660km) appear to correspond to changes in mineral structure due to the increased temperature and pressure with depth so this would strongly implicate mineral phase changes in deep earthquake sources. Down to 220 km the mantle is olivine + pyroxene [enstatite], from 220 – 410 km olivine + garnet [pyrope]. At 410 km olivine transforms to ringwoodite [the spinel form of olivine]. At the lower-upper mantle boundary at 660 km there is a further phase change to silicate perovskite (Mg,Fe,Al)SiO₃. Since the lower mantle comprises 55% of the Earth by volume this makes silicate perovskite the Earth’s most abundant mineral.

The task of my PhD was to investigate the source mechanics of deep earthquake to see if they had the same double-couple mechanism as the shallow earthquakes or whether they had a non-double-couple source and, due to mantle phase changes, to see if there was a volumetric change component to the source.

Most earthquake source mechanism determination was in those days, and still is today, undertaken using inversion methods which produce a best fit between the observed seismograms and synthetic ones calculated by computer. Some of the inversion solutions for large deep earthquakes had shown a significant non-double-couple component to their source, suggesting that the earthquakes may not be due to planar shear.

The technique that I was to use involved forward, rather than inverse modelling. It had been devised by my PhD supervisor for discriminating between natural earthquakes and underground nuclear explosions. By making measurements of the amplitudes of certain phases (P, S and the near-surface reflections pP and sP) with error bounds, all the possible source types and orientations of those sources that are compatible with those observations are saved. By combining the compatible sources for earthquake records from several recording stations you end up (hopefully) with only a very small range of possible sources and orientations.

The image in the header shows P, pP and sP phases together with a rare S-to-P conversion from the lower-upper mantle boundary S₆₆₀P

It should be remembered at this time that digital waveform data was in its infancy and my earthquake seismograms were taken from analogue records. These were kept on microfiche in the basement of the British Geological Survey in Edinburgh (next to the core store so it was always cold, even in summer). The correct microfiche had to be found for the right day, at the right time (decoding the time pips by eye – these are the white blobs next to the seismogram) at the right station, for the right component. The record was then blown up on a fiche reader and printed on a wet photocopier (so one finished up the day reeking of petroleum spirt). The earthquake data was also a minimum of two years old as it took that long to collect the records from the recording stations, copy them to microfiche and then distribute those copies. The contrast to today when I can watch earthquake seismograms from around the world on my computer desktop, pretty much in real time, could not be more stark.

Anyway, the upshot of the study was that I never found a deep earthquake that could not be explained with a non-double-couple or any significant volume change. They were all double couple, exactly like the shallow ones. Those earthquakes that had inversion mechanisms suggesting non-double-couple sources on closer examination of the actual seismograms revealed that they were actually multiple earthquakes with significant pulses of energy several seconds apart. I could analyse these sub-events and found that they were double couple but with slightly different orientations. This multiple shearing with different orientations was confusing the inversion solutions which at the time only used long period waveforms and could not distinguish the sub-events giving apparent non-double-couple solutions.

I could also use a similar method using the phase pulse lengths instead of the amplitudes to estimate the speed of the rupture and shape of the rupture surface. The up-going phases are much higher frequency than the down-going ones, showing a strong doppler shift. This means that the ruptures are travelling very fast, certainly faster than the speed of shear waves through the material. This is something different to shallow earthquakes, where rupture velocities supersonic to S-wave velocity are impossible.

So we have a geological conundrum. How is a material that should flow plastically accumulating enough stress to generate a magnitude 8.3 earthquake such as the one that occurred on June 9, 1994 636km beneath Bolivia and generates a shear mechanism indistinguishable from a shallow earthquake (other than perhaps by rupture velocity)?

There has to be some processes (probably involving mineral phase changes) that can cause some shear instability runaway condition that generates a supershear, rupturing at fast velocities generating deep earthquakes in a plastic material. What that process is uncertain, and something we may never know.

This work is over 20 years old and I’ve not been keeping up with the literature, so apologies if some/all of this is out of date. This was never written up other than in the PhD Thesis which maybe still available from the University of Wales