Accretionary Wedge #25 : Images

I’m late with last month’s accretionary wedge on images hosted at Highly Allochtonous (I’m writing this post between chairing exam boards).

As a quickie, I’m going to nick Clastic Detrius‘ idea and use my blog masthead image.

It is a seismogram from my PhD thesis. It comes from the pre-digital era where the original seismograms were recorded on light sensitive paper, a day at a time. These were then copied to microfiche and the copies sent round the world to a number of libraries. To get a record one had to travel to the library, find the set of microfiches for that day, then the station you wanted, then the fiche for the long period vertical component. You then had to load it into a microfiche reader and decode the time by eye. Each dot above the trace represents a minute, longer dashes represented hours (with some missing so you could work out what hour it was!). When you had found the right part of the record you then took a copy of the image. The system in the British Geological Survey in Edinburgh used a liquid petroleum based copier and after a day working in the microfiche library one reeked of petroleum vapour. It took about two years between the earthquake happening and the microfiches being lodged in the library. The contrast between 25 years ago when I had to travel from Cardiff to Edinburgh, look up at least two year old records by hand, copy them, bring them back to Cardiff and then digitise them by hand using software that I had written myself on an Apple IIe – and today when I can watch earthquake waves arrive in realtime from around the world sat at my computer screen – is staggering.

The long-period, vertical component seismogram in the image is from a mb 6.0 earthquake on September 21, 1981 274 km beneath the Kermadec Islands in the Pacific recorded at Adelaide, Australia. The direct P-wave arrives just before the second minute mark. A minute later a second downward kick indicates the arrival of pP, a wave that leaves the earthquake upwards as a P-wave, gets reflected back down from the surface near the epicentre and then travels back down through the mantle. The large arrival on the right-hand side of the image is sP. This leaves the source as an S-wave upwards and at the surface undergoes a phase conversion to a P-wave as well as a reflection and continues back down through the mantle as a P-wave. The interesting arrival here to me is the high frequency one between pP and sP which doesn’t appear on the standard travel-time tables. I tracked it down to being S670P, a wave that leaves the source downwards as an S-wave, hits the upper/lower mantle boundary at 670km depth and part converts to a P-wave that then travels on downwards through the mantle. A few years later I had a PhD student do her whole thesis on S670P, but it all started with this strange pulse on this seismogram.

I’m a great believer in observational seismology, actually looking at earthquake records rather than just pumping seismograms through big inversion programs. It is by looking at things closely and recognising when something strange is occurring that science advances.

Accretionary Wedge #24 : Heroes

This month’s accretionary wedge being hosted by Callan at Mountain Beltway is on the topic of heroes. There were several people that I could have written about, but in the end I felt had to go with a seismologist, so I have chosen Inge Lehmann.

There are many reasons why she is a hero, the discovery of the inner-outer core boundary and the 220km mantle discontinuity, her commitment to detailed, observational seismology, the difficulties she had to combat during the course of her career, being a woman who strived to established a scientific reputation in the first half of the twentieth century, but the thing that has always impressed me the most was the pure simplicity of the title of the paper in which the inner core boundary was described. These days there would be a long-winded title and about seventeen authors and a front cover of Nature. Her paper was simply entitled P’. That’s it, one letter and a punctuation mark (pronounced P-Prime). P’ is now better known as PKIKP, a P-wave traversing the mantle (P), outer core (K), inner core (I) and back out through outer core (K) and mantle (P).

Inge Lehmann
Inge Lehmann was born in Østerbro, Denmark on May 13, 1888. Her early education was at a school where boys and girls all did the same subjects, including rugby and needlepoint. She went to the University of Copenhagen to study mathematics and after passing her initial examinations went to University of Cambridge for a year. However, she burnt out from the pressures put on her to qualify for the mathematics degree course, and she returned to Denmark. She couldn’t face going back to university for a while and worked in an actuarial office for several years, gaining computational experience. Eventually, she returned to the University of Copenhagen in 1918 and attained a master’s degree in 1920.

Her seismology career started in 1925 helping to establishing seismic networks in Denmark and Greenland. In 1928 she was appointed as the first chief of seismology in the Royal Danish Geodetic Institute, a post she held until her retirement in 1953. She analysed and catalogued seismograms from Denmark and Greenland. She realised that the determination of epicentral data was not reliable and she correlated waveforms by eye to produce more robust interpretations. It was from this careful observation of seismic waveforms that she recognised that a distinct inner core was required to explain the interpretation on certain phases recorded at large epicentral distances.

Figure from Lehmann I, P', Bureau Central Seismologique International, Series A, Travaux Scientifiques, 14, 88, 1936.
The P’ paper was published in 1936, describing the inner-outer core boundary, now known as the Lehmann Discontinuity, and how the observed seismic waves were not the consequence of a diffraction, which was the accepted wisdom at the time.

In her later career she spent much of her time as a visiting scientist at Lamont-Doherty, Dominion Observatory, Caltech and Berkeley. She became an acknowledged authority on the structure of the upper mantle. Again by carefully studying the arrival times of certain earthquake phases, she proposed a sharp increase in seismic velocity at 220km and discovered a regional variability in the 400 km discontinuity. In fact, the 220km boundary was informally named the Lehmann discontinuity soon after its discovery, which almost precluded the formal naming of the more significant inner-outer core boundary after her in her centennial year.

Inge Lehmann died on February 21, 1993 at an age of almost 105. Shortly before her death she said that she had looked back on her life and ‘she was content. It had been a long and rich life full of victories and good memories’.


Bolt, B.

Carlowicz, M. Inge Lehmann (1888-1993)

Lehmann I, P’, Bureau Central Seismologique International, Series A, Travaux Scientifiques, 14, 88, 1936.

Accretionary Wedge #21 : Reaching out to someone

Jess at Magma Cum Laude is hosting this month’s Accretionary Wedge, asking “What kind of Earth Science outreach have you participated in?”

Actually, there is quite a lot of outreach that I do but one thing that I’ve actually referred to in some of my recent posts but realise that I’ve not explained, is the “Seismology for Schools” project.

kap-seis logo

The UK Schools Seismology project is coordinated by the British Geological Survey (BGS) and involves the donation of seismometers to schools to help promote geology and geophysics in the classroom. The BGS acts a hub organisation and then university geology departments (currently eighteen of them) act as mentors to schools in their region. I look after the schools in the northwest Midlands area with a group I dub “Keele and Partners – Seismology (or KAP-SEIS).

The project is nobly currently supported by the Petroleum Exploration Society of Great Britain (PESGB), Scottish Oil Club (SOC) and the British Geophysical Association (BGA). The money that the PESGB and SOC generously give often allows the universities to get match funding that increases the number of seismometers that we can donate. This year 118 seismometers will be given to UK schools. School staff are given training at a university, showing them how to set up the seismometer, how to analyse the data and how to use it in classroom teaching. We then provide back-up support and mentoring. (A full list of organisations supporting the project can be found here).

The seismometers involved are traditional horizontal “garden-gate” designs, either the SEP Seismometer System or the Rockwave HS-3. The reason for using these somewhat old fashioned designs is the instruments can also be used for teaching physics principles such as simple harmonic motion, damping and electromagnetic induction. They are also much nicer looking than the tin can packed with electronics that is a modern seismometer.

Examples of the data recorded by my seismometer at Keele can be found in my blog posts here, here, here and here.

The BGS provides web resources that help with earthquake phase arrival identification, earthquake location and also facilitates uploading data to IRIS. This allows data to be shared not just with other schools in the UK but with schools involved in similar programs in Ireland and the United States, and an increasing number of other schools around the world including in Germany, Spain, Switzerland, Namibia and South Africa.

It is a real shame that most earth science in UK schools is not taught by earth scientists (mostly chemists these days) and while their teaching is good it is quite difficult to inspire students in a subject that you have no specialism in (I know that I would have great difficulty teaching chemistry, let alone inspiring young chemists) so it is important that projects like this exist to help inspire the next generation of geophysicists.

I understand that all the seismometers for 2009 have been allocated, I’m about donate latest batch at a training day in December, but we strongly hope the the Petroleum Exploration Society of Great Britain and the Scottish Oil Club can continue their support in 2010 and we can give away even more next year and increase our coverage in areas that are currently not supported.

There are other outreach activities that I’m involved with such as devising geological trails for the public such as this for Cannock Chase and this for the Churnet Valley, both in Staffordshire.

I suppose that even this blog is a form of educational outreach.

[Note: the author is education officer for both the British Geophysical Association and the Staffordshire Regionally Important Geological/Geomorphological Sites group.]

Accretionary Wedge #20: Yet to discover

David Bressan at Cryology and Co is hosting Accretionary Wedge #20 and I’m late with this and don’t really have much time what with the start of teaching next week – sorry.

He asks …

What remains to be discovered for future earth scientists what we (still) don’t know about earth? What are the geological riddles that still lack answer – all questions are allowed – it could be a local anomaly, or a global phenomena, or something strange

Nasa blue marble 2005 photoshopped

I have always used the analogy that geology is like having just a few pieces of a thousand piece jigsaw and being asked what the whole picture is depicting. Each piece has to be examined in minute detail for the smallest clues to where it fits in the main image. It could be an edge or a corner which helps, but most of the time it is a middle part which doesn’t provide that much information. Do you have a piece of the sea or the sky to suggest where it might approximately fit in the bigger picture? Are you looking at the piece upside down? Occasionally you will be given a new piece to look at. If you are really lucky you might find that two pieces actually fit together. The thing is, as geologists, we are never going to get to be given all the pieces. Some have been lost down the back of the sofa for ever, others are unobtainable, never being in the box in the first place.

So what are we left to discover as geologists? Well I think pretty much everything. We have an incredibly young subject. Plate tectonics as a concept is less than fifty years old and it is just a good working hypothesis. It is total arrogance that we know pretty much everything about geology. We might be standing on the shoulders of giants but there will be even bigger titans to come to stand on ours. When they look back to our time they will think we knew virtually nothing.

Accretionary Wedge #19: Yes, we have a banana

It’s accretionary wedge time again an this month Dino Jim is asking us to ‘think outside the box’ when teaching geology. I seem to detect a food theme developing.

Here is a quick piece on using a banana as an analogue for rock deformation in general, and fault propagation folding in particular.

First take your banana and peel it.

Banana: Undeformed

Grasp an end in each hand leaving at least the central third free. Slowly move your hands towards each other.

Initially the banana will deform ductilely, and actually thicken. After the initial thickening, the banana will start to fold.

Banana: Thickening and Fold Initiation

As the fold develops into an anticline-syncline pair, note the extra compression on the inside of the folds generating buckling and extension on the outside of the folds generating tension cracking. If you look closely you can also see shearing starting to develop in the central limb between the two folds.

Banana: Folding developing

Deformation switches from ductile to brittle as shear failure through the central limb generates a thrust fault separating the hangingwall anticline from the footwall syncline.

Banana: fault propagation fold

And here is the real thing for comparison…

Broadhaven, Pembrokeshire fault propagation fold

You can see a gigapan and photosynth version of this structure in my previous blog post here.

Note: this isn’t my idea, I picked it up from Prof. Patrick James, Head of the School of Natural and Built Environments at the University of South Australia at a teaching and learning in geology conference.