Tectonics of the M7 earthquake near Christchurch, New Zealand

Posted on September 3, 2010 by Chris Rowan

A post by Chris RowanThe South Island of New Zealand has just been shaken by a large earthquake, reported as a magnitude 7.0 by the USGS. It appears to be quite a shallow rupture, on the Canterbury Plains close to Christchurch, and the focal mechanism indicates largely strike-slip motion.

Focal mechanism of September 3rd earthquake, and it's location with respect to the plate boundary in New Zealand

As the figure above illustrates, New Zealand is not just located on top of the boundary between the Pacific and Australian plates: it is located at a point where the nature of that plate boundary changes in some rather fundamental ways. The subduction zone running down the East Coast of the North Island terminates off the Northeast coast of the South Island, about 100 kilometres north of Christchurch, and gives way to a transform boundary cutting through the continental crust of the South Island, where the plate motions are accommodated by largely dextral strike-slip on the faults of the Marlborough Fault Zone (MFZ in the figure above) and the Alpine Fault (AP). Whilst this latest rupture clearly occurred some way south of both of these fault systems, the focal mechanism can be interpreted as showing as dextral strike-slip on an east-west oriented fault, suggesting that it is still linked to deformation at the plate boundary.

New Zealand is a region of distributed deformation: the relative motions between the Australian and Pacific plates are not accommodated on one or two faults in a narrow zone, but on many faults across a much wider zone. It is therefore perhaps not surprising to observe large earthquakes accommodating plate motions some distance from where the two plates actually meet. However, the occurrence of such earthquakes in this particular region of the South Island is probably also linked to ongoing changes in the nature of the plate boundary at the junction between the subduction zone and the continental transform. If you look at the displacement history of the individual faults in the Marlborough Fault zone, the northern faults are older, were more active in the geological past, and have quite small recent (in the geological sense of ‘the last few 100,000 years’) displacements; the southern faults are younger, and have much larger recent displacements. The most obvious explanation for these changes is that the most northern of the Marlborough faults was originally directly linked with the end of the subduction zone, but that these two structures moved out of alignment as the subduction zone moved south, causing new strands of the Marlborough Fault system to grow in order to more efficiently accommodate plate motions.

Growth of new plate boundary faults on the South Island of New Zealand in response to southward propagation of the subduction zone

This tectonic evolution is ongoing, and since the end of the subduction zone is now actually to the south of the southernmost and youngest of the Marlborough faults. Some of the plate boundary deformation is probably therefore being shunted into the region around Christchurch, where it needs to be accommodated by dextral strike-slip faulting. Eventually, over geological time, this deformation will lead to the formation of a new, more southerly strand of the Marlborough Fault system. It also means that earthquakes of this type of size are unlikely to be a one-off event in this area. Unsurprisingly, then, seismic surveys have identified a number of active faults beneath the recent sedimentary cover on the Canterbury plains (although they were identified in the linked study as reverse faults accommodating compression, strike-slip deformation is very difficult to identify if you only have a 2 dimensional cross section to work with).

Whilst this map of large historic earthquakes in New Zealand shows that earthquakes of this magnitude can occur pretty much anywhere in New Zealand, seismicity in this particular area has some particular hazards; it is close to a heavily populated region (Christchurch) built mainly on unconsolidated Quaternary sediments, which will intensify the potential shaking and damage to unreinforced buildings. Fortunately, whilst this earthquake appears to have caused a fair amount of damage, from the early reports casualties seem to be light.

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Categories: earthquakes, focal mechanisms, geohazards, tectonics

Diversity in the geosciences and the impact of social media

Posted on September 1, 2010 by Anne Jefferson

A post by Anne JeffersonResearchBlogging.orgOne year ago, Kim Hannula, Pat Campbell, Suzanne Franks, and I launched a survey about women geoscientists reading and writing in the blogosphere. We presented the results at the Geological Society of America meeting, and Kim wrote a great post summarizing and discussing our data. Then I took Kim’s post, polished it up with great wording and thinking suggestions from all of the co-authors and submitted it for publication. It went out to reviewers and a few months later, we were accepted for publication.

In the September issue of GSA Today, you can find our article on The Internet as a resource and support network for diverse geoscientists. We wrote the article with with the idea of reaching beyond the audience that already reads blogs (or attends education/diversity sessions at GSA), with the view that we might be able to open some eyes as to why time spent on-line reading and writing blogs and participating in Twitter might be a valuable thing for geoscientists to be doing. And, of course, we had some data to support our assertions.

GSA Today is an open-access journal, so everyone can and should go ahead and read the whole 2-page paper. But if you want a few highlights, here are some selections from the paper:

The online opportunities for mentoring, networking, and knowledge sharing may be particularly valuable for women and minority geoscientists. Virtual networks offer opportunities to provide support and reduce the professional isolation that can be felt in physical work environments where there are few colleagues of a similar gender, race, or ethnicity. …

Women reported professional and social benefits from reading blogs. We used a five-point scale (1: strongly agree; 3: neutral; 5: strongly disagree) to assess perceived benefits. Of the professional benefits, respondents were most positive about learning things outside their specialty (avg. 1.9), followed by learning within their specialty (avg. 2.3), learning about pedagogy (avg. 2.4), and learning about technology (avg. 2.5). Based on these responses, we conclude that these women blog readers perceive positive professional benefits from their online reading. This suggests that social and other online media could be strategically used to supplement the resources available to all geoscientists, regardless of their gender, ethnicity, geographic location, or employment status. …

Geoscience students perceived the strongest benefits from blog reading, while faculty most strongly agreed that blogs helped them find role models and normalize their experience by finding that many other faculty share their experiences and perspectives. Women in industry perceived less social benefit from blog reading than those in academia, but women in government were the most negative about their blog-reading experiences. In particular, their responses indicated that blog reading had not been helpful to them in finding role models. …

Blogs and other social media may provide a source of community and role models for women geoscientists and help in the recruitment and retention of women from undergraduate education to faculty or industry careers. Our survey results show that blogs are already providing valuable benefits to white, academic women geoscientists, but that existing social media networks could be doing a better job of supporting minority geoscientists and those outside academia. We believe that professional societies, employers, funding agencies, and individual geoscientists should recognize the potential value of social media for supporting a diverse geoscience community. To be effective, such recognition should be accompanied by policies that encourage geoscientists to actively participate in geoscience-related social media opportunities. …

As a white woman geoscientist in academia, I have definitely personally and professionally benefited from my blog reading and writing time. (I even have a publication to show for it!) But I would to love to hear more from minority and outside-of-academia geoscientists about what blogs, Twitter, and other internet-based forms of support could be doing to better support you. As you can see from the paragraph above, what we ended up advocating was that institutional support for blogging and blog-reading would help increase participation. We thought that, with increased participation, more minority and outside-of-academia geosciences voices would emerge, helping others find support, community, role models, and mentoring in voices similar to their own. Meanwhile those of us closer to the white/academic end of the spectrum could learn from all that a diverse geoscientist community has to offer.

One final note, I’m a newbie member of the Diversity in the Geosciences committee for the Geological Society of America. If you have ideas for how GSA could be doing a better job of promoting and supporting diversity off-line and/or on-line, please let me know.

Jefferson, A.J., Hannula, K.A., Campbell, P.B., & Franks, S.E. (2010). The Internet as a resource and support network for diverse geoscientists GSA Today, 20 (9), 59-61 : 10.1130/GSATG91GW.1

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Categories: bloggery, by Anne, publication

Stuff we linked to on Twitter last week

Posted on August 29, 2010 by Chris Rowan

A post by Chris RowanA post by Anne JeffersonAnother week, another link extravaganza for you to enjoy.

Blogs in motion

The National Ground Water Association joins the blogosphere with its creatively named NGWA Blog (http://info.ngwa.org/blog). Early posts suggest the blog will focus on the practical aspects of the hydrogeology profession.

Volcanoes, Earthquakes, Tectonics

Planets

Environmental

Water

(Paleo)climate

Fossils

General Geology

Interesting Miscellaney

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Categories: links

Friday Focal Mechanism

Posted on August 27, 2010 by Chris Rowan

A post by Chris RowanThe earthquake that particularly caught my eye this week occurred on Tuesday off the coast of Mexico:

Tuesday 24 August: M 6.1, off W Coast of Mexico, Depth 11 km

This focal mechanism is pure strike-slip: that is, it is the result of two sides of a fault moving laterally past each other, with no real compression or extension involved. A tectonic map of the area shows that it occured on the Rivera Fracture zone, a transform segment of the northern East Pacific Rise, just before it terminates in the Gulf of California. The dextral, or right-lateral sense of motion is consistent with the fracture zone being a tectonic link between different spreading segments of the ridge system, a concept that I have discussed before, regarding a similar earthquake in Iceland.

Plate boundaries off the west coast of Mexico

The other interesting thing about this area is that the East Pacific Rise is very close to the subduction boundary on the Mexican coast, and continuing eastward subduction means that the distance between them is shrinking all the (geological) time. The ridge segment at the eastern end of the Rivera Fracture zone is extremely close to being subducted, pretty much cutting off the section of oceanic crust to the northwest from the Cocos Plate to the southeast. In fact, deformation along the zones marked with the purple dotted lines mean that this area is effectively moving independently of the rest of the Cocos Plate, and can therefore be regarded as a separate plate: the Rivera Plate. This is what happens when a ridge gets subducted: large transform offsets eventually become the boundaries of little plates strung along the subduction zone, as the ridge segments that they originally linked to get subducted.

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Categories: earthquakes, focal mechanisms, tectonics

Yellowstone: what lies beneath

Posted on August 26, 2010 by Chris Rowan

A post by Chris RowanResearchBlogging.orgThe Yellowstone caldera is located over a ‘hotspot’, where volcanism – and in the case of Yellowstone, hydrothermal activity – is occurring far away from the plate boundaries where such things are normally found.

Plume above....plume below? Plume geyser in Yellowstone's Upper Geyser Basin. Photo: Chris Rowan 2010.

Hotspots are often linked to mantle plumes: if this is the case, the anomalous volcanism is the result of hot, buoyant rock from deep in the Earth rising up through the mantle to the base of the crust, and melting as it enters regions of (relatively) low pressure. As you may or may not be aware, there is a vigorous and long-running debate over whether this link is justified in all cases, with some arguing that mantle plumes may not exist at all (those interested in the gory details should check out mantleplumes.org). In the case of the Yellowstone hotspot, the linear hotspot track marked out by the chain of calderas along the Snake River Plain suggests that the hotspot remains stationary with respect to the North American plate moving over it, as you would expect if it was coming from deeper in the mantle. The oldest caldera on the track was active 16-17 million years ago, and is located in the same region as the Columbia River flood basalts, which also first started erupting about 17 million years ago; several other hotspots are also associated with flood basalts in this way, and they are interpreted as recording the moment when the voluminous head of an upwelling plume first reaches the base of the crust, triggering a large amount of melting and sending huge volumes of magma to the surface. It is possible that the Yellowstone hotspot is similarly connected to the Columbia River Basalts.

Geographical association between the earliest part of the Yellowstone hotspot path, and the Columbia River Basalts.

These features are suggestive of a possible plume origin for the Yellowstone hotspot but are far from definitive. In the latest attempt to settle the question, Obrebski et al. exploit the fact that there are more seismometers deployed across North America than ever before to produce the most detailed picture yet of the structure of the mantle beneath western North America. This picture is constructed using seismic tomography: an analysis of how the time that seismic waves from distant earthquakes arrive at a detector is changed by local variations in the temperature or composition of the material it is passing through. For example, if there is a region of hotter than normal material beneath a particular seismometer, the earthquake waves will travel more slowly and be detected later than expected; if it is colder than normal, the waves will move faster and arrive earlier than expected. By combining the signals from a large number of earthquakes, all of which take different routes through the mantle, and the records from a distributed grid of seismometers, you can produce a three dimensional picture of the structure of the mantle.

Obrebski et al.‘s latest model clearly images the ‘hot’ part of the hotspot, in the shape of a large, low velocity zone directly beneath the Yellowstone caldera. What is more, you can follow the low velocity zone downwards into the mantle, along a somewhat corkscrew path: it first descends to the northwest, before abruptly reversing its direction and descending to the southeast, and then changing direction again at about 800 km depth. This trail extends right to the base of the area modelled, 1200 km beneath the surface, almost half way to the core-mantle boundary, and much further than any potential plume has been traced beneath Yellowstone before.

Horizontal slice through the tomographic model at 200 km depth (left); the low velocity zone beneath the Yellowstone caldera follows a corkscrew path into the lower mantle (right). Source: Obrebski et al. (2010), Fig. 2.

But perhaps the most exciting thing about these new images is not what is there, but what isn’t. There has been a plate subducting eastwards beneath the Pacific Northwest since at least the Cretaceous period, and we would expect the tomographic model to pick up the cold subducted slab as a region of faster than average seismic velocities dipping to the east, and extending some way into the mantle. At shallow depths, the slab is easy to spot: in the horizontal slice through the model above, it is the the blue linear feature just inbound from the west coast. But as we look deeper in the mantle, we see something unexpected. A cross-section running E-W beneath Oregon (the top image in the figure below) reveals that there is no fast anomaly below about 300 km depth, even though there should be a slab there based on our understanding of the tectonic history of the subduction zone. A similarly oriented cross-section further to the south shows the slab extends much deeper beneath California and Nevada, to depths of at least 600 km. Curiously, however, further to the east there is another eastward dipping zone of fast (and presumably, cold) mantle, which looks a bit like a subducting slab but appears to be disconnected from any surface tectonics.

Vertical E-W cross sections showing the velocity structure of the mantle beneath (top) Oregon and (bottom) California and Nevada. Source: Obrebski et al. (2010)

Obrebski et al. argue that if the Yellowstone hotspot is indeed fed by a plume from the deep mantle, as these results seem to indicate, when it first ascended towards the surface it would have intersected with the subducting slab, and what we are seeing in the tomography is the aftermath of that interaction. Effectively, the upwelling plume burnt through the subducted plate beneath Oregon on its way to the surface, heating it up enough that it lost its rigidity and was assimilated into the asthenosphere, leaving isolated fragments further down dip that were no longer connected to the plate at the surface. Tectonic changes at the Cascadia subduction boundary around 19 million years ago, notably a large reduction in the rate of plate convergence, could be linked to this splintering of the subducted plate, and the reduction in slab pull that would have resulted. After overcoming this barrier to its upward passage, the plume head then reached the surface 2 million years later, causing the eruption of the Columbia River Basalts.

3-D plot showing the interaction between the subducting slab (blue) and the upwelling Yellowstone plume (yellow). Source: Obrebski et al. (2010)

This is all extremely cool in its own right, but I also find this quite interesting in the context of the whole mantle plume debate. This is hardly the first time that seismic tomography has been used to attempt to image mantle plumes beneath hotspots, and although they do occasionally seem to image conduits of hot material coming up from deep in the mantle, the dimensions of these features generally push at the limits of what is resolvable with tomography, raising the possibility that they could simply be artefacts of processing. But in this case, the tomography has also revealed that at some point in the past, material rising from the deep mantle completely disrupted the subduction system beneath the Pacific Northwest, and the debris of this past interaction is clearly associated with the current velocity anomaly beneath Yellowstone. This gives us more confidence that the ‘plume’ being imaged in this study is actually real.

Obrebski, M., Allen, R., Xue, M., & Hung, S. (2010). Slab-plume interaction beneath the Pacific Northwest Geophysical Research Letters, 37 (14) DOI: 10.1029/2010GL043489

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Categories: geophysics, paper reviews, volcanoes

Castle geology

Posted on August 23, 2010 by Anne Jefferson

A post by Anne JeffersonBeing a giant geo-nerd, I tend to pepper my travels with a lot of geologically or hydrologically interesting places. A recent trip brought me to the UK and included a meetup with my coblogger in Edinburgh. Being an American tourist, I also felt compelled to visit at least one castle during my time in the UK, so I dragged Chris to Edinburgh Castle…where we naturally we ended up talking about the geology.

Edinburgh Castle from Princes Street

Edinburgh Castle from Princes Street. The low area with gardens in the foreground used to be a loch/lake. (Photo by A. Jefferson)

First off, Edinburgh Castle sits atop the core of a Carboniferous (340 million year old) volcano, now called Castle Rock. The rock is impressively elevated above the surrounding glacially sculpted cityscape. The volcanic root formed a bit of a speed bump for Pleistocene glaciers moving from west to east. Castle Rock was more erosion resistant than surrounding sedimentary rocks, so it was left standing higher than the surroundings. But it also protected the sedimentary rocks in its lee – forming a giant crag and tail structure. The Royal Mile, which stretches from the Castle to Holyrood Palace rides on the tail of this structure – which gradually decreases in height and width away from the castle. In the Google Earth image below, I’ve maximized the vertical exaggeration and you can get some sense of it, though I can tell you that a more visceral understanding is achieved by walking up or down the Royal Mile.

Crag and tail of Edinburgh Castle and the Royal Mile

Crag and tail of Edinburgh Castle and the Royal Mile (image from Google Earth)

Within the castle itself, the glacial legacy is on display in the building stones, which show a remarkable diversity of lithologies, both those found locally and those farther afield in Scotland. It is likely that castle builders would have made use of the rocks left on the landscape when the glaciers retreated, but the ice caps would have given them plenty of variety. In the middle of the photo below, I think we are looking at the Old Red Sandstone, which has an important place in the history of geology and of paleontology.

Stonework on St. Margaret's Chapel, the oldest extant building in Edinburgh Castle

Stonework on St. Margaret's Chapel, the oldest extant building in Edinburgh Castle (photo by A. Jefferson)

The stone construction seemed to vary between buildings and even parts of buildings within the castle. The photo above is from St. Margaret’s Chapel, built in the 12th century, and you can clearly see how the big rocks are matrix-supported by a sand and gravel cement. This wall is actually even different from other walls on the same building, where stones are much more rectangular, probably cut, and there is much less matrix, as shown in the photo below. The informative castle guidebook suggests that the wall pictured above may have been originally part of another structure, onto which St. Margaret’s chapel was built adjointly. I should also point out that the walls in St. Margaret’s chapel are incredibly thick (1 m or more), so there’s a lot of stone work we’re not even seeing from the outside.

St. Margaret's chapel showing variety of stone work

St. Margaret's chapel showing variety of stone work (photo by A. Jefferson)

Portcullis Gate and the Argyle Tower

Portcullis Gate and the Argyle Tower (photo by A. Jefferson)


In other parts of the castle, and indeed in the surrounding city, it was fun to watch for places where old doorways or windows had been filled in or where additions had been added to stone buildings. These were usually pretty easily spotted by a change in the stone construction style. For example, in the photo on the left, you can see the main gated entrance to the castle (the Portcullis Gate), which was constructed in the late 16th century. On top of it is the Argyle Tower, one of the youngest buildings in the castle, constructed in 1887. To me it also looks like there might be an intermediate strata between the cut and tightly placed blocks immediately around the gate and the much more modern top part.

Wall adjacent to Lang Stairs

Wall adjacent to Lang Stairs. The plaque on the wall commemorates the successful recapture of the castle from the English in 1314. (photo by A. Jefferson)

One of the most impressive features of the castle was the way the stone architecture worked with the irregular topography of the bedrock surface. Walls had uneven bottoms, and even, in some places, sides, like this wall near the Lang Stairs.

Finally, I would be remiss if I didn’t point out one important hydrogeological influence on Edinburgh Castle and its history. As I mentioned, the castle is much higher than the surrounding topography (peak is 134 m above sea level) and made of dense volcanic rock. This has advantages from the defensive point of view, but a significant disadvantage from the “having enough water to stay alive” point of view. During peace times, castle residents presumably brought water up the hill from the loch or from wells in town. But during sieges, they were reliant on the Fore Well. This well, constructed in the 14th century, is an impressive 34 m deep (through solid rock! in the 14th century!), but only the bottom 3 m of the well are below the water table. The well could provide 11,000 liters – but that’s not a lot to supply a bunch of soldiers and their animals. During at least one siege, most of the deaths seem to be attributable to lack of adequate water, rather than the warfare itself.

So that’s what happens when you take an American geo/hydro nerd to Scotland…she looks at castles and thinks about rocks and water.

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Categories: by Anne, geology

Stuff we linked to on Twitter last week

Posted on August 22, 2010 by Chris Rowan

A post by Chris RowanA post by Anne JeffersonAs well as the two of us, there are many other earth, ocean and space scientists using Twitter these days: The AGU have more than 300 on their list. Listed below are the most interesting things we came across last week.

Blogs in motion

Volcanoes

Earthquakes & Tectonics

Fossils

(Paleo)climate

Water

Environmental

Planets

General Geology

Interesting Miscellaney

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Categories: links

Friday(ish) Focal Mechanisms: Samoa’s hidden rupture

Posted on August 21, 2010 by Chris Rowan

A post by Chris RowanResearchBlogging.orgIt seems that August is turning into ‘revisit past earthquakes’ month. Last week, Haiti; this week, the magnitude 8.1 earthquake that shook Samoa last September. Two Nature papers out this week unveil a slightly more complex story than we originally thought: a tale involving not just one, but two, or possibly even three, great earthquakes.

What remains undisputed is that a magnitude 8.1 earthquake ruptured the Pacific plate on the ‘trench slope’, just to the east of where it was being subducted beneath the Tonga microplate. It had an extensional focal mechanism, and was probably due to bending of the Pacific plate.

Initial interpretation of the Samoa earthquake of September 2009.

However, the aftermath of the earthquake provided several clues that this was not the whole story. Most of the aftershocks in the weeks that followed the main event were located not around the rupture point on the Pacific plate, but on the subduction megathrust, or even in the overiding Tongan plate – places where you would not expect the most significant stress changes to have occurred. The propogation of tsunami waves that caused great damage in Samoa and Northern Tonga, as charted by monitoring buoys, could not be modelled properly either – unless you assumed a thrust fault had generated the tsunami, rather than a normal fault as implied by the focal mechanism (shown in the figure above).

The initial focal mechanism for an earthquake is calculated primarily from the first motions – the behaviour of the first earthquake waves to arrive at seismometers around the world. However, if you want to properly model the size and shape of the fault plane that ruptured then you have to look at the whole train of seismic waves that arrived, from start to finish. When this was done, it was found that the full solution didn’t match up to the initial focal mechanism – in fact, there was no solution where a single, simple, rupture could produce the seismic signal from Samoa.

Solving this discrepancy requires an even closer look, which Lay et al have obliged us by taking. What they discovered was that the first minute or two of the arriving earthquake waves could be modelled by an extensional rupture on the trench slope, just as the initial focal mechanism suggested. After that, however, there was a significant divergence between what was expected from modelling such a rupture, and what was actually picked up by the global seismometer network. This suggested that the original earthquake had triggered another nearby, adding its own seismic voice to the cacophony. Further modelling demonstrated that you can generate a good fit to the observed seismograms if the subduction megathrust had ruptured at the same time. The best fit solution suggests it ruptured not once, but twice in quick succession, generating two magnitude 7.8 earthquakes. As a result of this almost instantaneous response, the seismic waves generated by the thrust events were mixed together with the seismic waves still being generated by the original earthquake, rather than showing up with their own right. But their signal can be extracted, and it explains the aftershock pattern – there are lots of aftershocks on the plate boundary because it actually ruptured as well.

Interpretation of Lay et al.. The detected earthquake signal is a composite of an extensional rupture on the trench slope, and thrusting on the subduction boundary.

But wait – there’s a second Nature paper too, which comes to similar, but not identical, conclusions. Beavan et al. start from the same point – that there was something a bit weird about this earthquake, particularly in the tsunami propagation patterns – but took a different route of investigation, opting to use GPS measurements to determine the post-seismic deformation. They found that sites situated on the most northerly of the Tonga islands had moved 3.5 cm to the east since the last set of GPS measurements in December 2005 – a vast discrepancy with the 0.8 cm of southwest movement that should have resulted from the extensional earthquake on the Pacific plate. The amount and direction of motion was much more consistent with a large compressional earthquake – of around magnitude 8.0 – on the subduction megathrust. Since no such event has been detected since 2005, they also conclude that this thrusting must have occurred within a few minutes of the trench slope earthquake on September 29th, and was responsible for generating the tsunami. This explains why the tsunami looked like it had been generated by a megathrust earthquake – it had been.

So where do these two papers differ? Beavan et al. are proposing one megathrust earthquake, while Lay et. al are proposing two, but this is less important than it probably seems. the total seismic energy release is pretty much the same in both cases, and the separate ruptures modelled by Lay et. al pretty much run into each other, which would make them difficult to separate in other datasets. A more significant difference is the proposed order of events: Lay et. al propose the extensional, trench slope earthquake occurred before, and triggered, the megathrust rupture(s); Beavan et al. think that it is far more likely that the subduction zone ruptured first, and this triggered the trench slope earthquake. However, they suggest that the megathrust event was a ‘slow’ earthquake, where fault movement is a slow grind rather than an abrupt jerk. Because the seismic waves generated by such event also build up slowly in amplitude, the initiation of the thrust earthquake was not detected; and by the time the signal was detectable, the larger earthquake on the trench slope had already been triggered and was obscuring it.

Interpretation of Beavan et al.. The thrusting occurs before the extensional rupture, but because it is a 'slow' earthquake it is not detected.

If I was forced to choose, I suspect that the seismic records used by Lay et al. should provide much better constraints on the sequence of events than the GPS and tsunami buoy records used by Beavan et al.. But if it was that clear-cut, I doubt both papers would have been published together like they have. Either way, these papers demonstrate just how sophisticated our analysis of earthquakes is becoming – and how much more sophisticated they might need to become before we fully understand the way the Earth deforms.

Lay, T., Ammon, C., Kanamori, H., Rivera, L., Koper, K., & Hutko, A. (2010). The 2009 SamoañTonga great earthquake triggered doublet Nature, 466 (7309), 964-968 DOI: 10.1038/nature09214

Beavan, J., Wang, X., Holden, C., Wilson, K., Power, W., Prasetya, G., Bevis, M., & Kautoke, R. (2010). Near-simultaneous great earthquakes at Tongan megathrust and outer rise in September 2009 Nature, 466 (7309), 959-963 DOI: 10.1038/nature09292

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Categories: earthquakes, focal mechanisms

Snowball Earth no problem for sponges

Posted on August 18, 2010 by Chris Rowan

A post by Chris RowanResearchBlogging.orgIn the debris surrounding 650 million year-old stromatolite reefs in South Australia, Adam Maloof and his group have discovered some rather unusual-looking fossils. They’re all about half a centimetre across, and are composed of red calcite (calcium carbonate), which indicates a biological origin. But they also have a range of apparently quite disparate shapes: as well as the wishbone and figure-of-8 (‘perforated slab’) morphologies shown in the image below, there were also rings and H’s (‘anvils’). In their Nature Geoscience paper, Maloof and his co-authors argue that all of these different forms are in fact cross-sections sliced at different angles through a single type of organism, with a complex three-dimensional shape. To demonstrate this, they took a polished section of the fossil bearing rock, scanned its surface, and traced the outline of any preserved fossil. They then ground off a fraction of a millimetre from the rock face, and scanned it again. Mimicking CT X-ray scans, all of the scanned outlines were then stacked together to reproduce the original shape of the fossilised organism.

Top: possible Australian sponge fossil: different shapes are due to the rock surface cutting through different planes of the original organism. Bottom: 3D reconstruction from sequential grinding and scanning, showing interior channel network.

Adam Maloof himself provides a nice, clear explanation of the ‘grind and scan’ method in this video, courtesy of New Scientist.

Adam Maloof describes how the 3D fossil sponge reconstructions were created

The most striking feature revealed by the 3D reconstructions is the network of internal channels that permeates these fossils, which is very reminiscent of the internal water canals of sponges. If this was the case, these would be, by a considerable temporal distance, the oldest animal fossils ever discovered. However, this classification is still tentative: although these fossils do seem to have a sponge-like organisation, they do not clearly possess features common to all other known sponges, living and fossil, such as their quasi-skeletal spicules. But while this claim should be approached with caution (for all I know, Chris Nedin is preparing a rebuttal as I type), if it is confirmed it is not an entirely unexpected discovery. To date, the confirmed physical fossil record of sponges, in the form of possible spicules found in Doushantuo Formation of southern China, stretches back only as far as the late Ediacaran, but molecular clock data suggests that some sponge groups diverged from a presumably spicule-bearing common ancestor at least 200 million years earlier than this, and that, perhaps due to some quirk of Neoproterozoic ocean chemistry, sponges existed during this period but their spicules were not preserved. And just last year sponge biomarkers – carbon compounds formed by the decomposition of organic molecules found only in sponges – were reported from 650-700 million year old rocks in Oman (which we discussed in the Podclast at the time).

So the evidence from numerous sources seems to be converging to suggest that sponges – the first animals – emerged much earlier than the beginning of the Cambrian. In fact, as the timeline in the figure below illustrates, these new fossils from Australia, and the biomarker evidence from Oman, are both found at a rather interesting point in Earth history: the period between the two hypothesised ‘Snowball Earth’ events, when the entire Earth may have completely frozen over for 10-20 million years. And if the molecular clock evidence is accurate, sponges might even have originated before the first proposed Snowball event 700-750 million years ago.

How recent discoveries have pushed the sponge fossil record back almost 100 million years.

This could be considered a paradox: a group of organisms apparently sailing through severe climatic events – a prolonged global deep freeze, followed by an intense supergreenhouse, in the most extreme models of the Snowball Earth – without much trouble at all. This is not the first example, either. Perhaps the ancestors of modern life were tougher critters than we give them credit for – or perhaps the Cryogenian was not quite as hard on Neoproterozoic life as some geologists believe.

Maloof, A., Rose, C., Beach, R., Samuels, B., Calmet, C., Erwin, D., Poirier, G., Yao, N., & Simons, F. (2010). Possible animal-body fossils in pre-Marinoan limestones from South Australia Nature Geoscience DOI: 10.1038/ngeo934

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Categories: Proterozoic, fossils, geology, paper reviews, past worlds

Stuff we linked to on Twitter last week

Posted on August 15, 2010 by Chris Rowan

A post by Chris RowanA post by Anne Jefferson

Blogs in motion

Not so much blogs in motion as blogs in multiplication this week. The GSA has unveiled Speaking of Geoscience; and NASA’s Earth Observatory’s Elegant Figures has kicked off with a fascinating post on visualising the Eyjafjallajokull ash cloud. Also note that DinoJim’s blog is rebranding as The Geology PAGE (Presenting Alternatives in Geoscience Education), and Geobulletin.org is the shiny new home of the stratigraphy.net geoblog aggregator.

Earthquakes & Tectonics

Fossils

(Paleo)climate

Water

Environmental

General Geology

Interesting Miscellaney

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Categories: links