GeoLog

Tectonics and Structural Geology

Imaggeo on Mondays: Measuring the wind direction

Imaggeo on Mondays: Measuring the wind direction

Remote, rugged, raw and beautiful beyond measure, the island of South Georgia rises from the wild waters of the South Atlantic, 1300 km south east of the Falkland Islands.

The Allardyce Range rises imposingly, south of Cumberland Bay, dominating the central part of the island. At its highest, it towers 2935 m (Mount Paget) above the surrounding landscape. In the region of 150 glaciers carve their way down the rocky peaks, toward craggy clifftops and the emerald green waters of the ocean.

Geologically speaking, the territory is unique. South Georgia sits atop the place where the South American Plate and the Scotia Plate slide past one another; exactly which can claim ownership of the mountainous outpost is highly debated.

As East and West Gondwana split, about 185 million years ago, South Georgia was pulled away from Tierra del Fuego, an archipelago off the southernmost tip of mainland South American, and experienced severe volcanism.

The island is so remote and exposed, it creates its own weather system. It sits in the path of very strong winds, the westerlies, which flow through the subtropical highs in the Southern Hemisphere.  As air flows over the high mountains of South Georgia, they generate atmospheric gravity waves (which transfer energy from the troposphere – the layer closest to the Earth’s surface – to the upper layers of the atmosphere, including the stratosphere, where the ozone layer is found). Atmospheric gravity waves are responsible for the transfer of considerable amounts of energy over large distances, and thus have a substantial impact on weather and climate.

It is precisely to study South Georgia’s atmospheric gravity waves that Andrew Moss, the author of today’s photograph, journeyed to the remote island back in January 2015 as part of the South Georgia Wave Experiment (SG-WEX). The project was led by the University of Bath, in collaboration with the British Antarctic Survey, the University of Leeds, and the UK Met Office. As part of this project Andrew worked with a colleague at the University of Bath (UK) to release radiosondes – a small, expendable instrument package that is suspended below balloon which measures pressure, temperature and humidity – to better understand the atmospheric conditions and measure atmospheric gravity-wave activity above the island.

The chilly, often cloudy and wet landscape is a wildlife haven. It is home to a staggering five million seals and 65 million seabirds.  The wildlife is so rich, on and off the island, that large swathes of South Atlantic waters surrounding South Georgia are protected and onshore activities which might disturb wildlife require permits.

“Over the course of the two-week field campaign, King Penguins, fur seals and elephant seals often surrounded us while we worked,” describes Andrew.  “During the trip, I captured a group of penguins, with their backs to the wind, clustered together on a windy day.”

References and further reading:

Imaggeo on Mondays: Atmospheric gravity waves (GeoLog, EGU Blogs, April 2017)

Hoffmann, L., Grimsdell, A. W., and Alexander, M. J.: Stratospheric gravity waves at Southern Hemisphere orographic hotspots: 2003–2014 AIRS/Aqua observations, Atmos. Chem. Phys., 16, 9381-9397, https://doi.org/10.5194/acp-16-9381-2016, 2016.

South Georgia & the South Sandwich Islands Government website

Radiosondes – a guide by the National Weather Service (NOAA)

Find out more about Andrew, here: https://www.researchgate.net/profile/Andrew_Moss8

September GeoRoundUp: the best of the Earth sciences from around the web

September GeoRoundUp: the best of the Earth sciences from around the web

Drawing inspiration from popular stories on our social media channels, as well as unique and quirky research news, this monthly column aims to bring you the best of the Earth and planetary sciences from around the web.

Major story and what you might have missed

This month has been an onslaught of  Earth and space science news; the majority focusing on natural hazards. Hurricanes, earthquakes and volcanic eruptions have been dominating headlines, but here we also highlight some other natural disasters which have attracted far fewer reports. Quickly recap on an action-packed month with our overview, complete with links:

Hurricanes

Thought the Atlantic hurricane season is far from over, 2017 has already shattered records: since 1st June 13 storms have been named, of which seven have gone onto become hurricanes and two registered as a category 5 storm on the Saffir-Simpson Hurricane Wind Scale. In September, Hurricanes Irma, Katia and Jose batter Caribbean islands, Mexico and the Southern U.S.; hot on the heels of the hugely destructive Hurricane Harvey which made landfall in Texas and Louisiana at the end of August. Images captured by NASA’s Operational Land Imager (OLI) on the Landsat 8 satellite show the scale of the damage caused by Hurricane Irma; while photos reveal the dire situation unfolding in Puerto Rico after Hurricane Maria.  OCHA, the United Nations Office for the coordinate of Human Affairs, released an infographic showing the impact the 2017 hurricane season has had on Caribbean islands (correct of 22nd September).

Earthquakes

At the same time, two powerful earthquakes shook Mexico in the space of 12 days causing chaos, building collapse and hundreds of fatalities.

Rumbling volcanoes

In the meantime, all eyes on the Indonesian island of Bali have been on Mount Agung which has already forced the evacuation of almost 100,000 people as the volcano threatens to erupt for the first time in 54 years. Unprecedented seismic activity around the volcano has been increasing, though no eruptive activity has been recorded yet.

Further south, the government of Vanuatu, a South Pacific Ocean nation, declared a state of emergency and ordered the evacuation of all 11,000 residents of Ambae island, as activity of its volcano, Manaro, increased. The New Zealand Defence Force (NZDF) sent an aircraft to fly over the volcano on Tuesday and discovered plumes of smoke, ash and volcanic rocks erupting from the crater.

Map of volcanic hazards for Ambae in Vanuatu. Credit: Vanatu Meteorology & Geo-hazard Department (vmgd).

The rainy season floods

The summer months mark the onset of the rainy season in regions of Sub-Saharan Africa which experience a savanna climate. Across the Arabian Sea, including the Indian subcontinent and Southeast Asia, also sees the onset of the monsoon.

Since June, widespread flooding brought on by heavy rainfall has left 56 dead and more than 185,000 homeless in Niger, one of the world’s poorest countries. But the crisis is not restricted to Niger, throughout the summer floods (and associated land and mudslides) in Africa are thought to have claimed 25 times more lives than Hurricane Harvey did.

Meanwhile Mumbai struggled when the heaviest rainfall since 2005 was recorded on 29th August, with most of Northern India experiencing widespread flooding. So far, the UN estimates that 1,200 people have lost their lives across Nepal, India and Bangladesh as a result of the rains. The Red Cross estimates that at least 41 million people have been affected by the flooding and causing the onset of a humanitarian crisis.

Record breaking temperatures and fires

Australia’s record-breaking spring heat (Birdsville, in Queensland’s outback, broke a weather record as temperatures hit 42.5C and Sydney recorded its hottest ever September day) combined with an unusually dry winter means the country is bracing itself for a particularly destructive bushfire season. Already fires rage, uncontrolled (at the time of writing), in New South Wales.

The western United States and Canada suffered one of its worst wildfire seasons to date. Earlier this month, NASA released a satellite image which showed much of the region covered in smoke. High-altitude aerosols from those fires were swept up by prevailing winds and carried across the east of the continent. By 7th September the particles were detected over Ireland, the U.K and northern France, including Paris.

Europe’s forest fire has been hugely devastating too. Much of the Mediterranean and the region North of the Black Sea continues to be in high danger of forest fires following a dry and hot summer. Fires are active in the Iberian Peninsula, Greece, and Germany (among others). Over 2,000 hectares were recently scorched by wildfires in the central mountainous area of Tejeda in Gran Canaria.

Links we liked

  • This month saw the end of NASA’s Cassini spacecraft and ESA’s Huygens probe’s spectacular journey to Saturn. After two decades of science, the mission ended on 15th September as the spacecraft crashed into the giant planet.
  • The last day of August marks the end of the Greenland snow melt season, so September was busy for scientists evaluating how the Greenland ice sheet fared in 2017.
  • “Few disciplines in today’s world play such a significant role in how society operates and what we can do to protect our future,” writes Erik Klemetti (Assoc. Prof. at Denison University), in his post on why college students should study geology.
  • The BBC launched The Prequel to its much anticipated Blue Planet II, a natural history progamme about the Earth’s oceans. Narrated by Sir Sir David Attenborough, the series will featured music by Hans Zimmer and Radiohead. The trailer is a true feast for the eyes. Don’t miss it!

The EGU story

Is it an earthquake, a nuclear test or a hurricane? How seismometers help us understand the world we live in.

Although traditionally used to study earthquakes, like the M 8.1 earthquake in Mexico,  seismometers have now become so sophisticated they are able to detect the slightest ground movements; whether they come from deep within the bowels of the planet or are triggered by events at the surface. But how, exactly, do earthquake scientists decipher the signals picked up by seismometers across the world? And more importantly, how do they know whether they are caused by an earthquake, nuclear test or a hurricane?

To find out we asked Neil Wilkins (a PhD student at the University of Bristol) and Stephen Hicks (a seismologist at the University of Southampton) to share some insights with our readers earlier on this month.

Mexico earthquakes: What we know so far

Mexico earthquakes: What we know so far

On Friday 8 September 2017 at 04:49 am UTC, a magnitude 8.1 earthquake hit off the coast of Mexico, 87 km SW of Pijijiapan. According to the U.S. Geological Survey, the epicentre was at 15.07 N, 93.72 W at a depth of about 69.7 km. Yesterday, another strong (magnitude 7.1) earthquake hit central Mexico, 55 km SSW of the city of Puebla and 120 km south of Mexico City.

Despite the lower magnitude, yesterday’s earthquake, which struck at a depth of 51 km, has caused widespread destruction. At the time of writing, official estimates put the death toll at 217 (according to Mexico’s National Coordinator for Civil Protection, Luis Felipe Puente), with shaking causing damage to and the collapse of hundreds of buildings in Mexico City and surrounding areas.

“The M 7.1 earthquake was much closer to Mexico City, a city build on a dried lake bed; this caused presumably (needs to be confirmed by data) much higher shaking in the densely populated capital then the larger, but farther M 8.1 event,” explains Martin Mai, President of the EGU’s Seismology Division.

“Both earthquakes were intraplate normal faulting events, not occurring on the interface between the subducting and overriding plates but rather inside the subducting plate,” adds Vala Hjorleifsdottir, a researcher at the National Autonomous University of Mexico.

These intraplate earthquakes generate relatively strong and rapid shaking, compared to their counterparts breaking the plate interface.  Furthermore, as the waves are generated deeper in the Earth, they do not travel through shallower material that damp them as they travel, and they are still strong when they arrive to the City of Mexico and neighbouring areas.  For these reasons, combined with their proximity to populated areas, these events can be more destructive than expected by their magnitude.

The U.S. Geological Survey estimated that significant causalities are likely in the region. Given the mix of vulnerable and earthquake resistant structures, the economic loss is also expected to be high. For more information visit impact pages of the event on the USGS website.

Six days after the latest earthquake, rescue workers are still search for victims among the rubble. This visual of Mexico City gives an impression of the scale of the devastation in the country’s capital city.

“Mexico City [is] built on a dried-out lake bed, or on ‘landfill’ of unconsolidated sediments.  The interaction between the incoming seismic waves and the sediments cause the waves to amplify and the duration of shaking to increase.  Both of these factors are devastating to buildings,” explains Hjorleifsdottir.

As to whether the two earthquakes are linked, scientists are fairly certain that the normal mechanisms which are known to trigger an earthquake after another didn’t come into play for the M 8.1 and the later M 7.1. At more than 600 km between the two quakes, they occurred, too far from one another. In addition, if shaking from an early earthquake is going to trigger a second, it is expected to happen shortly after the initial tremor, not 12 days later.

However, there are other mechanisms, which are less well understood, for example the triggering of earthquakes in hydrothermal areas and volcanoes, over large distances, for a period after large events.

“We believe this has to do with the behaviour of fluids in these areas, that promote the occurrence of earthquakes in these regions.  More research is needed to tell whether any of these other methods caused triggering of the second event,” says Hjorleifsdottir. Mai also adds: “It could be that stress changes caused by the M 8.1 event brought the fault (system) on which the M 7.1 earthquake happened closer to failure; but this requires detailed quantitative analysis”.

Editor’s note: Last updated 02.10.2017. This post will be update as more information about the earthquake becomes available.

With thanks to Martin Mai (EGU Seismology Division President), Vala Hjorleifsdottir, Paco Sánchez and Marco Calo (National Autonomous University of Mexico).

Further reading and resources:

U.S. Geological Survey overview of 19.09.2017 M 7.1 earthquake (includes interactive, shake and regional information maps)

U.S. Geological Survey overview of 08/09.2017 M 8.1 earthquake (includes interactive, shake and regional information maps, as well as finite fault results and moment tensor information)

Temblor blog post on M 7.1 earthquake

Temblor blog post on M 8.1 earthquake

Did Mexico dodge a bullet in last week’s M=8.1 earthquake? (Temblor blog post on dynamics of 8th September quake)

European-Mediterranean Seismological Centre information about yesterday’s earthquake

SSN (Mexico) page about yesterday’s earthquake (in Spanish)

GFZ GEOFON Global Seismic Network event page for yesterday’s earthquake

Mexico City, Before and After the Earthquake (New York Times visualisation)

Are Mexico’s two major earthquakes related, and what could happen next? (Temblor blog)

Shocked and shaken to the ground: An eyewitness report from Mexico City (Temblor blog)

Mexican Earthquakes: Chain Reaction or Coincidence? (Temblor blog)

Mapping Ancient Oceans

Mapping Ancient Oceans

This guest post is by Dr Grace Shephard, a postdoctoral researcher in tectonics and geodynamics at the Centre of Earth Evolution and Dynamics (CEED) at the University of Oslo, Norway. This blog entry describes the latest findings of a study that maps deep remnants of past oceans. Her open access study, in collaboration with colleagues at CEED and the University of Oxford, was published this week in the Nature Journal: Scientific Reports. This post is modified from a version that first appeared on the CEED Blog.

Quick summary:

There are several ways of imaging the insides of the Earth by using information from earthquake data. When these different images are viewed at the same time, a new type of map allows geoscientists to identify the most robust features. These deep structures are likely the remains of extinct oceans, known as slabs, that were destroyed hundreds of millions of years ago. The maps are computed at different depths inside the Earth and the resulting slabs can be resurrected back to the surface. Along with a freely available paper and website, the analysis yields new insights into the structure and evolution of our planet in deep time and space.

Earth in constant motion

The surface of the Earth is in constant motion and this is particularly true of the rocks found under the oceans. The crust – the outermost layer of the planet – is continually being formed in the middle of oceans, such as the Mid-Atlantic Ridge. In other places, older crust is being destroyed, such as where the Pacific Ocean is moving under Japan. A third type of locality sees the crust shifted along laterally, such as the San Andreas Fault in San Francisco. These three types of locations are often referred to as plate boundaries, and they connect up to divide the Earth’s surface into tectonic plates of different sizes and motions.

Where plates plunge into the mantle are termed subduction zones (red lines Figure 1, below). The configuration of these subduction zones has changed throughout geological time. Indeed, much of the ocean seafloor (blue area in Figure 1) that existed when the dinosaurs roamed the Earth has long since been lost into the Earth’s mantle and are now known as slabs. The mantle is the domain beneath the outer shell of our planet and extends to around 2800 km depth, to the boundary with the core.

The age and fabric of the seafloor contains some of the most important constraints in understanding the past configuration of Earth. However, the constant recycling of oceans means that the Earth’s surface as it is today can only tell us so much about the deep geological past – the innards of our planet hold much of this information, and we need to access, visualize, and disseminate it.

Figure 1. A reconstruction of the Earth’s surface from 200 Million years ago to present day in jumps of 10 Million years. Red lines show the location of subduction zones, other plate boundaries in black, plate velocities are also shown. Continents are reconstructed with the present-day topography for reference. Based on the model of Matthews et al. (2016; Global and Planetary Change). Credit: G Shephard (CEED/UiO) using GPlates and GMT software.

Imaging the insides

Using information from earthquake data, seismologists can produce images of the Earth’s interior via computer models – this technique is called seismic tomography. Similar to a medical X-ray scan that looks for features within the human body, these models image the internal structure of the Earth. Thus, a given seismic tomography model is a snapshot into the present-day structure, which has been shaped by hundreds of millions to billions of years of Earth’s history.

However, there are different types of data that can be used to generate these models and different ways they can be created, each with varying degrees of resolution and sensitivity to the real Earth structure. This variability has led to dozens of tomographic models available in the scientific arena, which all have slightly different snapshots of the Earth. For example, deep under Canada and the USA is a well-known chunk of subducted ocean seafloor (see ‘slab’ label in Figure 2). A vertical slice through the mantle for three different tomography models shows that while overall the models are similar, there are some slight shifts in its location and shape.

Importantly, seismic waves pass through subducted, old, cold oceanic plates more quickly than they do through the surrounding mantle (in the same way that sound travels faster through solids than air). It follows that these subducted slabs can be ‘imaged’ seismically (usually these slab regions show up as blue in tomography models such as in Figure 2 and as shown in this video by co-author Kasra Hosseini. The red regions might represent thermally hot features like mantle plumes).

Figure 2. Vertical slices through three different seismic tomography models under North America and the Atlantic Ocean (profile running from A to B). The blue region outlined by black dashed line is related to the so-called Farallon slab. While it is imaged in all three models the finer details of the slab geometry and depth are different. Model 1 is S40RTS (Ritsema et al., 2011), 2 is UU-P07 (Amaru, 2007) and 3 is GyPSum-S (Simmons et al., 2010).

For other geoscientists to utilize this critical information, for example to work out how continents and oceans moved through time, requires a spectrum of seismic tomography models to be considered. But several limiting questions arise:

Which tomography model(s) should be used?

Are models based certain data types more likely to pick up a feature?

How many models are sufficient to say that a deep slab can be imaged robustly?

Voting maps of the deep

To facilitate solutions to these questions, a novel yet simple approach was undertaken in the study. Different tomography models were combined to generate counts, or votes, of the agreement between models – a sort of navigational guidebook to the Earth’s interior (Figure 3).

Figure 3. An interactive 360° style image for the vote map at 1000 km depth. The black and red regions highlight the most robust features (high vote count = likely to be a subducted slab of ocean) and the blue regions are the least robust areas (low vote count). Coastlines in black for reference. Image: G Shephard (CEED/UiO) using 360player (https://360player.io/) and GMT software. More depth slices and options can be also imaged at our website.

A high vote count (black-red features in Figure 3) means that an increased number of tomography models agree that there could be a slab at that location. For the study in Scientific Reports the focus was on the oldest and deepest slabs, but the process can be undertaken for shallower and younger slabs, and for other features such as mantle plumes. The maps show the distribution of the most robust slabs at different depths – the challenge is to now try and verify the features and potentially link them to subduction zones at the surface back in time.

One way to achieve this is to assume that a subducted portion of ocean will sink vertically in the mantle, and then to apply a sinking rate to connect depth and time. This enables pictures that link the surface and deep Earth, like the cover image, to be made. A sinking rate of say, 1.2 centimeters per year, means that a feature that existed at the surface around 100 Million years ago might be found at 1200 km depth.

Many studies have started to undertake a similar exercise on both regional and global scales. However, because these vote maps are free to access, showcase a lot of different models and can be remade with a sub-selection of them, they serve as an easy resource for the community to continue this task.

Secrets in depth

A bit like dessert-time discussions about the best way to cut a cake, so too are the ways of imaging and analyzing the Earth (Figure 4). Do you slice it horizontally and see things that might correspond to the same age all over the globe? Or slice vertically from the surface to see a spectrum of ages (depths) at a given location? Or perhaps a 3-D imaging would be most insightful? Whichever choice is made for the vote maps, many interesting features are displayed.

Figure 4. Vote maps visualized using alternative imaging options on a sphere. Credit: G Shephard (CEED/UiO) using GPlates software

By comparing the changes in vote counts with depth, some intriguing results were found. An apparent increase in the amount of the slabs was found around 1000-1400 km depth. This could mean that about 130 Million years ago more oceanic basins were lost into the mantle. Or perhaps there is a specific region in the mantle that has “blocked” the slabs from sinking deeper for some period of time (for example, an increase in viscosity).

The vote maps and their associated depth-dependent changes hold implications on an interdisciplinary stage including through linking plate tectonics, mantle dynamics, and mineral physics.

Of course, the vote maps are only as good as the tomography models that they are comprised of – and by very definition, a model is just one way of representing the true Earth.

A resource for the community

Having accessed a variety of tomography models provided by different research groups or data repositories, this study was facilitated using open-source software (Generic Mapping Tools and GPlates).

An important component of reproducible science and advancing our understanding of Earth is to make datasets and workflows publicly available for further investigations.

An online toolkit to visualize seismic tomography data is being developed by the co-authors and a preliminary vote maps page is already online. Here, vote maps for a sub-selection of tomography models can be generated, including with a choice in colour scales and with overlays of plate reconstruction models. More functionality will soon be available – so watch this space!

By Grace Shephard, a postdoctoral researcher in tectonics and geodynamics at the Centre of Earth Evolution and Dynamics (CEED)

Contact information for more details: Grace Shephard – g.e.shephard@geo.uio.no

Full reference to the article, freely available to the public:

Amaru, M. L. Global travel time tomography with 3-D reference models,. Geol. Ultraiectina 274, 174 (2007).

Matthews, K.J. K.T. Maloney, S. Zahirovic, S.E. Williams, M. Seton, R.D. Müller. 2016. Global plate boundary evolution and kinematics since the late Paleozoic. Global and Planetary Change. v146. doi: 10.1016/j.gloplacha.2016.10.002

Ritsema, J., Deuss, A., van Heijst, H. J. & Woodhouse, J. H. S40RTS: a degree-40 shear-velocity model for the mantle from new Rayleigh wave dispersion, teleseismic traveltime and normal-mode splitting function measurements. Geophysical Journal International 184, 1223-1236, doi:10.1111/j.1365-246X.2010.04884.x (2011).

Simmons, N. A., Forte, A. M., Boschi, L. & Grand, S. P. GyPSuM: A joint tomographic model of mantle density and seismic wave speeds. Journal of Geophysical Research: Solid Earth 115, doi:10.1029/2010JB007631 (2010).