SEEING OUR EARTH 🌎🌍🌏
HOW GEOPHYSICS OPENS THE WINDOW TO OUR PLANET
IMAGING AND IMAGINING
There is much to see on our amazing planet, inside and out, and our eyes can only take in so much. To really understand Earth, we rely on technologies to extend our vision. Geophysical tools enable us to image the subsurface, detect particles in the air, and map out changes beneath our feet that are too subtle or too large to notice in a single lifetime.
HOME ON THE ROCKS
While seeing penguins close-up on the ground is exciting, seeing them from the air, via photos taken from an unoccupied aerial system (UAS), offers a much better and much less invasive View of their colony structures and a faster way to complete population counts. UAS survey photos were stitched together in a process called structure-from-motion to create a 3D model of this rookery on Ross Island, Antarctica.
Surveys enable researchers to understand population dynamics and the effects of factors such as climate change on polar ecology. Look for the gray fuzzy dots among the black dots to identify the fluffy chicks with their parents.
ACCELERATING ICE
Greenland is home to Earth’s second-largest ice sheet, with glaciers covering approximately 80% of its surface. Motion of this ice is slowest at the ice sheet’s center and accelerates in channels along the edges. The fastest glacier in Greenland is Jakobshavn Isbra, in the west. Researchers have clocked Jakobshavn Isbra at more than 17 kilometers per year, or over 46 meters per day (or 10.5 miles and 150 feet, respectively). Tracking Greenland’s speeding glaciers from the ground and satellites is critical for understanding current and future sea level rise.
TORN NOT BROKEN
In the middle of the North American continent, far from any active volcanoes or plate boundaries, massive lava flows indicate the geologic past was not like the present. What happened? Geophysics provides us some good clues. Gravity measurements and seismic surveys used to image the subsurface in the mid-continent show a double-armed structure reaching from Lake Superior southwestward and southeastward through the mid-western U.S. This structure is a failed rift — a place where the continent started to pull apart to form a new ocean, but then stopped. The evidence that remains offers a snapshot of one of plate tectonics’ most important processes — how continents form and break up.
RING AROUND THE HARD ROCKS
While most of the volcanoes along the Cascades Arc align neatly over the subducting Juan de Fuca plate, Mount St. Helens sits farther west. Why is that? A recent geophysical survey using a technique called magnetotellurics measured the electric resistivity of the subsurface and shows that Mount St. Helens sits directly atop a much larger ring of highly conductive rocks. This ring represents rocks of different composition and different strength than the surrounding crust. On the western Side, where Mount St. Helens is, it is a zone of weaker rocks surrounded by older, more rigid rocks. Magma rising farther to the east under the rigid rocks may divert westward through the weaker rocks to reach the surface, where it can erupt — thus offsetting Mount St. Helens from its siblings.
SHIFTING SANDS
Sand dunes as you’ve never seen them before? This aerial view of White Sands National Monument was created by calculating the difference of height of the dunes between two overflight LIDAR surveys, showing the direction and magnitude of erosion and deposition. Lidar, or Light Detection and Ranging, determines distance by calculating the travel time of reflected laser signals from either an airborne platform or a tripod, resulting in high-resolution elevation maps. Multiple surveys reveal changes in the land surface, like the migration of sand dunes. Imagery collected in September 2009 and June 2010. Deposition or upward movement is colored in blue. Erosion or downward motion is colored in red and yellow. The dunes are migrating towards the north-east, driven by a dominant wind direction from the southwest.
THE BREAKING EARTH
A ground-based rainbow? No, these bands of color show how much the ground moved in the Ridgecrest, California region as a result of two earthquakes in July 2019. Scientists use data from radar-equipped satellites to map changes in broad swaths of Earth’s surface. This image compares the path length (or slant range) of radar pulses traveling to the ground and back from one satellite pass (before the earthquake) to the next (after the earthquake). This method, InSAR, is used to measure changes in Earth’s shape down to the centimeter.
Each repetition of the light spectrum (from purple through yellow to green) represents 12 cm of motion along a direct path toward the satellite. The closer the bands, the more the motion — here, the bands cluster around the multiple faults that ruptured. Maximum offsets measured on the ground were up to 1.5 m, or 5 ft.
UNDER LAND AND SEA
Understanding subduction zones, where one tectonic plate dives under another, is particularly challenging because the contact between the two plates is often located beneath the ocean. Recent technological advances have enabled more deployments of seafloor instruments to extend our View outward from shore. One such array is the Alaska Amphibious Community Seismic Experiment, consisting of 105 seismometers underwater and on land along the Alaska Peninsula to image the forces beneath one of the most seismically and volcanically active regions on the planet.
LOOKING UP AND AHEAD
Hurricanes are powerful but picky — they can only travel to and maintain their power in areas with warm water, Low winds aloft, and high humidity into the mid-levels of the atmosphere. Measuring these and other factors enables hurricane track forecasts, and the more data, the better able scientists are to calculate possible trajectories. GPS allows us to ‘see’ the humidity in the atmosphere in real-time by measuring the delay moisture causes on the satellite signals received by GPS stations on the ground. These delays offer a map of high- and Iow-humidity areas, and how they change as the hurricanes pass through. Additionally, seismometers can be used to measure the strength of hurricanes by recording vibrations from waves that are impacting the ocean floor and the coast.
MYSTERY MOUNTAINS
Not all mountains form the same way, and getting at the root cause of mountains may mean getting to the roots of the mountains themselves. Using seismometers to image the structures beneath Earth’s surface can help us see what lies below. Here, scientists use seismometers to map out zones where seismic waves travel faster and slower beneath the Adirondacks, allowing us to ‘see’ underneath them. The work revealed a ‘slow’ zone, which may mean hotter materials are located there. One possibility? The blob is an upwelling from below that, together with thermal expansion, pushed up and formed the Adirondack Mountains.
ANCIENT ICE, CURRENT SEAS
Predicting future coastlines requires much more than knowing how fast current glaciers Will melt; we also need to factor in thermal expansion of the ocean and the vertical motions of our coasts. GPS is one tool for measuring the ups and downs of our land — much of which has to do with past glaciers. North America is still recovering from the last major glaciation by rebounding (going up relative to the Earth’s center) where it was previously being depressed by ice (in red) and relaxing and sinking where it bulged up around the ice’s edge (in the surrounding blue). The sinking of the Atlantic coast means preparing for faster and greater sea level rise than in neighboring areas.
HIGH RESOLUTION VISION
The deployment of EarthScope’s Alaska Transportable Array, a network of 280 new and contributing seismic stations, blanketing Alaska and western Canada, came at just the right time to capture two large earthquakes in northern Alaska and their aftershocks. These earthquakes are visible in the upper part of this plot of energy released during the earthquake sequence. Earthquakes as small as magnitude I yield important information about regional faults, tectonic stresses, and future earthquake hazard.
Source: unavco.org
