The Earth Story

The New Madrid Seismic Zone About once a year, residents of the counties at the border between Kentucky, Tennessee, Missouri, and Arkansas will feel the ground roll beneath their feet. This image maps out the location of earthquakes in this area over a 30-year period and clearly illustrates a major feature: the New Madrid Seismic Zone. This zone produces about 1 quake that can be felt per year in addition to many small earthquakes…and has historically produced really big ones.

Logarithmic We often say that earthquakes are logarithmic. On the moment magnitude earthquake scale, a magnitude 9 earthquake is 10x as powerful as a magnitude 8 earthquake, and so forth. This simple graph makes that point very elegantly.

The North Anatolian Fault – one of the world’s most dangerous It’s hard to say that there’s a single, most dangerous fault segment in the world, but if we started making a list, the North Anatolian fault would appear on it very quickly.

explainxkcd.com/wiki/index.php/2344:_26-Second_Pulse

Shaking Shutdown

Much of modern life makes noise that subtly shakes the earth below. Trucks moving along roads, hammers in a building going up, even people walking along the street. To a seismograph, these little vibrations are nothing but “noise” – a constant background shaking that is measured today as little wiggles in the electronic sensors in the device. At the beginning of 2020, something unprecedented happened.

This is an incredible landscape to view in the first place - the Sierra Nevada Mountains above Lone Pine California, just below Mt. Whitney - this hiker was taking panorama videos, and then an earthquake struck triggering rockfalls all around him.

Earlier today a strong earthquake and small tsunami struck the Alaskan coast. This video sums up the geology of that area and why it triggers earthquakes and volcanoes.

About 2 and a half weeks ago, Oaxaca Mexico was shaken by a strong earthquake. What do we know about this area of the world and how do we use seismic instruments to understand quakes like this one? Watch this video to learn more.

Thanks to plumes of dust from Africa, we're currently not seeing major Atlantic hurricanes, but we're in the middle of Atlantic Hurricane season so this video fits. Hurricanes produce waves that shake the ocean, and those waves are enough to slightly shake the ground as they approach continents. Those signals can be measured by modern seismic instruments and actually give information about the intensity of the hurricane as it is approaching land. This video explains how that is done!

The Mexican Subduction Zone Today, June 23 2020, there was a large magnitude earthquake near Oaxaca, Mexico, so today’s a good day for an overview of the geology of this part of Mexico. Mexico is a very active location tectonically, as seen in this map from a recent publication summarizing the geology of that area. It features a large volcanic arc, it is crossed by a number of smaller faults associated with the volcanic zones, and along its southern coastline, there are 2 oceanic plates that are gradually subducting beneath the continent.

Incredible video of waves in an extra-long pool during today’s Earthquake in Oaxaca, Mexico

Subtle shaking

The city of Pasadena, California is an interesting place. It is home to the USGS earthquake scientists who plan and coordinate emergency response for the Los Angeles area, it is just down the road from the Jet Propulsion Laboratory, and every year on New Year’s Day a suite of flower-bearing floats marches down Colorado Boulevard through the city. Combining several of those themes – this float in 2005 was produced by JPL and features a whole bunch of their spacecraft (How many can you ID?). It turns out, these floats also can be used for the other topic – testing seismic sensors.

Human activity (Cars, trains, people) vibrate the ground enough to produce a background shaking measurable by modern seismometers as a general "noise". Typically it peaks during the day when people are awake, and drops off in the evening and on weekends. This is a plot of recent seismic noise intensity shared by the Royal Observatory of Belgium; even though their people are working remotely they can still monitor the instruments, and you can see in the past week how much the background seismic noise has declined due to the Stay Home orders. Image credit: https://www.facebook.com/seismologie.be/

Summary of this morning’s Earthquake in Salt Lake City, Utah

https://www.forbes.com/sites/robinandrews/2020/03/18/magnitude-57-quake-just-rocked-utahs-salt-lake-city-heres-what-happened/

If you're looking for something to do with your day, here's 5 minutes where you can learn to read a seismogram.

INSAR

Radar is an extremely useful tool for monitoring changes in the Earth’s surface. Interferometric synthetic aperture radar, or INSAR, can accurately measure tiny changes in the Earth’s surface.

Radar waves pass through Earth’s atmosphere and will bounce off land surfaces. Radar is a form of light - electromagnetic radiation - so like light it is a type of wave with a frequency/wavelength. When radar bounces from the surface to a spacecraft, it arrives back at a specific point in each wave. It's difficult to turn small changes in wavelength from pixel to pixel into any useful information since they’re a function of many properties of the surface, but if 2 images taken close together in time are subtracted from each other, tiny changes from one frame to another stand out. Those tiny changes in the wavelength of the returning energy are reflected in changes in the colors on an INSAR interferogram like this one. As the wavelength of the returning energy changes, it either builds constructively on the waves in the previous pass or interferes destructively.

INSAR is a tool that can show changes in the Earth’s surface from one scene to the next – changes like deformation in a volcano or motion from an earthquake. If 2 closely timed radar images are available, earthquake motions can be understood and volcanic eruption warnings can be given, but doing so requires a satellite built to do the measurement.

Previous satellites could do these measurements, but they weren’t built to do rapid overpasses. A few years ago, the European Space Agency launched its Sentinel-1A satellite (http://on.fb.me/1B1f6Jj), the first in its next-generation series of Earth-observing spacecraft. That satellite carries radar specifically designed for INSAR – it will be able to image every point on Earth’s surface every 12 days.

This INSAR image shows ground deformation in an earthquake in Oaxaca , Mexico last year. The ground moved by a maximum  of about 40 centimeters in this earthquake, so each of the contours of color represents about 3 centimeters of ground motion. The points marked show estimates of the earthquake epicenter based on seismic measurements made by the USGS and Mexican seismic networks; clearly based on this image, the nearby seismic instruments from the Mexican seismic network produced a more precise estimate of the epicenter location.

-JBB

Image credit: EOS/ESA/NASA https://disasters.nasa.gov/oaxaca-mexico-earthquake-2018/insar-map-deformation-due-m72-oaxaca-earthquake https://eos.org/project-updates/earthquake-monitoring-gets-boost-new-satellite