Other Space Activities

Earthquakes (InSAR)

Dec 1, 2023

Applications

Earthquakes are the release of stresses in the Earth’s crust, caused by the slippage of tectonic plates along a fault line, plate subduction, subterranean heat flow, and volcanism. As the plates move past each other, they send seismic waves through the Earth. Earthquakes have devastating effects on lives and infrastructure, and are among the deadliest and costliest of natural disasters on the planet. Satellite observations provide a promising tool for rapidly detecting and monitoring earthquakes, facilitating quicker response and communication in times of crises. Remote sensing satellites provide data on seismic activity, land use and dynamic change, and damage assessments, while communications satellites provide essential support for disaster response. 5) 6)

Scientists are continuously developing methods for assessing seismic risks to better understand earthquakes and their impacts. Satellites with radar instruments can detect seismic activity from space, serving as a critical tool for measuring earthquakes. 1)

Small changes in the Earth’s topography can be measured to a high accuracy with Synthetic Aperture Radar (SAR) instruments. Radar imaging satellites send out radar pulses to the Earth’s surface and receive the reflected ‘echo’ that is returned. Multiple radar echoes gathered along the instrument’s swath and path of travel come together to produce a SAR image, which contains information about the distance each pulse travelled to the ground, revealing the 3D topography of the Earth’s surface. This imaging technique benefits from its capability to see through clouds and complete darkness, unlike traditional optical imagery that relies on reflected light and prime conditions. 1) 2)

Interferometric SAR (InSAR) or radar interferometry is used to measure changes in the Earth’s topography as a result of various processes, including seismic activity. SAR is produced at contiguous bands that range from 1 - 30 cm in wavelengths, and these longer wavelengths are what allow radar pulses to penetrate various media. Dividing distance the pulse travels by its wavelength yields its phase. Interferometry involves inspecting the difference in phase between two or more waves, and in the case of InSAR, comparing radar signals by stacking SAR images on top of each other. Any change in phase (i.e. from millimetre-scale shifts of the Earth’s crust) produces an interference pattern, and facilitates the production of interferograms. 3)

Figure 1: InSAR principles. Illustrates how changes in the Earth’s topography between satellite passes introduces a phase shift in measurements, facilitating the production of interferograms. (Image credit: Geoscience Australia)

Spaceborne InSAR enables the precise spotting of earthquake faults, active volcanoes and the slowly drifting tectonic plates, as well as monitoring glacial movement, landslides and man-made subsidence, identifying aquifers, and tracking oil and gas extraction across the globe. 4)

More on InSAR can be found in ESA’s InSAR Principles Guidelines manual.

Large earthquakes not only deform the Earth’s crust, but they also cause small changes in local gravity fields. Gravitational field strength varies around the planet’s surface, and ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission was tasked to map these variations with high precision. These variations in gravity are often due to the inhomogeneous distribution of material inside the planet. Earthquakes force the movement of this material by tens of kilometres, which causes small changes in local gravity. If originating underwater, earthquakes can change the shape of the seabed, which displaces water and affects the ocean surface topography as well as gravity. 7)

Figure 2: Variations in the gravity field around Japan after the 2011 magnitude 9.0 earthquake, measured by GOCE and GRACE. Gravity field is measured in milliEötvös (mE) - equivalent to 10-12 s-2. The ‘beach ball’ marks the epicentre. (Image credit: DGFI/TU Delft)

It was revealed that GOCE ‘felt’ sound waves in space from an earthquake, through analysis of the devastating 9.0 magnitude quake that struck East of Japan’s Honshu Island in March 2011. High-resolution vertical gravity gradients measured by GOCE over Japan show that the quake had ruptured the gravity field, sending out ripples of low frequency sound - called infrasound - into the atmosphere. The sound waves created perturbations of air density in the thermosphere, detected by GOCE as it crossed the wavefront. GOCE’s findings are also consistent with observations from the US-German Gravity Recovery and Climate Experiment (GRACE) mission. 7) 8)

Animation of GOCE’s measurements of the March 2011 Tohoku earthquake can be viewed from this link.

Tectonic movements can also be tracked by navigation satellites paired with ground-based receiver stations. Global Navigation Satellite Systems (GNSS) can determine the magnitude of large earthquakes by measuring the displacement of the ground at receiver stations close to the fault. Seismometers are much more sensitive to seismic activity, but high magnitude earthquakes can force measurements ‘off the charts’. Thus the combination of GNSS and seismometers, be it ground or space based, complement each other well. GNSS data can also supplement InSAR data, filling in the gaps between passes over an area. 10)

Example Products

Interferograms

Satellites perform interferometric SAR scans of Earth’s surface to produce paired or multiple image interferograms, revealing changes in a landscape across large areas in high detail. Interferograms are visualised with optical interference patterns of light being reflected together, forming a colourful and comprehensive map. The fringes of interference between the phase-shifted SAR images represent changes in the Earth’s surface due to seismic activity. 3) 12)

Figure 3: Interferogram of the Bay of Naples, Italy created from the European Remote-Sensing Satellite -1 (ERS-1) InSAR data. (Image credit: F.Rocca/Politechnic University of Milan, and ESA, 3))

Figure 3 shows an image consisting of two composite images, the wider image being a backscatter-intensity radar image, and the smaller, colourful one being an interferogram of Vesivius and its surroundings. The coloured fringes represent changes in topography.

Digital Elevation Models (DEMs)

DEMs created with lidar (light detection and ranging) data are used to monitor and analyse the aftermath of earthquakes. Multi-temporal datasets and DEMs are compared from before and after an earthquake, to assess damage and ground movement. Repeat lidar imagery can be used to assess collapsed buildings and other damage to infrastructure. DEMs are a more detailed and insightful tool than traditional imagery for damage assessment following natural disasters, as 3D changes can be measured, extracting more information from the site. Height information from lidar pulses indicate if a building has collapsed or if the Earth has moved, by creating a DEM and comparing it to a pre-earthquake DEM. 11) 12)

Aspects typically assessed with lidar data following an earthquake are crustal movement, building damage and landslides. All these measurements can assist first responders, emergency services and local governments.

Figure 4: Digital Surface Model (DSM) comparison from before (Jan 2013) and after (Apr 2016) the Kumamoto earthquake over the Minimami-Aso village, Kumamoto prefecture, Japan. (Image credit: National Research Institute for Earth Science and Disaster Resilience (NIED))

As well as using lidar instruments, spaceborne SAR instruments can be used to produce DEMs.

Figure 5: Sentinel-1 DEM - interferogram pair from after the 2019 earthquake sequence in Mindanao, Philippines (Image credit: COMET)

Related Missions

Copernicus: Sentinel 1

The ESA Sentinel-1 constellation consists of two radar imaging satellites, Sentinel-1A launched in April 2014 and Sentinel-1C launching in March 2024, succeeding the anomalous end to Sentinel-1B’s mission in late 2021. The identical satellites carry the C-band SAR instrument, with Stripmap (SM), Interferometric Wide Swath (IW), Extra Wide Swath (EW), and Wave (WV) imaging modes. Sentinel-1 is a key mission for monitoring seismic activity around the Earth with its InSAR capabilities, as well as for monitoring sea and land ice, mapping of forest, water and soil management. Sentinel-1 is the first space component of the Copernicus programme.

GOCE (Gravity field and steady-state Ocean Circulation Explorer)

GOCE was an ESA geodynamics and geodetic satellite mission that provided valuable data to advance understanding of the Earth’s interior, determine the stationary gravity field, and provide a high-accuracy global height reference system - the geoid. Launched in March 2009, GOCE became the first spaceborne seismometer after detecting infrasound waves from the March 2011 Japanese earthquake with its accelerometer. GOCE’s mission ended in November 2013.

Envisat

Envisat was an ESA research mission launched in March 2002 with the aim to study and monitor the Earth’s environment on various scales, succeeding the ERS-1 (European Remote Sensing-1) mission. The mission carried the Advanced SAR (ASAR) and the Radar Altimeter-2 (RA-2) instruments to gather InSAR data, facilitating production of displacement maps and interferograms of tectonic activity around Earth. Envisat operated for 10 years, providing science with a wealth of data on our planet’s health and climate change.

NISAR (NASA-ISRO Synthetic Aperture Radar)

NASA and the Indian Space Research Organisation (ISRO) are together developing NISAR, a mission planned to globally measure the causes and effects of land surface changes. The 2024-launching mission will be the first satellite to use two different radar frequencies (L-band and S-band) to measure changes in the planet’s surface less than a centimetre across. This allows NISAR to measure a wide range of changing events, from glacial movement to the dynamics of earthquakes and volcanoes.

GRACE (Gravity Recovery And Climate Experiment)

GRACE was a US-German dual-minisatellite satellite-to-satellite tracking (SST) geodetic mission that operated from March 2002 to October 2017. The mission made K/Ka-band ranging measurements between GRACE-1 and -2 to detect small changes in satellite separation, facilitating gravity field measurements of the Earth with unprecedented accuracy. GRACE provided crucial information about Earth’s dynamic processes like earthquakes, and was succeeded by the GRACE-FO (Follow-On) mission.

GRACE | GRACE-FO

QuakeSat

QuakeSat was a US research nanosatellite operating from June 2003 to December 2004, with the objective to measure extremely low frequency (ELF) magnetic signal data as a potential earthquake precursor.

DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions)

Demeter was a French space agency (CNES) microsatellite mission that studied ionospheric and electromagnetic perturbations caused by natural phenomena like earthquakes, operating from June 2004 to December 2010.

References

1) “Earthquakes,” ESA, Observing the Earth, URL: https://www.esa.int/Applications/Observing_the_Earth/Earthquakes

2) “Earthquake monitoring with radar satellites,” ESA, Observing the Earth, URL: https://www.esa.int/ESA_Multimedia/Videos/2015/02/Earthquake_monitoring_with_radar_satellites

3) “How does interferometry work?,” ESA, Observing the Earth, URL: https://www.esa.int/Applications/Observing_the_Earth/How_does_interferometry_work

4) “Widening Envisat’s InSAR view,” ESA, Observing the Earth, August 2008, URL: https://www.esa.int/Applications/Observing_the_Earth/Widening_Envisat_s_InSAR_view

5) “How are earthquakes detected, located and measured?,” British Geological Survey, URL: https://www.bgs.ac.uk/discovering-geology/earth-hazards/earthquakes/how-are-earthquakes-detected/

6) Harrison, Paul et al, (2014). “Earthquake Monitoring and Response from Space: The TREMOR Concept,” International Space University, URL: https://www.researchgate.net/publication/33422430_Earthquake_Monitoring_and_Response_from_Space_The_TREMOR_Concept

7) “Earth’s gravity scarred by earthquake,” ESA GOCE, December 2013, URL: https://www.esa.int/Applications/Observing_the_Earth/FutureEO/GOCE/Earth_s_gravity_scarred_by_earthquake

8) “Earthquake ‘felt’ in space, ESA, March 2013, URL: https://www.esa.int/ESA_Multimedia/Videos/2013/03/Earthquake_felt_in_Space

9) “GOCE: the first seismometer in orbit”, ESA March 2013, URL: https://www.esa.int/Applications/Observing_the_Earth/FutureEO/GOCE/GOCE_the_first_seismometer_in_orbit

10) “GNSS for Earth Observation,” Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics, URL: https://comet.nerc.ac.uk/gnss-earth-observation/

11) Yamazaki, F.; Liu, W.; Horie, K. Use of Multi-Temporal LiDAR Data to Extract Collapsed Buildings and to Monitor Their Removal Process after the 2016 Kumamoto Earthquake. Remote Sens. 2022, 14, 5970. https://doi.org/10.3390/rs14235970

12) Hajeb, M.; Karimzadeh, S.; Matsuoka, M. SAR and LIDAR Datasets for Building Damage Evaluation Based on Support Vector Machine and Random Forest Algorithms—A Case Study of Kumamoto Earthquake, Japan. Appl. Sci. 2020, 10, 8932. https://doi.org/10.3390/app10248932

13) “SAR (ERS) Interferometry,” ESA earth online, URL: https://earth.esa.int/eogateway/instruments/sar-ers/interferometry

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (eoportal@symbios.space).