Earth, Planets and Space | 2021
L-band Synthetic Aperture Radar: Current and future applications to Earth sciences
Abstract
© The Author(s) 2021. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Synthetic Aperture Radar (SAR) has been around for more than 40 years. The first civilian-use SAR satellite was SeaSat, launched by National Aeronautics Space Administration (NASA) in 1978 to monitor Earth’s oceans. In early 1990s, we realized that SAR Interferometry (InSAR) is a powerful tool in delineating Earth’s topography and deformation (e.g., Massonnet et al. 1993) because SAR can image broad areas with a high spatial resolution on the order of 10 m or less without a requirement of ground-based instruments. The advantage of SAR as a powerful imaging tool is also because it works day and night regardless of clouds. SAR images taken from L-band satellites complement satellites at shorter wavelengths such as Cand X-bands. L-band SAR has lower resolution than Cand X-band images but are more coherent over time, especially in vegetated regions. L-band SAR data are typically easier to unwrap because of higher coherence and fewer fringes, but more susceptible to ionospheric effects. L-band SAR satellites for InSAR include JERS-1 (1992– 1998), ALOS (2006–2011), ALOS-2 (2014–present), and SAOCOM-1 (2017–present), which have a recurrence time of order a few weeks. Such recurrence times do not allow monitoring with high temporal resolution. However, after the launch of ALOS-4, NISAR, and Tandem-L in the next few years, we will be able to monitor Earth’s surface every few days with L-band SAR. Therefore, the time is ripe to review what we have learned from previous and ongoing L-band missions, either spaceborne or airborne and what we expect to learn from future missions. This special issue consists of 11 papers on various topics, including earthquake and volcano deformation, land subsidence, landslide, and permafrost. SAR images revealed that an earthquake rupture is almost always more complicated than a shear failure on a planar fault. SAR images have demonstrated that an earthquake rupture accompanies off-fault cracks and lineaments. Fujiwara et al. (2020) reviewed the observations of such secondary features associated with several onshore earthquakes in the last 25 years. The geometry of the faults on which triggered shallow slip occurs is variable; some faults are parallel to the main fault, and some are oblique or perpendicular to the main fault. However, they found some common features that tend to occur on pre-existing weakness. Therefore, a large earthquake in the future would also trigger shallow slips at the same location. Hashimoto (2020) and Ishitsuka et al. (2020) investigated the 2016 Kumamoto earthquake (Mw = 7.0). Hashimoto (2020) investigated the first two years of postseismic deformation from ALOS-2 images. He noticed that the L-band SAR images are coherent even if image pairs are separated for 2 years. He also noticed that some images are susceptible to ionospheric perturbations. The observed deformation field is complex; the time constant of displacement decay varies between 30 to 600 days in different locations, indicating that the observed deformation results from multiple origins. Afterslip with a mixed right-lateral normal faulting mechanism explains much of the observed deformation field, but the afterslip cannot explain observed deformation in some areas (Table 1). Open Access