Objectives Moderate earthquakes generally produce weaker and more spatially limited elastic deformation than large earthquakes. However, when the source is shallow and the near-surface medium is strongly heterogeneous, centimeter- to decimeter-scale deformation may occur within a narrow near-field zone. Such deformation is commonly characterized by high spatial gradients, low coherence, and irregular spatial patterns, making it difficult to detect using conventional C-band interferometric synthetic aperture radar (InSAR) observations. We investigate the capability of high-resolution L-band InSAR for identifying intense near-field deformation associated with shallow moderate earthquakes.

Methods A sensitivity analysis was conducted by considering three related aspects: the conversion of interferometric phase to line of sight (LOS) displacement controlled by radar wavelength, the dependence of phase uncertainty on interferometric coherence and equivalent number of looks, and the phase gradient conditions required for reliable phase unwrapping. Based on this framework, the theoretical detectability and spatial resolvability of near-field high-gradient deformation were evaluated. Two shallow moderate earthquake cases in South China were then analyzed, including the 2021 Bijie M 4.5 earthquake in Guizhou Province and the 2026 Liuzhou M 5.2 earthquake swarm in Guangxi Zhuang Autonomous Region. Sentinel-1 C-band synthetic aperture radar (SAR), advanced land observing satellite-2 (ALOS-2) phased array type L-band SAR-2 (PALSAR-2), and Lutan-1 (LT-1) L-band SAR data were processed and compared in terms of wrapped phase continuity, coherence distribution, LOS displacement, deformation profiles, and deformation gradients.

Results The theoretical analysis indicates that short-wavelength SAR provides higher phase sensitivity under high-coherence and low-gradient deformation conditions, whereas L-band SAR exhibits greater tolerance to high deformation gradients because larger LOS displacements correspond to fewer phase cycles. The case studies demonstrate that this advantage is particularly important in vegetated, mountainous, and Karst regions. For the Bijie earthquake, Sentinel-1 failed to provide a stable coseismic deformation signal, whereas ALOS-2 detected a near-field deformation zone approximately 3 km in length, with LOS displacement ranging from approximately -15 cm to 10 cm. For the Liuzhou earthquake swarm, Sentinel-1 revealed only weak and partially decorrelated signals, whereas LT-1 recovered a clearer near-field deformation pattern with a maximum LOS displacement close to 20 cm. The deformation peaks, narrow transition zones, and concentrated gradient regions were better preserved in the high-resolution L-band results.

Conclusions High-resolution L-band InSAR is well suited for detecting intense near-field deformation caused by shallow moderate earthquakes in complex surface environments. Its advantage is not determined by wavelength alone, but by the combined effect of longer wavelength, better coherence preservation, lower fringe density and fine spatial sampling. These results indicate that high-resolution L-band SAR can provide important observations for near-field deformation identification, damage interpretation and shallow geological process analysis.