Discovery and characterization of earthquakes on immature faults with InSAR
Date
2024
Authors
Sethanant, Israporn
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Abstract
Fault structural maturity describes how fault zones evolve with incremental slip, and may control certain aspects of earthquake rupture behaviour. Immature faults are harder to identify and characterize than mature faults by virtue of their subtle to non-existent topographic and geophysical expressions, and thus pose distinct hazards and risks. Nonetheless, the advent of satellite-borne Interferometric Synthetic Aperture Radar (InSAR) surface deformation mapping has revolutionized our ability to characterize large earthquakes that occur on these hidden faults. By doing so, we can start to recognize common attributes to immature faults that may have important implications for seismic hazard. This is the central theme of this dissertation.
After introducing how InSAR works, I describe how to interpret surface deformation maps for various faulting styles. To explore relations between structural maturity and rupture behaviour, I then employ InSAR to characterize recent earthquakes along immature strike-slip faults in two contrasting tectonic settings: the Eastern California Shear Zone/Walker Lane of California and Nevada, USA, and the Shan Plateau in Laos, Thailand and Myanmar. The main foci are the 2020 Mw 6.5 Monte Cristo Range, Nevada earthquake and the 2019 Mw 6.2 Sainyabuli, Laos earthquake, respectively. In both cases, I tried to characterize the full complexity of the earthquakes by working with collaborators to incorporate independent seismological constraints.
We found certain rupture behaviours common to the Monte Cristo Range and Sainyabuli earthquakes that may be linked to fault maturity. Both mainshocks had short (∼20–30 km) fault lengths and pronounced (∼80–90%) shallow slip deficits (SSD) that may reflect off-fault deformation ∼5–10 km from the primary faults, consistent with relocated aftershocks. However, InSAR slip profile comparisons with other strike-slip earthquakes suggest that SSD is controlled more by earthquake magnitude than maturity, with smaller events having the greatest range in SSD and larger ones propagating to the surface with more consistency. The Monte Cristo Range earthquake also involved an unusual configuration of crossing faults, likely due to the early evolution stage of the host fault zone.
The limited geomorphic expression along modelled faults of the Monte Cristo Range and Sainyabuli events raises an alarming concern for seismic hazard assessment (SHA). Since immature faults are hard to map, they are often overlooked in hazard calculations. However, SHA practitioners may benefit from knowledge of rupture geometries and behaviours provided by observational studies like this dissertation, to help define seismic sources and select appropriate ground motion models for regional and site-specific SHA.
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Keywords
Earthquake, Seismology, Immature faults, InSAR, Remote-sensing, Seismic hazard, Strike-slip earthquakes