Remote sensing investigations of immature faulting and distributed coseismic deformation
Date
2023-09-07
Authors
Gaudreau, Élyse
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Abstract
Anticipating future earthquake behavior requires active faults to be accurately mapped as well as a wealth of knowledge on their physical properties and tectonic histories, but many incipient faults are unknown until after a damaging earthquake strikes. Earthquakes on so-called immature faults are also associated with near-surface slip that is distributed over zones hundreds to thousands of meters wide, rather than focused along a narrow fault with a clear surface expression as are their mature counterparts. This results in an underestimation of seismic hazard, as field surveys cannot capture the full coseismic displacement field, and typical fault slip inversions are unable to model this distributed deformation.
In this thesis, a suite of cutting edge geodetic remote sensing techniques --- Interferometric Synthetic Aperture Radar (InSAR), photogrammetry and correlation of historical optical imagery --- are exploited to investigate what controls the localization of surface slip and the consequences of common modeling assumptions such as approximating a fault as a series of infinitely thin planes in an elastic halfspace. In doing so, I also shed light on the active tectonics of three study regions --- northern Alaska, southern California and the Tibetan plateau --- with important implications for seismic hazard assessments.
The 1971 Mw 6.6 San Fernando, California, 2018 Mw 6.4 and Mw 6.0 Kaktovik, Alaska, and 2021 Mw 7.4 Maduo, China earthquakes are three examples of major earthquakes to have ruptured immature faults that were previously unknown or considered inactive, demonstrating that incipient faults pose a significant seismic hazard. The coseismic displacement field of the 1971 San Fernando, California earthquake, extracted from historical aerial imagery, reveals in unprecedented detail the complexity of the surface deformation. There is a discrepancy between seismic data and surface (geologic and geodetic) displacement data that may be explained by the strong dynamic stresses inferred from nearby accelerometer recordings. For the 2018 Kaktovik, Alaska earthquakes, a combined analysis of InSAR and seismological data reveals that deformation in this region is accommodated by a complex network of unmapped faults that may be influenced by inherited Cordilleran structures. A survey of well-studied earthquakes from across the Tibetan plateau suggests that inherited structures may have also played a significant role in the 2021 Maduo earthquake, possibly enabling its unusually long rupture and therefore large magnitude.
Surface displacements mapped with optical imagery and InSAR show that surface slip during the 1971 San Fernando, California earthquake and multiple recent earthquakes in the Tibetan plateau was distributed over zones hundreds of meters wide. I show that this may partly explain why InSAR-derived coseismic slip models systematically underestimate near-surface slip, with the degree of bias partly dependent on fault complexity and thus likely influenced by fault maturity. I also demonstrate that independently inverted geodetic slip models are unreliable in the shallowest 1 km of the crust, suggesting that the proportion of this shallow slip deficit that can be attributed to distributed, inelastic surface deformation cannot be meaningfully estimated.
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Keywords
Remote sensing, Earthquakes, InSAR, Optical imagery, Fault maturity