| Summary: | The high-resolution source imaging of large earthquakes is vital for understanding earthquake physics, seismogenic environments, and seismic hazards. Back-projection (BP) is a commonly used technique to reveal how large earthquakes release high-frequency (HF) seismic energy, which is the primary source of severe ground shaking. This dissertation focuses on enhancing the resolution of BP to advance our knowledge of earthquakes.
I first investigate how various factors, including earthquake depth, focal mechanism complexity, and source-side 3D structure, can introduce uncertainties in BP results. Then a novel travel-time calibration strategy is proposed to improve BP resolution by lessening the influence of source-side 3D structures on travel times. The calibrated BP is employed to image the rupture processes of two crustal strike-slip events: the 2002 Mw7.9 Denali and the 2021 Mw7.4 Maduo earthquakes, both featured with clear surface ruptures. Our results demonstrate excellent consistency with independent observations, including surface ruptures and near-fault seismic data.
The calibrated BP method is further applied to study several large earthquakes, including the the 2019 Mw8.0 Peru earthquake, 2021 Mw8.1 Kermadec earthquake and the 2023 Mw7.7&Mw7.7 earthquake doublet. Comparing BP result with the coseismic slip distribution of the Peru earthquake suggests strong frequency-dependent rupture, with the largest slip patch radiating weakest HF seismic energy, suggesting strong dynamic weakening which could be attributed to thermal pressurization.
The 2021 Kermadec earthquake unusually opened the slab-mantle contact, with HF seismic energy emanating from the up-dip boundary of the coseismic slip and located right beneath the slab-forearc Moho
intersection, contrasting with typical megathrust earthquakes. This phenomenon suggests a novel seismogenic environment in the Kermadec region and the HF radiation is linked to with strong material contrast transitioning from slab-crust to slab-mantle contact, possibly associated with serpentinization in the forearc mantle wedge.
The 2023 Turkey earthquake doublet exhibits diverse rupture dynamics. The Mw7.8 event dislocates a plate boundary fault asymmetrically with a super-shear and generalized Rayleigh propagating speeds to the northwest and southwest, respectively. Conversely, the Mw7.6 event ruptures intraplate faults bilaterally in a more symmetrical manner, with both super-shear rupture speeds toward the east and west. The different symmetries during the Mw7.8 and 7.7 events can be attributed to strong and moderate material contrasts across the hosting faults, respectively, highlighting the significant influence of fault zone velocity structures on rupture dynamics.
The newly developed method significantly advances our understanding of earthquake physics and seismogenic environments. Moreover, it holds promise for near real-time high-resolution source imaging of large earthquakes in seismically active regions.
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