Date of Award

2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Specialization

Marine Geology and Geophysics

Department

Oceanography

First Advisor

Meng (Matt) Wei

Abstract

Oceanic transform faults (OTFs) are long being recognized as hosting fewer and smaller earthquakes. On average, only 15% of the accumulated strain energy on OTFs are released through earthquakes. Detailed studies on a handful of OTFs suggests that their seismic distributions are highly segmented along-strike and favor the thick seismogenic patch model or multi-mode. Therefore, in Manuscript 1, we test this hypothesis at a global scale on 138 OTFs. We find that creeping segments are ubiquitous and alone can explain the deficiency of the earthquakes on OTFs. On a global scale, the thick seismogenic patch model is most consistent with the observation above. Moreover, 59 out of 71 OTFs with good bathymetry data have creeping segments not associated with geological structures, indicating along-strike variation of fault properties may be the main reason for the existence of those creeping segments.

Besides having persistent seismic segments, large characteristic earthquakes on OTFs often occurs quasi-periodically from modern seismic observations and sometimes synchronized earthquake cycles on nearby faults or fault segments are well documented. To further understand the mechanisms behind these synchronized earthquakes on OTFs, in Manuscript 2, we simulate a 2D elastic fault in the framework of rate- and state-dependent friction and demonstrate that the synchronization between two nearby asperities (20 km x 5 km), separated by 10 km, can be achieved via afterslip when the intervening barrier is relatively weak or through static stress interaction when the barrier patch is strong. We also find that the width of the barrier patch is more important than its strength in determining the synchronization behavior. Furthermore, interaction between two asperities promotes partial rupture and leads the fault system to switch between synchronized and desynchronized.

Earthquake synchronization over more distant asperities than the one studied above requires a mechanism that can reach the far field, such as viscoelastic stress transfer, which has received much less attention and is still poorly understood. In Manuscript 3, we build a three-dimensional numerical model of seismic cycles in the framework of rate- and state-dependent friction with a brittle layer overlaying a viscoelastic mantle with a nonlinear rheology to simulate earthquake cycles on separate asperities. We find that in general viscoelastic stress transfer can promote earthquake synchronization between two distant seismic asperities. Even if the asperities are separated by 30 km, synchronization is still possible for a viscosity of the underlying mantle of 1017 Pa s, which can be attained by dislocation creep or transient creep during the postseismic. Considering the similarities in tectonic and structural settings, viscoelastic stress transfer and earthquake synchronization may also occur, as observed in Manuscript 1, at the St. Paul and Menard OTFs on the Mid-Atlantic and Pacific-Antarctic ridges, respectively.

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