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The Secondary Zone of Subsidence (SZS) During Subduction Zone Interseismic Deformation and Its Implications for Megathrust Earthquake Potential

Description: 

Understanding interseismic vertical deformation helps to evaluate the potential of megathrust earthquakes, especially in areas where horizontal constraints are inadequate. In a viscoelastic Earth, a fundamental component of this deformation is the development of a Secondary Zone of Subsidence (SZS) around the volcanic arc, in addition to the Primary Zone of Subsidence (PSZ) above the locked zone of the megathrust. To explain the physics of the SZS, we construct finite element models of repeating megathrust earthquakes. We show that the near-arc area initially undergoes uplift during postseismic stress relaxation but changes to subsidence about halfway into the interseismic period. The late-interseismic subsidence rate of the SZS is higher for a longer earthquake recurrence interval or lower mantle viscosity but is of the order of 10% of the subduction rate. In contrast, the commonly used elastic locking models fail to predict the presence of the SZS and thus can lead to incorrect understanding of earthquake potential.

Our models are validated by a comparison with geodetic observations at various subduction zones that are presently at different stages of their earthquake cycles, including southern Chile (early interseismic), Nankai (mid interseismic), and Japan Trench prior to 2011 (late interseismic). The importance of understanding SZS is demonstrated by examining the Lesser Antilles subduction zone, where the forearc is submarine, and GNSS measurements along the volcanic island arc cannot tell whether there is contractile strain associated with megathrust locking. However, the GNSS measurements indicate that the arc area is subsiding at about 2 mm/yr, 10% of the subduction rate. We propose that the ongoing arc subsidence is evidence for SZS which indicates that the megathrust is locked in a late stage of interseismic deformation. The implication is that this subduction zone is prone to megathrust rupture in the future, requiring a revision of seismic and tsunami hazard assessment in this region.

Session: Temporally Variable Records of Earthquake Behavior and Considerations for Seismic Hazard Analyse [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Kelin

Student Presenter: No

Invited Presentation: 

Poster Number: 36

Authors