Dynamics and Near‐Field Surface Motions of Transitioned Supershear Laboratory Earthquakes in Thrust Faults. Issue 3 (23rd March 2022)

• Record Type: Journal Article
• Authors: Tal, Yuval; Rubino, Vito; Rosakis, Ares J.; Lapusta, Nadia
• Is Part Of: Journal of geophysical research. Volume 127:Issue 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-23
• Subjects: thrust faults -- dynamic ruptures -- laboratory earthquakes -- ground motion -- digital image correlation -- supershear; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB023733
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: We study how the asymmetric geometry of thrust faults affects the dynamics of supershear ruptures and their associated trailing Rayleigh ruptures as they interact with the free surface, and investigate the resulting near‐field ground motions. Earthquakes are mimicked by propagating laboratory ruptures along a frictional interface with a 61° dip angle. Using an experimental technique that combines ultrahigh‐speed photography with digital image correlation, we produce sequences of full‐field evolving measurements of particle displacements and velocities. Our full‐field measurement capability allows us to confirm and quantify the asymmetry between the experimental motions of the hanging and footwalls, with larger velocity magnitudes occurring at the hanging wall. Interestingly, because the motion of the hanging wall is generally near‐vertical, while that of the footwall is at dip direction shallower than the dip angle of the fault, the horizontal surface velocity components are found to be larger at the footwall than at the hanging wall. The attenuation in surface velocity with distance from the fault trace is generally larger at the hanging wall than at the footwall and it is more pronounced in the vertical component than in the horizontal one. Measurements of the rotations in surface motions confirm experimentally that the interaction of the rupture with the free surface can be interpreted through a torqueing mechanism that leads to reduction in normal stress near the free surface for thrust earthquakes. Nondimensional analysis shows that the experimental measurements are consistent with larger‐scale numerical simulations as well as field observations from thrust earthquakes. Plain Language Summary: The asymmetric interaction of thrust earthquakes with the Earth's surface leads to complex dynamic behavior and strongly asymmetric ground motions. Near‐fault measurements from such earthquakes are rare and do not allow for detailed characterization of the earthquake rupture and the associated near‐field ground motions. In this study, we create controlled ruptures in a laboratory set‐up mimicking the thrust fault earthquake process. We utilize a unique, optical, ultrahigh‐speed imaging technique to observe such updip laboratory earthquakes at high spatial resolution and in real time, and to analyze their complex dynamic interactions with the free surface. Such a study would be difficult to achieve in the field because of the typical spatial sparsity of the recorded data. The experiments allow us to quantify the differences in ground motion between the two sides of the fault, the decrease of ground motion with distance from the fault, and the dynamic surface rotations. Moreover, the experimental observations enable us to directly relate the measured near‐field ground motion to the state of the earthquake rupture on the fault. Key Points: We characterize laboratory thrust ruptures as they interact with the free surface after transitioning to supershear at various distances Our full‐field analysis enables studying the relationship between near‐field ground motion and the dynamics of the ruptures on the fault Velocity magnitudes are larger at the hanging wall, but the horizontal velocities are larger at the footwall because of rotations