9:45 AM - 10:00 AM
[SCG61-16] Unintuitive Displacement Fields Caused by Horizontal and Thrust Faults in an Elastic-Viscoelastic Layered Half-Space
Keywords:displacement field, viscoelastic relaxation, landform development
The displacement field caused by fault motion is often difficult to intuitively understand. In the case of an infinite elastic medium, the displacement field is symmetrical with respect to the fault, which can be easily understood by considering the force system of a double-couple. In the case of an elastic half-space, however, the situation is significantly different. For example, the surface displacement caused by a reverse fault in an elastic half-space shows an asymmetrical pattern, where the uplift of the hanging wall is much larger than the subsidence on the footwall. This behavior can be understood by considering the influence of the free surface. Furthermore, Okada (2003) demonstrated that in some cases of reverse faulting, subsidence does not occur on the foot wall. When the medium includes a viscoelastic layer, it would be more difficult to intuitively understand the deformation field caused by a fault motion.
In this study, we calculate the displacement field due to faulting in an elastic-viscoelastic layered half-space and consider the deformation mechanism. We use the semi-analytical solution derived by Fukahata & Matsu’ura (2005, 2006) for the calculation.
Koide & Fukahata (2023, 2024; JpGU) presented the vertical displacement fields on the Earth's surface resulting from a reverse fault placed at shallow and deep parts in the upper (elastic) layer. When a reverse fault is placed in the shallow part, the hanging wall uplifts due to elastic deformation; subsequently, it subsides due to viscoelastic relaxation. When a reverse fault is placed in the deep part of the elastic layer, overall uplift occurs due to elastic deformation; subsequently, further significant uplift occurs due to viscoelastic relaxation. Generally, the elastic layer behaves like a plate after viscoelastic relaxation because the viscoelastic layer behaves like a fluid. When a reverse fault exists only in either the shallow or deep part of the elastic layer, nonuniform shortening occurs with respect to the depth, which causes the elastic plate to bend, resulting in the deformation field mentioned above.
We also calculate displacement fields caused by a horizontal fault in the elastic layer. The horizontal fault is placed at a depth in the middle of the elastic layer. When we give a fault motion with the upper side of the horizontal fault moving to the left and the lower side to the right, uplift occurs around the left end of the fault and subsidence near the right end due to elastic deformation. After viscoelastic relaxation, further uplift and subsidence occur. The internal deformation field after viscoelastic relaxation, upward displacements occur on the left side of the fault and downward displacements on the right side. As mentioned above, the elastic layer behaves as an elastic plate after viscoelastic relaxation. Therefore, considering that the thickness of the elastic medium is finite in the vertical direction while it is infinite in the horizontal direction, it is suggested that the vertical component of the double-couple forces significantly contributes to the deformation. Thus, antisymmetric vertical displacement occurred at both ends of the fault.
In this study, we calculate the displacement field due to faulting in an elastic-viscoelastic layered half-space and consider the deformation mechanism. We use the semi-analytical solution derived by Fukahata & Matsu’ura (2005, 2006) for the calculation.
Koide & Fukahata (2023, 2024; JpGU) presented the vertical displacement fields on the Earth's surface resulting from a reverse fault placed at shallow and deep parts in the upper (elastic) layer. When a reverse fault is placed in the shallow part, the hanging wall uplifts due to elastic deformation; subsequently, it subsides due to viscoelastic relaxation. When a reverse fault is placed in the deep part of the elastic layer, overall uplift occurs due to elastic deformation; subsequently, further significant uplift occurs due to viscoelastic relaxation. Generally, the elastic layer behaves like a plate after viscoelastic relaxation because the viscoelastic layer behaves like a fluid. When a reverse fault exists only in either the shallow or deep part of the elastic layer, nonuniform shortening occurs with respect to the depth, which causes the elastic plate to bend, resulting in the deformation field mentioned above.
We also calculate displacement fields caused by a horizontal fault in the elastic layer. The horizontal fault is placed at a depth in the middle of the elastic layer. When we give a fault motion with the upper side of the horizontal fault moving to the left and the lower side to the right, uplift occurs around the left end of the fault and subsidence near the right end due to elastic deformation. After viscoelastic relaxation, further uplift and subsidence occur. The internal deformation field after viscoelastic relaxation, upward displacements occur on the left side of the fault and downward displacements on the right side. As mentioned above, the elastic layer behaves as an elastic plate after viscoelastic relaxation. Therefore, considering that the thickness of the elastic medium is finite in the vertical direction while it is infinite in the horizontal direction, it is suggested that the vertical component of the double-couple forces significantly contributes to the deformation. Thus, antisymmetric vertical displacement occurred at both ends of the fault.