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[SCG55-P11] Unintuitive Displacement Field Caused by Fault Motion 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. However, 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 foot wall. This 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 is more difficult to intuitively understand the deformation field due to 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.
We compute the vertical displacements on the Earth's surface resulting from a reverse fault placed at shallow, middle, 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; after that 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; after that further significant uplift occurs due to viscoelastic relaxation. These results indicate that the understanding that viscoelastic relaxation occurs to restore gravitational equilibrium is not always correct.
To understand the characteristics of these displacement fields physically, we also calculate internal deformations. When we put a reverse fault in the shallow part of the elastic layer, after viscoelastic relaxation, downward displacements occurred overall around the fault. In contrast, when we put a reverse fault in the deep part of the elastic layer, after viscoelastic relaxation, upward displacements occur overall around the fault. Generally, after viscoelastic relaxation, the elastic layer behaves like a plate because the viscoelastic layer becomes like a liquid. Considering that the force exerted on a reverse fault is horizontal compression, when a reverse fault exists only in either the shallow or deep part of the elastic layer, unbalanced shortening occurs, which cause the elastic plate to bend, resulting in the deformation field mentioned above.
We also calculate displacement fields caused by horizontal faults in the elastic layer. When we give a fault motion with the upper part of the horizontal fault moving to the left and the lower part to the right, uplift occurs near the left end of the fault and subsidence near the right end due to elastic deformation. After viscoelastic relaxation, further uplift and subsidence occur. Examining 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, after viscoelastic relaxation the elastic layer behaves as an elastic plate. Therefore, considering that the thickness of the elastic medium is finite in the vertical direction but infinite in the horizontal direction, it is suggested that the vertical component of the double-couple forces significantly contributes to the deformation. Thus, symmetric vertical displacement occurred at both edges 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.
We compute the vertical displacements on the Earth's surface resulting from a reverse fault placed at shallow, middle, 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; after that 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; after that further significant uplift occurs due to viscoelastic relaxation. These results indicate that the understanding that viscoelastic relaxation occurs to restore gravitational equilibrium is not always correct.
To understand the characteristics of these displacement fields physically, we also calculate internal deformations. When we put a reverse fault in the shallow part of the elastic layer, after viscoelastic relaxation, downward displacements occurred overall around the fault. In contrast, when we put a reverse fault in the deep part of the elastic layer, after viscoelastic relaxation, upward displacements occur overall around the fault. Generally, after viscoelastic relaxation, the elastic layer behaves like a plate because the viscoelastic layer becomes like a liquid. Considering that the force exerted on a reverse fault is horizontal compression, when a reverse fault exists only in either the shallow or deep part of the elastic layer, unbalanced shortening occurs, which cause the elastic plate to bend, resulting in the deformation field mentioned above.
We also calculate displacement fields caused by horizontal faults in the elastic layer. When we give a fault motion with the upper part of the horizontal fault moving to the left and the lower part to the right, uplift occurs near the left end of the fault and subsidence near the right end due to elastic deformation. After viscoelastic relaxation, further uplift and subsidence occur. Examining 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, after viscoelastic relaxation the elastic layer behaves as an elastic plate. Therefore, considering that the thickness of the elastic medium is finite in the vertical direction but infinite in the horizontal direction, it is suggested that the vertical component of the double-couple forces significantly contributes to the deformation. Thus, symmetric vertical displacement occurred at both edges of the fault.