10:45 〜 12:15
[SGD02-P14] Mechanical Modeling of Makran Megathrust and its splay faults for prediction of slip components
キーワード：Deformation pattern, Seismogenic/seismic shaking potential, Imbricate thrust fault system, Seismic attributes, Mechanical modeling, Limit Analysis
Pakistan lies on the region where three plates collide with each other along with a triple junction near Karachi city. In southwest, the Arabian plate subducts beneath the Eurasian plate off the southern coast of Iran and Pakistan. At the western end of the subduction megathrust, it is connected to the Zagros thrust system of Iran through Minab fault, whereas it is connected to the southern Chaman fault system along with Murray Ridge transtensional zone at its eastern end. The Makran Subduction Zone (MSZ) has produced the world's largest arc-trench gap (500-600 km) and wedge system with splay faults (both on and offshore), having ~7 km thick sedimentary strata.
The MSZ is poorly understood, yet potentially the largest source of earthquakes and tsunami hazards in southern Pakistan. The increasing convergence rates between the Arabia and the Eurasia moving from west to east across the subduction zone initiates the slip partitioning into the megathrust motion. This differential convergence divides the MSZ into eastern and western parts with high and low seismicity, respectively. Seismicity shows that only Makran offshore region has experienced at least six large earthquakes (M>7) rupturing the plate boundary in the last 500 years, including the tsunamigenic 1945 M8.1 event (the second worst tsunami in the Indian Ocean after 2004 Sumatra tsunami). Following the 1945 earthquake, tsunami runups of upto 15 m were reported at Pasni and Ormara, 1.4 m at Karachi, and 2 m at Mumbai (Byrne et al., 1992; Heidarzadeh et al., 2008). This zone has the potential to generate 8.7– 9.2 (Mw) earthquakes (Smith et al., 2013; Frohling and Szeliga, 2016).
By interpreting digital seismic profiles and its attributes, we investigate the deformation patterns and seismogenic potential of the study area. By application of mechanical modelling based on seismic geometry using limit analysis approach, we aimed to understand the seismic shaking potential with subsequent tsunami generation at frontal interface and to retrieve the frictional properties. The seismic profiles analysis demonstrates that MSZ has an imbricate fault system with seaward verging thrusts and the velocity analysis suggests lateral continuity in the frontal region.
Seismic attributes highlight the fractured and porosity zones having possible high pore pressure, which greatly reduce friction values along the megathrust interface. The effective friction (µeff) is estimated to be ≦0.12 to slip along the subduction interface. Our analysis suggests that low friction properties in the frontal area are one of major reason of landward dipping thrust faults .i.e., µeff ~0.12–0.04 from back to frontal interface are required to reproduce the observed fault system. For wedge geometry, we run different simulations to estimate the frontal horizontal and vertical components of slip. Assuming the convergence rate of 3-4 cm/yr, the calculated slip deficit in last 77, 158, and 257 years, after 1945, 1864 and 1765 earthquakes along offshore ruptured patches of eastern MSZ is 2.31-3.08 m, 4.74-6.32 m, and 7.71-10.28 m, respectively. A frontal uplift ranging from 0.69 ± 0.37 to 3.02 ± 1.45 m can be expected, if the next rupture occurs along the previously ruptured areas. Splay faults have high uplift as compared to the plate interface and would amplify the uplift of thick water column (~3000 m) and can generate a large tsunami in the coastal areas.
The MSZ is poorly understood, yet potentially the largest source of earthquakes and tsunami hazards in southern Pakistan. The increasing convergence rates between the Arabia and the Eurasia moving from west to east across the subduction zone initiates the slip partitioning into the megathrust motion. This differential convergence divides the MSZ into eastern and western parts with high and low seismicity, respectively. Seismicity shows that only Makran offshore region has experienced at least six large earthquakes (M>7) rupturing the plate boundary in the last 500 years, including the tsunamigenic 1945 M8.1 event (the second worst tsunami in the Indian Ocean after 2004 Sumatra tsunami). Following the 1945 earthquake, tsunami runups of upto 15 m were reported at Pasni and Ormara, 1.4 m at Karachi, and 2 m at Mumbai (Byrne et al., 1992; Heidarzadeh et al., 2008). This zone has the potential to generate 8.7– 9.2 (Mw) earthquakes (Smith et al., 2013; Frohling and Szeliga, 2016).
By interpreting digital seismic profiles and its attributes, we investigate the deformation patterns and seismogenic potential of the study area. By application of mechanical modelling based on seismic geometry using limit analysis approach, we aimed to understand the seismic shaking potential with subsequent tsunami generation at frontal interface and to retrieve the frictional properties. The seismic profiles analysis demonstrates that MSZ has an imbricate fault system with seaward verging thrusts and the velocity analysis suggests lateral continuity in the frontal region.
Seismic attributes highlight the fractured and porosity zones having possible high pore pressure, which greatly reduce friction values along the megathrust interface. The effective friction (µeff) is estimated to be ≦0.12 to slip along the subduction interface. Our analysis suggests that low friction properties in the frontal area are one of major reason of landward dipping thrust faults .i.e., µeff ~0.12–0.04 from back to frontal interface are required to reproduce the observed fault system. For wedge geometry, we run different simulations to estimate the frontal horizontal and vertical components of slip. Assuming the convergence rate of 3-4 cm/yr, the calculated slip deficit in last 77, 158, and 257 years, after 1945, 1864 and 1765 earthquakes along offshore ruptured patches of eastern MSZ is 2.31-3.08 m, 4.74-6.32 m, and 7.71-10.28 m, respectively. A frontal uplift ranging from 0.69 ± 0.37 to 3.02 ± 1.45 m can be expected, if the next rupture occurs along the previously ruptured areas. Splay faults have high uplift as compared to the plate interface and would amplify the uplift of thick water column (~3000 m) and can generate a large tsunami in the coastal areas.