Constraints on Element Mobility during Deformation within the Seismogenic Zone, Shimanto Belt, Japan

AbstractThe geometries (i.e., dip angles) of active faults from the surface to the seismogenic zone are the most important factors used to evaluate earthquake ground motion, which is crucial for seismic hazard assessments in urban areas. In Osaka, a metropolitan city in Japan, there are several active faults (e.g., the Uemachi and Ikoma faults), which are inferred from the topography, the attitude of active faults in surface trenches, the seismic reflection profile at shallow depths (less than 2 km), and the three-dimensional distribution of the Quaternary sedimentary layers. The Uemachi and Ikoma faults are N–S-striking fault systems with total lengths of 42 km and 38 km, respectively, with the former being located ~ 12 km west of the latter; however, the geometries of each of the active faults within the seismogenic zone are not clear. In this study, to examine the geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone, we analyze the development of the geological structures of sedimentary layers based on numerical simulations of a two-dimensional visco-elasto-plastic body under a horizontal compressive stress field, including preexisting high-strained weak zones (i.e., faults) and surface sedimentation processes, and evaluate the relationship between the observed geological structures of the Quaternary sediments (i.e., the Osaka Group) in the Osaka Plain and the model results. As a result, we propose geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone. When the friction coefficient of the faults is ~ 0.5, the dip angles of the Uemachi and Ikoma faults near the surface are ~ 30°–40° and the Uemachi fault has a downward convex curve at the bottom of the seismogenic zone, but does not converge to the Ikoma fault. Based on the analysis in this study, the dip angle of the Uemachi fault zone is estimated to be approximately 30°–40°, which is lower than that estimated in the previous studies. If the active fault has a low angle, the width of the fault plane is long, and thus the estimated seismic moment will be large.

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Frictional properties of anorthite (feldspar): implications for the lower boundary of the seismogenic zone

Earth Planets and Space ◽

10.1186/s40623-020-01271-6 ◽

2020 ◽

Vol 72 (1) ◽

Author(s):

Koji Masuda

Keyword(s):

Dominant Role ◽

Lower Boundary ◽

Earthquake Magnitude ◽

Seismogenic Zone ◽

Effective Pressure ◽

Velocity Dependence ◽

Brittle Deformation ◽

Frictional Properties ◽

Depth Extent ◽

Mineral Constituents

Abstract Earthquake magnitude is closely related to the depth extent of the seismogenic zone, and higher magnitude earthquakes occur where the seismogenic zone is thicker. The frictional properties of the dominant mineral constituents of the crust, such as feldspar-group minerals, control the depth extent of the seismogenic zone. Here, the velocity dependence of the steady-state friction of anorthite, the calcic endmember of the feldspar mineral series, was measured at temperatures from 20 to 600 °C, pore pressures of 0 (“dry”) and 50 MPa (“wet”), and an effective pressure of 150 MPa. The results support previous findings that the frictional properties of feldspar play a dominant role in limiting the depth extent of the seismogenic zone. This evidence suggests that brittle deformation of anorthite may be responsible for brittle fault movements in the brittle–plastic transition zone.

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Thermal squeezing of the seismogenic zone controlled rupture of the volcano-rooted Flores Thrust

Science Advances ◽

10.1126/sciadv.abe2348 ◽

2021 ◽

Vol 7 (5) ◽

pp. eabe2348

Author(s):

Karen Lythgoe ◽

Muzli Muzli ◽

Kyle Bradley ◽

Teng Wang ◽

Andri Dian Nugraha ◽

...

Keyword(s):

Thermal Structure ◽

Critical Role ◽

Volcanic Edifice ◽

Seismic Array ◽

Temperature Gradients ◽

Seismogenic Zone ◽

Arc Volcano

Temperature plays a critical role in defining the seismogenic zone, the area of the crust where earthquakes most commonly occur; however, thermal controls on fault ruptures are rarely observed directly. We used a rapidly deployed seismic array to monitor an unusual earthquake cascade in 2018 at Lombok, Indonesia, during which two magnitude 6.9 earthquakes with surprisingly different rupture characteristics nucleated beneath an active arc volcano. The thermal imprint of the volcano on the fault elevated the base of the seismogenic zone beneath the volcanic edifice by 8 km, while also reducing its width. This thermal “squeezing” directly controlled the location, directivity, dynamics, and magnitude of the earthquake cascade. Earthquake segmentation due to thermal structure can occur where strong temperature gradients exist on a fault.

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