Steady‐State Effective Normal Stress in Subduction Zones Based on Hydraulic Models and Implications for Shallow Slow Earthquakes

The spatial distribution of effective normal stress, σₑ, is essential for understanding the fault motion. Although Rice (1992, https://doi.org/10.1016/s0074-6142(08)62835-1) proposed a steady-state solution for a vertical strike-slip fault zone with constant fluid properties, models that are based on the concept by Rice (1992, https://doi.org/10.1016/s0074-6142(08)62835-1) and are applicable for other tectonic settings have not yet been developed. Such a model is particularly important in subduction zones because the relationship between low σₑ and slow earthquakes is often discussed. To quantitatively examine the causes of a local decrease in σₑ on a shallow region of the subduction zone, we performed model calculations that incorporated mechanisms characteristic to subduction zones. Our basic model, which considers the effect of smectite dehydration and the mechanical effect of subduction, yields results that are consistent with those reported by Rice (1992, https://doi.org/10.1016/s0074-6142(08)62835-1): the gradient of σₑ remarkably decreases with the increase in depth, whereas the realistic fluid properties rule out nearly constant σₑ at depth. We obtained a monotonic increase in σₑ with the increase in depth for the physically sound solutions and failed to generate a local decrease in σₑ . The presence of a splay fault and fluid leakage though it cannot decrease σₑ locally. We found that a local decrease in permeability decreased σₑ locally around an impermeable zone and, thus, possibly led to the occurrence of shallow slow earthquakes. The water release caused by the dehydration reaction may not be the dominant factor, although smectite dehydration releases silica and promotes its precipitation

Kinetic effect of heating rate on the thermal maturity of carbonaceous material as an indicator of frictional heat during earthquakes

Abstract Because the maximum temperature reached in the slip zone is significant information for understanding slip behaviors during an earthquake, the maturity of carbonaceous material (CM) is widely used as a proxy for detecting frictional heat recorded by fault rocks. The degree of maturation of CM is controlled not only by maximum temperature but also by the heating rate. Nevertheless, maximum slip zone temperature has been estimated previously by comparing the maturity of CM in natural fault rocks with that of synthetic products heated at rates of about 1 °C s−1, even though this rate is much lower than the actual heating rate during an earthquake. In this study, we investigated the kinetic effect of the heating rate on the CM maturation process by performing organochemical analyses of CM heated at slow (1 °C s−1) and fast (100 °C s−1) rates. The results clearly showed that a higher heating rate can inhibit the maturation reactions of CM; for example, extinction of aliphatic hydrocarbon chains occurred at 600 °C at a heating rate of 1 °C s−1 and at 900 °C at a heating rate of 100 °C s−1. However, shear-enhanced mechanochemical effects can also promote CM maturation reactions and may offset the effect of a high heating rate. We should thus consider simultaneously the effects of both heating rate and mechanochemistry on CM maturation to establish CM as a more rigorous proxy for frictional heat recorded by fault rocks and for estimating slip behaviors during earthquake

Slip parameters on major thrusts at a convergent plate boundary: regional heterogeneity of potential slip distance at the shallow portion of the subducting plate

Abstract Understanding variations of slip distance along major thrust systems at convergent margins is an important issue for evaluation of near-trench slip and the potential generation of large tsunamis. We derived quantitative estimates of slip along ancient subduction fault systems by using the maturity of carbonaceous material (CM) of discrete slip zones as a proxy for temperature. We first obtained the Raman spectra of CM in ultracataclasite and pseudotachylyte layers in discrete slip zones at depths below the seafloor of 1–4 km and 2.5–5.5 km, respectively. By comparing the area-under-the-peak ratios of graphitic and disordered bands in those Raman spectra with spectra of experimentally heated CM from surrounding rocks, we determined that the ultracataclasite and pseudotachylyte layers had been heated to temperatures of up to 700 and 1300 °C, respectively. Numerical simulation of the thermal history of CM extracted from rocks near the two slip zones, taking into consideration these temperature constraints, indicated that slip distances in the ultracataclasite and pseudotachylyte layers were more than 3 and 7 m, respectively. Thus, potential distance of coseismic slip along the subduction-zone fault system could have regional variations even at shallow depth (≤ 5.5 km). The slip distances we determined probably represent minimum slips for subduction-zone thrusts and thus provide an important contribution to earthquake preparedness plans in coastal areas facing the Nankai and Sagami Troughs

MOESM2 of Kinetic effect of heating rate on the thermal maturity of carbonaceous material as an indicator of frictional heat during earthquakes

Additional file 2. Peak decomposition method for Raman spectra. Description of data: Representative Raman spectrum (800 °C, fast-rate heating experiment) with peaks decomposed for calculation of the ID/IG ratio. Our ID/IG ratios roughly correspond to ID1/ID2 ratios calculated by several prior studies (e.g., Furuichi et al. 2015; Ito et al. 2017)

MOESM1 of Kinetic effect of heating rate on the thermal maturity of carbonaceous material as an indicator of frictional heat during earthquakes

Additional file 1. Numerical simulation results for the heating experiments. Description of data: Simulated experimental relationships between a temperature and b heating rate and time for a 100-Âľm-diameter particle of CM

MOESM2 of Slip parameters on major thrusts at a convergent plate boundary: regional heterogeneity of potential slip distance at the shallow portion of the subducting plate

Additional file 2. Temperature development related to the distance away from a slip zone. Results of numerical simulation showing displacement-temperature development during earthquake at 3 mm away from the slip zone of the Emi major thrust with depth of 2 km and maximum temperature of 700 °C. Other parameters are referred from Table 1

Python code for automatic peak deconvolution of Raman spectra of carbonaceous material for application to the geothermometer of Aoya et al. (2010)

<p>By running the Python code in the deposited zip file, one can perform automatic peak deconvolution of Raman spectra of carbonaceous material. Based on the obtained Raman spectral parameters, the metamorphic temperature of the rocks of interest can be calculated using the geothermometer of Aoya et al. (2010, Journal of Metamorphic Geology). Simple instructions for users can be found in the Readme.txt. The codes have been developed by Kaneki et al. (submitted to Progress in Earth and Planetary Science).</p&gt

Characteristics of Fault Rocks Within the Aftershock Cloud of the 2014 Orkney Earthquake (M5.5) Beneath the Moab Khotsong Gold Mine, South Africa

Cores recovered during the International Continental Scientific Drilling Program project "Drilling into Seismogenic zones of M2.0 to M5.5 earthquakes in deep South African Gold Mines" include fault breccia from within the aftershock cloud of the 2014 Orkney earthquake (M5.5). The breccia and surrounding intrusive rocks, probably lamprophyres rich in talc, biotite, calcite, and amphibole, had high magnetic susceptibilities owing to the presence of magnetite. All of these characteristics can be attributed to fluid-related alteration. Both the breccia and the lamprophyres had low friction coefficients and showed evidence of velocity strengthening, which is inconsistent with the occurrence of earthquakes. Variable amounts of talc, biotite, calcite, and amphibole within the lamprophyres might have produced complex frictional properties and spatial heterogeneity of fault stability. The altered lamprophyres may be the host rocks of the 2014 Orkney earthquake, but frictional complexity may have governed the magnitudes of the main- and aftershocks and their distributions.ISSN:0094-8276ISSN:1944-800