Testing the seismic quiescence hypothesis through retrospective trials of alarm-based earthquake prediction in the Kurile–Japan subduction zone

We make trial binary forecasts for the Kurile–Japan subduction zone for the period 1988–2014 by hypothesizing that seismic quiescence (i.e., the absence of earthquakes of M ≥ 5 for a minimum period of Tq) is a precursor of a large (7.5 ≤ Mw < 8.5) earthquake in the coming period Ta within a radius R of the quiescence. We evaluate the receiver-operating-characteristic diagram constructed using a range of forecast models specified by (Tq, R, Ta). A forecast experiment targeting eight large earthquakes in the studied spacetime suggests that the risk of a large earthquake is modestly (probability gain G ~ 2) but significantly (p-value less than 5%) heightened for several years following a long quiescent period of Tq ≥ 9 years, within several tens of kilometers of the quiescence. We then attempt cross-validation, where we use half the data for training [i.e., optimization of (Tq, R, Ta)] and the remaining half for evaluation. With only four target earthquakes available for evaluation of the forecasts in each of the learning and evaluation periods, our forecast scheme did not pass the cross-validation test (with a criterion that the p-value is less than 5%). Hence, we cannot formally deny the possibility that our positive results for the overall period are a ghost arising from over-fitting. However, through detailed comparison of optimal models in the overall test with those in the cross-validation tests, we argue that severe over-fitting is unlikely involved for the modest G of ~ 2 obtained in the overall test. There is thus a reasonable chance that the presently tested type of quiescence will pass the cross-validation test when more target earthquakes become available in the near future. In the meantime, we find that G improves to ~ 5 when target earthquakes are limited to 8 ≤ Mw < 8.5, though we cannot say anything about the possible involvement of over-fitting because we have only three such very large target earthquakes.

Roles of Hydrothermal Circulations on Heat Flow in the Fore-arc, Northeast Japan Subduction Zone, A Numerical Modeling Study.

&lt;p&gt;Heat flow in the fore-arc, Northeast Japan shows characteristic highs and lows in the seaward and landward regions of the trench axis, respectively, compared to 50 mW/m&lt;sup&gt;2&lt;/sup&gt; that is constrained from the corresponding half-space cooling model (135 Ma). For example, the high average of 70 mW/m&lt;sup&gt;2&lt;/sup&gt; at the 150-km seaward region from the trench was observed while the low average of 30 mW/m&lt;sup&gt;2&lt;/sup&gt; at the 50-km landward region was. To explain the differences between the constraints and observations of the heat flow, previous studies suggested that the high heat flow in the seaward region results from the reactivated hydrothermal circulations in the oceanic crust of the Pacific plate along the developed fractures by the flexural bending prior to subduction. The low heat flow is thought to result from thermal blanket effect of the accretionary prism that overlies the cooled subducting slab by the hydrothermal circulations. To understand heat transfer in the landward region of the trench, a series of two-dimensional numerical models are constructed by considering hydrothermal circulations in the kinematically thickening accretionary prism that overlies the converging oceanic crust of the Pacific plate where hydrothermal circulations developed prior to subduction. The model calculations demonstrate no meaningful hydrothermal circulations when the reasonable bulk permeability of the accretionary prism(&lt;10&lt;sup&gt;-14&lt;/sup&gt;m&lt;sup&gt;2&lt;/sup&gt;) is used; the thermal blanket effect significantly hinders the heat transfer, yielding only the heat flow of 10 mW/m&lt;sup&gt;2&lt;/sup&gt; in the landward region, much lower than the average of 30 mW/m&lt;sup&gt;2&lt;/sup&gt;. This indicates that other mechanisms such as the expelled pore fluid by compaction of the accretionary prism play important roles in the heat transfer across the accretionary prism.&lt;/p&gt;

Influence of Regional Tectonic and Rheological Structure on the Seismic Cycle for Northeast Japan

&lt;p&gt;The subduction zone is a natural laboratory for studying the seismic cycle. On March 11, 2011, in the central part of the Japan subduction zone, the strongest Mw=9.0 Tohoku earthquake occurred, terminating a seismic cycle that lasted about 1200 years. We analyzed two decades of GNSS observations at 1400 GEONET stations to reveal the peculiarities of the tectonic and rheological structure of the Japan subduction zone which driven such a long-term seismic cycle. We consider GNSS data within the framework of a generalized approach, including the assessment of the coupling of the interplate interface before the earthquake, the construction of a model of the distributed displacement in the source zone, and the study of postseismic processes characterizing the relaxation of elastic stresses in the vicinity of the source.&lt;/p&gt;&lt;p&gt;As a result, we found that in the last year before the earthquake, there was an increase in the rates of elastic deformation of the continental margin and a corresponding increase in the interplate coupling. To study the process of the release of elastic energy during the Tohoku earthquake, we built a model of the distributed slip in the source. We used different earth models during inversion of GNSS data to study the impact of the regional tectonic and rheological structure and confirm the resilience of our inversion technique. We used GNSS data to build a model of pure afterslip in the first six months after the Tohoku earthquake and a model of afterslip combined with the short-term viscoelastic relaxation to estimate the relative contributions of these postseismic processes to the observed displacement field. Long-term postseismic time series of GNSS displacements were used to build the model of viscoelastic relaxation in the asthenosphere following the Tohoku earthquake. To estimate the transition time of the subduction zone to the steady-state of elastic stress accumulation we constructed a forecast of attenuation of viscoelastic stresses in the asthenosphere on the basis of our viscoelastic relaxation model.&lt;/p&gt;&lt;p&gt;We also studied the possible block structure of the Japanese Islands and its impact on the seismic cycle performing cluster analysis of GNSS displacement data at different stages of the seismic cycle.&lt;/p&gt;&lt;p&gt;This study was supported by the Russian Science Foundation (project 20&amp;#8211;17-00140).&lt;/p&gt;

3D anisotropic structure of the Japan subduction zone

How mantle materials flow and how intraslab fabrics align in subduction zones are two essential issues for clarifying material recycling between Earth’s interior and surface. Investigating seismic anisotropy is one of a few viable technologies that can directly answer these questions. However, the detailed anisotropic structure of subduction zones is still unclear. Under a general hexagonal symmetry anisotropy assumption, we develop a tomographic method to determine a high-resolution three-dimensional (3D) P wave anisotropic model of the Japan subduction zone by inverting 1,184,018 travel time data of local and teleseismic events. As a result, the 3D anisotropic structure in and around the dipping Pacific slab is firstly revealed. Our results show that slab deformation plays an important role in both mantle flow and intraslab fabric, and the widely observed trench-parallel anisotropy in the forearc is related to the intraslab deformation during the outer-rise yielding of the subducting plate.

Slab-derived fluid evolution induced from oxygen and hydrogen isotopes compositions of blueschist-facies phengites

&lt;p&gt;Phengite is the most common metamorphic mineral in H&lt;em&gt;P&lt;/em&gt;-UH&lt;em&gt;P&lt;/em&gt;&amp;#160;metasedimentary rocks, which can convey H&lt;sub&gt;2&lt;/sub&gt;O,&amp;#160;LILEs&amp;#160;(especially K, Ba, Cs and Rb), Li, B and N in their structure formed at depths up to 300 km. The breakdown of phengite in a downgoing oceanic slab would cause fluid-induced element transport into the overlying mantle wedge. We have investigated the&amp;#160;&lt;sup&gt;2&lt;/sup&gt;H/&lt;sup&gt;1&lt;/sup&gt;H (D/H) and&amp;#160;&lt;sup&gt;18&lt;/sup&gt;O/&lt;sup&gt;16&lt;/sup&gt;O ratios of twenty-four phengite separates from pelitic schists of the Devonian&amp;#8211;Carboniferous Renge Belt (SW Japan), Permian Shaiginsky Complex (Far East Russia) and Cretaceous Sambagawa Belt (SW Japan).&lt;/p&gt;&lt;p&gt;We found the presence of the very light hydrogen isotope (&amp;#948;D &lt; &amp;#8211;95&amp;#8240;) in blueschist-facies phengites in the three different metamorphic belts. For example, phengite from the lawsonite- and epidote-grade metasedimentary schists of the Osayama Serpentinite M&amp;#233;lange (OSM) of the Renge Belt are characterized by negative hydrogen isotope compositions (&amp;#948;D values relative to VSMOW) ranging from &amp;#8211;113 to &amp;#8211;93.9&amp;#8240; and oxygen isotope compositions (&amp;#948;18O values relative to VSMOW) ranging from +12.9 to +14.6&amp;#8240;.&lt;/p&gt;&lt;p&gt;High-Si features and K&amp;#8211;Ar ages of the investigated phengites deny the possibility of meteoric-hydrothermal alteration to have caused the low &amp;#948;D values. The light values might be attributed to isotopic fractionation during progressive metamorphic dehydration.Assuming a meamorphic temperatures range of 250&amp;#8211;350&amp;#176;C for the OSM schists, the inferred metamorphic fluid compositions in blueschist-facies depth for that fossil slab had a range of &amp;#948;D = ~&amp;#8211;40 to &amp;#8211;75&amp;#8240; and &amp;#948;18O = ~+13 to +15&amp;#8240;. These values are significantly lighter than the slab-fluid induced from the Arima hot spring water in a forearc region of modern SW Japan subduction zone. Our study suggests that&amp;#160;slab-derived fluids in ancient Pacific-type subduction zone are characterized by light hydrogen isotope and that the&amp;#160;phengite breakdown can affect&amp;#160;hydrogen isotope of nominally anhydrous minerals (NAMs) in the deep mantle.&lt;/p&gt;

Updated attenuation tomography of Japan subduction zone

SUMMARY We determine an updated model of P- and S-wave attenuation (Qp, Qs) tomography of the Japan subduction zone using an improved inversion scheme, and derive the first Qp/Qs model of the study region. We establish a system of observation t* equations by taking 1/Q values at 3-D grid nodes arranged in the study volume as unknown parameters. This scheme can eliminate model errors caused by ignoring high-order terms of the Taylor expansion in the Q-format scheme adopted by previous studies. The inversion problem is considered as a quadratic programming problem with bound constraints for best fitting the observed t* data in a least-squares sense. The 3-D attenuation model is obtained by using a limited-memory Broyden–Fletcher–Goldfarb–Shanno algorithm for bound constrained optimization. Because this inversion scheme uses bound constraints to avoid negative Q values, damping regularization is not needed, hence the ranges of Qp and Qs values obtained are less affected by human subjectivity. The subducting Pacific and Philippine Sea slabs exhibit high-Q and high Qp/Qs, whereas the mantle wedge beneath the volcanic front and back-arc area shows low-Q and low Qp/Qs. Both Qp and Qs vary in a range of 10–2000, and the Qp/Qs ratio changes from 0.4 to 1.4. Our results reveal a narrow high-Qp/Qs belt that is coincident with the distribution of volcanic and non-volcanic low-frequency micro-earthquakes (M 0.0–2.5), which may reflect high water-saturation anomalies that are probably associated with slab-derived fluids and responsible for the generation of low-frequency micro-earthquakes.