Earthquake Engineering Research Institute

A logarithmic function is the popular model of temporal evolution of afterslip, derived from the rate-and-state friction law (RSF) under the steady-state assumption (Marone+1991JGR). Relaxing this assumption, self-accelerating aseismic slip is predicted prior to subsequent decay even with velocity velocity-strengthening setting (i.e., a–b> 0; PerfettiniAmpuero2008JGR). The only natural observation example of such an accelerating stage of afterslip following large earthquakes is the case of the 2003 Tokachi-oki earthquake (M 8.0) in Japan, presented by Fukuda2009JGR (F09) with the data analysis performed by LarsonMiyazaki2008EPS (LM08). They reported that the early postseismic deformation emerged ~1 hour after the mainshock. We revisit this earthquake’s early postseismic deformation with a modern kinematic GNSS processing workflow by Gipsy-X v2.3 because many default and/or recommended settings and products have evolved from the time when these previous works were carried out. This revisit will align the Tokachi-oki case with other earthquake cases analyzed by GNSS processing strategies closer to ours than LM08’s.Among all the parameters/settings of GNSS processing we tested, the most impactful parameter was the position random walk (RW) parameter. We tested a wide range of values from 1 to 1e-5 m/sqrt(s) for this parameter with switching to the white noise during the mainshock and the M 7.1 largest aftershock (1.3-h later). Comparing our test results with F09’s dataset, the largest mismatch was found between the mainshock and the 7.1 largest aftershock when we attempted to reproduce F09’s cumulative displacements. During this interevent window, F09’s dataset shows tiny deformation, while our solutions show significant deformation. On the other hand, our test solutions exhibit the acceleration at similar timings as F09’s, with the RW parameter same as F09’s (1e-5 m/sqrt(s)), but our cumulative displacements are much smaller than F09’s after the largest aftershock coseismic step was removed. This is because of a trade-off between early postseismic deformation and the largest aftershock step, caused by the very tight RW not allowing sites to move other than at the coseismic timing. Therefore, we recommend careful testing position RW parameter to accurately resolve early postseismic deformation, rather than taking a value introduced in other studies. With our test results, we concluded that no parameters could satisfactorily reproduce the early postseismic deformation presented in F09; in other words, the acceleration of early afterslip reported in F09 was absent in our solutions. Our results imply that the transition between the interseismic and postseismic stage of velocity strengthening faults would happen within several minutes at the longest, implying that the very beginning of afterslip is concurrent with the dynamic ruptures of the mainshock.

The Ontong Java Plateau (OJP) is thought to have formed through large-scale Cretaceous volcanism, however the process of the massive volcanism remains largely unknown. Shito et al. [2025] explored the ascent process of the thermochemical plume and its impact on the physicochemical properties of the preexisting lithosphere. Based on the high-frequency seismic wave analysis revealed that the internal structure of the lithosphere beneath the OJP is a hybrid structure comprising dike swarms that are superimposed on the laminar structure. Moreover, the lithosphere exhibits lower seismic wave velocities than normal oceanic lithosphere, suggesting that the lithospheric mantle was physicochemically altered by the intrusion of dike swarms filled with magma from a large-scale thermochemical plume.This study employed two-dimensional tomography analysis to estimate lateral variations in dike density. The model parameter is relative energy reduction of So wave to Po wave and the data is Po and So wave envelope. As the first step, the two-dimensional tomography was performed under the assumption that the observed Po/So energy ratio is a simple integral value along the great circle path. The results revealed the presence of an area in the central part of the OJP where the reduction in So wave energy is significantly greater compared to Po waves. This suggests it corresponds to an area with high dike density and, also suggests the possibility that this location is the center of a massive eruption.Future research aims to examine quantitative relationship between dike density and energy reduction of Po and So wave and the validity of the linearity, and to estimate the two-dimensional distribution of dike density using more appropriate methods.

Over the past two decades, seismic hazard modeling has advanced along two complementary frontiers: empirical probabilistic frameworks, which systematically capture uncertainty through statistical inference, and physics-based simulation platforms, which directly compute ground motions from the governing equations of wave propagation. This project seeks to unify these two worlds by developing an end-to-end integration between OpenQuake and CyberShake, thereby creating a new generation of seismic hazard models that are globally extensible, probabilistically complete, and physically consistent. CyberShake has been under active development for more than a decade, demonstrating its robustness and scientific maturity through extensive implementations in California. It performs a physics-based probabilistic seismic hazard analysis (PSHA), replacing traditional empirical Ground Motion Prediction Equations (GMPEs) with full 3D numerical simulations of seismic wave propagation. Built upon the UCERF2/3 Earthquake Rupture Forecasts, CyberShake computes hazard curves directly from synthetic seismograms generated via Strain Green’s Tensors and thousands of stochastic rupture variations. This approach enables non-ergodic, site-specific hazard estimation and has set a global benchmark for high-fidelity hazard computation. However, its application has remained geographically limited: both the ERF and 3D velocity models were designed specifically for California, requiring extensive datasets that are rarely available elsewhere. Conversely, OpenQuake, developed by the Global Earthquake Model (GEM) Foundation, provides a fully open-source, Python-based framework for probabilistic seismic hazard and risk analysis. It serves as the computational backbone of large-scale hazard models such as the European Seismic Hazard Model 2020 (ESHM20), which integrates decades of regional expertise into a unified and statistical representation. OpenQuake provides a complete probabilistic framework to build Earthquake Rupture Forecasts (ERFs) that combine declustered catalogs, background seismicity, and multi-branch logic trees, ensuring a balanced and uncertainty-aware representation of regional tectonics. Furthermore, its ecosystem extends seamlessly to vulnerability and exposure modules, enabling the translation of hazard into actionable risk assessments and resilience planning.This project will establish a direct pipeline from OpenQuake’s event-based results to the generation of an ERF compatible with CyberShake’s simulation framework, ensuring moment–rate consistency. By doing so, it will enable CyberShake simulations to be performed for regions beyond California, extending its use to Europe based on the knowledge contained in the ESHM20. The first pilot region is Istanbul, Turkey, a densely populated metropolis located near the western termination of the North Anatolian Fault. Our initial results show that the workflow is already functioning at the prototype level: we have developed a unified 3D velocity model for the Istanbul region by combining available tomographic models with local datasets; generated preliminary event-based rupture catalogs from ESHM20 using OpenQuake; and demonstrated early convergence behavior in hazard curves, indicating that the rupture sampling strategy is statistically robust. These initial results demonstrate the feasibility of the integration approach and indicate that the essential elements needed for a CyberShake-ready ERF are already in place.

Summary In recent years, distributed acoustic sensing (DAS) has enabled the observation of strain over tens of kilometres at metre-level intervals by using optical fibre as a sensor. This study presents an analytical solution for the cross-spectrum of ambient noise with DAS data acquired from arbitrarily shaped and/or multiple fibre-optic cables, with the aim of estimating subsurface S-wave velocity structures using the spatial autocorrelation (SPAC) method. Our formulation accounts for both isotropic and anisotropic wave incidence. The analytical cross-spectrum depends on the angles between the horizontal direction connecting the two measurement points and the axial strain directions at the two points. This study demonstrates that both Rayleigh and Love waves contribute to the cross-spectrum, and that their contributions vary in a complex manner depending on the cable geometry, seismic velocity structure, interstation distance between observation points, and source amplitudes. By using this analytical solution, an integrated analysis combining the SPAC method and the ambient noise tomography method is applicable to DAS data acquired from arbitrarily shaped and/or multiple cables. In addition, the analytical expression considering anisotropic wave incidence will be useful for correcting travel-time anomalies caused by source heterogeneity. The application of our formulation to DAS data from winding or multiple cables will facilitate high-resolution and precise imaging of the three-dimensional structure.

Abstract The Alpine fault is a major transpressive plate boundary in southwestern New Zealand known on paleoseismological grounds to produce great (Mw∼8) earthquakes on <300 yr timescales. The fault is now late in its typical and well-determined interseismic period, having last ruptured in a great earthquake in 1717 C.E. Between October 2021 and November 2023, the Southern Alps Long Skinny Array (SALSA) consisting of 43 broadband and 4 short-period seismometers, was installed with ∼10 km interstation spacing along a 450 km stretch of the Alpine fault, between Maruia and Milford Sound/Piopiotahi. SALSA’s geometry was designed to enable the synthesis of virtual earthquakes representing slip at arbitrary points on the Alpine fault as part of research into the range of far-field ground motions that might result from geologically plausible rupture scenarios. Virtual earthquakes provide a means of evaluating ground motions from a much larger number of kinematic rupture scenarios than is computationally tractable using full-waveform simulations. In addition to providing closely spaced noise measurements, SALSA has yielded novel recordings of microseismicity and tremor along most of the onshore extent of the Alpine fault and will enable ongoing research into the present-day state of the fault and future rupture behavior. Here, we describe the rationale for deploying SALSA and present initial results and analysis that illustrate the dataset and indicate avenues for future research.

This study experimentally investigates a cement-free rubberised geopolymer ultra-high performance concrete (GUHPC), incorporating waste tyre rubber as a partial replacement for sand across controlled size fractions (medium, coarse) and hybrid gradations at 10–30 % replacement levels. The fresh and mechanical properties were evaluated, alongside sustainability metrics such as embodied CO 2 and material cost. The findings revealed that rubber substitution above 20 % led to significantly reduced workability and prolonged setting time. At 30 % replacement, the 28-day compressive and flexural strengths declined by 30 % and 17 % (from 164.3 to 113.9 MPa, and from 15.3 to 12.8 MPa, respectively). However, compressive energy absorption and early-age flexural toughness were notably enhanced. Hybrid rubber mixes minimised strength loss, raised toughness indices, and restrained matrix-penetrating cracks relative to single-size counterparts, attributed to enhanced packing and aggregate skeleton continuity. The alkaline geopolymer matrix improved rubber hydrophilicity and interfacial bonding, mitigating strength reductions compared to cement-based rubberised concretes. Microstructural analysis revealed strong matrix-fibre bonding in the control, evident mechanical interlocking at serrated medium rubber interfaces, and weak bonding with fibre debonding at coarse rubber surfaces. Moreover, replacing 10–30 % of sand with rubber reduced embodied CO 2 by 26–79 % and decreased material cost by 10 %. Within the scope of fresh, mechanical, and sustainability metrics, the 20 % hybrid rubber mix provided the best performance balance, with a 53 % carbon footprint reduction while retaining high strength and ductility. These results underscore the potential of rubberised GUHPC for sustainable infrastructure and protective applications such as impact-resistant façade panels, energy-dissipating safety barriers, and durable pavement overlays, where high toughness and resilience are critical performance requirements. • Geopolymer-based UHPC (GUHPC) was developed using waste tyre crumb rubber (CR). • GUHPC with 20 % CR addition exhibited compressive and flexural strengths of 134.4 and 14.0 MPa, respectively. • Rubberised GUHPC showed smaller strength losses than cement-based concrete due to better CR bonding in the alkaline matrix. • Medium CR maintained partial interlock via serrated surfaces, while coarse CR showed smooth, weakly bonded inclusions. • Replacing 30 % silica sand with CR reduced GUHPC cost by 9.7 % and embodied CO 2 by 79 %.

Existence of a heterogeneous attenuation structure in northeast Japan has long been documented. Despite this, attenuation structure around the plate boundary in the offshore region could not be examined thoroughly owing to a lack of data. After the 2011 Mw 9.1 Tohoku-oki earthquake, a seafloor network of 150 observatories, known as S-net, was established spanning the entire Japan Trench subduction zone to reinforce infrastructure for earthquake and tsunami early warnings. In the present research, we computed S-wave Fourier spectra of accelerograms from the seafloor and land stations and applied the principle of seismic tomography to retrieve three-dimensional attenuation structure (Qs). Our results showed a prominent low-to-moderate Qs zone near the upper surface of the oceanic plate in the seafloor area, most likely indicating the presence of water-rich oceanic crust and subducted sediments. We also found that the Qs structures in both onshore and offshore regimes are moderately to strongly frequency-dependent. A comprehensive understanding of the frequency-dependent behavior of attenuation structure remains elusive. We anticipate that the findings presented here will contribute to resolving pertinent issues in the field of solid earth science and to a deeper understanding of earthquake hazards in subduction zones.