High accuracy earthquake locations are critical for understanding seismogenic structures and estimating the earthquake hazard. However, for offshore regions, especially when station coverage is sparse or unfavorable, locating earthquake hypocenters is challenging. Here we modified our newly developed earthquake location workflow and applied it to a far-offshore earthquake sequence occurred in 2023, south of Izu Peninsula, Japan, ~150 km away from the nearest shoreline, close to Hachijojima Island. The workflow provides a well constrained cluster geometry and depth, using P and S-phases, as well as pP depth-phases, recorded at land seismic stations. Our results reveal a sharp north-east dipping earthquake cluster, which agrees well with the focal mechanisms estimated by the National Research Institute for Earth Science and Disaster Resilience. The estimated high p-value of the Modified Omori Law for aftershock decay, along with the increase in the background seismicity rate revealed by the application of a non-stationary Epidemic-Type Aftershock Sequence model, may indicate the swarm character of the sequence and the presence of aseismic forcing, such as crustal fluids within the Philippine Sea Plate that promoted the earthquake activity. Coulomb stress change estimations indicate that the occurrence of the 2023 Hachijojima sequence is unlikely to have been statically triggered by previous earthquakes in the area, however the mainshock may have been triggered by a ~10 minutes preceding foreshock. The normal faults located at a depth of around 25 to 45 km, along which the sequence has occurred, can be explained by the tensional regime along the Izu-Bonin arc.

A document by Yihuan Ruan. Click on the document to view its contents.

Clarifying the relationship between regular earthquakes and slow fault slip is essential for understanding the mechanisms behind seismic activity. We hypothesize that the background seismic activity around the Guerrero seismic gap in Mexico is partially triggered by interplate slow-slip events (SSEs). Consequently, we present an extension of the spatio-temporal epidemic-type aftershock sequence (ETAS) model, which incorporates background seismicity as a piecewise constant function over time. In this study, Global Navigation Satellite System (GNSS) data is employed to identify the occurrence periods of SSEs, thereby delineating the intervals during which changes in background seismicity may occur. Due to the technical complexity of performing inference with an inhomogeneous ETAS model, this work employs a penalized maximum likelihood inference method using the Expectation-Maximization (EM) algorithm. This approach also permits the inference of the branching process for aftershocks, thus enabling the estimation of the genealogy between earthquakes. This information could be utilized to decluster earthquakes. This study elucidates how the background seismicity increases during the periods of the Guerrero SSEs, which allows for a more comprehensive understanding of seismic activity and the relationship between slow and fast earthquakes in Mexico. Our new model can be applied not only in Mexico but also at plate boundaries worldwide to quantify the impact of SSEs on seismic activity.

Detecting crustal deformation during transient deformation events at offshore subduction zones remains challenging. The spatiotemporal evolution of slow slip events (SSEs) on the offshore Hikurangi subduction zone, New Zealand, during February–July 2019, is revealed through a time-dependent inversion of onshore and offshore geodetic data that also account for spatially varying elastic crustal properties. Our model is constrained by seafloor pressure time series (as a proxy for vertical seafloor deformation), onshore continuous Global Navigation Satellite System (GNSS) data, and Interferometric Synthetic Aperture Radar (InSAR) displacements. Large GNSS displacements onshore and uplift of the seafloor (10-33 mm) require peak slip during the event of 150 to >200 mm at 6-12 km depth offshore Hawkes Bay and Gisborne, comparable to maximum slip observed during previous seafloor pressure deployments at north Hikurangi. The onshore and offshore data reveal a complex evolution of the SSE, over a period of months. Seafloor pressure data indicates the slow slip may have persisted longer near the trench than suggested by onshore GNSS stations in both the Gisborne and Hawkes Bay regions. Seafloor pressure data also reveal up-dip migration of SSE slip beneath Hawke Bay occurred over a period of a few weeks. The SSE source region appears to coincide with locations of the March 1947 Mw 7.0–7.1 tsunami earthquake offshore Gisborne and estimated Great earthquake rupture sources from paleoseismic investigations offshore Hawkes Bay, suggesting that the shallow megathrust at north and central Hikurangi is capable of both seismic and aseismic rupture.

Understanding the interaction between tectonic plates from geodetic data is relevant to the assessment of seismic hazard. To shed light on that prevalently slow aseismic interaction, we developed a new static-slip inversion strategy, the ELADIN (ELastostatic ADjoint INversion) method, that uses the adjoint elastostatic equations to compute the gradient of the cost function. To handle plausible slip constraints, ELADIN is a 2-step inversion algorithm. First it finds the slip that best explains the data without any constraint, and then refines the solution by imposing the constraints through a Gradient Projection Method. To obtain a selfsimilar, physically-consistent slip distribution that accounts for sparsity and uncertainty in the data, ELADIN reduces the model space by using a von Karman regularization function that controls the wavenumber content of the solution, and weights the observations according to their covariance using the data precision matrix. Since crustal deformation is the result of different concomitant interactions at the plate interface, ELADIN simultaneously determines the regions of the interface subject to both stressing (i.e., coupling) and relaxing slip regimes. For estimating the resolution, we introduce a mobile checkerboard that allows to determine lower-bound fault resolution zones for an expected slip-patch size and a given stations array. We systematically test ELADIN with synthetic inversions along the whole Mexican subduction zone and use it to invert the 2006 Guerrero Slow Slip Event (SSE), which is one of the most studied SSEs in Mexico. Since only 12 GPS stations recorded the event, careful regularization is thus required to achieve reliable solutions. We compared our preferred slip solution with two previously published models and found that our solution retains their most reliable features. In addition, although all three SSE models predict an upward slip penetration invading the seismogenic zone of the Guerrero seismic gap, our resolution analysis indicates that this penetration might not be a reliable feature of the 2006 SSE.

The triggering of large earthquakes on a fault hosting aseismic slip or, conversely, the triggering of slow slip events (SSE) by passing seismic waves involves seismological questions with important hazard implications. Just a few observations plausibly suggest that such interactions actually happen in nature. In this study we show that three recent devastating earthquakes in Mexico are likely related to SSEs, describing a cascade of events interacting with each other on a regional scale via quasi-static and/or dynamic perturbations. Such interaction seems to be conditioned by the transient memory of Earth materials subject to the “traumatic” stressing produced by the seismic waves of the great 2017 (Mw8.2) Tehuantepec earthquake, which strongly disturbed the aseismic slip beating over a 650 km long segment of the subduction plate interface. Our results imply that seismic hazard in large populated areas is a short-term evolving function of seismotectonic processes that are often observable.

Isolating the source of non-tidal oceanographic noise in seafloor pressure data is critical for improving the use of these data for seafloor geodetic applications. Residuals between nearby bottom pressure records have typically been used to remove the non-tidal components, as these are largely common-mode. To evaluate the similarities between pairs of observed bottom pressure records at a range of water depths, we calculate the standard deviations of the time series of residuals between data from all site pairs, recorded during a recent experiment offshore New Zealand. Similar to a recent study offshore Cascadia, we find that the magnitude of the standard deviation depends more on relative water depth than the distance between sites. This confirms that non-tidal components are more similar along isobaths even if the distance between sites is large. We show that the depth range varies with the depth of the deeper site of the pairs under restrictions.

We conducted dynamic viscoelastic measurements of three clay minerals in a solid–liquid two-phase state: kaolinite, illite, and smectite with water. These constituents of concentrated (dense) suspensions were investigated using a high-temperature and high-pressure rheometer, to understand tectonic and non-tectonic phenomena in the shallow part of a fault system, such as shallow slow slip events in subduction zones, and landslides on fault or bed planes. We observed shear strain rate dependencies of phase angles of both dynamic stress and strain waveforms on the rheometer at varying temperature and pressure. The pressure and temperature dependence of the viscoelastic properties of the system can be qualitatively understood by applying the Zwanzig–Mountain theory. The local packing fraction change owing to dynamic oscillations affects the changing viscoelastic properties in systems such as shallow fault systems.

In this study, the role of fluids in a slow slip events (SSEs) regime was examined by investigating the rheological properties of the solid-liquid two-phase system on a laboratory scale. We specifically investigated how the rheological properties change with the shear rate when the granular layer is fluid-rich. We used a velocity-controlled rheometer to apply shear to a liquid-saturated granular layer. The results show that the liquid-saturated granular layer’s shear viscosity depended on the liquid viscosity and exhibited pseudoplastic behavior. Additionally, the saturated granular layered exhibited hysteresis, which increased as the viscosity of the liquid decreased. After a quantitative discussion through Bagnold (1954) framework, we propose a fault model that includes a fluid-rich granular layer.