Geophysical Journal International

SummaryRealistic models of earthquake sequences can be simulated by assuming faults governed by rate-and-state friction embedded in an elastic medium. Exploring the possibility of using such models for earthquake forecasting is challenging due to the difficulty of integrating Partial Differential Equation (PDE) models with sparse, low-resolution observational data. This paper presents a machine-learning-based reduced-order model (ROM) for earthquake sequences that addresses this limitation. The proposed ROM captures the slow/fast chaotic dynamics of earthquake sequences using a low-dimensional representation, enabling computational efficiency and robustness to high-frequency noise in observational data. The ROM’s efficiency facilitates effective data assimilation using the Ensemble Kalman Filter (EnKF), even with low-resolution, noisy observations. Results demonstrate the ROM’s ability to replicate key scaling properties of the sequence -namely the magnitude-frequency, moment-duration, and moment-area relationships- and to estimate the distributions of fault slip rate and state variable, enabling predictions of large events in time and space with uncertainty quantification. These findings underscore the ROM’s potential for forecasting and for addressing challenges in inverse problems for nonlinear geophysical systems.

SummaryThis study introduces a novel inversion approach to resolve the layered anisotropic structure of the Earth’s crust and upper mantle using harmonic patterns observed in Ps receiver functions (RFs). Designed for dense seismic networks, our method effectively captures the complexity and deformation of subsurface structures by leveraging the decomposition of RF patterns into five harmonic functions representing distinct terms in a harmonic regression model. Our approach combines residual weighting at individual and coherency weighting between stations to reduce noise and enhance structural signal, followed by a direct inversion of harmonic patterns alongside a multi-phase stack. This process is optimized using a Markov chain Monte Carlo framework to iteratively explore model space and define posterior distributions, which yields robust model parameter estimates with quantified uncertainties. We validated our approach with both synthetic models and real data from the dense SEISConn seismic transect in Northern Connecticut. The synthetic test highlighted the reliability of coherency weighting in reducing noise. Real data analysis revealed anisotropic and structural features consistent with geological expectations and previous studies, including shallow anisotropy in western Connecticut, a shallowing Moho towards the Hartford basin, and indications for a west-dipping layer beneath the Moho in the lithospheric mantle. Our approach offers a promising tool for revealing the details of anisotropic features beneath dense seismic profiles, facilitating insights into tectonic history and lithospheric deformation.

SummaryLarge-scale geodetic monitoring of volcanic earthquakes is essential for understanding the physical mechanisms governing volcanic activity and magma migration. Recently, a significant earthquake swarm occurred around the Scotia plate. Geodetic data of 14 permanent GNSS stations on the Antarctic Peninsula, King George Island, the South Sandwich Islands, and South America were collected and analyzed, to monitor the crustal deformation of King George Island, the expansion of Bransfield Strait, the drift of Scotia plate, and sea level anomalies. Tidal data from four permanent tide stations were analyzed to monitor sea level anomalies. Results showed that after earthquakes the King George Island’s movement speed increased tenfold and its direction altered by 90 degrees. Land surface fluctuations in southeast King George Island were observed a year before the earthquakes, followed by continuous uplift. A combinatorial model including a point pressure source and expanding dike fit well with new geodetic monitoring data, revealing the impact of volcanic activity on this region. Geodetic monitoring and modeling quantitatively depicted the pre-seismic, co-seismic, and post-seismic phases of geological changes, providing new evidence and insights into the complex geological structures.

SummaryA 220 km long active seismic profile crossing the Saronic and Corinth Gulfs was performed using 35 4C Ocean Bottom Seismographs (OBS) and 4 3C stand-alone land stations. We recorded shots fired along line at every 120 m from a 48-l airgun array of 51 bar-m power. The velocity model was developed by first break tomographic inversion, followed by kinematic and dynamic ray tracing. The final velocity model was used to prestack depth migrate the Common Receiver Gathers. Moho depth below the central Corinth Basin is located at 32 km, thinning to 22 km below the Lechaio Gulf, at the transition to the Saronic Basin. In the Saronic domain Moho is found at 19 to 21 km depth, whereas below we identified a low velocity upper mantle of Vp 7.5 to 7.6 km/s, extending to 34 km depth. This is a low velocity asthenosphere wedge that intruded from the Cyclades region below the Saronic Gulf, driving the volcanic activity. It does not extend in the Corinth Rift domain to the west, where Pn is 8.0 km/s. Two major extensional detachments were mapped along the profile: one to the NW in the central Corinth Gulf, east of Galaxidi, corresponding to the onshore Itea-Amfissa Detachment, having a throw of more than 2000 m; the other to the SE, west of Agios Georgios Island, separates the Cycladic Metamorphic Core Complex from the non-metamorphic internal nappes of the Hellenides. Thrusting to the NW observed in the SE Saronic Basin has doubled the thickness of the high velocity limestones. At 20 km SE of Agios Georgios Island a major dextral strike slip fault was mapped. Normal faults with more than 1 km throw are observed around the Isthmus of Corinth trending E-W and WNW-ESE. Maximum thickness of 2000 m of Middle Miocene-Quaternary sediments (Vp 2.1–2.8 km/s and 3.4 km/s) is observed in the Corinth Gulf and minimum of 200 m of Quaternary sediments above the western Cyclades. Beneath the volcanoes of Aegina, Methana, Paphsanias, and Sousaki the crust has a lateral Vp velocity increase of nearly 3 per cent, whereas depth migrated data show high reflectivity, indicating magma intrusions through the crust. Mesozoic limestones with Vp 5.2 to 5.8 km/s and 1800 to 2000 m thickness occur along the profile, corresponding probably to the Tripolis external carbonate platform, whereas limestones and flysch with Vp 4.3 to 5.5 km/s are overlying, probably corresponding to pelagic sequences like the Pindos nappe. The Saronic domain is characterized by arc-parallel NW-SE structures, hosting the volcanic arc, and is tectonically controlled by magma uplift and thermally triggered deformation. The Corinth domain is affected by E-W transverse-oblique structures of a rapidly developing continental rift, and deformation driven by fracturing of a brittle crust of the Central Hellenic Shear Zone.