Cynthia Ebinger

Rifts that initiate in mechanically strong, stable continental lithosphere are characterized by M > 5 earthquakes at depths >35 km near or below the crust mantle interface. Current models for deep rift zone earthquakes invoke elevated pore pressures associated with magmatism, and rapid stressing from magma intrusions. We evaluate the role of static stress changes caused by lateral density contrasts (magma intrusions) in the upper mantle and crust‐mantle interface on mantle earthquakes. We use numerical models of static stress changes, and test models against seismic data from the Tanganyika rift, East Africa which has upper mantle earthquakes and locally elevated crustal Vp/Vs. The earthquake source mechanisms from these ML2.4–2.8 earthquakes show steep nodal planes and little correlation to crustal source mechanisms and E–W extension direction. Likewise, the direction of fast splitting from seismic anisotropy studies is oblique to predictions from earthquake source mechanisms. The models of density contrasts (magma intrusions) explored in this study predict differential stresses of 1–10 MPa, stresses consistent with average earthquake stress drops. The local stress field rotations around the edges of intrusive bodies can explain the variable source mechanisms and match the local rotation of shear wave splitting direction, providing a plausible mechanism for mantle earthquakes in cratonic rifts. By analogy, exhumed mantle from rift zones reveals pseudotachylites near intrusions. Metasomatic reactions in contact aureoles broaden density contrasts and reduce upper mantle strength, potentially enabling extension of initially cold, strong continental lithosphere.

Seasonal land motion can be caused by processes above or below Earth's surface, often linked to natural changes in the hydrological cycle. In coastal deltaic systems, the coupling of water level changes between rivers and aquifers may cause significant surface deformation, but this process is poorly understood. In this research, we show that land motion can be a proxy for groundwater level changes in layered and semi‐confined aquifers, with implications for other delta systems worldwide. We investigate the processes driving the >15 mm seasonal deformation in the Mississippi River Delta near Baton Rouge, Louisiana. We consider elastic deformation due to surface loading and poroelastic deformation caused by changes in groundwater levels. The underlying aquifer system is formed by almost‐independent sands, crossed by the Mississippi and Amite Rivers, and cut by the Baton Rouge fault, which is a leaky barrier. We quantify seasonal deformation using Sentinel‐1 Interferometric Synthetic Aperture Radar (InSAR), between 2016 and 2022 and validate the results with Global Navigation Satellite Systems (GNSS) data. We find that the amplitude of the seasonal response has spatial variations related to the distance to the Mississippi River and the Baton Rouge Fault. We identify which aquifer layers are in phase with the observations and are thus most likely to cause poroelastic deformation. Our results are supported by hydraulic properties from the literature for the aquifer system. We conclude that seasonal motion in the area is dominantly driven by the poroelastic response to Mississippi River level changes that recharge the shallower aquifer layers.

The East African Rift provides a natural laboratory to study the influence of pre-existing lithospheric thin spots on the development of rifting and hotspot tectonism. Below the Ethiopian Rift and elevated Ethiopian Plateau, extensive magmatic and thermal modification due to Eocene-Oligocene flood basalt magmatism and Miocene-Recent rifting has resulted in slow lithospheric mantle velocities (< 4.1km/s; Dugda et al., 2007, JGR). In contrast, below the previously rifted, lower-lying Turkana Depression to the south, the lithospheric mantle appears relatively unmodified (4.2-4.8 km/s; Kounoudis et al., 2023, EPSL), despite being underlain by hot, mantle plume material. Important in this picture are detailed constraints on the lithosphere-asthenosphere boundary (LAB).Why the Turkana Depression, and particularly the failed Anza Rift terranes, remained resistant to thermal and magmatic modification, is debated. Although the Turkana Depression was a lithospheric thin spot at the onset of plume magmatism, Cenozoic rifting is now circumnavigating, not exploiting, the Anza Rift terranes (Musila et al., 2023, G3). Lithospheric thin spots therefore don't necessarily mark weak zones that are exploited by subsequent rifting and magmatism. One hypothesis for the apparently refractory nature of the Anza lithosphere is that Mesozoic rifting removed easily fusible phases, suppressing subsequent melting and associated strain localisation (Kounoudis et al., 2025, Nature).To test this geodynamic scenario, we calculated teleseismic S-to-p receiver functions and examined lithospheric thickness variations in the Turkana Depression, where the contrast between fast, relatively unmodified lithospheric mantle and slow, partially molten, plume-infiltrated asthenosphere is expected to provide impulsive S-to-p conversions at the LAB. We observe that the least impulsive and shallowest LAB conversions are associated with Miocene-Recent rift zones, and isolated shield volcanoes. Elsewhere, sharper and deeper S-to-p conversions attest to a lithosphere that has resisted thermo-mechanical modification.

We analyze Interferometric Synthetic Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS) data to characterize ground deformation and dike opening associated with the May 2021 Nyiragongo eruption. Despite documented eruptions in 1977 and 2002, Nyiragongo's magmatic system and its interaction with regional rifting remain poorly understood. Here, we use Sentinel‐1 (ESA Copernicus) and ALOS‐2 (JAXA) InSAR data and GNSS observations from the KivuGNet network, that cover the period of the eruption and dike intrusion. Deformation is dominated by a co‐eruptive dike intrusion extending over 25 km, accompanied by ground fissures of about 1.5 m opening in Goma (Democratic Republic of Congo) and Gisenyi (Rwanda). Joint inversions of four interferograms with GNSS data resolved a maximum dike opening near 10 m and a volumetric increase of about 180 Mm ³ . The inferred dike geometry aligns with observed ground deformation and seismicity, underscoring the coupling between magmatic and tectonic processes.

Lithospheric thin zones, such as recently failed rifts, are generally assumed to be weak spots where magmatism and deformation can concentrate during rifting and large igneous province development ^1–3 . Yet, the Turkana Depression in East Africa, the site of the failed 66-million-year-old Anza Rift, did not experience the widespread flood magmatism seen on the adjacent Ethiopian Plateau, despite being a lithospheric thin spot when the region encountered hot plume material around 45 million years ago ⁴ . Here we jointly invert surface-wave and receiver function data to constrain crustal and upper-mantle seismic structure below the Depression to evaluate lithospheric thermo-mechanical modification. Evidence for thick lower crustal intrusions, ubiquitous below the uplifted Ethiopian Plateau ^5,6 , is comparatively lacking below the Depression’s failed Anza Rift system, which ongoing East African rifting is circumnavigating, not exploiting. The mantle lithosphere below the Depression has also retained its cool, fast-wavespeed ‘lid’ character, contrasting the Ethiopian Plateau. Volatile depletion during failed Anza rifting probably rendered the thinned lithosphere refractory without later rejuvenation. Subsequent rifting and magmatism thus initiated away from the still-thin Anza Rift, in regions where fertile lithosphere enabled melting and the sufficient lowering of plate yield strength. Areas of thinned lithosphere are thus not necessarily persistent weak zones where significant extension and magmatic provinces will develop.

Seasonal land motion can be caused by processes above the surface or underground. These processes often are linked to natural changes in the hydrological cycle. Seasonal deformation can be elastic due to surface loading or poroelastic caused by changes in subsurface fluid storage. We investigate the processes driving the > 15 mm seasonal deformation in the Baton Rouge, Louisiana region which is intersected by the Mississippi and the Amite Rivers. The aquifer system is formed by vertical sands that are almost independent of each other, and it is crossed by the E-W striking Baton Rouge normal fault, which is a leaky barrier. We quantify seasonal motions using Sentinel-1 InSAR time series acquired between 2016-2022. InSAR precision was validated using local GNSS data. We compare the deformation with modeled motions from non-tidal loads (hydrological, oceanic, and atmospheric), river water levels, and modeled groundwater levels. We find that none of the non-tidal loads contributed significantly to the seasonal deformation. The amplitude of the seasonal response has spatial variations related to the distance to the Mississippi River and the Baton Rouge fault. We also identify which layers of the aquifer system are in phase with the observations and are thus most likely to cause poroelastic deformation. The temporal lags between deformation and changes in groundwater levels depend on which aquifer sand is driving the motion. We conclude that seasonal ground deformation in the area is dominantly driven by the poroelastic response to Mississippi River level changes that recharge the shallower aquifer layers.

The spatiotemporal characteristics of infrequent back‐arc rifting events and their relationships to volcanic unrest as well as other transient processes within the subduction system are not well known. We report 10 spatio‐temporal swarms of earthquakes that occurred along ∼175 km of the northern and central Taupō Volcanic Zone (TVZ) March–September 2019. The swarms lack clear mainshock‐aftershock distributions, suggesting involvement of pressurized fluids. The most energetic swarms occurred beneath the southwestern flank of Whakaari/White Island volcano (WI) and were accompanied by elevated SO₂ emissions and ∼8 mm southwest displacement of the edifice. Focal mechanisms of the WI swarms suggest horizontal dilation in the direction of tectonic extension achieved by slip along networks of closely spaced, NE‐striking normal faults linked by oblique‐slip to strike‐slip faults. Given the small GNSS displacements and upward‐migrating swarms, we favor the interpretation that magmatic volatiles were released along faults in response to changes in crustal stress. Based on the punctuated, cascading nature of swarms along the northern TVZ, we hypothesize that faults and magmatic systems are fluid‐rich, experiencing aseismic creep, and critically stressed, raising the possibility that small changes in crustal stress from slow slip along the Hikurangi subduction zone caused a reduction in the minimum horizontal stress. This change occurred along the entire northern TVZ over ∼5 months, potentially contributing to widespread volcanic unrest. The probable release of magmatic fluids from shallow magma bodies in the vicinity of WI between May and June argues against a causal relation to the 9 December 2019 WI eruption.

We analyze Synthetic Aperture Radar Interferometry (InSAR) and Global Navigation Satellite System (GNSS) data to characterize ground deformation and dike opening associated with the May 2021 Nyiragongo eruption. Despite well-documented eruptions in 1977 and 2002, Nyiragongo’s magmatic system and its interaction with regional rifting remain poorly understood. Here, we use Sentinel-1 (ESA Copernicus) and ALOS-2 (JAXA) InSAR data and GNSS observations, from the KivuGNet network, covering the period of the eruption and dike intrusion. Deformation is dominated by a southward-propagating co-eruptive dike intrusion extending over 25 km, accompanied by surface ruptures and ground fissures of about 1.5 meters in Goma (Democratic Republic of Congo) and Gisenyi (Rwanda). Joint inversions of four interferograms with GNSS data resolved a maximum dike opening near 10 meters and a volumetric increase of about 180 Mm ³ . The inferred dike geometry aligns with observed ground deformation and seismicity, underscoring the coupling between magmatic and tectonic processes.