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The East African Rift System (EARS) is an active continental rift zone that hosts a diverse range of processes that result in recent and ongoing crustal deformation. To analyze these processes, we used Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) to complement the sparse GNSS velocity field with high resolution measurements of line-of-sight (LOS) deformation between 2015 and 2022. A broad survey approach was employed to detect new signals within the central section of the EARS, and to establish a baseline for future studies focused on tectonic, volcanic, and/or hydrologic deformation and change spanning geodetic time scales. With this regional approach, we were able to resolve signals varying over time-spans of a few weeks to several years that have magnitudes greater than ∼1 cm/yr. Our resulting deformation field shows a diverse range of signals related to the processes listed above as well as other unknown sources. This includes up to 8.5 cm/yr of groundwater-related subsidence in Nairobi, up to 70 cm of LOS displacement during the eruption of Nyiragongo in 2021, and steady uplift in the Manyara basin with a rate of 2.8 cm/yr. We update previously published InSAR measurements of Mount Suswa volcano, and the geothermal fields in Olkaria. We also show LOS change ranging between 2 and 5 mm/yr over a distances of approximately 200 km spanning the Kenya Rift with the greatest differential rates occurring south of Lake Turkana. More generally, our results highlight the ambiguity of relying solely on the sparse network of GNSS for studies such as those characterizing tectonic motion and rift opening. Such studies are likely missing many important signals and/or includes sites that are contaminated with signals unrelated to the target process. • InSAR complements the GNSS velocity field in the EARS, revealing high-resolution LOS deformation between 2015 and 2022. • Identified a diverse range of volcanic, tectonic, and anthropogenic deformation signals. • Illuminated the limitations of sparse GNSS networks for comprehensive deformation analysis. [ABSTRACT FROM AUTHOR]

The changes in physical properties leading up to a volcanic eruption provide clues to processes occurring within volcanoes and may reveal premonitory signals. The Sierra Negra shield volcano, located in the Galápagos Islands, erupted on 26 June 2018 after months of continued inflation and escalating earthquake activity. We applied ambient noise interferometry to five months of continuous seismic data from 12 broadband stations to calculate crustal shear‐wave velocity changes before and during the eruption. Using the Moving Window Cross‐Spectral technique and a pre‐eruption stack of ambient seismic data as reference, we found a −0.27% decrease in velocity 17 days before the eruption in station‐pairs that pass beneath the caldera's north‐eastern sector. Sensitivity kernels suggest that the velocity changes of this precursory signal are greatest at depths corresponding to the shallow sill (∼2 km) beneath the wide caldera. Our results, considered in light of geodetic, seismicity, and petrological results, suggest that this velocity decrease is in part caused by dilatation from a ML 4.8 earthquake, and degassing after a possible magma intrusion below or at the base of the sill. The precursory velocity decrease within the sill region offers an important tool for forecasting future eruptions at Sierra Negra. Plain Language Summary: Geoscientists use seismic methods to detect changes in the characteristics of volcanoes that could indicate increasing risk of eruption. The Sierra Negra volcano, located in the Galápagos Islands, erupted on 26 June 2018. Using time variations in seismic signals mainly generated by the ocean's interaction with the shoreline, we found a decrease in the velocity of seismic waves about 2 weeks before the eruption. The decrease is largest when these waves travel through Sierra Negra's shallow magma reservoir. This velocity reduction started on June 9th, the same day as the second‐largest pre‐eruptive earthquake recorded since 2017, and ended one day before the eruption, on June 25th. A slight increase in velocity was observed on May 13th and lasted 4 days. We interpret this increase as a magma pulse that destabilizes the volcanic system. The combined effect of this magma intrusion and the magnitude 4.8 earthquake altered the volcano's state and favored the start of the eruption. The technique that we used could forecast future eruptions at Sierra Negra and other basaltic island volcanoes. Key Points: A velocity variations analysis provides important evidence of pre‐eruptive physical processes inside the volcanic edificeUsing ambient noise interferometry we obtained a precursor signal for the 2018 Sierra Negra eruption, which offers a forecasting toolThe velocity anomaly is spatially constrained to station‐pairs near the initial eruptive fissures, and is caused by extensional stress [ABSTRACT FROM AUTHOR]

Located about 1000 km west of continental Ecuador, Sierra Negra is a shield volcano with a large summit caldera and is one of the most active volcanoes in the Galapagos archipelago. The unrest of the volcano began when it started to inflate after the previous eruption in 2005. Seismic activity increased gradually during this period. The activity escalated with a magnitude 5.3 earthquake at 09:15 UTC on 26th June 2018. Following the earthquake was a few hours of seismic quiescence. At about 17:15 UTC, an intense seismic swarm commenced and later tremor became dominant. Since then, eruptive fissures were observed at the north flank of the volcano, which marked the onset of the eruption. The sequence of events was recorded by a temporary network of 14 broadband seismometers deployed in April 2018. Here we present results of volcanic tremor location using a seismic amplitude ratio method with data from the temporary network and 3 nearby permanent stations. Preliminary results show that the tremor source was located deep, the northwest of the caldera and was stable in space until the earthquake swarm commenced. After that, the source moved towards the caldera and became shallower. It finally localized at the north flank of the caldera where the eruptive fissures were observed. We aim to determine whether the tremor movement represents dike opening and/or magma migration. [ABSTRACT FROM AUTHOR]

Sierra Negra has a shallow, elliptical caldera (7×10 km) and is one of the most active volcanoes in the Galapagos Archipelago, located approximately 1000 km west of continental Ecuador. The most recent unrest of Sierra Negra started when an increase of seismicity was recorded at permanent seismic stations on the islands in July 2017. The increase ended with a magnitude 5.3 earthquake on 26th June 2018 at 09:15 UTC. An intense seismic swarm commenced after a few hours of relative seismic quiescence, and later evolved into tremor. Eruptive fissures were observed at the north flank of the volcano, which marked the onset of the eruption (around 20:00 UTC).While the seismic amplitude ratio method suggested different persistent source locations throughout the day of the eruption day the structures that produce these sources are still unclear. Using time-lapse local earthquake tomography (before and after the eruption time) together with the seismic amplitude method could help in the further understanding of the geodynamics of the volcano. In this study we performed a 3D seismic tomography inversion for the Sierra Negra area using P-wave arrival times of the local earthquakes recorded at 14 temporary and 5 permanent stations. The preliminary results illustrate two relatively low velocity anomalies reaching the surface where the seismic amplitude method documented the persistent sources. [ABSTRACT FROM AUTHOR]