A nearly 20-year hiatus in major seismic activity in southern California ended on 4 July 2019 with a sequence of intersecting earthquakes near the city of Ridgecrest, California. This sequence included a foreshock with a moment magnitude (Mw) of 6.4 followed by a Mw 7.1 mainshock nearly 34 hours later. Geodetic, seismic, and seismicity data provided an integrative view of this sequence, which ruptured an unmapped multiscale network of interlaced orthogonal faults. This complex fault geometry persists over the entire seismogenic depth range. The rupture of the mainshock terminated only a few kilometers from the major regional Garlock fault, triggering shallow creep and a substantial earthquake swarm. The repeated occurrence of multifault ruptures, as revealed by modern instrumentation and analysis techniques, poses a formidable challenge in quantifying regional seismic hazards.

The existence of elongated, shallow magma chambers beneath the axes of fast-spreading mid-ocean ridges is well established 1-8 . Yet, at slow-spreading ridges such shallow and elongated magma chambers are much less evident 9,10 . Simple thermal models 8 therefore predict that spreading velocity and magma supply may provide the main controls on magmachamber depth and morphology. Here we use interferometric synthetic aperture radar data to investigate the dynamics of the magma chamber beneath the slow-spreading Erta Ale segment of the Ethiopian Rift. We show that an eruption from Alu-Dalafilla in November 2008 was sourced from a shallow, 1 km deep, elongated magma chamber that is divided into two segments. The eruption was probably triggered by a small influx of magma into the northern segment. Both segments of the magma chamber fed the main eruption through a connecting dyke and both segments have been refilling rapidly since the eruption ended. Our results support the presence of independent sources of magma supply to segmented chambers located along the axes of spreading centres 11 . However, the existence of a shallow, elongated axial chamber at Erta Ale indicates that spreading rate and magma supply may not be the only controls on magma-chamber characteristics.Shallow (<2 km depth) elongated axial magma chambers have been detected at many places on fast-spreading ridges 1-7 , but magma chambers have been found at only a few slow-spreading ridges as deeper isolated magma pockets 9,10 . The depth and continuity of these chambers is thought to depend on spreading rate and magma supply 8 .Typically, an axial high and continuous shallow magma chamber (at ∼1-2 km depth) characterizes ridges spreading faster than ∼40 mm yr −1 (refs 1,2,8), whereas a median valley and isolated deeper chambers (≥3 km deep) are found at slower-spreading ridges 5,9,10 . Elongated axial magma chambers are typically 250-4,000 m wide 1,3,4 and tens of kilometres long. In contrast, the magma chambers at slow-spreading ridges are isolated, nearspherical bodies. Magma chambers at ∼3 km depth have been inferred both in Krafla 12 and Askja 13 , Iceland. At the Dabbahu segment in Afar, both the Gabho and Dabbahu volcanoes are also associated with axisymmetric chambers between 3 and 5 km depth 14 . The ∼120-km-long Erta Ale spreading segment separates the Nubian plate from the Danakil microplate in the Afar region of Ethiopia. It forms a continuous ridge made up of distinct volcanic centres 15 (Fig. 1a,b), including the Erta Ale volcano, which hosts an active lava lake 15,16 . The spreading rate at this latitude is estimated at ∼12 mm yr −1 (ref. 17). In November 2008, an eruption occurred in the Alu-Dalafilla volcanic centre that lies 30 km north of the

The 25 April 2015 M w 7.8 Gorkha earthquake caused more than 8000 fatalities and widespread building damage in central Nepal. The Italian Space Agency's COSMO-SkyMed Synthetic Aperture Radar (SAR) satellite acquired data over Kathmandu area four days after the earthquake and the Japan Aerospace Exploration Agency's Advanced Land Observing Satellite-2 SAR satellite for larger area nine days after the mainshock. We used these radar observations and rapidly produced damage proxy maps (DPMs) derived from temporal changes in Interferometric SAR coherence. Our DPMs were qualitatively validated through comparison with independent damage analyses by the National Geospatial-Intelligence Agency and the United Nations Institute for Training and Research's United Nations Operational Satellite Applications Programme, and based on our own visual inspection of DigitalGlobe's WorldView optical pre-versus postevent imagery. Our maps were quickly released to responding agencies and the public, and used for damage assessment, determining inspection/imaging priorities, and reconnaissance fieldwork.

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The 5–9 March 2011 Kamoamoa fissure eruption along the east rift zone of Kīlauea Volcano, Hawai`i, followed months of pronounced inflation at Kīlauea summit. We examine dike opening during and after the eruption using a comprehensive interferometric synthetic aperture radar (InSAR) data set in combination with continuous GPS data. We solve for distributed dike displacements using a whole Kīlauea model with dilating rift zones and possibly a deep décollement. Modeled surface dike opening increased from nearly 1.5 m to over 2.8 m from the first day to the end of the eruption, in agreement with field observations of surface fracturing. Surface dike opening ceased following the eruption, but subsurface opening in the dike continued into May 2011. Dike volumes increased from 15, to 16, to 21 million cubic meters (MCM) after the first day, eruption end, and 2 months following, respectively. Dike shape is distinctive, with a main limb plunging from the surface to 2–3 km depth in the up‐rift direction toward Kīlauea's summit, and a lesser projection extending in the down‐rift direction toward Pu`u `Ō`ō at 2 km depth. Volume losses beneath Kīlauea summit (1.7 MCM) and Pu`u `Ō`ō (5.6 MCM) crater, relative to dike plus erupted volume (18.3 MCM), yield a dike to source volume ratio of 2.5 that is in the range expected for compressible magma without requiring additional sources. Inflation of Kīlauea's summit in the months before the March 2011 eruption suggests that the Kamoamoa eruption resulted from overpressure of the volcano's magmatic system.

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Sierra Negra volcano in Isabela island, Galápagos, erupted from October 22 to October 30 in 2005. During the 8 days of eruption, the center of Sierra Negra's caldera subsided about 5.4 meters. Three hours prior to the onset of the eruption, an earthquake (Mw 5.4) occurred, near the caldera. Because of the large and complex phase gradient due to the huge subsidence and the earthquake, it is difficult to form an interferogram inside the caldera that spans the eruption. The deformation is so large and spatially variable that the approximations used in existing InSAR software (ROI, ROI_PAC, DORIS, GAMMA) cannot properly coregister SAR image pairs spanning the eruption. We have developed here a two‐step algorithm that can form intra‐caldera interferograms from these data. The first step involves a “rubber‐sheeting” SAR image coregistration. In the second step we use range offset estimates to mitigate the steep phase gradient. Using this new algorithm, we retrieve an interferogram with the best coverage to date inside the caldera of Sierra Negra.

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