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SUMMARY The Crandall Canyon Mine, Utah, collapse in 2007 August resulted in a total of nine fatalities. We processed data from the ALOS satellite acquired before and after the collapse to quantify surface subsidence associated with the collapse to better understand the collapse process. The deformationshowsasteepV-shapedpatternofsubsidencewithslightasymmetry.Wecompare the fit of four different models that simulate the subsidence pattern. The first two models use elastic half-space rheology. We find that collapse alone cannot explain the observations, and a component of normal faulting is required to fit the data. The second set of models simulates collapseinelasto-plasticmedia.Onlyasmallcomponentofnormalfaultingisrequiredinthese models. We suggest that considering elasto-plastic material behaviour is particularly important for shallow deformation modelling, where microfractures and other non-elastic rheology are common. Disregarding this material behaviour can lead to biased model parameter estimates.

SUMMARY In the early stages of continental rifting, extension takes place by normal faulting, while in mature continental rifts dyke intrusion dominates. Little is known about the nature of the transition between fault-controlled and dyke-controlled extension or about the processes in an intermediate setting. Here, we present observations of the temporal and spatial evolution of surface displacements during the 2007 July 14–August 4 rifting episode in Northern Tanzania, an immature section of the East African Rift. The ground deformation initiated with subsidence that can be attributed to ∼40 cm of normal motion on a NE striking fault. Following July 17, deformation was dominated by the intrusion of ∼7-km-long dyke. Dyke opening increased gradually to a total of ∼2.4 m. From July 21, the collapse of a shallow graben above the fault dominated the near-field displacements. Comparison to the 2007 Dabbahu dyke, Afar, which occurred in a more mature rift, shows an order-of-magnitude scale difference in dyke length. Using numerical models of dyke propagation, we attribute this to the size and depth of the magma chamber; in immature rifts the thick crust and slow spreading rate favour small, deep magma chambers, forming short, buried dykes, whereas in mature rifts the thinner crust and faster spreading rate favour large, shallow magma chambers and long, erupting dykes. Observing the pattern of active processes in the East African Rift is key to understanding the development of rift systems and passive margins elsewhere.

SUMMARY We investigate the 2007 Pisco, Peru, earthquake (M8.0) using seismological and geodetic data. Analysis of teleseismic records indicates that the earthquake is characterized by a steadily increasing moment rate, with the maximum occurring between about 55 and 70 s and the associated slip occurring about 40 km south of the epicentre. We combine InSAR data from L-band (ALOS) and C-band (Envisat) satellites to produce a 3-D image of the ground displacements caused by the earthquake. Distributed slip inversions of the interferograms identify a single large patch of slip in the location of the maximum moment rate identified by seismology. Despite a clear spatial coincidence between the location of the coseismic slip and the anomalously high topography of the Paracas Peninsula, we find no evidence that the earthquake was directly responsible for motion on any upper plate faults.

SUMMARY We present a new surface velocity field for Baja California using GPS data to test the rigidity of this microplate, calculate its motion in a global reference frame, determine its relative motion with respect to the North American and the Pacific plates, and compare those results to our estimate for Pacific‐North America motion. Determination of Pacific Plate motion is improved by the inclusion of four sites from the South Pacific Sea Level and Climate Monitoring Project. These analyses reveal that Baja California moves as a quasi-rigid block but at a slower rate in the same direction, as the Pacific Plate relative to North America. This is consistent with seismic activity along the western edge of Baja California (the Baja California shear zone), and may reflect resistance to motion of the eastern edge of the Pacific Plate caused by the ‘big bend’ of the San Andreas fault and the Transverse Ranges in southern California.

OVSICORI The subduction plate interface along the Nicoya Peninsula, Costa Rica, generates damaging large (Mw > 7.5) earthquakes. We present hypocenters and 3-D seismic velocity models (VP and VP/VS) calculated using simultaneous inversion of P- and S-wave arrival time data recorded from small magnitude, local earthquakes to elucidate seismogenic zone structure. In this region, interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes. Plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern Nicoya Peninsula, respectively. Microseismicity offset across the plate suture of East Pacific Rise-derived and Cocos-Nazca Spreading Center derived oceanic lithosphere is ∼5 km, revising earlier estimates suggesting ∼10 km of offset. Interplate seismicity begins downdip of increased locking along the plate interface imaged using GPS and a region of low VP along the plate interface. The downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental Moho intersection, possibly due to the onset of ductile behaviour. Slow forearc mantle wedge Pwave velocities suggest 20–30 per cent serpentinization across the Nicoya Peninsula region while calculated VP/VS values suggest 0–10 per cent serpentinization. Interpretation of VP/VS resolution at depth is complicated however due to ray path distribution.We posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes. La interfaz de placas de subducción a lo largo de la Península de Nicoya, Costa Rica, genera grandes terremotos dañinos (Mw > 7,5). Presentamos hipocentros y modelos tridimensionales de velocidad sísmica (VP y VP/VS) calculados utilizando la inversión simultánea de datos de tiempos de llegada de ondas P y S registrados en terremotos locales de pequeña magnitud para dilucidar la estructura de la zona sismogénica. En esta región, la microsismicidad del ciclo intersísmico no define únicamente el alcance potencial de ruptura de los grandes terremotos. La microsismicidad de la interfaz de placas se extiende de 12 a 26 y de 17 a 28 km bajo el nivel del mar debajo del sur y norte de la península de Nicoya, respectivamente. El desplazamiento de la microsismicidad a través de la sutura de la placa de la litosfera oceánica derivada del East Pacific Rise y del Cocos-Nazca Spreading Center es de ~5 km, lo que revisa estimaciones anteriores que sugieren ~10 km de desplazamiento. La sismicidad entre placas comienza echado abajo del aumento del bloqueo a lo largo de la interfaz de la placa fotografiada con GPS y una región de baja VP a lo largo de la interfaz de la placa. El borde echado hacia abajo de la microsismicidad de la interfaz de la placa ocurre echado hacia arriba de la losa oceánica y la intersección del Moho continental, posiblemente debido al inicio del comportamiento dúctil. Las velocidades lentas de la onda P de la cuña del manto del antearco sugieren una serpentinización del 20 al 30 por ciento en toda la región de la península de Nicoya, mientras que los valores calculados de VP/VS sugieren una serpentinización del 0 al 10 por ciento. Sin embargo, la interpretación de la resolución VP/VS en profundidad es complicada debido a la distribución de la trayectoria de los rayos. Postulamos que la cuña del manto del antearco está serpentinizada regionalmente, pero aún puede sostener la ruptura durante los terremotos más grandes de la zona sismogénica. Universidad Nacional, Costa Rica University of California-Santa Cruz, USA Georgia Institute of Technology, USA University of California-San Diego, USA University of Miami, USA IFM-GEOMAR, Alemania Observatorio Vulcanológico y Sismológico de Costa Rica