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We present a new tomographic model of the mantle in the area of the 2010 M8.8 Maule earthquake and surrounding regions. Increased ray coverage provided by the aftershock data allows us to image the detailed subducting slab structure in the mantle, from the region of flat slab subduction north of the Maule rupture to the area of overlapping rupture between the 1960 M9.5 and the 2010 M8.8 events to the south. We have combined teleseismic primary and depth phase arrivals with available local arrivals to better constrain the teleseismic earthquake locations in the region, which we use to conduct nested regional–global tomography. The new model reveals the detailed structure of the flat slab and its transition to a more moderately dipping slab in the Maule region. South of the Maule region, a steeply dipping relic slab is imaged from ∼200 to 1000 km depth that is distinct from the moderately dipping slab above it and from the more northerly slab at similar depths. We interpret the images as revealing both horizontal and vertical tearing of the slab at ∼38°S to explain the imaged pattern of slab anomalies in the southern portion of the model. In contrast, the transition from a horizontal to moderately subducting slab in the northern portion of the model is imaged as a continuous slab bend. We speculate that the tearing was most likely facilitated by a fracture zone in the downgoing plate or alternatively by a continental scale terrane boundary in the overriding plate.

SUMMARY We use measurements at 35 GPS stations in northern Central America and 25 seismometers at teleseismic distances to estimate the distribution of slip, source time function and Coulomb stress changes of the Mw = 7.3 2009 May 28, Swan Islands fault earthquake. This event, the largest in the region for several decades, ruptured the offshore continuation of the seismically hazardous Motagua fault of Guatemala, the site of the destructive Ms = 7.5 earthquake in 1976. Measured GPS offsets range from 308 millimetres at a campaign site in northern Honduras to 6 millimetres at five continuous sites in El Salvador. Separate inversions of geodetic and seismic data both indicate that up to ∼1 m of coseismic slip occurred along a ∼250-km-long rupture zone between the island of Roatan and the eastern limit of the 1976 M = 7.5 Motagua fault earthquake in Guatemala. Evidence for slip ∼250 km west of the epicentre is corroborated independently by aftershocks recorded by a local seismic network and by the high concentration of damage to structures in areas of northern Honduras adjacent to the western limit of the rupture zone. Coulomb stresses determined from the coseismic slip distribution resolve a maximum of 1 bar of stress transferred to the seismically hazardous Motagua fault and further indicate unclamping of normal faults along the northern shore of Honduras, where two M > 5 normal-faulting earthquakes and numerous small earthquakes were triggered by the main shock.

SUMMARY Families of similar earthquakes at shallow depths occurring over multiple timescales have been identified prior to and during the 2005–2006 eruption of Augustine Volcano. The use of conventional and double-difference location methods failed to result in stable locations, due primarily to noisy site conditions as well as unfavourable station geometry for the Alaska Volcano Observatory stations. Previous work found that relatively robust estimates of the median arrival time difference between station pairs could be determined and used a constant velocity half-space model, true station elevations and a variant of the method of hyperbolas to estimate locations for four of the clusters. We have carried out a more extensive location analysis, including data from nine clusters and incorporating a linear gradient velocity model obtained from preliminary forward modelling of the data. Our location results show a progressive shallowing of the clusters with time. Clusters prior to 2005 December lie near sea level, whereas those in 2005 December and 2006 January concentrate about 500 m above sea level (asl) prior to the explosive eruptions and about 500 and 1200 m asl, respectively, (essentially at the surface) afterwards. These results are consistent with geodetic observations and modelling.

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

SUMMARY Taking advantage of the increased ray coverage due to seismicity following the 2004 December and 2005 March great earthquakes, an improved iterative regional–global tomographic method was applied to the Sumatra–Andaman and adjacent regions to better constrain the 3-D mantle velocity heterogeneity in the region. Velocity and hypocentral parameters were iteratively perturbed to sharpen the image of the subducted slab. Several iterations were performed, and the effects of source mislocation were considered in the iterative process, an issue usually neglected in global tomography. We find that source relocation between iterations increases the amplitudes of slab anomalies and sharpens the definition of slab geometry beyond what can be achieved by a fixed-source iterative inversion alone. In addition, extensive restoration tests of synthetic data were conducted that emphasize enhancements obtained by our iterative process. These tests show significant increases in amplitude and decreased smearing of synthetic slab features. Thus, when applied to the real data, similar enhancements are inferred in the resulting model, which better illustrates the complex slab geometry in the upper-mantle and transition zone regions along the Sumatra, Andaman and Burma subduction zones.