ALBERT

Description: Selected multi-proxy and accurately dated marine and terrestrial records covering the past 2000 years in the Iberian Peninsula (IP) facilitated a comprehensive regional paleoclimate reconstruction for the Medieval Climate Anomaly (MCA: 900–1300 AD). The sequences enabled an integrated approach to land–sea comparisons and, despite local differences and some minor chronological inconsistencies, presented clear evidence that the MCA was a dry period in the Mediterranean IP. It was a period characterized by decreased lake levels, more xerophytic and heliophytic vegetation, a low frequency of floods, major Saharan eolian fluxes, and less fluvial input to marine basins. In contrast, reconstruction based on sequences from the Atlantic Ocean side of the peninsula indicated increased humidity. The data highlight the unique characteristics of the MCA relative to earlier (the Dark Ages, DA: ca 500–900 years AD) and subsequent (the Little Ice Age, LIA: 1300–1850 years AD) colder periods. The reconstruction supports the hypothesis of Trouet et al. (2009), that a persistent positive mode of the North Atlantic Oscillation (NAO) dominated the MCA.

Description: On February 27, 2010, the Central Chilean margin ruptured over a length of _400 km in the Mw 8.8 Maule earthquake. The international seismological community responded quickly by organising the International Maule Aftershock Deployment (IMAD) consisting of more than 140 seismological stations from Chile, Germany, France, the USA and the UK. This land seismic network is complemented by 30 ocean bottom seismometers in the northern portion of the rupture, operating from September to December 2012. Similar efforts were carried out by the geodetic community, installing more than 65 cGPS stations and an even larger number of campaign sites. Last but not least surveys of coastal uplift and surface faulting provide constraints on the immediate coseismic response as well as on the longer term evolution of the margin. In the MARISCOS project (MAule eaRthquake: Integration of Seismic Cycle Observations and Structural investigations) seismological, geodetic and geological approaches are combined in order to link coseismic slip, the postseismic response, and the longer term properties of the margin. We have created a bulletin of over 16000 events with low epicentral uncertainties. Seismic activity occurs in 4 main groups: (1) Normal faulting outer rise events at depths between the surface and 30 km depth. (2) a dipping 70-80 wide band along the whole rupture zone, thin in cross-section. Most of the events in this band are consistent with plate interface seismicity, but a kink in cross-section suggests the existence of a splay fault forming the shallowest part. This band is separated from the trench by a 50 km aseismic zone and is approximately terminated by the coastline on the landward side (at least to the north of the main shock epicentre), likely corresponding to the plate interface-continental Moho intersection at depth. (3) elongated clusters of seismicity at 40-50 km depth and with plate interface focal mechanisms, which occur below the continental Moho. (4) Pronounced crustal seismicity, most prominently normal faulting seismicity with strike oblique to the trench occur at the northern limit of the rupture zone. The northern part of the rupture zone is imaged with local earthquake tomography and shows elevated vp/vs values (_1.85) in the western part of the intense crustal seismicity. Further seismicity occurs at intermediate depth range (80-120 km) and shallowly in the volcanic arc. Improved models of coseismic and postseismic slip were computed based on the high density geodetic data and with a realistic plate geometry and elasticity structure. The postseismic response over the first 420 days is characterised by elongated patches of afterslip downdip of the coseismic slip in the rupture zone north of the hypocentre, which spatially largely coincides with the main plate interface seismicity (group 2). The equivalent moment of the afterslip is much larger than the cumulative seismic moment of the aftershocks, but although there is a close temporal correspondence in the decay of afterslip and seismicity, the slip of some aftershocks might be larger than the cumulative afterslip. A deeper patch of afterslip to the south of the coseismic slip is not associated with significant seismicity. Based on the detailed aftershock locations, we have implemented dynamic station corrections for backprojection of the main shock using stations in the US, Antarctica, and Africa. Using this calibration we are able to image coherent energy at frequencies above 2-4 Hz. Similar to other investigators we find that higher frequency energy release is found downdip of the lower frequency release and geodetic slip, but contrary to some published work we locate the HF release for the northern part of this rupture near the downdip end of the main aftershock zone (group 2), updip of the deep band (group 3) for rupture times 〉 50 s. Before 50 s, we find rupture further downdip nearer group 3 seismicity at the deepest part of seismogenic plate interface. Taken together, these results indicate that rather than being either velocity weakening (unstable, Seismogenic) or velocity strengthening (stable, creeping), the plate interface over large areas can switch between both modes of frictional behaviour and is maybe over large areas in a conditionally stable regime, where fluid diffusion can control the variable behaviour.

Description: The excellent spatial coverage of continuous GPS stations in the region affected by the Maule Mw=8.8 2010earthquake, combined with the proximity of the coast to the seismogenic zone, allows us to model megathrust afterslip on the plate interface with unprecedented detail. We invert post-seismic observations from continuous GPS sites to derive a time-variable model of the first 420 days of afterslip. The afterslip pattern appears to be transient and non-stationary, with the cumulative afterslip pattern being formed from afterslip pulses. Frequency analysis of the slip rate for each patch of the interface model shows that the region of the interface with the highest aftershock density also has the most variable slip rate suggesting that afterslip pulses and aftershocks are closely related in time and space. Changes in static stress on the plate interface from the co- and post seismic slip cannot explain the aftershock patterns, suggesting that another process – perhaps fluid related - is controlling aftershocks.We use aftershock data to quantify the seismic coupling distribution during the postseismic phase. Comparison of the postseismic behaviour to interseismic locking reveals that highly locked regions do not necessarily behave asrate-weakening in the postseismic period.

Description: On 27 February 2010 the Mw 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. The postseismic phase was monitored by a network of temporary seismic stations (IMAD) and GPS stations along and around the whole rupture zone. Here, we examine the relation between the spatial-temporal properties of the aftershock distribution and postseismic displacements from GPS. Using scaling relations derived from global data, we calculate the slip and size of individual aftershocks and relate these to preliminary afterslip models. Events larger than Mw=5 result in slip of more than _20 cm and some of these events presumably exceed locally the cumulative afterslip from 2 months period. Along the whole rupture the small patches ruptured by the aftershocks do not fill up the whole rupture, which is reflected by the observation that the cumulative moment of aftershock seismicity is significantly smaller than the total moment released by afterslip. The time-series of Earth’s surface postseismic displacements analyzed here show rapid transient deformation immediately following the Maule earthquake. Results show a first order linear relationship between cumulative displacement with the cumulative number of aftershocks. Similar relations have been observed for other earthquakes. This indicates that both processes decay obeying a similar physical law. Due to the higher number of GPS stations along the Maule 2010 rupture the ratio of displacement (afterslip) per aftershock event might allow to infer rheological properties when mapped along the rupture zone. Furthermore, we discuss the relation between the azimuths of co-seismic, interseismic and postseismic displacement vectors and compare these with the strike directions of focal mechanisms. We compare the results from the Maule 2010 rupture area with the 2005 Nias and the 2011 Tohoku-Oki earthquakes.