ALBERT

Description: The Maule earthquake (2010 February 27, M w 8.8, Chile) broke the subduction megathrust along a previously locked segment. Based on an international aftershock deployment, catalogues of precisely located aftershocks have become available. Using 23 well-located aftershocks, we calibrate the classic teleseismic backprojection procedure to map the high-frequency seismic radiation emitted during the earthquake. The calibration corrects traveltimes in a standard earth model both with a static term specific to each station, and a ‘dynamic’ term specific to each combination of grid point and station. The second term has been interpolated over the whole slipping area by kriging, and is about an order of magnitude smaller than the static term. This procedure ensures that the teleseismic images of rupture development are properly located with respect to aftershocks recorded with local networks and does not depend on accurate hypocentre location of the main shock. We track a bilateral rupture propagation lasting ~160 s, with its dominant branch rupturing northeastwards at about 3 km s –1 . The area of maximum energy emission is offset from the maximum coseismic slip but matches the zone where most plate interface aftershocks occur. Along dip, energy is preferentially released from two disconnected interface belts, and a distinct jump from the shallower belt to the deeper one is visible after about 20 s from the onset. However, both belts keep on being active until the end of the rupture. These belts approximately match the position of the interface aftershocks, which are split into two clusters of events at different depths, thus suggesting the existence of a repeated transition from stick-slip to creeping frictional regime.

Description: An automatic procedure is presented to retrieve rupture parameters for large earthquakes along the Sunda arc subduction zone. The method is based on standard array analysis and broadband seismograms registered within 30°–100° epicentral distance. No assumptions on source mechanism are required. By means of semblance the coherency of P waveforms is analysed at separate large-aperture arrays. Waveforms are migrated to a 10°×10° wide source region to study the spatio-temporal evolution of earthquakes at each array. The multiplication of the semblance source maps resulting at each array increases resolution. Start, duration, extent, direction, and propagation velocity are obtained and published within 25 min after the onset of the event. First preliminary results can be obtained even within 16 min. Their rapid determination may improve the mitigation of the earthquake and tsunami hazard. Real-time application will provide rupture parameters to the GITEWS project (German Indonesian Tsunami Early Warning System). The method is applied to the two M8.0 Sumatra earthquakes on 12 September 2007, to the M7.4 Java earthquake on 2 September 2009, and to major subduction earthquakes that have occurred along Sumatra and Java since 2000. Obtained rupture parameters are most robust for the largest earthquakes with magnitudes M≥8. The results indicate that almost the entire seismogenic part of the subduction zone off the coast of Sumatra has been ruptured. Only the great Sumatra event in 2004 and the M7.7 Java event on 17 July 2006 could reach to or close to the surface at the trench. Otherwise, the rupturing was apparently confined to depths below 25 km. Major seismic gaps seem to remain off the coast of Padang and the southern tip of Sumatra.

Description: Understanding the effects of major hydrogeological controls on hyporheic exchange and bank storage is essential for river water management, groundwater abstraction, restoration and ecosystem sustainability. Analytical models cannot adequately represent complex settings with, for example, transient boundary conditions, varying geometry of surface water–groundwater interface, unsaturated and overland flow, etc. To understand the influence of parameters such as (1) sloping river banks, (2) varying hydraulic conductivity of the riverbed and (3) different river discharge wave scenarios on hyporheic exchange characteristics such as (a) bank storage, (b) return flows and (c) residence time, a 2-D hydrogeological conceptual model and, subsequently, an adequate numerical model were developed. The numerical model was calibrated against observations in the aquifer adjacent to the hydropower-regulated Lule River, northern Sweden, which has predominantly diurnal discharge fluctuations during summer and long-lasting discharge peaks during autumn and winter. Modelling results revealed that bank storage increased with river wave amplitude, wave duration and smaller slope of the river bank, while maximum exchange flux decreased with wave duration. When a homogeneous clogging layer covered the entire river–aquifer interface, hydraulic conductivity positively affected bank storage. The presence of a clogging layer with hydraulic conductivity 〈 0.001 m d−1 significantly reduced the exchange flows and virtually eliminated bank storage. The bank storage return/fill time ratio was positively related to wave amplitude and the hydraulic conductivity of the interface and negatively to wave duration and bank slope. Discharge oscillations with short duration and small amplitude decreased bank storage and, therefore, the hyporheic exchange, which has implications for solute fluxes, redox conditions and the potential of riverbeds as fish-spawning locations. Based on these results, river regulation strategies can be improved by considering the effect of certain wave event configurations on hyporheic exchange to ensure harmonious hydrogeochemical functioning of the river–aquifer interfaces and related ecosystems.

Description: Understanding the effects of major hydrogeological controls on hyporheic exchange and bank storage is essential for river water management, groundwater abstraction, restoration and ecosystem sustainability. Analytical models cannot adequately represent complex settings with, for example, transient boundary conditions, varying geometry of surface water–groundwater interface, unsaturated and overland flow, etc. To understand the influence of parameters such as (1) sloping river banks, (2) varying hydraulic conductivity of the riverbed and (3) different river discharge wave scenarios on hyporheic exchange characteristics such as (a) bank storage, (b) return flows and (c) residence time, a 2-D hydrogeological conceptual model and, subsequently, an adequate numerical model were developed. The numerical model was calibrated against observations in the aquifer adjacent to the hydropower regulated Lule River, Northern Sweden, which has predominantly diurnal discharge fluctuations during summer and long-lasting discharge peaks during autumn and winter. Modelling results revealed that bank storage increased with river wave amplitude, wave duration and smaller slope of the river bank, while maximum exchange flux decreased with wave duration. When a homogeneous clogging layer covered the entire river–aquifer interface, hydraulic conductivity positively affected bank storage. The presence of a clogging layer with hydraulic conductivity 〈 0.001 m d−1 significantly reduced the exchange flows and virtually eliminated bank storage. The bank storage return/fill time ratio was positively related to wave amplitude and the hydraulic conductivity of the interface and negatively to wave duration and bank slope. Discharge oscillations with short duration and small amplitude decreased bank storage and, therefore, the hyporheic exchange, which has implications for solute fluxes, redox conditions and the spawning potential of riverbeds. Based on these results, river regulation strategies can be improved by considering the effect of certain wave event configurations on hyporheic exchange to ensure harmonious hydrogeochemical functioning of the river–aquifer interfaces and related ecosystems.

Description: The Maule earthquake (2010 February 27, Mw 8.8, Chile) broke the subduction megathrust along a previously locked segment. Based on an international aftershock deployment, catalogues of precisely located aftershocks have become available. Using 23 well-located aftershocks, we calibrate the classic teleseismic backprojection procedure to map the high-frequency seismic radiation emitted during the earthquake. The calibration corrects traveltimes in a standard earth model both with a static term specific to each station, and a ‘dynamic’ term specific to each combination of grid point and station. The second term has been interpolated over the whole slipping area by kriging, and is about an order of magnitude smaller than the static term. This procedure ensures that the teleseismic images of rupture development are properly located with respect to aftershocks recorded with local networks and does not depend on accurate hypocentre location of the main shock. We track a bilateral rupture propagation lasting ∼160 s, with its dominant branch rupturing northeastwards at about 3 km s−1. The area of maximum energy emission is offset from the maximum coseismic slip but matches the zone where most plate interface aftershocks occur. Along dip, energy is preferentially released from two disconnected interface belts, and a distinct jump from the shallower belt to the deeper one is visible after about 20 s from the onset. However, both belts keep on being active until the end of the rupture. These belts approximately match the position of the interface aftershocks, which are split into two clusters of events at different depths, thus suggesting the existence of a repeated transition from stick-slip to creeping frictional regime.

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: Since its first application on Sumatra-Andaman earthquake, back-projection analysis has been widely exploited to infer the time-evolution of the rupture fronts of mega-earthquakes. In this technique, selected seismic phases recorded at teleseismic distances by a network of sensors are shifted according to a possible source position and a velocity model, and a multichannel version of the cross-correlation function is estimated. In this way, the time dependent map of the seismic energy emission in the source area can be inferred. We have back-projected the mainshock of Maule earthquake (Mw 8.8), which nucleated on 27/02/2010 in central Chile and is one of the largest earthquakes recorded in modern times. We have analyzed P phases filtered in the frequency range (0.4-3) Hz recorded by three seismic arrays located in US, Africa and Antarctica. Relative time shifts between sensors (inferred by maximizing the cross-correlation function) have been estimated with respect to a 1D global velocity model (ak135) and have been refined introducing two corrections, a static correction anda dynamic correction. The former is the time shift induced by local effects in the sensor area, whereas the latter is the correction associated with the source-sensor path and is mostly affected by medium properties in the source area. We have inferred these two corrections by analyzing the waveforms of 23 aftershocks and foreshocks with high magnitude (〉5.3). In detail, static correction was chosen as the mean time shift averaged over all the events recorded by one station, while dynamic correction was the remaining part of the travel time after removing the 1Dmodel travel time and the static correction. Moreover, dynamic corrections (and hence the complete travel times)have been interpolated over all the source area by Kriging, a spatial interpolation method. Results show that high-frequency seismic energy emission mostly occurs along the coastline with a general northward migration during the event. Specifically, in the first minute of the rupture process, the energy emission occurs southerly from or close to the epicenter. Afterwards, seismic emission moves northwards, with a gap with respect to the first emission zone, and a further northward migration occurs till the end of emission. Both the spatial gap of seismic emission and the northward migration are in line with the results of other studies in the same area, whereas we find a shallower emission area and different emission features in the zone close to the epicenter. Results for different frequency bands and the analysis of secondary maxima of energy emission are being investigated. In particular, we are shifting towards higher frequencies looking at the frequency bands (1-4) Hz and (2-8) Hz. The former band displays an emission pattern similar to that of (0.4-3) Hz, but with a sharper gap of about 50 Km; the latter band hows coherent arrivals only during the first 80 s, with a clear energy emission south of the epicenter at the onset of the event and preserving the northward migration afterwards.

Description: An automatic procedure is presented to retrieve rupture parameters for large earthquakes along the Sunda arc subduction zone. The method is based on standard array analysis and broadband seismograms registered within 30°–100° epicentral distance. No assumptions on source mechanism are required. By means of semblance the coherency of P waveforms is analysed at separate large-aperture arrays. Waveforms are migrated to a 10°×10° wide source region to study the spatio-temporal evolution of earthquakes at each array. The multiplication of the semblance source maps resulting at each array increases resolution. Start, duration, extent, direction, and propagation velocity are obtained and published within 25 min after the onset of the event. First preliminary results can be obtained even within 16 min. Their rapid determination may improve the mitigation of the earthquake and tsunami hazard. Real-time application will provide rupture parameters to the GITEWS project (German Indonesian Tsunami Early Warning System). The method is applied to the two M8.0 Sumatra earthquakes on 12 September 2007, to the M7.4 Java earthquake on 2 September 2009, and to major subduction earthquakes that have occurred along Sumatra and Java since 2000. Obtained rupture parameters are most robust for the largest earthquakes with magnitudes M≥8. The results indicate that almost the entire seismogenic part of the subduction zone off the coast of Sumatra has been ruptured. Only the great Sumatra event in 2004 and the M7.7 Java event on 17 July 2006 could reach to or close to the surface at the trench. Otherwise, the rupturing was apparently confined to depths below 25 km. Major seismic gaps seem to remain off the coast of Padang and the southern tip of Sumatra.

Description: The Maule earthquake (2010 February 27, Mw 8.8, Chile) broke the subduction megathrust along a previously locked segment. Based on an international aftershock deployment, catalogues of precisely located aftershocks have become available. Using 23 well-located aftershocks, we calibrate the classic teleseismic backprojection procedure to map the highfrequency seismic radiation emitted during the earthquake. The calibration corrects traveltimes in a standard earth model both with a static term specific to each station, and a ‘dynamic’ term specific to each combination of grid point and station. The second term has been interpolated over the whole slipping area by kriging, and is about an order of magnitude smaller than the static term. This procedure ensures that the teleseismic images of rupture development are properly located with respect to aftershocks recorded with local networks and does not depend on accurate hypocentre location of the main shock. We track a bilateral rupture propagation lasting ∼160 s, with its dominant branch rupturing northeastwards at about 3 kms−1. The area of maximum energy emission is offset from the maximum coseismic slip but matches the zone where most plate interface aftershocks occur. Along dip, energy is preferentially released from two disconnected interface belts, and a distinct jump from the shallower belt to the deeper one is visible after about 20 s from the onset. However, both belts keep on being active until the end of the rupture. These belts approximately match the position of the interface aftershocks, which are split into two clusters of events at different depths, thus suggesting the existence of a repeated transition from stick-slip to creeping frictional regime.