Description: Chile is one of the most seismically active countries in the world with a M〉8 earthquake occurring approximately every ten years. The recent 27th February 2010 (Mw=8.8) earthquake released the energy accumulated in the Concepción-Constitución seismic gap, contained since the last 1835 earthquake. The hypocenter was located at the subduction interface of the Nazca plate under the South American plate, near the coast of Talcahuano, at a depth of 30 km. The rupture area extended approximately for 450 km long, from the Arauco Peninsula northwards to Pichilemu. Hundreds of aftershocks have taken place in the rupture area, the largest of which was the Pichilemu M=6.9 earthquake on 11th March. A volcano-surrounding network was installed in Llaima and Villarrica volcanoes in Southern Chile in November, 2009. Each network comprises one Broad Band station, one “Ocean” Bottom Station and three Short-Period stations arranged in a circular disposition. The seismological data recording at the two volcanoes is still ongoing until April 2011. Data examples and first results from the analysis of the data recorded from November, 2009 until April, 2010 are presented. The mainshock, foreshocks and aftershocks were retrieved, and are being separated from the local seismicity in the records. During the same time frame, volcanic events are identified to characterize the volcano seismicity at Llaima and Villarrica volcanoes, to correlate this activity with degassing emissions (SO2) measured with mini-DOAS. Any influence of the Maule earthquake on the volcanoes behaviour is also investigated based on the seismic records.

Description: Non-linear elasticity has recently been considered as a source of scattering, therefore contributing to the coda of seismic waves, in particular for the case of explosive sources. This idea is analysed further here, theoretically solving the expression for the envelope of coda waves generated by a point moment tensor in order to compare with earthquake data. For weak non-linearities, one can consider each point of the non-linear medium as a source of scattering within a homogeneous and linear medium, for which Green's functions can be used to compute the total displacement of scattered waves. These sources of scattering have specific radiation patterns depending on the incident and scattered P or S waves, respectively. In this approach, the coda envelope depends on three scalar parameters related to the specific non-linearity of the medium; however these parameters only change the scale of the coda envelope. The shape of the coda envelope is sensitive to both the source time function and the intrinsic attenuation. We compare simulations using this model with data from earthquakes in Taiwan, with a good fit.

Description: The M8.8 Maule earthquake of 27 Feb. 2010 in Chile was the largest earthquake that ruptured a mature seismic gap in a subduction zone, monitored with a dense space-geodetic network. This provides an image of the pre-seismically locked state of the plate interface of unprecedented high resolution, allowing for an assessment of the spatial correlation of interseismic locking and coseismic slip. Here we use GPS observations spanning the decade preceding the 2010 Maule earthquake to derive the pre-seismically surface deformation. Additionally, we use static coseismic surface displacements of 42 campaign GPS sites, InSAR data and land level changes to constrain the co-seismic slip distribution. Inter- and coseismic surface velocities are modeled using a spherical and layered finite element model (FEM) of the Andean subduction zone, including topography, bathymetry and realistic plate configurations as compiled from recent geophysical transects. According to our analysis, the 2010 Maule earthquake ruptured a part of the plate margin that was accumulating stresses across a heterogeneously locked interface in the final few years of an earthquake cycle. Despite differences in resolution and complexity, our and published slip distributions show a first-order pattern of two high-slip patches (asperities) north and south of the epicenter and separated by a low-slip zone 50-100 km wide. The rupture stopped in areas that were highly locked before the earthquake but where pre-stress had been significantly reduced by overlapping twentieth-century earthquakes. The largest recorded coastal uplift of up to 2 m occurred in the Arauco Peninsula, with peak horizontal displacements of 5 m at the Santa María Island. In the preliminary slip model presented we relate this uplift to elastic deformation caused by an asperity with slip of up to ~8 m in the southernmost part of the rupture. This slip component has not been seen by teleseismic models published so far presumably because of the network configuration, low slip velocities, simplified fault geometry, or bilateral propagation of the rupture. Inspection of the similarity between coseismic slip distributions and the pre-seismic locking reveals a high degree of coherence: The two asperities which ruptured in 2010 appeared to have been nearly fully locked before the earthquake. Between these asperities, the rupture bridged the zone that was not fully locked with consistent low coseismic slip. Based on the delicate balance between the 2010 coseismic slip and theoretically accumulated slip deficit since 1835 (which can be calculated under the assumption that the pre-seismic locking pattern is representative of the entire interseismic period) it appears that the observed locking pattern has been temporally largely invariant over the seismic cycle. Our observation suggests that coseismic slip heterogeneity at the scale of single asperities, and therefore the seismic potential of future great earthquakes, can be anticipated by foregoing geodetic observations.

Description: The Maule earthquake of 27th February 2010 (Mw=8.8) affected about 500 km of the Nazca-South America plate boundary in south-central Chile producing spectacular crustal deformation. This earthquake occurred on a mature seismic gap, which presented a high degree of plate locking in the decade preceding the event. Shaking, tsunami inundation and coastal uplift patterns show similarities with its predecessor in 1825, suggesting that both events ruptured an analogous segment of the plate boundary. Here, we present estimates of static coseismic surface displacements measured by a dense GPS network and used them in conjunction with published geodetic data to obtain an updated, higher-resolution slip model of the 2010 Maule earthquake. The use of a finite element model that introduced the main geometrical complexities of the Chile subduction zone allowed us to compare the spatial relation of slip patterns before and during the 2010 Maule earthquake with tectonic features of the forearc. The combination of the data sets provided a good resolution, indicating that most of the slip was well resolved. Coseismic slip was concentrated north of the epicenter with up to 16 m of slip, whereas to the south it reached over 10 m within two minor patches. The theoretical accumulated slip deficit including the 1960 earthquake slip that overlapped 150 km of the 2010 rupture; the 1928 and 1985 events; as well as heterogeneous plate locking suggests that the 2010 event closed the seismic gap. Slip deficit distribution shows an apparent local overshoot that highlight cycle-to-cycle variability, which has to be taken into account when anticipating future events from interseismic observations. Rupture propagation was obviously not affected by bathymetric features of the incoming plate. Instead, splay faults in the upper plate seem to have limited rupture propagation in the updip and along- strike directions. Additionally, we found that along-strike gradients in slip are spatially correlated with geometrical inflections of the megathrust. Our study suggests that persistent tectonic features may control strain accumulation and release along subduction megathrusts inducing a seismotectonic segmentation persistent over several seismic cycles.

Description: On 27 February 2010 theMw 8.8 Maule earthquake in Central Chile ruptured a well known seismic gap, which last broke in 1835. Shortly after the mainshock, Chilean agencies (UC Santiago, UC Concepción) and the international seismological community (USA (IRIS), France (IPGP), UK (University of Liverpool), Germany (GFZ)) installed a total of 142 portable seismic stations along the whole rupture zone in order to capture the aftershock activity. Most stations were in the field until September 2010, with a subset remaining until January 2011; the UK stations will remain in the field beyond this time. The data from the initial deployment are open and are being distributed through the IRIS and GEOFON data centres. We will present preliminary aftershock distributions based on automatic detection algorithms. In total, for the period between March and September 2010 we detected _60,000 locatable earthquakes, of which we form a subset of _7,000 events with high quality locations. The depth of events in the high quality subset is generally well constrained such that the plate interface is clearly defined, and can be separated from overriding plate seismicity. First order features that can be identified are: 1.) A pronounced cluster of seismicity is apparent at 25-35 km depth and 50-120 km perpendicular distance from the trench (with some NS variation). 2.) A secondary band of seismicity can be identified at 40-50 km depth and _150-160 km perpendicular trench distance and between 34_ and 37_S. Although the secondary band lies along the continuation of the primary one, it is clearly separated from it by a gap with sparse seismicity. 3.) Intense crustal seismicity is found in the region of Pichilemu. This region hosted the strongest aftershock (Mw=6.9), a normal faulting event with NW strike. The aftershocks extend from the plate interface to the surface and are aligned on a NNW-SSE oriented band in map view. 4.) An isolated shallow cluster of crustal seismicity occurs beneath the volcanic arc (36.42_S, 71.1_W) near Laguna del Dial. Ongoing research is concerned with calculating first motion focal mechanisms for the larger events and improving locations by relative location methods.

Description: The information contained in the high frequency seismic wave field extends beyond travel times. Omnipresent coda waves reflect the structural heterogeneity of the medium that leads to complex propagation paths of scattered waves. Analysis of these coda waves can therefore help to characterize the structure of the medium on length scales that is not accessible by traditional tomographic techniques. Wave propagation at regional distances has been used successfully in this context. Seismologie - Naturgefahren und Geophysik – 281 Here we focus on global distances. We present an algorithm to model the high frequency seismic energy propagation in a spherically symmetric Earth model such as ak135 with additional small scale heterogeneity. The algorithm solves the radiative transfer equation by means of a Monte-Carlo simulation of individual energy particles. The simulation results are contrasted with envelope stacks of global teleseismic records.

Description: We use Global Positioning System (GPS) velocities and kinematic Finite Element models (FE-models) to infer the state of locking between the converging Nazca and South America plates in South-Central Chile (36[degree sign]S - 46[degree sign]S) and to evaluate its spatial and temporal variability. GPS velocities provide information on earthquake-cycle deformation over the last decade in areas affected by the megathrust events of 1960 (Mw= 9.5) and 2010 (Mw= 8.8). Our data confirm that a change in surface velocity patterns of these two seismotectonic segments can be related to their different stages in the seismic cycle: Accordingly, the northern (2010) segment was in a final stage of interseismic loading whereas the southern (1960) segment is still in a postseismic stage and undergoes a prolonged viscoelastic mantle relaxation. After correcting the signals for mantle relaxation, the residual GPS velocity pattern suggests that the plate interface accumulates slip deficit in a spatially and presumably temporally variable way towards the next great event. Though some similarity exist between locking and 1960 coseismic slip, extrapolating the current, decadal scale slip deficit accumulation towards the ~ 300-yr recurrence times of giant events here does neither yield the slip distribution nor the moment magnitude of the 1960 earthquake. This suggests that either the locking pattern is evolving in time (to reconcile a slip deficit distribution similar to the 1960 earthquake) or that some asperities are not persistent over multiple events. The accumulated moment deficit since 1960 suggests that highly locked patches in the 1960 segment are already capable of producing a M ~ 8 event if triggered to fail by stress transfer from the 2010 event.

Description: Different factors might affect the propagation of seismic waves producing scattering, including heterogeneities and non-linear elasticity. A key difference between these two factors is the dependence of the strength of the scattered waves on the strength of the incident wave, being linear for the former and non-linear for the latter. A detailed study of the TIPTEQ data, where about a hundred explosions were recorded on 180 three-component stations in the distance range of approximately 0–100 km, shows that this dependence is non-linear. Data were analysed in the following way: (i) the envelope of bandpass filtered data between 10 and 40 Hz was obtained for a large number of stations from different distance ranges and charge sizes of shots, (ii) for these distances we modelled the envelope considering the non-linear elasticity. The shapes of the theoretical and observed envelopes were in general very similar. A scale factor for each case was obtained considering the best fit of its complete envelope and (iii) since this scale factor depends mainly on the size of the explosion, we computed the ratio (R) of the scale factor (sf) for different sizes of explosions at the same distance. Finally, varying the distance between 0 and 50 km and (iv) we computed the power (p) of the dependence of the ratio (R) on the ratio of the charge sizes [ ]. For the complete data set we obtain a value of p= 2.5 ± 0.9 , which is clearly greater than 1. This shows that non-linear elasticity is an important factor in the contribution to seismic wave scattering in the frequency range of 10–40 Hz.