ALBERT

Description: SUMMARY We explore a recently developed procedure for kinematic inversion based on an elliptical subfault approximation. In this method, the slip is modelled by a small set of elliptical patches, each ellipse having a Gaussian distribution of slip. We invert near-field strong ground motion for the 2004 September 28 M w 6.0 Parkfield, California, earthquake. The data set consists of 10 digital three-component 18-s long displacement seismograms. The best model gives a moment of 1.21 × 10 18 N m, with slip on two distinct ellipses, one with a high-slip amplitude of 0.91 m located 20 km northwest of the hypocentre. The average rupture speed of the rupture process is ∼2.7 km s −1 . We find no slip in the top 5 km. At this depth, a lineation of small aftershocks marks the transition from creeping above to locked below, in the interseismic period. The high-slip patch coincides spatially with the hypocentre of the 1966 M w 6.0 Parkfield, California, earthquake. The larger earthquakes prior to the 2004 Parkfield earthquake and the aftershocks of the 2004 earthquake ( M w 〉 3) also lie around this high-slip patch, where our model images a sharp slip gradient. This observation suggests the presence of a permanent asperity that breaks during large earthquakes, and has important implications for the slip deficit observed on the Parkfield segment, which is necessary for reliable seismic hazard assessment.

Description: SUMMARY We study the scaling of spectral properties of a set of 68 aftershocks of the 2007 November 14 Tocopilla ( M 7.8) earthquake in northern Chile. These are all subduction events with similar reverse faulting focal mechanism that were recorded by a homogenous network of continuously recording strong motion instruments. The seismic moment and the corner frequency are obtained assuming that the aftershocks satisfy an inverse omega-square spectral decay; radiated energy is computed integrating the square velocity spectrum corrected for attenuation at high frequencies and for the finite bandwidth effect. Using a graphical approach, we test the scaling of seismic spectrum, and the scale invariance of the apparent stress drop with the earthquake size. To test whether the Tocopilla aftershocks scale with a single parameter, we introduce a non-dimensional number, , that should be constant if earthquakes are self-similar. For the Tocopilla aftershocks, C r varies by a factor of 2. More interestingly, C r for the aftershocks is close to 2, the value that is expected for events that are approximately modelled by a circular crack. Thus, in spite of obvious differences in waveforms, the aftershocks of the Tocopilla earthquake are self-similar. The main shock is different because its records contain large near-field waves. Finally, we investigate the scaling of energy release rate, G c , with the slip. We estimated G c from our previous estimates of the source parameters, assuming a simple circular crack model. We find that G c values scale with the slip, and are in good agreement with those found by Abercrombie and Rice for the Northridge aftershocks.

Description: We present a detailed study of tsunami-induced tilt at in-land sites, to test the interest and feasibility of such analysis for tsunami detection and modelling. We studied tiltmeter and broadband seismometer records of northern Chile, detecting a clear signature of the tsunamis generated by the 2007 Tocopilla ( M  = 7.6) and the 2010 Maule ( M  = 8.8) earthquakes. We find that these records are dominated by the tilt due to the elastic loading of the oceanic floor, with a small effect of the horizontal gravitational attraction. We modelled the Maule tsunami using the seismic source model proposed by Delouis et al. and a bathymetric map, correctly fitting three tide gauge records of the area (Antofagasta, Iquique and Arica). At all the closest stations (7 STS2, 2 long-base tiltmeters), we correctly modelled the first few hours of the tilt signal for the Maule tsunami. The only phase mismatch is for the site that is closer to the ocean. We find a tilt response of 0.005–0.01 μm at 7 km away from the coastline in response to a sea level amplitude change of 10 cm. For the Maule earthquake, we observe a clear tilt signal starting 20 min before the arrival time of the tsunami at the nearest point on the coastline. This capability of tilt or seismic sensors to detect distant tsunamis before they arrive has been successfully tested with a scenario megathrust in the southern Peru-northern Chile seismic gap. However, for large events near the stations, this analysis may no longer be feasible, due to the large amplitude of the long-period seismic signals expected to obscure the loading signal. Inland tilt measurements of tsunamis smooth out short, often unmodelled wavelengths of the sea level perturbation, thus providing robust, large-scale images of the tsunami. Furthermore, tilt measurements are not expected to saturate even for the largest run-ups, nor to suffer from near-coast tsunami damages. Tiltmeters and broadband seismometers are thus valuable instruments for monitoring tsunamis in complement with tide gauge arrays.

Description: We study the distribution of the aftershocks of Tocopilla M w 7.7 earthquake of 2007 November 14 in northern Chile in detail. This earthquake broke the lower part of the seismogenic zone at the southern end of the Northern Chile gap, a region that had its last megathrust earthquake in 1877. The aftershocks of Tocopilla occurred in several steps: the first day they were located along the coast inside the co-seismic rupture zone. After the second day they extended ocean-wards near the Mejillones peninsula. Finally in December they concentrated in the South near the future rupture zone of the Michilla intermediate depth earthquake of 2007 December 16. The aftershock sequence was recorded by the permanent IPOC (Integrated Plate Boundary Observatory in Chile) network and the temporary task force network installed 2 weeks after the main event. A total of 1238 events were identified and the seismic arrival times were directly read from seismograms. Initially we located these events using a single event procedure and then we relocated them using the double-difference method and a cross-correlation technique to measure time differences for clusters of aftershocks. We tested a 1-D velocity model and a 2-D one that takes into account the presence of the subducted Nazca Plate. Relocation significantly reduced the width of the aftershock distribution: in the inland area, the plate interface imaged by the aftershocks is thinner than 2 km. The two velocity models give similar results for earthquakes under the coast and a larger difference for events closer to the trench. The surface imaged by the aftershocks had a length of 160 km. It extends from 30 to 50 km depth in the northern part of the rupture zone; and between 5 and 55 km depth near the Mejillones peninsula. We observed a change in the dip angle of the subduction interface from 18° to 24° at a depth of 30 km. We propose that this change in dip is closely associated with the upper limit of the rupture zone of the main event. We also studied the focal mechanisms of the aftershocks, most of them were thrust events like the mainshock. As the aftershock activity was significantly reduced, on 2007 December 13, an M L 6.1 event occurred offshore of the Mejillones peninsula reactivating the seismicity. Three days later the Michilla intraslab earthquake of M w 6.8 ruptured an almost vertical fault with slab-push mechanism. The aftershocks locations of this event define a planar zone about 11 km in depth, situated right bellow the subduction interface.

Description: The tsunami resonance inside basins (closed or semi-enclosed) depends on the period of the incident waves, reflection and energy dissipation, characteristics of the boundary and the geometry of the basin. When waves continuously enter the basin, they cause abnormal water level fluctuations and produced damage if their periods are close to the periods of free oscillation of the basin. These resonant oscillations inside harbours, bays, or other semi-enclosed or closed basins can have a direct influence on the management of harbours, shipping and coastal uses. So, it is important to determine these free oscillations. These resonant characteristics have been observed in the Marquesas, an archipelago prone to tsunami amplification, during the last three tsunamis (Samoa 2009, Maule 2010, Tohoku 2011). These events were recorded by the two tide gauges located in the Marquesas. In this archipelago, there are two monitored bays : the first one is located in Hiva Oa Island (Tahauku Bay) and the second in Nuku Hiva Island (Taihoae Bay). For all these tsunamis, more than 3 d of water tidal resonance were recorded. In this work, we make a free oscillation analysis of the Marquesas Archipelago using real tide gauge data, simulated tsunami data and theoretical computations. During the last century, this archipelago was hit by several trans-Pacific tsunamis. It is interesting to note that, following different tsunami reports the bays responded differently depending on the earthquake source region. For example, Tahauku and Atuona, two bays in Hiva Oa Island only 1 km apart, have different responses depending on the tsunami, as it was observed in 1946 (Aleutian earthquake) and 1960 (Chile earthquake). For this reason, we study the azimuthal dependence of the excitation of the free oscillation modes, and we show that some azimuths enhance tsunami amplification.

Description: We numerically investigate the effect of thermal pressurization (TP) on fault branch behaviour during dynamic rupture propagation, a situation likely to occur during large earthquakes at subduction interfaces. We consider a 2-D mode II (in-plane) rupture in an infinite medium that propagates spontaneously along a planar main fault and encounters an intersection with a pre-existing branching fault. The fault system is subjected to uniform external stresses. We adopt the values used by Kame et al . We use the 2-D boundary integral equation method and the slip-weakening friction law with a Coulomb failure criterion, allowing the effective normal stress to vary as pore pressure changes due to TP. We reveal that TP can alter rupture propagation paths when the dip angle of the main fault is small. The rupture propagation paths depend on the branching angle when TP is not in effect on either of the faults, as described by Kame et al. However, the dynamic rupture processes are controlled more by TP than by the branching angle. When TP is in effect on the main fault only, the rupture propagates along the main fault. It propagates along the branch when TP is in effect on both faults. Finally, we considered the case where there is a free surface above the branch fault system. In this case, the rupture can propagate along both faults because of interaction between the free surface and the branch fault, in addition to TP on the main fault.

Description: 〈span class="paragraphSection"〉〈div class="boxTitle"〉Abstract〈/div〉We study the seismic source of the 2015 (〈span style="font-style:italic;"〉M〈/span〉〈sub〉w〈/sub〉 6.7) Jujuy, Argentina intermediate depth earthquake. We first constrain the fault plane by using a teleseismic inversion and by determining the aftershock distribution. Then, we perform kinematic and dynamic inversions to retrieve the parameters that control the rupture process, using data at regional distances, and modelling the source as an elliptical patch. Best models suggest a subshear rupture propagation with a duration of ∼5 s. Results from the dynamic modelling suggest a stress drop of 11.87 MPa and a fracture energy rate of 2.95 MJ m〈sup〉−2〈/sup〉, which are slightly less but of the same order as those of other events of similar size. Finally, we perform a Monte-Carlo inversion to explore the behaviour of the frictional parameters in the solution space, and then we compare our results with other intraslab events. We find that the 〈span style="font-style:italic;"〉κ〈/span〉 parameter (ratio between strain energy and fracture energy) and the relation between seismic moment and stress drop are similar for all the considered events.〈/span〉