The space-time distribution of slip of the 17 August 1999 İzmit earthquake is investigated by inverting synthetic aperture radar (SAR) interferometry and Global Positioning System (GPS) data, together with teleseismic broadband and nearfield strong-motion records. Surface offsets are used as an added constraint. Special emphasis is given to analysis of the resolution of the different data sets. We use a four-segment finite fault model and a nonlinear inversion scheme, allowing slip to vary in amplitude, direction, and duration, as well as variable rupture velocity. From the inversion of synthetic data, we find that the best spatial resolution can be expected in the upper half of the fault model (above 12 km), where coverage of the interferometric SAR data is good (western half of the rupture), and near the GPS and strongmotion stations. Teleseismic data are found to have a lower resolution that is more evenly distributed over the fault model. The joint inversion of all the data sets has an increased resolving power compared with the separate inversions and gives a more robust description of the space and time distribution of slip. Our study shows the importance of resolution tests in evaluating the reliability of earthquake kinematic models, and it confirms that an excellent fit of a single kind of data does not necessarily imply a good retrieval of the kinematic properties of an earthquake. The İzmit rupture, which is almost pure right-lateral strike-slip faulting, is dominated by the bilateral breaking of a central asperity located between 29.7Њ E (about 10 km west of the city of Gölcük) and 30.4Њ E (eastern margin of Sapanka Lake), with slip reaching 6-8 m in the depth range 6-12 km. The western termination of the rupture is found near the city of Yalova, but large slip ends around 29.7Њ E (about 10 km east of Hersek Delta). A second area of large slip is required by all the data sets further east toward the city of Düzce, between 30.7Њ E and 31.1Њ E (Karadere and Düzce faults). This eastern slip zone, which is separated from the main central asperity by an area of greatly reduced slip, is less well constrained by the data. However, a strong-motion station near the city of Düzce helps to locate a high-slip patch near 31.1Њ E in the depth range 6-12 km. The total seismic moment resulting from the joint inversion is 2.4 ‫ן‬ 10 27 dyne cm. Most of the energy release occurred in a short time, less than 15 sec, corresponding to the bilateral breaking of the central asperity. Rupture propagation is relatively uniform and fast toward the west, with a rupture velocity close to 3.5 km/sec. Propagation of large slip toward the east is initially slower, but it accelerates during a short time interval about 10 sec after rupture nucleation. Eastward progression then slows down to less than 2 km/sec after 15 sec, and rupture almost vanishes in amplitude ca. 20 sec after initiation. Rupture propagation then proceeds on the easternmost Karadere and Düzce fault segments, east of 30.7Њ E, from 22 to ca. 50 sec. Supershear ruptu...

International audienceThe shallow depth underthrust earthquake of February 27, 2010 (Mw 8.8) ruptured the subduction plate interface in central Chile between 34°S and 38°S. We retrieve the spatial and temporal distribution of slip during this mega-earthquake through a joint inversion of teleseismic records, InSAR and High Rate GPS (HRGPS) data. Additionally, our model is shown to agree with broadband surface waves. Rupture initiated at about 32 km depth and propagated bilaterally resulting in two main slip zones located SSW and NNE of the hypocenter. Nucleation did not take place within or at the edge of one of these main asperities, but in between. During the first 30s, slip propagated predominantly southwards. Later on, the rupture evolved more slowly and more symmetrically. Eventually, the northern asperity became predominant with maximum slip reaching about 20 m. Most of the seismic moment was released within 110s, a relatively short time, explained by the bilateral propagation. The overall average rupture velocity is 2.6 km/s but propagation occurred initially faster towards the south (3.2 km/s). Large slip did not reach the trench, a result consistent with the moderate size of the tsunami. Down-dip, rupture stopped at about 50 km depth, in agreement with the lower limit of the locked zone inferred by Ruegg et al. (2009) from pre-seismic GPS data

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SUMMARYNew neotectonic observations, along with a detailed aerial photograph analysis, allow a new interpretation of the recent tectonic behaviour of the outer forearc in northern Chile between 22.5°S and 24.5°S (Antofagasta region). Both the Coastal Cordillera and the Mejillones Peninsula are under E-W extension. Normal faults dipping east with an almost N-S orientation are predominant. Large-scale Neogene to Recent deformation is characterized by vertical uplift and subsidence related to normal faulting. Recent kinematics along the Atacama Fault System, including the Atacama Fault itself, are controlled by the regional extensional stress regime, and the predominant component of recent displacement along the Atacama Fault is vertical (normal). However, strike-slip components are also observed, but with moderate offsets, no larger than one to two tens of metres. The sense of shear, and the ratio between vertical and horizontal offsets, vary coherently with the fault azimuth, compatible with E-W extension. Oblique-slip faulting, with normal and left-lateral offsets of similar amplitude, is observed on the most northeasterly oriented segment of the Atacama Fault. This small left-lateral component of displacement and the strike-slip motion predicted by models of strain partitioning of the convergence along the Atacama Fault are in opposite senses. This occurs because the stress regime in the coastal region is not compressional but extensional. Very fresh recent ruptures are numerous, especially between 23°S and 24°S, and provide clear indications of the occurrence of moderate to large continental earthquakes during the Late Quaternary. As pointed out by several authors, indirect lines of evidence suggest active subduction erosion and underplating along the margin of northern Chile. We propose that part of the long-term extension is due to the broadscale flexure of the outer forearc. This flexure may be controlled by offshore subsidence caused by subduction erosion near the trench, and by onshore uplift related to the underplating of eroded low-density material beneath the Coastal Cordillera. Neotectonic observations show that the state of stress in the outer forearc remains extensional (in a deviatoric sense) in the long term. However, it is proposed that the state of stress has a strong time-dependent component, which is intimately linked to the subduction seismic cycle. The M w =8.0 Antofagasta earthquake of 1995 July 30 showed that large subduction earthquakes produce E-W extension in the coastal region. The overall E-W deviatoric extension in the outer forearc should be reduced by interseismic contraction in the period separating two subduction earthquakes. This reduction means that continental faults will remain locked and aseismic in the interseismic period of the subduction cycle, in agreement with microseismic observations, which indicate an absence of shallow crustal seismicity. We assume that the amount of extension produced by subduction earthquakes in the coastal area is larger than the cumulative inter...

Accurate and fast magnitude determination for large, shallow earthquakes is of key importance for post-seismic response and tsumami alert purposes. When no local real-time data are available, which is today the case for most subduction earthquakes, the first information comes from teleseismic body waves. Standard body-wave methods give accurate magnitudes for earthquakes up to Mw= 7–7.5. For larger earthquakes, the analysis is more complex, because of the non-validity of the point-source approximation and of the interaction between direct and surface-reflected phases. The latter effect acts as a strong high-pass filter, which complicates the magnitude determination. We here propose an automated deconvolutive approach, which does not impose any simplifying assumptions about the rupture process, thus being well adapted to large earthquakes. We first determine the source duration based on the length of the high frequency (1–3 Hz) signal content. The deconvolution of synthetic double-couple point source signals—depending on the four earthquake parameters strike, dip, rake and depth—from the windowed real data body-wave signals (including P, PcP, PP, SH and ScS waves) gives the apparent source time function (STF). We search the optimal combination of these four parameters that respects the physical features of any STF: causality, positivity and stability of the seismic moment at all stations. Once this combination is retrieved, the integration of the STFs gives directly the moment magnitude. We apply this new approach, referred as the SCARDEC method, to most of the major subduction earthquakes in the period 1990–2010. Magnitude differences between the Global Centroid Moment Tensor (CMT) and the SCARDEC method may reach 0.2, but values are found consistent if we take into account that the Global CMT solutions for large, shallow earthquakes suffer from a known trade-off between dip and seismic moment. We show by modelling long-period surface waves of these events that the source parameters retrieved using the SCARDEC method explain the observed surface waves as well as the Global CMT parameters, thus confirming the existing trade-off. For some well-instrumented earthquakes, our results are also supported by independent studies based on local geodetic or strong motion data. This study is mainly focused on moment determination. However, the SCARDEC method also informs us about the focal mechanism and source depth, and can be a starting point to study systematically the complexity of the STF

[1] Using a joint inversion of seismological waveforms and ground displacement observations, we estimate several parameters of the fault geometry and rupture process of the Mw = 6.9 May 21, 2003 Boumerdes-Zemmouri earthquake. The relocated epicenter is considered as a known parameter. Total rupture length, rupture duration, and maximum slip are 55 km (from 3.4°E to 4.0°E), 12 s, and 3 m. The modeled south dipping reverse fault, oriented ENE-WSW outcrops a few km offshore which is consistent with the absence of observed surface rupture inland. Two shallow and relatively localized slip zones are found, on both sides of the hypocenter. To the SW, between Boumerdes and Zemmouri, slip is concentrated between 11 and 2 km depth. To the NE, between Zemmouri and Dellys, slip is concentrated between 6 km depth and the sea floor. Various resolution tests indicate that our model is well constrained by the available data, and help understanding which data constrains each parameter of the model.

We have identified an active normal fault in the epicentral area of the Basel (Switzerland) earthquake of 18 October 1356, the largest historical seismic event in central Europe. The event of 1356 and two prehistoric events have been characterized on the fault with geomorphological analysis, geophysical prospecting, and trenching. Carbon-14 dating indicates that the youngest event occurred in the interval 610 to 1475 A.D. and may correspond to the 1356 Basel earthquake. The occurrence of the three earthquakes induced a total of 1.8 meters of vertical displacement in the past 8500 years for a mean uplift rate of 0.21 millimeters per year. These successive ruptures on the normal fault indicate the potential for strong ground movements in the Basel region and should be taken into account to refine the seismic hazard estimates along the Rhine graben.

The Rhône River Valley in France, a densely populated area with many industrial facilities including several nuclear power plants, was shaken on November 11th 2019, by the Mw 4.9 Le Teil earthquake. Here, we report field, seismological and interferometric syntheticaperture radar observations indicating that the earthquake occurred at a very shallow focal depth on a southeast-dipping reverse-fault. We show evidence of surface rupture and up to 15 cm uplift of the hanging wall along a northeast-southwest trending discontinuity with a length of about 5 km. Together, these lines of evidence suggest that the Oligocene La Rouvière fault was reactivated. Based on the absence of geomorphic evidence of cumulative compressional deformation along the fault, we suggest that it had not ruptured for several thousand or even tens of thousands of years. Our observations raise the question of whether displacement from surface rupture represents a hazard in regions with strong tectonic inheritance and very low strain rates.