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Candela T.,Joseph Fourier University | Renard F.,Joseph Fourier University | Renard F.,University of Oslo | Schmittbuhl J.,Institute Of Physique Du Globe Of Strasbourg | And 2 more authors.
Geophysical Journal International | Year: 2011

We present analysis of the spatial correlations of seismological slip maps and fault topography roughness, illuminating their identical self-affine exponent. Though the complexity of the coseismic spatial slip distribution can be intuitively associated with geometrical or stress heterogeneities along the fault surface, this has never been demonstrated. Based on new measurements of fault surface topography and on statistical analyses of kinematic inversions of slip maps, we propose a model, which quantitatively characterizes the link between slip distribution and fault surface roughness. Our approach can be divided into two complementary steps: (i) Using a numerical computation, we estimate the influence of fault roughness on the frictional strength (pre-stress). We model a fault as a rough interface where elastic asperities are squeezed. The Hurst exponent, characterizing the self-affinity of the frictional strength field, approaches, whereis the roughness exponent of the fault surface in the direction of slip. (ii) Using a quasi-static model of fault propagation, which includes the effect of long-range elastic interactions and spatial correlations in the frictional strength, the spatial slip correlation is observed to scale as, whererepresents the Hurst exponent of the slip distribution. Under the assumption that the origin of the spatial fluctuations in frictional strength along faults is the elastic squeeze of fault asperities, we show that self-affine geometrical properties of fault surface roughness control slip correlations and that. Given thatfor a wide range of faults (various accumulated displacement, host rock and slip movement), we predict that. Even if our quasi-static fault model is more relevant for creeping faults, the spatial slip correlations observed are consistent with those of seismological slip maps. A consequence is that the self-affinity property of slip roughness may be explained by fault geometry without considering dynamical effects produced during an earthquake. © 2011 The Authors Geophysical Journal International © 2011 RAS.


Lin J.,Woods Hole Oceanographic Institution | Stein R.S.,U.S. Geological Survey | Meghraoui M.,Institute Of Physique Du Globe Of Strasbourg | Toda S.,Kyoto University | And 2 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2011

The essential features of stress interaction among earthquakes on en echelon thrusts and tear faults were investigated, first through idealized examples and then by study of thrust faulting in Algeria. We calculated coseismic stress changes caused by the 2003 Mw = 6.9 Zemmouri earthquake, finding that a large majority of the Zemmouri afterslip sites were brought several bars closer to Coulomb failure by the coseismic stresses, while the majority of aftershock nodal planes were brought closer to failure by an average of ∼2 bars. Further, we calculated that the shallow portions of the adjacent Thenia tear fault, which sustained ∼0.25 m slip, were brought >2 bars closer to failure. We calculated that the Coulomb stress increased by 1.5 bars on the deeper portions of the adjacent Boumerdes thrust, which lies just 10-20 km from the city of Algiers; both the Boumerdes and Thenia faults were illuminated by aftershocks. Over the next 6 years, the entire south dipping thrust system extending 80 km to the southwest experienced an increased rate of seismicity. The stress also increased by 0.4 bar on the east Sahel thrust fault west of the Zemmouri rupture. Algiers suffered large damaging earthquakes in A.D. 1365 and 1716 and is today home to 3 million people. If these shocks occurred on the east Sahel fault and if it has a ∼2 mm/yr tectonic loading rate, then enough loading has accumulated to produce a Mw = 6.6-6.9 shock today. Thus, these potentially lethal faults need better understanding of their slip rate and earthquake history. Copyright 2011 by the American Geophysical Union.


Grandin R.,Ecole Normale Superieure de Paris | Grandin R.,CNRS Paris Institute of Global Physics | Jacques E.,CNRS Paris Institute of Global Physics | Nercessian A.,CNRS Paris Institute of Global Physics | And 7 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2011

Seismicity released during lateral dike intrusions in the Manda Hararo-Dabbahu Rift (Afar, Ethiopia) provides indirect insight into the distribution and evolution of tensile stress along this magma-assisted divergent plate boundary. In this paper, 5 dike intrusions among the 14 that form the 2005-present rifting episode are analyzed with local and regional seismic data. During dike intrusions, seismicity migrates over distances of 10-15 km at velocities of 0.5-3.0 km/h away from a single reservoir in the center of the rift segment, confirming the analogy with a slow spreading mid-ocean ridge segment. Comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression. Dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward, suggesting an asymmetry of tension in the brittle-elastic lithosphere. Seismicity during dike injections is concentrated near the propagating crack front. In contrast, faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions. The seismic energy released during dike intrusions in the Manda Hararo Rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion. The low level of seismic energy accompanying dike intrusions, despite their significant volumes, is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary. Copyright 2011 by the American Geophysical Union.


Kanamori H.,California Institute of Technology | Rivera L.,Institute Of Physique Du Globe Of Strasbourg | Lee W.H.K.,862 Richardson Court
Geophysical Journal International | Year: 2010

History of instrumental seismology is short. Seismograms are available only for a little more than 100 years; high-quality seismograms are available only for the last 50 years and the seismological database is very limited in time. To extend the database, seismograms of old events are of vital importance. Many unusual earthquakes are known to have occurred, but their seismological characteristics are poorly known. The 1907 Sumatra earthquake is one of them (1907 January 4, M= 7.6). Gutenberg and Richter located this event in the outer-rise area of the Sunda arc. This earthquake is known to be anomalous because of its extensive tsunami, which is disproportionate of its magnitude. The tsunami affected the coastal areas over 950 km along the Sumatran coast. We investigated this earthquake using the historical seismograms we could collect from several seismological observatories. We examined the P-wave arrival times listed in the Strassburg Bulletin (1912) and other station bulletins. The scatter of the Observed-Computed traveltime residuals ranges from-30 to 30 s, too large to locate the event accurately. The uncertainty of the epicentre estimated from an S-P grid-search relocation study is at least 1° (110 km). We interpreted the Omori seismograms from Osaka, Mizusawa and Tokyo, and the Wiechert seismograms from Göttingen and Uppsala by comparing them with the seismograms simulated from modern broad-band seismograms of the 2002, 2008 and two 2010 Sumatra earthquakes which occurred near the 1907 earthquake. From the amplitude of Rayleigh waves recorded on the Omori seismograms we conclude that the magnitude of the 1907 earthquake at about 30 to 40 s is about 7.8 (i.e. 7.5 to 8.0). The SH waveforms recorded on the Göttingen and Uppsala seismograms suggest that the 1907 earthquake is a thrust earthquake at a shallow depth around 30 km. The most likely scenario is that the 1907 earthquake initiated on the subduction interface, and slowly ruptured up-dip into the shallow sediments and caused the extensive tsunami. Although their quantity and quality are limited, historical seismograms provide key quantitative information about old events that cannot be obtained otherwise. This underscores the importance of preserving historical seismograms. © 2010 The Authors Journal compilation © 2010 RAS.


Tsai V.C.,California Institute of Technology | Tsai V.C.,U.S. Geological Survey | Hayes G.P.,U.S. Geological Survey | Hayes G.P.,Synergetics Inc. | Duputel Z.,Institute Of Physique Du Globe Of Strasbourg
Geophysical Research Letters | Year: 2011

Since the work of Kanamori and Given (1981), it has been recognized that shallow, pure dip-slip earthquakes excite long-period surface waves such that it is difficult to independently constrain the moment (M0) and the dip (δ) of the source mechanism, with only the product M0 sin(2δ) being well constrained. Because of this, it is often assumed that the primary discrepancies between the moments of shallow, thrust earthquakes are due to this moment-dip tradeoff. In this work, we quantify how severe this moment-dip tradeoff is depending on the depth of the earthquake, the station distribution, the closeness of the mechanism to pure dip-slip, and the quality of the data. We find that both long-period Rayleigh and Love wave modes have moment-dip resolving power even for shallow events, especially when stations are close to certain azimuths with respect to mechanism strike and when source depth is well determined. We apply these results to USGS W phase inversions of the recent M9.0 Tohoku, Japan earthquake and estimate the likely uncertainties in dip and moment associated with the moment-dip tradeoff. After discussing some of the important sources of moment and dip error, we suggest two methods for potentially improving this uncertainty. Copyright 2011 by the American Geophysical Union.


Badt N.,Ben - Gurion University of the Negev | Hatzor Y.H.,Ben - Gurion University of the Negev | Toussaint R.,Institute Of Physique Du Globe Of Strasbourg | Sagy A.,Geological Survey of Israel
Earth and Planetary Science Letters | Year: 2016

We study the evolution of slip surface topography using direct shear tests of perfectly mating surfaces. The tests are performed under imposed constant normal stress and constant slip rate conditions, to a sliding distance comparable to the roughness scale of the studied surfaces. Prismatic limestone blocks are fractured in tension using four-point bending and the generated surface topographies are measured using a laser profilometer. The initially rough fracture interfaces are tested in direct shear while ensuring a perfectly mating configuration at the beginning of each test. The predetermined sliding distance in all tests is 10 mm and the sliding velocity is 0.05 mm/s. A constant normal stress is maintained throughout the tests using closed loop servo control. The range of normal stresses applied is between 2 MPa and 15 MPa. After shearing, the surface topographies are re-scanned and the geometrical evolution is analyzed. We find that surface roughness increases with increasing normal stress: under normal stresses below 5 MPa the surfaces become smoother compared to the original geometry, whereas under normal stresses between 7.5 MPa and 15 MPa the surfaces clearly become rougher following shear. Statistical spectral analyses of the roughness profiles indicate that roughness increases with length-scale. Power spectral density values parallel to the slip orientation are fitted by power-law with typical power value of 2.6, corresponding to a Hurst exponent of 0.8, assuming self-affine roughness. This power value is consistent for the post-sheared surfaces and is obtained even when the original surface roughness does not follow initially a power-law form. The value of the scaling-law prefactor however increases with increasing normal stress. We find that the deformation associated with shearing initially rough interlocked surfaces extends beyond the immediate tested surface, further into the intact rock material. The intensity of the damage and its spatial distribution clearly increase with increasing normal stress. Wear loss is measured by subtracting the post-shear surface from the pre-shear surface matrices using known reference points. Our measurements indicate that wear loss and roughness evolution are both positively correlated with the mechanical shear work applied during the experiments. We argue, therefore, that normal stress plays a significant role in the evolution of interlocked surfaces, such as geological faults, and strongly affects the energy partitioning during slip. © 2016 Elsevier B.V.


Ameglio L.,Airborne | Jacobs G.,Airborne | Von Ludwig J.,Airborne | Munschy M.,Institute Of Physique Du Globe Of Strasbourg
Leading Edge (Tulsa, OK) | Year: 2011

Over the past 25 years, airborne geophysics has become an effective technology for mapping various natural resources targets and signatures on the Earth's surface and subsurface. © 2011 Society of Exploration Geophysicists.


Dujardin J.R.,Institute Of Physique Du Globe Of Strasbourg | Bano M.,Institute Of Physique Du Globe Of Strasbourg
Near Surface Geoscience 2012 | Year: 2012

Most Ground Penetrating Radar (GPR) measurements are performed on nearly flat areas. If strongly dipping reflections and/or diffractions are present in the GPR data, a classical migration processing step is needed in order to determine the geometries of shallow structures. Nevertheless, standard migration routine is not suitable for GPR data collected on areas showing variable and high topographic relief. To account for the topographic variations the GPR data are, in general, corrected by applying static shifts instead of using an appropriate topographic migration which would place the reflectors at their correct locations with the right dip angle. In this paper we present an overview of Kirchhoff migration and show the importance of topographic migration in the case where the depth of the target structures is of the same order as the relief variations. Examples of synthetic and real GPR data are shown to illustrate the efficiency of the topographic migration.


Grandin R.,Ecole Normale Superieure de Paris | Socquet A.,CNRS Institute of Earth Sciences | Doubre C.,Institute Of Physique Du Globe Of Strasbourg | Jacques E.,CNRS Paris Institute of Global Physics | C.P. King G.,CNRS Paris Institute of Global Physics
Earth and Planetary Science Letters | Year: 2012

Magmatic accretion at slow-spreading mid-ocean ridges exhibits specific features. Although magma supply is focused at the centre of second-order segments, melts are episodically distributed along the rift toward segment ends by lateral dyke intrusions. It has been previously suggested that an along-axis downward topographic slope away from the magma source is sufficient to explain lateral dyke propagation. However, this cannot account for the poor correlation between dyke opening and surface elevation in the 2005-2010 series of 14 dyke intrusions of Afar (Ethiopia). Using mechanical arguments, constrained by both geodetic and seismological observations, we propose that the large dykes that initiate near the mid-segment magma source are attracted toward segment ends as a result of a thickening of the elastic-brittle lithosphere in the along-rift direction. This attraction arises from the difference of elastic resistance between the segment centre where the lithosphere is thermally weakened by long-term focusing of melts, and comparatively "colder", hence stronger segment ends. The axial topographic gradient in magmatic rifts may be more likely explained as an incidental consequence of these variations of along-axis elastic-brittle thickness, rather than the primary cause of lateral dyke injections. © 2011 Elsevier B.V.


Larnier H.,Institute Of Physique Du Globe Of Strasbourg | Sailhac P.,Institute Of Physique Du Globe Of Strasbourg | Chambodut A.,Institute Of Physique Du Globe Of Strasbourg
Earth, Planets and Space | Year: 2016

Magnetotelluric (MT) data consist of the sum of several types of natural sources including transient and quasiperiodic signals and noise sources (instrumental, anthropogenic) whose nature has to be taken into account in MT data processing. Most processing techniques are based on a Fourier transform of MT time series, and robust statistics at a fixed frequency are used to compute the MT response functions, but only a few take into account the nature of the sources. Moreover, to reduce the influence of noise in the inversion of the response functions, one often sets up another MT station called a remote station. However, even careful setup of this remote station cannot prevent its failure in some cases. Here, we propose the use of the continuous wavelet transform on magnetotelluric time series to reduce the influence of noise even for single site processing. We use two different types of wavelets, Cauchy and Morlet, according to the shape of observed geomagnetic events. We show that by using wavelet coefficients at clearly identified geomagnetic events, we are able to recover the unbiased response function obtained through robust remote processing algorithms. This makes it possible to process even single station sites and increase the confidence in data interpretation. © 2016 Larnier et al.

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