The 2013 M_w 7.7 Balochistan Earthquake: Seismic Potential of an Accretionary Wedge

Great earthquakes rarely occur within active accretionary prisms, despite the intense long‐term deformation associated with the formation of these geologic structures. This paucity of earthquakes is often attributed to partitioning of deformation across multiple structures as well as aseismic deformation within and at the base of the prism (Davis et al., 1983). We use teleseismic data and satellite optical and radar imaging of the 2013 M_w 7.7 earthquake that occurred on the southeastern edge of the Makran plate boundary zone to study this unexpected earthquake. We first compute a multiple point‐source solution from W‐phase waveforms to estimate fault geometry and rupture duration and timing. We then derive the distribution of subsurface fault slip from geodetic coseismic offsets. We sample for the slip posterior probability density function using a Bayesian approach, including a full description of the data covariance and accounting for errors in the elastic structure of the crust. The rupture nucleated on a subvertical segment, branching out of the Chaman fault system, and grew into a major earthquake along a 50° north‐dipping thrust fault with significant along‐strike curvature. Fault slip propagated at an average speed of 3.0  km/s for about 180 km and is concentrated in the top 10 km with no displacement on the underlying décollement. This earthquake does not exhibit significant slip deficit near the surface, nor is there significant segmentation of the rupture. We propose that complex interaction between the subduction accommodating the Arabia–Eurasia convergence to the south and the Ornach Nal fault plate boundary between India and Eurasia resulted in the significant strain gradient observed prior to this earthquake. Convergence in this region is accommodated both along the subduction megathrust and as internal deformation of the accretionary wedge

Transient self-potential anomalies associated with recent lava flows at Piton de la Fournaise volcano (Réunion Island, Indian Ocean)

International audienceSelf-potential signals are sensitive to various phenomena including ground water flow (streaming potential), thermal gradients (thermoelectric potential), and potentially rapid fluid disruption associated with vaporization of water. We describe transient self-potential anomalies observed over recent (< 9 years) lava flows at Piton de la Fournaise volcano (Reunion Island, Indian Ocean). Repeated self-potential measurements are used to determine the decay of the self-potential signals with time since the emplacement of a set of lava flow. We performed a 9 km-long self-potential profile in February 2004 in the Grand Brûlé area. This profile was repeated in July–August 2006. The second repetition of this profile crossed eight lava flows emplaced between 1998 and 2005 during seven eruptions of Piton de la Fournaise volcano. The self-potential data show clear positive anomalies (up to 330 mV) and spatially correlated with the presence of recent lava flows. The amplitude of the self-potential anomalies decreases exponentially with the age of the lava flows with a relaxation time of not, vert, similar 44 months. We explain these anomalies by the shallow convection of meteoric water and the associated streaming potential distribution but we cannot exclude possible contributions from the thermoelectric effect and the rapid fluid disruption mechanism. This field case evidences for the first time transient self-potential signals associated with recent volcanic deposits. It can be also a shallow analogue to understand the variation of self-potential signals in active geothermal areas and transient self-potential signals associated with dike intrusion at larger depths. The empirical equation we proposed can also be used to diagnose the cooling of recent lava flow on shield volcanoes

Toward Forecasting Volcanic Eruptions using Seismic Noise

During inter-eruption periods, magma pressurization yields subtle changes of the elastic properties of volcanic edifices. We use the reproducibility properties of the ambient seismic noise recorded on the Piton de la Fournaise volcano to measure relative seismic velocity variations of less than 0.1 % with a temporal resolution of one day. Our results show that five studied volcanic eruptions were preceded by clearly detectable seismic velocity decreases within the zone of magma injection. These precursors reflect the edifice dilatation induced by magma pressurization and can be useful indicators to improve the forecasting of volcanic eruptions.Comment: Supplementary information: http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguier_SI.pdf Supplementary video: http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguierMovieVolcano.av

Aseismic slip and seismogenic coupling along the central San Andreas Fault

International audienceWe use high-resolution Synthetic Aperture Radar- and GPS-derived observations of surfacedisplacements to derive the first probabilistic estimates of fault coupling along the creeping section of theSan Andreas Fault, in between the terminations of the 1857 and 1906 magnitude 7.9 earthquakes. Usinga fully Bayesian approach enables unequaled resolution and allows us to infer a high probability ofsignificant fault locking along the creeping section. The inferred discreet locked asperities are consistentwith evidence for magnitude 6+ earthquakes over the past century in this area and may be associated withthe initiation phase of the 1857 earthquake. As creeping segments may be related to the initiation andtermination of seismic ruptures, such distribution of locked and creeping asperities highlights the centralrole of the creeping section on the occurrence of major earthquakes along the San Andreas Fault

The 2012 Sumatra great earthquake sequence

The equatorial Indian Ocean is a well known place of active intraplate deformation defying the conventional view of rigid plates separated by narrow boundaries where deformation is confined. On 11 April 2012, this region was hit in a couple of hours by two of the largest strike-slip earthquakes ever recorded (moment magnitudes Mw=8.6 and 8.2). Broadband seismological observations of the Mw=8.6 mainshock indicate a large centroid depth (∼30 km) and remarkable rupture complexity. Detailed study of the surface-wave directivity and moment rate functions clearly indicates the partition of the rupture into at least two distinct subevents. To account for these observations, we developed a procedure to invert for multiple-point-source parameters. The optimum source model at long period consists of two point sources separated by about 209 km with magnitudes Mw=8.5 and 8.3. To explain the remaining discrepancies between predicted and observed surface waves, we can refine this model by adding directivity along the WNW–ESE axis. However, we do not exclude more complicated models. To analyze the Mw=8.2 aftershock, we removed the perturbation due to large surface-wave arrivals of the Mw=8.6 mainshock by subtracting the corresponding synthetics computed for the two-subevent model. Analysis of the surface-wave amplitudes suggests that the Mw=8.2 aftershock had a large centroid depth between 30 km and 40 km. This major earthquake sequence brings a new perspective to the seismotectonics of the equatorial Indian Ocean and reveals active deep lithospheric deformation

Strain budget of the Ecuador–Colombia subduction zone: A stochastic view

The 2016 Pedernales earthquake (M_W=7.8) ruptured a portion of the Colombia–Ecuador subduction interface where several large historical earthquakes have been documented since the great 1906 earthquake (M=8.6). Considering all significant ruptures that occurred in the region, it has been suggested that the cumulative moment generated co-seismically along this part of the subduction over the last century exceeds the moment deficit accumulated inter-seismically since 1906. Such an excess challenges simple models with earthquakes resetting the elastic strain accumulated inter-seismically in locked asperities. These inferences are however associated with large uncertainties that are generally unknown. The impact of spatial smoothing constraints on co-seismic and inter-seismic models also prevents any robust assessment of the strain budget. We propose a Bayesian kinematic slip model of the 2016 Pedernales earthquake using the most comprehensive dataset to date including InSAR and GPS offsets, tsunami waveforms, and kinematic records from high-rate GPS and strong-motions. In addition, we use inter-seismic geodetic velocities to produce a probabilistic inter-seismic coupling model of the subduction interface. Our stochastic co-seismic and inter-seismic solutions include the ensemble of all plausible models consistent with our prior information and that fit the observations within uncertainties. The analysis of these model ensembles indicates that an excess of co-seismic moment during the 1906–2016 period is likely in Central Ecuador only if we assume that 1942 and 2016 earthquakes are colocated. If this assumption is relaxed, we show that this conclusion no longer holds given uncertainties in co- and inter-seismic processes. The comparison of 1942 and 2016 teleseismic records reveals large uncertainties in the location of the 1942 event, hampering our ability to draw strong conclusions on the unbalanced moment budget in the region. Our results also show a heterogeneous coupling of the subduction interface that coincides with two slip asperities in our co-seismic model for the 2016 Pedernales earthquake and with the location of historical ruptures in 1958, 1979 and 1998. The spatial variability in coupling and complexity in earthquake history suggest strong heterogeneities in frictional properties of the subduction megathrust

Single station Monitoring of Volcanoes Using Seismic ambient noise

Seismic ambient noise cross correlation is increasingly used to monitor volcanic activity. However, this method is usually limited to volcanoes equipped with large and dense networks of broadband stations. The single station approach may provide a powerful and reliable alternative to the classical “cross-stations” approach when measuring variation of seismic velocities. We implemented it on the Piton de la Fournaise in Reunion Island, a very active volcano with a remarkable multi-disciplinary continuous monitoring. Over the past decade, this volcano was increasingly studied using the traditional cross-correlation technique and therefore represents a unique laboratory to validate our approach. Our results, tested on stations located up to 3.5 km from the eruptive site, performed as well as the classical approach to detect the volcanic eruption in the 1-2 Hz frequency band. This opens new perspectives to successfully forecast volcanic activity at volcanoes equipped with a single 3-component seismometer

Citrate adsorption can decrease soluble phosphate concentration in soil : experimental and modeling evidence

The adsorption/desorption of phosphate (PO4) on soil minerals is a major process regulating soluble phosphate concentrations (i.e. phosphorus availability) and ultimately PO4 bio-availability. Release of citrate by roots is widely recognized as an effective biological mechanism for increasing available phosphorus (P) in soil. However, interactions between citrate and PO4 are poorly understood and little investigated in soils. Using surface complexation modeling we recently predicted that citrate adsorption can decrease available P in soils depending mainly on soil type and on citrate, exchangeable calcium, and soil organic carbon concentrations. The most pronounced decrease was predicted in chromic cambisols. The intention of the present study was to verify the accuracy of our predictions against experimental data measured in a real chromic cambisol, and to improve our understanding of PO4-controlling processes through surface complexation modeling. The addition of a low to moderate citrate concentration (<50 mu M) effectively decreased available P. In contrast, a concentration of 100 mu M produced an increase of available P. The maximum decline of available P was observed at 20 mu M of citrate. The agreement between simulated and measured values of P availability and total dissolved Ca concentrations was excellent at a citrate concentration ranging from 0 to 50 mu M. Our model substantially underestimated the increase of available P measured at 100 mu M of citrate. This discrepancy showed the presence of another PO4-controlling process, which revealed to be the dissolution of P-containing minerals, most probably illite. This study confirmed that the release of citrate in soils and its subsequent adsorption onto minerals can produce the decrease of P availability. Such citrate-induced decreases of available P can be observed after addition of a relatively low concentration. The threshold concentration was 20 mu M in the chromic cambisol investigated here. This validation should have serious implications in the prospect of rhizosphere management for a better PO4 acquisition by plants through citrate release, as an adverse effect of citrate can be expected, depending on soil properties and citrate concentration. This study also further demonstrated the ability of surface complexation models to reproduce and predict dissolved concentrations of various ions in soil solutions as a function of environmental conditions

Energy and protein values of lucerne leaf meal in growing pigs

Forage legumes efficiently utilise the growing season and represent a relevant way to ensure soil cover and to produce high protein yields. Among the range of forage, leaf fraction of lucerne would be considered as a valuable protein source for swine. Two experiments were thus conducted to determine the energy and the protein values of lucerne leaf meal (LLM) in growing pigs. In the first experiment, total tract digestibility coefficient (TTD) of energy was measured on a total of 25 pigs (60 kg BW) randomly allotted to five different dietary treatments: 0, 5, 10, 15 and 20% of LLM was incorporated in a wheat-soybean meal diet. On a dry matter basis, the average crude protein (CP), lysine, crude fibre and gross energy contents of LLM were 25.3%, 1.09%, 15.5% and 19.3 MJ/kg, respectively. After 10-days adaptation period, faeces and urines were collected during seven consecutive days. In the second experiment, the apparent (AID) and standardized (SID) ileal digestibility coefficient of AA were measured on five pigs (32.4 kg) fitted with ileo-rectal anastomosis and arranged in a 5×5 Latin square design with 6 periods and five diets containing 0, 5, 10, 15 and 20% of LLM. Pigs were fed each of the five diets during one 7-days period, and ileal digesta were collected on d 5, 6 and 7. On the 6th period, all the pigs were fed with a free protein diet for the measurement of the basal ileal endogenous. As expected, the TTD of energy linearly decreased with increasing inclusion of LLM in the diet. On average, the TTD value and the digestible energy content of leaf meal were 45.4% and 8.5 MJ/kg. Increasing the rate of incorporation of LLM reduced the AID for crude protein and amino acids. This reduction was linear (P<0.05). On average, the AID and SID of lysine were 57.7 and 62.5%, respectively. These low digestibility of energy and lysine are probably explained by the presence of anti-nutritional factors (saponins, polyphenols, etc.) at a high concentration in the LLM