SUMMARY
On 2013 September 24, an earthquake of magnitude Mw 7.7 occurred on the Hoshab Fault in southern Pakistan, south of the 650-km-long Chaman Fault, within the eastern Makran accretionary wedge. ...This earthquake was caused predominantly by strike-slip faulting. To quantify the post-seismic deformation following this large earthquake, we used ascending and descending Sentinel-1 data over the period 2014 November–2017 December. The deformation time-series over these 3 years shows that the post-seismic deformation was not linear over the time. To determine the mechanisms that may have driven post-seismic deformation, such as after-slip or a combination of after-slip and viscous relaxation, we explored some models, including after-slip only, and a combination of after-slip and viscous relaxation, to identify the best fit between the observed and simulated deformation time-series. Our results indicate that after-slip was the main mechanism controlling the post-seismic deformation. The introduction of a basal flat fault in the modelling improves the fit between the model results and the time-series obtained from the satellite images This basal fault is located at a depth of 18 km, has a northward dip of 7° and corresponds to the basal décollement level of the Makran accretionary prism.
Grímsvötn Volcano is the most active volcano in Iceland, and its last three eruptions were in 1998, 2004, and 2011. Here we analyze the displacement around Grímsvötn during these last three eruptive ...cycles using 10 GPS stations. The observed displacements in this region generally contain a linear component of tectonic and glacio‐isostatic origin, in agreement with the previously estimated values of plate motions and vertical rebound. Larger amplitude deformation observed close to Grímsvötn at the GFUM continuous GPS station clearly reflects a major volcanic contribution superimposed on a tectonic component. We estimate and subtract the tectonic trend at this station using regional observed displacement. The direction and pattern of the residual volcanic displacement (for coeruptive and intereruptive periods) are consistent for all three of these eruptive cycles. The posteruptive inflation is characterized by an exponential trend, followed by a linear trend. In this study, we explain this temporal behavior using a new analytic model that has two connected magma chambers surrounded by an elastic medium and fed by a constant basal magma inflow. During the early posteruptive phase, pressure readjustment occurs between the two reservoirs, with replenishment of the shallow chamber from the deep chamber. Afterward, due to the constant inflow of magma into the deep reservoir, the pressurization of the system produces linear uplift. A large deep reservoir favors magma storage rather than surface emission. Based on displacement measured at GFUM station, we estimate an upper limit for the radius of the deep reservoir of ∼10 km.
Key Points
GPS time series for the last three intereruptive cycles of Grimsvotn are analyzed
We propose a model with two connected magmatic chambers, with basal magma inflow
The vertical deformation limits the lateral extension of the deep reservoir
The morphological boundary between the Himalayas and the foreland plain is well expressed and most often corresponds to the frontal emergence of the Main Himalayan Thrust (MHT). This boundary is ...affected by surface ruptures during very large Himalayan earthquakes (Mw > 8) that regularly induce (with a recurrence of the order of 500 to 1200 years) the uplift of the foothills relative to the plain.
However, a thrust-fold system is hidden beneath the plain and is displayed by the seismic profiles of oil companies in east/central Nepal and by H/V passive geophysical techniques in Darjeeling. Its long-term kinematic evolution is slow, with a tectonic uplift of the hanging wall that is lower than the subsidence rate of the foreland basin, that is, less than approximately half a millimetre per year. During phases of low sedimentation controlled by climatic fluctuations, the morphological surfaces of the piedmont are incised by large rivers for several tens of metres; therefore, structures hidden under the sediments emerge slightly in the plain.
The evolution of the hidden structures corresponds to an embryonic thrust belt mainly affected by a long-term shortening rate of 1.4 +2.5/−1.2 mm·yr−1, that is, 2–20% of the shortening rate of the entire Himalayan thrust system. Nonetheless, the details of the deformation associated with the embryonic thrust belt are still poorly understood. Several deformation components could affect the central Himalayan and Darjeeling piedmonts. i) Any slow steady-state deformation, such as layer parallel shortening (LPS) is not detected by Global Navigation Satellite System (GNSS) data, and such deformation would therefore absorb less than 0.5 mm·yr−1. The geodetic data that suggest the aseismic growth of some of the structures are highly controversial. ii) For the rest of the deformation of the embryonic thrust wedge, it is yet to be proven whether deformation occurs during rare great earthquakes affecting the piedmont during medium earthquakes and/or during post-seismic deformation related to great earthquakes. The amplitude of this long-term low deformation is too limited to significantly reduce the seismic hazard in the seismic gaps of the Himalayan belt. iii) In some portions of the Himalayan front, such as Darjeeling (India), the thrust deformation related to great earthquakes propagates several tens of kilometres south of the morphological front in the zone previously affected by the long-term low deformation. It induces multi-metre surface ruptures in the piedmont and a mean shortening of 8.5 ± 6.2 mm·yr−1. iiii) Pre-existing faults in the bedrock of the Indian craton, often oblique to the Himalayan structures, are locally reactivated beneath the foreland plain with low deformation rates.
•New analysis of episodic and permanent GNSS data and coupling estimation along the MHT using a realistic geometry.•Interseismic deformation of Himalayas of Nepal before the 2015 Gorkha ...earthquake.•Estimation of interseismic slip along the West Nepal Faults system.
We analyze episodic (1995–2010) and permanent GNSS data quantifying interseismic velocities in the Himalaya of Nepal so as to constrain spatial variations in coupling along the Main Himalayan Thrust. To estimate this coupling, we model the MHT with the help of cross sections allowing us to constrain its changes in geometry from far-western Nepal to central and eastern Nepal. We determine that the upper flat of the MHT is nearly totally locked, the crustal ramp is partially locked, and that free slip is localized only along the lower flat north of the Himalaya. This location of increased coupling probably corresponds to the brittle/ductile change in rheology along the MHT and explains the location of very large (greater than the Gorkha 2015 event) earthquake epicenters north of the crustal ramp. Including the simulation of the Western Nepal Fault System in the continuation of the Karakorum fault does not improve simulation of the interseismic velocity field. The 25 April Gorkha earthquake nucleated in a highly coupled part of the upper flat of the MHT and propagated eastward along a less coupled part of the MHT.
We present a new kinematic and strain model of an area encompassing the Calabrian and Hellenic subduction zones, western Anatolia and the Balkans. Using Haines and Holt's (1993) method, we derive ...continuous velocity and strain rate fields by interpolating geodetic velocities, including recent GPS data in the Balkans. Relative motion between stable Eurasia and the western Aegean Sea is gradually accommodated by distributed N‐S extension from Southern Balkans to the Eastern Corinth Gulf, so that the westward propagation of the North Anatolian Fault (NAF) throughout continental Greece or Peloponnesus is not required. We thus propose that the NAF terminates in north Aegean and that N‐S extension localized in the Corinth Gulf and distributed in Southern Balkans is due to the retreat of the Hellenic slab. The motion of the Hyblean plateau, Apulia Peninsula, south Adriatic Sea, Ionian Basin and Sirte plain can be minimized by a single rigid rotation around a pole located in the Sirte plain, compatible with the opening the Pelagian rifts (2–2.5 mm/yr) and seismotectonics in Libya. We interpret the trenchward ultraslow motion of the Calabrian arc (2–2.5 mm/yr) as pure collapse, the Calabrian subduction being now inactive. In the absolute plate motion reference frame, our modeled velocity field depicts two toroïdal crustal patterns located at both ends of the Hellenic subduction zone, clockwise in NW Greece and counter‐clockwise in western Anatolia. We suggest the NW Greece toroïdal pattern is the surface expression of a slab tear and consequent toroïdal asthenospheric flow.
Key Points
Propagation of Anatolian Fault throughout continental Greece is not required
Extension in the Corinth Gulf and in the Balkans is due to slab retreat
Absolute velocity field is toroidal at both ends of the Hellenic subduction
The Holocene tectonic activity of Latin American countries is poorly constrained because of the short time span of the instrumental record and the lack of any seismic calendar during pre-colonization ...times. Therefore, some areas with low and diffused seismicity have been catalogued as a “seismic gap”. It has been suggested that the northernmost segment of the Boconó Fault in Venezuela is one of these areas, although a recent MW 7.4 event that occurred in 1812 has been well documented by a historical investigation. In this study, we mapped the historical events triggered by the Boconó Fault using a paleoseismological investigation, a geological interpretation of high-resolution satellite images, field mapping as well as ground-penetrating radar (GPR) profiles at selected trench sites. Our results suggest that at least three seismic events had produced surface ruptures in the trench site based on the exposed trench stratigraphy as well as the results of radiocarbon dating and age models: i) a latest event (E3) that occurred between 1545 and 1825 CE, ii) a previous event (E2) that occurred between 1483 and 1743 CE and iii) an older event (E1 before 1456–1636 CE). The E3 event might correspond to the 1812 major earthquake. These results further support the fact that the 1812 event was triggered by the Boconó Fault and that the rupture reached the surface. The potential slip deficit along the studied segment ranges from 1.7 to 2.3 m and corresponds to Mw ~7, assuming that the 1812 CE earthquake released all of the strain stored earlier. As the time span between events E2 and E3 is short (<360 years), it is worth to pay a great attention to the seismic risk assessment for a densely inhabited Boc-e fault segment.
•Identification and dating of the three latest earthquakes on the northernmost segment of the Boconó Fault•Geomorphic and stratigraphic observation of the primary surface rupture related to the deadly 1812 Mwi 7.4 historical event•The slip deficit ranges between 1.7 to 2.3 m, assuming that the 1812 AD earthquake released all of the strain stored earlier
This work analyses terraces formation from the case of Albanian rivers. An allostratigraphy study of the fluvial terraces is combined with new numerical dating. 30 \(\mbox {}^{14}\)C and 4 \(\mbox ...{}^{10}\)Be new dated sites along four rivers and 45 ages previously acquired along three other rivers were used to define terrace chronologies at the scale of the whole Albania. Few terrace remnants are related to stages older than the last glacial period and are older than \(194\pm 19\) ka. Terrace level (T1) includes plain-like terraces and T1 is related to a rapid succession of valley incision and valley fill that occurred during the warm Holocene climatic optimum. The other nine terrace levels (T2 to T10) formed during the last glacial period (MIS 5d to end of MIS 2). Terraces T2, T6 and T7 formed nearly synchronously with interstadial transitions toward warmer and wetter conditions. The formation of terraces T3, T4, T5 and T8 (\({<}\)60 ka) coincide with the warm climatic excursions of the Heinrich events. This result suggests that these short climatic events strongly punctuate the geomorphologic dynamics of rivers in mountainous areas.
We used episodic GNSS measurements to quantify the present‐day velocity field in the northwestern Himalaya from the Himalayan foreland to the Karakoram Range. We report a progressive N‐S ...compressional velocity gradient with two noticeable exceptions: in the Salt Range, where important southward velocities are recorded, and in Nanga Parbat, where an asymmetrical E‐W velocity gradient is recorded. A review of Quaternary slip along active thrusts both in and out of sequence allows us to propose a 14 mm/yr shortening rate. This constraint, together with a geometrical model of the Main Himalayan Thrust (MHT), allows us to propose estimations of the slip distributions along the active faults. The lower flat of the MHT is characterized by ductile slip, whereas the coupling increases along the crustal ramp and along the upper flat of the MHT. The basal thrust of the Potwar Plateau and Salt Range presents weak coupling, which is interpreted as the existence of a massive salt layer forming an excellent décollement. In the central part of the frontal Salt Range, the velocities suggest the existence of a southward horizontal flux in the massive salt layer. The simulations also suggest that the velocities recorded in Nanga Parbat can be explained by active westward thrusting along the fault that borders the massif to the west. Simulations suggest that the slip along this fault evolves with depth from 5 mm/yr ductile slip near the MHT to no slip along the upper part of the fault.
Key Points
Estimation of interseismic deformation using GNSS velocities
Estimation of coupling along the Main Himalayan Thrust in northwestern Himalaya
Quantification of GNSS velocities in northwestern Himalaya and Karakorum ranges
The Boconó fault is a strike‐slip fault lying between the North Andean Block and the South American plate which has triggered at least five Mw > 7 historical earthquakes in Venezuela. The North ...Andean Block is currently moving toward NNE with respect to a stable South American plate. This relative displacement at ~12 mm yr−1 in Venezuela (within the Maracaibo Block) was measured by geodesy, but until now the distribution and rates of Quaternary deformation have remained partially unclear. We used two alluvial fans offset by the Boconó fault (Yaracuy Valley) to quantify slip rates, by combining 10Be cosmogenic dating with measurements of tectonic displacements on high‐resolution satellite images (Pleiades). Based upon a fan dated at >79 ka and offset by 1350–1580 m and a second fan dated at 120–273 ka and offset by 1236–1500 m, we obtained two Pleistocene rates of 5.0–11.2 and <20.0 mm yr−1, consistent with the regional geodesy. This indicates that the Boconó fault in the Yaracuy Valley accommodates 40 to 100% of the deformation between the South American plate and the Maracaibo Block. As no aseismic deformation was shown by interferometric synthetic aperture radar analysis, we assume that the fault is locked since the 1812 event. This implies that there is a slip deficit in the Yaracuy Valley since the last earthquake ranging from ~1 to 4 m, corresponding to a Mw 7–7.6 earthquake. This magnitude is comparable to the 1812 earthquake and to other historical events along the Boconó fault.
Key Points
We measured the two Pleistocene slip rates along the Boconó fault using 10Be dating and Pleiades images (5.0–11.2 mm yr−1 and <20 mm yr−1)
Boconó fault in the Yaracuy Valley accommodates 40 to 100% of the relative displacement between the Maracaibo Block and the South American plate
The slip deficit in the Yaracuy Valley ranges from 1 to 4 m along the Boconó fault since the MWI 7.4 event in 1812
We study translation, rotation, and strain of active tectonic blocks in Northeastern Venezuela from the Global Navigation Satellite System (GNSS) observations. Since the installation of the geodetic ...network in 2003, one of the goals was to place at least three observation sites at each tectonic block to study the deformation of each one. Based on this premise, we define at least seven blocks: Bergantín and Caripe blocks south of the El Pilar Fault (EPF), and Cariaco Gulf, Land bridge, Paria, North Peninsula, and Margarita Island blocks north of the EPF. Our preferred block modeling shows angular rotations from 0.02 to 0.29° Ma−1. It is known that the EPF concentrates the active deformation in this region of the Caribbean-South American plate boundary. However, the existent rotation could accommodate part of the motion. The strain rate tensors (SRT) indicate NW-SE compression and NE-SW extension for the western blocks. To the east, the 4-Land bridge block keeps the NW-SE compression but shows a decrease in the extensional component. The 5-Paria blocks show a complete inversion in the sense of semi-axis. Additionally, we evaluate the possibility of different motions in Margarita block calculating translational vector, rotational velocity, and strain from three GNSS sites at each side thereof. Our results show remarkable similarities for the Macanao Peninsula and Eastern Margarita Island, pointing to both belonging to a single block.
•The post-Middle Miocene rotation is comparable to present-day rotation in the SDI.•The Caripe block and northern peninsula Araya show anti-clockwise rotation.•Strain tensor rates are mainly NW-SE compressive and NE-SW extensional.•The eastern and western Margarita Island comprises a single tectonic block.•The Paria Peninsula shows a different pattern of stress.
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