ALBERT

Description: We present fully dynamic generic three-dimensional laboratory models of progressive subduction with an overriding plate and a weak subduction zone interface. Overriding plate thickness ( T OP ) is varied systematically (in the range 0–2.5 cm scaling to 0–125 km) to investigate its effect on subduction kinematics and overriding plate deformation. The general pattern of subduction is the same for all models with slab draping on the 670 km discontinuity, comparable slab dip angles, trench retreat, trenchward subducting plate motion, and a concave trench curvature. The narrow slab models only show overriding plate extension. Subduction partitioning ( v SP⊥ /( v SP⊥ + v T⊥ )) increases with increasing T OP , where trenchward subducting plate motion ( v SP⊥ ) increases at the expense of trench retreat ( v T⊥ ). This results from an increase in trench suction force with increasing T OP , which retards trench retreat. An increase in T OP also corresponds to a decrease in overriding plate extension and curvature, because a thicker overriding plate provides more resistance to deform. Overriding plate extension is maximum at a scaled distance of ~200–400 km from the trench, not at the trench, suggesting that basal shear tractions resulting from mantle flow below the overriding plate primarily drive extension rather than deviatoric tensional normal stresses at the subduction zone interface. The force that drives overriding plate extension is 5–11% of the slab negative buoyancy force. The models show a positive correlation between v T⊥ and overriding plate extension rate, in agreement with observations. The results suggest that slab rollback and associated toroidal mantle flow drive overriding plate extension and backarc basin formation.

Description: The 290-km-long, Nayband strike-slip fault bounds the western margin of the Lut block and cuts across a region thought to have been quiescent during the last few millennia. Cl-36 cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of cumulative geomorphic offsets are used to derive the long-term slip rate. The measured offsets at two sites along the fault range between 9 ± 1 m and 195 ± 15 m with ages from 6.8 ± 0.6 ka to ∼ 100 ka, yielding minimum and maximum bounds of late Pleistocene and Holocene slip rates of 1.08 and 2.45 mm yr -1 , respectively. This moderate slip rate of 1.8 ± 0.7 mm yr -1 , averaged over several earthquake cycles, is compared to the paleoseismic record retrieved from the first trench excavated across the fault. Combining the paleoseismic evidence with 18 OSL ages obtained from this trench site demonstrates the occurrence of at least four large (M w  ∼ 7) earthquakes during the last 17.4 ± 1.3 ka and of two older earthquakes, one before ∼ 23 ka and another before 70 ± 5 ka. The exposed sediment succession also indicates a significant gap at the end of MIS-2 and the beginning of MIS-1. The age of the most recent regional incision is accurately bracketed between 6.1 ka and 7.4 ka. Sediments from the last ∼ 7 ka contain evidence of the three younger earthquakes. Interestingly, the penultimate and antepenultimate events occurred between 6.5 ± 0.4 ka and 6.7 ± 0.4 ka within a time interval lasting at most 1 ka whereas the most recent earthquake occurred within the last millennium. Such an irregular earthquake occurrence suggests the seismic behavior of the Nayband fault is not strictly time dependent but possibly related to clustering. From this and taking into account the occurrence of the most recent earthquake within the last 800 years, the imminence of an earthquake along the Nayband fault cannot be discarded. Although the most recent surface-rupturing event seems to have occurred after AD 1200, this event went unnoticed in the historical records. This provides a marked illustration of the incompleteness of the historical seismic catalogs in Central Iran, challenging any assessment of regional seismic hazard without appropriate geologic and geochronological information. Large and infrequent earthquakes are characteristic of the seismic behavior of the slow-slipping strike-slip faults slicing Central and Eastern Iran. Also, the slip rates summed across Central and Eastern Iran from the Iran Plateau up to the Afghan lowlands appear in agreement with the most recent GPS data.

Description: A seven-year time series of satellite radar images over Unimak Island, Alaska-site of Westdahl Volcano, Fisher Caldera and Shishaldin Volcano-was processed using a model-free Persistent Scatterer Interferometry (PSI) technique assisted by Numerical Weather Prediction (NWP) model. The deformation-only signals were optimally extracted from atmosphere-contaminated phase records. The reconstructed deformation time series maps are compared with campaign and continuous Global Positioning System (GPS) measurements as well as Small Baseline Subset (SBAS) InSAR results for quality assessment and geophysical interpretation. We observed subtle surface inflation at Westdahl Volcano that can be fit by a Mogi source located at approximately 3.6 km north of Westdahl peak and at depth of about 6.9 km that is consistent to the GPS estimated depth for the 1998 to 2001 time period. The magma chamber volume change decays during the period of 2003 to 2010. The deformation field over Fisher Caldera is steadily subsiding over time. Its best-fit analytical model is a sill source that is about 7.9 km in length, 0.54 km in width, and locates at about 5.5 km BSL underneath the center of Fisher Caldera with strike angle of N52°E. Very little deformation was detected near Shishaldin peak, however, a region approximately 15 km east of Shishaldin, as well as an area at the Tugamak range at about 30 km northwest of Shishaldin, show evidence for movement towards the satellite, with a temporal signature correlated with the 2004 Shishaldin eruption. The cause of these movements is unknown.

Description: [1]  We present a new tomographic model of radially anisotropic shear-velocity variations in the Earth's mantle based on a new compilation of previously published datasets and a variable block parameterization, adapted to local ray-path density. We employ ray-theoretical sensitivity functions to relate surface-wave and body wave data with radially anisotropic velocity perturbations. Our database includes surface-wave phase delays from fundamental modes up to the 6 th overtone, measured at periodsbetween 25 and 350 s, as well as cross-correlation traveltimes of major body-wave phases. Prior to inversion we apply a crustal correction using the crustal model CRUST2.0 [ Bassin et al., 2000] and we account for azimuthal anisotropy in the upper mantle using ray-theoretical corrections based on a global model of azimuthal anisotropy. While being well correlated with earlier models at long spatial wavelength, our preferred solution, savani , additionally delineates a number of previously unidentified structures, due to its improved resolution in areas of dense coverage. This is because the density of the inverse grid ranges between 1.25 ° in well sampled to 5 ° in poorly sampled regions, allowing us to resolve regional structure better than it is typically the case in global S -wave tomography. Important features of our model include: (i) A distinct ocean-continent anisotropic signature in the uppermost mantle; (ii) an oceanic peak in above average ξ  〉 1 which is shallower than in previous models and thus in better agreement with estimates of lithosphere thickness; (iii) a long wavelength pattern of ξ  〈 1 associated with the large low-shear-velocity provinces in the lowermost mantle. Furthermore we conduct a comprehensive comparison between various published isotropic and anisotropic upper- and whole-mantle tomographic models to identify regions in which anisotropic images have reached a stage of maturity, comparable to that of their isotropic counterparts.