ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉By coupling with the ground, wind causes ground motion that appears on seismic records as noise across a wide bandwidth. This wind‐generated noise can drown out important features such as small earthquakes and prevent observation of normal modes from large earthquakes. Because the wind field is heterogeneous at local scales due to structures, diurnal heating, and topography, wind‐induced seismic noise may be different on seismometers installed just meters apart. We investigated the spatial variability of wind‐induced noise using two weather sensors separated by ∼100  m and collocated with one deep borehole and four near‐surface broadband seismometers. We found that at longer periods (〉5  s), increasing wind speed causes noise increases on the horizontal components of seismometers. Although this has been previously observed, we also measured γ2 coherences of less than 0.2 between the wind speed, wind direction, and the pressure recorded by our weather stations. We further observed a loss of coherence between the vertical components of our seismometers from an 8–20 s period. The amplitude of the drop in coherence appears to depend on the substrate surrounding the seismometer. Based on two previously developed theoretical models, we found that the local material surrounding the sensor could be amplifying the wind‐generated noise. We also investigated the frequency dependence of wind‐induced noise and found that the dominant source of high‐frequency seismic noise at some sites could be anthropogenic rather than induced by wind. In addition, we estimated the linear relationships between the root mean square (rms) of wind speed and rms seismic velocity for each sensor, finding substantial variability between different installments. A more detailed understanding of the complex processes by which wind‐induced noise is generated can inform the installation of sensors and the development of methods for mitigation of these effects, thus improving the overall quality of seismic data.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉By coupling with the ground, wind causes ground motion that appears on seismic records as noise across a wide bandwidth. This wind‐generated noise can drown out important features such as small earthquakes and prevent observation of normal modes from large earthquakes. Because the wind field is heterogeneous at local scales due to structures, diurnal heating, and topography, wind‐induced seismic noise may be different on seismometers installed just meters apart. We investigated the spatial variability of wind‐induced noise using two weather sensors separated by ∼100  m and collocated with one deep borehole and four near‐surface broadband seismometers. We found that at longer periods (〉5  s), increasing wind speed causes noise increases on the horizontal components of seismometers. Although this has been previously observed, we also measured γ2 coherences of less than 0.2 between the wind speed, wind direction, and the pressure recorded by our weather stations. We further observed a loss of coherence between the vertical components of our seismometers from an 8–20 s period. The amplitude of the drop in coherence appears to depend on the substrate surrounding the seismometer. Based on two previously developed theoretical models, we found that the local material surrounding the sensor could be amplifying the wind‐generated noise. We also investigated the frequency dependence of wind‐induced noise and found that the dominant source of high‐frequency seismic noise at some sites could be anthropogenic rather than induced by wind. In addition, we estimated the linear relationships between the root mean square (rms) of wind speed and rms seismic velocity for each sensor, finding substantial variability between different installments. A more detailed understanding of the complex processes by which wind‐induced noise is generated can inform the installation of sensors and the development of methods for mitigation of these effects, thus improving the overall quality of seismic data.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We carry out a new probabilistic seismic hazard analysis (PSHA) for Nepal. The 2015 Mw 7.8 Gorkha, Nepal, earthquake (hereafter the Gorkha earthquake) highlights the seismic risk in Nepal, allows better characterization of the geometry of the Main Himalayan thrust (MHT), and enables comparison of recorded ground motions with predicted ground motions. These new data, together with recent paleoseismic studies and geodetic‐based coupling models, allow for good parameterization of the MHT’s characteristics. Other faults in Nepal remain less well studied. Unlike previous PSHA studies in Nepal that are exclusively area‐based, we use a mix of fault and area source models to describe six seismic sources in Nepal. For each source, the Gutenberg–Richter (GR) a‐ and b‐values are found, and the maximum magnitude earthquake (Mmax) estimated, using a combination of earthquake catalogs, moment conservation principles, and similarities to other tectonic regions. We use OpenQuake to carry out the analysis, and estimate peak ground acceleration (PGA) at 2% and 10% probability of exceedance in 50 yrs, along with hazard curves at various locations. We find that PGA reaches 0.6g at 10% probability of exceedance in 50 yrs and is high over most of Nepal. In contrast to previous seismic hazard models, our hazard is high in southern Nepal and fairly evenly distributed across the west‐northwest–east‐northeast direction. The MHT is the principal seismic hazard in Nepal so we study the effects of changing several parameters associated with this fault and find that uncertainties in a and b‐values have a much more significant effect on estimated PGA than the geometry or Mmax. We compare the results from a modeled Gkha earthquake scenario using different ground‐motion prediction equations (GMPEs) with observations, and find that none of the trialled GMPEs fully account for all the features observed. Developing a region‐specific GMPE would be a next step for future seismic‐hazard analysis in Nepal.〈/span〉

Description: 〈span〉High-resolution elevation surveys of deformed late Pleistocene shorelines and new luminescence dating provide improved constraints on spatiotemporal patterns of distributed slip between normal and strike-slip faulting in southern Owens Valley, eastern California. A complex array of five subparallel faults, including the normal Sierra Nevada frontal fault and the oblique-normal Owens Valley fault, collectively form an active pull-apart basin that has developed within a dextral transtensional shear zone. Spatiotemporal patterns of slip are constrained by post−IR-IRSL (post-infrared−infrared stimulated luminescence) dating of a 40.0 ± 5.8 ka highstand beach ridge that is vertically faulted and tilted up to 9.8 ± 1.8 m and an undeformed suite of 11−16 ka beach ridges. The tectono-geomorphic record of deformed beach ridges and alluvial fans indicates that both normal and dextral faulting occurred between the period of ca. 16 and 40 ka, whereas dextral faulting has been the predominant style of slip since ca. 16 ka. A total extension rate of 0.7 ± 0.2 mm/yr resolved in the N72°E direction across all faults in Owens Lake basin is within error of geodetic estimates, suggesting extension has been constant during intervals of 10〈sup〉1〈/sup〉−10〈sup〉4〈/sup〉 yr. A new vertical slip rate of 0.13 ± 0.04 m/k.y. on the southern Owens Valley fault from deformed 160 ± 32 ka shoreline features also suggests constant slip for intervals up to 10〈sup〉5〈/sup〉 yr when compared to paleoseismic vertical slip rates from the same fault segment. This record supports a deformation mechanism characterized by steady slip and long interseismic periods of 8−10 k.y. where the south-central Owens Valley fault and Sierra Nevada frontal fault form a parallel fault system.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The modern physiography of central Turkey is dominated by the 1-km-high Central Anatolian Plateau and the Central Tauride mountains that form the southern plateau margin. These correspond to a Cretaceous–Eocene backarc extensional province and forearc fold-thrust belt, respectively. The extent to which the morphology of the Miocene plateau was inherited from the physiography of the Cretaceous–Eocene subduction zone that assembled the Anatolian crust has not been tested but is important if we are to isolate the signal of Miocene and younger subduction dynamics in the formation of the modern plateau margin. There is no known stratigraphic record of the post-Eocene pre-Miocene evolution of the Taurides. We therefore collected rock samples across the Taurides and used zircon (U-Th)/He (ZHe), apatite (U-Th)/He (AHe), and apatite fission-track (AFT) low-temperature thermochronometers to constrain cooling; we interpret these thermochronometers to signal erosional exhumation. We use inverse thermal modeling to aid interpretation of our results and find that: (1) thermochronometers across the Taurides were reset as a result of heating by the emplacement of the Antalya and Bozkır nappes; (2) AFT and ZHe Eocene cooling ages are related to structurally driven uplift and erosional exhumation on major thrust culminations; (3) dispersed AHe ages record low rates of Oligocene–early Miocene cooling and hence low rates of erosional exhumation; and (4) fast rates of cooling were determined for samples along the margin of the Köprüçay Basin. We interpret that early Miocene cooling is a signal of active erosion of the western Central Taurides at a time of marine sedimentation in the Mut Basin on the southern Central Taurides, and these differing histories may reflect evolution above the Antalya and Cyprus slabs. Our thermochronological data, the enigmatic development of the Antalya Basin, and thrusting within the basin may be explained as the surface expression of stepwise delamination of the Antalya slab from the Tauride hinterland to its current position below the Gulf of Antalya since early Miocene time over a distance of ∼150 km.〈/span〉

Description: 〈span〉The modern physiography of central Turkey is dominated by the 1-km-high Central Anatolian Plateau and the Central Tauride mountains that form the southern plateau margin. These correspond to a Cretaceous–Eocene backarc extensional province and forearc fold-thrust belt, respectively. The extent to which the morphology of the Miocene plateau was inherited from the physiography of the Cretaceous–Eocene subduction zone that assembled the Anatolian crust has not been tested but is important if we are to isolate the signal of Miocene and younger subduction dynamics in the formation of the modern plateau margin. There is no known stratigraphic record of the post-Eocene pre-Miocene evolution of the Taurides. We therefore collected rock samples across the Taurides and used zircon (U-Th)/He (ZHe), apatite (U-Th)/He (AHe), and apatite fission-track (AFT) low-temperature thermochronometers to constrain cooling; we interpret these thermochronometers to signal erosional exhumation. We use inverse thermal modeling to aid interpretation of our results and find that: (1) thermochronometers across the Taurides were reset as a result of heating by the emplacement of the Antalya and Bozkır nappes; (2) AFT and ZHe Eocene cooling ages are related to structurally driven uplift and erosional exhumation on major thrust culminations; (3) dispersed AHe ages record low rates of Oligocene–early Miocene cooling and hence low rates of erosional exhumation; and (4) fast rates of cooling were determined for samples along the margin of the Köprüçay Basin. We interpret that early Miocene cooling is a signal of active erosion of the western Central Taurides at a time of marine sedimentation in the Mut Basin on the southern Central Taurides, and these differing histories may reflect evolution above the Antalya and Cyprus slabs. Our thermochronological data, the enigmatic development of the Antalya Basin, and thrusting within the basin may be explained as the surface expression of stepwise delamination of the Antalya slab from the Tauride hinterland to its current position below the Gulf of Antalya since early Miocene time over a distance of ~150 km.〈/span〉