Sea surface height variations in the Mindanao Dome region in response to the northern tropical Pacific winds

Sea surface height (SSH) variability in the Mindanao Dome (MD) region is found to be one of the strong variations in the northern Pacific. It is only weaker than that in the Kuroshio Extension area, and is comparable to that in the North Pacific Subtropical Countercurrent region. Based on a 1.5-layer reduced gravity model, we analyzed SSH variations in this region and their responses to northern tropical Pacific winds. The average SSH anomaly in the region varies mainly on a seasonal scale, with significant periods of 0.5 and 1 year, ENSO time scale2-7years, and time scale in excess of 8 years. Annual and long-term variabilities are comparably stronger. These variations are essentially a response to the northern tropical Pacific winds. On seasonal and ENSO time scales, they are mainly caused by wind anomalies east of the region, which generate westward-propagating, long Rossby waves. On time scales longer than 8 years, they are mostly induced by local Ekman pumping. Long-term SSH variations in the MD region and their responses to local winds are examined and discussed for the first time .     

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Research on meteorological thresholds of drought and flood disaster: a case study in the Huai River Basin,China

Together with affected areas of crops from 1978 to 2008, the daily precipitation of 110 stations located in the Huai River Basin during 1959–2008 was used to study the critical conditions when drought and flood occur, based on which the quantitative relationship between the critical condition and the affected area of crops was further studied. Based on the research on the hazard-formative factor of precipitation and the damage degree of crops, the spatial-temporal characteristics of disasters were analyzed, the drought and flood disaster-causing threshold was determined, and the quantitative relationship between the disaster-causing threshold and affected area of crops was established. The results are as the follows: (1) During 1959–2008, extreme precipitation levels were high in the eastern and western part of the Huai River Basin and were low in its central part; the spatial distribution of the coefficient of variation (CV) differed greatly from average extreme precipitation: the series of most stations were located in the central basin, and especially there was a positive trend in Anhui and Henan Provinces. (2) The cumulative precipitation during the disaster period of each station was divided by its mean cumulative precipitation during the same period in 1959–2008 to obtain the disaster-causing threshold, which has shown a good effect on reflecting the actual grade and affected areas in disasters. (3) The relationship among disaster grade, disaster-causing threshold and damage area of crops was established; this threshold can be used as a tool for agricultural disaster assessment and early warning, and can effectively improve the ability to prevent and mitigate disaster in the Huai River Basin. (4) The disaster-causing threshold can be an important input parameter for hazard assessment; other underlying surface indicators can be good supplements for determining the threshold in hazard assessment.     

Decoding Neoarchaean to Palaeoproterozoic tectonothermal events in the Rangnim Massif,North Korea: regional correlation and broader implications

Any knowledge about Archaean to Palaeoproterozoic magmatic and metamorphic events in North Korea has the potential to make a significant difference to our understanding of the early tectonic configuration and evolution of East Asia. This zircon U–Pb dating and Hf isotopic study documents multiple Neoarchaean to Palaeoproterozoic tectonothermal events from the meta-igneous complex in the Machollyong ‘Group’ of the Rangnim Massif. Two tonalitic-trondjemitic gneiss samples record a crystallization age of meta-igneous protoliths at ca. 2.56 Ga and multiple migmatization and metamorphism from 2.52 to 1.85 Ga. A meta-dolerite sample yields a magmatic emplacement age of ca. 1.83 Ga. In situ zircon Hf isotopic data indicate that most zircons from the gneiss samples have ε_Hf(t) values from –16.9 to + 3.1 and crustal model ages from 2.84 to 3.73 Ga, whereas magmatic zircons from the meta-dolerite dike record ε_Hf(t) values from –5.2 to + 5.2 and model ages of 2.05–2.44 Ga. The first-recognized Neoarchaean tonalitic-trondjemitic migmatite complex in the Rangnim Massif, together with previously identified tonalitic-trondhjemitic-granodioritic (TTG) rocks in the Rimjingang Belt and the coeval counterparts in western Gyeonggi massif, represents the oldest crustal nuclei in the Korean Peninsula. The multiple tectonothermal events in this study present reliable evidence not only for attesting to consanguinity of the basement between the Korean Peninsula and the North China Craton but also for defining the influence scope of the late Palaeoproterozoic orogeny in the Korean Peninsula.     

Petrogenesis and geodynamic setting of the Triassic granitoid plutons in West Qinling,China: insights from LA-ICP-MS zircon U–Pb ages,Lu–Hf isotope signatures and geochemical characteristics of the Zhongchuan pluton

West Qinling is one of the most important parts of the Qinling orogenic belt and includes acidic–intermediate plutons and many types of ore deposits. In this article, we collected geochemical and geochronological data for the Triassic granitoid plutons of West Qinling and found that nearly all plutons share the similar features with the Zhongchuan pluton. We present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb ages, major and trace element geochemistry, and zircon Hf isotope systematics for the granites of the Zhongchuan pluton to elucidate the evolution of granitoid plutons in West Qinling during the Triassic. LA-ICP-MS zircon U–Pb dating indicates that the Xujiaba and Guandigou units formed at 220.1 ± 1.2 and 215.9 ± 0.85 Ma, respectively, reflecting the time of the Late Triassic. The rocks of the Zhongchuan pluton are metaluminous to weakly peraluminous and have a high-K calc-alkaline to shoshonite series with high SiO₂ (63.59–76.22%) and low P₂O₅ (0–0.2%) concentrations, a high K₂O/Na₂O ratio (1.18–17.92), a high differentiation index (78.45–93.04) and a medium A/CNK ratio (0.98–1.69). The zircon Hf isotope dating indicates that the Xujiaba and Guandigou units have an inhomogeneous ε_Hf(t) (?4.425 to 1.067 for Xujiaba and ?4.920 to 2.042 for Guandigou) and two-stage Hf model ages (1123–1531 Ma for Xujiaba and 1115–2342 Ma for Guandigou). The geochemical and isotopic data imply that the granites of each unit share the same origin. They probably derived from the partial melt of metagreywackes and then mixed with the mantle-derived magma. Based on the regional geological history, petrographic characteristics and new geochemical and isotopic data of the Zhongchuan pluton, we suggest that the Triassic magma was derived from the partial melts of metagreywackes and was influenced by the mantle-derived melt during the collision of the Yangtze and Qinling plates.     

Long-term statistics of extreme tsunami height at Crescent City

Historically, Crescent City is one of the most vulnerable communities impacted by tsunamis along the west coast of the United States, largely attributed to its offshore geography. Trans-ocean tsunamis usually produce large wave runup at Crescent Harbor resulting in catastrophic damages, property loss and human death. How to determine the return values of tsunami height using relatively short-term observation data is of great significance to assess the tsunami hazards and improve engineering design along the coast of Crescent City. In the present study, the extreme tsunami heights observed along the coast of Crescent City from 1938 to 2015 are fitted using six different probabilistic distributions, namely, the Gumbel distribution, the Weibull distribution, the maximum entropy distribution, the lognormal distribution, the generalized extreme value distribution and the generalized Pareto distribution. The maximum likelihood method is applied to estimate the parameters of all above distributions. Both Kolmogorov-Smirnov test and root mean square error method are utilized for goodness-of-fit test and the better fitting distribution is selected. Assuming that the occurrence frequency of tsunami in each year follows the Poisson distribution, the Poisson compound extreme value distribution can be used to fit the annual maximum tsunami amplitude, and then the point and interval estimations of return tsunami heights are calculated for structural design. The results show that the Poisson compound extreme value distribution fits tsunami heights very well and is suitable to determine the return tsunami heights for coastal disaster prevention.     

Receiver function imaging in strongly laterally heterogeneous crust: Synthetic modeling of BOLIVAR data

We resolve a large (~20 km) discrepancy in Moho depth determined from PdS receiver functions (RFs) and from active source seismic profiling in the complex Caribbean-South American plate boundary zone in eastern Venezuela. As part of the BOLIVAR experiment 20 broadband stations were deployed along an active source profile to record teleseisms. Using the extremely heterogeneous crustal model obtained from active source data, we generated 2D finite-difference elastic wave synthetics and from them calculated receiver functions and CCP stacks. We compare the observations with synthetic sections that have been spatially sampled at 0.25 km to 40 km. The densely sampled synthetics show that several events in the field data that were originally interpreted as the Moho are multiple reflections within sedimentary basins. Where the Moho has the steepest dip under the plate boundary the CCP stacks fail to image the Moho well, regardless of the density of spatial sampling. A suitable spatial sampling criterion for clearly imaging the lower crust and Moho is to overlap Fresnel zones by 50% at Moho depth, which for the 1 Hz receiver functions examined here, requires an instrument spacing of 15–20 km, with the actual field data density ranging from 20 km to 100 km.     

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Research on the Historical Data of the 1585 A. D. South Chaoxian Earthquake and Its Seismogenic Structure

In 1585, a Ms53/4 earthquake occurred in the south of Chaoxian county, Anhui Province. The parameters of this earthquake were reported differently in various versions of earthquake catalogues. According to detailed textual research on the historic records of this earthquake, the epicenter location of the earthquake was further confirmed by means of seismo-geological field investigations in the Chaohu-Tongling region along the western Yangtze River valleys. Shallow seismic prospecting and drilling methods were applied in studying the buried fault. The possibility of the existence of seismogenic faults and fault activity in the western Yangtze River area were analyzed in depth, and the causative tectonic background of the 1585 Ms53/4 south Chaoxian earthquake was studied. The results of this study indicate that the Yanjiaqiao-Fengshahu fault, which was active in the early to mid-Pleistocene, is possibly the causative structure of this earthquake.To identifying the seismogenic structure of the 1585 south Chaoxian earthquake will help gain more knowledge about the tectonic background of moderate and small earthquake activity in Eastern China.     

Ocean eddy momentum fluxes at the latitudes of the Gulf Stream and the Kuroshio extensions as revealed by satellite data

Eddy momentum fluxes, i.e. Reynold stresses, are computed for the latitude bands of the Gulf Stream and Kuroshio extensions using 13 years of data from the merged satellite altimeter product of Le Traon et al. The spatial pattern and amplitude of the fluxes is remarkably similar to that found by Ducet and Le Traon using the 5 years of data that were available to them. In addition to updating the work of Ducet and Le Traon, we provide new insight into the role played by the underlying variable bottom topography, both for determining the structure of the eddy momentum fluxes seen in the satellite data and for influencing the way these fluxes feedback on the mean flow. While there is no clear evidence that eddies locally flux momentum into the eastward jets of the Gulf Stream and Kuroshio extensions, a clearer picture emerges after zonally integrating across each of the North Atlantic and North Pacific basins. We argue that the eddy momentum fluxes do indeed drive significant transport, a conclusion supported by preliminary results from a 3-D model calculation. We also present evidence that in the North Pacific, the Reynolds stresses are important for driving the recirculation gyres associated with the Kuroshio extension, taking advantage of new data from both observations and high-resolution model simulations.