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Mw 6.3 and 6.0 earthquakes occurred on January 27 and 29 in 2020 in the southeastern Solomon Islands which is one of the most seismically active areas in the southern Pacific. To investigate the seismogenic mechanism and structure of the southeastern Solomon Islands, we locate the foreshocks-main-shock-aftershocks sequence of two moderate earthquakes recorded by the regional seismic network. In this study, we establish the new database and locate earthquakes for analyzing the seismogenic structures and deriving the new regional 1D velocity model. Based on the special distribution of the foreshock-aftershock sequence, the interaction of subduction and transform zones between the Pacific and the Australia Plates leads to the near-vertical dip-slip tear structure in the southeastern Solomon Islands. Confirmed with PREM and the new 1D velocity model for testing the robustness of the earthquake locations, the seismic gap at depths from 25 to 35 km is observed as the “jelly sandwich” rheology. In addition, seismic events with large-amplitude, high-frequency signals could be observed in the fore-arc area is due to waves that guided by the subducted Australia plate. In order to improve the capability of earthquake detection, we generate templates from foreshocks-aftershocks sequence to detect repeating or near-repeating seismicity. Our study provides unprecedented seismic data and velocity model for the study area that could benefit understanding detailed structure beneath the region and promoting the initial reference model for locating earthquakes and seismic tomography.

In this study, we conduct a multi-disciplinary study, including geophysics, geochemistry, and geology, in an attempt to reconstruct geothermal geological model(s) at the shallow 3 km, for a project of potential geothermal power generation at the Hongchailin site in the Ilan plain of northeastern Taiwan. Our geophysical techniques include seismic imaging from natural earthquakes as well as ambient noise. Three seismic arrays deployed at different time periods in the past decade were used. We also incorporate geophysical imaging results from previous studies, in particular a series of seismic reflection profiles. For the magnetotelluric (MT) technique, two major surveys have been conducted, including 2015-2018 AMT by National Central University and 2021-2022 MT by GERD and Academia Sinica. Three test holes were drilled around the Hongchailin site in 2016-2019. Logging and on-site measurements were conducted at increment depths, including stratigraphy and rock types, P/T measurements, fractures analyses, geochemical analyses (e.g., hydrogen/oxygen/helium isotope, fluid inclusion, etc.), and so on. Based on regional geology and structures, and incorporating geophysical subsurface imaging, we reconstruct geothermal geological models in line with detailed geological cross sections at the shallow 3 km level. We interpret that there exits a shallow geothermal reservoir within the massive quartz sandstone layers (Szeleng sandstone) at 1-2 km depth with the downhole temperature of 80-100 C. Geologically, the reservoir is located at the regional Songlo syncline and its south limb; and geophysically, it corresponds to a relatively low resistivity area. Isotope results show that the cool meteoric water came from nearby higher altitude mountain area, then flew through the Szeleng sandstone, which plunges 1-2 km depth below the Ilan plain. Hot fluid is interpreted to be derived from deeper heat source and to be upflowed along a N-S trending vertical faults system near the Hongchailin area. In addition, two E-W trending major faults, identified by the seismic reflection profiles, seem to be acted as hydrothermal fluid barriers to confine the hot fluid within the reservoir area.

The late Holocene lacustrine environmental evolution and paleo-typhoon records are poorly understood around lowland Liyu Lake and mountain Tunlumei Pond in eastern Taiwan. In this study, we use records of diatom populations, magnetic susceptibility, total organic carbon, carbon-to-nitrogen ratio, and δ13C from two lacustrine sediment cores in eastern Taiwan to reconstruct the paleoenvironment and the paleo-typhoon records during the late Holocene. Significant changes in lithology and in multi-proxy data in the record from lowland Liyu Lake (LYL) show that LYL became hydrologically isolated by 2850 cal BP, possibly as a result of landslide-induced alluvial fan formation. Synchronous increases of the diatom-inferred lake level proxy for LYL and regional precipitation proxies reflect a strengthened East Asia summer monsoon since 1600 cal BP. Eutrophication of the water in LYL is also inferred, and this is interpreted to be related to agricultural activities, which provides evidence for changes in land use during the past 200 years. The occurrence of diatom valves in sediment of the mountain Tunlumei Pond (TLM) indicates that the pond area has been stable since 760 cal BP, which reflects an increase in the local precipitation. The decrease of diatom-inferred pH indicates an increase in the input of acid runoff from the watershed during typhoon-triggered heavy rainfall. Both lacustrine records suggest that the typhoon intensity increased during the early Little Ice Age. The sediment records in northeastern Taiwan also suggest an increase in typhoon activity during the late Little Ice Age. The asymmetric pattern of typhoon intensity during the last 1000 years is interpreted to reflect the control exerted by anomalies in both global temperature and the El Nino-Southern Oscillation intensity on typhoon tracks over several centuries.

The Taiwan orogenic belt has high precipitation and high exhumation rates (3–6 km/Myr), making it a natural laboratory for understanding how erosion, tectonics, and climate interact. Understanding how rock exhumation rates have evolved since the late Miocene is key to unraveling climate–tectonic interactions. In this study, we present 8 new zircon fission track (ZFT) and (Usingle bondTh)/He (ZHe) cooling ages (including 2 unreset ages) from 5 samples from 1700 to over 3300 m elevation around the highly incised Dajia River in the core of the Hsuehshan Range (termed the mid-HR in this study) in central Taiwan. Two of the ZFT and ZHe ages (5.3 Ma and 2.6 Ma) are the oldest ages recorded in this region. These ages suggest that the cooling rate of the mid-HR is lower than that of the Backbone Range and Yushan. With these and previously published cooling ages, we modeled the exhumation and cooling history using a 2-D thermal diffusion equation with time-varying erosion. The ZFT and ZHe ages are best explained by a steadily increasing exhumation rate up to 3.4 + 0.5/−0.3 km/Myr from 6.5 to 1 Ma. The exhumation rate in the last million years is not as well constrained but appears to have increased at a slower rate. Our findings differ from those of previous studies that propose that the exhumation rate increased rapidly in the last 1–2 Myr. We propose that the steady increase in the exhumation rate in the mid-HR indicates increasing topographic relief, which is likely caused by progressive crustal thickening.

The impacts of anthropogenic forcing on Indian summer monsoon (ISM) rainfall are obscure, partly due to limited availability of highly resolved hydroclimate proxy records as well as the highly regionalized nature of precipitation. Here, we report an annually-resolved speleothem oxygen isotope record from Xianren Cave, southwestern China, which represents rainfall change over the broad ISM region. We find that the region has endured at least six decadal-scale weak monsoon events in the past three hundred years. One of them, lasting from the early to mid 19th century, shares the similar gradual, persistent trend as the most recent decline in ISM rainfall and both have a magnitude substantially larger than the others dominated by natural variability. This early weak monsoon event occurred during a historical time of intensive deforestation in the region. We conclude that the ISM trend could have been altered by the changes in land-use and land-cover since the early 19th century. Ministry of Education (MOE) National Research Foundation (NRF) Published version We are in debt to Dr. Ming Tan who initiated the research in Xianren Cave with a grant from the National Natural Science Foundation of China (NSFC-41272204). This work has been possible thanks to the support by the Earth Observatory of Singapore via its funding from the National Research Foundation Singapore (NRF), the Singapore Ministry of Education (MOE) under the Research Centres of Excellence initiative. X. W. acknowledges the financial support from an MOE Tier 2 grant (MOE2019-T2-1-174) and an NRF grant (NRF2017NRF-NSFC001-047). U-Th dating at the High-Precision Mass Spectrometry and Environment Change Laboratory was supported by grants from the Science Vanguard Research Program of the Ministry of Science and Technology, Taiwan, ROC (109-2123-M-002-001 to C.-C.S.), the Higher Education Sprout Project of the Ministry of Education, Taiwan, ROC (109L901001 to C.-C.S.), and the National Taiwan University (110L8907 to C.-C.S.). K. L. acknowledges the financial support (SKLLQG1815) from State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, China.

Previous paleoclimatic studies by Asian cave records have shown that variability in the Asian summer monsoons is well correlated with local summer insolation, North Atlantic climate, and the Greenland stadial-interstadial cycles on millennial to orbital time-scales. However, it remains unclear whether a similar agreement exists on short timescales. With 230Th dates in precision as good as ±62 years, we present a decadal-resolved δ18O record covering 30.0–22.7 thousand years ago (ka) from a stalagmite of Son La Province, northwestern Vietnam as a proxy record of the Asian summer monsoon (ASM). On millennial timescales, our data show strong coherence with Chinese and North Indian speleothem δ18O records, suggesting that the ASM has synchronously responded over a broad region to Dansgaard–Oeschger (D-O) events in the North Atlantic. We observe that the onset timing and structure of D-O 4 varied spatially. The event commenced earlier and less abrupt at sites located with a connection to the modern Intertropical convergence Zone (ITCZ) than sites off the influence from ITCZ. It implies that the tropics may play a critical role in the global climate system. In addition, our decadal-resolution record clearly demonstrated a few centennial events with a duration of ~200 years, which probably correlates with the Suess cycle of solar activity. Published version

The source of fluvial deposits in terraces provides important information about the catchment fluvial processes and landform evolution. In this study, we propose a novel approach that combines high-resolution Itrax-XRF scanning and Canonical Discriminant Analysis (CDA) to identify the source of fine-grained fluvial terrace deposits. We apply this approach to a group of terraces that are located on the hanging wall of the Chihshang Fault in eastern Taiwan with two possible sources, the Coastal Range on the east and the Central Range on the west. Our results of standard samples from the two potential sources show distinct ranges of canonical variables, which provided a better separation ability than individual chemical elements. We then tested the possibility of using this approach by applying it to several samples with known sediment sources and obtain positive results. Applying this same approach to the fine-grained sediments in Chihshang terraces indicates that they are mostly composed of Coastal Range material but also contain some inputs from the Central Range. In two lowest terraces T1 and T2, the fine-grained deposits show significant Central Range component. For terrace T4, the results show less Central Range input and a trend of decreasing Central Range influences up section. The Coastal Range material becomes dominant in the two highest terraces T7 and T10. Sediments in terrace T5 appear to have been potentially altered by post-deposition chemical alteration processes and are not included in the analysis. Our results show that the change in source material in the terraces deposits was relatively gradual rather than the sharp changes suggested by the composition of the gravels and conglomerates. We suggest that this change in sources is related to the change in dominant fluvial processes that controlled by the tectonic activity.

We derive erosion rates from detrital zircon fission-track ages and cosmogenic nuclide concentrations from sediments from the modern rivers of eastern Taiwan in order to investigate how surface erosional processes vary in space and time across the young arc-continent collisional orogen of Taiwan. Taiwan is characterized by rapid rates of exhumation, a fluvial and landslide-dominated landscape, high seismicity, high relief and frequent typhoons. The obliquity between the convergence direction and the trend of the plate boundary provides a gradient in uplift and variations in longevity of orogenic activity with a young, immature orogen in the south, a mature orogen in central and northern Taiwan, and perhaps even the cessation of orogeny in the far north. The modern zircon fission-track detrital record is consistent with basement ages that show that much of the orogen is eroding at high rates with basin-wide mean zircon fission-track cooling ages as young as 0.9 Ma. The erosion rates derived from concentrations of cosmogenic nuclides ( 10 Be) provide erosion rates averaged over much shorter timescales, but these two proxies provide estimates of erosion rates that are within error of each other across most of the collisional belt. Erosion rates are lowest in the immature zone of the orogen (

The long-lived radionuclide 129 I (half-life: 15.7 × 10 6 yr) is well-known as a useful environmental tracer. At present, the global 129 I in surface water is about 1–2 orders of magnitude higher than pre-1960 levels. Since the 1990s, anthropogenic 129 I produced from industrial nuclear fuels reprocessing plants has been the primary source of 129 I in marine surface waters of the Atlantic and around the globe. Here we present four coral 129 I time series records from: 1) Con Dao and 2) Xisha Islands, the South China Sea, 3) Rabaul, Papua New Guinea and 4) Guam. The Con Dao coral 129 I record features a sudden increase in 129 I in 1959. The Xisha coral shows similar peak values for 129 I as the Con Dao coral, punctuated by distinct low values, likely due to the upwelling in the central South China Sea. The Rabaul coral features much more gradual 129 I increases in the 1970s, similar to a published record from the Solomon Islands. The Guam coral 129 I record contains the largest measured values for any site, with two large peaks, in 1955 and 1959. Nuclear weapons testing was the primary 129 I source in the Western Pacific in the latter part of the 20th Century, notably from testing in the Marshall Islands. The Guam 1955 peak and Con Dao 1959 increases are likely from the 1954 Castle Bravo test, and the Operation Hardtack I test is the most likely source of the 1959 peak observed at Guam. Radiogenic iodine found in coral was carried primarily through surface ocean currents. The coral 129 I time series data provide a broad picture of the surface distribution and depth penetration of 129 I in the Pacific Ocean over the past 60 years.

manuscript accepted on 2019 August 5. Contains original data as Table 1 and Figs. 1-6

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The Solomon megathrust along the Western Solomon Arc generated two megathrust earthquakes in the past decade (Mw 8.1 in 2007 and Mw 7.1 in 2010). To investigate the interseismic deformation and inferred coupling on the megathrust, we deployed the first continuous GPS network in the Western Solomon Islands. Our 2011-2014 GPS data and the back-slip inversion model show coupling ratio as high as 73% along the southeastern 2007 rupture segment, but only 10% on average along the segment of 2010 event. Based on the spatial distribution of coseismic slip, aftershock clusters, derived coupling pattern and paleo-geodetic records, we discovered the former as a semi-permanent asperity and the latter as a potential megathrust barrier. We propose that a characteristic earthquake of magnitude not less than Mw 8 will recur in an interval of 100 or more years by either single or doublet earthquake.

An MW 8.1 earthquake occurred in 2007 and induced a tsunami that hit the Western Solomon Islands and caused casualties. This motivated us to deploy a seismic network around the rupture zone of the 2007 event. To investigate the seismic velocity structure of the Western Solomon Islands, we select seismograms of teleseismic events recorded by our seismic network. Joint inversion of P-wave receiver functions and surface-wave group velocity dispersion curves is used to estimate station-based 1-D velocity models. The resulting velocity models show a highly variable crustal structure across the region. The Moho depths beneath the stations range from 25 to 40 km. A low-velocity zone (LVZ) is observed at most seismic stations in this work. Our study provides preliminary station-based seismic velocity models for the study region, and more stations will be deployed in the continuing project. An integrated 3-D velocity model will be determined in the future.

Taiwan experiences high deformation rates, particularly along its eastern margin where a shortening rate of about 30 mm/yr is experienced in the Longitudinal Valley and the Coastal Range. Four Sacks-Evertson borehole strainmeters have been installed in this area since 2003. Liu et al. (2009) proposed that a number of strain transient events, primarily coincident with low-barometric pressure during passages of typhoons, were due to deep-triggered slow slip. Here we extend that investigation with a quantitative analysis of the strain responses to precipitation as well as barometric pressure and the Earth tides in order to isolate tectonic source effects. Estimates of the strain responses to barometric pressure and groundwater level changes for the different stations vary over the ranges −1 to −3 nanostrain/millibar(hPa) and −0.3 to −1.0 nanostrain/hPa, respectively, consistent with theoretical values derived using Hooke's law. Liu et al. (2009) noted that during some typhoons, including at least one with very heavy rainfall, the observed strain changes were consistent with only barometric forcing. By considering a more extensive data set, we now find that the strain response to rainfall is about −5.1 nanostrain/hPa. A larger strain response to rainfall compared to that to air pressure and water level may be associated with an additional strain from fluid pressure changes that take place due to infiltration of precipitation. Using a state-space model, we remove the strain response to rainfall, in addition to those due to air pressure changes and the Earth tides, and investigate whether corrected strain changes are related to environmental disturbances or tectonic-original motions. The majority of strain changes attributed to slow earthquakes seem rather to be associated with environmental factors. However, some events show remaining strain changes after all corrections. These events include strain polarity changes during passages of typhoons (a characteristic that is not anticipated from our estimates of the precipitation transfer function) that are more readily explained in terms of tectonic-origin motions, but clearly the triggering argument is now weaker than that presented in Liu et al. (2009). Additional on-site water level sensors and rain gauges will provide data critical for a more complete understanding, including the currently unresolved issue of why, for some typhoons, there appears to be a much smaller transfer function for precipitation-induced strain changes.

Reconstructed coral skeletal Pb : Ca and Ba : Ca ratios from the Northern Coast of Lanyu Islet are shown to be sensitive indicators of land–use practices which took place on the islet during the past 30 years. Coral skeletal Pb : Ca and Ba : Ca both increased dramatically. Pb : Ca ratios are very sensitive to biomass burning on Lanyu Islet while Ba : Ca generally respond to rainfall variations except in case of instantaneous flooding. The method presented here provides a quantitative technique for identifying human effects that can be used for coastal land management and offers a way to study the environment during prehistoric times, where no written records are available.

The mountain building processes in Taiwan are currently among the most rapid in the world. However, the spatial and temporal dynamics of this orogen are still poorly resolved within the

We propose an improved method for precise sulfur isotopic measurements by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) in conjunction with a membrane desolvation nebulization system. The problems of sulfur loss through the membrane desolvation apparatus are carefully quantified and resolved. The method overcomes low intrinsic sulfur transmission through the instrument, which was initially 1% when operating at a desolvation temperature of 160 °C. Sulfur loss through the membrane desolvation apparatus was resolved by doping with sodium. A Na/S ratio of 2 mol mol −1 produced sulfur transmissions with 98% recovery. Samples of 3 nmol (100 ng) sulfur achieved an external precision of ±0.18‰ (2 SD) for δ 34 S and ±0.10‰ (2 SD) for Δ 33 S (uppercase delta expresses the extent of mass-independent isotopic fractionation). Measurements made on certified reference materials and in-house standards demonstrate analytical accuracy and reproducibility. We applied the method to examine microbial-induced sulfur transformation in marine sediment pore waters from the sulfate-methane transition zone. The technique is quite versatile, and can be applied to a range of materials, including natural waters and minerals.

Hazards from gravity-driven instabilities on hillslope (termed ‘landquake’ in this study) are an important problem facing us today. Rapid detection of landquake events is crucial for hazard mitigation and emergency response. Based on the real-time broadband data in Taiwan, we have developed a near real-time landquake monitoring system, which is a fully automatic process based on waveform inversion that yields source information (e.g., location and mechanism) and identifies the landquake source by examining waveform fitness for different types of source mechanisms. This system has been successfully tested offline using seismic records during the passage of the 2009 Typhoon Morakot in Taiwan and has been in online operation during the typhoon season in 2015. In practice, certain levels of station coverage (station gap 106 m3 and area > 0.20 km2) are required to ensure good performance (fitness > 0.6 for successful source identification) of the system, which can be readily implemented in other places in the world with real-time seismic networks and high landquake activities.

We present a reconstructed lithologic column compiled from a series of lacustrine outcrops along a tributary of the Nyang River, a major tributary of the Yarlung-Tsangpo in southeast Tibet. The deposits were preserved between terraces at altitudes of 2950–3100 m asl. The stratigraphic record features at least two sets of coarsening-upward sequences depicting episodic aggradation and progradation of a glacially dammed lake related delta. Recognized facies changes illustrate the evolution cycles of depositional environments from pro-delta, delta front, to delta plain. Radiocarbon and optically stimulated luminescence dates reveal an aging-downward trend in stratigraphic order and provide an approximate timeline for the formation of glacially dammed lakes in late Pleistocene. This result reflects that the Zelunglung Glacier had progressively advanced to block the Yarlung-Tsangpo river and the dam materials had stepwise stacked up to an altitude of 3095 m asl during Marine Oxygen Isotope Stages 4 to 2.

Taiwan is particularly sensitive to changes in monsoonal precipitation and to the frequency of typhoon-induced heavy precipitation events. Many parts of this mountainous subtropical island have high erosion rates that produce high sedimentation rates in lakes and offshore ocean basins, and evidence suggests that these rates have varied in the past. A high-resolution 2 ky record of changes in sediment delivery to Dahu, a floodplain lake in northeastern Taiwan, that suggests changes in precipitation has been inferred from organic matter, pollen, and spores in a 12 m sediment core and from the organic matter compositions of soil, catchment plants, and fluvial suspended sediment. From 0 to 1400 AD, the lake experienced repeated flood events, principally resulting from overflows of the “old” Ilan River, one of tributaries of the Lanyang River, that were briefly interrupted ~ 1000 AD by decreased precipitation and/or only localized rainfall. The record of heavy precipitation can be linked to the northward shift of the ITCZ. During the early Little Ice Age, 1400–1600 AD, local erosion fluctuated in association with deforestation and farming, during which the ITCZ shifting varied as well. A decrease in precipitation since 1600 AD is linked with the southward migration of the ITCZ.

The structural setting and deformation history are very important aspects of understanding the frequent earthquakes and assessing the hazards. In this paper, we interpret two new seismic profiles that were collected after the Wenchuan Mw7.9 earthquakes occurred on May, 2008. Our interpretation has integrated four wells, three seismic profiles and the surface geology to reveal structural characteristics of the central Longmen Shan thrust. The drilled wells reveal that there are two detachments within the upper Triassic and the lower Triassic at the Longmen Shan thrusts. The shallow detachment fault transferred its slip to the basin and made another deformation inner the Sichuan basin. Between the shallow detachments, many faults developed and the major structures at the thrust front have formed as structural wedges. Both the drilled wells and seismic interpretation show the upper Triassic repeated in the depth. The shortening at the central Longmen Shan thrust is approximately 30%–37%. According to geological survey and the seismic interpretation, the foreland basin was developed during the late Triassic and the angular unconformity between the lower Jurassic and the upper Triassic at the Longmen Shan front. Several different tectonic events can be identified since the Mesozoic, which indicated that the Longmen Shan has a complex evolution history. In our model, the major Beichuan fault (F2) and the Pengguan fault (F3) at the central Longmen Shan dip steeply near the surface but are more gentle-dipping at depth. Both of the shallow thrusting and the basement shortening made contribution to the rapid uplift of the Longmen Shan during the Cenozoic. The co-seismic ruptured fault is like a branchy fault that developed on the major fault system. The Wenchuan Mw7.9 earthquake and the Lushan Mw6.7 earthquake may be related to the reaction of the basement structures.

We document the 30 ka cumulative slip history and long-term slip vector azimuth on the northern Chelungpu fault based on a series of fault-bend folded alluvial terraces and draw quantitative relationships between geological structure, deformation observed from the geomorphology, and coseismic displacements during the 1999 Mw = 7.6 Chi-Chi earthquake. In our study area, three main terrace levels show progressive folding by kink band migration in relation to the underlying fault geometry, forming a main N-S fold scarp up to ~193 m high and secondary E-W scarps. Detailed analysis using 5 m resolution digital elevation model allows us to characterize the scarp morphology and quantify the deformation parameters, namely, terrace heights, fold scarp relief, and fold limb width and slope angle. The 3-D deformation of the highest terrace, dated by optically stimulated luminescence at 30.2 ± 4.0 ka, enables to simultaneously determine amplitude and azimuth of the long-term slip vector based on scarp relief. The long-term slip vector, oriented N338° ± 6°, is found to parallel the Chi-Chi coseismic displacements in this area. Cumulative slip and dating results yield a constant slip rate of 17.7 ± 2.2 mm/a in the direction N338° ± 6°. Late Quaternary shortening rates observed at four sites vary along strike in a similar way to Chi-Chi coseismic displacements. Together with the collinearity of long-term and coseismic slip vectors at our study site, this suggests that Chi-Chi earthquake is a characteristic earthquake for the Chelungpu thrust with recurrence interval ~470 years. We also discuss implications for the regional and long-term distribution of shortening in the central Western Foothills.

We used aerial photos taken before and after the 21 September 1999, M_w 7.6, Chi-Chi earthquake in central Taiwan to measure the near-field ground deformation. A total of 12 pairs of images were processed with Co-registration of Optically Sensed Images and Correlation to produce a horizontal displacement map of a 10 km × 10 km area near Tsaotun. Using pairs of images with different viewing angles, both the horizontal and vertical slip across the fault zone can be measured. Our measurements when resampled into lower resolution are consistent with lower resolution measurements of horizontal displacements obtained from SPOT images, as well as with vertical displacements obtained from repeated leveling measurements and field observations. Horizontal strain is strongly localized along the Chelungpu fault (CLPF) and along a secondary scarp that runs parallel to the CLPF about 2 km to the east, the Ailiao fold scarp (ALF). This pattern closely matches the surface ruptures mapped in the field. Horizontal strain across CLPF correlates remarkably well with the topographic features produced by long-term deformation. The cumulative horizontal shortening across the CLPF and ALF amounts to 4.9 ± 0.4 and 6.1 ± 0.6 m, respectively, and fault-parallel displacement is 3.4 ± 0.4 m. The pattern of surface strain is consistent with the interpretation of the ALF as a fold scarp formed over an active axial hinge zone. This study shows that, even in this compressional setting, most surface deformation is localized within narrow fault zones or active axial hinges.

In the last few decades optically stimulated luminescence (OSL) dating has become an important tool in geochronological studies. The great advantage of the method, i.e. dating the depositional age of sediments directly, can be impaired by incomplete bleaching of grains. This can result in a scattered distribution of equivalent doses (DE), leading to incorrect estimation of the depositional age. Thoroughly tested protocols as well as good data analysis with adequate statistical methods are important to overcome this problem. In this study, samples from young fluvial sand and flood plain deposits from the Elbe River in northern Germany were investigated to compare its depositional ages from different age models with well-known historical dates. Coarse grain quartz (100–200 μm and 150–250 μm) and polymineral fine grains (4–11 μm) were dated using the single aliquot regenerative (SAR) dose protocol. The paleodose (DP) was calculated from the DE data set using different approaches. Results were compared with the development of the Elbe River, which is well-documented by historical records and maps covering the last 1,000 years. Depending on the statistical approach it can be demonstrated that depositional ages significantly differ from the most likely depositional age. For the investigated coarse grain quartz samples all ages calculated from the MAM-3UL, including their uncertainties, are within the historical documented age. Results of the polymineral fine grain samples are overestimating the historically documented depositional age, indicating undetectable incomplete bleaching. This study shows the importance of using an adequate statistical approach to calculate reliable OSL ages from fluvial sediments.

Taiwan is considered the archetypical orogen in the development of critical wedge models of mountain building; however, the issue of how arc collision progressed along the margin remains poorly understood. To resolve this, the detrital archive of orogenesis preserved in Coastal Range rocks of eastern Taiwan was used to reconstruct the erosional response of arc collision. The spatial and temporal distribution of exhumation recorded in this detrital record is consistent with southwards progression of arc–continent collision in a punctuated rather than sequential manner with deposition confined to segmented foredeep basins. Apparent increases in sedimentation rates in Taiwan during the Pleistocene may not reflect increased erosion, but instead may be due to the collisional margin architecture. Supplementary materials: Analytical details and dating results are available at www.geolsoc.org.uk/SUP18693.

[1] Distributed activity of geomorphic processes with different spatiotemporal scales is hard to monitor in detail with conventional methods but might be detected with seismometers. From July to September 2010, we deployed 14 seismometers to evaluate the ability of a two-dimensional array with small interstation distances (11 km) to continuously monitor geomorphic processes in a mountain catchment (370 km2) in Taiwan. Spectral analysis of seismic records highlights different sources with high-frequency content (>1 Hz), consistent with hillslope and river processes. Using a common detection algorithm and a location technique based on the timing of seismic amplitude, we have located 314 near-surface events, most of which (69%) occurred during daily convective storms. Event activity was positively correlated with the precipitation intensity, but this relation was not uniform in the catchment. High-resolution satellite images and air photos did not show geomorphic change during the study, which did not have any episodes of extreme precipitation. A majority of events (61%) were collocated with preexisting geomorphic features (landslide scars, gullies) within the uncertainty on location (9% of interstation distance). The combination of event location and timing suggests a geomorphic source of recorded signals and most events had the seismic characteristics of rockfall, debris avalanches, or slides. Reactivation of prior erosion sites by such processes is difficult to detect with imagery, but can possibly be resolved by seismic monitoring. When proven, this approach will allow a spatially comprehensive survey of geomorphic activity at the catchment scale, with temporal detail sufficient to evaluate the exact (meteorological) conditions under which process events occur.

Late Quaternary shortening along the length of the Western Foothills of Taiwan highlights the tectonic segmentation of the foreland and raises questions about the relationship between erosion and the thickness of synorogenic foreland basin fill, and their influence on thrust kinematics. We compare measurements of shortening with geodetic observations and numerical model results, and relate these to regional topography. Predictions of shortening from numerical modeling and observed strain gradients within central Taiwan are generally similar in terms of their scaling and kinematic behavior. Within this framework, the current deformation field is likely related to the inheritance of older passive-margin structures in the foreland, as well as to the easily erodible nature of the 5-km-thick sequence of synorogenic sediment accreted at the leading edge of the orogen in the last 1.1 m.y. Additionally, available constraints on the timing of recent activity of faults suggest that infilling of piggy-back or wedge-top basins there occurred rapidly, contemporaneously with the activation of the frontal thrust and a possible increase in the rate of shortening across the adjacent thrust sheet toward the center of the orogen.

We discuss a radiocarbon study of sediment samples collected from Nanfu terrace in western Taiwan. From these, we extracted humic acids (HA) and humin from the very fine and coarse grain-size fractions using a standard acid-alkali-acid pretreatment. The humin extracts were combusted at 400 and 1100 °C by stepped-combustion, to yield a low-temperature (LT) carbon component and a high-temperature (HT) carbon component. We compare the ages of the LT and HT humin fractions to the HA fractions, in samples collected at 2 depths within the Nanfu terrace. As in previous stepped-combustion studies on sediments, we find that the HA ages are the youngest on average, and overlap the LT ages, and that the carbon contained in the HT fraction is always distinctly older than the LT and HA ages. To better understand the relationship between 14C age and combustion temperature, we conducted an incremental stepped-combustion experiment with one of the samples (1E) using 50 °C steps that ranged from 300 to 1100 °C. The 14C results of the stepped-combustion products show a clear division between 2 isotopically identifiable carbon constituents, from carbon released below 400 °C and carbon released above 550 °C. By comparing the δ13C and 14C results, we find evidence for a third carbon isotopic component in the humin that is released when combusted at ∼500 °C.

The 2008 Ms 8.0 Wenchuan earthquake of China ruptured two large faults along the Beichuan-Yingxiu thrust belt and the Pengguan thrust belt at the Longmen Shan Orogenic belt, eastern margin of the Tibetan Plateau. Our study focused on the structural geometry at the Pengguan thrust belt located at the frontal zone of the Central Longmen Shan. In this paper, in order to identify the co-seismic fault at the Pengguan thrust belt many 2-D/3-D seismic reflection profiles have been interpreted by integrating with the geological maps, drilling data, DEM data and the surface ruptures. The results show that there were three major faults and other branch faults at the Pengguan thrust belt. The major faults have formed as imbricates and associated minor duplexes within the shallow strata. The Pengguan fault has a steep dip at the shallow surface and a gentle dip at depth. Both the Beichuan-Yingxiu fault and the Pengguan fault were developed on the shallow detachment in the central segment of Longmen Shan. The third (No. ③) Pengguan fault is coincident with the co-seismic surface ruptures. It was a pre-existing fault and re-activated during the Wenchuan earthquake. Three-dimensional geometries show that the No. ③ fault is heterogeneous and it has controlled the slip of co-seismic ruptures. Based on the regional geology, the 3-D fault geometry and the surface ruptures, the frontal zone of the Central Longmen Shan has been divided into the Bailu segment and the Hanwang segment. Our studies suggest that the Pengguan fault has been active since the Cenozoic. The frontal zone of the Central Longmen Shan thrust belt has potential for future earthquakes.

The north- to northeast-trending Dien Bien Phu fault (DBPF) zone appears to the south of the Red River fault (RRF) zone, sharing the spatial alignment of the Xianshuihe–Xiaojiang fault (XXF) and extending over a distance of 150 km from Yunnan, China, through northwest Vietnam into Laos. Although the DBPF is one of the most conspicuous active fault systems in Indochina, it is less studied than the RRF and XXF, and its quantified kinematics remain mostly unknown. Our detailed modern fault trace mapping, compiled from topographic maps, stereographic aerial photos, ASTER satellite imageries, and field reconnaissance reveals new information on the fault geometry, the slip magnitude and distribution along the fault, and the relationship between river-channel offset and fault activity. The geometry of the modern DBPF is complex, consisting of single strands and stepovers. Abundant geomorphic expressions along the DBPF illustrate that the modern fault is dominated by sinistral motion and the present left-lateral component of motion is also clearly demonstrated by the existence of numerous rivers offsets. Multiple offsets of geomorphic features along the fault are recognized and reconstructed, and the largest sinistral displacement on the DBPF is ca. 12.5 km based on drainage network restoration. Because sinistral motion likely initiated approximately 5 Ma, the most probable Pliocene-to-present average slip rate on the DBPF is on the order of 2.5 mm/yr. Based on the slip rate, the DBPF could generate an earthquake with a magnitude greater than M w 7 and a recurrence interval of 500 to 1000 yr. The combined GPS velocity fields observed from northwest Vietnam and south China reveal approximately 2 to 3 mm/yr left-lateral slip across the DBPF, and significant east–west extension exists in the western crustal block of DBPF. Although the boundary fault system of the active crust rotation around the Eastern Himalayan Syntaxis (i.e., XXF) does not cut the RRF, the Pliocene-to-present activity along the DBPF favors the possibility that the tectonic shear has been transmitted across the RRF and taken up on the DBPF. The results of this study suggest that the modern DBPF zone acts as a reactivated fault, with a different slip sense from its previous phase, and plays a role as an eastern boundary of the crustal deformation in northern Indochina.

article i nfo Article history: We estimated the source rupture of the 2008 Wenchuan Earthquake (Ms 8), China, based on a back- projection of seismic waves to their source plane, using data from regional broadband arrays in Taiwan and northern Vietnam. Observations from these arrays, located at different azimuths, were processed to eval- uate the spatio-temporal rupture behavior of the fault. The seismic energy spot converted from windowed array waveforms was back-projected to the rupture plane of the earthquake, to image the instantaneous slip on the fault plane. Rupture processes were reconstructed based on the imaged time-dependent seismic energy radiating from the earthquake fault plane. The high station density of both arrays enabled a detailed examination of the rupture processes. The results indicate that the 2008 Wenchuan Earthquake had a rupture duration of approximately 100 s, with major asperities radiating energy at 50 s after the initiation of rupture. The location of asperities is in good agreement with sections of the earthquake fault along which major dam- age occurred, such as along the Yingxiu-Beichuan Fault. The radiated seismic energy shows a complex spatial distribution on the fault plane. Our analysis indicates that the rupture initiated at the epicenter and extended to the northeast for approximately 280 km. The average rupture velocity of this earthquake was approxi- mately 2.8 km/s.

The long-lived radionuclide

We have investigated the structure beneath the Tatun volcanic zone north of Taipei metropolitan area of Taiwan, area of five million people. We used the data collected from a seismic network deployed for 5 years over the volcanic zone. This plus another data from Taiwan regional networks allow us to carry out tomographic inversions for Vp, Vp/Vs and Qp structures beneath the Tatun volcanic zone. Based on our results and other geological, tectonic, and seismic findings, we reconstruct the structural evolution of the crust in the Tatun volcanic zone, and discuss the implication to the surrounding faults, fractured zones, and discuss potential future volcanic activities. From the tomographic results, there appear to exist a tube-shaped, highly fractured ancient magma passage with high seismic velocities that parallel to the Chinshan fault, and magma passage extends to the southeast at the depth about 20 km. This structure suggests plutonic intrusion passage beneath the Tatun volcano group that may have been associated with the earlier subduction of the Philippine Sea plate, melting of the subducted plate at depth has generated the magma intrusion that has brought about the Tatun volcanic activities. The high seismicity today also implies a highly fractured crust due to the hydrothermal activities and induced crustal stress. The hydrothermal fluid-rich upper crust as indicated by the low Vp/Vs ratio may have important bearing on the potential hazards associated with the two active faults cutting through both the Taipei Basin as well as the Tatun volcanic groups.