Earthquakes of the Nepal Himalaya : towards a physical model of the seismic cycle

Home to hundreds of millions of souls and land of excessiveness, the Himalaya is also the locus of a unique seismicity whose scope and peculiarities still remain to this day somewhat mysterious. Having claimed the lives of kings, or turned ancient timeworn cities into heaps of rubbles and ruins, earthquakes eerily inhabit Nepalese folk tales with the fatalistic message that nothing lasts forever. From a scientific point of view as much as from a human perspective, solving the mysteries of Himalayan seismicity thus represents a challenge of prime importance. Documenting geodetic strain across the Nepal Himalaya with various GPS and leveling data, we show that unlike other subduction zones that exhibit a heterogeneous and patchy coupling pattern along strike, the last hundred kilometers of the Main Himalayan Thrust fault, or MHT, appear to be uniformly locked, devoid of any of the “creeping barriers” that traditionally ward off the propagation of large events. The approximately 20 mm/yr of reckoned convergence across the Himalaya matching previously established estimates of the secular deformation at the front of the arc, the slip accumulated at depth has to somehow elastically propagate all the way to the surface at some point. And yet, neither large events from the past nor currently recorded microseismicity nearly compensate for the massive moment deficit that quietly builds up under the giant mountains. Along with this large unbalanced moment deficit, the uncommonly homogeneous coupling pattern on the MHT raises the question of whether or not the locked portion of the MHT can rupture all at once in a giant earthquake. Univocally answering this question appears contingent on the still elusive estimate of the magnitude of the largest possible earthquake in the Himalaya, and requires tight constraints on local fault properties. What makes the Himalaya enigmatic also makes it the potential source of an incredible wealth of information, and we exploit some of the oddities of Himalayan seismicity in an effort to improve the understanding of earthquake physics and cipher out the properties of the MHT. Thanks to the Himalaya, the Indo-Gangetic plain is deluged each year under a tremendous amount of water during the annual summer monsoon that collects and bears down on the Indian plate enough to pull it away from the Eurasian plate slightly, temporarily relieving a small portion of the stress mounting on the MHT. As the rainwater evaporates in the dry winter season, the plate rebounds and tension is increased back on the fault. Interestingly, the mild waggle of stress induced by the monsoon rains is about the same size as that from solid-Earth tides which gently tug at the planets solid layers, but whereas changes in earthquake frequency correspond with the annually occurring monsoon, there is no such correlation with Earth tides, which oscillate back-and-forth twice a day. We therefore investigate the general response of the creeping and seismogenic parts of MHT to periodic stresses in order to link these observations to physical parameters. First, the response of the creeping part of the MHT is analyzed with a simple spring-and-slider system bearing rate-strengthening rheology, and we show that at the transition with the locked zone, where the friction becomes near velocity neutral, the response of the slip rate may be amplified at some periods, which values are analytically related to the physical parameters of the problem. Such predictions therefore hold the potential of constraining fault properties on the MHT, but still await observational counterparts to be applied, as nothing indicates that the variations of seismicity rate on the locked part of the MHT are the direct expressions of variations of the slip rate on its creeping part, and no variations of the slip rate have been singled out from the GPS measurements to this day. When shifting to the locked seismogenic part of the MHT, spring-and-slider models with rate-weakening rheology are insufficient to explain the contrasted responses of the seismicity to the periodic loads that tides and monsoon both place on the MHT. Instead, we resort to numerical simulations using the Boundary Integral CYCLes of Earthquakes algorithm and examine the response of a 2D finite fault embedded with a rate-weakening patch to harmonic stress perturbations of various periods. We show that such simulations are able to reproduce results consistent with a gradual amplification of sensitivity as the perturbing period get larger, up to a critical period corresponding to the characteristic time of evolution of the seismicity in response to a step-like perturbation of stress. This increase of sensitivity was not reproduced by simple 1D-spring-slider systems, probably because of the complexity of the nucleation process, reproduced only by 2D-fault models. When the nucleation zone is close to its critical unstable size, its growth becomes highly sensitive to any external perturbations and the timings of produced events may therefore find themselves highly affected. A fully analytical framework has yet to be developed and further work is needed to fully describe the behavior of the fault in terms of physical parameters, which will likely provide the keys to deduce constitutive properties of the MHT from seismological observations.

Variations in northeast Asian environments over the last 350,000 years reconstructed from pollen records from the northwest Pacific Ocean

EXTRACT (SEE PDF FOR FULL ABSTRACT): Recent analyses of terrestrial (pollen) and marine microfossils (foraminifera and radiolaria) in cores V28-204 and RC14-99 from the northwest Pacific Ocean extend the continuous, chronostratigraphically-controlled records of the regional vegetation of the Pacific coast of Japan and offshore marine environments through three full glacial cycles. The high-resolution pollen time series show systematic relationships between fluctuations in Japanese vegetation and global ice volume over the last 350 kyr. ... Comparison with solar insolation at 30°N and with an index of orbital parameters suggests that variation in northeast Asian summer monsoon intensity is related to orbital forcing.

Palaeoenvironmental changes from pollen record in deep sea core PC-1 from northern Okinawa Trough, East China Sea during the past 24 ka

A pollen record of core PC-1 from the northern Okinawa Trough, East China Sea (ECS), provides information on vegetation and climate changes since 24 cal. kaBP. A total of 103 samples were palynologically analyzed at 8 cm intervals with a time resolution of 230 a. Four pollen zones are recognized: zone I (812-715 cm, 24.2-21.1 cal. kaBP), zone II (715-451 cm, 21.1-15.2 cal. kaBP), zone III (451-251 cm, 15.2-10.8 cal. kaBP), zone IV (251-0 cm, 10.8-0.3 cal. kaBP), corresponding to Late MIS 3, Last Glacial Maximum (LGM), deglaciation and Holocene, respectively. The LGM is characterized by the dominance of herbs, mainly Artemisia, and high pollen influx, implying an open vegetation on the exposed continental shelf and a cool and dry climate. The deglaciation is a climate warming stage with Pinus percentage increased and Artemisia percentage decreased and a rapid sea-level rise. The Holocene is characterized by predominance of tree pollen with rapid increase in Castanea-Castanopsis indicating the development of mixed evergreen and deciduous broad-leaved forest and a warm, humid climate. Low pollen influx during the Holocene probably implies submergence of the continental shelf and retreat of the pollen source area. The vegetation indicated by pollen assemblage found in this upper zone is consistent with the present vegetation found in Kyushu, Japan. Originating from the humid mountain area of North Luzon of the Philippines, Tasmania and New Zealand, Phyllocladus with sporadic occurrence throughout PC-1 core probably suggests the influence of Palaeo-Kuroshio Current or intense summer monsoon. The observed changes in Pinus and Herbs percentage indicate fluctuations of the sea level, and high Pinus percentage corresponds to high sea level. Spectrum analysis of the pollen percentage record reveals many millennial-scale periodicities, such as periodicities of 6.8, 3.85 2.2, 1.6 ka.

Cold event at 5 500 a BP recorded in mud sediments on the inner shelf of the East China Sea

A 700-year record (1.0-1.5 a resolution) of the East Asian winter monsoon (EAWM), based on grain-size analysis and AMS(14)C dating of Core EC2005 from the inner-shelf mud wedge of the East China Sea (ECS), was compared with the Dongge stalagmite delta O-18 record during the mid-Holocene. The upper muddy section of Core EC2005 has been formed mainly by suspended sediments derived from the Changjiang (Yangtze) River mouth since 7.3 ka BP. High precipitation and a strengthened EAWM might have played key roles in the high sedimentation rate (1 324-1 986 cm/ka) between 5.9-5.2 ka BP. The EAWM strengthened when the Asian summer monsoon weakened, especially around 5 500 a BP, which corresponded to a worldwide cold event. The EAWM during the mid-Holocene shows statistically significant solar periodicities at 62 and 11 a. The 5 500 a BP cold event might be resulted from orbital forcing and changes in solar activity.

The Shandong mud wedge and post-glacial sediment accumulation in the Yellow Sea

Two well-defined deltaic sequences in the Bohai Sea and in the South Yellow Sea represent post-glacial accumulation of Yellow River-derived sediments. Another prominent depocenter on this epicontinental shelf, a pronounced clinoform in the North Yellow Sea, wraps around the northeastern and southeastern end of the Shandong Peninsula, extending into the South Yellow Sea. This Shandong mud wedge is 20 to 40 m thick and contains an estimated 300 km(3) of sediment. Radiocarbon dating, shallow seismic profiles, and regional sea-level history suggest that the mud wedge formed when the rate of post-glacial sea-level rise slackened and the summer monsoon intensified, at about 11 ka. Geomorphic configuration and mineralogical data indicate that present-day sediment deposited on the Shandong mud wedge comes not only from the Yellow River but also from coastal erosion and local rivers. Basin-wide circulation in the North Yellow Sea may transport and redistribute fine sediments into and out of the mud wedge.

PALEOMONSOONS OF CHINA OVER THE LAST 130,000 YEARS - PALEOMONSOON RECORDS

The characteristics of the modern monsoon climate of China may be used as clues for recognizing the records of paleomonsoon climate. The present paper deals primarily with the various paleomonsoon records of the last 130,000 years in the southeast monsoon area. These records mainly come from the following three fields: (i) the historical, (ii) the geological, including loess-paleosol sequence, deserts, lakes, snowlines, timberlines, the phenomena of continental desertization and so on, and (iii) the biological, presented by vegetation a.d mammals. Among these records, the loess-paleosol sequence in the Loess Plateau reflects a climatic history characterized by alternation of two different climatic periods when the Asian winter monsoon and summer monsoon showed pronounced effects on environment, respectively.

PALEOMONSOONS OF CHINA OVER THE LAST 130,000 YEARS - PALEOMONSOON VARIATION

Based upon the effect of land-sea interaction on the paleomonsoon variation and the time series of climatic proxy-indicators, the historical Asian monsoon variation over the last 130,000 and 18,000 years has been reconstructed with an emphasis on the basic characteristics of summer monsoon circulation. The monsoon-climatic cycles and associated model of environmental development over the central and eastern China are proposed and the mechanism of paleomonsoon variation of China preliminarily discussed. The variation of East Asian monsoon circulation should be regarded as a regional result of both solar-radiation changes and the global glacial-interglacial cycles. The episodic uplifting of the Qinghai-Xizang Plateau since the late Miocene has to a large extent controlled the forming and evolution of the paleomonsoon circulation of China.

Responses of summertime extreme wave heights to local climate variations in the East China Sea

We detected the responses of summertime extreme wave heights (H-top10, average of the highest 10% of significant wave heights in June, July and August) to local climate variations in the East China Sea by applying an empirical orthogonal function analysis to Htop10 derived from the WAVEWATCH- III wave model driven by 6 hourly sea surface wind fields from ERA-40 reanalysis over the period 1958-2002. Decreases in H-top10 in the northern East China Sea ( Yellow Sea) correspond to attenuation of the East Asian Summer Monsoon, while increases in the south are primarily due to enhancement of tropical cyclone activities in the western North Pacific.

Intraseasonal variability of Indian Ocean sea surface temperature during boreal winter: Madden-Julian Oscillation versus submonthly forcing and processes

[ 1] Intraseasonal variability of Indian Ocean sea surface temperature (SST) during boreal winter is investigated by analyzing available data and a suite of solutions to an ocean general circulation model for 1998 - 2004. This period covers the QuikSCAT and Tropical Rainfall Measuring Mission (TRMM) observations. Impacts of the 30 - 90 day and 10 - 30 day atmospheric intraseasonal oscillations (ISOs) are examined separately, with the former dominated by the Madden-Julian Oscillation (MJO) and the latter dominated by convectively coupled Rossby and Kelvin waves. The maximum variation of intraseasonal SST occurs at 10 degrees S - 2 degrees S in the wintertime Intertropical Convergence Zone (ITCZ), where the mixed layer is thin and intraseasonal wind speed reaches its maximum. The observed maximum warming ( cooling) averaged over ( 60 degrees E - 85 degrees E, 10 degrees S - 3 degrees S) is 1.13 degrees C ( - 0.97 degrees C) for the period of interest, with a standard deviation of 0.39 degrees C in winter. This SST change is forced predominantly by the MJO. While the MJO causes a basin-wide cooling ( warming) in the ITCZ region, submonthly ISOs cause a more complex SST structure that propagates southwestward in the western-central basin and southeastward in the eastern ocean. On both the MJO and submonthly timescales, winds are the deterministic factor for the SST variability. Short-wave radiation generally plays a secondary role, and effects of precipitation are negligible. The dominant role of winds results roughly equally from wind speed and stress forcing. Wind speed affects SST by altering turbulent heat fluxes and entrainment cooling. Wind stress affects SST via several local and remote oceanic processes.

海气热通量与东亚夏季风的关系

本文采用基于风切变的季风指数确定了多年季风爆发的时间；使用实测资料计算分析了2008年南海季风爆发前后海气通量的特征；基于COARE3.0算法，用NCEP2中海气要素再分析资料计算了海气热通量场并与NCEP2中原始热通量场进行了比较；利用EOF方法得到季风爆发早晚年份海气热通量场的时空特征；利用SVD方法分析了热通量场及海温场与季风的关系，初步探讨了海气热通量的变化影响季风爆发的过程和机理。结果表明： 1、2008年南海夏季风爆发期间热带气旋对海气要素的影响较大。动量交换系数与热量交换系数是风速的函数，曲线在风速为4m/s时有一个转折的过程。 2、海温的变化超前于季风爆发时间和强度的变化；热通量的变化超前于海温的变化。热通量通过海温这一中间过程对季风产生作用。 3、季风爆发时间和强度的变化受前期2至3个月时黑潮区域热通量变化的影响。此海域的热通量较大的时候，其后的季风爆发偏早、偏强；反之，季风爆发偏晚、偏弱。

西太平洋—印度洋—青藏高原气候系统在南海夏季风爆发过程中的作用

本文利用美国国家环境预测中心和国家大气研究中心(NCEP/NCAR—National Centers for Environmental Prediction/National Center for Atmospheric Research)的位势高度、气温、风速等大气资料、欧洲中期天气预报中心 （ECMWF—European Centre for Medium-Range Weather Forecasts—ERA-40）的雪深资料、美国国家海洋大气管理局（NOAA—National Oceanic and Atmospheric Administration）的海表温度（SST）资料、美国Scripps海洋研究所的上层海洋热含量资料等，采取相关分析、合成分析、经验正交函数分析、小波分析和小波交叉谱分析等统计分析方法，系统深入地讨论了西太平洋—印度洋—青藏高原气候系统在南海夏季风爆发过程中的作用。得到的主要结论如下： 1. 西太平洋和印度洋在南海夏季风爆发过程中起着关键作用 利用1951－1998年多种大气海洋资料，分析研究结果表明，西太平洋（暖池热含量）、印度洋（纬向风）在南海夏季风爆发中起关键的调控作用：以1970年为界，1970年之前，印度洋起主要作用，1970年之后西太平洋起主要作用，这主要是1970前后北极涛动有明显跃变的原因，这种跃变决定了印度洋在南海夏季风爆发中是否起决定作用（西风异常或东风异常），进而，决定了有利于或不利于南海夏季风的爆发。 1970年之前，北极涛动指数为负值，海陆温差（海上气温减大陆气温）是负值，大陆气温偏高，印度洋气温相对偏低，印度洋出现西风异常，有利于南海夏季风早爆发。在此期间，与印度洋SST密切相关的南印度洋偶极子的变化也与南海夏季风的爆发紧密相联。当南印度洋为正偶极子（西南印度洋SST为正异常，印度洋其他区域的SST为负异常）时，北印度洋为西风异常，南海夏季风爆发偏早；南印度洋为负偶极子（西南印度洋SST为负异常，印度洋其他地区的SST为正异常）时，北印度洋为东风异常，南海夏季风爆发偏晚。 1970年之后，北极涛动指数为正值，海陆气温差为正值，印度洋的状态不利于南海季风爆发；在这种情况下，西太平洋暖池的热含量则成为控制南海夏季风爆发的主要原因：暖池变暖的年份，即 La Niña 年，南海夏季风爆发早（强），反之，当暖池变冷的年份，即El Niño年，南海季风爆发晚（弱），即，南海夏季风爆发的早（强）晚（弱）与ENSO事件密切相关。 2.青藏高原春季积雪对南海夏季风爆发有重要的影响 1958-2003年青藏高原3月积雪厚度与南海夏季风爆发时间存在着很好的正相关。青藏高原3月积雪厚度偏厚时，其500毫巴以上的气温偏低，上层海陆之间的气温差是正值，南亚高压向西北方向的移动速度变慢，上层东风偏弱，西太平洋地区的上层辐散和下层辐合变弱，西太平洋暖池热含量偏少，南海夏季风爆发偏晚（弱）。同时，下层850毫巴东印度洋异常大气是东风和跨赤道反气旋对，南海被东风异常所控制，这种大气环流形势不利于南海夏季风的爆发；青藏高原3月积雪厚度偏薄时，其500毫巴以上的气温偏高，上层海陆之间的气温差是负值，上层南亚高压在南亚地区建立较早，上层东风偏强，西太平洋地区的上层辐散和下层辐合偏强，西太平洋暖池热含量偏多，南海夏季风爆发偏早（强）。同时，下层850毫巴东印度洋低层大气是西风异常和跨赤道气旋对，南海被西南风异常所控制，有利于南海夏季风的爆发。 研究结果还表明，青藏高原春季的积雪与厄尔尼诺事件存在着密切的关系。在厄尔尼诺鼎盛期的冬季，各种条件都有利于青藏高原的降雪，从而，来年春天的积雪则变厚，不利于南海季风的爆发。 3. 南海夏季风爆发的预测 1970年之后，西太平洋暖池的热含量与南海夏季风的爆发早晚有非常好的负相关。据此，我们可以通过西太平洋暖池热含量的变化来预测南海夏季风的爆发。通过暖池区海洋上层400米热含量的分析研究，我们找到了西太平洋暖池热含量变化的代表站点（以3N，138E为中心的1°×1°范围），其热含量变化能很好代表整个西太平洋暖池热含量的变化（相关系数大于0.85）。在此基础上，文章用1993-2007年热带大气海洋浮标列阵（TAO-Tropical Atmosphere Ocean-array）中最靠近该站点的浮标（2N, 137E）资料验证了上述选择站点的代表性和相应的预测能力。1993-2004年TAO浮标（2N, 137E）3月上层400米和500米海洋热含量与南海夏季风爆发时间的相关系数分别是-0.75，-0.73，置信度均超过99%；用1993-2007年4月份TAO浮标（2N, 137E）上层400米和500米海洋热含量与南海夏季风爆发时间作相关则相关系数均为-0.83，置信度超过99%。因此，我们可以通过3月或者4月份该TAO浮标（2N, 137E）的热含量来预测当年南海夏季风爆发的早（强）晚（弱）。 总之，南海夏季风爆发以1970年为界存在明显的年代际变化，1970年之前，主要受印度洋控制，1970年之后，南海夏季风爆发主要受控于太平洋（西太平洋暖池），这种变化是由北极涛动年代际变化引起的，。青藏高原春季积雪也对南海夏季风有重要影响，但主要受ENSO控制。因此，我们认为西太平洋—印度洋—青藏高原气候系统在南海夏季风爆发中起着重要的调控作用：西太平洋的作用当属第一位，印度洋的作用居第二，青藏高原的作用最弱。

大气季节内振荡对东亚夏季风活动的影响研究

利用ERA40逐日再分析资料、NCEP/NCAR2逐日再分析资料、中国740个测站日降水资料、上海台风研究所提供的西太平洋热带气旋资料、Kaplan等重建的月平均SSTA资料、NOAA逐日长波辐射（OLR）等资料，应用离散功率谱分析、带通滤波、EOF分析等统计方法，研究了东亚夏季风(EASM）的移动特征、东亚地区季节内振荡(ISO)的基本特征、季节内振荡对东亚夏季风活动的影响、季节内振荡对东亚夏季风异常活动的影响机理。主要结论如下： （1）综合动力和热力因素定义了可动态描述东亚夏季风移动和强度的指数，并利用该指数研究了东亚夏季风的爆发和移动的季节内变化及其年际和年代际变化特征。研究发现，气候平均东亚夏季风前沿分别在28候、33候、36候、38候、40候、44候出现了明显的跳跃。东亚夏季风活动具有显著的年际变率，主要由于季风前沿在某些区域异常停滞和突然跨越北跳或南撤引起，造成中国东部旱涝灾害频繁发生。东亚夏季风的活动具有明显的年代际变化，在1965年、1980年、1994年发生了突变，造成中国东部降水由“南旱北涝”向“南涝北旱”的转变。 （2）东亚季风区季节内变化具有10~25d和30~60d两个波段的季节内振荡周期，以30-60d为主。存在三个主要低频模态，第一模态主要表征了EASM在长江中下游和华北地区活动期间的低频形势；第二模态印度洋-菲律宾由低频气旋式环流控制，主要表现了ISO在EASM爆发期间的低频形势；第三模态主要出现在EASM在华南和淮河活动期间的低频形势。第一模态和第三模态是代表东亚夏季风活动异常的主要低频形势。 （3）热带和副热带地区ISO总是沿垂直切变风的垂直方向传播。因此，在南海-菲律宾东北风垂直切变和副热带西太平洋北风垂直切变下，大气热源激发菲律宾附近交替出现的低频气旋和低频反气旋不断向西北传播，副热带西太平洋ISO以向西传播为主。中高纬度地区，乌拉尔山附近ISO以向东、向南移动或局地振荡为主；北太平洋中部ISO在某些情况下向南、向西传播。 （4）季风爆发期，伴随着热带东印度洋到菲律宾一系列低频气旋和低频反气旋， 冷空气向南输送，10~25天和30~60天季节内振荡低频气旋同时传入南海加快了南海夏季风的爆发。在气候态下，ISO活动表现的欧亚- 太平洋（EAP）以及太平洋-北美（PNA）低频波列分布特征(本文提出的EAP和PNA低频波列与传统意义上的二维定点相关得到的波列不同)。这种低频分布形式使得欧亚和太平洋中高纬度的槽、脊及太平洋副热带高压稳定、加强，东亚地区的低频波列则成为热带和中高纬度ISO相互作用影响东亚夏季风活动的纽带。不同的阶段表现不同的低频模态，30~60d低频模态的转变加快了EASM推进过程中跳跃性；30-60d低频模态的维持使得EASM前沿相对停滞。 （5）30-60d滤波场，菲律宾海域交替出现的低频气旋和低频反气旋不断向西北传播到南海-西太平洋一带。当南海-西太平洋地区低频气旋活跃时，季风槽加强、东伸，季风槽内热带气旋（TC）频数增加；当南海-西太平洋低频反气旋活跃时，季风槽减弱、西退，TC处于间歇期，生成位置不集中。 （6）在El Nino态下，大气季节内振荡偏弱，北传特征不明显，但ISO由中高纬度北太平洋中部向南和副热带西太平洋向西的传播特征显著，东亚地区ISO活动以第三模态为主，EASM集中停滞在华南和淮河流域，常伴随着持续性区域暴雨的出现，易造成华南和江淮流域洪涝灾害，长江和华北持续干旱。在La Nina态下，大气季节内振荡活跃，且具有明显的向北传播特征，PNA低频波列显著，东亚地区ISO活动以第一模态单峰为主；EASM主要停滞在长江中下游和华北地区，这些地区出现异常持续强降水，华南和淮河流域多干旱；在El Nino态向La Nina态转换期，ISO活动以第一模态双峰为主，长江中下游常常出现二度梅。

三门峡宋家店黄土－古土壤序列的地层学研究及其古环境记录

The Sanmen Gorge area is located in the southernmost margin of the Chinese Loess Plateau with well developed eolian deposit sequence for the past 2.6 Ma, providing a key site for further understanding of the evolution history of the East Asian monsoon since late Pliocene. This study attempted to characterize the stratigraphy and paleoclimate record of the loess-paleosol sequence in the Songjiadian section. The work involved includes systematic field investigation, paleomagnetic and rock magnetic analyses, grain size and major chemical composition analyses, and multiple proxy measurements of magnetic susceptibility, color reflectance and the ratio of CBD-dissolvable iron to the total iron (FeD/FeT). By comparisons of the Songjiadian section with well studied loess sections in the west of the Sanmen Gorge, the spatial variations of the East Asian monsoon was evaluated for some periods during which typical loess or paleosols developed. The following conclusions have been obtained. 1. Stratigraphic correlation and paleomagnetic result demonstrate that the loess-paleosol sequence in the Songjiadian section was accumulated from 2.6Ma, and is generally a complete and continuous loess sequence. However, notable differences from type loess sections have been identified for a few loess and paleosol units, featured by absence or anomalous thickness in the Songjiadian section. 2. Magnetic susceptibility and chromaticity records clearly reveal the loess-paleosol cycles, and indicate that the Sanmen Gorge area has been warmer and more humid than the Lingtai and Jingchuan sections in the western central Loess Plateau since the Early Pleistocene. 3. Grain size distribution patterns are typical of eolian dust, and show a great similarity between various units of loess and paleosols, and between the S32 and the underlying Red Clay through the Songjiadian profile, suggesting the eolian origin for the loess, paleosols and the Red Clay. 4. Comparison of the FeD/FeT curves from different loess sections indicates a stronger chemical weathering in the Songjiadian section and notable enhancement around 1800, 800 and 600 ka BP, implying the strengthening of the East Asian monsoon during these periods. In contrast, it was weakened at 1100 ka BP. Generally, the summer monsoon shows a gradually decreasing trend during the entire Pleostocene, but the spatial pattern typified by an increasing trend in weathering intensity from north to south remained the same. 5. The loess unit L9 in the Songjiadian section displays two geomagnetic field anomalies with the midpoint ages of 0.917 and 0.875 Ma respectively, with a segment of 12 ka. They are demonstrated to be equivalent to the Santa Rosa and Kamikatsura geomagnetic excursions. 6. Magnetite is the main magnetic carrier for both loess and paleosols. Maghemite concentration is higher in paleosols than in loess, and is an important carrier for the enhanced magnetic susceptibility in paleosols. Magnetic fabric analysis suggests a dominant N-S wind direction prevailing in the L9 and L15, while the summer winds were dominantly in NNE-SSW direction during the S8 period, notably differing from previous studies.