Tectonophysics 584 (2013) 1–6

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Preface

Great earthquakes in the 21st century and geodynamics of the Tibetan Plateau

Pei-Zhen Zhang a,⁎, Eric Robert Engdahl b

a State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, Chinab Department of Physics, University of Colorado, Boulder, CO 80309, USA

⁎ Corresponding author.E-mail address: peizhen@ies.ac.cn (P.-Z. Zhang).

0040-1951/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.tecto.2012.11.001

a b s t r a c t

a r t i c l e i n f o

Available online 7 November 2012

Keywords:Wenchuan earthquakeTibetan PlateauGeodynamicsEarthquake disaster reduction

What are the geodynamic processes that caused these deadly earthquakes? Why have these earthquakescaused so much damage? What are the key lessons that we have learned from these devastating earth-quakes? Answers to these questions will significantly enhance not only our understanding of earthquakeoccurrence but also our ability to reduce seismic hazard. Under the framework of bi-lateral cooperation onearthquake sciences between China and USA, the Second Bi-Lateral Workshop on Earthquake Sciences washeld in Chengdu, Sichuan Province, China, from April 22 to 25, 2011. Among the goals of this workshopwas a review of recent advances in the study of great earthquakes and the exchange of ideas on earthquakedisaster reduction. The principle theme of the workshop was “Great Earthquakes in the 21st Century andGeodynamics.” This Special Issue contains a total of 24 papers presented during the workshop. The contribu-tions cover a wide-range of topics associated with the theme. This preface summarizes the main points of thepapers presented in this issue.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The 21st century began with numerous great earthquakes, resultingin great casualties, economic losses, and social disturbances. The 2001Kunlunmountain earthquake (Ms=8.1) raised the curtain on this periodof vigorous earthquake activity. The 2004 to 2005 Sumatra earthquakesand associated tsunami caused more than 240,000 deaths. The 2008Wenchuan earthquake killed more than 80,000 people and resulted inmore than a billion dollars in economic losses (Liu-Zeng et al., 2009;Xu et al., 2009; Zhang et al., 2010). In January 12, 2010, an earthquakeof magnitude only Mw=7.0 occurred in Haiti and caused more than223,000 casualties (Bilham, 2010). A little more than a month later,in February 27, 2010, another great earthquake shook the Earth witha magnitude of Mw=8.8 in Chile (Madariaga et al., 2010; Moreno etal., 2010). The 14 April 2010 Mw=6.9 Yushu earthquake occurred~400 km west of the 2008 Wenchuan earthquake in central Tibetwhich killed more than 2000 people (Chen et al., 2010). In 11 March2011, a great Tohoku-Oki earthquake of Mw=9.0 struck Japan, andthe associated tsunami damaged the Nuclear Power Plants that in turncaused a world-wide crisis of nuclear radiation leakage (Kato et al.,2012; Simons et al., 2011).

What are the geodynamic processes that caused these deadlyearthquakes? Why have these earthquakes caused so much damage?What are the key lessons that we have learned from these devastatingearthquakes? Answers to these questions will significantly enhance

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not only our understanding of earthquake occurrence but also ourability to reduce seismic hazard. Under the framework of bi-lateralcooperation on earthquake sciences between China and USA, theSecond Bi-Lateral Workshop on Earthquake Sciences was held inChengdu, Sichuan Province, China, from April 22 to 25, 2011. Amongthe goals of this workshop was a review of recent advances in thestudy of great earthquakes and the exchange of ideas on earthquakedisaster reduction. The principle theme of the workshop was “GreatEarthquakes in the 21st Century and Geodynamics.”

2. Workshop of great earthquakes in the 21st centuryand geodynamics

Theworkshopwaswell attended by 43 participants from the UnitedStates and 63 from China. A two-day pre-workshop field trip wasconducted to visit surface ruptures associatedwith the 2008Wenchuanearthquake. The presentations were organized into two main themes.

2.1. Great earthquakes in the 21st century

Great earthquakes during the first decade of the 21th century havekilled more than 500,000 people and caused tremendous economicloss. These earthquakes differ from each other in terms of their tectonicenvironment and geodynamic processes, yet there are also some com-mon features in both the rupture processes and the disaster scenarios.The complex thrust faulting associated with the 2008Wenchuan earth-quake, for example, appears to differ substantially from the strike-slipfaulting of the Haiti earthquake. Despite its lower magnitude, the Mw7.0 Haiti earthquake causedmore casualties than theMs 8.0Wenchuan

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earthquake. The 2010 Yushu earthquake occurred in a remote andsparsely populated region of the interior of Tibet, but also caused signif-icant human and property losses. Intensive studies following thesegreat earthquakes have resulted in abundant observational data andnew insights into the geodynamic processes of these earthquakes;comparisons of these studies will help to improve our ability to reduceseismic losses in future great earthquakes. Recent advances in newtechnologies such as GPS, InSAR, seismic array and space-based obser-vations, among others, have extended our ability to monitor crustalmotion, strain accumulation, and the behavior of seismogenic faults as-sociated with devastating earthquakes at various timescales. Advancesin laboratory and numerical simulation have also enabled important in-sights to the mechanics of rupture initiation, propagation and arrest ofearthquake ruptures. This theme focused on, but was not restricted tothe following topics:

■ Tectonic background of great earthquakes in both interplate andintraplate settings;

■ Coseismic surface ruptures and displacements associated witheach of these great earthquakes;

■ Earthquake rupturing processes constrained by seismological andgeodetic data;

■ Active tectonics, paleoseismology and seismic hazard assessment.

2.2. Continental deformation: structure, deformation, and geodynamics

Great earthquakes in continental interiors are generallymore devas-tating to human life and social development than oceanic earthquakes,but their causes are poorly understood. In contrast to localized defor-mation along oceanic plate boundaries, continental earthquakes and ac-tive faults are distributed in a vast region with variety of tectonic styles,and their spatial and temporal occurrence, as shown in China andCentral Asia, are much more irregular than those of plate boundaryearthquakes. Such irregularity, to a large extent, may be attributed tothe heterogeneous physical properties of continental crustal and litho-sphere, which are usually thick and weak, with layered rheologicalstructure, distributed fault systems, and relatively low rate of loading.To better explore the style, geometry, rate, kinematics and dynamicsof continental tectonics and earthquakes, this theme focused on the fol-lowing topics:

■ Fundamental differences between plate boundary earthquakesand those in diffuse plate boundary zones (such as western US,Tibetan Plateau) and in mid-continent's (such as North Chinaand central-eastern US);

■ Spatial correlation between continental seismicity with crust,sub-crustal lithosphere, and upper mantle structures in China,US, and elsewhere

■ Relationship between the style and rate of crustal deformationand rheological properties of continental lithosphere;

■ Growth, uplift, and dynamic process of the Tibetan Plateau andCentral Asia;

■ Patterns of continental deformation revealed by active faults,earthquake activity, seismological and GPS observations, and ter-restrial laser scanning.

3. This volume

This Special Issue contains a total of 24 papers presented duringthe workshop. The contributions cover a wide-range of topics associ-ated with the two themes. Below we summarize the main points ofpapers presented in this issue.

The 2008 Wenchuan earthquake is no doubt the central issue ofthe workshop, not only because this event caused such huge casual-ties and economic loss, but also because the seismogenic LongmenShan fault slips slowly and had been assigned a modest seismic risk(Burchfiel et al., 2008; Densmore et al., 2007; Zhang et al., 2010).

For the Special Issue Pei-Zhen Zhang was invited to write a reviewpaper on the tectonic and geodynamic background of the 2008Wenchuan earthquake. Through integrated studies on active faults,GPS crustal deformation, and geophysical structure, Zhang (2013–thisissue) shows that deformation in the Western Sichuan is governed byinteractions among three crustal blocks (Songpan, Chuandian, andSouth China) of distinct rheological properties under a tectonic frame-work with the eastward growth of the “soft” Eastern Tibet beingblocked by the “hard” lithosphere of the South China block. The uppercrust of the three blocks is dominated by brittle deformation, whereasthe ductile flow of the lower crust would cause a dragging movementbetween the brittle upper crustal blocks. The relative motions amongthe brittle upper crustal blocks cause strain accumulations among theirbounding faults to generate large earthquakes. The 2008 Wenchuanearthquake is a consequence of these geodynamic processes. Deforma-tion of the Western Sichuan region can thus be described in terms ofcombined model of rigid block movement and continuous deformation.

3.1. The 2008 Wenchuan earthquake

Unlike papers in the Special Issue of Tectonophysics on the 2008Wenchuan earthquake in 2010 (Yin, 2010) which mainly reportsearly studies on coseismic deformation and structures of the LongmenShan fault, 8 papers in this volume on the 2008Wenchuan earthquakereport follow-up studies on the mechanism and paleoseismology ofthis great event.

TheWenchuan earthquake Fault Scientific Drilling (WFSD) startedjust 178 days after the 2008 Ms 8.0 earthquake. Li et al. (2013–thisissue) reported initial results from the first hole (WFSD-1) at a finaldepth of 1201.15 m. They find that Principle Slip Zone (PSZ) ofthe 2008 Wenchuan earthquake located at ~590 m-depth with1 cm-wide fresh fault gouge, as indicated by logging data such astemperature, natural gamma ray, p-wave velocity and resistivity,combined with the fresh appearance, magnetic susceptibility, and mi-crostructure of the gouge. The Wenchuan earthquake slip plane has adip angle of ~65° above a depth of ~590 m. There are at least 12 faultzones in the entire core profile, including the Yingxiu–Beichuan faultzone, with a minimum fault zone width of ~100 m. The distributionof fault gouge that is several meters thick, the location of theWenchuan earthquake's PSZ, and the thickness of fresh gouge, allimply a correlation between the width of the fault zone and the num-ber of seismic events.

To study the pre-earthquake background stress in the region of theWenchuan earthquake, Luna and Hetland (2013–this issue) used aBayesian probabilistic estimation to find that the coseismic slip of theWenchuan earthquake is consistent with a constant orientation of prin-cipal stresses along the strike of the Longmen Shan fault zone. The in-ferred maximum compressive stress direction is sub-horizontal andapproximately east–west trending. The intermediate compressivestress is sub-horizontal and north–south trending, and is most likelyabout 30% the magnitude of the most compressive stress. The leastcompressive stress is near-vertical.

Using a numerical simulation approach, Rohrbach et al. (2013–thisissue) verified the seismogenesis-associated seismic velocity and atten-uation variations in ambient noise measurements. First, they dividedthe seismogenic process into six phases. Then, using a finite differencetime domain method they generated 30-minute low frequency ambi-ent noise over a vertical profile of 200×45 km and recorded it with a90-station array on the surface. Next, they processed the synthetic am-bient noise records to get the auto-correlation function (ACF),cross-correlation function (CCF), Rayleigh wave dispersion curve, andhorizontal-to-vertical spectral ratio (H/V). Finally, they examined thetemporal variation of these parameters versus the phases of theseismogenic process and found the most pronounced changes occurin the phase with the largest velocity drop and the recovery phasedirectly before the mainshock.

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To answer the question why the great Wenchuan earthquake oc-curs on such a slow slipping fault as the Longmen Shan, Zhu andZhang (2013–this issue) simulated inter- and coseismic deformationwithin one seismic cycle using a visco-elastic finite element method.The model results show that, in order to fit the observed interseismicdeformation, a “soft” lower crust and upper mantle beneath EasternTibet and a very strong lithosphere of the Sichuan basin are needed.The computed strain rates accumulate much faster in the lower crustof the eastern Tibet than that in other regions during interseismicperiod. The “soft” lower crust in the eastern Tibet and the “strong”Sichuan basin do not accumulate high elastic strain energy. However,the rapid ESED accumulation takes place within the upper crustaround the Longmen Shan fault zone, which plays an important rolein generation of the Wenchuan earthquake. They also show that slipaccelerations on the gently dipping lower section decrease normalstress and increase shear stress on the steeply dipping upper sectionto generate the great 2008 Wenchuan earthquake.

The recurrence interval of the 2008 Wenchuan earthquake hasbeen an important concern of geoscientists. Previous studies gavea wide range of recurrence intervals from ~1,000 years (Lin et al.,2010) to 10,000 years (Burchfiel et al., 2008). Ran et al. (2013–thisissue) find three paleoseismic events (including theWenchuan earth-quake) within the past 6000 years through a trench investigation,geomorphic analysis and radiocarbon dating. They estimate that anaverage recurrence interval of great earthquakes is at most3000 years, each with similar coseismic vertical displacements ofabout 2 m. They further suggest that the strain accumulation and de-formation across the Longmenshan fault zone is a relatively stableand continuous process since late Quaternary.

There is a ~7 km long Xiaoyudong surface rupture orientedNW340° that is almost perpendicular to the major rupture zone ofthe 2008 Wenchuan earthquake. In order to understand what rolethe Xiaoyudong rupture played in the tectonic evolution of theLongmen Shan thrust zone, Chen et al. (2013–this issue) conducted de-tailed studies of geomorphology, paleoseismology, and coseismic dis-placement measurements. They suggest that the Xiaoyudong ruptureis not a passive tear fault but an active participator of slip partitioningon multiple faults within the Longmen Shan fault zone. The temporalconcurrence of a paleoearthquake on the Yingxiu–Beichuan fault sug-gests that the recurrence behavior of multi-strand rupturing alongthe Longmen Shan fault zone may conform to the characteristicslip-patch model, rather than the characteristic earthquake model.

The 2008 Wenchuan earthquake did not rupture the southernsegment of the Longmen Shan fault. The seismic risk of the southernsegment has been of concern to many seismologists. M. Wang et al.(2013–this issue) have studied one fault strand, the Qionghai faultwithin the southern segment of the Longmen Shan fault zone. Theyshow that the 50-km-long Qiongxi thrust fault (QTF) is currently ac-tive. Trench investigations, coupled with interpretations of seismicreflection profiles and radiocarbon results, show that a recentsurface-rupturing earthquake occurred on the Qionghai fault duringthe Late Ming to Qing Dynasty, between AD 1600 and 1800. Thus,the seismic hazard of the southern segment of the Longmen Shanfault cannot be overlooked.

Based on the analysis of historical earthquake that have occurred inthe vicinity of the Longmen Shan fault zone, Chen and Hsu (2013–thisissue) point out that there are additional seismogenic faults across theLongmen Shan belt, including those in the hinterland and in the fore-land. They notice that the occurrence of moderate-sized historicalearthquakes resulted in a wide region of reported damage in theSichuan basin, calling attention to the seismic hazard potential in theheavily populated Chengdu basin. They further warn that longrecurrence-intervals of great events, estimated from trenching of collu-vium along the ruptures of the 2008 sequence, should not be taken asreliable estimates for somewhat smaller, but nonetheless highly de-structive events along the Longmen Shan thrust belt.

3.2. The Yushu earthquake of 14 April 2010

The 14 April 2010 Mw 6.9 Yushu earthquake ruptured the north-western segment of the Ganzi–Yushu fault in Qinghai Province,China. Though this earthquake occurred in a sparsely populated areaof central Tibetan Plateau, more than 2000 people were killed becausethe earthquake fault ruptured through the capital town of Yushu,which is a population center. Three papers present coseismic andinterseismic studies on the seismogenic Yushu fault based on InSARand GPS observations.

Qu et al. (2013–this issue) obtained the coseismic deformationfield of the Yushu earthquake using L band and C band SAR images.The inversion results show that the slip distributions constrained bythe two kinds of InSAR data depict two concentrated regions of slipdistribution. One is located near the Jiegu town on the southeasternsegment of the Yushu fault with a large area and a maximum slip of2.4 m, and the other is at the epicenter on the northwestern segmentof the fault with a relatively small area. They suggest slip distributionfrom a joint inversion of the two kinds of SAR data to be the mostrobust model compared to results constrained by L band SAR dataor C band SAR data alone.

Zhang et al. (2013–this issue) applied a joint inversion ofteleseismic data and InSAR measurements to obtain a robust ruptureprocess and slip distribution of the 2010 Yushu earthquake, throughreducing the trade-off between slip timing and location. They suggestthat InSAR data can resolve fault slip better at depth range of0–15 km whereas teleseismic data give better resolution near thehypocenter. Their final joint inversion results show that the 2010Yushu earthquake has a rupture time of around 20 seconds, duringwhich 90% of the seismic moment has been released. Two peakenergy-releasing moments occur at 8 and 12 seconds after the earth-quake initiation. They also find two peak-slip asperities, one near thehypocentral area in a depth range of 10–15 km; the other distributedin a large near surface area at the eastern segment.

Using GPS data obtained from 1999 to 2007 in the vicinity of theGanzi–Yushu fault, Y-Zh. Wang et al. (2013–this issue) estimatesthe slip rates of the Fenghuoshan and Ganzi–Yushu–Xianshuihefault system. Their results agree with geological estimates of thefault slip rates, and show a progressive increase of shear motionfrom northwest to southeast across segments of the Ganzi–Yushu–Xianshuihe fault zone, implying variations in the transfer and absorp-tion of deformation in different regions in and around the Tibetanplateau.

Using an improved tomography algorithm Pei et al. (2013–thisissue) has obtained high resolution seismic velocity structure andazimuthal anisotropy around the epicenter of the 14 April 2010Ms=7.1 Yushu earthquake. The most striking result is a high velocityanomaly with large anisotropy at the epicenter on the Yushu–Garzefault. The main rupture originated within this high velocity anomalyand propagated southeastward into a low velocity anomaly withsmall anisotropy at Yushu. They suggest that these results demon-strate a clear example that lateral variation and anisotropy in seismicstructures of the upper crust controlled the origination (stress accu-mulation) and rupture propagation of the 2010 Yushu earthquakeand distribution of aftershocks as well.

3.3. Other earthquakes and hazards

The 2008 Yutian Mw 7.1 earthquake occurred in a junction area ofthe Altyn Tagh, Karakax and Kunlun fault systems with an elevationof more than 5000 m asl. High-resolution satellite image interpretationand field investigation (X. Xu et al., 2013–this issue) indicate that thesurface rupture zone produced by the Yutian earthquake is ~31 kmlong along the NS-trending Yulong Kashgar fault. The surface rupturezone consists of four different types of surface ruptures with bothnormal- and oblique-slip components. The maximum left-lateral slip

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and vertical offset measured in the field are ~3.6 m and ~3.3 m, respec-tively. X. Xu et al. (2013–this issue) argue that the normal faulting andextension associated with the Yutian earthquake support the eastwardblock-like motion model where deformation takes place mainly alongthe block boundaries delineated bymega-strike-slip faults in the north-ern Tibetan Plateau.

Based on his experiences working in many developing countriesBilham (2013–this issue) discusses five specific reasons why lossesfrom earthquakes continue to rise despite increasingly sophisticatedmethods to estimate seismic risk throughout the world. He points outthat one of the trends in current seismic hazard studies is that sophisti-cated methodologies to evaluate risk are inappropriate in regionswhere strain rates are low, andwhere historical data are short comparedto the return time of damaging earthquakes (Bilham, 2013–this issue).The scientific community has remained largely unaware of the impor-tance of these impediments to the development and application of ap-propriate seismic resistant codes, and is ill equipped to address them.

3.4. Deep crustal structures of Tibet's lithosphere

A 350-km long seismic wide-angle reflection/refraction profilewas conducted across the central Qaidam basin, from the northernmargin of the East-Kunlun Shan to the southern margin of the QilianShan. Zhao et al. (2013–this issue) finds that the crust of Qaidambasin has several significant features. (1) The sedimentary fill of theQaidam basin reaches a maximum thickness of 8 km, and the basinshape mirrors the uplifted Moho. (2) The crystalline crust is thickestbeneath the northern margin of the basin towards the Qilian Shan(58–62 km) and thinnest beneath the center of the basin (52 km).Variations in crustal thickness are caused primarily by thickness var-iations in the lowermost layer of the crust. (3) Poisson's ratio andP-wave velocity values suggest that the Qaidam crust has essentiallya felsic composition with an intermediate layer at its base. Based onthe crustal structure reported here, they suggest that late Cenozoicconvergence is accommodated by thick-skinned tectonic deforma-tion, with thickening involving the entire crust across the Kunlun–Qaidam–Qilian system.

Y-X. Wang et al. (2013–this issue) present active-source seismicdata recorded along a 1600-km-long profile crossing the southernTarim basin, the western flank of the South-Qilian Shan, the north-eastern margin of Qaidam basin, East-Kunlun Shan, Songpan–Ganziterrane, and Sichuan basin. Several significant crustal features are re-vealed by this long profile. (1) The crustal thickness varies considerablyalong the profile, varying from 48 km to 70 km. (2) North of the Kunlunfault variations in crustal thickness are mainly caused by variationsin lower crustal thickness, whereas south of the Kunlun fault theyare caused by thickness variations throughout the crust. (3) North ofthe Kunlun fault they detect a mid-crustal low-velocity zone that isnot apparent south of the fault. (4) The Kunlun fault seems to act as acompositional boundary for the lower crust, with a Poisson's ratio of0.29 north of the fault (Kunlun–Qaidam terrane) and 0.26 south of thefault (Songpan–Ganzi terrane). Measured Poisson's ratio and P-wavevelocity values suggest that the lower crust throughout the Tibetanplateau (South-Qilian Shan, margins of the Qaidam Basin, East-KunlunShan, Songpan–Ganzi terrane) is of intermediate composition. Thus theNE Tibetan plateau along their profile is missing a mafic lower crustallayer.

Z-J. Xu et al. (2013–this issue) developed a joint inversion scheme tojointly use surface wave dispersion and receiver function data todetermine the crustal and upper mantle S velocity model under theHi-CLIMB array. They find that the Indian crust is observed to subductand underplate the Tibetan crust between the Yalu-zangbu suture andthe Banggong-Nujiang suture, at a latitude of about 31.5°. The crustthickens from about 50 km below the Indian foreland to over 75 kmsouth of Banggong-Nujiang suture and turns shallower at around65 km under the Qiangtang block with two zones of complicated Moho

structure. They also show that a low velocity zone in a form of discontin-uous patches is present in themid-crust inmost parts of the profile underthe plateau with up to more than a 10% velocity reduction. The shape ofthis low velocity structure calls for a revision of the channel flowmodel.

To test models of the distribution of strength in the lithosphere,Levin et al. (2013–this issue) present a compilation of published andnewly determined focal mechanisms for northwestern Tibet and theadjacent Tarim basin that, taken together, characterize the stresswithin the crust and possibly the uppermostmantle. They also explorethe deformation of the upper mantle using published and newlyanalyzed measurements of shear wave splitting in teleseismiccore-refracted waves. They find close similarity between deformationdirections within the crust of the plateau andwithin the upper mantleon both sides of the boundary. A plausible explanation of such similar-ity would be the coupling of the crust and the upper mantle, with noweak zone being present in the lower crust. They further postulatethat a scenario for such coupling would involve an extension of adeformation zone associated with the Altyn Tagh fault into the upper-most mantle, making this fault zone very similar to a plate boundary.

The Red River fault is one of major strike-slip faults in the south-eastern border of the Tibetan Plateau. In order to understand whatrole this fault plays in the tectonic deformation of southeastern Asia,Huang et al. (2013–this issue) obtained the first local seismic tomog-raphy with a stepwise inversion using P and Pn phases recorded by anewly deployed portable broadband seismic network. They show thatthe Red River fault is a lithospheric structure penetrating to at leastthe uppermost mantle with mantle thermal anomalies. In general, thecrust of northern Vietnam appears to be weak and sits on a relativelyhot uppermost mantle, showing a long and complex thermo tectonichistory. A mid-lower crustal segmentation of the Red River fault isalso proposed to reconcile the discrepancies recently observed betweenYunnan province and northern Vietnam.

3.5. Crustal deformation and kinematics

To investigate whether the deformation in Tibetan Plateau is dis-tributed broadly or localized on a few major faults, Z-Q. Zhang et al.(2013–this issue) established a kinematic model comprised of 14 ro-tating, elastic–plastic blocks to represent the modern deformation ofeastern Tibet and neighboring regions. Block rotations, fault slip,and permanent strain rates within the blocks were constrained byinverting GPS velocities, slip vector azimuths derived from earth-quakes, and geologic slip rates. Their inversion results suggest thatthe internal straining of the blocks takes up about half of theeast-southeast motion than that due to faulting along the blockboundaries and that the internal shortening of blocks is nearly twotimes larger than that due to faulting along the block boundaries inthe N-S shortening component. A primary feature of the kinematicsin eastern Tibet is that faulting on a finite number of faults takes upthe major part of NW-SE shear while the contraction of the block in-terior takes up a major part of N-S compression.

He et al. (2013–this issue) use 3D finite element models incorpo-rating a fault as a Coulomb-type friction zone to investigate mechan-ical relation between crustal rheology and long-term deformation ofthe main active fault systems in the northeastern margin of the Tibet-an plateau. These models are constrained by the GPS velocity fieldand available geological slip rates. Crustal rheology is simplified asan elastobrittle upper crust and a viscoelastic lower crust. Their re-sults show that slip partitioning around the northeastern Tibetanboundary is related mechanically to low fault friction.

To understand the kinematic pattern of active deformation innortheastern margin of the Tibetan plateau, Zheng et al. (2013–thisissue) investigated Late Quaternary geological slip rates and the GPSdecadal slip rates along the major active faults in the Qilian Shan re-gion. They show that active tectonic deformation has been partitionedinto strike-slip along the Haiyuan–Qilian fault and reverse faulting

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along many range-front faults on the flanks of Qilian Mountain. Themajor strike-slip fault has a slip rate of 4–5 mm/year that decreases to-ward both ends to thicken the crust there, whereas the slip rates on thereverse faults are less than 1 mm/year. They conclude that the distribu-tion of slip rates from both late Quaternary geological records and thecurrent GPS observations suggest a decrease in slip rate, a redistribu-tion of strain, and the transformation of displacement along themajor strike slip faults into crustal shortening, basin formation andmountain uplift that characterize present-day deformation of thenorthern margin of the Tibetan Plateau.

Liu et al. (2013–this issue) presented results from 47 new apatitefission track (AFT) samples primarily from two vertical transects of1.5–2.5 km relief located on the northern flank of the North QinlingMountain near the northeastern corner of the Tibetan Plateau. Corre-lations between AFT ages and both elevation and track lengths, com-bined with thermal modeling of representative samples, reveal thatthe North Qinling experienced two major stages of Cenozoic exhuma-tion accelerations: minor acceleration in the period between ~35 and~25 Ma; and relatively rapid exhumation since ~10 Ma. They inter-pret these two stages of rapid cooling to be two episodes of upliftingof the Qinling Mountain that might be related to outward growth ofthe Tibetan Plateau.

Acknowledgments

The workshop of “Great Earthquakes in the 21st Century andGeodynamics” was sponsored by China Earthquake Administration(CEA), National Science Foundation of China (NSFC), US Geological Sur-vey (USGS), and US National Science Foundation (NSF). We thank HuChunfeng and Zhao Ming from CEA, Yu Sheng and Chen Huai fromNSFC, Walter Mooney from USGS, and Leonard Johnson from NSF forplanning the Sino-US bilateral workshops in both Boulder (2008),USA and Chengdu (2010), China. State Key Laboratory of EarthquakeDynamics, Institute of Geology, CEA organized the workshop and thefield trips. We thank Ran Yongkang, Han Bing, Yin Gongming, YanYan, Zhang Shuping, Qi Guili, Jia Haibo, Xu Jinjin, Lu Zhiyong, Du Bingand others for their hard work that made the workshop and field tripsuccessful and enjoyable. Special thanks go to Ms. Yan Yan for hercheerful efforts before and during theworkshop, and also in the processof preparation of this Special Issue. We also thank Mian Liu, a chief ed-itor of the journal, and editorial teamof the journal for their enthusiasmof this Special Issue. We would like to acknowledge the significant con-tribution by all of the reviewers without whom this Special Issue wouldnot be possible. We acknowledge financial support from the PublicService Funds for Earthquake Studies (201008003), National NaturalScience Foundation of China (41381320057), the State Key Laboratoryof Earthquake Dynamics (LED2008A01), USGS, and NSF.

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