尼泊尔Ms8.1地震构造背景及对周边地区地震活动趋势的影响

分析印度板块与欧亚板块俯冲带、尼泊尔Ms8.1地震震中区地震构造及地震发震构造,讨论主震对余震触发及余震的时空强特征和地震对周边地区地震活动趋势的影响。初步分析认为,主边界断裂为本次地震的发震构造,属于低角度逆冲断层地震;余震分布范围与震源破裂面积和方式基本一致,具有向东迁移的时空特征;西藏定日Ms5.9、聂拉木Ms5.3地震不属于尼泊尔地震余震,是应力扰动的结果;沿俯冲带向东至喜马拉雅东构造结是大震发生的危险地段。     

青藏高原西部蛇绿岩中玻安岩(boninite)及其地质意义

简要报道了在青藏高原西部班公湖蛇绿岩带和狮泉河蛇绿岩带之间的火山岩中识别出的玻安岩。玻安岩是俯冲岩浆作用的产物,在青藏高原东部及周边地区已有玻安岩或玻安岩系岩石存在的报道。结合西部出现的玻安岩说明青藏高原地区俯冲作用普遍存在。玻安岩的形成时代为J₂-K₁,埃达克岩的形成时代为K₁,由J₂-K₁,的玻安岩至K₁的埃达克岩反映冈底斯北部岛弧带由不成熟的洋内弧向成熟岛弧转变的过程,它们是中生代冈底斯北部大陆水平生长(侧向增生)的记录。     

洋脊和年轻洋壳俯冲与埃达克岩生成的热模拟

新生代以来 ,环太平洋周边分布的埃达克岩 (Adakite)主要与年轻洋壳俯冲时在 70～ 90km深处的部分熔融有关。利用数值方法 ,模拟了洋壳俯冲的热演化过程并讨论了脱水、熔融对埃达克岩浆活动的影响。结果表明 :仅在活动海岭俯冲前后约 10Ma内 ,年轻的、热的俯冲海洋板片在 75～85km深度范围内 ,温度升高至 82 5～ 10 0 0℃脱水 ,导致年轻洋壳中角闪岩部分熔融 ,形成埃达克岩(Adakite)。而一般洋壳俯冲在 10 0km以下深度才脱水 ,由于脱水区压力较高洋壳自身不能熔融 ,水进入上覆地幔楔状体导致部分熔融 ,形成安山岩 (Andesite     

Tectonics and sedimentation around Kashinosaki Knoll: A subducting basement high in the eastern Nankai Trough

Abstract   When seamounts and other topographic highs on an oceanic plate are subducted, they cause significant deformation of the overriding plate and may act as asperities deeper in the seismogenic zone. Kashinosaki Knoll (KK) is an isolated basement high of volcanic origin on the subducting Philippine Sea Plate that will soon be subducted at the eastern Nankai Trough. Seismic reflection imaging reveals a thick accumulation of sediments (∼1200 m) over and around the knoll. The lower portion of the sedimentary section has a package of high-amplitude, continuous reflections, interpreted as turbidites, that lap onto steep basement slopes but are parallel to the gentler basement slopes. Total sediment thickness on the western and northern slopes is approximately 40–50% more than on the summit and southeastern slopes of KK. These characteristics imply that the basal sedimentary section northwest of KK was deposited by infrequent high-energy turbidity currents, whereas the area southeast of KK was dominated by hemipelagic sedimentation over asymmetric basement relief. From the sediment structure and magnetic anomalies, we estimate that the knoll likely formed near the spreading center of the Shikoku Basin in the early Miocene. Its origin differs from that of nearby Zenisu Ridge, which is a piece of the Shikoku Basin crust uplifted along a thrust fault related to the collision of the Izu–Bonin arc and Honshu. KK has been carried into the margin of the Nankai Trough, and its high topography is deflecting Quaternary trench turbidites to the south. When KK collides with the accretionary prism in about 1 My, the associated variations in sediment type and thickness around the knoll will likely result in complex local variations in prism deformation.     

热带外太平洋通过海洋过程对热带太平洋的影响

利用MOM2海洋环流模式分别研究了年际和年代际尺度上热带外太平洋通过海洋过程对热带太平洋的影响.利用1945~1993年COADS资料的表面通量强迫海洋模式,积分46 a作为模式控制试验.取热带外太平洋海表面强迫为气候值做敏感性试验.将控制试验结果减去敏感性结果就可以得到热带外太平洋通过海洋过程对热带太平洋的影响.结果表明,年际尺度上,主要是海洋波动过程起作用,但总的海洋过程的影响较小.年代际尺度上,主要是副热带环流输送起作用,其中包括平均环流输送的温度异常以及异常环流输送的平均温度,并且后者贡献相对较大.年际和年代际尺度上,热带太平洋的温度异常主要发生在密度跃层附近.研究发现热带外南太平洋对热带和热带外太平洋之间的水量交换有影响,而热带外北太平洋的影响较弱.     

Estimating economic losses of midrise reinforced concrete shear wall buildings in sedimentary basins by combining empirical and simulated seismic hazard characterizations

Studies of recorded ground motions and simulations have shown that deep sedimentary basins can greatly increase the intensity of earthquake ground motions within a period range of approximately 1–4 s, but the economic impacts of basin effects are uncertain. This paper estimates key economic indicators of seismic performance, expressed in terms of earthquake‐induced repair costs, using empirical and simulated seismic hazard characterizations that account for the effects of basins. The methodology used is general, but the estimates are made for a series of eight‐ to 24‐story residential reinforced concrete shear wall archetype buildings in Seattle, WA, whose design neglects basin effects. All buildings are designed to comply with code‐minimum requirements (i.e., reference archetypes), as well as a series of design enhancements, which include (a) increasing design forces, (b) decreasing drift limits, and (c) a combination of these strategies. As an additional reference point, a performance‐based design is also assessed. The performance of the archetype buildings is evaluated for the seismic hazard level in Seattle according to the 2018 National Seismic Hazard Model (2018 NSHM), which explicitly considers basin effects. Inclusion of basin effects results in an average threefold increase in annualized losses for all archetypes. Incorporating physics‐based ground motion simulations to represent the large‐magnitude Cascadia subduction interface earthquake contribution to the hazard results in a further increase of 22% relative to the 2018 NSHM. The most effective of the design strategies considered combines a 25% increase in strength with a reduction in drift limits to 1.5%.     

钦-杭结合带在中生代构造转折事件以前的板块构造机制

钦州湾—杭州湾结合带是位于扬子与华夏两大古陆块中间的巨型构造结合带,在演化成西太平洋活动大陆边缘之前,经历了多期次的构造-岩浆事件。扬子和华夏板块于新元古代通过四堡造山运动(1 000~880 Ma)沿江山—绍兴断裂,经赣东北、湘中至钦州湾地区发生碰撞-拼合事件,拼合界线大致位于钦州湾—杭州湾结合带内,形成"两陆夹一盆"的主要格局。后碰撞过程经历了造山后岩浆活动和大陆拉张裂解两个过程,在结合带形成广阔的拉张盆地,加里东期(460~410 Ma)以及印支期(250~200 Ma)发生的碰撞-拼合事件导致扬子和华夏地块多次再造,引发强烈的构造-岩浆活动,并形成了华南统一的沉积环境。受西太平洋板块俯冲影响,华南地区中生代构造转折事件(125~140 Ma)使华南地区主要构造背景由碰撞挤压调整为岩石圈减薄,成为华南最重要的岩浆活动和成矿期。根据内部结构的不均一性和演化历史差异,钦州湾—杭州湾结合带可分为北段、中段和南段3段。其中,中段与传统南岭大体一致;北段为南岭以北地区,即绍兴—江山—萍乡一带;南段为南岭以南地区,大致与云开隆起—十万大山盆地相当。年代学和地球化学研究显示,在云开地块西缘的一系列变质基性岩、超基性岩和变质基性火山岩形成于新元古代洋中脊(MORB)或者岛弧(ITA)特征的构造环境,最近在岑溪一带发现形成于加里东期(441 Ma)的变质火山岩同样具有MORB型地球化学特征。在十万大山两侧发现早中生代的酸性火山岩和流纹岩具有典型岛弧型火山岩地球化学特征。可见结合带南段曾经存在古老洋壳,先后经历了新元古代、加里东期和印支期的碰撞造山事件,与北段演化历史具有一致性。     

Differential subduction and exhumation of crustal slices in the Sulu HP-UHP metamorphic terrane: insights from mineral inclusions, trace elements, U-Pb and Lu-Hf isotope analyses of zircon in orthogneiss

Based on new evidence the Sulu orogen is divided from south‐east to north‐west into high‐pressure (HP) crustal slice I and ultrahigh‐pressure (UHP) crustal slices II and III. A combined set of mineral inclusions, cathodoluminescence images, U‐Pb SHRIMP dating and in situ trace element and Lu‐Hf isotope analyses was obtained on zircon from orthogneisses of the different slices. Zircon grains typically have three distinct domains that formed during crystallization of the magmatic protolith, HP or UHP metamorphism and late‐amphibolite facies retrogression, respectively: (i) oscillatory zoned cores, with low‐pressure (LP) mineral inclusions and Th/U > 0.38; (ii) high‐luminescent mantles (Th/U < 0.10), with HP mineral inclusions of Qtz + Grt + Arg + Phe + Ap for slice I zircon and Coe + Grt + Phe + Kfs + Ap for both slices II and III zircon; (iii) low‐luminescent rims, with LP mineral inclusions and Th/U < 0.08. Zircon U‐Pb SHRIMP analyses of inherited cores point to protolith ages of 785–770 Ma in all seven orthogneisses. The ages recorded for UHP metamorphism and subsequent retrogression in slice II zircon (c. 228 and c. 215 Ma, respectively) are significantly older than those of slice III zircon (c. 218 and c. 202 Ma, respectively), while slice I zircon recorded even older ages for HP metamorphism and subsequent retrogression (c. 245 and c. 231 Ma, respectively). Moreover, Ar‐Ar biotite ages from six paragneisses, interpreted as dating amphibolite facies retrogression, gradually decrease from HP slice I (c. 232 Ma) to UHP slice II (c. 215 Ma) and UHP slice III (c. 203 Ma). The combined data set suggests decreasing ages for HP or UHP metamorphism and late retrogression in the Sulu orogen from south‐east to north‐west. Thus, the HP‐UHP units are interpreted to represent three crustal slices, which underwent different subduction and exhumation histories. Slice I was detached from the continental lithosphere at ～55–65 km depth and subsequently exhumed while subduction of the underlying slice II continued to ～100–120 km depth (UHP) before detachment and exhumation. Slice III experienced a similar geodynamic evolution as slice II, however, both UHP metamorphism and subsequent exhumation took place c. 10 Myr later. Magmatic zircon cores from two types of orthogneiss in UHP slices II and III show similar mid‐Neoproterozoic crystallization ages, but have contrasting Hf isotope compositions (εHf_(～785) = ?2.7 to +2.2 and ?17.3 to ?11.1, respectively), suggesting their formation from distinct crustal units (Mesoproterozoic and Paleoproterozoic to Archean, respectively) during the breakup of Rodinia. The UHP and the retrograde zircon domains are characterized by lower Th/U and ¹⁷⁶Lu/¹⁷⁷Hf but higher ¹⁷⁶Hf/¹⁷⁷Hf(t) than the Neoproterozoic igneous cores. The similarity between UHP and retrograde domains indicates that late retrogression did not significantly modify chemical and isotopic composition of the UHP metamorphic system.     

大陆碰撞造山带的两类橄榄岩——以柴北缘超高压变质带为例

论述了大陆俯冲碰撞带中地幔橄榄岩的基本特征和成岩类型,并重点讨论柴北缘超高压变质带中不同性质的橄榄岩及其成因。根据岩石学特征,我们确定柴北缘超高压带中发育有两种类型的橄榄岩:(1)石榴橄榄岩,岩石类型包括石榴二辉橄榄岩、石榴方辉橄榄岩、纯橄岩和石榴辉石岩,是大陆型俯冲带的标志性岩石。金刚石包裹体、石榴石和橄榄石的出溶结构、温压计算等均反映其来源深度大于200km。地球化学特征表明该橄榄岩的原岩是岛弧环境下高镁岩浆在地幔环境下堆晶的产物。(2)大洋蛇绿岩型地幔橄榄岩,与变质的堆晶杂岩(包括石榴辉石岩、蓝晶石榴辉岩)和具有大洋玄武岩特征的榴辉岩构成典型的蛇绿岩剖面,代表大洋岩石圈残片。这两类橄榄岩的确定对了解柴北缘超高压变质带的性质和构造演化过程有重要意义。     

Preliminary estimation of the tsunami hazards associated with the Makran subduction zone at the northwestern Indian Ocean

We present a preliminary estimation of tsunami hazard associated with the Makran subduction zone (MSZ) at the northwestern Indian Ocean. Makran is one of the two main tsunamigenic zones in the Indian Ocean, which has produced some tsunamis in the past. Northwestern Indian Ocean remains one of the least studied regions in the world in terms of tsunami hazard assessment. Hence, a scenario-based method is employed to provide an estimation of tsunami hazard in this region for the first time. The numerical modeling of tsunami is verified using historical observations of the 1945 Makran tsunami. Then, a number of tsunamis each resulting from a 1945-type earthquake (M _w 8.1) and spaced evenly along the MSZ are simulated. The results indicate that by moving a 1945-type earthquake along the MSZ, the southern coasts of Iran and Pakistan will experience the largest waves with heights of between 5 and 7 m, depending on the location of the source. The tsunami will reach a height of about 5 m and 2 m in northern coast of Oman and eastern coast of the United Arab Emirates, respectively.