Global-scale analysis of seismic activity prior to 2004 Sumatra-Andaman mega-earthquake

Following our experience in intermediate-term monitoring of seismicity before large earthquakes worldwide and in some regions we apply a similar approach to the analysis of seismicity on global scale several decades before the December 26, 2004 Sumatra–Andaman mega-earthquake. The Earth lithosphere is being considered as a single whole, representing the ultimate level of the complex Earth's hierarchy. The study brings up the following questions: Are there any anomalies of the global seismicity behaviour observed in advance of the mega-earthquake? Do these anomalies correspond to the global scale seismic patterns similar to those detected on the regional scale before great, major, and strong events? In other words, does the Earth lithosphere considered as a single whole show up an approach of the mega-earthquake in the way typical for events on the smaller levels of hierarchy? The results of our investigation favour the positive answer to the questions. Specifically we found that during the decade before the Sumatra–Andaman mega-earthquake the Earth lithosphere reveals, at least in intermediate-term scale, classical symptoms of instability, which can be depicted by known precursory seismic patterns. These are: (i) transformation of frequency–magnitude distribution, (ii) change in the rate of seismic activity, and (iii) depth redistribution of activity. Moreover changes of dependencies between magnitudes of different types are detected at the same time. The observed global scale patterns of collective behaviour of seismicity may indicate the state of criticality of the Earth lithosphere before the Sumatra–Andaman mega-earthquake.     

5.9 Mw, 18th June 2010 earthquake and fault segment linkage at Andaman – A study based on macroseismic survey,GPS geodesy and Coulomb stress changes

18th June, 2010 5.9 M_w earthquake at North Andaman triggered along NW–SE pre-existing fault with reverse fault mechanism. Macroseismic survey and GPS geodesy reveal maximum damages following NE–SW trend due to normal fault mechanism. Coulomb stress modeling for post- and inter-seismic earthquakes after the 2004 mega-earthquake show different stages of fault segment linkage at North Andaman. The present earthquake has been explained as co-shock due to asiesmic soft linkage of fault propagation.     

Magnetic anomalies over the Andaman Islands and their geological significance

The Andaman Islands form part of the outer-arc accretionary sedimentary complex belonging to the Andaman–Sumatra active subduction zone. The islands are characterized by thick cover of Neogene sediments along with exposed ophiolite rocks at few places. A regional magnetic survey was carried out for the first time over the Andaman Islands with a view to understand the correlation of anomaly signatures with surface geology of the islands. The residual total field magnetic anomaly maps have revealed distinct magnetic anomalies having intermediate to high amplitude magnetic signatures and correlate with the areas over/close to the exposed ophiolite rocks along the east coast of north, middle and the south Andaman Islands. The 2D modelling of magnetic anomalies along selected E–W profiles across the islands indicate that the ophiolite bodies extend to a depth of about 5–8 km and spatially correlate with the mapped fault/thrust zones.     

GPS-derived crustal deformation in southwestern China

The Sichuan – Yunnan region is divided into nine active secondary crustal blocks, based on several GPS repeat surveys at more than 200 GPS sites during the period 1999 – 2005. Velocities of the nine secondary blocks are calculated and analysed. The strain field within the area related to the 2004 Sumatra – Andaman earthquake event is also analysed. Results indicate that the crustal movement in the northern and western areas of the Sichuan – Yunnan region is stronger than that in the south and east. The horizontal velocities change from 19 – 20 mm/y in the northern and the central rhombic block to 11.7 mm/y in the southern rhombic block. The orientations of block motion vary from 99° in the north to 126 – 150° in the central area and 156 – 188° in the south, implying that the motion of the Sichuan – Yunnan rhombic block is dominated by a clockwise rotation. The velocity differences between blocks inside and outside the rhombic block are about 6.5 – 7.7 mm/y in the northern and central Sichuan – Yunnan region. The southeastward extrusion rate of the Tibetan Plateau shows a remarkable downtrend of up to 47% along the Xianshuihe Fault, suggesting an increase in strain accumulation and hence an area prone to strong earthquakes. The horizontal coseismic deformation caused by the Mw9.0 Sumatra – Andaman earthquake is <10 mm with a south-southeast orientation towards the earthquake epicentre. The dilatational strain rates from coseismic displacements reveal a possible interaction between the extrusion from the Tibet plateau interior and the underthrust effects from the Sumatra – Andaman earthquake.     

The Mw 7.5 2009 Coco earthquake, north Andaman region

The recent 10 August 2009 Coco earthquake (Mw 7.5), the largest aftershock of the giant 2004 Sumatra Andaman earthquake, occurred within the subducting India plate under the Burma plate. The Coco earthquake nucleated near the northwestern edge of the 2004 Sumatra-Andaman earthquake rupture under the unruptured updip segment of the plate boundary interface. The earthquake with predominant normal motion on approximately north-south to northeast-southwest oriented plane is very similar to the 27 June 2008 Little Andaman earthquake which occurred in the South Andaman region near the trench. We provide the only available estimate of coseismic offset due to the 2009 Coco earthquake at a survey-mode GPS site in the north Andaman, located about 60 km south of the Coco earthquake epicentre. The not so large coseismic displacement of about 2 cm in the ESE direction is consistent with the earthquake focal mechanism and its magnitude. We suggest that, like the 2008 Little Andaman earthquake, this earthquake too occurred on one of the approximately north-south to northeast-southwest oriented steep planes of the obliquely subducting 90°E ridge which was reactivated in normal motion after subduction, under the favourable influence of coseismic and ongoing postseismic deformation due to the 2004 Sumatra-Andaman earthquake. Another notable feature of this earthquake is its relatively low aftershock productivity. We suggest that the earthquake occurred very close to the aseismic region of the Irrawaddy frontal arc of very low seismicity where pre-existing faults are not so critically stressed and because of which the earthquake could trigger only a few aftershocks in its immediate vicinity.     

Trend and spatial distribution of rainfall & rainy days over Andaman & Nicobar Islands

The present study examines the characteristics and climatological features of daily rainfall data over Andaman & Nicobar Islands. Analysis of rainfall data reveals a large monthly deviation over the northern latitudes as compare to southern latitudes of Andaman & Nicobar Islands. Also, it is found that rainfall increases from north to south latitudes in all the seasons except monsoon, where a reverse pattern exists. In trend analysis, a statistically significant decreasing trend (confidence level >95?%) is observed for yearly rainfall and rainy days over the region. Analysis of daily rainfall intensity for each year shows increasing trend for frequency of rather heavy rain (35.6?C64.4?mm) and significant decreasing trend for frequencies of light rain (2.5?C7.5?mm), and very heavy rain (>124.5?mm) over the region. Many times, very heavy rain events are associated with cyclonic disturbances affecting Andaman & Nicobar Islands region. The analysis of cyclonic disturbances over the region reveals a stronger and more significant decreasing trend. So, one of the causes for decreasing trend in very heavy rain over Andaman & Nicobar Islands may be due to significant decreasing frequency of cyclonic disturbances affecting this region.     

Seismogenic sources in the Bay of Bengal vis-à-vis potential for tsunami generation and its impact in the northern Bay of Bengal coast

Geodynamic status, seismo-tectonic environment, and geophysical signatures of the Bay of Bengal do not support the occurrence of seismogenic tsunami. Since thrust fault and its intensity and magnitude of rupture are the key tectonic elements of tsunamigenic seismic sources, the study reveals that such characteristics of fault-rupture and seismic sources do not occur in most of the Bay of Bengal except a small segment in the Andaman–Nicobar subduction zone. The inferred segment of the Andaman–Nicobar subduction zone is considered for generating a model of the deformation field arising from fluid-driven source. The model suggests local tsunami with insignificant inundation potential along the coast of northern Bay of Bengal. The bathymetric profile and the sea floor configuration of the northern Bay of Bengal play an important role in flattening the waveform through defocusing process. The direction of motion of the Indian plate makes an angle of about 30° with the direction of the opening of Andaman Sea. The opening of Andaman Sea and the direction of plate motion of the Indian plate results in the formation of Andaman trench where the subducting plate dives more obliquely than that in the Sunda trench in the south. The oblique subduction reduces significantly the possibilities of dominant thrust faulting in the Andaman subduction zone. Further, north of Andaman subduction in the Bengal–Arakan coast, there is no active subduction. On the otherhand, much greater volume of sediments (in excess of 20 km) in the Bengal–Arakan segment reduces the possibilities of mega rupture of the ocean floor. The water depth (≈1,000 m) along most of the northern Bay of Bengal plate margin is not optimum for any significant tsunami generation. Hence, very weak possibility of any significant tsunami is suggested that based on the interpretation of geodynamic status, seismo-tectonic environment, and geophysical signatures of the Andaman subduction zone and the Bengal–Arakan coast.     

Revealing the coastal event-history of the Andaman Islands (Bay of Bengal) during the Holocene using radiocarbon and OSL dating

Earthquakes that trigger tsunamis are of great geological, ecological and socio-economic importance. The knowledge of the recurrence interval of these events will give information about the hazard for a region. Coastal sediments on the Andaman Islands located in the eastern Bay of Bengal were investigated to find evidence for palaeotsunamis and palaeoearthquakes. Fieldwork was conducted on Red Skin Island and North Cinque Island, south of South Andaman. Sediment material from event-layers was dated by optically stimulated luminescence and radiocarbon dating method. The results show evidence possibly for one earthquake at about 1,000 or 3,000 years before the present together with deposits from possible tsunamis and storms. The complex pattern of co- and post-seismic uplift and subsidence of the Andaman Islands is reflected in the investigated sections and made it possible to reconstruct an event-history for the last 3,000 years.     

Dynamic estimate of seismic danger based on multifractal properties of low-frequency seismic noise

A new method of dynamic estimate of seismic danger is presented which is based on estimating multifractal properties of low-frequency seismic noise. The efficiency of the method is illustrated by the analysis of seismic noise from broadband seismic network F-net in Japan. The analysis of multifractal properties of low-frequency seismic noise from Japan seismic network F-net since the beginning of 1997 allowed a hypothesis about approaching Japan Islands to a future seismic catastrophe to be formulated at the middle of 2008. The base for such a hypothesis was statistically significant decreasing of multi-fractal singularity spectrum support width mean value. The peculiarities of correlation coefficient estimate within 1 year time window between median values of singularity spectra support width and generalized Hurst exponent allowed to make a decision that starting from July 2010, Japan come to the state of waiting strong earthquake. This prediction of Tohoku mega-earthquake, initially with estimate of lower magnitude as 8.3 only (at the middle of 2008) and further on with estimate of the time beginning of waiting earthquake (from the middle of 2010), was published in advance in a number of scientific articles and abstracts on international conferences. The analysis of seismic noise data after Tohoku mega-earthquake indicates increasing probability of the 2nd strong earthquake within the region where the north part of Philippine Sea plate is approaching island Honshu (Nankai Trough). This region is characterized by relatively low values of singularity spectrum support width which is an indicator of seismic danger. In one paper (Sobolev in Izv Phys Solid Earth 47:1034–1044, 2011), the low-frequency seismic noise at the same range of periods was investigated retrospectively using data from the stations of broadband network IRIS which are located around the epicenter of Tohoku mega-earthquake with a distance up to 1,200 km. It was shown that the variance of the noise and the number of high-amplitude asymmetric impulses were grown dramatically before the event for stations which are located within the radius up to 500 km from the epicenter.     

Magnetic fabrics of Paleozoic rocks across the Lachlan Transverse Zone from eastern New South Wales ∗

The anisotropy of magnetic susceptibility (AMS) was systematically measured for samples collected across the Lachlan Transverse Zone in the Eastern Subprovince of the Lachlan Orogen, New South Wales. Although the degree of anisotropy is usually moderate to low, it can be shown that the origin of the magnetic fabric is generally composite. Many localities are witness to a tectonic influence in addition to a magnetic foliation preserved from the time of rock formation (compaction). Furthermore, some localities indicate the presence of superimposed magnetic fabrics, potentially associated with a Silurian east–west direction of shortening, and a younger north–south (?) direction of shortening. Finally, the progressive southwards change in orientation of the magnetic lineation in the Molong area from north–south to east–west and then back to north–south again south of the Lyndhurst–Neville Fault suggests that the Lachlan Transverse Zone coincides with, and reflects, a major cross-structure in the Eastern Subprovince. AMS is thus a powerful tool to help map the fabric of Paleozoic rocks in the Tasmanides. Additional data will be required to help obtain a comprehensive picture of the tectonic history of the region.     

伸展作用序列及其深部背景: 以晚中生代以来鲁西隆起和济阳坳陷为例

露头区野外地质调查、隐伏区地质与地球物理资料研究表明,晚中生代-新生代鲁西隆起区和济阳坳陷区正断层发育,包括陡倾斜的控凹边界断层和缓倾斜的滑脱断层两类,两者构成伸展滑脱半地堑,且滑脱构造在隆起北部和坳陷南部最发育。K Ar和FT测年结果指示伸展断层的发育时间为176~103 Ma、67～49 Ma和42～25 Ma 3个时期。隆起区、坳陷区陡断层分别在中地壳22 km 和15 km左右变平,成为拆离滑脱断层。构造物理模拟表明,在伸展＋塑性物质上涌机制下隆起区和坳陷区正断层均具有由南向北的发育极性,大致对应中侏罗世-早白垩世、古新世-早始新世、中始新世-渐新世3个发育阶段,且伴随控凹断层发育的同时,断块掀斜引起滑脱断层同步发育。层析成像表明中生代早期扬子板块沿作为转换断层的郯庐断裂以近EW向与华北板块俯冲的残留体可能导致晚中生代地幔物质上涌,新生代地幔上涌则可能与太平洋板块与欧亚板块俯冲及印欧板块碰撞的远程效应有关。研究区正断层受控于地幔物质上涌+伸展作用,以齐河-广饶断层为界呈前展式分别由南向北发育,并控制着坳陷区油气的形成、运聚和分布向北迁移。     

Aftershock Propagation Characteristics During the First Three Hours Following the 26 December 2004 Sumatra-Andaman Earthquake

Within three hours of the mainshock rupture of the 26 December 2004 Sumatra-Andaman earthquake, 45 aftershocks occurred that are distributed all along the mega-thrust fault plane and also along the West Andaman fault. Seven of these aftershocks struck sequentially and unilaterally from the mainshock in the south towards north within 2h 9m 50.76s indicating an overall rate of aftershock propagation to the tune of 167 meters/sec. Seismic moment calculated from fault parameters gives a value of 1.2 × 10³⁰ dyne cm. Three separate fault segments are identified from distribution of aftershocks with propagation rates 330, 250 and 85 meters/sec in the southern, central and northern segments. These 7 unilaterally propagating shocks along the mega-thrust are probably not aftershocks of the mainshock rather these are sequentially triggered shocks each rupturing a small segment of the fault. Location of the mainshock and several aftershocks are guided by several lithospheric hinge faults identified previously.     

Aftershock Propagation Characteristics During the First Three Hours Following the 26 December 2004 Sumatra-Andaman Earthquake

Within three hours of the mainshock rupture of the 26 December 2004 Sumatra-Andaman earthquake, 45 aftershocks occurred that are distributed all along the mega-thrust fault plane and also along the West Andaman fault. Seven of these aftershocks struck sequentially and unilaterally from the mainshock in the south towards north within 2h 9m 50.76s indicating an overall rate of aftershock propagation to the tune of 167 meters/sec. Seismic moment calculated from fault parameters gives a value of 1.2 × 10³⁰ dyne cm. Three separate fault segments are identified from distribution of aftershocks with propagation rates 330, 250 and 85 meters/sec in the southern, central and northern segments. These 7 unilaterally propagating shocks along the mega-thrust are probably not aftershocks of the mainshock rather these are sequentially triggered shocks each rupturing a small segment of the fault. Location of the mainshock and several aftershocks are guided by several lithospheric hinge faults identified previously.     

滇东地区煤田构造格局与煤系赋存规律的研究

滇东地区位于特提斯和环太平洋构造体系的交接部位,煤田构造格局的总体特征是呈NNE向条带状展布。区内断层以NNE、NE向为主,受主要断裂影响,煤田在南北向呈带状,东西向呈段状展布,各煤矿区在平面上呈NE向带状分布。研究区煤系赋存受基底主干断裂的控制作用明显,变形强度由北向南逐渐增强,北部地区以褶皱构造为主,断层较少,煤层多保存于向斜构造中,保存较完整,煤层埋藏较深;南部受弥勒-师宗断裂带影响,断裂构造极为发育,对煤层破坏严重,煤系赋存不稳定。     

CHANNELS FROM THE UPPER CARBONIFEROUS COAL MEASURES OF SOUTH WALES

Sandstone-filled channels occur widely in the Upper Carboniferous (Westphalian) rocks of South Wales. Subsurface and outcrop studies of channels and associated structures in three formations within the Westphalian sequence reveal significant regional variations in orientation, transverse profile, geometry and lithology of fill. The pattern of channel-orientation in each formation is consistent with the system of palaeocurrents derived from independent studies of cross-bedding and indicates that in the lowest Westphalian (Basal Coal Measures) derivation was principally from the north and east, with intermittent supply from the south, whereas in the Upper Westphalian material was derived mainly from the south and east but with substantial contributions from the north.
Formation of the channels is ascribed to fluvial erosion and filling. The mutually consistent regional variations in channel-parameters are compared with the conspic- uous changes in lithofacies and interpreted as indicating diRerences in Auviohydraulic locale within the original basin of sedimentation.     

焦家断裂带紫罗姬家以南的延伸问题探讨

焦家断裂带是胶西北地区三大著名的金成矿带之一,一般认为焦家断裂带为龙（口）莱（州）断裂带南段的高家庄子紫罗姬家段。根据近几年的研究成果,认为焦家断裂带在紫罗姬家以南,沿195＆#176;～200＆#176;方位延伸至紫罗綦家与黄家之间分为3支：第一分支沿紫罗綦家—赵官庄—前单家—张家埠至曹家埠村北;第二分支（主断裂带）沿紫罗綦家村东南—大尹家—西尹—西罗台村南至曹家埠村南;第三分支自紫罗綦家与黄家村之间向南延伸至史家村西之后,再向南去向不明,推测逐渐尖灭的可能性较大。     

Regional conductance map of Andaman and Nicobar region

To image the electrical conductivity distribution, fluxgate magnetometers are operated at five sites in Andaman and Nicobar region. Transfer functions are estimated for the period range 8–128 min, from nighttime transient geomagnetic variations, using robust regression analysis. The observed induction arrows in Andaman Islands are found to point towards east despite deep sea located towards its west. This indicates that fore-arc basin (Andaman–Nicobar deep) is more conducting than the region of outer non-volcanic Island arc.Thin sheet model requires the conductance of 10,000–35,000 S (with increase conductivity towards the south) for explaining the observed induction pattern. The observed induction pattern at Andaman–Nicobar stations can be explained in terms of high conducting Cretaceous–Tertiary sediments filling the Andaman–Nicobar deep. High conductivity over Invisible bank has been attributed to the partial melts/volatile fluids derived from the subducting Indian plate that are intruding into the eastern margin of fore-arc basin through the West Andaman Fault (WAF).The induction pattern at Great Nicobar station (Campbell Bay) may be related to the highly conducting sediments filling the Mergui basin along with mafic intrusions. Also crustal transition occurs below the Mergui Terrace at the Malayan coast contributing to the enhanced conductivity anomaly.     

青藏高原东南缘晚第三纪盐源构造逸出盆地的沉积特征与构造控制

晚第三纪盐源盆地位于夹持于鲜水河断裂与红河断裂之间的川滇块体的东南缘，并夹于木里弧与盐源弧形构造之间。为东西向展布并向南东凸出的弧形盆地，盆地中充填了巨厚的同构造期的磨拉石，是一个由顶底不整合面限制的构造层序，具有总体先向上变细后向上变粗变浅的完整沉积旋回；盆地自南而北的充填样式总体显示为冲积扇（水下扇）－深湖－湖沼－河流冲积平原，为一个南厚北薄楔形盆地，沉只特征表明该盆地具有单断张性盆地的充填特性，川滇块体向南东挤出作用使该区由原来的挤压状态下逆冲系统转变为引张状态下的向南东的构造逃逸系统，从而在川滇块体内形成晚第三纪盐源盆地，盆地的长轴垂直于川滇块体南东向挤出方向。因此，晚第三纪盐源盆地是大陆块体侧向挤出作用的沉积响应，沉积物的时代研究表明川滇块体这次挤出构造事件出现的时间为晚中新世－上新世，其与Tapponnier(1986）大陆块体侧向多期挤逸模式最后一期挤出事件相对应。     

Seismic Cycles and Trend Predictions of Earthquakes in Sumatra–Andaman and Burmese Subduction Zones using Temporal b-value and Hurst Analysis

The annual b-value fluctuation patterns in Burmese subduction zone and Andaman–Sumatra subduction zone are evaluated from earthquake data (January 1990 to June 2016; Mw ³ 4.3) to identify seismic cycles with sequential dynamic phases as described in the ‘elastic failure model’ of Main et al. (1989). Two seismic cycles have been identified in Andaman–Sumatra subduction zone, one started in 1990 and ended on 2004 with occurrence of great Sumatra earthquake (Mw 9.0) and the other started in 2005 and continuing till date with the phase of crack coalescence and fluid diffusion (3A&B). Similarly, the subduction zone of Burma shows evidence of one incomplete seismic cycle within 1990–2016 and presently undergoing the crack coalescence and fluid diffusion (3A&B) phase. The analysis has prompted to subdivide the area into thirteen smaller seismic blocks (A to M) to analyse area specific seismic trend and vulnerability analysis employing Hurst Statistics. Hurst plots with the dynamic phases of ‘elastic failure model’ of earthquake generation is compared to assess the blocks with high seismic vulnerability. The analysis suggest that north Andaman zone (block G) and north Burma fold belt (block M) are seismically most vulnerable. Moreover, the seismic vulnerability of Tripura fold belt and Bangladesh plain (block K) is equally high.     

Geothermobarometry, P–T paths and metamorphic field gradients of high-pressure rocks from the Adula Nappe, Central Alps

Metabasic rocks from the Adula Nappe in the Central Alps record a regional high‐pressure metamorphic event during the Eocene, and display a regional variation in high‐pressure mineral assemblages from barroisite, or glaucophane, bearing garnet amphibolites in the north to kyanite eclogites in the central part of the nappe. High‐pressure rocks from all parts of the nappe show the same metamorphic evolution of assemblages consistent with prograde blueschist, high‐pressure amphibolite or eclogite facies conditions followed by peak‐pressure eclogite facies conditions and decompression to the greenschist or amphibolite facies. Average PT calculations (using thermocalc ) quantitatively establish nested, clockwise P–T paths for different parts of the Adula Nappe that are displaced to higher pressure and temperature from north to south. Metamorphic conditions at peak pressure increase from about 17 kbar, 640 °C in the north to 22 kbar, 750 °C in the centre and 25 kbar, 750 °C in the south. The northern and central Adula Nappe behaved as a coherent tectonic unit at peak pressures and during decompression, and thermobarometric results are interpreted in terms of a metamorphic field gradient of 9.6 ± 2.0 °C km^?1 and 0.20 ± 0.05 kbar km^?1. These results constrain the peak‐pressure position and orientation of the nappe to a depth of 55–75 km, dipping at an angle of approximately 45° towards the south. Results from the southern Adula Nappe are not consistent with the metamorphic field gradient determined for the northern and central parts, which suggests that the southern Adula Nappe may have been separated from central and northern parts at peak pressure.