Seismic Sequence in Carbonate Rocks By Vibrational Liquefaction

On the basis of the records of strong seismic events taking place in soft carbonate sediments, a new seismic sequence system of vibrational liquefaction is established, which consists of a series of units, such as escaped structure of micrite veins and liquefied deformation formed in the period of seismic liquefaction, land subsidence structure after liquefaction, tsunamic hummocky and turbidite produced by seismic events, This sequence is a generalization and summation of field observation in vast areas, which shows the whole process of a strong seismic event and provides an unified theoretical explanation. The pattern of the seismic sequence by vibrational liquefaction provides one of correlation standards for geologists in the field to discriminate events in carbonate sequences. Based on the pattern of seismic sequence, the authors first advance a new conception of the Palaeo-Tanlu (Tancheng-Lujiang) Zone and discuss primarily its geological significations.

Seismic sequence and depositional evolution of slope basins in mid-northern margin of the South China Sea

As one of the biggest marginal seas in the western Pacific margin, the South China Sea （SCS） experienced continental rifting and seafloor spreading during the Cenozoic. The northern continental margin of the SCS is classified as a passive continental margin. However, its depositional and structural evolution remains controversial, especially in the deep slope area. The lack of data hindered the correlation between continental shelf and oceanic basin, and prevented the establishment of sequence stratigraphic frame of the whole margin. The slope basins in the mid-northern margin of SCS developed in the Cenozoic; the sediments and basin infill recorded the geological history of the continental margin and the SCS spreading. Using multi-channel seismic dataset acquired in three survey cruises during 1987 to 2004, combined with the data of ODP Leg 184 core and industrial wells, we carried out the sequence stratigraphic division and correlation of the Cenozoic in the middle-northern margin of SCS with seismic profiles and sedimentary facies. We interpreted the seismic reflection properties including continuity, amplitude, fi＇equency, reflection terminals, and 15 sequence boundaries of the Cenozoic in the study area, and correlated the well data in geological age. The depositional environment changed from river and lake, shallow bay to open-deep sea, in correspondence to tectonic events of syn-rifting, early drifting, and late drifting stages of basin evolution.

Characteristics of Seismic Sequences and Early Tendency Judgment for Yutian M_S7.3 Earthquake in 2014

The basic parameters,seismogenic structure and seismic sequences characteristics of the Yutian MS7. 3 earthquake on February 12,2014 are introduced and compared to the Yutian MS7. 3 earthquake in 2008. The results show that the MS5. 4 earthquake is regarded as an immediate foreshock of the Yutian MS7. 3 main shock. The frequency of strong aftershock sequences was low and their number declined quickly,and the maximum aftershock was a MS5. 7 earthquake. According to analysis of the historical earthquake sequence type,and parameter of h-value,b-value and energy release ratio between main shock and sequence etc.,we found the preliminary conclusion that the Yutian MS7. 3 earthquake sequence in 2014 was a foreshock-main shock-aftershock type.

Tracing the Forecast of Earthquakes Based on the Seismicity Characteristics of the Chi-Chi Strong Earthquake Swarm Sequence

The data of earthquakes with M ≥3 0 during the 7 years from September 21, 1993 to September 20, 2000 recorded by the Taiwan Central Weather Bureau (CWB) show that there were 6 types of clear characteristics of seismicity during the Chi Chi strong earthquake swarm of September 21 These 6 types of characteristics are (1) foreshock types, (2) seismic gaps, (3) seismic bands, (4) clustering activity of foreshocks and signal shock, (5) quiescence before the main shock and (6) secondary aftershocks in the aftershock sequence. Using the procedures for analyzing the yearly strong earthquake tendency, further tracing based on the earthquake sequence characteristics, and taking the Chi Chi earthquake sequence as an example, tracing analysis of the earthquake tendency was attempted using the shorter time range of monthly rather than in a yearly time scale. An attempt was made to establish the procedures for tracing analysis of shallow focus earthquakes in the seismic belt of western Taiwan. It is hoped that this can provide an analystical method for approaching the short imminent time scale of seismometry based earthquake forecasting.

Seismic Facies in a Deepwater Area of a Marine Faulted Basin:Deepwater Area of the Paleogene Lingshui Formation in the Qiongdongnan Basin

In recent years, deep water areas have become popular exploration fields because of their abundant hydrocarbon resource potential. There are only relatively poor planar seismic profiles and no wells for deepwater areas of the Lingshui Formation in the Qiongdongnan Basin. A lot of faults developed and strata are fragmented due to high temperatures and high pressure, and this has resulted in dim sequence boundaries. Based on seismic data of the deepwater area and well data of bordering shallow water areas, Lingshui Formation was divided into four third class sequences; namely SI, SII, SIII and SIV, and the three-dimensional isochronous stratigraphic framework of the Lingshui Formation in the studied area was shaped. Based mainly on seismic attributes such as amplitude, continuity, internal structure and external shape, six typical seismic facies were identified, including mat-shaped, filling, wedge-shaped, foreset, moundy-shaped and lenticular-shaped, and a seismic facies distribution map was subsequently drawn. With studies on wells of bordering shallow water areas, regional sedimentary characteristics, and isopach map as references, sedimentary planar distribution features were analyzed. The deepwater area of the Lingshui Formation has mainly developed littoral and shallow sea. Sandstone bodies of fan delta, braided river delta, slope fan, basin floor fan, and turbidite fan are at an interdigitate junction to marine hydrocarbon source rocks and thus are favorable prospecting targets.

Characteristics of ambient stress values for micro－earthquake sequences in Western Yunnan Earthquake Prediction Experimental Field

According to the source velocity spectrum model suggested by Chen and Duda (1993), the method of directly measuring the maximum value <Vc (cm)>max of the velocity spectrum from the seismic moment M0 has been studied. The authors have measured the seismic moment M0 and source mechanical parameters of three microearthquake sequences, the Er' yuan, Maidi and Songgui sequences, by use of the digital data of these sequences obtained by near-field observation in the Western Yunnan Earthquake Prediction Experimental Field (WYEPEF). The magnitudes ML of the three micro-earthquake sequences are mostly between 1. 0 and 2. 5, the measured seismic moment M0 is between 10(10)N·m and 10(13)N·m, and the rupture radius a of the source between 90 m and 270 m. The study of the ambient stress parameters τ0 and Δσ shows that the ground motion peak velocity r·v is a function of the ambient stress parameters τ0 and Δσ, while the ambient stresses are related to the seismotectonics and the media states in turn. The values of ambient stress parameters τ0 and Δσ for the three earthquake sequences are between 0. 5-5. 0 MPa for the high stress earthquake swarm (Maidi sequence),between 0. 3-1. 5 MPa for the moderate stress swarm (Songgui sequence), and between 0. 01-0. 5 MPa for the low stress swarm (Er' yuan sequence). We have further discussed the dependence of the high, moderate and low stress states on the tectonics and on the difference in media state. This study can help us to understand deeply the relation that the features of ambient stress states bear with the seismogenic process and the potential earthquake risk zone. Besides, the authors have also found that there is an obvious difference between the moment scaling laws for large and small earthquakes. The scaling law between the peak velocity r·v and the seismic moment M0 is r·v∝M0 0.6 when M0>10(14) N·m, but is r·v∝M0(1/2) when M0<10(14) N·m.

The space and time distribution characteristics of the shear stress field for the sequence of the Wuding earthquake

Follow Chen and Duda's model of spectral fall-off of ω~3, the dependence of peak parameters of ground motion, peak displacement d_m, peak velocity vin and peak acceleration a_m, upon the environment stress τ_o-values are studied using near source seismic digital recordings for the sequence of the Wuding, Yunnan, M = 6.5 earthquake, in which, as a new thought, the peak parameters are assumed to be related to the medium Q-value. Three formulae for estimating the environment stress τ_o-values by the peak parameters of three types of ground motions are derived. Using these formulae, the environment stress τ_o-values are calculated for the sequence of the Wuding earthquake. The result show that τ_o-values calculated by the three formulae are constant largely, the averages of τ_o are in the range of 5.0-35 MPa for most earthquakes. It belongs to the high-stress earthquakes sequence: the high-stress values are restricted to the relatively small area closely near to the epicenter of the main shock. The fine distribu tion structure for the contours of the environment stress τ_o-values is related closely to the strong aftershocks. The analysis of spatial and temporal feature of To-values suggests that the earthquakes sequence in a rupture process generated at the specific intersection zone of seismo-tectonics under high-stress background.

Extensional structures of the Nan'an Basin in the rifting tip of the South China Sea: Implication for tectonic evolution of the southwestern continental margin

Nan'an Basin is a giant hydrocarbon basin,but its tectonic division scheme and associated fault systems has not been well understood.Based on newly acquired seismic data from the southwestern margin of the South China Sea,this study analyzed the structural units,tectonic feature and geodynamics of the sedimentary basin.The new data suggests that the Nan0 an Basin is a rift basin oriented in the NE-SW direction,rather than a pull-apart basin induced by strike-slip faults along the western margin.The basin is a continuation of the rifts in the southwest South China Sea since the late Cretaceous.It continued rifting until the middle Miocene,even though oceanic crust occurred in the Southwest Subbasin.However,it had no transfer surface at the end of spreading,where it was characterized by a late middle Miocene unconformity(reflector T3).The Nan'an Basin can be divided into eight structural units by a series of NE-striking faults.This study provides evidences to confirm the relative importance and interplay between regional strike-slips and orthogonal displacement during basin development and deformation.The NE-SW-striking dominant rift basin indicates that the geodynamic drivers of tectonic evolution in the western margin of the South China Sea did not have a large strike-slip mechanism.Therefore,we conclude that a large strike-slip fault system did not exist in the western margin of the South China Sea.

Multi-scale expression of spatial activity anomalies of earthquakes and its indicative significance on the space and time attributes of strong earthquakes

The noise modei based on 6 trous wavelet algorithm produces a multi-scale expression of image through the combination of wavelet transform and a testing modei of statistical significance. This kind of expression not only gives the formation and location of image structure on different scales, but also eliminates the influence of noise. Since the algorithm does not need any priori hypotheses, it is suitable for the data with complex structure. The research line is employed in this paper to analyze the spatial activity of earthquake. The method of how to recognize and describe the multi-scale space activity of earthquake is emphatically discussed in this paper. Taking typical sequences in Southwest China as research cases, we systematically study the structure characters of spatial activity of earthquake on different scales. Results show that multi-scale space structure to some extent possesses indicative effect on strong epicenters. And the foreshock anomalies of Songpan seismic sequence also reveal interesting pattern during the spatial-temporal evolvement.

Hydrocarbon Potential of Pre-cenozoic Strata in the North Yellow Sea Basin

The North Yellow Sea Basin ( NYSB ), which was developed on the basement of North China (Huabei) continental block, is a typical continental Mesozoic-Cenozoic sedimentary basin in the sea area. Its Mesozoic basin is a residual basin, below which there is probably a larger Paleozoic sedimentary basin. The North Yellow Sea Basin comprises four sags and three uplifts. Of them, the eastern sag is a Mesozoic-Cenozoic sedimentary sag in NYSB and has the biggest sediment thickness; the current Korean drilling wells are concentrated in the eastern sag. This sag is comparatively rich in oil and gas resources and thus has a relatively good petroleum prospect in the sea. The central sag has also accommodated thick Mesozoic-Cenozoic sediments. The latest research results show that there are three series of hydrocarbon source rocks in the North Yellow Sea Basin, namely, black shales of the Paleogene, Jurassic and Cretaceous. The principal hydrocarbon source rocks in NYSB are the Mesozoic black shale. According to the drilling data of Korea, the black shales of the Paleogene, Jurassic and Cretaceous have all come up to the standards of good and mature source rocks. The NYSB owns an intact system of oil generation, reservoir and capping rocks that can help hydrocarbon to form in the basin and thus it has the great potential of oil and gas. The vertical distribution of the hydrocarbon resources is mainly considered to be in the Cretaceous and then in the Jurassic.

Methods for Determining the Epicenter of Strong Earthquake

The analysis of seismic sequences is the primary objective for the study of the evolution of seismicity in a particular area, in order to determine a greater awareness of its seismogenic potential. The eventual determination of the epicenters of future earthquakes associated with the expected magnitude can be the tool to better seismic prevention. In this paper, we present some procedures for epicenter prediction of a strong earthquake, developed after a careful analysis of the fluctuations of latitude and longitude values in time and space and distance, between seismic events occurred in a specific area. By analyzing several seismic sequences, whose data have been taken from the numerous catalogs on seismicity, we noticed that the epicenters of the earthquakes that precede the strongest ones, tend to converge towards the epicenter where the strong earthquake will happen, following a pattern and a repetitive directional trend. Analysis of the pattern and trend, which represent the fluctuation of events and distances between pairs of earthquakes, has allowed us to localize the epicentral area of a future earthquake, which more reliably complements the other forecasting methods we have developed in the past. Retrospective tests performed on past seismic sequences have shown that the predictive procedures developed are able to identify in a simple way and in the short term, the area where a strong earthquake is most likely to occur.

Morphology, sedimentary characteristics, and origin of the Dongsha submarine canyon in the northeastern continental slope of the South China Sea

The Dongsha submarine canyon is a large canyon belonging to a group of canyons on the northeastern South China Sea margin Investigation of the Dongsha canyon is important for understanding the origin of this canyon group as well as the transport mechanism of sediments on the margin, and the evolution of the Taixinan foreland basin and the associated Taiwan orogenic belt. In this study, the morphology, sedimentary characteristics, and origin of the Dongsha canyon were investigated by inte- grating high-resolution multi-channel seismic reflection profiles and high-precision multibeam bathymetric data. This is a slope-confined canyon that originates in the upper slope east of the Dongsha Islands, extends downslope in the SEE direction, and finally merges with the South Taiwan Shoal canyon at a water depth of 3000 m. The total length and average width of the canyon are around 190 and 10 km, respectively. Eleven seismic sequence boundaries within the canyon fills were identified and interpreted as incision surfaces of the canyon. In the canyon fills, four types of seismic facies were defined： parallel onlap fill, chaotic fill, mounded divergent facies, and migrated wavy facies. The parallel onlap fill facies is interpreted as altemating coarser turbidites or other gravity-flow deposits and fine hemipelagic sediments filling the canyon valley. The chaotic fill faci- es is presumed to be debrites and/or basal lag deposits filling the thalwegs. The mounded divergent and migrated wavy seismic facies can be explained as canyon levees consisting mainly of overspilled fine turbidites and sediment waves on the levees or on the canyon-mouth submarine fans. Age correlation between the sequence boundaries and the ODP Site 1144 data suggests that the Dongsha canyon was initiated at approximately 0.9 Ma in the middle Pleistocene. Mapping of the canyon indicates that the canyon originated at the upstream portion of the middle reach of the modem canyon, and has been continuously expanding both upstream and downstream by retrogressive erosion, incision, and deposition of turbidity currents and other gravity transport processes. The ages of the sequence boundaries representing major canyon incision events are in good agreement with those of global sea-level lowstands, indicating that sea-level changes may have played an important role in the canyon＇s development. The Dongsha canyon developed in a region with an active tectonic background characterized by the Taiwan up- lifting and the development of the Taixinan foreland basin. However, no evidence suggests that the canyon formation is directly associated with local or regional faulting and magmatic activities. Turbidity currents and other gravity transport processes （includ- ing submarine slides and slumps） may have had an important influence on the formation and evolution of the canyon.