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 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.

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.

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.

Estimation of seismic stress drop from the peak velocity of ground motion

According to the source dislocation model suggested by Brune(1970), the authors have calculated the displacement spectra of S wave and source parameters of the Heqing M S 5 3 earthquake sequence, using the digital data of this sequence obtained in the Western Yunnan Earthquake Prediction Experimental Field (WYEPEF). Based on this calculation we have studied the dependence of the peak velocity ( rv ) of ground motion on the seismic stress drop Δ σ . From the seismic scaling law we obtained ( rv )∝Δ σ 2/3 , thus the three formulae of calculating seismic stress drop Δ σ using the peak velocity parameters can be derived: lg( rv)=d 1+13lg M 0+23lgΔ σ ; lg( rv) =d 2+13 M L+23lgΔ σ ; lgΔ σ =-1 0+1 5lg( rv ) Assuming that the average stress drop Δ σ =3.0×10 6 Pa for great and small earthquakes, then the constants d 1=-3 88 and d 2=-0 38 are determined by the observational data of the Heqing M S5 3 sequence. Results of the source parameters for this sequence show that the seismic moment M 0 is between 10 11 N·m and 10 15 N·m, the rupture radius a of the source is between 200 m and 600 m, the stress drop Δ σ is between 0 1 MPa and 10 MPa and the average stress drop Δ σ =3 7 MPa calculated from the peak velocity parameter of the ground motion. Δσ values measured from these scaling relations are basically in agreement with the results given by Brune′s method( 1970). Results of this study show that the dependence of the ground motion peak velocity parameter (rv) on the stress drop Δσ is even stronger than that on the seismic moment M 0 .

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.

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.