Fractal analysis of major faults and fractal dimension of lineaments in the Indo-Gangetic Plain on a regional scale

The Indo-Gangetic Plain(IGP)is one of the most seismically vulnerable areas due to its proximity to the Himalayas.Geographic information system(GIS)-based seismic characterization of the IGP was performed based on the degree of deformation and fractal dimension.The zone between the Main Boundary Thrust(MBT)and the Main Central Thrust(MCT)in the Himalayan Mountain Range(HMR)experienced large variations in earthquake magnitude,which were identified by Number-Size(NS)fractal modeling.The central IGP zone experienced only moderate to low mainshock levels.Fractal analysis of earthquake epicenters reveals a large scattering of earthquake epicenters in the HMR and central IGP zones.Similarly,the fault fractal analysis identifies the HMR,central IGP,and south-western IGP zones as having more faults.Overall,the seismicity of the study region is strong in the central IGP,south-western IGP,and HMR zones,moderate in the western and southern IGP,and low in the northern,eastern,and south-eastern IGP zones.

On the physical model of earthquake precursor fields and the mechanism of precursors＇timespace distribution（Ⅲ）──anomalies of seismicity and crustal deformation and their mechanisms when a strong earthquake is in prep

By studying the seismicity pattern before 37 earthquakes with M≥6. 0 in North China and the pattern of crustal deformation in the Capital Area from 1954 to 1992, some abnormal characteristics of these patterns before strong earthquakes have been extracted. A comparison has been made between the anomalies of these two kinds of Patterns. From the results we can know the following. ① Before a strong earthquake, the seismicity will strengthen and the crustal deformation rate will increase. ② Several years before a strong earthquake, there will be seismic gaps and deformation gaps around the epicenter of the quake. ③ The dynamic parameters of patterns all show a decrease in information dimension. This means that the crustal deformation has become more and more localized with time and it gives an important indication showing that a strong earthquake is in preparation. At the end of the paper, the physical mechanisms of the abnormal patterns of seismicity and crustal deformationhave been explained in a unified way in terms of the earthquake-generating model of a inhomogeneous strongbody in inhmogeneous media.

Seismic stress perturbation and triggering patterns induced by the 2016 Central Italy earthquake sequences

Numerous shallow earthquakes, including 24 th August Amatrice, 26 th October Visso, and 30 th October Norcia earthquakes, ruptured the segments of Mount Vettore-Gorzano fault system in the central Apennines(Italy) in 2016. In order to investigate the stress perturbation and triggering patterns among the earthquake sequences, we introduce a more realistic nonplanar coseismic fault geometry model, which improve the rupture model by assimilating relocated aftershocks and the GPS observations. We adopt the seismic slip inversion program of the steepest descent method(SDM) to create the detailed coseismic rupture models and optimize Coulomb Failure Stress model by varying the coefficient of friction and received fault parameters. The results indicate that the nonplanar fault geometry model is more reflective of the deep slip of the coseismic rupture than planar model. As evidenced by the coseismic Coulomb stress changes caused by the three mainshocks at different depth slices, the stress loading mainly distributes on the active fault zones and the stress changes can well explain the spatial distribution of aftershocks. The first large Amatrice mainshock accelerates the occurrence of the Mw 5.9 Visso and Mw 6.6 Norcia earthquakes, with the positive stress changes at the hypocenter exceeding the stress triggering threshold(0.010×10^(6) Pa) and up to 0.015×10^(6) and 0.257×10^(6) Pa, respectively. Furthermore, the Mw 5.9 Visso earthquake as well encourages the occurrence of the Mw 6.6 Norcia event with the increased stress changes of 0.052×10^(6) Pa on the hypocenter. It is concluded that the stress transfer and accumulation play crucial roles on the linkage triggering mechanism among the mainshock-mainshock and mainshockaftershocks. Noteworthily, the cumulative stress changes on the southwest segment of the Norcia Fault(NF), the southeast parts of the Montereale Fault System(MFS) and Mount Gorzano Fault(MGF) of the main regions are up to(1.5~3.5) ×10^(6) Pa. The cumulative stress changes have not been released sufficiently by aftershocks, which may increase the seismic hazard in those regions.

The fracture characteristic of the 1989 Datong-Yanggao earthquake sequence

This paper, first, analyzed the seismic activity patterns of the Datong-Yanggao earthquake sequence by 4 stages,finding that three times earthquakes of Mis were located in a same NNE seismically active zone and two times located in 2 NWW seismically active zones. Then the focal-mechanism solutions of 12 earthquakes of M≥4 and 114 earthquakes of M≥1. 0 in various zones were obtained, a tectonic analysis was made for these data, the obtained fracture characteristic of this sequence is: the conjugate fracture combination with the NNE right lateral faults as its trunk and the two NWW left lateral faults as its branches. Finally, it is pointed out that there is an obvious difference between the seismic fracture and the tensile dip-slip tectonic activity of the main body of the seismic region.

Application of digital image processing to the determination of spatial distribution of earthquakes

This paper has proposed a practical method for determining the spatial distribution pattern of earthquakes by means of digital image processing and has given some calculation results. This method can overcome the artificial arbitrariness which is usually inevitable in determining the spatial distribution of earthquakes. Meanwhile, the form of seismic gaps and the method for identifying seismic gaps have also been discussed. It should be pointed out that the method proposed in this paper is a new idea in this respect. However, this method is still unable to determine the seismic gap uniquely. In order to identify the real seismic gap, comprehensive analysis of the variation of other relevant parameters (e. g., the b-value, etc. ) should also be made. The results are as follows; Thephenomenon that an area of high seismic activity is surrounded by low seismicity areas, i. e., the seismic clustering pattern is the general form of spatial distribution of earthquakes; whereas a low seismicity area surrounded by high seismicity areas, i. e., the seismic gap pattern, is only an important form of spatial distribution of earthquakes. Earthquakes often occur in a certain seismic clustering area or on its margin.

Two-step interface and velocity inversion—— Study o e of the Tangshan seismic region

This paper studies the computation method of two step inversion of interface and velocity in a region. The 3 D interface is described by a segmented incomplete polynomial; while the reconstruction of 3 D velocity is accomplished by the principle of least squares in functional space. The computation is carried out in two steps. The first step is to inverse the shape of 3 D interface; while the second step is to do 3 D velocity inversion by distributing the remaining residual errors of travel time in accordance with their weights. The data of seismic sounding in the Tangshan Luanxian seismic region are processed, from which the 3 D structural form in depth of the Tangshan seismic region and the 3 D velocity distribution in the crust below the Tangshan Luanxian seismic region are obtained. The result shows that the deep 3 D structure in the Tangshan seismic region trends NE on the whole and the structure sandwiched between the NE trending Fengtai Yejituo fault and the NE trending Tangshan fault is an uplifted zone of the Moho. In the 3 D velocity structure of middle lower crust below that region, there is an obvious belt of low velocity anomaly to exist along the NE trending Tangshan fault, the position of which tallies with that of the Tangshan seismicity belt. The larger block of low velocity anomaly near Shaheyi corresponds to a denser earthquake distribution. In that region, there is an NW trending belt of high velocity anomaly, probably a buried fault zone. The lower crust below the epicentral region of the Tangshan M S=7.8 earthquake is a place where the NE trending belt of low velocity anomaly meets the NW trending belt of high velocity anomaly. The two sets of structures had played an important role in controlling the preparation and occurrence of the M S=7.8 Tangshan earthquake.