Research on the relationship between geophysical structural features and earthquakes in Mid-Yunnan and the surrounding area

In this study, we analyzed the gravity and, magnetic characteristics, and the occurrence of a fault zone and discussed the relationships between the two locations. The results reveal that the subsurface structures strikes are different compared with those in the research region. In other words, the geophysical advantageous directions from the gravity and magnetic anomalies are not the same as those caused by the surface structures. The local horizontal gradient results from the gravity and magnetic anomalies show that the majority of earthquakes occur along an intense fault zone, which is a zone of abrupt gravity and negative magnetic change, where the shapes match very well. From the distribution of earthquakes in this area, we find that it has experienced more than 11 earthquake events with magnitude larger than Ms7.0. In addition, water development sites such as Jinshajiang, Lancangjiang, and the Red River and Pearl River watersheds have been hit ten times by earthquakes of this magnitude. It is observed that strong earthquakes occur frequently in the Holocene active fault zone.

Comparison of River Terraces in the Middle Reach Valleys of the Yellow River and Analysis on the Multi-Gradational Features of Tectonism in the Formation of Terrace Series

Where the Yellow River flows through the Haiyuan-Tongxin arc-form tectonic region on the northeastern side of the Qinghai-Xizang (Tibet) Plateau, as many as 10～21 basis and erosion terraces have been produced, among which the biggest altitude above river level is 401m and the formation age of the highest terrace is 1.57 Ma B.P. Based on comparative analysis of the Yellow River terraces located separately in the Mijiashan mountain, the Chemuxia gorge, the Heishanxia gorge and the other river terraces in the vast extent of the northern part of China, it has been found that the tectonic processes resulting in the formation of the terrace series is one of multi-gradational features, i.e., a terrace series can include the various terraces produced by tectonic uplifts of different scopes or scales and different ranks. The Yellow River terrace series in the study region can be divided into three grades. Among them, in the first grade there are 6 terraces which were formed separately at the same time in the vast extent of the northern part of China and represent the number and magnitude of uplift of the Qinghai-Xizang Plateau since 1.6 Ma B. P.; in the second grade there are 5 terraces which were separately and simultaneously developed within the Haiyuan-Tianjingshan tectonic region and represent the number and magnitude of uplift of this tectonic region itself since 1.6Ma B.P.; in the third grade there are 10 terraces which developed on the eastern slope of the Mijiashan mountain and represent the number and amplitude of uplift of the Haiyuan tectonic belt itself since 1.6Ma B.P. Comparison of the terrace ages with loess-paleosoil sequence has also showed that the first grade terraces reflecting the vast scope uplifts of the Qinghai-Xizang Plateau are very comparable with climatic changes and their formation ages all correspond to the interglacial epochs during which paleosoils were formed. This implies that the vast extent tectonic uplifts resulting in river down-cutting are closely related to the warm-humid climatic periods which can also result in river downward erosion after strong dry and cold climatic periods, and they have jointly formed the tectonic-climatic cycles. There exists no unanimous and specific relationship between the formation ages of the second and third grade terraces and climatic changes and it is shown that the formation of those terraces was most mainly controlled by tectonic uplifts of the Tianjingshan block and the Haiyuan belt. The river terraces in the study region, therefore, may belong to 2 kinds of formation cause. One is a tectonic-climatic cyclical terrace produced jointly by vast extent tectonic uplifts and climatic changes, and the terraces of this kind are extensively distributed and can be well compared with each other among regions. Another is a pulse-tectonic cyclical terrace produced by local tectonic uplifts as dominant elements, and their distribution is restricted within an active belt and can not be compared with among regions.

Relocation of hypocenters from DOMERAPI and BMKG networks： a preliminary result from DOMERAPI project

Merapi volcano located in central Java, Indonesia,is one of the most active stratovolcanoes in the world. Many Earth scientists have conducted studies on this volcano using various methods. The geological features around Merapi are very attractive to be investigated because they have been formed by a complex tectonic process and volcanic activities since tens of millions of years ago. The southern mountain range, Kendeng basin and Opak active fault located around the study area resulted from these processes. DOMERAPI project was conducted to understand deep magma sources of the Merapi volcano comprehensively. The DOMERAPI network was running from October 2013 to mid-April 2015 by deploying 46 broad-band seismometers around the volcano. Several steps, i.e., earthquake event identification,arrival time picking of P and S waves, hypocenter determination and hypocenter relocation, were carried out in this study. We used Geiger＇s method（Geiger 1912） for hypocenter determination and double-difference method for hypocenter relocation. The relocation result will be used to carry out seismic tomographic imaging of structures beneath the Merapi volcano and its surroundings. For the hypocenter determination, the DOMERAPI data were processed simultaneously with those from the Agency for Meteorology, Climatology and Geophysics（BMKG） seismic network in order to minimize the azimuthal gap. We found that the majority of earthquakes occurred outside the DOMERAPI network. There are 464 and 399 earthquakes obtained before and after hypocenter relocation, respectively. The hypocenter relocation result successfully detects some tectonic features, such as a nearly vertical cluster of events indicating a subduction-related backthrust to the south of central Java and a cluster of events to the east of Opak fault suggesting that the fault has an eastward dip.

Tectonics and Mechanisms of Uplift in the Central Uplift Belt of the Huimin Depression

The Huimin（惠民） depression is a third-level tectonic element of the Bohai（渤海） Bay basin in eastern China.The central uplift belt is the most important oil and gas accumulation zone in the depression,but the lack of adequate geological studies in the area has greatly hindered exploration and development.In this article,using seismic data,fracture mechanics,and a combination of data on fault growth indices and fault throws,we present an analysis of tectonic activity in the central uplift belt and adjacent regions.The amount of extension is calculated along balanced N-S cross-sections,along with the thickness of strata eroded from the fourth,third,and second members of the Shahejie（沙河街） Formation（Es4-Es2） in the uplift belt,by analyzing porosity and stratigraphic correlations.In addition,uplift features are described,and their timing and processes of formation are analyzed and dis-cussed.The results indicate that strike-slip and extensional tectonic movements coexisted,with the effects of the latter most obvious.The spatial and temporal nature of the extensional move-ments is varied：fault activity during the period Ek-Es4 was the strongest on the northern Ningnan（宁南） fault,and activity in the western part of the area was stronger than that in the east,which in turn was stronger than that in the central region;during Es3-Es2,the strongest fault movements were along the eastern part of the middle Linyi（临邑） fault,and activity in the western part of the area was rela-tively weak,whereas in the mid-west it was the strongest.The extensional movements were a response to the activity of the faults.The sediments in the lower part of the fourth member of the Shahejie For-mation（Es4x） show that Es4 was the time when the central belt first began to be uplifted strongly.Uplift was uneven during the Paleogene：the western part of the area was uplifted continuously,while the mid-eastern area underwent alternating periods of uplift and subsidence.During the Paleogene,a number of different tectonic features developed in the central uplift belt at different times and are manifested as follows：during the period Ek-Es4,a gentle slope was formed as part of the Zizhen（滋镇） sag;during Es3-Es2,the northern part of the central uplift belt continued to display a gentle slope as part of the Zizhen sag,but the southern region developed an steep slope as part of the Linnan（临南） sag.There are close interrelationships between uplift,strike-slip,and extension within the central uplift belt of the Huimin depression,as is manifest by the areas of strongest extension being uplifted most rapidly,and also eroded the most.