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Late quaternary activity research of the northern marginal fault of Emei Platform, Shanxi Province
Abstract
Based on the 1:50 000 geological mapping of active fault, the paper investigates the stratum, topography and faulted landforms of the northern marginal fault of Emei Platform, and preliminarily divides the northern marginal fault of Emei Platform into three sections by two stepovers near Tanjiazhuang Village and Nanliu Village according to different fault activity of each section. At west of Tanjiazhuang Village is a loess platform, and the high terrain scarp can be seen from the northern margin. The height of scarp decreases progressively and the slope becomes gentle westwards at the place between Nanchi Village and Xikang Village, and to the place near Xiaoliang town, we cannot see obvious terrain scarps. The faulted sections can only be seen in the gullies which cross the terrain scarp at the south of Guozhuang Village and Tanjiazhuang Village. The fault dislocates the Pliocene red clay and the middle Pleistocene Lishi loess and covered by Malan loess; continuous paleosoil can be seen across the terrain scarp in some gullies. These indicate that in this section the fault was active in the early middle Pleistocene and its activity becomes weaker or no longer active after that. The fault in the section between Tanjiazhuang Village and Nanliu Village can be divided into three parts by Shidian Village and Jinming Village, which are named, from west to east in sequence according to each faulted landform, the northern marginal fault of lacustrine terrace, the piedmont fault of Zijin Mountain and the northern marginal fault of loess platform. The fault transition area between each part is continuous and the fault is in linear distribution, so we see the whole fault section as having the same activity. In this section the Holocene diluvial fan is faulted. At least two plaeoearthquake events happened since Holocene, and the latest activity is in(2.00~1.29) ka BP according to Renzhuang trench and Jinsha trench, which can be well compared with former researches. The fault slip rate is over 0.33 mm/a in the section south of Maguduo Village and is more than 0.36 mm/a according to Renzhuang trench since the later period of the late Pleistocene. In the section between Nanliu Village and Xizhangpo Village, the fault distributes along the frontal edge of the diluvial platform and is covered by thick loess. A 50~200 m high linear terrain scarp formed due to the activity of fault can be seen along the frontal edge especially in the part between Xunwang Village and Xulu Village. At north of Wuzhai Village, the height of scarp decreases progressively and to the place near Xizhangpo Village, the terrain scarp cannot be seen clearly. In this section, Malan loess is faulted, which indicates that this fault section has been active since the late Pleistocene, but the evidence of Holocene fault activity has not been obtained yet due to the non-development of Holocene stratum. The fault slip rate is no less than 0.1 mm/a since the late Pleistocene according to the faulted section at south of Xunwang Village.
The Fen–Wei rift zone (FWRZ) of North China is an important zone of active crustal deformation representing a transition from extrusion tectonics related to the Tibetan Plateau to subduction tectonics related to the potential far-field influence of the west Pacific plate. In this study, we determined the kinematic constraints of active crustal deformation in the FWRZ, which are fundamental for forecasting seismicity. NeoKinema, a kinematic finite-element model, was employed to estimate the long-term fault slip rates, distributed crustal deformation field, and on- and off-fault strain-rate fields in the FWRZ by fitting updated geological fault slip rate, geodetic GPS velocity, and principal compressive stress direction datasets. Our results show that the FWRZ is a characteristic low-strain kinematic setting, with most active faults exhibiting slip rates of less than 1 mm/a. The total sinistral shear rate from the southern Ordos block to the Qinling Mountains is approximately 1 mm/a, indicating limited tectonic extrusion along the EW-trending Qinling Mountains. Additionally, the central Shanxi rift exhibits prominent dextral shear of ∼0.5 mm/a that decreases toward its north and south ends, corresponding to crustal extension of 1.1–1.2 mm/a in the Datong and Yuncheng basins, respectively. However, this significant crustal extension cannot be solely attributed to terminal effects caused by dextral shear in the central Shanxi rift. A comparison between predicted seismicity and historical earthquake records reveals some remarkable seismic gaps, particularly in the Datong, Hancheng, and Yuncheng basins, indicating higher seismic potential in these locations. This study provides insights into the long-term crustal deformation processes and regional seismic potential of the FWRZ.
The Shanxi rift system is one of the most active intraplate tectonic zones in the North
China Block, resulting in devastating seismicity. Since 1303, the rift has experienced fifteen
Ms ≥ 6.5 earthquakes. Aiming at a better understanding of Coulomb stress evolution and its
relationship with the seismicity in the rift system, we investigated the Coulomb stress changes
due to coseismic slip and post-seismic relaxation processes following strong earthquakes as
well as the interseismic tectonic loading since the 1303 Hongdong Ms = 8.0 earthquake. Our
calculation applies a specified regional medium model, takes the gravity effect into account
and uses the fault geometry of the next event as the receiver fault in a given calculation. Our
results show that nine out of 12Ms ≥6.5 earthquakes since the 1303 Hongdong earthquake and
more than 82 per cent of small-medium instrumental events after the 1989 Datong-Yanggao
Ms = 6.1 earthquake fall into the total stress increased areas. Our results also reveal the
different roles of the coseismic, post-seismic and interseismic Coulomb stress changes in the
earthquake triggering process in the Shanxi rift system. In a short period after a strong event,
the stress field changes are dominated by coseismic Coulomb stress due to sudden slip of the
ruptured fault, while in the long term, the stress field is mainly dominated by the accumulation
of interseismic tectonic loading. Post-seismic stress changes play an important role by further
modifying the distribution of stress and therefore cannot be ignored. Based on the current
stress status in the Shanxi rift system, the Linfen basin, southern and northern Taiyuan basin,
Xinding basin and the north part of the rift system are identified as the most likely locations of
large events in the future. The results of this study can provide important clues for the further
understanding of seismic hazard in the Shanxi rift system and thus help guiding earthquake
risk mitigation efforts in the future.
Key words: Earthquake dynamics; Earthquake interaction, forecasting, and prediction;
Seismicity and tectonics; Continental tectonics: extensional; Asia.
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