Regeneration and anti-migration of sand waves associated with sand mining in the Taiwan Shoal

Sand waves in the Taiwan Shoal are characterized by two distinct spatial scales. Giant sand waves have a length of2 kilometers with height between 5 m and 25 m, whilst small sand waves is less than 100-m long with height less than 5 m between giant sand wave peaks(crests). A series of five high-resolution multi-beam echo-sounding surveys between 2012 and 2020 in the middle of Taiwan Shoal indicated that artificial dredging on the giant sand waves had caused sand wave reform and evolution. Overall, the removal of giant sand waves significantly affected the migration of small sand waves adjacent to the dredging site, with the latter on both sides of the former appear to migrate towards the dredging pit. Moreover, in the dredging area, new sand waves emerged with wavelength much smaller than the original giant sand waves, while the convergent pattern of the small sand waves tends to store and form the giant sand waves, which might spread far beyond the survey period.

Differences of polygonal faults related to upper Miocene channels:a case study from the Beijiao sag of Qiongdongnan basin,South China Sea

Deep-water coarse-grained channels are embedded within a polygonal fault tier,and the polygonal faults(PFs)present non-polygonal geometries rather than classic polygonal geometry in plan view.However,PFs present differences when they encounter deep-water(coarse-grained vs.fine-grained)channels with different lithology,which has not been further studied to date.Three-dimensional(3D)seismic data and a drilling well from Beijiao sag of Qiongdongnanbasin,South China Sea were utilized to document the plan view and cross-sectional properties of the PFs and their differences and genetic mechanism were investigated.Results show that,first,PFs can be divided morphologically into channel-segmenting PFs and channel-bounding PFs in plan view.The former virtually cuts or segments the axes of channels in highand low-amplitudes,and the latter nearly parallels the boundaries of the channels.Both are approximately perpendicular to each other.Secondly,channel-bounding PFs that related to low-amplitude channels are much longer than those of high-amplitude ones;channel-segmenting PFs related to low-amplitude channels are slightly longer than the counterparts related to high-amplitude channels.Lastly,the magnitudes(e.g.,heights)of the PFs are proportional to the scales(e.g.,widths and heights)of low-amplitude channels,whereas the magnitudes of the PFs are inversely proportional to the scales of high amplitude channels.Coarse-grained(high amplitude)channels act as a mechanical barrier to the propagation of PFs,whereas fine-grained(low-amplitude)channels are beneficial to the propagation and nucleation of PFs.Additionally,the genetic mechanism of PFs is discussed and reckoned as combined geneses of gravitational spreading and overpressure hydrofracture.The differences of the PFs can be used to reasonably differentiate coarse-grained channels from fine-grained channels.This study provides new insights into understanding the different geometries of the PFs related to coarse-grained and fine-grained channels and their genetic mechanism.

Three-dimensional S-wave velocity structure of the crust and upper mantle for the normal fault system beneath the Yinchuan Basin from joint inversion of receiver function and surface wave

A series of parallel normal faults are distributed in the Helan Mountain-Yinchuan Basin tectonic belt,where a historical M8.0 earthquake occurred.It is rare that such a great earthquake occurs in a normal fault system within the continent.To deeply understand the fine structure of the normal fault system,we deployed 104 broadband temporary stations near the system,collected data from permanent stations and other temporary stations nearby,and obtained the high-precision threedimensional S-wave velocity structure beneath 206 stations via joint inversion of receiver function and surface wave.A typical graben-in-graben feature bounded by four major faults was identified in the Yinchuan Basin.We analyzed the seismicity in the normal fault system and found a seismic strip in the southern part of the basin,where there are significant changes in the sedimentary thickness,which is speculated to be the southern boundary of the normal fault system.There are significant differences in the crustal thickness and velocity structure in the crust on both sides of the boundary between the Helan Mountain and the Yinchuan Basin,and a low-velocity zone was identified in the upper mantle beneath this boundary,which could be related to the fact that the Helan Mountain-Yinchuan Basin tectonic belt is located between the Alxa Block and the Ordos Block.The M8.0 Yinchuan-Pingluo earthquake occurred at the junction of four major faults in the Yinchuan Basin,which was located in the high-velocity zone near the velocity transition zone at the basin-mountain boundary.The low-velocity zone in the upper mantle beneath this boundary may have promoted the nucleation of this earthquake.Based on evidence from geological drilling,micro seismicity,the regional stress field,and the velocity models obtained in this study,it is inferred that the eastern piedmont fault zone of the Helan Mountain was the seismogenic fault of the 1739 M8.0 Yinchuan-Pingluo earthquake.