Preface to the special issue on complicated features on seismic sources and deep structures

Earthquake is a kind of natural phenomenon in the Earth. Earthquake seismology has been developed quickly since modern digital seismometers were applied to observations. From seismic records, seismologists study source properties of the earthquakes and detect multi-scale structural variations in the crust, mantle, and core. Earthquake is complicated, including seismicity and seismic sources. (Lay et al., 2018; Chen YT et al., 2019; Zhang X et al., 2019; Gao Y, 1996). The lateral variations of deep structures of the earthquake zones are associated with tectonic evolution and regional deformation (Yao HJ, 2020; Huang JL and Zhao DP, 2006; Zhang ZJ et al., 2011; Bao XW et al., 2013). In order to understand the complexity of seismic sources and deep structures, we organized this special issue on complicated features on seismic sources and deep structures in the journal of Earthquake Science, which is sponsored by the Seismological Society of China and the Institute of Geophysics, China Earthquake Administration.

Direct source of the earthquake hazard is the strong motion caused by a buried fault or the fracturing of ground surface produced by a shallow slip, and the hazard consequence is usually associated closely with the complexity of earthquake rupture process. Therefore, understanding the rupture processes which produce the ground strong motion or generate fracturing is extremely essential to seismic hazard mitigation all the time. In this special issue, there are three papers contributing new understandings on the kinematic process of a special earthquake initiated at the top mantle, on the effect of the nucleation and slipping depths and on the dynamic process of two landslides.

The M_W6.7 Iburi earthquake 2018 that shocked Hokkaido is a destructive inland earthquake. Ren CM et al. resolved the kinematic rupture process of this M_W6.7 earthquake by a joint inversion of strong motion and InSAR observations. Yang HF et al. reviewed shallow destructive earthquakes. The depths of earthquake occurrence and the large slip distribution are critical for seismic hazard assessment. Whether the large slip of seismic source is near the ground surface will result in the difference in seismic hazard. Landslide recorded by seismometers could be considered as a non-tectonic event of shallow complicated seismic source. Li WY et al. used seismic records of two river-blocking landslides in Jinsha River in 2018 to study complex dynamics of landslides and considered the better understanding of physical processes is beneficial to disaster mitigation. These three papers present complicated features of seismic sources.

Strong and large earthquakes frequently occur in China. There are lots of large faults through Chinese mainland, including strike-slip faults as well as reverse faults. For instance, the Longmenshan fault is a large reverse fault in the east margin of Tibetan Plateau where occurred Wenchuan M_S8.0 in 12 May 2008, and the Tancheng-Lujiang fault (Tanlu fault in short) is a large strike-slip fault in the eastern China where occurred Tancheng M8.5 in 25 July 1668. Seismicity, tectonics and structure in the east margin of Tibetan Plateau have been studied relatively much recent years (Li QS et al., 2020). However, the research on the Tanlu fault in the eastern China is far from sufficient. In this special issue, there are five papers focusing on deep structures, three of which are contributing new understandings on the seismic and magnetotelluric structures of the Tanlu fault.

Xu J et al. presented the high-resolution tomographic results by the passive seismic and magnetotelluric linear array data to characterize subsurface structures beyond 2 km depths in the southern segment of Tanlu fault, north side of Chao lake. Receiver functions are effective to image Moho depths and physical characteristics. Hong DQ et al. obtained the lateral variation of Moho depths beneath the central and southern segment of the Tanlu fault zone and discussed the relationship to tectonic units, such as the Dabie orogenic belt, boundary between North China block and South China block. Li LL et al. imaged the thicknesses of sediment, crystalline crust and Moho, as well as the v_P/v_S ratios beneath the whole Tanlu fault zone and the adjacent area, such as Songliao basin, Hehuai basin and Subei basin. The variations from north to south as well as from west to east suggested that the variation is closely related to deep tectonic movements.

Shao XH et al. adopted seismic records shot by the airgun in the Anhui segment of Yangtze river, the new data source developed quickly in recent years (Chen Y et al., 2017), to image the 3D isotropic and azimuthally anisotropic velocity structures at depths of 1–10 km around the middle-lower Yangtze river using the first arrivals from airgun sources. It shows the ore-related high-velocity anomalies and complicated azimuthal anisotropy related to regional faults. Li Y et al. used shear-wave splitting analysis to obtain preliminary seismic anisotropy in the upper crust in the southern segment of Xiaojiang fault zone, by the seismic stations of the temporary linear seismic broadband array and the permanent network. It presented the difference of spatial distribution of seismic anisotropy, as reported somewhere else before (Zhao B et al. 2012), and discussed its tectonic implications. Last two papers touched the topic of seismic anisotropy, a developing research field of seismology.

This special issue is publishing eight papers, three of which are seismic sources, five of which are seismic structures. It suggests that complicated seismic sources from shallow subsurface to the mantle, complicated 3D tomography and seismic anisotropy are all important scientific topics. To be specific, the internal interface in the lithosphere, the deep structure beneath the fault, new observation data and improved data process techniques are brought into focus. Last, we would like to thank all the authors for their timely contributions to this special issue, thankful to editor-in-charge and all reviewers for their warmly contributions.