The relationship between crust-lithosphere structures and seismicity on the southeastern edge of the Tibetan Plateau

Journal Pre-proof The relationship between crust-lithosphere structures and seismicity on the southeastern edge of the Tibetan Plateau

Xingqian Xu, Lijun Su, Junzhe Liu, Wanhuan Zhou, Xin Qu PII:

S0040-1951(19)30415-9

DOI:

https://doi.org/10.1016/j.tecto.2019.228300

Reference:

TECTO 228300

To appear in:

Tectonophysics

Received date:

6 February 2019

Revised date:

10 November 2019

Accepted date:

21 November 2019

Please cite this article as: X. Xu, L. Su, J. Liu, et al., The relationship between crustlithosphere structures and seismicity on the southeastern edge of the Tibetan Plateau, Tectonophysics(2019), https://doi.org/10.1016/j.tecto.2019.228300

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© 2019 Published by Elsevier.

Journal Pre-proof

The Relationship Between Crust-lithosphere Structures and Seismicity on the Southeastern Edge of the Tibetan Plateau

Xingqian Xu1，Lijun Su2,3,4*，Junzhe Liu2，Wanhuan Zhou5，Xin Qu6

1 College of Water Conservancy, Yunnan Agricultural University, Kunming, Yunnan 650201, P. R. China.

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2 Key Laboratory of Mountain Hazards and Earth Surface Process, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu, Sichuan 610041, China.

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3 CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China.

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4 University of Chinese Academy of Sciences, Beijing 100049, China. 5 Department of Civil and Environmental Engineering, Faculty of Science and Technology,

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University of Macau, Macau, China.

College of Civil and Architecture Engineering, Anyang Institute of Technology, Henan

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455000, P. R.China.

Key Points:

The escape flow of hot asthenospheric material beneath the plateau moving roughly

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

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Corresponding author: [email protected] (L. Su).

eastward is blocked by the thick Sichuan Basin. 

Seismicity have a good consistency with crust-lithosphere deformation.

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Discontinuities in the lithosphere could be used as a reference parameter for seismic hazard analysis and long-term earthquake prediction.

Keywords LAB, S wave receiver function, Southeastern Tibet, Longmenshan(LMS), Seismicity

Manuscript submitted to Tectonophysics for possible publication as an article 6 February, 2019

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1 Introduction The southwestern mountain area of China and the southeastern edge of the Tibetan Plateau have a sharp rise and fall of topography with complex geological structures and extreme development of deep faults (e.g., Molnar and Tapponnier, 1975; England and Molnar, 1997; Mao et al., 2005; Zhang et al., 2009). Since Quaternary, especially the late Quaternary, this area has experienced deep fault activity: frequent earthquakes have occurred, and

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considerable loss of life and property has been caused by extremely destructive earthquakes

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(Teng et al., 2008; Wu et al., 2008; Liu et al., 2013). The depth and topography of the Moho and the lithosphere-asthenosphere boundary (LAB) are the most important parameters to

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understand the crust-lithosphere structure and seismicity (Kenbett and Engdahl, 1991; Kind et

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al., 2012; Hu et al., 2013).

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To recognize the movement status and characteristics of hot plastic fluid in the upper mantle, the depth and undulating variation of the lithosphere should be accurately identified

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(Royden et al., 2008; Zhang et al., 2010; Hu et al., 2011, 2012). In an attempt to solve the key

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technical issue of obtaining the high-resolution lithosphere thickness, a number of studies have been conducted using several methods of body wave tomography (Huang et al., 2002;

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Wang et al., 2003), surface wave dispersion (Hu et al., 2008a; Li et al., 2008), SKS anisotropy measurements (Wang et al., 2008), the P receiver function (Yang et al., 2009; Zhang et al., 2010; Hu et al., 2013), magnetotelluric sounding (Li et al., 2014; Cheng et al., 2015), global positioning system (GPS) observations (Zhang et al., 2004; Gan et al., 2007) and finite element simulation (Wang et al., 2007). Most investigations focused on typical issues, such as the crust and lithosphere deformation, the mantle hot material transport processes and the dynamic mechanism below the Qinghai-Tibet Plateau. The earth’s crust is dragged by the hot plastic fluid in the uppermost mantle, and the lithosphere is formed as only a weak interface between the lower crust and upper mantle lid

Journal Pre-proof above the asthenosphere in seismic earth models. Tomography and the surface wave dispersion method could be applied to detect the structure of the crust and mantle at a large scale and low resolution. Zhang et al (2007) obtained the depth distribution of the crust, the lithosphere and the asthenosphere by using surface wave dispersion with group velocity dispersion inversion for the Qinghai-Tibet Plateau and its adjacent areas, but the scale was too large. Li et al. (2010) found that the movement direction of the Sichuan-Yunnan block was prevented by the thick and hard lithosphere below the Sichuan basin when using ambient

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noise Rayleigh wave phase velocity tomography technology. Both horizontal and vertical

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resolutions are low for surface wave tomography with respect to lithosphere detection (Hu et

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al., 2008b; 2012). Body wave tomography has relatively high resolution but is not sensitive to

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the vertical velocity, and the vertical resolution is limited (McKenzie and Priestley, 2008; Priestley and Tilmann, 2009). A numerical simulation is applicable for linear and nonlinear

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problems to handle complex physical problems with the advantages of low cost, rapid

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calculation and clear physical concepts. Wang et al (2007) established a three-dimensional finite element model to discuss the dynamic mechanism in the Sichuan-Yunnan region by

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simulating the current crustal movement and stress distribution; however, with too many

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assumptions, the reliability of the results from this method is rather difficult to verify. In the last decades, the P-wave function technique has been used to calculate a relatively accurate crust thickness, but the depth of the lithosphere calculated by this method is not convincing (Langston, 1977; Lawrence and Shearer, 2006; Hu et al., 2015a). Yang et al (2009) obtained the crustal thickness and S-wave velocity structure in western Sichuan using the P-wave function and found that this region is a tectonic environment for large earthquakes in the future. However, the above studies indicated that there is probably a certain relationship between the crust-lithosphere structure and seismicity. With respect to the P-wave receiver function, the Ps converted wave is often submerged

Journal Pre-proof in the multiple reflected waves from shallow discontinuities. Consequently, the Ps converted wave signal is very weak, or even false, and is difficult to identify for deep discontinuity detection. The depth of the lithosphere interface was accurately calculated by the S-wave receiver function method, which could eliminate the disadvantages of the P-wave receiver function in terms of lithosphere detection (e.g., Li et al., 2000; Kumar et al., 2005; Yuan et al., 2006; Hu et al., 2012, 2015a). Sp converted waves will generate as the S-wave propagates through interfaces within the crust-mantle transition zone. Due to the fact that the S-wave

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velocity is faster than the P-wave velocity, Sp converted waves reach the station prior to the

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S-waves (Hu et al., 2011; Kind et al., 2012). All other multiple reflected waves from shallow

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discontinuities followed the S-wave. The Sp converted wave is relatively independent without

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any interference of the reflected waves. By contrast, the S-wave receiver function method is the most effective way to obtain the depth of the crust-lithosphere structure.

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In this study, we use S receiver functions from 51 permanent broad-band seismic stations

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deployed in the Sichuan region to estimate the crust-lithosphere structure. We then discuss the relationship between crust-lithosphere structures and seismicity from 1970 to 2017, obtained

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from digital seismic records, on the southeastern edge of the Tibetan Plateau.

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2 Tectonic background

The black rectangle in Figure 1 shows the global geographic location of the study area in the upper left corner, located on the southeastern edge of the Tibetan Plateau. The light blue dotted line represents the main faults, including the Longmenshan fault, Xianshuihe fault, Anninghe fault, Nujiang fault, Jingshajiang-Red river fault and Lijiang-Jinhe fault; the study area is divided into three parts: the Songpan-Ganze fold (SG), Sichuan Basin (SB) and Sichuan-Yunnan terrene (SY) (Wang and Burchfiel, 2000). The red inverted triangle indicates the distribution of seismic monitoring stations in Sichuan Province, and black uppercase letters represent station names. The uniform and intensive layout of seismic monitoring

Journal Pre-proof stations provides a solid foundation for lithosphere depth research. The collision between the Indian plate and the Eurasian plate resulted in the uplift of the Qinghai-Tibet Plateau and the eastward flow of hot mantle material, which caused the Songpan-Ganze fold and Sichuan-Yunnan rhombic block to strongly deform over a long period (Klemperer, 2006; Royden et al., 2008; Zhang et al., 2010). The crust and lithosphere deformation will inevitably lead to the enhancement of surface seismic activity. Complex tectonics and large-scale active fault distribution are the main tectonic characters of this

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region, especially for the Longmenshan (LMS) fault, where two rare earthquakes (“5.12”

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Wenchuan Ms 8.0 earthquake in 2008 and “4.20” Lushan Ms 7.0 earthquake in 2013)

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occurred. The two earthquakes were probably related to the crust and lithosphere deformation

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with the release of massive accumulation energy. Such catastrophic earthquakes illustrate the necessity of our study on the crust and lithosphere deformation.

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3 The method

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3.1 The basic theory

The teleseismic body wave contains three types of signal information, i.e., source time

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function, focal orientation and near-source structure, as well as the crust-mantle structure

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instrument response near the surface station. If the seismic source and lower mantle propagation effect could be calculated or eliminated from the seismic waveform, the rest of the waveform only indicates the structure of the Earth’s media structure information below the station (Langston, 1977; Vinnik, 1977). Figure 2 shows the ray paths of the P-wave and S-wave, as well as the S-wave theory receiver function. The body wave from the Earth's interior encountered the interfaces in the crust-mantle, being accompanied by reflection, refraction and transmission. This will generate the converted waves Ps, PpPs, PsSs, Sp and so on. The wave phase immediately after the direct S-wave is the Sp converted wave without multiple reflection or refraction waves (Hu et

Journal Pre-proof al., 2011, 2012, 2015). The crust and lithosphere Sp converted phase should be paid more attention to accurately determine the crust and lithosphere interface while avoiding multiple wave interference. The S-wave receiver function time axis was reversed for comparison with the P-wave receiver function, which left only the Sp phase after the direct wave (Kumar et al., 2005; Yuan et al., 2006; Hu et al., 2011). The most critical step of the S-wave receiver function was to suppress the initial direct wave amplitude as much as possible and ultimately strengthen the

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converted phase amplitude. According to seismic ray theory, the specific method was to

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suppress the time-zero S-wave amplitude close to 0 through the adjustment of the incidence

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angle or seismic wave velocity. This method overcame the limitations of the P-wave receiver

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function and solved the problem that the lithosphere converted phases were very difficult to identify from the stacked phase of the multiple converted waves.

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3.2 The S-wave receiver function method

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In order to obtain the S receiver function, the S component is deconvoluted from the P component. In this way, the effect of the source and path is removed, and the conversion

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interface beneath a station is constrained. To remove the contribution of the free surface and

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to enhance the conversion phases, an effective method is to isolate P, SV and SH waves in the local coordinates by rotating the ZNE components to the ZRT coordinates (Reading et al., 2003; Svenningsen and Jacobsen, 2004).The velocity of the S wave can be calculated from the horizontal slowness and the apparent angle of the incidence wave（Hu et al.,2011）. It can be estimated from the radial receiver function (RRF) and the vertical receiver function (ZRF) as follows (Svenningsen et al., 2007). As seen from the above discussion, the most important step of S receiver function processing is to isolate the S and P components from the original earthquake signals using the transformation.The key step is to estimate the velocity of the S wave at the free surface for extracting the P and S components.

Journal Pre-proof 3.3 The data processing flow The receiver function calculation is a method to predict radial and tangential earthquake records using the vertical component of the movement with the focus on equivalence assumption. The simplest method is to divide the tangential and radial Fourier spectrum by the vertical recorded Fourier spectrum in the frequency domain and then transform the results to the time domain with Fourier transform. The obtained time-domain waveform is the receiver function (Yuan et al., 2006). Two horizontal components of the three teleseismic

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components were rotated into radial and tangential components, respectively, and then the

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radial receiver function and tangential receiver function were obtained by deconvolution

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between the vertical component and two horizontal components. Deconvolution eliminates

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the impact of the source and mantle propagation path on the teleseismic body waveform; thereby the receiver function is only related to the media structure beneath the stations and

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basically has nothing to do with the seismic source and propagation path. The specific

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calculation process is shown in Figure 3.

All waveform data from 51 seismic stations were processed with SAC (Seismic Analysis

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Code) software developed by the Lawrence Livermore National Laboratory (LLNL) at the

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University of California. The main difference between the S receiver function and P receiver function is the selection of teleseismic events (epicenter distance greater than 60 km) and the time axis reversion. The two main critical steps of S-wave receiver function data processing are coordinate rotation and deconvolution (Hu et al., 2011, 2012; Svenningsen et al., 2007). Finally, the source time function and instrument response are removed from the original records of the three seismic components. 3.4

An example of single station data processing Figure 4(a) shows the stacked time series profile of the S-wave receiver function at

station REG (Ruoergai) using 12 teleseismic events around the world; each teleseismic event

Journal Pre-proof was processed using the flow in Figure 3. The stacked profile selected the initial amplitude minimum receiver functions, which were obtained by adjusting the seismic wave velocity. Finally, the qualified receiver functions were stacked together, and this final receiver function is shown on the top of Figure 4(a). The stacked receiver function effectively strengthened the Sp converted wave signal. The black crest indicates that the arrival time of the Moho is 7.58 s; the largest gray trough indicates the lithosphere interface conversion phase is 15.53 s. As seen above, the S receiver function method effectively suppressed multiple reflection waves and

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strengthened the weak converted wave signal. The result is clearer and more reliable than the

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P receiver function.

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The stacked time series profile transformed from the time domain in figure 4(a) was

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transformed to the depth domain in figure 4(b) through time-depth conversion. As seen in figure 4(b), it is clear that the crust thickness is 55.334 km and the lithosphere thickness is

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119.58 km beneath the REG station. Two black dotted lines indicate the crust and lithosphere

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interface, and both the black crest and gray trough are the total peak amplitudes. The black positive phase indicates an S-wave velocity increase at the crust interface, and the gray

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negative phase indicates that the S-wave velocity decreases rapidly at the boundary between

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the lithosphere and asthenosphere. The difference between the positive phase and the negative phase illustrates that the seismic wave response is distinct for different elastic media in the Earth's interior with respect to being able to distinguish the layered structure of the Earth's structure. 4 Results and discussion 4.1 Tomography of the crust and upper mantle Based on S-wave receiver function theory and methods, thousands of receiver functions of all global teleseismic events received from 51 seismic stations were calculated to obtain the stacked receiver function profiles and crust-mantle depth profiles beneath these stations. Table

Journal Pre-proof 1 lists the arrival times of the S or Sp converted waves and the depth of the crust and lithosphere beneath all stations in the study area. A Cartesian coordinate system XYZ on the earth’s surface was established when setting the Z axis direction towards the Earth’s sphere center to be positive. Therefore, all depth values are positive in Table 1. As shown in figure 5, the respective thickest and thinnest crusts are approximately 73.511 km beneath the YJI (Yajiang) station and 34.72 km beneath the XCO (Xichong) station in the Sichuan basin. The average depth of the crust is approximately 65 km beneath the Ganze block and the

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Sichuan-Yunnan block and 45 km in the Sichuan Basin. Generally, the crustal depth from

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SRFs beneath this area is quite consistent with that from PRFs (Zhang et al., 2009; Wang et al.,

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2010). The depth and the topography of the Moho change obviously around both the

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Longmenshan fault and the Anninhe-Zemuhe fault.

By contrast, the respective thickest and thinnest lithosphere values are 180.103 km

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beneath the MGU (Meigu) station in the Sichuan basin and 99.33 km beneath the SPA

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(Songpan) station in the Songpan-Ganze fold (figure 6). The topography of LAB undulates sharply, increasing from 120 km beneath the northern Sichuan Basin to 170 km beneath the

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southern area. The most important finding revealed by figure 6 is that the western and

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northwestern Sichuan Basin is surrounded by a shallow LAB belt (100~120 km). A total of 25 field observation stations are located in the relatively flat Sichuan Basin, and 14 and 12 field seismic stations are located in the Sichuan-Yunnan block and Ganze block, respectively. The distribution of stations is uniform and intensive in the Sichuan Basin. There are 12 stations on the Ganze block with a uniform layout. Fewer stations are located in the Sichuan-Yunnan block, but these are not uniform. To obtain more and better quality original seismic records, more field observation stations should be built in this region. As shown in figure 7, the Sp piercing point locations at a depth of 120 km for all seismic events are computed using the IASP91 model to investigate the lateral variation of the

Journal Pre-proof lithosphere. The positions of profiles in the study area are labeled as AB, CD, EF, GH, IJ and KL. The sections along AB, CD, EF, GH, IJ and KL are formed by stacking individual traces with piercing points at a depth of 120 km within a window of 1×1°. All individual traces are corrected to a reference distance of 67° before summation. Thick black dashed lines represent a positive phase and a negative phase is interpreted from the Moho and the LAB in each section. An abrupt change of the Moho is obvious approximately 15 km beneath the LMS in the sections AB, CD and EF. The crustal thickness varied by approximately 10-20 km from

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beneath the SG to under the Sichuan Basin around the Lijiang–Jinhe fault and

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Anninhe-Zemuhe fault in the sections GH, IJ and KL. The lateral variation of the LAB was

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dramatic from 150 km under the SG to 120 km under the LMS with a step of approximately

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40 km in all sections. The results calculated by SRFs are consistent with the observations made by Zhang et al (2009) and Wang et al (2010). The average density and perturbation of

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the upper crust, middle crust, lower crust and uppermost mantle of the LMS in Luo et al (2009)

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verified our calculated results. From the viewpoint of isostatic balance, the thinner LAB belt around the Sichuan Basin corresponds to an uplift of high-density asthenosphere that requires

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a lower density LAB above it (Hu et al., 2011).

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4.2 SKS/SKKS splitting and GPS velocity measurements With respect to previous studies, a shear wave splits into two components with different speeds when it travels through an anisotropic layer in the earth. The strength of anisotropy and the thickness of the anisotropic layer have a strong effect on the time delay between the faster waves and slower waves. Seismic anisotropy may arise from the mineral lattice preferred orientation within the crust or crystal alignment due to asthenosphere flow (Meissner et al., 2002). As shown in figure 8, SKS and SKKS have been widely used to detect the mantle anisotropy with seismological datasets in eastern Tibet (Li et al., 2011; Chen et al., 2013; Hu et al., 2015; Yang et al., 2017). S-wave splitting indicated that the movement of

Journal Pre-proof asthenosphere flow was an important indicator of seismic activity. In addition, ongoing GPS measurements provide evidence for the deformation of the continental lithosphere in eastern Tibet (Zhang et al., 2004). The material beneath the plateau was moving roughly eastward with a certain speed. The direction of the rapid S wave is almost parallel to the GPS velocity and the absolute plate motion (APM) (Kreemer et al., 2003). In general, SKS and SKKS splitting, GPS measurements and APM have a good corresponding relationship with the variation of the crust-lithosphere on the southeastern edge of the Tibetan Plateau. Thus, our

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calculated crust-lithosphere structure from the S receiver function is consistent with previous

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studies.

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4.3 The relationship between crust-lithosphere deformation and seismicity

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To investigate the relationship between crust-lithosphere deformation and seismicity, all of the 518 seismic events that exceeded Ms 4.5 were collected from the Chinese State

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Seismological Scientific Data Sharing Center when it was established with digital earthquakes

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records in 1970. As shown in figure 9, the dark red dashed line indicates the major faults that developed in this region; the solid circles with different sizes and colors represent 518

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earthquakes of magnitude 4.5≤Ms≤8.0. The black contour lines represent the thickness

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distribution of the Moho in the study area, and the contour interval is 10 km. The light and dark gray areas are in the range of 44 ~ 52 km and 60~68 km, respectively of the Moho depth. Some catastrophic earthquakes in recent years have been labeled in the study area, such as the Wenchuan “5.12” earthquake in 2008, with a magnitude of Ms 8.0, and the Lushan “4.20” Ms 7.0 earthquake in 2013. There is a certain correlation between the distribution of moderate earthquakes and the variation of the Moho depth in figure 9. The study area was divided into two approximately symmetrical parts by the dividing line of a narrow strip in the longitude range of 102°~104°. As known by statistical analysis in figure 10, the location distribution of moderate

Journal Pre-proof earthquakes shows two peak regions with the variation of the Moho depth in the range of 44~52 km and 60~68 km corresponding to the light and dark gray areas, respectively, in figure 9. There are 217 moderate earthquakes that account for 41.89% of total earthquakes in the light gray area; these are listed in table 2. Moreover, there are 157 moderate earthquakes in the dark gray area that account for 30.31% of total earthquakes. An additional 144 moderate earthquakes that accounted for 27.79% of total earthquakes are located in the blank area of the study region.

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By contrast, a more obvious corresponding correlation between the distribution of

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moderate earthquakes and the variation of the LAB depth is visible in figure 11. Combined

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with the statistical analysis in figure 12, the most important finding is that the location

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distribution of earthquakes shows one peak region with the variation of LAB depth in the range of 130~150 km. There are 321 moderate earthquakes that account for 62.15% of the

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total earthquakes in this gray area; these are listed in table 3. Shear wave splitting and GPS

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measurements suggested that the hot asthenosphere escape flow material beneath the plateau moved roughly eastward around the Sichuan Basin with a certain speed (Zhang et al., 2004;

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Yang et al., 2017). The crust-lithosphere structure has been dramatically deformed due to the

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movements of the hot escape flow, which was blocked by the thick Sichuan Basin, and a shallow LAB belt is shown in figure 11. The shallow LAB belt indicated that some disastrous earthquakes may occur in this deformed area within a range of 130~150 km in the future. To prove this issue, this study shows the locations of 6 disastrous earthquakes (Ms>6.0) in the last ten years. They are the Wenchuan “5.12” earthquake in 2008, with a magnitude of Ms 8.0 (Xu et al., 2008), the Panzhihua Ms 6.1 earthquake in 2008 (Wang et al., 2011), the Lushan “4.20” Ms 7.0 earthquake in 2013 (Li et al., 2013), the Ludian Ms 6.5 earthquake in 2014 (Xu et al., 2014), the Kangding Ms 6.3 earthquake in 2014 (Xie et al., 2017) and the Jiuzhaigou Ms 7.0 earthquake in 2017 (Li et al., 2017). As can be seen in figure 11, another 5

Journal Pre-proof earthquakes are located in the shallow LAB belt of 130~150 km in addition to the Jiuzhaigou earthquake. The relationship between lithosphere deformation and seismic activity illustrates that most of the moderate earthquakes are distributed around the Sichuan Basin, especially on the boundary between Sichuan and Yunnan provinces with a dramatic deformation region of the lithosphere. In essence, deep lithosphere tectonic movement affects the Earth's shallow seismic activity. Therefore, the lithosphere thickness can be used as a parameter to estimate

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seismic activity and to conduct large-scale seismic zone division. In different intensity seismic

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zones, different anti-seismic parameters should be used in the design of buildings and

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construction. The accuracy of the seismic zone division directly affects the safety of

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construction projects. Using lithosphere thickness as a parameter is of great importance when conducting seismic zone division.

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5 Conclusions

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The Sichuan Basin is located in a north-south seismic belt, where a series of large faults developed with dramatic seismic activity. For equal magnitude earthquakes, a shorter

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epicenter distance and a shallower focal depth of the earthquake result in greater damage. It is

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widely accepted that seismic activity is controlled by the tectonics in the crust without considering the influence of the deep hot escape flow beneath the upper mantle, which has a significant influence on seismic activity. Consequently, this study obtained the lithosphere thickness beneath 51 seismic stations with an S-wave receiver function technique on the southeastern edge of the Tibetan Plateau to investigate the relationship between lithosphere deformation and seismic activity. The major findings of our research are as follows. (i) The S-wave receiver function technique has a unique advantage to effectively detect the lithosphere interface with higher accuracy and resolution compared with other methods: the low-resolution surface wave, large-scale body tomography and P-wave receiver function.

Journal Pre-proof (ii) The Moho and LAB interface undulates dramatically with a large deformation gradient, especially on the Longmenshan fault belt and the Sichuan-Yunnan junction. The thick and hard Sichuan Basin is surrounded by a shallow LAB belt in the range of 130-150 km. The average lithosphere depth is approximately 160 km beneath the Sichuan Basin. In addition, the lithosphere thickness of the northwestern and the southeastern corners should be investigated by further study as a result of the lack of seismic stations and earthquake records. (iii) The statistical correlation of the crust-lithosphere structure and distribution of

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historical seismic events indicates that the most dramatic deformation area is mainly

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concentrated around the Sichuan Basin. The Moho depth ranges of 44~52 km and 60~68 km

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correspond to 72.2% of the total number (518) of moderate earthquakes. The LAB depth

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range of 130~150 km corresponds to 62.15% of the total number (518) of moderate earthquakes. These results suggest that seismicity has good consistency with crust-lithosphere

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deformation, especially for some disastrous earthquakes that occurred in the last ten years.

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(iv) The lithosphere thickness obtained by the S-wave receiver function method could be used as one of the reference factors for seismic zone division and seismic hazard assessment,

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as well as for earthquake early warning. A greater number and better seismic records will be

network.

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acquired in lithosphere depth research with the development of a seismic observation station

Acknowledgments

We would like to thank LetPub (www.letpub.com) for providing linguistic assistance during the preparation of this manuscript.This study has been financially supported by the National Natural Science Foundation of China under Grant 41867040 , the Yunnan Youth Fund program (2016FD030) and the Key Program of the Chinese Academy of Sciences (KZZD-EW-05-01) . Special acknowledgement should be also given to the 100 Talents Programme of The Chinese Academy of Sciences. All figures were generated with GMT

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Journal Pre-proof The crust-lithosphere structure, especially the discontinuities such as Moho and lithosphere-asthenosphere boundary (LAB), is important in the investigation of geodynamic process implications and tectonic evolution of the lithosphere and regional seismic activity.We stacked the S receiver functions from 51 permanent broad-band stations to investigate the crust-lithosphere structures beneath the southeastern edge of the Tibetan Plateau and further discussed the mechanism of lower-crust earthquakes, the regional tectonics deformation characteristics, and the relationship between the two discontinuities and seismicity in this region. The results shows that the Moho depth increases from 40~52 km beneath the Sichuan Basin to 56~74 km on the both western side of the Longmenshan (LMS) fault and

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Anninhe-Zemuhe fault. The LAB depth ranges from 130 to 170 km beneath the Sichuan

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Basin, and a shallow belt ranges from 100 km to 130 km around the Sichuan Basin. On the Moho depth contour map, 374 moderate earthquakes that accounted for 72.2%, corresponding

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to the depth range of 44~52 km and 60~68 km on the both sides of a narrow strip (longitude 102°~104°). On the LAB depth contour map, 321 earthquakes accounted for 62.15% in the

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depth range of 130~150km. The variation of LAB corresponds to the increasing crustal thickness, intensive lower crustal earthquakes ( hypocenters about 60km and magnitude

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4.0≤Ms≤4.9 ) and surface abrupt elevations.It implied that Normal strong earthquakes generate the lower crustal earthquakes or aftershocks that resulted in the shear zones where

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fluid-induced metamorphic transformations, the eclogitization of dry granulite, and further

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and seismicity.

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affect the crustal thickness and deformation to control the spatial distribution of deep faults

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CRediT author statement Xingqian Xu：write the manuscript, calculated s receiver functions, draw Figures. Lijun Su： ensure the descriptions are accurate and response to reviewers. Junzhe Liu ： the analysis of correlation between crust-lithosphere and seismicity.

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Wanhuan Zhou ： the data collection of Shear wave splitting and GPS

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measurements.

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Xin Qu：the data collection of earthquakes in the study region.

Journal Pre-proof Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Figure 1. The field seismic station layout in the study region. F1—Longmenshan fault; F2—Xianshuihe–Anninghefault; F3—Jinshajiang-Red river fault; F4—Nujiang-Lancangjiang fault; F5—Lijiang–Jinhe fault; F6—Anninhe-Zemuhe fault; SG—Songpan-Ganze fold; SB—Sichuan Basin; SY—Sichuan-Yunnan terrene.

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Figure 2. The seismic ray path and S-wave theoretical receiver function.

Figure 3. The data processing flow of the S-wave receiver function.

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Figure 4. The stacking profiles of the receiver function and depth conversion at the REG (Ruoergai)

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station.

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Figure 5. Contour map of the Moho thickness (units: km). The dashed lines represent the faults, and the

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triangles represent the stations.The label names of the faults and geological blocks are shown in Figure 1.

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Figure 6. Contour map of the LAB thickness (unit: km). The dashed lines represent the faults, and the

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triangles represent the stations. The label names of the faults and geological blocks are shown in Figure 1.

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Red arrows indicate possible crustal flow channels estimated from depth of the LAB.

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Figure 7. (a) Piercing points of the Sp phase at a depth of 120 km, represented by the red crosses. The lines AB, CD, EF, GH, IJ, and KL denote the profiles, of which the size of a stacking bin is 1°× 1°. (b) Bin stacking of the SRFS and elevation variation along AB, and ; (c) Bin stacking of the SRFS and elevation variation along CD; (d) Bin stacking of the SRFS and elevation variation along EF; (e) Bin stacking of the SRFS and elevation variation along GH; (f) Bin stacking of the SRFS and elevation variation along IJ; (g) Bin stacking of the SRFS and elevation variation along KL. The converted phases from the Moho and the LAB are interpreted with thick black dotted line on the all depth profiles. Small red circles represent the earthquakes with magnitude 4.0 ≤ Ms ≤ 4.9, big circles represent the earthquakes with magnitude 5.0 ≤ Ms

Journal Pre-proof ≤ 5.9, and the earquakes with the white question mark are the typical lower-crust earthquakes about 50-70km in the region. The label names of the faults and geological blocks are shown in Figure 1. The stacking bin is 1°× 1°with 0.5 overlap. The traces are corrected to a reference distance of 67°before

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Figure 8. Topographic map displaying mantle anisotropy from SKS or SKKS (black bars) measured in

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previous studies ( Sol et al.,2007, Li et al., 2011; Chen et al., 2013). The black bars centered on the

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seismic stations (white circles) are aligned with the fast S wave direction and have lengths proportional to the time delay. The velocity of GPS stations with respect to stable Eurasia (red arrows) (Zhang et al., 2004), and the Absolute Plate Motion (APM, thick white arrows) calculated using the GSRM V1.2 model in Kreemer et al.(2003), which were plotted for comparison. The dark blue lines represent the faults in the study region.

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Figure 9. Contour map of the Moho thickness (unit: km).

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The dashed lines represent the faults, and the triangles represent the stations. The circles denote the distribution of earthquakes (1970~2017) in the study region.

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Figure 10. The statistical relationship between the Moho thickness and the number of

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earthquakes (4.5≤Ms≤8.0) in the study region .

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Figure 11. Contour map of the LAB thickness (unit: km). The dashed lines represent the faults, and the

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triangles represent the stations. The circles denote the distribution of earthquakes (1970~2017) in the study

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Figure 12. The statistical relationship between the lithosphere thickness and the number of

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earthquakes(4.5≤Ms≤8.0) in the study region.

Journal Pre-proof Table 1. The depth of the crust and lithosphere beneath the stations in the study area Station Location Trace

Moho

Moho

LAB

LAB

(sec)

(km)

(sec)

(km)

Station Location Trace

Moh

(sec

AXI

SB

20

5.6

41

18.08

139

27

MLI

SY

12

9.74

2

BTA

SY

18

8.06

59

21.59

166

28

MNI

SB

8

7.13

3

BYD

SB

10

6.62

48

19.28

148

29

MXI

SG

13

7.10

4

BZH

SB

10

6.73

49

17.93

138

30

PGE

SY

16

8.00

5

DFU

SG

10

8.36

61

15.44

119

31

PWU

SG

13

7.28

6

EMS

SB

9

7.97

58

20.81

160

32

PZH

SY

9

7.46

7

GZA

SG

13

7.55

55

17.15

132

33

QCH

SG

13

6.41

8

GZI

SY

10

9.11

67

20.15

155

34

REG

SG

14

7.58

127

35

RTA

SG

14

8.57

156

36

SMI

SB

15

8.24

108

37

SMK

SY

13

7.49

180

38

SPA

SG

14

7.91

124

39

WCH

SG

10

6.98

149

40

WMP

SB

12

7.16

175

41

XCH

SY

11

9.17

HLI

SY

11

7.46

55

16.49

HMS

SB

12

5.78

42

20.27

11

HSH

SG

12

7.76

57

14.09

12

HWS

SB

10

6.08

44

23.37

13

HYS

SB

11

7.26

53

16.13

14

JJS

SB

11

6.92

51

15

JLI

SB

10

6.11

45

16

JLO

SY

10

8.78

64

24.11

186

42

XCO

SB

10

4.93

17

JMG

SB

10

6.05

44

17.48

135

43

XHA

SB

9

6.29

18

JYA

SB

10

7.64

56

19.58

151

44

XJI

SG

11

9.08

19

LBO

SB

12

7.85

57

21.62

166

45

XSB

SB

11

7.64

5.84

43

12.68

100

46

YGD

SB

12

6.62

SY

21

10.0

LD4

SB

12

21

LGH

SY

12

22

LTA

SY

14

23

MBI

SB

16

24

MDS

SB

9

25

MEK

SG

26

MGU

SB

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8.5

62

12.89

100

47

YJI

9.65

70

14.69

113

48

YYC

SY

11

7.10

6.65

24.14

186

49

YYU

SY

23

8.81

57

20.39

157

50

YZP

SB

11

6.86

11

7.37

54

17.42

134

51

ZJG

SB

11

6.44

10

7.22

53

23.39

180

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49

7.76

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Ration

40~44

21

4.05%

44~48

88

16.99%

48~52

129

24.9%

52~56

60

11.58%

56~60

57

11.0%

60~64

91

17.57%

64~68

66

12.74%

68~72

6

Total

518

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Depth range(km)

Table 3. The earthquake distribution at different lithosphere depth ranges The number of earthquakes

Ration

33

6.37%

25

4.83%

110~120

23

4.44%

120~130

15

2.9%

130~140

173

33.40%

140~150

148

28.57%

150~160

31

5.98%

160~170

42

8.12%

＞170

28

5.41%

Total

518

100%

＜100

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Depth range(km)

The highlights of this study as follows: 

The Moho and LAB discontinuities depth display the opposite distribution characteristics around the Sichuan Basin.

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LAB could be used as an indicator for the size and direction of lower crustal flow.



Lower crustal earquakes linked to both the aftershocks of normal earquakes and the lower crustal phase transformation.

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Seismicity have a good consistency with crust-lithosphere deformation.

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