Earthquake Reaearch in China  2019, Vol. 33 Issue (2): 174-185     DOI: 10.19743/j.cnki.0891-4176.201902006
Characteristics of Seismic Wave Propagation in the Binchuan Region of Yunnan Using a Dense Seismic Array and Large Volume Airgun Shots
SHE Yuyang1, YAO Huajian1,2, WANG Weitao3, LIU Bin1
1. Laboratory of Seismology and Physics of Earth's Interior & School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;
2. Mengcheng National Geophysical Observatory, University of Science and Technology of China, Mengcheng 233500, Anhui, China;
3. Key Laboratory of Seismic Observation and Geophysical Imaging, Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
Abstract: The Binchuan region of Yunnan is a structurally complex region with mountains, basins, and active faults. In this situation, seismic wave propagation exhibits complex characteristics due to strong heterogeneity of underground media instead of following the great-circle path. In order to obtain a high-resolution shallow crustal structure, a dense seismic array was deployed during March 21 to May 30, 2017 in this area. To better understand the complexities of seismic wave propagation in this region, we perform array-based frequency-domain beamforming analysis and single-station based polarization analysis to investigate the characteristics of seismic wave propagation, using airgun-generated P-wave signals recorded by dense array stations in this experiment. The results from these two methods both reveal similar but complex characteristics of seismic wave propagation in the Binchuan basin. The azimuth anomalies off the great-circle path are quite large with values up to 30°, which is caused by strong structural heterogeneity in the very shallow crust. Our research provide a better understanding of the complex geologic structures in this area and provide guidance for detecting concealed faults and distribution of velocity anomalies.
Key words: Airgun signal     Body waves     Wave propagation     Binchuan basin     Beamforming     Polarization analysis

INTRODUCTION

The Binchuan basin in Yunnan is a typical transtensional structure located in a pull-apart basin formed since the Late Cenozoic (Luo Ruijie et al., 2015). This basin is surrounded by high mountains and lakes, with a large elevation difference (about 1000m). Many faults with different trending and movement patterns are distributed in this area, especially the Red River fault (RRF) and the Chenghai fault (CHF), which are the two major faults accommodating significant tectonic deformation and posing serious seismic hazards in this region (Allen C. R. et al., 1984). Due to its special tectonic location and complex geological structure, many earthquakes have occurred in this region and neighboring areas. The largest earthquake occurred on the CHF was the 1515 M7.7 Yongsheng earthquake (Huang Xiaolong et al., 2018). According to the positive correlation between the intensity of seismic and fault activities, former studies considered the Binchuan area as a M7.0 earthquake potential area (Wang Fan et al., 2015). Therefore, the investigation of subsurface structure is necessary to help us better understand the complex fault zone structure and strong motion characteristics.

During the 1980s, numerous seismic experiments have been conducted in Yunnan, such as wide-angle seismic reflection and refraction studies (Zhang Zhongjie et al., 2005; Zhang Xi et al., 2009), receiver function analysis (e.g., Hu Jiafu et al., 2005; Wang Chunyong et al., 2010), seismic tomography (e.g., He Zhengqin et al., 2004; Yao Huajian et al., 2008; Huang Zhouchuan et al., 2015), and joint seismic inversion (e.g. Liu Qiyuan et al., 2014; Bao Xuewei et al., 2015). In addition to 3-D structures, the China Earthquake Administration (CEA) has built a Fixed Airgun Signal Transmission Station (FASTS) in Binchuan, Yunnan in 2011 (Wang Baoshan et al., 2012; Chen Yong et al., 2017) to better capture temporal velocity and attenuation changes of fault zones due to earthquakes because of high repeatability of waveforms generated by airgun sources. During March 21 to May 30, 2017, a 2-D short-period dense array (about 400 portable seismic stations) was also performed by the CEA (Fig. 1) (Wang Baoshan et al., 2018; Xu Y. et al., 2018) around the Binchuan basin in Yunnan. The purpose of this experiment was to obtain the high-resolution shallow crustal structure of the Binchuan basin and surrounding area as well as to understand complexities of seismic wave propagation and amplification in this region.

 Fig. 1 Topography, faults and station locations in the study area Station locations are shown as triangles deployed by Institute of Geophysics, China Earthquake Administration. Red and blue triangles are the stations we used in this study. Waveforms of blue triangles stations are shown in Fig. 2. Yellow star represents the Binchuan FASTS. CHF: Chenghai Fault, RRF: Red river Fault, SCB: Sichuan Basin

 Fig. 2 (a) Recorded waveform data in the 2-8Hz frequency band from the Binchuan FASTS to the 12th sub-dense array stations (blue triangles in Fig. 1) after linear stack. (b) Aligned waveforms for beamforming analysis. Gray shaded window in (b) represents the time window for beamforming analysis

Seismic rays would follow the great-circle path in the horizontally isotropic layered medium. However, in real situations, seismic wave propagation exhibits complex characteristics due to strong heterogeneity of underground media with mountains and basins, for instance, in the Binchuan area. In this study, we intend to focus on investigating the characteristics of seismic wave propagation in this region using airgun signals recorded by dense array stations in this experiment (Wang Baoshan et al., 2018). Two classical methods are used in this study, the first method is the beamforming method (e.g., Rost S. et al., 2002) based on dense array stations. The second is the polarization analysis method (e.g., Jurkevics A., 1988) based on single station waveforms.

1 DATA AND PREPROCESSING

The seismic data we analyze is part of the dense seismic experiment, carried out at Binchuan in 2017 by CEA. Airgun shots produced by an airgun array (four airguns with a volume of 2000 inch3 each) fired from the Binchuan FASTS were used in this study. During the observation of this dense array in the experiment, ~1400 airgun shots were fired. The observation system consists of 400 portable short-period seismic stations deployed in a rectangular region (Fig. 1), which spans an area about 25.6°—25.9°N and 100.3°—100.7°E. Considering the limitation of research methods we used, we only chose 67 stations for their dense distribution (see stations in red and blue in Fig. 1).

Because the airgun source is repeatedly fired at the same location, it can generate highly repeatable seismic waves (e.g., Lin Jianmin et al., 2008; Chen Yong et al., 2017). We can therefore enhance the signal-to-noise ratio (SNR) of the waveform data and obtain high-quality seismic waveforms through stacking. We cut the vertical-component data from the excitation time to 20s later, removed the mean and trend of each trace, and linearly stacked all the waveforms recorded at each receiver. Previous studies have shown that the dominant frequency band of the excited airgun-shot signal is between 3Hz and 6Hz (e.g., Lin Jianmin et al., 2008; Yang Wei et al., 2013). Therefore, we band-pass-filtered the waveform data in the 0.5-10.0Hz frequency band (or the 0.1-2.0s period band) to suppress noise. An example of waveforms from the Binchuan FASTS after the linear stack is shown in Fig. 2(a).

2 METHOD AND RESULTS

In this section, we will use two methods to analyze the propagation direction of the P waves that are excited by the Binchuan FASTS. The first method is the array-based frequency-domain beamforming analysis (e.g, Rost S. et al., 2002), which will determine the average back azimuth of the incoming (plane) wave to the dense array. The second method is the single-station based polarization analysis (e.g., Jurkevics A., 1988), which directly determines the back azimuth of the incoming wave to the station. In each subsection, we will first introduce each method and then provide the results.

2.1 Beamforming Analysis

The Beamforming method (e.g., Rost S. et al., 2002) in the frequency domain was used to calculate the apparent slowness vectors of the first arrival P waveforms generated by the airgun shots after stacking. This method is based on the far-field plane wave propagation hypothesis. We performed a gird search in the apparent slowness space (slowness and back azimuth) and intended to maximize the beamforming energy of the aligned P waveforms.

For each selected slowness Si and back azimuth θj at grid node (i, j), we can calculate the phase delay Δφk, ij at each station k for central frequency ω as

 $\Delta \varphi_{k, i j}=\omega \cdot \Delta t_{k, i j}$ (1)

where Δtk, ij represents the time delay for each station k with respect to the reference location. In order to ensure the integrity of effective signal in the limited time window, we first aligned the waveforms before doing beamforming analysis (Fig. 2(b)). So the time delay Δtk, ij can be divided into two parts: time delay δt1k, ij caused by the location of each station and the time shift δt2k for each trace for aligning the waveforms:

 $\Delta t_{k, i j}=\delta t_{1 k, i j}+\delta t_{2 k}$ (2)
 $\delta t_{1 k, i j}=l_{x} S_{i, x} \sin \left(\theta_{j}\right)+l_{y} S_{i, y} \cos \left(\theta_{j}\right)$ (3)

where lx and ly are the x and y component of the distance between station k and the array center, and Si, x and Si, y are, respectively, the x and y component of the slowness for node (i, j).

The position of the node where the maximum beamforming energy P(ω) is reached provides an estimate of the apparent slowness and back azimuth of the incoming wave front.

 $P(\omega)=\left|\sum_{k=1}^{N} X_{k}(\omega) \cdot e^{-i \Delta \varphi_{k, i j}}\right|^{2}$ (4)

In order to apply this method to array data, we have to select an adequate set of parameters, including the filter, time window and apparent slowness grids. We used a bandpass, zero-phase Butterworth filter in the 2-8Hz frequency band, where most of the energy is concentrated for the airgun signal. We selected a window length of 0.5s for the analysis. This window represents about 2.5 periods at the dominant frequency of 5Hz, which has been suggested as the optimum window length for this type of analysis (Almendros J. et al., 1999). The apparent slowness and azimuth grid interval are chosen to be 0.001s/km and 1°, respectively.

To satisfy the plane wave hypothesis, we only selected twelve sub-arrays (67 stations) from the dense part of the 2-D array of Binchuan to perform beamforming analysis. In this situation, we assume a homogeneous structure under each sub-dense array. These sub-dense arrays are distributed from north to south along the Binchuan Basin. The propagation distance is in the range of 7-19km. Each sub-array consists of ~7 stations in a circular region with a radius of about 1.5km. Although we have chosen the denser part of this Binchuan array, the distances between stations are still too large. That will result in a problem of spatial aliasing, that is, the beamforming energy images are not clean enough, typically with multiple peaks. There will be relatively important secondary peaks produced by this spatial aliasing. To ensure the reliable results on spatial aliasing, we did synthetic tests for each sub-array using theoretical waveforms. Fig. 3 is an example of synthetic tests. As we see small-aperture arrays show wider peaks (Fig. 3(c)), resulting in lower resolution and larger uncertainties, whereas large-aperture arrays with sparse station distribution display a narrow main peak with some secondary peaks produced by spatial aliasing (Fig. 3(b)). After doing this test, we can find only one peak in northwest represents the true main peak for wave propagation direction and slowness (Fig. 3(e)), and the others are all produced by spatial aliasing.

 Fig. 3 Synthetic tests for 7th sub-dense array (see Fig. 4 for the location) (a) Relative station locations for 7th sub-array (blue triangles) and the added artificial stations (green and red triangles). (b)-(e) Results of synthetic tests for those stations in (a): (b)blue stations, (c)red stations, (d)green stations, (e)all stations. The interval of slowness and back azimuth shown in beamforming results by the black dashed lines are 0.1s/km and 45°, respectively. The ranges for slowness and back azimuth for beamforming analysis are 0.05-0.5s/km and 0°-360°, respectively

 Fig. 4 Beamforming results for the 12 selected sub-dense arrays Red star and triangles represent the Binchuan FASTS and stations. The yellow triangle represents the station used to perform the polarization analysis in Fig. 5. The black circles show the range of each sub-array. The black lines and red bars represent the great circle path and the real back azimuth for seismic wave propagation from the beamforming analysis. Beamforming power results for each sub-array is placed at the right of the corresponding sub-array. The white circles point out the true main peak for wave propagation direction

 Fig. 5 Particle motion analysis (a) 3-component records by a station in the 12th sub-array, in the 2-8Hz frequency band. The R and T components which are rotated to the great circle path and the real path are shown in this figure, respectively. The gray shaded window in this figure represent the time window for polarization analysis. (b) Particle motion of the waveforms (rotate to the great circle path) in the gray shaded window in (a). (c) Particle motion of the waveforms after rotation using the back azimuth calculated by polarization analysis. BZ: back azimuth

The procedure described above allows us to calculate the average back azimuth that characterizes the first arrivals of P waveforms generated by the airgun shots to the sub-seismic arrays. To extract the most significant solutions among this data set, we consider only those stable and reliable solutions. Fig. 4 shows a summary of the results for the twelve selected sub-arrays. In this figure we place the beamforming result at the right of the corresponding sub-array. We also mark the great-circle path and the real direction of seismic wave propagation to the sub-array in order to illustrate a visual representation of the ray-bending. We observe the existence of back azimuth anomalies from 0° to 30°. The variation angle depends on the position of the sub-array. The regions of the 3rd-6th and 8th-12th sub-array show major anomalies of wave propagation direction.

2.2 Polarization Analysis

The analysis of multicomponent recordings can provide an estimate of polarization properties of seismic arrivals. Knowledge of the polarization state can be used to distinguish between surface wave arrivals and body waves, or more generally for the purpose of wavefield decomposition. The method we used is based on the analysis of the covariance matrix of multicomponent recordings and principal component analysis using singular value decomposition.

The processing is carried out in the time domain. The polarization within a time window is estimated as follows. Let us consider the vector valued signal S(t)=[Sx(t), Sy(t), Sz(t)]T. We assume that this signal represents a three-component seismic recording. Here we associate Sx(t), Sy(t), Sz(t) with the usual radial (inline), transverse, and vertical components of a triaxial recording. Polarization analysis of S(t) can be carried out by eigen analysis of the cross-energy matrix M, defined from the data vector as (Flinn E.A., 1965; Kanasewich E.R., 1981; Jurkevics A., 1988; Jackson C. M. et al., 1991; Kulesh M. et al., 2007):

 ${\bf M}(\xi)=\left[\begin{array}{ccc}{I_{x x}(\xi)} & {I_{x y}(\xi)} & {I_{x z}(\xi)} \\ {I_{y x}(\xi)} & {I_{y y}(\xi)} & {I_{y z}(\xi)} \\ {I_{z x}(\xi)} & {I_{y z}(\xi)} & {I_{z z}(\xi)}\end{array}\right]$ (5)

In the equation above

 $I_{k m}(\xi)=\frac{1}{T} \int\limits_{\xi-T / 2}^{\xi+T / 2}\left[S_{k}(\tau)-\mu_{k}(\xi)\right]\left[S_{m}(\tau)-\mu_{m}(\xi)\right] d \tau$ (6)

where k, m=(x, y, z).The above covariance matrix is computed within a time window of length T and around its time centre ξ; μk(ξ) is the mean value of the signal component Sk in the window T.

The eigenanalysis performed on M(ξ) yields the principal component decomposition of the energy for the time window T. Such a decomposition produces three eigenvalues λ1(ξ)≥λ2(ξ)≥λ3(ξ) and three corresponding eigenvectors vk(ξ) that fully characterize the magnitudes and directions of the principal components of the ellipsoid that approximates the particle motion in the considered time window. Purely rectilinear ground motion (P-wave) has only one nonzero eigenvalue. The azimuth can be expressed by:

 $\alpha(\xi)=\arctan \left(v_{1, y}(\xi) / v_{1, x}(\xi)\right)$ (7)

To effectively use the standard covariance method for polarization analysis, one needs to start by selecting the appropriate length of the time window T. Selecting this window may not be easy. If the window length is too short, the stability of the polarization analysis will be low due to a higher sensitivity to local information, whereas we can hardly find the polarity because of the insensitivity to effective signals with too long a window length. The optimal length of the time window must depend on the main frequency of the effective signal in the time window. We selected the window length based on three criteria: (1) only one wave arrives in the time window; (2) the SNR (signal to noise ratio) should be the largest; (3) the effective signal must be completely contained.

Fig. 5 gives an example of the polarization analysis. We plot the seismic records which are rotated to the great circle path and the real path (estimate by polarization analysis), respectively (Fig. 5(a)). The particle motions in time window also shown in Figs. 5(b) and 5(c). We can clearly find that the particle motion of transverse (T) component is corrected to zero in time window after rotated to the real path.

Fig. 6 shows the result of the polarization analysis for each station. We find that the back azimuth anomaly of the P-wave propagation for each station is nearly the same as the beamforming result of the corresponding sub-array, which confirms that both methods yield reliable estimates of wave propagation direction. There exists small differences for some stations, which may be caused by the quality of effective signals.

 Fig. 6 Results of the polarization analysis for each station The red star and blue triangles represent the Binchuan FASTS and stations. The black lines show the great circle path for seismic wave propagation. The red bars and the orange arrows represent the beamforming results and polarization analysis results, respectively
3 DISCUSSION

In this study we used 12 sub-seismic arrays to measure the back azimuth anomalies of the first arrivals generated by the Binchuan FASTS. Different values of back azimuth anomalies have been detected from beamforming and polarization analysis. As we know, in laterally homogeneous media, seismic rays are confined in a vertical plane that includes the source and the receiver (or approximately the sub-array centre). Thus the expected back azimuth anomaly is zero everywhere. Any deviation from zero indicates the presence of lateral heterogeneities that affect the ray path.

Apparently, the Binchuan region is not laterally homogeneous. The seismic wave fronts are affected by these heterogeneities. Seismic waves speed up in high-velocity regions and slow down in low-velocity areas, producing distorted wave fronts and twisted ray paths. For example, if a low-velocity body was located between the source and the receiver, seismic rays would follow the fastest path, turning around the low-velocity body instead of across it.

As we have discussed above, those detected azimuth anomalies indicate low-velocity bodies (sediments in the Binchuan basin) between FASTS and stations or high-velocity bodies (crystalline rocks of the mountains) besides the theoretical path. According to numerical simulation of ray propagation based on a simple 1D velocity model, we find that the depth of P-wave transmissions will not be greater than 1km at the current propagation distance (< 20km). It means that all these azimuth anomalies we observed are attributed to the heterogeneities in the very shallow crust (< 1km).

Apart from the evidence about the highly heterogeneous structure of the Binchuan basin, we have to emphasize that we are using waveform data provided by an active seismic experiment. We are therefore dealing with high-frequency and very shallow seismic source. In this sense, our azimuth anomalies estimated from the data set may constitute a more complicated situation. In general, high-frequency seismic waves are more affected by topography and heterogeneities than low-frequency waves, due to shorter wavelengths. Moreover, the shallow structure is expected to be more heterogeneous with large velocity variations.

We should point out that there might be some component azimuth errors caused by careless deployments, which is important in single station polarization analysis. In order to validate the reliability of results, we use the teleseismic body waves from an MW6.8 earthquake in the South Pacific region (epicentral distance ~8, 500km) to evaluate this issue. Fig. 7 shows the histograms of azimuth anomalies from the polarization analysis using airgun shots and teleseismic events. Calculated results indicate that more than two-thirds of stations (45 stations) show at most 5 degrees, about one-fifth of stations (14 stations) show not exceeding 10 degrees and only 8 stations show more than 10 degrees deviation from the larger than 5 degrees but circle path (Fig. 7(b)). Therefore, the azimuth errors caused by careless deployments have little influence on polarization analysis.

 Fig. 7 Histograms of azimuth anomalies (a) The statistic distribution of polarization analysis results in Fig. 6(b) The distribution of azimuth errors estimated by the teleseismic event (Lon=167.3767°, Lat=-14.5884°, MW=6.8)
4 CONCLUSIONS

In this study, the polarization analysis based on single station and beamforming analysis based on dense array both reveal similar but complex characteristics of seismic wave propagation in the Binchuan basin. This situation is due to the strong structural heterogeneity of underground media. Our research can provide a better understanding of the complex geologic structures in this area and provide guidance for detecting concealed faults and distribution of velocity anomalies. In the future, combining the characteristics of seismic wave propagation with high-resolution tomography can provide a reliable model for this complex region. The obtained propagation directions of seismic waves can also be used to access the reliability of future tomographic models in this region.

ACKNOWLEDGEMENTS

The authors thank the editors and two anonymous reviewers for valuable comments that helped improve this article.

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