Sediment thickness variations of the Tangshan fault zone in North China from a dense seismic array and microtremor survey

Journal of Asian Earth Sciences 185 (2019) 104045

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Sediment thickness variations of the Tangshan fault zone in North China from a dense seismic array and microtremor survey

T

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Feng Baoa, Zhiwei Lia, , Baofeng Tianb, Liaoliang Wangc, Guanghong Tuc a

State Key Laboratory of Geodesy and Earth’s Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China Institute of Geophysics, China Earthquake Administration, Beijing 100081, China c Guangzhou Marine Geological Survey, China Geological Survey, Guangzhou 510075, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Dense seismic array Ambient noise H/V spectral ratio Site effects Tangshan Fault

Reliable knowledge of sediment thickness is beneficial for investigating buried active faults and improving seismic hazard assessments, especially in regions with recent strong earthquakes. In this study, we investigate the basement of sediments around the 1976 Ms 7.8 Tangshan earthquake region, North China using the microtremor horizontal-to-vertical spectral ratio (H/V) method. For the ambient vibration survey, we deploy a dense seismic array (comprising 152 seismographs with inter-station distances of approximately 1–4 km) and obtain the resonance frequency from each seismograph, which indicates the sediment thickness. The H/V method provides a potential tool for inferring the shallow subsurface structure. Based on the relations between resonance frequency and sediment thickness from permanent borehole seismic stations' records, we recover the sediment thickness around the Tangshan fault zone, which varies from less than 200 m in the northern region to more than 800 m in the southern region. Our results are generally consistent with those of previous drilling and geological studies. Variations in the thickness of sediments indicate that the buried fault system has experienced significant modifications over time, which is partially due to predominant tectonic control by the NE-SW trending Tangshan Fault. The obtained thickness of sediments provide an important basis for earthquake strong motion simulations, sediment crustal corrections for travel time tomography and active fault investigations in the Tangshan fault zone.

1. Introduction The Tangshan Ms 7.8 earthquake in 1976 was one of the most devastating earthquakes in the last 100 years worldwide (Butler et al., 1979). The Tangshan fault zone, in turn, is one of the most seismically active areas in North China. This zone is situated at the intersection of the Yanshan Uplift and North China depression basin (Fig. 1a) and in the eastern Zhangjiakou-Bohai Seismic Belt, which has experienced historically famous destructive earthquakes (such as the 1679 SanhePinggu M 8 great earthquake), in the eastern Taihang Uplift on the southern margin of Yanshan and on the northern margin of the North China block (Wang et al., 2004; Liu et al., 2011b; Wang et al., 2017). This block formed in the early to middle Eocene (Ye et al., 1985) and seems to be divided into rhombic and triangular sub-blocks by a variety of dominant fault systems with different trends, including NNE, NW, and nearly E-W. The NNE-trending faults are the most prominent (Ye et al., 1985). Historical records reveal that many destructive earthquakes have occurred on these faults (Chen and Nábelek, 1988). The

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NNE-trending Tangshan Fault, which is regarded as the seismogenic fault of the great 1976 Tangshan earthquake (Zeng et al., 1988; Guo et al., 2011; Li et al., 2018), is located within one of the rhombic subblocks defined by four fault belts: Fengtai-Yejituo Fault, LuanxianLaoting Fault, Ninghe-Changli Deep Fault and Jiyunhe Fault. The bedrocks in the Tangshan fault zone were uplifted in the Mesozoic and Early Cenozoic (Guo et al., 2011), and sediments of hundreds of meters in thicknesses cover nearly the entire region (Fig. 1b). Mesozoic and Tertiary sediments are missing in the area, and the Quaternary system lies directly on top of upper Palaeozoic strata (Ye et al., 1987; Liu et al., 2007; Yu et al., 2011). The Tangshan Fault is a very large intra-continental strike-slip fault and consists of a series of en echelon fissures that form a typical flower structure in the sediments, incising and disturbing the lower crust and the crust-mantle transition zone (Liu et al., 2011a). The 1976 Ms 7.8 Tangshan earthquake produced a NNE-trending surface rupture that could be traced for approximately 8 km. The maximum horizontal and vertical displacements were 2.3 and 0.7 m, respectively (Guo et al., 2011). The focal

Corresponding author. E-mail address: [email protected] (Z. Li).

https://doi.org/10.1016/j.jseaes.2019.104045 Received 23 January 2019; Received in revised form 24 September 2019; Accepted 26 September 2019 Available online 28 September 2019 1367-9120/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Geological setting and historical earthquakes. (a) Regional seismic structural setting and area seismicity for historical earthquakes with magnitude > 5 (grey circles). The populous cities are represented (square points), and the scale of the Tangshan fault zone in this study is also indicated (black square). The blue triangles are the permanent seismic stations with borehole seismometers. The red lines show active faults, including F1 (Fengtai-Yejituo Fault), F2 (Luanxian-Laoting Fault), F3 (Ninghe-Changli Deep Fault), F4 (Jiyunhe Fault) and F5 (Tangshan Fault). (b) Seismicity after the great 1976 Tangshan earthquake for events with magnitude > 3 (open circles) and Quaternary geological map, in which grey blocks represent bedrock, orange blocks represent Pleistocene deposits (Qp), and khaki blocks represent Holocene deposits (Qh). The red dashed lines provide more details about the major faults (Liu et al., 1982; Guo et al., 2017). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

expensive, especially when the depth to bedrock is greater than 75 m (Liu et al., 2011a; Chandler and Lively, 2016). Therefore, the current understanding of the thickness and infill geometry of the sedimentary cover in the Tangshan fault zone is still limited. Given the poor knowledge of the sedimentary architecture and ongoing seismic activity, providing detailed and integrated geometry of the sediment thickness and the topography of the basement of sediments around the Tangshan fault zone is imperative. In this study, we deployed a dense seismic array and carried out microtremor surveys to determine the site resonance frequencies for the sediment thicknesses and the delineation of the sediment-bedrock interface. The basement topography under the sediment shows that the Tangshan fault zone is characterized by complex bedrock morphology, which can be divided into several sections with significantly different geometries and potential active fault systems. This study also provides a low-cost, reliable and efficient approach to investigating the basement depth beneath sediments.

mechanism solution of the main shock indicates mostly right-lateral strike-slip motion with some dip-slip components (west side up). Locations of the aftershocks revealed at least three fault segments, each with a different orientation (Chen and Nábelek, 1988; Nábelek et al., 1987). The complicated Tangshan earthquake sequence included strikeslip, thrust and normal-fault events (Butler et al., 1979). The variations in sediment thickness and basement of sediments in earthquake regions are crucial for seismic hazard analysis because the amplification of local ground motion controlled by lithological properties of the sedimentary infill and sharp seismic velocity contrasts (Field and Jacob, 1993; Hartzell et al., 2016). Previous seismic studies have shown that shaking by destructive earthquakes at sedimentary sites can cause extraordinarily more serious damage than that at bedrock sites (e.g., Shaw and Suppe, 1996; Fukushima et al., 2000; Fletcher and Wen, 2005). This difference is attributed to site effects, such as frequency-dependent amplification and increased duration and spatial disparity of ground motion in both the near and far fields (FloresEstrella et al., 2007; De Luca et al., 2005). Moreover, sediments generally have low seismic velocities, which can cause significant time delays for both P and S waves when the thickness of the sediments is up to a few hundred metres or more (Ni et al., 2014). Without correction, this low-velocity sedimentary layer leads to large biases in the seismic travel time tomography for the deeper crust (Bell et al., 2015). Therefore, studying the geometry of the basement of sediments is important, especially for active fault-controlled basements of sediments in densely populated metropolitan regions. These regions are vulnerable to devastating earthquakes; therefore, knowledge about sedimentary properties is helpful for enhancing risk assessments for infrastructure (Özalaybey et al., 2011; Maresca et al., 2012; Ni et al., 2014). Reliable studies on sediment thickness still present challenging problems in areas such as the Tangshan fault zone. Previous studies have shown that active faults in this fault zone are covered by thick sediments and the thickness of Quaternary deposits varies from less than 100 m to more than 800 m (Guo et al., 2011; Li et al., 2018; Bao et al., 2018). However, traditional borehole drilling survey penetrating the whole stratal package in this region is limited due to high costs. Most boreholes in the Tangshan fault zone are shallower than 100 m, and only a few boreholes have been drilled to the basement, where the overlying sediment is thicker than 500 m (Guo et al., 2011). Shallow seismic reflection and refraction surveys using active sources are also

2. Data and methods 2.1. Dense seismic array A dense seismic array with 152 seismometers (Fig. 2) was deployed between January and March 2017. These seismometers were distributed principally following an approximately regular rectangular pattern. The array aperture was 40 km, and the inter-station distance ranged from 1 to 4 km. These instruments were all three-component seismometers with two horizontal and one vertical component. The seismometers were buried ~0.3 m underground to reduce the impact of wind. Their responses were electronically extended to achieve a flat velocity response from 0.2 to 100 Hz. Sampling was performed at 100 samples per second (sps) using a 24-bit built-in data acquisition system working in continuous mode. 2.2. H/V survey Single-station microtremor horizontal-to-vertical spectral ratios (H/ V) have been widely applied to investigate site effects (Nakamura, 1989; Lermo and Chávez-García, 1993; Le Brun et al., 2001; Panou et al., 2005; Chen et al., 2009; Bonnefoy-Claudet et al., 2009), shallow 2

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2.3. Relationship between the resonance frequency and sediment thickness Assume that the subsurface velocity structure model is composed of a stack of homogeneous and isotropic horizontal layers overlying a homogeneous half-space. The observed resonance frequencies ( fr ) of the sedimentary layers are linked to their thicknesses h (i.e., depths to ¯S through the folseismic bedrock) and average shear-wave velocity V lowing formula (Ibs-von Seht and Wohlenberg, 1999; Bao et al., 2018; Lachet and Bard, 1994):

fr =

¯s 1 V = 4h 4T

(1)

¯S is the shear-wave travel time T from bottom to top of a where h/V layer. The Vs profile in the sedimentary layer at depth z obeys the following rule:

VS (z ) = VS0 (1 + Z )a

(2)

where Vs0 is the surface shear-wave velocity in m/s, Z = z/z0 (with z0 = 1 m) and the value of “a” quantifies the vertical velocity gradient (Faust, 1951; D'Amico et al., 2008). Considering that VS (z ) is equal to dz /dt , the shear-wave travel time, T, taken by a shear wave to reach the top of the bedrock is as follows:

Fig. 2. Distribution of seismic stations for H/V surveying (red triangles) and a drill hole (blue symbol) for accuracy control (Long, 2010). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

h

T=

∫ VSd(zz ) = 0

subsurface seismic properties (Bodin and Horton, 1999; Parolai et al., 2002; Benjumea et al., 2011) and sediment-bedrock structures (Ibs-von Seht and Wohlenberg, 1999; Castellaro and Mulargia, 2009; Wang et al., 2009; Gabas et al., 2016) in a variety of geological settings (Sukumaran et al., 2011; Imposa et al., 2016; Yan et al., 2018) that exhibit large impedance contrasts in material properties, such as density and shear-wave velocities. The reliability of H/V has been verified in detecting the impedance contrast between loose deposits and seismic bedrock (Chatelain et al., 2008; Haghshenas et al., 2008; Chen et al., 2009; Bao et al., 2018). The microtremor wavefield measured by a three-component seismometer and the calculation of the ratio between its horizontal and vertical Fourier spectra are required in this method. The peak frequency in the H/V curve represents the fundamental resonance frequency that can be associated with the depth of the main seismic impedance contrast and the average shear-wave velocity above the contrast interface (Lachet and Bard, 1994; Fah et al., 2001). The operating length usually ranges from 30 min to several hours to obtain a robust estimate of the microtremor wavefield characteristics (Civico et al., 2017; Bao et al., 2018). The processing of the individual measurement recordings generally includes windowing of short duration, selection of time windows to exclude strong transients and smoothing of the calculated amplitude spectrum (Konno and Ohmachi, 1998). Furthermore, the amplitude spectra of the two horizontal components can be combined in several ways before calculating the H/V curve, including calculating the squared average, total horizontal energy, and directional energy. In this study, the horizontal spectrum is calculated by the squares average of the amplitude spectrum of the NS and EW components. In general, the H/V method has been successfully used to infer the thickness of Quaternary deposits inside basins where a strong seismic impedance contrast exists at the interface between the soft sediment and the hard bedrock (Ibs-von Seht and Wohlenberg, 1999; Langston and Horton, 2014). The effectiveness of the method and reliability of the instrument, including instrumental considerations and stability of H/V analysis, have been discussed in a previous study by Bao et al. (2018). Extensive tests have been conducted to evaluate and verify the reliability of the H/V results, which are not repeated in detail here.

1 (1 + h)(1 − a) − 1 VS0 (1 − a)

(3)

If h ≫ 1 and h(1 − a) ≫ 1, then the approximate relationship (D'Amico et al., 2008) is as follows:

T≈

h(1 − a) VS0 (1 − a)

(4)

By substituting Eq. (4) into Eq. (1), it is possible to derive the thickness from the frequency:

V (1 − a) ⎞1/(1 − a) −1/(1 − a) ⎛ b ⎞1/(1 − a) −1/(1 − a) = h = ⎛ S0 fr fr 4 ⎝4⎠ ⎠ ⎝

(5)

where the value b represents VS0 (1 − a) . Eq. (5) also shows that the h − fr relationship can be established using a non-linear power-law formula, such as h = Afr B , in which the values of A and B are constant coefficients solved indirectly by two other constants, a and b. Such a relationship provides a practical method of estimating sediment thickness and has been developed in a number of studies (Ibs-von Seht and Wohlenberg, 1999; Parolai et al., 2002; Hinzen et al., 2004; Özalaybey et al., 2011). ¯S is formulated as follows: Replacing T from Eq. (4), V a ¯S = h = VS0 (1 − a)h = bha V T

(6)

¯s − h relationship can also be established using another Thus, the V non-linear power-law formula with similar constant coefficients of a and b. In this study, the coefficients a and b were empirically determined ¯S and h derived from by a power-law function regression between V borehole seismometer records from permanent seismic stations in North China (Fig. 1a). At a borehole three-component seismic station, two pulses with comparable amplitudes and the same polarity can be observed frequently on radial and tangential seismograms of local events. The interval between these two pulses is almost identical for different events, suggesting a shear-wave structural origin (Hauksson et al., 1987; Fukushima et al., 1992; Jongmans and Malin, 1995). The first pulse is well established as a direct S-wave from the earthquake, while the second pulse is due to surface reflection, and they are denoted as S1 and S2, respectively. These two seismic phases provide a good method of resolving near-surface P-wave (Liu et al., 2011c) and S-wave (Shen 3

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Table 1 Information on borehole stations in the North China depression basin, collected from previous studies (Shen et al., 2010; Liu et al., 2011c). The distribution of the permanent seismic stations with borehole seismometers is shown in Fig. 1a. Sta

Dep (m)

ΔTS2 − S1 (s)

¯ s (m/s) V

¯ p/V ¯s V

Sta

Dep (m)

ΔTS2 − S1 (s)

¯ s (m/s) V

¯ p/V ¯s V

ZHX NLS JIZ CIQ FHY HEJ JTG QIG YOQ WUQ HBT BET WAK XAZ WEA CAD CGZ CHH

150 160 173 174 185 241 241 241 241 244 245 246 246 246 266 247 247 247

0.86 0.86 0.99 0.95 0.97 1.24 1.22 1.37 1.19 1.34 1.29 1.38 1.28 1.21 1.28 1.28 1.29 1.38

349 372 349 366 381 389 395 352 405 364 380 357 384 407 416 386 383 358

5.1 5.1 4.3 4.8 4.4 4.4 4.9 5.1 4.4 4.8 4.5 5.1 4.4 4.5 4.3 4.6 4.8 5.3

CHT FTZ HAG NHZ TJH XZZ ZTZ LUT SJZ XIZ DAG ANK DOT DZG JIH T23 XIJ TAH

247 247 247 247 247 247 247 248 248 248 249 271 273 291 277 275 283 480

1.33 1.23 1.46 1.31 1.41 1.33 1.30 1.34 1.32 1.39 1.38 1.32 1.41 1.22 1.45 1.49 1.28 1.84

371 402 338 377 350 371 380 370 376 357 361 411 387 477 382 369 443 522

4.9 4.7 5.2 4.2 5.0 4.9 4.8 5.0 4.9 5.6 5.3 5.1 4.7 5.3 4.7 4.7 4.1 4.1

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.04 0.01 0.03 0.05 0.05 0.05 0.01 0.03 0.03 0.05 0.02 0.01 0.02 0.01 0.02 0.02 0.03 0.01

2h ¯S V

(7)

Thus, the average shear-wave velocity was measured. Because previous related studies were published in Chinese (Shen et al., 2010; Liu et al., 2011c), in this study, the data for borehole station depth and shear-wave velocity are reorganized in Table 1. The average shear-wave velocity and corresponding station depth are fitted with a non-linear power-law equation (6) to obtain the following regression relationship:

¯S (h) = V

111h0.25

3. Results

(8)

3.1. H/V measurements

The coefficient of determination (R-squared) of the fit is 0.97 (Fig. 3a). Based on this relationship, we can obtain the coefficients a and b to establish h- fr according to Eq. (5):

h (fr ) = 83fr −1.33

Typical H/V curves obtained from the dense seismic array in the Tangshan fault zone are shown in Fig. 4. According to the shapes of the curves, the H/V analysis reveals three typical cases, including single, double and no peak H/V curves. In the northern Tangshan fault zone, the measurements at station TS101 (deployed in Heishan Hill, shown in Fig. 2) and station TS106 (deployed in Weishan Hill, shown in Fig. 2) are performed above solid rock. Therefore, these two sites do not display amplification (approximately flat H/V curves) corresponding to the sediments. Stations TS109, T042 and TS045 are deployed near the

(9)

Based on borehole information as deep as 1219 m in the western Lower Rhine Embayment (Germany), Ibs-von Seht and Wohlenberg (1999) obtained another relation expressed as follows:

h (fr ) = 96fr −1.388

0.05 0.02 0.01 0.02 0.01 0.02 0.04 0.05 0.04 0.02 0.01 0.01 0.01 0.03 0.03 0.05 0.03 0.02

Using Eq. (10), the sediment thickness in metropolitan Beijing is estimated based on a H/V survey and coincides with major geomorphological and neotectonic expressions in the area (Chen et al., 2009). Fig. 3b shows a comparison between our Eqs. (9) and (10). These two relations display estimates of overburden thickness declining with increasing frequency. The sediment thickness from Eq. (9) in our study is slightly thinner at the same frequency. In addition, the sediment thickness in the southern Tangshan depression was roughly estimated ¯s = 500 m/s refer¯S/4fr and V by Bao et al. (2018) with equations h = V ring to the average shear-wave velocity from the 480 m-deep TAH borehole station (Table 1). With this relation, the bedrock basement becomes deeper at frequencies higher than 0.5 Hz.

et al., 2010) velocities and attenuation structures (Chong et al., 2009) of the Quaternary sediment in the North China depression basin. According to ray theory, the travel time difference between the direct S-wave (TS1) and the near-surface reflected S-wave (TS2) can be approximated as follows:

ΔTS2 − S1 = TS2 − TS1 =

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

(10)

Fig. 3. (a) Shear-wave average velocity curve obtained from permanent borehole station (blue triangles shown in Fig. 1a) recordings. The optimal fitting power-law equation curve (black line) relating station depth to shear-wave average velocity has a coefficient of determination (R-squared) of 0.97. The black dots and scales represent the shear-wave average velocities and variances, which are calculated by the average travel-time differences in the S1 and S2 seismic phases of earthquake sequences (Chong et al., 2009; Hauksson et al., 1987). (b) Resonance frequency to sediment thickness relationships according to the powerlaw (the thick solid line) and inverse pro¯ s is portion (the thin solid line; the value of V 500 m/s based on the study of Bao et al., 2018) equations in this study, and the dotted-dashed line corresponds to another relation by Ibs-von Seht and Wohlenberg (1999). 4

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Fig. 4. Examples of typical site H/V results (see Fig. 2 for station locations; these station names are all labelled in Fig. 2) as observed at northern stations: (a) TS101, (b) TS106 and (c) TS109; at middle stations: (d) TS042, (e) TS045 and (f) TS060; and at southern stations: (g) TS022, (h) TS004 and (i) TS020. Within each panel, the black line represents the H/V curve with standard deviation (grey shaded area).

of 250–500 m/s (Liu et al., 2011c, Bao et al., 2018), the sediment thickness in most regions has a range of 20–850 m as predicted by Eq. (1). We observe a maximum thickness of ~900 m in the southern Tangshan depression, and the thickness varies in accordance with the topography and surface geology, reaching close to zero where the bedrock crops out. Sediment thickness distributions are calculated for the study area based on our derived power-law equation (9) in Fig. 5b and the inverse proportion equation in Fig. 5c. The sediment thickness map for the study area is also compiled based on sparse geological drillings (Liu et al., 1982) in Fig. 5d. The H/V-derived sediment thickness presented in this study generally coincides with the results of previous geological surveys in the whole region. For example, the sediment is much thinner on the northern side and much thicker on the southern side of the study area. Moreover, the new sediment thickness maps provide more details with much smaller undulations over the entire area than those in previous studies. These small-scale fluctuations appear to be reasonably consistent with the surface morphology and subsurface geology in this area. A comparison of the H/V-derived thicknesses and the drilling survey reveals some inconsistencies, especially in the deepest depocentre location. A dense calibration seismic profile was conducted across the entire disputed region of the southern study area with an inter-station distance of ~1 km perpendicular to the Tangshan Fault, and it revealed two significant seismic impedance interfaces at depths of ~100 m and 300–800 m. Related research results have verified good consistency with the seismic reflection interfaces from shallow seismic reflection exploration and deep seismic reflection profiling (Bao et al., 2018).

centre of the study area and have the characteristics of a clear amplification peak frequency above 0.7 Hz. In the southern part of the study area, the occurrence of two amplification frequencies between 0.15 and 3 Hz has also been observed in previous work (Bao et al., 2018); one peak is between 0.15 and 0.60 Hz, and the other peak is ~1 Hz. This type of curve is illustrated in Fig. 4f–i. 3.2. Resonance frequency distribution To explore the variations in the resonance frequencies, which are defined as the lowest frequencies of spectral peaks from the computed H/V curves, in relation to the basement structure, we prepare a resonance frequency map for the study area by gridding the resonance frequency values over the measurement points (Fig. 5a). The high resonance frequencies (close to 3 Hz or higher) are tightly concentrated in the north-central Tangshan region around and within Dachenshan, Weishan and Changshan Hills and northern Heishan Hill, and these hills have bedrock outcrops and correspond geologically to the Tangshan Uplift and the southern margin of the Yanshan Uplift. A sharp frequency gradient is observed at the edge of the hard rock outcrop concentration area. However, the low resonance frequencies are distributed in the southern vast region (0.15–0.6 Hz, indicated by red colours) and on both sides of the central Tangshan Uplift. The Jinggezhuang Rift is in the west, and the Kaiping Rift is in the east. The lowest frequencies are found in the Caobo region in the southern part of the study area, within the depression. 3.3. Sediment thickness By analysing the H/V curves, we find that the predominant resonance frequency has a range of 0.15–3 Hz (Fig. 5a) in most parts of the Tangshan fault zone. Referring to the average shear-wave velocity

3.4. Sp-converted phases from local earthquakes The Tangshan fault zone has a well-developed impedance contrast 5

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Fig. 5. (a) Resonance frequency (in Hz) distribution with H/V microtremor survey sites (white crosses) and (b–d) Quaternary thicknesses (in metres) around the Tangshan fault zone. Panels (b) and (c) are maps showing the thickness of the sedimentary infill derived from the H/V resonance frequency by different frequencythickness relationships (see upper left label of each panel). (d) Contour map showing the thickness of the Quaternary sediment from sparse geological drillings (Liu et al., 1982).

station must be smaller than its focal depth. This requirement is imposed to ensure that the Sp phase can be observed with a high signal-tonoise ratio and that waveform complexity due to interactions with the details of the deeper structure of the basin can be avoided (Langston, 2003). At 6:31 a.m. on March 4, 2017 (local time), a magnitude 1.6 local earthquake occurred at the centre of the dense seismic array with the epicentre relocated at latitude 39.547°N, longitude 118.276°E and depth 17 km (Fig. 6a). The velocity seismograms for this event exhibit clear P, S and Sp phases (Fig. 6b–e). The phase identified as Sp appears as a high-frequency arrival with strong energy similar to the direct P phase on the vertical component preceding the S phase, whereas the direct S phase is also clear on the two horizontal components. Thus, the characteristics of these seismograms strongly indicate that the observed Sp phase is converted from the direct S phase at the high impedance interface between the sediments and bedrock and that all arrivals follow nearly vertical travel paths beneath the stations.

across the sediment-bedrock interface, especially in the south, as revealed by the microtremor H/V survey in this study, and this result implies that the thick sedimentary cover with large impedance contrast should also have a well-defined signature in the recordings of local earthquakes. Such seismic signals are often found in the recordings of local earthquakes made over thick sedimentary basins in the form of converted and/or reverberated phases generated at the sediment-bedrock interface (Chen et al., 1996; Wang and Klaus, 2002; Langston, 2003). Differential travel times between these phases and direct P and/ or S phases are primarily sensitive to the thickness and average velocity of the sedimentary cover. Among these phases, Sp, converted from the direct S-wave to a P-wave, is the most commonly observed phase and has been used to investigate the sediment-bedrock interface in many studies (Mandal, 2007; Langston et al., 2009; Chang et al., 2010; Chopra et al., 2010). In this study, the epicentral distance from the local event to the 6

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Fig. 6. (a) Location of the local earthquake is circled in black. The seismic stations for verifying sediment thickness are cruciform in black. Recordings of vertical (Z), north–south (N) and east–west (E) horizontal components at different epicentral distances for sites TS038 (b), TS060 (c), TS050 (d) and TS040 (e) for this local earthquake, illustrating P, S and Sp seismic phases.

the same site. The log shows that almost the entire 60 m consists of unconsolidated media, including mud, clay, sand and gravel (Fig. 7a). Below these Quaternary unconsolidated sediments are hard Carboniferous strata (Guo et al., 2011). Therefore, the consistency between the H/V pseudo-depth and well logging data thicknesses indicates that the adapted frequency-thickness relationship yields a good confidence level in the Tangshan fault zone.

To check the validity of our bedrock depth estimation based on microtremor results, we compare the results between the H/V and Sp methods beneath stations TS038, TS040, TS050 and TS060 (Fig. 6). We use the following equation that relates the travel times of the TS and TSp phases to the thickness of sediments h overlying bedrock for a nearvertical incident S-wave:

h=

¯S (TS − TSp ) V ¯S/V ¯P 1−V

(11)

4.2. Uncertainties of different frequency-thickness relationships

¯ P and V ¯S are the average P- and S-wave velocities of the sediwhere V ¯S/V ¯ P and V ¯S in Eq. (11) are derived ments, respectively. The values of V from the studies of Shen et al. (2010) and Liu et al. (2011c) listed in ¯S for the shallow sediment to 500 m/s and calculate a Table 1. We set V ¯ P/V ¯S value of 4.78 ± 0.14 from Table 1 (Bao et al., 2018; Wang et al., V 2016). These values result in sediment thicknesses of approximately 696 m beneath TS038, 506 m beneath TS060, 652 m beneath TS050, and 747 m beneath TS040 according to Eq. (11). The evaluation of the frequency and sediment thickness relationship developed in Eq. (9) yields sediment thicknesses of 622 m (TS038), 474 m (TS060), 661 m (TS050) and 706 m (TS040). Therefore, the Sp phase observations can provide additional complementary constraints on the sediment thickness in the Tangshan fault zone.

First, we display the difference in sediment thickness estimated between the non-linear power-law equation (9) and the inverse proportion equation. Fig. 8a shows that the difference between these two relationships is less than 50 m in the central and northern regions of the study area. However, the difference in the southern area close to the Caobo region is larger but no more than 150 m. In addition, we show differences in sediment thickness derived from the non-linear powerlaw equation (9) and sparse geological drillings (Fig. 8b). The difference is less than 100 m in the north-central region and less than 150 m in the south-eastern region. In contrast, the difference is up to 300 m in the southwestern area. 4.3. Sediment thickness and basement of sediments

4. Discussion Based on the resonance frequency around the Tangshan fault zone, the infill thickness is calculated for the whole area with the H/V frequency-thickness relationship. The sediment thickness around the Tangshan fault zone is characterized by thinning in the north and thickening in the south. Thickness variations indicate a highly complex subsurface characterized by the presence of three main buried depocentres: Jinggezhuang, Kaiping, and southern Tangshan. A string of hills with a sharp thickness gradient in the northern area separates the Jinggezhuang Rift from the Kaiping Rift. The Jinggezhuang Rift presents a roughly NE-SW elongated shape, which shows the inferred sediment thickness deepening in the south and

4.1. Borehole confirmation and resolution analyses To confirm the H/V results, one seismic station was installed on the surface of borehole TZC1 (longitude 118.17°E, latitude 39.62°N, in Fig. 2 and Fig. 5a, referring to Long, 2010) and acquired 2-h ambient seismic data. The predominant frequency of this borehole station is 1.44 Hz (Fig. 7b). The interpretation of the bedrock depth is 65 ± 15 m from its peak frequency considering the different frequency-thickness relationships in this study. The H/V pseudo-depth fits well with the thickness provided by the well logging data from the borehole drilled at 7

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Fig. 7. (a) Column from a drill hole down to 70 m depth (named TZC1, modified from Long, 2010). (b) Corresponding microtremor H/V curve with a 2-hour duration by means of a station deployed in the same position as illustrated in Fig. 2.

reconstructing the buried interface between the infill deposits and the top of the pre-Quaternary substratum, this work obtains the basement of sediments and the relationships between the fault system and the sediment thickness. The obtained H/V ratio site response is compared with the shallow geology from the borehole. The reliability of the sediment thickness values is verified by comparison with geological and local Sp-converted phase estimated results. In this study, an effective and reliable method is provided to obtain sediment thickness results and thus is of great significance for simulating earthquake strong motions and investigating active faults. The recovered buried basement morphology shows a complex subsurface architecture in the Tangshan fault zone, which is characterized by topographic highs and lows that represent tectonic uplifts and depositional districts. The main depositional district is recognized south of the Tangshan fault zone, which is characterized by distinctively different infill thickness. As a whole, variations in the thickness of the deposits show that the Tangshan fault zone deepens lengthwise from ~0 (bedrock) to nearly 200 m in the northern area and to a maximum thickness more than 800 m in the southern area. Several identified buried tectonic structures are compatible with the fault arrangement recognized at the surface. The subsurface model highlights that the Tangshan fault zone has experienced considerable modifications in its architecture over time, and its overall geometry reveals tectonic control by the faults that are part of the NNE-striking extensional Tangshan Fault.

reaching a maximum value of ~200 m. The deepest depocentre is located in the southern Tangshan depression, and it is characterized by a maximum thickness exceeding 800 m. The elevation of seismic bedrock is interpreted from sediment thickness and surface elevation (Fig. 9). The interpretation indicates an obviously uneven terrain that is high in the north from ~100 to ~−100 m and low in the south from −300 to lower than −800 m, where the seismic bedrock relief may vary rapidly within ~200 m and agrees well with the spatial characteristics revealed by deep seismic reflection profiling (Bao et al., 2018). Beneath the unevenly thick sedimentary cover, the basement is extensively fractured. A pronounced depression in the seismic bedrock occurs in the Caobo region, east of Jianzigu Village. The highest elevation of the bedrock is interpreted in the north and corresponds to local hills with rocky outcrops. Changshan Hill and Weishan Hill, which are the northern boundary of the alluvialproluvial basin, are both relatively low hills on the southern margin of the Yangshan uplift. Dachengshan Hill is a relic hill of basement rock distributed within the study area and forms a south-westward extension of Changshan Hill and Weishan Hill. 5. Conclusions A dense seismic array and H/V microtremor survey provide a reliable sediment thickness model of the Tangshan fault zone. By

Fig. 8. Uncertainties of different frequency-thickness relationships. (a) Thickness differences between the results of the non-linear power-law equation (9) and the inverse proportion equation in the Tangshan fault zone. (b) Thickness differences between measurements based on the non-linear power-law equation (9) and sparse geological drillings (Liu et al., 1982). 8

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Fig. 9. Basement of sediments and main geological tectonic units.

Declaration of Competing Interest 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. Acknowledgements The authors are grateful to the staff of the Tangshan Central Seismic Station of the Earthquake Administration of Hebei Province for their valuable collaboration and support with the microtremor survey. We appreciate the editor and reviewers for their constructive comments. We thank Professor Sidao Ni for his helpful comments. We also thank Yongjun Zou and Wen Tian for their meaningful discussions. This work was supported by grants 2018YFC1504202, NSFC41674065, and NSFC41404052 and the China-ASEAN Marine Geosciences Research and Disaster Reduction Initiative Project (121201002000150022). The study is also supported by the China Earthquake Science Experiment project, China Earthquake Administration (2019CSES0102) and the State Key Laboratory of Geodesy and Earth’s Dynamics. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jseaes.2019.104045. References Bao, F., Li, Z., Yuen, D., Zhao, J., Ren, J., Tian, B., 2018. Shallow structure of the Tangshan fault zone unveiled by dense seismic array and horizontal-to-vertical spectral ratio method. Phys. Earth Planet. Inter. 281, 46–54. Bell, S.W., Ruan, Y., Forsyth, D.W., 2015. Shear velocity structure of Abyssal Plain sediments in Cascadia. Seism. Res. Lett. 86 (5), 1247–1252. Benjumea, B., Macau, A., Gabas, A., Bellmunt, F., Figueras, S., Cires, J., 2011. Integrated geophysical profiles and H/V microtremor measurements for subsoil characterization. Near Surf. Geophys. 9 (5), 413–425. Bodin, P., Horton, S., 1999. Broadband microtremor observation of basin resonance in the Mississippi embayment, Central US. Geophys. Res. Lett. 26 (7), 903–906. Bonnefoy-Claudet, S., Baize, S., Bonilla, L.F., Berge-Thierry, C., Pasten, C., Campos, J., Volant, P., Verdugo, R., 2009. Site effect evaluation in the basin of Santiago de Chile using ambient noise measurements. Geophys. J. Int. 176, 925–937. Butler, R., Stewart, G.S., Kanamori, H., 1979. The July 27, 1976 Tangshan, China

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