Three‑Dimensional Seismic Velocity Structure in the Sichuan Basin, China

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Three‑dimensional seismic velocity structure in the Sichuan basin, China

Wang, Maomao; Hubbard, Judith; Plesch, Andreas; Shaw, John H.; Wang, Lining

2016

Wang, M., Hubbard, J., Plesch, A., Shaw, J. H., & Wang, L. (2016). Three‑dimensional seismic velocity structure in the Sichuan basin, China. Journal of Geophysical Research: Solid Earth, 121(2), 1007‑1022. doi:10.1002/2015JB012644 https://hdl.handle.net/10356/88913 https://doi.org/10.1002/2015JB012644

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Journal of Geophysical Research: Solid Earth

RESEARCH ARTICLE Three-dimensional seismic velocity structure 10.1002/2015JB012644 in the Sichuan basin, China

Key Points: Maomao Wang1,2, Judith Hubbard1,3,4, Andreas Plesch1, John H. Shaw1, and Lining Wang5,6 • We present a high-resolution seismic velocity model for Sichuan basin 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA, 2School of Earth • ﬂ Vp structure re ects complex tectonic 3 and sedimentation process Sciences and Engineering, Nanjing University, Nanjing, China, Earth Observatory of Singapore, Nanyang Technological 4 5 • The model applied effectively in the University, Singapore, Asian School of the Environment, Nanyang Technological University, Singapore, Research Institute seismic hazard assessment of Petroleum Exploration and Development, PetroChina, Beijing, China, 6School of Earth and Space Sciences, Peking University, Beijing, China

Supporting Information: • Figures S1–S4 • Figure S1 Abstract We present a new three-dimensional velocity model of the crust in the eastern margin of the • Figure S2 Tibetan Plateau. The model describes the velocity structure of the Sichuan basin and surrounding thrust • Figure S3 belts. The model consists of 3-D surfaces representing major geologic unit contacts and faults and is • Figure S4 • Data Set S1 parameterized with Vp velocity-depth functions calibrated using sonic logs. The model incorporates data • Data Set S2 from 1166 oil wells, industry isopach maps, geological maps, and a digital elevation model. The geological • Data Set S3 surfaces were modeled based on structure contour maps for various units from oil wells and seismic • Data Set S4 reﬂection proﬁles. These surfaces include base Quaternary, Mesozoic, Paleozoic, and Proterozoic horizons.

Correspondence to: The horizons locally exhibit major offsets that are compatible with the locations and displacements of M. Wang, important faults systems. This layered, upper crustal 3-D model extends down to 10–15 km depth and [email protected] illustrates lateral and vertical variations of velocity that reﬂect the complex evolution of tectonics and

sedimentation in the basin. The model also incorporates 3-D descriptions of Vs and density for sediments that are Citation: obtained from empirical relationships with Vp using direct measurements of these properties in borehole logs. Wang, M., J. Hubbard, A. Plesch, To illustrate the impact of our basin model on earthquake hazards assessment, we use it to calculate ground J. H. Shaw, and L. Wang (2016), Three- dimensional seismic velocity structure motions and compare these with observations for the 2013 Lushan earthquake. The result demonstrates the in the Sichuan basin, China, J. Geophys. effects of basin ampliﬁcation in the western Sichuan basin. The Sichuan CVM model is intended to facilitate fault Res. Solid Earth, 121, 1007–1022, systems analysis, strong ground motion prediction, and earthquake hazards assessment for the densely doi:10.1002/2015JB012644. populated Sichuan region.

Received 5 NOV 2015 Accepted 30 JAN 2016 Accepted article online 4 FEB 2016 1. Introduction Published online 27 FEB 2016 The Sichuan basin lies along the eastern margin of Tibetan Plateau and is one of the most densely populated and industrialized areas in China. The basin is surrounded by a series of active thrust and strike-slip fault systems, including the Longmen Shan, Xianshuihe, Minjiang, and Huaying Shan fault systems, which have experienced historical seismicity (Figure 1) [Burchﬁel et al., 1995, 2008; Deng et al., 1994; Allen et al., 1991; Wen et al., 2008].

On 12 May 2008, the 2008 Mw 7.9 Wenchuan earthquake struck the western boundary of the Sichuan basin, caused more than 80,000 fatalities, left more than 1.5 million people homeless, and produced enormous economic losses. Thus, accurate seismic velocity models in the Sichuan basin and adjacent Longmen Shan are crucially important for understanding local crustal structures and improving regional seismic hazard assessment. There have been several previous studies investigating the seismic velocity structure of the eastern margin of Tibetan Plateau, focused on the Longmen Shan, Xianshuihe fault zone, Sichuan basin, and Kangdian block [e.g., Wang et al., 2007; Li et al., 2009; Wang et al., 2009; Lei and Zhao,2009;Zhang et al., 2010; Pei et al., 2010; Robert et al., 2010; Xu et al., 2010; Liu et al., 2014; Wang et al., 2015]. These studies are mainly based on teleseismic waveform data or deep seismic imaging such as active source seismic refraction or wide-angle reflection experiments. These results provide constraints on crust and upper mantle velocity structures beneath the eastern margin of the Tibetan Plateau. However, due to sparse seismic ray coverage within the Sichuan basin, these models and data sets commonly have limited resolution within the sedimentary basin. Regional tomographic models thus lack a precise representation of the basin velocity structure, which can have a significant effect on ground motions due to basin amplification [e.g., Olsen, 2000; Komatitsch et al., 2004; Graves,2008].Asa consequence, numerical simulations of seismic wave propagation and resulting ground motion forecasts in

©2016. American Geophysical Union. the Sichuan basin remain poor quality, generally failing to match strong motion acceleration and Fourier All Rights Reserved. amplitude spectra in the transverse component displacement seismograms [Chang et al., 2012].

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Figure 1. Geological map showing of the Sichuan basin and surrounding mountain belts, as well as data sets that are used to constrain our new velocity model. Geologic map is referenced from the 1:500000 digital geological map of China, China Geological Survey [CGS, 2003].

To develop our new model of wave speed structure, we employ the concept of a Unified Structural Representation (USR). For the past decade, research groups in the South California Earthquake Science Center (SCEC) have pioneered the development of a USR for Southern California [Shaw et al., 2015]. The USR comprises two interdependent models, termed the Community Fault Model (CFM) [Plesch et al., 2007] and Community Velocity Model (CVM) [Magistrale et al., 1996, 2000; Süss and Shaw, 2003; Kohler et al., 2003; Plesch et al., 2009; Shaw et al., 2015]. The CFM provides detailed locations and displacements of major active fault zones in three dimensions that are defined from seismic reflection profiles, relocated seismicity, well data, and geologic cross sections. The CVM includes a high-resolution model of basin velocity structure that is consistent with the locations and displacements of major faults in the CFM. These basin structures are then embedded in crust and upper mantle tomography models. Sediment velocities are derived using direct velocity measurements from petroleum exploration, such as sonic logs and seismic stacking velocities [e.g., Magistrale et al., 1996, 2000; Süss and Shaw, 2003]. This type of integrative model enables more accurate calculations of ground motion. For example, the ground motion in the Los Angeles region can be accurately modeled down to a period of 2 s within the sedimentary basins [Komatitsch et al., 2004]. In this paper, we employ the USR concept [Shaw et al., 2015] to develop a CVM that describes the seismic wave speed structure in the Sichuan basin. The model extends from 27.5° to 34.5°N and from 100° to 110°E and encompasses the Sichuan basin and surrounding fold-and-thrust belts (including the Longmen Shan, Micang Shan, Daba Shan, Eastern Sichuan, and Kangdian), as well as the part of the Kunlun and Xianshuihe strike-slip fault systems in the eastern Tibetan Plateau (Figure 1). The model was developed in GoCAD, a three-dimensional geologic computer-aided design tool that facilitates the modeling of geologic surfaces, the analysis of spatially distributed data, and the modeling or simulation of properties [Mallet, 1992]. The geological surfaces used in the model are based on industry structure contour and isopach maps for various units augmented by stratigraphic picks in oil and gas wells and seismic reflection profiles. These surfaces are

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consistent with the locations and displacements of major fault system that are deﬁned by a companion Community Fault Model [Hubbard et al., 2012]. This new P wave seismic velocity model is then compared with a regional tomography model derived from teleseismic waveform data.

2. Tectonics and Sedimentation The Sichuan basin is located to the east of the Longmen Shan fold-and-thrust belt at the eastern margin of Tibetan Plateau (Figure 1). To the west of the Longmen Shan, the large, triangular Songpan-Garzê basin contains 5–15 km thick, highly deformed Triassic turbiditic sediments [Burchfiel et al., 1995]. The Songpan-Garzê basin formed as a remnant ocean basin due to the collision of the South and North China blocks [Yin and Nie, 1993]. To the east of the Longmen Shan, the Sichuan basin is filled with up to 10 km of Late Proterozoic (Sinian) to Quaternary sedimentary deposits lying above the Pre-Sinian metamorphic igneous basement of the Yangtze Craton in the South China block [Burchfiel et al., 1995; Chen and Wilson, 1996; Jia et al., 2006]. The architecture and evolution of the Sichuan basin are largely controlled by surrounding thrust belts, including the Longmen Shan belt to the west, the Daba Shan to the north, the Eastern Sichuan belt to the east, and the Kangdian belt to the south (Figure 1). The basin developed as a large intracratonic basin from Sinian to middle Late Triassic times when a thin, incomplete succession of shallow-marine and nonmarine sediments were deposited [Burchfiel et al., 1995; Jia et al., 2006]. The basin transitioned to fully terrestrial during the Late Triassic Indosinian orogeny, due to the closing of the Paleo-Tethys [Chen and Wilson, 1996]. During the collision, the basinward thrusting of the Longmen Shan formed the Sichuan foreland basin [Burchfiel et al., 1995; Jia et al., 2006; Li et al., 2003]. The foreland sediments are >4 km thick in the west and gradually thin to the southeast [Li et al., 2003; Jia et al., 2006]. In the Early Jurassic, the basin entered a period of tectonic quies- cence. Lower Jurassic rocks overlie the Late Triassic thrust faults and folds in the basin and exhibit a relatively constant thickness throughout the basin margin. From Middle Jurassic to Early Cretaceous, the depocenter shifted to the northwestern Sichuan basin [Meng et al., 2005]. This shift was caused by flexure in the northwest basin driven by the formation of the Daba Shan fold-and-thrust belt, which accommodated continued convergence of the North and South China blocks after the Triassic closure of the Paleo-Tethys [Ratschbacher et al., 2003; Dong et al., 2013]. Early Tertiary sediments, consisting of fluvial lacustrine red sandstone and siltstone [Burchfiel et al., 1995; Chen and Wilson, 1996; Jia et al., 2006], are restricted to the southwestern part of the Sichuan basin and conformably overlie Cretaceous rocks. In the late Cenozoic, the Longmen Shan uplifted rapidly and formed the steepest topography along the margin of the Tibetan Plateau [Kirby et al., 2002]. This formed a thin (up to 700 m deep) foreland basin filled with Quaternary sediments along the western part of the Sichuan basin (Figure 1) [Burchfiel et al., 1995; Hubbard and Shaw, 2009; Hubbard et al., 2010]. Most sedimentary deposits exposed elsewhere in the basin are of Mesozoic age. Thus, the region includes two distinct subbasins, a Quaternary basin localized along the foreland of the Longmen Shan and a much broader area containing Mesozoic through Precambrian strata. Our model distinguishes these basin structures, as they have substantial differences in sediment wave speeds and densities.

3. Data and Modeling Method To develop the 3-D seismic velocity model, we created a database of seismic reflection profiles, well tops, maps, and cross sections that define major geologic interfaces and various types of direct velocities measurements largely from petroleum and hydrologic exploration. We used these constraints to develop triangulated surface representations of the major stratigraphic interfaces and the top of crystalline basement. These interfaces, along with topography, were used to develop a 3-D volumetric description of the basin sediments that is consistent with the locations and displacements of major faults in the CFM [Hubbard et al., 2012]. Within the 3-D model framework, we developed velocity-depth functions from well log data and then used them to parameterize the velocity structure. The development of these model components is described in the following sections.

3.1. Geologic Interfaces We ﬁrst constructed a 3-D volumetric description of the basin sediments using constrains from topography, geologic interfaces, and the top of basement. The major geological interfaces included in the 3-D model are the base of the Quaternary, Mesozoic, Paleozoic, and Proterozoic sediments. The base of the Proterozoic sections deﬁnes the top basement surface. These chronostratigraphic units represent important lithologic

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boundaries and are readily defined in wells and seismic reflection profiles. Thus, these geological interfaces serve as effective markers to define the basin geometry and also will guide our parameterization of velocity structure. An example of a seismic reflection profile that was used to define the basement horizon and stratigraphic layers within the sedimentary basin is shown in Figure 2. The geometry and depth of the base Quaternary surface were derived largely from isopach maps constrained by hydrologic exploration in the 1970s [Chengdu Hydrology Research fl fi fi Figure 2. Sample data of seismic re ection pro le used to de ne the top (CDHR) Team, 1977; Qian and Tang, crystalline basement surface and major stratigraphic horizons within the 1997; Li and Guo, 2008]. The sedimentary basin. TWTT, two-way travel time. Quaternary rocks are mainly fluvial sediments, and they include consoli- dated conglomerate, sandstone, siltstone, and mudstone, as well as unconsolidated sand and gravel. The depth of the Mesozoic, Paleozoic, and Proterozoic units are constrained by industry structure contour and isopach maps that we calibrated and refined using horizon picks from 1166 exploration and production wells (Figure 1). The Cretaceous to Late Triassic section consists of lacustrine sandstone, siltstone, and coal, while the underlying Triassic strata are marine carbonates and shales, locally containing evaporites. Paleozoic rocks can be divided into three major sequences based on lithology and sedimentary distribution. Cambrian to Ordovician rocks are mainly shallow marine carbonates with minor amounts of terrigenous clastic rocks; these units extend across the Sichuan basin. Silurian, Devonian, and Carboniferous rocks are present in the Longmen Shan range front, with a thickness of 2000–4000 m, but are absent over most of the remainder of the Sichuan basin. Permian rocks are mainly shallow marine limestone and dolomite and contain some sandstone and conglomerate near the base of the section. The Proterozoic rocks penetrated by wells in the basin are upper Sinian deposits (Dengying Formation), dominated by 300–1200 m thick micrite and dolomite carbonates. The base of the sedimentary section as defined in this study is the contact between these Sinian sedimentary rocks and mid-Proterozoic metamorphic igneous rocks. The top of the crystalline basement horizon defined in this study was compiled from published isopach maps [Editorial Group of Petroleum Geology of Sichuan Oil Field (EGPGSOF), 1989], data from recent petroleum wells, and interpretation on seismic reflection profiles (e.g., Figure 2) [Jia et al., 2006; Hubbard et al., 2010; Li et al., 2014; Guo et al., 2013; Wang et al., 2014]. Based on these constraints, we developed contour maps representing the base of the Quaternary, Mesozoic, Paleozoic, and Proterozoic units for the Sichuan CVM, as shown in Figures 3–6. These contour maps of horizons were then imported into Gocad, a 3-D geological modeling application [Mallet, 1992]. Within Gocad, the horizons are represented as triangulated surfaces (t-surfs), which consist of nodes or points (x, y,andz) associated with one another in triangular elements. The surfaces are locally cut by major faults as represented in the Sichuan Community Fault Model (CFM) [Hubbard et al., 2012]. These faults have the most pronounced impact on the horizons and basin shape along the Longmen Shan range front, where they truncate and offset Mesozoic, Paleozoic, and Proterozoic horizons (Figures 4–6). In contrast, many of the faults within the Sichuan basin are blind, and therefore, their locations and displacements are represented by overlying folds (Figures 4–6). Major strike-slip faults also produce uplifted regions along their traces within the Tibetan Plateau.

3.2. Velocity Parameterization We utilized 36 sonic logs from petroleum wells in the Sichuan basin to describe the seismic velocities in the basin. The locations of petroleum wells with sonic logs are illustrated in Figure 1. Sonic logs record direct measurements of interval transit time between a source and receiver on opposite ends of a logging tool

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and thus provide measures of the P wave velocity under lithostatic pressure. These data are typically used in the petroleum industry to determine porosity, density, and lithology. Sonic velocities are sensitive to compaction and cementation of the rock. The distance between source and receiver is typically small (1–2m),andthetoolismovedalong the borehole, recording continuously. The transmitter emits in a sonic-to- ultrasonic range, and the receiver records P wave arrivals. The sonic logs measure seismic velocities in μs/ft; these measure the interval transit time of a compressional sound wave travel- ing through 1 ft (0.3048 m) of formation. In this study, the sonic measurements that recorded interval transit times are directly converted to P wave interval velocities using the method:

6 VpðÞm=s ¼ 10 ðÞ0:3048=ΔT ; ΔT ¼ T R2 T R1; (1)

where Vp is the P wave interval velocity and ΔT are the sonic wave travel time Figure 3. Structural contour map of the base of the Quaternary section in the between two receivers TR2 and TR1. Sichuan basin. Note that contoured depths are above sea level. Red lines are Figure S1 in the supporting informa- the faults in the Sichuan region. The modeled surfaces, horizons, and grids are provided in simple ASCII files in the Universal Transverse Mercator tion shows an example of the velocity projection (UTM 48 zone (102–108) in the Northern Hemisphere, WGS 84 data available in one well from the projection system). Sichuan basin. The high-resolution (<1 m) measurements show a wide scatter, reflecting variations in lithology, fracture patterns, and fluid content. To describe the general velocity trend in the basin, we want to define the smooth velocity trends along the vertical dimension. For this purpose, the P wave interval velocity data are smoothed by mean filtering with a 30 m moving window in the slowness domain, which is tested to effectively reduce data divergence. Figure 7 shows all velocity measurements from sonic logs that are included in this study. The wells sample basin depths between 50 and 6000 m, and the measurements are grouped by the geologic unit they represent based on formation tops within these wells, which were also used to define the horizons described in the

preceding section. The data show that most velocities (Vp) in the basin range from 3000 to 6500 m/s, and Vp generally increases with depth in the upper 1500 m. The Mesozoic units (Cretaceous, Jurassic, and Triassic) all show similar velocity ranges and average values with depth. Thus, for the purpose of developing velocity- depth functions, we combine the Mesozoic units together, yielding four major stratigraphic packages: (1) Cenozoic alluvium, (2) Mesozoic sedimentary and metasedimentary rocks, (3) Paleozoic sedimentary and metasedimentary rocks, and (4) Proterozoic sedimentary and metamorphic rocks. Cenozoic alluvium is thin (0–500 m) and limited primarily to the southwestern basin. These sediments have velocities ranging from 1

to 2.5 km/s, shown by the Vp measurements from wells and stacking velocities (Figure S2). Mesozoic sedimentary rocks are thick (2600–9800 m) and widely exposed at the surface and in boreholes in the basin. Their velocities vary from about 3 to 6 km/s. Paleozoic sedimentary and metamorphic rocks are also common, exposed primarily in the Longmen Shan and in regions to the north, east, and south of the basin. These units vary in thickness from 400 to 4100 m and have velocities from about 5 to 7 km/s. Proterozoic sedimentary

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Figure 4. Structural contour map of the base of the Mesozoic section in the Sichuan basin. Red lines are the faults in the Sichuan region.

Figure 5. Structural contour map of the base of the Paleozoic section in the Sichuan basin. Red lines are the faults in the Sichuan region.

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Figure 6. Structural contour map of the base of the Proterozoic section in the Sichuan basin. Red lines are the faults in the Sichuan region.

and metamorphic rocks crop out in narrow regions around the boundary of the basin but are widely disturbed within the basin. Two exploration wells penetrate the base of the Sinian, recording velocities of about 6.0–7.0 km/s. To parameterize our basin velocity model, we establish velocity-depth functions for these different geological units. The data used to constrain these functions consist of more than 1000, 30 m averaged sonic velocity measurements. For each geologic unit we base our velocity-depth function on an empirically derived shear

wave velocity (Vs)profile for generic rocks established by Boore et al. [1997]. In order to relate this Vs profile to our Vp measurements, we use Brocher’s [2005] empirically derived Vp-Vs function to develop a template Vp-depth profile. Finally, we use the data in each geologic interval to fitascaleandshiftparametertothis Vp-depth profile template using a least squares regression. This results in four distinct velocity-depth functions as shown below:

¼ : : ; Vp_Qu 0 85VpBrocher-1 1 (2) ¼ : þ : ; Vp_Mes 0 57VpBrocher 2 2 (3) ¼ : þ : ; Vp_Pal 0 9VpBrocher 1 1 (4) ¼ : þ : ; Vp_Pro 0 32VpBrocher 4 5 (5)

Vp_Brocher ¼ 0:9409 þ 2:0947Vs 3 0:8206Vs2 þ 0:2683Vs 4 0:0251Vs ; (6)

where Vp_Qu, Vp_Mes, Vp_Pal, and Vp_Pro represent the P wave velocity in km/s for Quaternary, Mesozoic, Figure 7. P wave velocity measurements for Quaternary, Cretaceous, Jurassic, Triassic, Paleozoic, and Sinian strata derived from 36 sonic logs Paleozoic, and Proterozoic units in the within the Sichuan basin. basin, respectively (Figure 8). Each

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velocity-depth curve (4–6) exhibits low

velocities at depths < 1km, with Vp gradually increasing with depth between 1 and 4 km. The gradient in

Vp decreases further at depths greater than 4 km. Moreover, these velocity- depth functions clearly illustrate that average velocities increase as a function of sedimentary age. This reflects the fact that the deeper and older rockshavebecomemorecompacted than the shallow and younger rocks. We calculate standard deviations of the variance between the data and model for Quaternary, Mesozoic, Paleozoic, and Proterozoic in Figure S3 to quantify- ing model uncertainty. Our modeling of Figure 8. P wave velocities within the sedimentary basin, distinguished velocity with depth for each geologic by age (Quaternary, Mesozoic, Paleozoic, and Proterozoic), with best fit unit involves smoothing the P wave Vp-depth functions. interval velocity data (30 m sample) of the sonic logs. Thus, the final model reflects the average values of the measured velocity structure. However, clearly, other factors—such as the occurrence of local velocity structures that are not sampled by the wells—may contribute to greater uncertainties. Ultimately, the comparison of synthetic waveforms generated using the model with natural earthquake records will provide the most robust way of formally assessing model accuracy.

3.3. Velocity (Vp, Vs) and Density Calibration

In addition to P wave speed (Vp), S wave speed (Vs), and density (ρ) are generally required to simulate seismic wave propagations and calculate strong ground motions for earthquake hazard assessment. In particular, strong ground motions are

sensitive to the local Vs structure [Brocher, 2005]. Thus, it is important to establish empirical relationship among

Vp, Vs, and density in order to parameterize our model. To assess the Vp-Vs relationship, we use Vertical Seismic Proﬁle (VSP) log data from a petroleum exploration well (Qibei_101) in the Sichuan basin that independently mea-

sures both Vp and Vs. VSP is a technique used to correlate borehole seismic measurements with surface seismic data [Hardage, 1985]. VSP velocity measurements of VSP are recorded in a vertical borehole using geophones insidethewellboreandasourceat the surface near the well. The VSP data used in this study contain continuous P and S wave measurements from about 200 to 5200 m [Yao and He, 2006] (Figure 9). To constrain the scaling Figure 9. P wave and S wave velocities measurements derived from vertical seismic proﬁle (VSP) data from the Qibei_101 well in the relationship between Vp and Vs, we use Sichuan basin. a least squares technique to ﬁt the

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Figure 10. Comparison between observed Vs and Vp and the modeled Vs. The observed S and P wave velocities are from the Qibei_101 well in the Sichuan basin. The error bars showing on the modeled Vs is the standard deviation of the variance between the data and model for Vs.

observed shear wave data to a function based on a empirical Vs-Vp relation [Brocher, 2005] by linear regression:

Vs_best_fit ¼ 0:8706VsBrocher þ 147:85; (7) 2 3 4 Vs_Brocher ¼ 0:7858 1:2344Vp þ 0:7949Vp 0:1238Vp þ 0:0064Vp ; (8)

where Vs_best_fit is the predicted shear wave velocity based on a given Vp. And Vs_Brocher (km/s) is defined using the empirical Vs-Vp relation from Brocher [2005] (Figure S4). Finally, the comparison between observed Vs and Vp and the best fit Vs is shown in Figure 10. The difference between the best fit Vs curve and the observed Vs curve is minimal, confirming that the empirical relationship we applied is effective for the Sichuan basin.

For local density-Vp scaling relations, we analyzed six borehole logs that record

both Vp and density (ρ) measurements. The well log data show that the rock density roughly increases with the P wave speed. We compared the original density measurements with the empirical density

with Vp of Brocher [2005] and linear regression of the density with Vp: 3 2 ρ g=cm ¼ 1:6612Vp 0:4721Vp 3 4 þ 0:0671Vp 0:0043Vp 5 þ 0:000106Vp ; (9) 3 2 ρ g=cm ¼ 0:0665Vp þ 2:1971; R ¼ 0:1608 (10)

where Vp ranges from 1.5 to 8.5 km/s. Figure 11 shows Brocher’s relationship Figure 11. The empirical relationship (V and ρ) with the density logging p and the linear regression (V and density) data in the Sichuan basin. The relationship is from Brocher [2005]. The p light blue area represents the ±10% error of the empirical relationship with the binned density logging data. from Brocher [2005]. Since only slight differences between

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modeled and observed density are present,weadopttheBrocher’s relationship

of ρ-Vp (10) to parameterize the density structure in the basin. 3.4. Assembling the Three-Dimensional Seismic Velocity Model We combine the geological surfaces and velocity functions to develop a 3-D velocity model that consists of a regular grid (x, y,andz) parameterized

with Vp, Vs, and density (ρ) values. The Figure 12. Perspective view of the geological surfaces used in the model. 3-D velocity model (Figure 12) is populated from 27.5 to 34.5°N and from 100 to 110°E and from the surface to 22 km depth below sea level in the vertical dimension, covering the entire Sichuan basin and adjacent regions. The modeled surfaces, horizons, and grids are provided in simple ASCII ﬁles in the Universal Transverse Mercator projection (UTM 48 zone (102–108) in the Northern Hemisphere, WGS 84 projection system). Using a GoCAD voxet (a three-dimensional regular and cubic grid), we construct a three-dimensional ortho- gonal, spatial array of data cells that are applied to produce and represent the 3-D seismic velocity model for the Sichuan basin. The voxet cell size is 200 m by 200 m in the horizontal plane and 100 m vertically. P wave velocities values for each cubic grid cell were assigned using the velocity-depth functions of 3–6 that are described in section 3.2. This upper crustal model extends down to ~10–15 km depth, although the maximum depth of sediments is about 12 km. For the purpose of comparison and illustration, we embed the basin model into a regional tomographic model [Lei and Zhao, 2009]. Figure 13 shows a perspective view of the Sichuan basin and adjacent tectonic blocks in the 3-D velocity model, including the sedimentary section above the basement tomographic model. To represent the shear velocity at ground surface, we use Vs30 (the average shear velocity down to 30 m depth) as the near surface of the velocity model. Vs30 is obtained from geological or topographic gradients to represent shear wave speed for seismic conditions mapping [Allen and Wald, 2009]. In this study, Vs30 was generated using the Global Vs30 Map Server, which derives maps of seismic site conditions using topographic slope as a proxy [Wald and Allen, 2007; Allen and Wald, 2009].

4. Model Description Our complete 3-D seismic velocity model for the Sichuan basin, which extends from the surface to 22 km depth, is shown in perspective view in Figure 13a. We illustrate the velocity structure with sections that pass through the central part of the 3-D seismic velocity model (Figures 13b and 13c). The sections show the velocity distributions of four individual geological layers, confirming that the general trend of velocity roughly increases with depth, with the sediments velocities ranging from 2000 to 7000 m/s. The base of the sedimentary section in the 3-D seismic velocity model shows significant topography, reflecting the multi- stage tectonic history and deformation of the region. Moreover, the base of the sediments is a high-velocity zone, from 6500 to 7000 m/s. Notably, these velocities are significantly higher than those present in the regional tomographic model [Wang et al., 2009; Lei and Zhao, 2009]. Previous studies [Zhong et al., 2013] and our analysis of well logs confirm that these high velocities correspond to the late Sinian (Precambrian) Dengying Formation, which consists of 300–1200 m dolomite rocks with very low porosities. This high-velocity interval may produce an inversion relative to the wave speeds in the underlying basement rocks and thus have a significant impact on wave propagation in the basin. The velocity section of Figure 13b also clearly demonstrates the asymmetric shape of the basin, with the thickest Cenozoic and Mesozoic sections located in the western part of the basin in the foreland of the Longmen Shan. Farther to the west, the base of the sediments in the Songpan-Garzê block reaches an averaged depth of about 16 km, which is consistent with the previous estimates for the thickness of the Mesozoic to Proterozoic sediments from the deep seismic reflection profile [Guo et al., 2013]. These two basins and corresponding low-velocity structures are separated by the Longmen Shan thrust belt, which is represented

WANG ET AL. SEISMIC VELOCITY MODEL OF SICHUAN BASIN 1016 Journal of Geophysical Research: Solid Earth 10.1002/2015JB012644

Figure 13. (a) Perspective view of the 3-D P wave velocity model for the Sichuan basin and adjacent regions in Sichuan Providence, China. (b) Cross section showing Vp across the 3-D velocity model, from Songpan-Garzê to Sichuan basin. (c) Cross section showing Vp across the 3-D velocity model, from the Daba Shan thrust belt to the Kangdian belt through the Sichuan basin.

by an ~ 40 km wide zone with high seismic velocities. These higher velocities in the Longmen Shan reﬂect the Phanerozoic sedimentary rocks and crystalline basement that have been brought up to the shallow crust and surface by the extensive thrusting in this region. Figure 13c shows the seismic velocity in the sedimentary depression in the northeastern Sichuan basin, which is consistent with the foreland basin formed by the Daba Shan fold-and-thrust belt in the Jurassic to Cretaceous [Ratschbacher et al., 2003; Dong et al., 2013]. We next describe the model’s 3-D velocity structure using a series of depth slice from 400 m to 8000 m below sea level (Figure 14, negative numbers here refer to elevations above sea level). The seismic velocity distribution as viewed in these depth slices is largely controlled by basin structure and age of the sediments. The depth slice at 400 m shows the base of the small Quaternary foreland basin ﬁlled with low-velocity (1500–2800 m/s) sediments (Figure 14). This Quaternary basin has a maximum thickness of 650 m, with an averaged thickness of the 200–300 m [Li and Guo, 2008]. Velocity structures to the southeast within the basin at this 390 m depth level generally correspond to anticlines and related thrust faults formed above a series

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Figure 14. Horizon sections across the 3-D velocity model in the Sichuan basin and adjacent regions. Depth slices are in meters below sea level.

of detachments that extend from the Longmen Shan into the basin (Figure 14) [Hubbard et al., 2010; Wang et al., 2014]. Depth slices between 5000 and 8000 m highlight the sedimentary foredeeps of the Longmen Shan in the west and the Daba Shan fold-and-thrust belt in the northeast that ﬁlled with Late Triassic and Late Jurassic to Cretaceous terrigenous clastics, respectively. At about 3000 m depth, high-velocity regions begin to appear in the central and southern basin, and these features broaden with depth (Figure 14). These features correspond to broad, contractional anticlines. One of the largest of these structures, the Weiyuan anticline, is cored by Precambrian metasedimentary and igneous rocks and underlain by a northwest dipping thrust ramp. Thrust slip along this ramp uplifts the entire sedimentary section above the anticline, yielding higher seismic velocities than the surrounding area. The borders of the Sichuan basin also show considerable change with depth. Down to depths of about 400 m, the basin borders are generally ﬁxed in their locations and laterally juxtapose sediment velocities of 4000–5000 m/s, corresponding to Mesozoic rocks, with 6000–7000 m/s crystalline basement (Figures 13 and 14). Below 4000 m, the basin decreases in extent and becomes separated into subbasins due to the basement uplift (Figure 13). However, given proportional increases in both sediment and basement velocities, the velocity step across the basin edges continues to be about 2000 m/s (Figure 14).

5. Applications In this section, we explore the implications of the Sichuan velocity model for seismic hazards assessment by

using it to calculate ground motions for the 2013 Mw 6.6 Lushan earthquake that occurred in the Longmen Shan fold-and-thrust belt. Speciﬁcally, we use attenuation relationships to calculate ground motions and compare these with observations from the 2013 earthquake.

The Mw 6.6 Lushan earthquake occurred on the Range Front blind thrust in the southern segment of the Longmen Shan thrust belt [Wang et al., 2014; Li et al., 2014]. The event is one of the best recorded earthquakes in Sichuan region, because of the expansion of seismic network in the region after the 2008 Wenchuan earthquake. Strong ground motions triggered by the Lushan earthquake, includes peak values of ground acceleration, velocity, and displacement, are well recorded in 114 new generation strong ground motion monitoring stations [Wen and Ren, 2014].

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Figure 15. Peak ground acceleration (PGA) calculated using a modiﬁed version of the attenuation relationship of Field [2000] and our basin model shown in color base map. The generated PGA map is the natural logarithm of the strong ground motion (cm/s2). These values are compared with observed ground motions data of the Lushan earthquake from Wen and Ren [2014].

To calculate the anticipated ground motions from this event using our velocity model, we use a modified form of the attenuation relationship of Field [2000] that accounts for basin depth and velocity. The modified form of empirical attenuation relationship of peak ground acceleration (PGA) calculation that for the Sichuan basin is as follows: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi V μ ¼ InðÞ¼ PGA b þ b ðÞþM 6 b ðÞM 6 2 þ b ln r2 þ h2 þ b ln s þ 0:0000675dα 0:14; 1 2 3 5 v V a (11) cm I ¼ 3:66 log PGA in 1:66; MM 10 s2 μ where μ = ln(PGA)(thus PGA = e ); M = magnitude of the earthquake; r = distance from earthquake epicenter

(km); Vs = velocity at which earthquake waves travel at 30 m depth, in m/s, Vs30; d = depth of sedimentary basement; and b1 = 0.872, b2 = 0.442, b3 = 0.067, b5 = 0.96, bv = 0.154, h = 8.9, Va = 760 (m/s), and α = 1.4. The sedimentary basement depth (d)isdefined as the depth of S wave speed travel at 2500 m/s, which is defined relative to the 2.5 km/s shear wave velocity isosurface in the SCEC 3-D velocity model [Magistrale et al., 2000]. In Southern California, Field [2000] shows that the basin depth has a stronger influence on the PGA amplification than site classification. During our application for the Sichuan basin, we found that the 2.5 km/s shear wave velocity surface in the Sichuan CVM is shallow and almost flat and thus cannot effectively reflect the actual basin basement shape. This is because of the Mesozoic rocks that occupy the majority of the Sichuan basin have relative higher velocity than the younger rocks that tend to fill basins in southern California where the relationship was established. Thus, we choose to modify the original relationship by substituting the interface between the Mesozoic and Paleozoic rocks for the original value d (isosurface = 2.5 km/s). This substitution yields values of both PGA range and distribution when compared with observations and shown in Figure 15. The generated PGA map in Figure 15 is the natural logarithm of the strong ground motion (cm/s2). In particular, the results show the asymmetric distribution of PGA distribution relative to the earthquake epicenter in the range front of south

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Longmen Shan (Figure 15). Specifically, stronger ground motions extend to the east and northeast from the epicenter into the sedimentary basin, which is generally consistent with recorded data from strong-motion seismograph network (Figure 15) [Wen and Ren, 2013; Chen et al., 2013; Mooney and Wang, 2014]. This illustrates the well-known effect of basin amplification and suggests that our new basin model can be used to quantify these hazards through both attenuation relationships and numerical wave propagation studies. From the equation (11), the results of PGA calculation are largely controlled by earthquake source, basin structure, and Vs30. The geological surfaces that were used to construct basin structure are from oil wells and seismic reflection profiles and thus have the least uncertainties. However, in the mountain belt region, the geological interfaces are mainly constrained from published geological structures and contours, with limited constraints from wells and seismic reflection profiles. Thus, the model may have relative large uncertainties in this region. In addition to this, Vs30 is obtained from topographic gradients to represent shear wave speed for seismic conditions mapping in the basin. Previous analysis shows that the Vs30 values are consistent with records from wells in Sichuan region [Feng,2013].However,ageology-based Vs30 map would likely help to represent shallow subsurface velocity structures and thus will improve the result of strong ground motions calculations.

6. Summary We present a new, high-resolution 3-D seismic velocity model for the Sichuan basin that incorporates constraints on basin shape derived from 1166 oil wells, more than 600 seismic reflection profiles, 14 industry isopach maps, and topography data. These data sets were used to constrain the geometry of four major geologic interfaces (base Quaternary, base Mesozoic, base Paleozoic, and base Proterozoic or top basement) that were used to define the basin shape and guide the parameterization of internal velocity

structures. Vp-depth profiles were developed for each of these geologic units using sonic logs from 36 wells sampling depth from 50 to 6000 m. New Vp to Vs and Vp to density scaling laws were developed using VSP and density log data and used to define all of these properties throughout the basin model. The velocity structures in the model reflect the complex tectonic evolution and variable sedimentary fill of the Sichuan basin. We confirm the presence of a low seismic velocity Quaternary basin beneath the highly density population Chengdu Plain that may amplify seismic waves and thus pose a significant seismic hazard. This small Quaternary basin is embedded within a larger basin structure that contains Mesozoic through Proterozoic sediments with generally high velocities (3500–6000 m/s). We document the presence of a very high velocity 6500–7000 m/s layer at the base of the sedimentary sequence that has not been resolved in previous tomography studies and will likely yield a strong velocity inversion with depth that impacts wave propagation. Finally, we use an attenuation relationship to calculate the ground motions for the 2013 Lushan earthquake based on our new seismic velocity model. The results show basin amplification in the western Sichuan basin, which is consistent with records from strong-motion stations. We suggest that our velocity model can be used in wave propagation studies to further assess seismic hazards in this region.

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