Abstract:

In 1668, a major M_W8.5 earthquake struck Tancheng, located in the central-southern segment of the Tanlu fault zone. Jointly resolving P-wave velocities in the crust and uppermost mantle can improve constraints on uppermost-mantle velocity, which is essential for understanding the tectonic mechanisms of strong earthquakes in this fault system and the deep seismogenic setting. Here we perform a joint inversion for crustal and uppermost-mantle P-wave velocity beneath the central-southern Tanlu fault zone using travel times of the first-arriving Pg and Pn phases, together with secondary Pg phases recorded by the seismic network. Incorporating secondary Pg arrivals increases crustal ray coverage and substantially improves the resolution and accuracy of the uppermost-mantle velocity structure.
The resulting P-wave model shows that near-surface(0km)high-velocity anomalies broadly correspond to mountain ranges, whereas low-velocity anomalies are mainly associated with sedimentary basins. Arc-shaped, belt-like high-velocity anomalies(5.40~5.54km/s) are observed in the northern Jiaodong Peninsula, the Taihang Mountains, and the Sulu orogenic belt, particularly in the Huaibei-Jining-Tai'an region. In contrast, low-velocity anomalies occur beneath Wuhan(Jianghan Basin), Hefei-Liuan(Hefei Basin), and Juxian(Shandong), and a prominent NS-trending low-velocity belt extends along the Shangqiu-Heze-Puyang-Liaocheng corridor(4.80~4.89km/s). At depths of 10~15km, high velocities dominate beneath the Dabie orogenic belt, Huainan-Bengbu, the Luxi uplift, and the Jiaodong Peninsula, whereas pronounced low-velocity anomalies appear beneath the Yangzhou-Changzhou-Suzhou to Shanghai-Nantong sector of the Subei Basin and the southern Yellow Sea. At ~15km depth, velocity structure differs markedly across the Tanlu fault zone: The western side from Juxian southward to the Dabie orogen exhibits strong high-velocity anomalies, while the eastern Subei-southern Yellow Sea Basin and the Xinyi-Lianyungang area are characterized by low velocities.
At ~20km depth, the velocity pattern changes relative to the upper and middle crust and shows strong along-strike segmentation within the Tanlu fault zone. North of Tancheng, a low-velocity anomaly is present; from Tancheng to Sihong, a high-velocity anomaly reaches a maximum of 6.39km/s; from Sihong to Lujiang, low velocities dominate, with a minimum of 6.03km/s near Jiashan. The Dabie orogenic belt exhibits a high-velocity anomaly of ~6.43km/s. At depths of 25~30km, which mainly reflect the middle-lower crust, high-velocity anomalies dominate beneath the Dabie orogen, the Taihang Mountains, and northern Jiangsu-southern Yellow Sea, with additional localized highs near Jiaozhou(Shandong) and Suqian(Jiangsu). Low-velocity anomalies occur in the Tancheng-Anqiu-Weifang segment of the Tanlu fault zone and also along the Liaocheng-Jinan corridor and near Jiashan and Wuhu.
Pn velocities in the central-southern Tanlu fault zone are strongly heterogeneous. North of Tancheng, low Pn velocities are mainly distributed on the eastern side of the fault zone, whereas south of Hefei they occur predominantly on the western side. High Pn velocities appear on the eastern side of the fault zone in the Tancheng-Jiashan region. Such heterogeneity likely reflects lateral variations in mechanical strength at the top of the upper mantle beneath different segments of the fault zone.
Overall, crustal velocity anomalies correlate with surface geomorphology and the distribution of major faults. In particular, velocity anomalies at 10~15km depth delineate the strike of the Tanlu fault zone. Across the central-southern Tanlu fault zone, the South China Plate and the Lower Yangtze Block display a clear crustal velocity contrast. In the upper-middle crust, low velocities prevail west of the Tanlu fault zone and high velocities to the east; in the lower crust, pronounced high velocities occur east of the Tanlu fault zone and south of the Jiashan-Xiangshui Fault, whereas low velocities dominate to the west. The relative strength contrast between upper and lower crust inferred from the spatial relationship between seismicity and velocity anomalies may play an important role in controlling earthquake occurrence. By incorporating a 3D crustal model, the joint inversion enhances the resolution of uppermost-mantle structure, revealing low-velocity anomalies flanking the epicentral region of the 1668 Tancheng M_W8.5 earthquake and strong uppermost-mantle heterogeneity beneath the central-southern Tanlu fault zone. Integrating crustal and uppermost-mantle velocity patterns, we divide the central-southern Tanlu fault zone into three segments—north of Tancheng, Tancheng to Jiashan, and south of Jiashan—consistent with previous segmentation studies.

Key words: Tanlu fault zone, joint inversion, P-wave velocity, crust and uppermost mantle structure

摘要：

郯庐断裂带中南段历史上曾发生过1668年山东郯城8.5级大地震, 同时获取该区域地壳上地幔顶部速度结构有助于提高上地幔顶部速度结构的精度, 对研究断裂带强震发生的构造机制、 深部孕震构造背景有重要意义。文中利用台网记录的初至Pg、 Pn波和续至Pg波震相走时联合反演郯庐断裂带中南段地壳上地幔顶部的P波速度结构。结果显示, 该区域内地壳速度结构异常分布与地表构造、 区域大型断裂分布较吻合, 尤其是地壳10~15km深度的速度异常清晰刻画了郯庐断裂带的走向。郯庐断裂带中南段两侧的华北板块和下扬子块体在地壳中存在较明显速度差异。在中上地壳, 郯庐断裂带西侧以最大为6.32km/s的高速异常为主, 东侧以<6.10km/s的低速异常为主; 下地壳在断裂带以东、 淮阴-响水口断裂以南表现出显著高速异常, 断裂带以西则表现出低速异常特征。区域地震分布与速度异常显示, 中上地壳与中下地壳速度结构的相对强弱对地震有重要的控制作用。研究区上地幔顶部Pn波速度在7.65~8.50km/s之间, 高速、 低速异常分布较为明显, 显示出研究区上地幔顶部结构具有较强的不均匀性。考虑地壳三维结构的联合反演提高了上地幔顶部速度结构的精细程度, 不仅显示出郯城8.5级发震位置两侧存在典型的低速异常, 同时在郯庐断裂带中南段两侧的上地幔顶部存在较为显著的速度异常。地壳和上地幔顶部速度结构分布均显示郯城8.5级大地震发生位置附近存在显著的速度异常区, 结合地壳上地幔顶部速度结构可将郯庐断裂带中南段分为郯城以北、 郯城—嘉山段, 嘉山以南3段, 这与前人研究的分段结果较为一致。

关键词: 郯庐断裂带, 联合反演, P波速度结构, 地壳上地幔顶部结构