Yibing Dong1,3, Sidao Ni2, David A. Yuen4, Zhiwei Li2 1 University of Science and Technology of China, , China. 2 Institute of Geodesy and Geophysics, Chinese Academy of Sciences, , China. T51D-0507 3 Earthquake Agency, , China. 4 Department of Earth Sciences, University of Minnesota, Minneapolis, U.S.A. AGU Fall Meeting, New Orleans, December 2017 [email protected]

Crustal rheology from

The North China Basin is a Mesozoic-Cenozoic continental rift basin on the eastern In the North China Basin, there is a layer of sediment on the crystalline basement. Relocation needs reliable velocity model. Based on the previous study results (Duan focal depths North China Craton. It is the central region of craton destruction, also a very And various seismic phases are observed due to conversion of waves at the interface, et al., 2016), we use a half-space model with a overlying sedimentary layer. Using the It is suggested that the depth-frequency distribution can shed light on the seismically active area suffering severely from devastating earthquakes, such as the such as the Sp wave, i.e. the P wave converted from the incident S wave, and the Ps initial Vp of 1.6~1.8 km/s and initial Vs of 0.3~0.4 km/s with smooth positive crustal rheology (Brace and Kohlstedt 1980). A comparison of Yield Strength 1966 M7.2 earthquake, the 1967 Hejian M6.3 earthquake, and the 1976 wave, i.e. the S wave converted from the incident P wave (Fig. 3a). When the gradient versus depth in the sediment, and the crustal average Vp of 6.15 km/s and the Envelopes (YSE) with observed earthquake depth distribution may improve M7.8 earthquake (Fig. 1a). We found remarkable discrepancies of depth seismograph is installed in the borehole, the converted wave will consist of the direct average Vp/Vs ratio of 1.74 in the half-space. For stations with different sedimentary understanding of the rheology of the lower crust. So we perform a comparison distribution among the three earthquakes, for instance, the Xingtai and Tangshan wave and its surface reflected wave, as well as the P waves (Fig. 3b). thickness, we uses different models, such as, modWEA for the station WEA, between the resolved Depth-Frequency Distribution (DFD) and the YSE in the earthquakes are both upper-crustal earthquakes occurring between 9 and 15 km on modWUQ for WUQ, and modNHZ for stations including CGZ, HEJ, JIH, NHZ, QIG, North China (Zhou and He, 2003). The comparison (Fig. 7a) shows a good fit depth, but the depth of the Hejian earthquake was reported of about 30~72 km, ranging WAK, and YOQ (Fig. 5). at depth of about 20 km, where the two envelopes both reach the peak value. from lowermost crust to upper mantle. Seismological investigation reveals that most However, some discrepancies remain near the upper (10 km) and lower (30 km) earthquakes in the Eastern Continental China occur between 7 and 21 km deep and 24 bounds of the depth profile. The YSE indicates too high a strength at shallow km is the lower bound of seismic activity (Zhang et al, 2002). Therefore, the focal and greater depths compared with the observed distribution. depth in the region near the Hejian earthquake need to be investigated. Then we collect seismograms of 44 local events with high SNR during 2008 to 2016 beneath the Hejian Seismic Zone according to the catalog provided by the China Earthquake Networks Center. It shows that (Fig. 1b), the epicenters are belt-like distributed nearly in the NNE direction , basically the same with the orientation of the fault systems. From the catalog, there are ~25% of events occurring in the depths from 24 to 29 km, with the maximum depth of 29 km, near the uppermost mantle. The depth distribution is different from that of the Eastern Continental China, and we suspect that the focal depths may not be well-resolved. Fig. 5 Velocity models in this study.

Fig. 7 Comparison between the Yield Strength Envelope (YSE) in the North China (NC) and the Depth-Frequency Distributions (DFD) in the Hejian Seismic Zone (HSZ) (a), and comparison between the YSE in the Baikal Rift Systems (BRS) and the DFDs in the HSZ (b). Black dashed NEIC The first stage of relocation is to resolve the accurate epicenters of events employing line is the YSE. Gray and red polyline denotes the depth-frequency distribution before and after Hypo2000. To analyze the influence of the model uncertainty on the depth relocation in the HSZ, respectively. Horizontal lines delineate the theoretical depth of brittle- ductile transitions. CEDC determination, we use Crust1.0 and modWEA for relocation, respectively. The results reveal that, the influence of the model uncertainty on the depth determination is greater Since the YSE in the North China seems to not fit well with the observed DFD, Fig. 3 (a) Ray paths of the local Ps and Sp converted waves. Triangle indicates seismic station. (b) Ray paths of than that on the epicentral determination. It is notable that, after relocation, there still the direct P wave (P1) and its surface reflected wave (P2). (c) Vertical (Z), radial (R), and tangential (T) we proceed to test whether variations of crustal rheological model could be This study exists earthquakes in the depths of 24~27 km. These depths need to be examined with components of seismograms of a M3.0 earthquake at station WEA. Arrivals of phases are labeled on each responsible for the observed discrepancies. From investigations on the lower waveform. (d) Three components of seismograms of a M2.5 earthquake at station NHZ. the converted waves. crustal earthquakes in some other continental areas, we find the similarity between the depth distribution in the Baikal Rift Systems (BRS) and the North China Basin (NCB). Therefore, we perform a comparison between the DFD in the NCB and the YSE in the BRS (Fig. 7b). We find a good fit between the two envelopes, including that: (1) the depth distribution of earthquakes is almost at Fig. 1 (a) Seismicity map with magnitudes M6.0 above from 1966 to 1976 in the North China Basin. (b) Synthetic waveforms show that (Fig. 4), the Sp wave is characterized by the first order proportional to the strength profile, (2) the dominant depth (~20 km) Seismicity map of the 44 selected events with magnitudes between M1.0~3.0 during 2008 to 2016 in following features: (1) its energy is mainly concentrated on the vertical component, of events fits well with the peak in the strength profile around 20 km, and that the Hejian Seismic Zone. Black triangles denote seismic stations, and black lines give the fault system. with a stronger amplitude and lower frequency compared to P wave; (2) the travel- (3) the seismicity cut-off depth (25 km) fits well with the brittle-ductile Main faults: F1, Cangdong Fault; F2, Dachengdong Fault; F3, Niutuozhen Uplift Eastern Fault; F4, Daxing Uplift Eastern Fault. time difference between Sp and P wave almost linearly correlates with focal depth, transition in the BRS. thus capable of providing tight constraints on the depth; (3) the arrival-time difference between the Sp and P wave increases with the epicentral distance, hence the depth uncertainty due to the epicentral distance uncertainty should be taken into Conclusions Methods account; (4) the arrival-time difference between the Sp and S wave is almost a constant at near epicentral distances, providing constraints to the shallow velocity Ø The converted wave is effective in accurately resolving focal depths for local structure beneath the station. earthquakes in sedimentary regions.

Ø According to the good fit between the depth-frequency distribution in the North Hypo2000 Epicenter CENC Hypo2000 Hypo2000 China Basin (NCB) and the Yield Strength Envelope (YSE) in the Baikal Rift catalog with Crust1.0 with modWEA Converted waves Systems (BRS), we infer that, (1) the seismogenic thickness is ~25 km in the NCB and the main deformation mechanism is brittle fracture; (2) the temperature is Fig. 6 Depth distribution before and after relocations along latitudes (a) and longitudes (b). Gray, white, red, moderate in the seismogenic zone of crust and relative high below 25 km. and green circles denote location of earthquakes from the calalog, Hypo2000 with Crust1.0, Hypo2000 with Crustal Depth modWEA, and locally converted waves, respectively. Histogram for the 44 events with depths from the Crustal catalog (c), Hypo2000 with Crust1.0 (d), Hypo2000 with modWEA (e), and locally converted waves (f). velocity frequency rheology model distribution References Employing epicenters from Hypo2000 with modWEA, we obtain well-resolved focal depths of these earthquakes by locally converted waves. The comparison of the depth Zhang, G.M., Wang, S.Y., Li, L., Zhang, X.D., Ma, H.S., 2002. Focal depth research of earthquakes in Mainland China: distribution before and after the relocations (Fig. 6) indicate that, (1) the initial depth Implication for tectonics. Chinese Science Bulletin 47(12), 969-974. Locally distribution is relatively scatterred, with ~25% of events lie in depths of 24~29 km Focal Duan, Y.H., Wang, F.Y., Zhang, X.K., Lin, J.Y., Liu, Z., Liu, B.F.,Yang, Z.X., Guo, W.B., Wei, Y.H., 2016. Three- converted and maximum depth of 29 km; (2) compared with the initial and the Crust1.0-based waves depth result, the depths from Hypo2000 with modWEA distribute more compact, with the dimensional crustal velocity structure model of the middle-eastern north China Craton (HBCrust1.0). Sci. China Earth dominant depths of 20~22 km, ~7% of events lie in depths of 24~27 km and Sci. 59(7), 1477-1488. DOI: 10. 1007/s11430-016-5301-0. maximum depth of 27 km; (3) the depths from Sp converted wave distribute even Brace, W F, Kohlstedt, D.L., 1980. Limits on crustal stress imposed by laboratory experiments. Journal of Geophysics Fig. 4 Synthetic waveforms at the epicentral distance of 14 km. From left to right is radial, vertical and tangential more compact, with the dominant depths of 18~20 km, ~5% of events lie in depths of Research 85, 6248-6252. Fig. 2 The roadmap of research method. component respectively, integrated to displacement and bandpassed with the frequency range of 2~5Hz. Gray, green, red, and blue dotted lines denote the theoretical arrival-time of the P, Ps, Sp and S wave, respectively. 24~25 km and maximum depth of 25 km.