Locally Converted Waves And Applications In Resolving Focal Depth In Sedimentary Basins

Sedimentary basins are usually economically developed and densely populated regions all over the world. However, some of the basins are quite seismically active,suffering severely from devastating earthquakes, such as the Bohai Bay Basin in China, the Los Angeles Basin in America, the lacustrine plain of Lake Texcoco in Mexico, and the Kansai region in Japan. The focal depth is also a key factor for seismic disaster in continental regions, and it is generally accepted that the shallower earthquake will cause the heavier damage. Therefore, study on the distribution characteristics of focal depths will provide references for seismic fortification and disaster assessment in sedimentary basins. It is also suggested that reliable focal depths can shed light on the crustal rheology, hence to improve our understanding of the local seismogenic mechanism. In brief, it would be helpful to employ the method resolving accurate focal depths in sedimentary regions to promote researches on the seismicity and its tectonic and rheological implications.The focal depth is an important parameter of seismology. It provides significant reference for researches including lithospheric rheology, seismogenic mechanics,seismotectonics, hazard evaluation and etc.. Seismological investigations have revealed that depth-frequency distribution of shallow earthquakes correlates with the crustal rheology inside continents. The lithospheric rheological property plays a key role in the seismogenic thickness, and the well-resolved focal depths of earthquakes will in turn shed light on the regional rheology. However, it is a challenge for the arrival-time-based methods to obtain accurate focal depths in sedimentary regions.The shallow velocity structure of the sediment has a strong effect on the seismic wave-field, leading to a low resolution depth determination. Moreover, some commonly used depth phases are easier to be identified in the bedrock outcrop regions.It would be helpful to develop a new method to obtain the well-resolved focal depths in the sedimentary regions.From seismograms in sedimentary basin, the locally converted waves generated at the sedimentary interface are often observed. In such regions, there is a layer of sediment on the crystalline basement, various seismic phases are observed due to conversion of waves at the interface, such as the Sp wave, i.e. the P wave converted from the incident S wave, and the Ps wave, i.e. the S wave converted from the incident P wave. The Sp wave is characterized by the following features: (1) its energy is mainly concentrated on the vertical component, with a stronger amplitude and lower frequency compared to P wave; (2) the travel-time difference between Sp and P wave is linearly correlated with the focal depth, providing tight constraints to the depth. We select the July 4, Wenan M5.1 earthquake as an example to examine the efficiency of our method. The resolved depth by the Sp wave at station WEA is ?15 km, consistent with that from the CAP method (Huang, 2009). Tests indicate that, (1)the resolved depth from locally converted waves increases almost linearly with the epicentral distance, hence we can use the epicentral uncertainty to represent the depth uncertainty, and that (2) for one earthquake, the resolved depths from different types or different epicentral distances of converted wave are identical, so we suggest the employment of that with smaller distance and higher SNR to improve the accuracy of focal depth. As a conclusion, we suggest that, the method of converted waves is effective in resolving focal depths of local events in sedimentary regions, where low shear velocity may severely bias focal depth. Besides, there are some requirements for the method, including: (1) a good understanding of the velocity model; (2) the well-resolved epicenter; and (3) the clear converted phase in the seismograms.In the CENC catalog during 2008 to 2016 at the Bohai Bay Basin, we find some probable lower crustal earthquakes with focal depths deeper than 25 km. However, It is suggested that most continental earthquakes occur in the upper crust, while few in the lower crust. Therefore, we suspect that the focal depths of these events may not be well-resolved. To investigate the causes of the abnormal depths, we collect 44 seismograms of local events with high SNR according to the catalog, obtain accurate epicenters with Hypo2000 method and well-resolved focal depth with locally converted waves. The depth distribution shows abundance of earthquakes in depth interval of ?20 km, with some events (-23%) in the lower crust, but absence of seismicity deeper than 25 km, and the mean depth uncertainty is ?2 km. Moreover,the comparison between relocation results reveal that, for the employing of the arrival-time-based location method in sedimentary regions, the influence of the model uncertainty on the depth determination is greater than that on the epicentral determination, hence a reliable crustal model will be helpful to improve the depth accuracy.The distribution characteristics of seismicity in the Bohai Bay basin (BBB)indicate that the earthquakes may be related to both the differential activity of Neotectonic units and the Moho upheaval. We also find a good fit between the depth-frequency distribution in the BBB and the Yield Strength Envelope (YSE) in the Baikal Rift Systems (BRS). And there are geological investigations indicating the tight correlation of tectonic evolution between the BBB and BRS. Accordingly, we infer that (1) the seismogenic thickness is ?25 km in the NCB; (2) the main deformation mechanism is brittle fracture in the seismogenic region; and (3) the temperature is moderate in the seismogenic zone of crust and relative high below 25 km. And we also suggest the employing of our method in other sedimentary regions which are seismically active to improve the understanding about the local crustal rheology.In this study, we have developed a method with converted waves to resolve focal depths of local earthquakes in sedimentary basins. Some case analysis and modeling tests show the validity, stability and applicability of the method. With the method, we have obtained the distribution characteristics of focal depths beneath the Bohai Bay Basin, providing some constrains to the regional crustal rheological properties. In the future, we will employ the method in other sedimentary basins with severe earthquake hazard, to get the distribution of focal depths and the regional crustal rheology. It will be helpful to improve the understanding of the seismogenic mechanism of sedimentary regions, providing important references for earthquake prevention and disaster reduction to the governments.