Internal Structure Of The Lunar Nearside Based On Moonquake Tomography

The study on lunar internal structures not only is a significant scientific issue for the origin and evolution of the moon, but also has a great impact on the control of the spacecraft orbit and the selection of the landing site on the moon. The previous study suggested that the lunar internal structures are similar to that on the earth, which is divided into crust, mantle and core, but the moon is a celestial body with almost no tectonic activities. In fact, the lunar core is similar to the terrestrial core consisting of the liquid outer core and solid inner core. The lowermost mantle is partial melt, heated by earth tide or/and radioactive element, resulting in tectonic activity. This phenomenon indicates that there exists the thermodynamic process in the moon nowaday.The study of the lunar internal structures can reveal the phase state of the lunar internal material, process of differentiation and so on, which provides reference for studying the lunar evolution and formation. In addition, similar to the earth, the variation of the lunar internal structures would influence topography and material distribution. Thus, the study of the lunar internal structures can help us to understand the formation of the basin, highland, and the dichotomy between nearside and farside.The previous investgation of the lunar internal structures were mainly based on the lunar surface and space observation. In order to study lunar internal structures better, many kinds of methods adopted including seismology, gravity, heat flow and so on. A recent detection on the lunar shallow subsurface structures is based on the observation from Lunar Penetrating Radar onboard Chang’E-3.Among all of the methods above, the best one to study the internal structures in large scale is seismology. Seismic wave has strong penetrating power, which can obtain the information from the crust to the core. Moreover, seismic wave can effectively infer the material property of the moon, because it is sensitive to temperature, pressure and composition variation.The study based on Apollo seismic experiment suggested that the lunar internal structure is divided into crust, mantle and core, whose corresponding material components are obtained. Then seismic tomography was conducted in 2008 for the first time to detect the lunar internal structure. However, the division of the internal structures was not accurate enough. Due to the limitation of moonquake and seismic station number, early research study can only provide the 1-D velocity model, and rough 3-D seismic tomography. Thus, this thesis aims to 1) locate more moonquakes,2) conduct more accurate 3-D moonquake tomography using the recent velocity model built by myself and more moonquakes,3) analyze the lateral heterogeneity of the crust and mantle, and 4) answer the question about the distribution of KREEP terrane and the velocity discontinuity in the lunar mantle.The main work of this thesis are 1) locating more than 200 moonquakes using Geiger location method,2) comparing the distribution and mechanism between moonquake and earthquake,3) using Fast Marching Method and Subspace Inversion to do tomography, (4) analyzing the KREEP terrane, mascon, and velocity model.Several conclusions are summarized as follows:(1) The moonquakes location precision of this thesis is greater than that in early study by using my velocity model, which indicates that the velocity model used in this thesis is more accurate than others. Among the located moonquakes in previous study, maybe only 3 of them are locate on the farside. Comparison on the distribution and mechanism of moonquake and earthquakes indicates that many differences between them.(2) Although the velocity of the moon increases with depth, it varies obviously in lateral and vertical, which indicates that the significant heterogeneities may exist in the lunar interior. P-velocity is lower in the crust and middle mantle, and higher in upper mantle and lower mantle. Similar to P-velocity, S-velocity is lower in crust and middle mantle, and higher in upper mantle. Howere, the range of lower S-velocity region is smaller than that of the lower P-velocity region. The lower mantle is characterized with large area with lower S-velocity of large scale.(3) Lower P-velocity and S-velocity areas in the crust, extending to 200km deep, mainly concentrate at Procellarum KREEP terrane. This lower velocity area is consistent with the Procellarum KREEP terrane with high abundance Th, which may indicate that heat-producing element Th caused the heat flow values in KREEP terrane is higher than the surrounding areas, resulting in the lower velocity in this area.(4) The tomography results show that there are high velocity anomalies in Mare Imbrium and Mare Serenitatis, but no high velocity channels corresponding to these anomalies are found. Several reasons are given.1) Low resolution of our results at shallow depths due to limited shallow moonquakes make the detection of the high velocity channels difficult.2) The mascons of Mare Imbrium and Mare Serenitatis were the results of the filling out by extraneous objects which impacted the moon other than mantle upwelling and magma filling.3) The mascons of Mare Imbrium and Mare Serenitatis were caused by mantle upwelling and magma filling, but the magma channels closed during the lunar evolution process.(5) The 1-D velocity models including average velocity model and velocity models of typical positions were extracted from tomographic results. By comparison, the average velocity model is similar to the model published by Garcia in 2011. Howere, the velocity of models of typical positions varies obviously, which may suggest that the velocity discontinuity doesn’t appear in global but local area.(6) Some knowledge is obtained.1) Significant heterogeneities exist in the lunar interior. The heterogeneities in lunar crust and upper mantle may be caused by impacts and magma activities during the lunar evolution preocess. The heterogeneities in the lunar mantle may be caused by mantle overturn.2) The lower velocity anomalies at Procellarum KREEP terrane may indicate that the radioactive element are still producing thermal energy, which results in the heat flow values difference between Procellarum KREEP terrane and highland.3) Although the layering structure of the moon is similar to that of the earth, comparison on the velocity models of the earth and the moon suggest that the moon has its unique feature. The tomography results and the velocity models published show that the extremely low velocity zone similar to the earth is not found, which may indicate there is no asthenosphere in the lunar interior and the abundance of the radioactive element is not high enough to melt the lunar mantle under present temperature and pressure conditions.Due to the limitation of the number of moonquakes and stations, there are several problems to be solved in the future.(1) In order to improve the resolution of the tomography, more moonquake arrivals or more moonquake phase data are needed.(2) Owing to the small number of shallow moonquakes, the resolution of shallow structures in tomography is much lower than that in the deep regions. Coda wave of moonquakes and surface wave data can be used to obtain the shallow structures with high resolution.(3) Joint inversion of P and S wave data or epicenter and velocity can be considered in future work.This thesis consists of 8 chapters. The first chapter mainly states the research background and contents. The second chapter shows the landform, internal structures, layering structure and material composition of the moon. The third chapter analyzes moonquake signals and states the classification method for moonquakes. The fourth chapter shows seismic phase identification of the moonquake. The fifth chapter introduces the moonquake location method and tomography theory. The sixth chapter discusses moonquake location result, and compares the distribution and mechanism of moonquake and earthquake. The seventh chapter demonstrates moonquake tomography results and gives their geological interpretation. The eighth chapter, last chapter, summarizes all works of this thesis.