Fine-scale Structure Of The Earth’s Inner Core Boundary And Aftershocks Of North Korea’s 2017 Nuclear Test

Seismic structure and seismic sources are two important and closely related topics in seismology.The understanding of seismic sources is important for studies of seis?mic structure and the knowledge of seismic structure can also help studies of seismic sources.In this thesis,we will present three studies related to these two topics,i.e.1)studying fine-scale structure of the Earth’s inner core boundary using pre-critically reflected PKiKP waves;2)determining locations and source mechanisms of the four small seismic events occurred after North Korea’s 2017 nuclear test using various seis-mic observations;3)detecting small seismic events in North Korea nuclear test site region from May 2017 to April 2018,and investigating the characteristics of aftershock sequence triggered by North Korea’s 2017 nuclear test.Fine-scale structure of the Earth’s inner core boundary(ICB)is important for our understanding of the inner core solidification processes,the origin of the Earth’s mag-netic field,and the composition of the outer core.We collected 1,263 pairs of high-quality PKiKP and PcP observations from seismic waveform data recorded by Hi-net in Japan,which sample the ICB beneath central China,southeast Japan Sea,west North Pacific Ocean and southwest Okhotsk Sea.PKiKP-PcP traveltime residuals reveal that there is no significant ICB topography in the study region,with an estimated upper bound of ICB topographic height change of 1.5 km.Waveform differences between PKiKP and PcP reveal a localized 4-8 km thick zone across the ICB region beneath southwest Okhotsk Sea,while the surrounding regions exhibit a sharp ICB.Our results present the first seismological evidence for the existence of localized mushy zone at the Earth’s ICB.North Korea’s September 3,2017 nuclear test was followed by several small seis-mic events,which had caught the attention of the scientific community,the media,and the public.We use relative time differences and relative polarities between event pairs,and body wave and surface wave observations,in combination with waveform model-ing techniques,to determine the relative locations and source mechanisms of the four small events.Our study reveals that the first event occurring 8.5 minutes after the 2017 test is an on-site collapse toward the nuclear test center from 440 ± 260 m northwest of the test site,with its source best represented by a near-vertical single force,with an azimuth of～150° and a dip angle of 70°-75°,and the later events are a naturally earth-quake swarm,located～8.4 ± 1.7 km north of the test site within a region of 520 m,with a focal depth of at least 2.4 km and a focal mechanism of nearly pure strike slip along the north-south direction with a high dip angle of 50°-90°.North Korea’s 2017 nuclear test was followed by a series of small seismic events.However,a complete catalog of event information and nature of these events remain unknown.Using match-and-locate and single-station sliding cross-correlation meth-ods,we conduct a survey of small seismic events from four months before and eight months after the 2017 nuclear.We report 88 seismic events occurring after the 2017 test,indicating intense seismicity.However,no seismic events are detected before the 2017 test.These events are confined in a region of 2 km,8.4 km north of the test site,delineating a near north-south strike direction of the associated fault.Theoretical stress field calculations indicate these small events are aftershock sequences triggered by the 2017 nuclear test.