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Ding, Shiban,Ni, Sidao,Kim, YoungHee,He, Xiaohui Elsevier 2019 Physics of the earth and planetary interiors Vol.289 No.-
<P><B>Abstract</B></P> <P>The seismic velocity structure of the crust-mantle transition zone (CMTZ) provides key constraints on crustal dynamics, and can be studied with Pn wave which propagates horizontally along the Moho discontinuity. In this study, we first explore an effect of velocity-gradient structures in the CMTZ through synthetic Pn waveforms, and demonstrate that the shape of Pn changes from step-like to pulse-like when the CMTZ becomes from sharp to a gradient velocity structure, respectively. We then use Pn waveforms of the regional earthquakes (the July 2012 Mw 4.8 earthquake in east China and the September 2016 Mw 5.1 earthquake in South Korea) to examine the CMTZ structures to the west and east of the Yellow Sea and thus to constrain tectonic affinity between the eastern China and southern Korean Peninsula. The Pn waveforms from the Mw 4.8 event in east China show that the Moho in southern Yangtze Craton is sharp whereas there may be a gradient structure in CMTZ with a 6–10 km thickness in northern Yangtze Craton. The Pn waveforms from the Mw 5.1 event in South Korea show that the CMTZ in South Korea is also sharp, similar to the southern Yangtze Craton. Observed difference in CMTZ structures suggest that Sulu Orogenic Belt may extend along the north of the Gyeonggi Massif in Korea.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Pn waveforms are sensitive to the gradients of crust-mantle transition zone (CMTZ). </LI> <LI> Southern Korea features sharp Moho while eastern China Moho shows variability. </LI> </UL> </P>
Yun, Sukyoung,Ni, Sidao,Park, Minkyu,Lee, Won Sang Blackwell Publishing Ltd 2009 Geophysical journal international Vol.179 No.1
<P>SUMMARY</P><P>Parameters of earthquake sequences, for instance location and timing of foreshocks and aftershocks, are critical for understanding dynamics of mid-ocean ridge and transform faults. Whole sequences including small earthquakes in the ocean cannot be well recorded by land-based seismometers due to large epicentral distances. Recent hydroacoustic studies have demonstrated that <I>T</I> waves are very effective in detecting small submarine earthquakes because of little energy loss during propagation in Sound Fixing and Ranging channel. For example, an <I>M<SUB><SMALL>W</SUB></SMALL></I> 6.2 (2006 March 6, 40.11°S/78.49°E) transform-fault earthquake occurred at the Southeastern Indian Ocean Ridge, but National Earthquake Information Center only reported three aftershocks in the first following week. We applied cross-correlation method to hydroacoustic data from the International Monitoring System arrays in the Indian Ocean to examine the whole earthquake sequence. We detected 14 aftershocks and none foreshock for the earthquake and locations of these aftershocks show an irregular pattern. From the observation, we suggest that the feature could be caused by complicated transcurrent plate-boundary dynamics between two overlapped spreading ridges that is possibly explained by the bookshelf faulting model.</P>
Kim, YoungHee,He, XiaoHui,Ni, SiDao,Lim, Hobin,Park, Sun‐,Cheon Seismological Society 2017 Bulletin of the Seismological Society of America Vol.107 No.5
<P>Two earthquakes (M-w 5.1 and 5.5) ruptured branches of the Yangsan fault system in Gyeongju, South Korea, on 12 September 2016. Aftershocks, including a notable M-w 4.3 earthquake on 19 September 2016, were clustered around the epicenters of the first two events. The M-w 5.5 earthquake is considered the largest earthquake in South Korea to have occurred during the modern instrumental recording period since 1978. Although there is no apparent surface rupture, these earthquakes have greatly shaken South Korea, leaving both physical and societal impacts. In this study, we determine the source mechanism and rupture directivity using regional seismic-waveform data to understand the earthquake source processes. Based on the waveform inversion, we report that the mainshock (M-w 5.5 event) is a strike-slip event with two nodal planes 117 degrees/84 degrees/21 degrees and 24 degrees/69 degrees/173 degrees at a depth of 14 km. The inversion also demonstrates that the mainshock event ruptured against the 24 degrees seismogenic fault plane to the south-southwest, with a rupture length of similar to 4.3 km. This rupture propagation direction agrees well with the spatial distribution of relocated aftershock events and reported seismic intensities.</P>