P Wave Velocity Structure in The Inner Core Constrained by PKP(BC)-PKP(DF) Travel Time Differences Measured at Dense Seismic Array in Japan

A complete understanding about the velocity structure in the inner core is very important to determine the precise inner core rotation rate and study the dynamic processes in it. The measurement of differential travel times of two core phases, such as PKP(BC) and PKP(DF), is useful to study the inner core velocity structure because it is less sensitive to heterogeneities in the mantle and the possible mislocations of the events, as well. To detect the small scale variations in P wave velocity in the inner core, we measured differential travel times of PKP(BC) and PKP(DF) for events occurred in South America and recorded at dense broadband seismic network (F-net) in Japan. Their epicentral ranges are between about 145° and 155° and they are appropriate to detect PKP(BC) and PKP(DF) phases simultaneously. Our preliminary results over several years of the data show that the gradient of observed differential travel times with epicentral distance is clearly less steep than theoretical one based on 1D reference Earth model (IASP91). We systematically analyze these differences in an effort to determine the precise P wave velocity structure in the inner core and more differential travel time data for other events and station pairs will be used to give more constraints on the possible small scale velocity changes in the inner core.