The Global Navigation Satellite System network in Japan detected transient crustal deformation along the Nankai trough, Japan, from June 2018. Time-dependent inversion analysis shows that a long-term slow slip event in northern Hyuga-nada Sea along the Nankai trough, Japan, started in June 2018 and decayed in October 2018. From October 2018, a slip area appeared in the Bungo channel and expanded to northern Hyuga-nada Sea and southwest Shikoku at the time of the maximum slip rate. The slip area in the middle of the Bungo channel started to abate around May 2019, with the slip in neighboring areas nearly stopping in August 2019. The estimated rupture propagation is different from those in the past Bungo channel SSEs, in which rupture propagated southwestward from the Shikoku side to the Kyushu Island side at the time of the maximum slip rate. Furthermore, the rupture in northern Hyuga-nada Sea preceded the Bungo channel SSE and reappeared together with the Bungo channel SSE at the time of the 2018–2019 event, though the northern Hyuga-nada Sea SSE followed the 2009–2011 Bungo channel SSE. There is a possibility that the differences in the rupture propagation and recurrence interval from the past events are due to the 2016 Kumamoto earthquake. The adjacent locked area along the Nankai trough subduction zone is a well-known seismic gap and the 2018–2019 SSE changed the stress state in favor of the occurrence of nearby subduction earthquakes.
The latency of UT1 measurement with Very Long Baseline Interferometry (VLBI) has been greatly reduced by using e-VLBI technology. VLBI observations on the baseline formed by the Kashima 34-m and the Onsala 20-m radio telescopes achieved ultra-rapid UT1 measurements, where the UT1 result was obtained within 30 min after the end of the observing session. A high speed network and a UDP-based data transfer protocol 'Tsunami' assisted the high data rate and long-distance data transfer from Onsala to Kashima. The accuracy of the UT1 value obtained from the 1-h single baseline e-VLBI experiment has been confirmed to be as the same level with the rapid combined solution of Bulletin-A. The newly developed technology is going to be transferred to the regular intensive VLBI sessions, and it is expected to contribute to the improved latency and accuracy of UT1 data.
Radio frequency (RF) direct sampling is a technique used to sample RF signals that are higher than the sampling rate, without the use of a frequency converter and an anti-aliasing filter. In the case of geodetic VLBI, the RF frequency is at most 9 GHz. Recently, a digital sampler with high sensitivity at RF frequencies greater than 10 GHz was developed. The sampler enables us to evaluate the use of the RF direct sampling technique in geodetic VLBI. RF direct sampling has the potential to make the system simple and stable because, unlike a conventional system, analog frequency converters are not used. We have developed two sets of RF direct sampling systems and operated them on Kashima and Tsukuba baseline (about 50 km length) in Japan. At first, we carried out the VLBI experiment only for X band (8 GHz) signals and successfully got the first fringes. Aliased signals could be discriminated through correlation processing. Then, we adopted RF direct sampling for mixed signals, i.e., S band (2 GHz) and X band signals are combined with each other to make a geodetic VLBI observation. We carried out a 24 hr geodetic VLBI session on 2011 October 19 and succeeded in fringe detection for both S and X bands. After correlation processing, baseline analysis was carried out and we got results consistent with those obtained by conventional VLBI.
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