Research for deep back-arc structure in Mariana by Japan-US joint long-term seismic observation

2004年4月和5月对美国-日本联合马里亚纳长期地震观测部署的LTOBS进行了回收。虽然部署了58个LTOBS，但总体数据恢复率约为80%。5月20日至21日，美方负责人wiens教授访问日本，双方就数据交换和分析进行了讨论。初步分析资料，确定震源，发现沿弧后扩张中心和弧前大蛇形海山周围有地震活动。由于在2001年的初步研究实验中也发现了这些，因此假定它们是稳定的活动。利用1年的实测资料，对震源和三维速度结构进行了同步反演。由于最初检测到3000多个事件，即使没有先验信息，也可以对太平洋板块俯冲进行成像。得到了更精确的震源分布，证实了陆地测量站确定的系统震中位移。这些结果在2005年12月的AGU会议上发表。与美国科学家的合作研究与上一年相同。此外，7月在马里亚纳对未回收的LTOBS进行了潜水回收，成功回收了与本研究相关的3只LTOBS。至此，两次实验得到的可用数据全部可用，并完成了日美之间的最终数据交换。从2001年的数据中，今年又恢复了2个低空点，重新对整个到达数据进行核对，然后再次进行同步反演。由于增加的LTOBS改善了台站分布，高分辨率地揭示了弧后浅层地震活动。双地震带也表现出精细的构造，从其上平面蛇形海山下方开始。2003年获得的新资料主要在美国进行了s波分裂、面波分析、震源确定和体波层析成像等分析。这些结果通过音频和海报在AGU会议上进行了介绍。少

FY2004Recovery of LTOBS deployed for the US-JP joint long-term seismic observation in Mariana started in 2004 was performed in April and May. Although 58 LTOBS were deployed, the overall data recovery was about 80%. On 20-21 May, the US side chief, Prof.Wiens visited Japan and we discussed about the data exchange and analysis. Preliminary analysis of this data, hypocenter determination, seismic activities along the spreading center in the back-arc and around the large serpentine seamount in the fore-arc were found. As these were also found in the pilot study experiment in 2001, these are assumed as steady activities. Farther more, a simultaneous inversion for hypocenter and 3-D velocity structure had been performed too by using arrival data for one year. Due to more than 3000 events originally detected, it could image the subducting Pacific plate even if without priori information of it. More precise hypocenter distribution obtained certified the systematic shift of epicenters determin … More ed by land stations. These result was presented in the AGU meeting in Dec.FY2005Cooperative study with US scientists was continued as same in the previous year. Further more, a dive recovery for un-recovered LTOBS in Mariana was conducted in July, and 3 LTOBS related with this study were successfully recovered. So that, all usable data obtained from two experiments became available, and the final data exchange between Japan and USA was also finished. From the 2001 data with 2 more LTOBS recovered this year, whole arrival data was checked again, then the simultaneous inversion was performed again. As additional LTOBS improved the station distribution, shallow seismic activity in the back-arc was revealed with high resolution. The double seismic zone also shows a fine structure, which starts from beneath the serpentine seamount of its upper plane. The new data obtained in 2003 was analyzed mainly in US side such as S-wave splitting, surface wave analysis, hypocenter determination and body wave tomography. These results were presented in the AGU meeting by aural and posters. Less