Japan Geoscience Union Meeting 2018

Presentation information

[EJ] Oral

S (Solid Earth Sciences) » S-GD Geodesy

[S-GD01] Gravity and Geoid

Wed. May 23, 2018 10:45 AM - 12:15 PM A09 (Tokyo Bay Makuhari Hall)

convener:Takayuki Miyazaki(Geospatial Information Authority of Japan), Keiko Yamamoto(National Astronomical Observatory of Japan), Chairperson:Nawa Kazunari, Nisijima Jun

11:00 AM - 11:15 AM

[SGD01-08] Gravity changes due to the 2016 Kaikoura earthquake

*Yoichi Fukuda1, Hiroshi Takiguchi2, Jun Nishijima3, Takahito Kazama1, Vaughan Stagpoole 4, Grant O'Brien4 (1.Department of Geophysics, Graduate School of Science, Kyoto University, 2.Japan Aerospace Exploration Agency, 3.Department of Earth Resources Engineering, Graduate School of Engineering, Kyushu University, 4.Institute of Geological and Nuclear Science, New Zealand)

Keywords:Kaikoura earthquake, gravity changes, crustal movements, relative gravity measurements, absolute gravity measurements

The 2016 Kaikoura earthquake occurred at 11:02 on Nov. 13, 2016 UTC (00:02 on Nov. 14, 2016 NZDT) in the South Island of New Zealand. Its epicentre was located about 60 km south-west of Kaikoura at a depth of 15 km. The magnitude 7.8 (Mw) earthquake lasted about 2 minutes. One of the characteristics of the earthquake was its complicated ruptures towards north-east and large crustal movements along major faults. In-SAR analysis, GNSS measurements, as well as in-situ observations revealed that more than 8 meters uplift was occured as a block motion along the fault boundaries. Also 1-2 meters uplift was widely observed along the north-east coastal area.

Since the gravity changes due to the vertical crustal movements provide the information about the subsuface density changes, which would be usefull for considering the rupture mechanism, we have conducted relative gravity measurements at bench marks along State Highway 1 (SH1) from Blenheim to Christchurch. The obtained gravity values were compered with those of the 1980's precise gravity network. Although the uncertainties of the comparisons were not small, the results clearly show the gravity changes due to the widespread uplift. In particular, the gravity changes around Ward are significant and a step of the gravity change of about 0.5 mGal is clearly detected across the Kekerengu Fault.

In addition to the relative gravity measurements, we conducted absolute gravity measurments at 2 sites in Marlborough region in Sep. 2017, and we plan to reoccupy the same points in April-May 2018 to detect the possible gravity changes due to postseismic deformation. We also plan to conduct GNSS survey at the gravity bench marks to evaluate height changes more precisely. These data will be used to estimate the ratio between the gravity changes vs height changes, which should provide useful information about the mechanism of the crustal movements.

This study was partially supported by JSPS KAKENHI Grant No. 15H05205.