[S-CG39] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG39-P08] Increase of tremor-rate in Marlborough region (New Zealand) after the 2016 Mw 7.8 Kaikōura earthquake.

*Pierre Romanet¹, Florent Aden-Antoniow², Ryosuke Ando¹, Stephen C Bannister³, Calum John Chamberlain⁴, Yoshihisa Iio⁵, Satoshi Matsumoto⁶, Tomomi Okada⁷, Richard H. Sibson⁸, Akiko Toh¹, Satoshi Ide¹ (1.The University of Tokyo, School of Science, Earth and Planetary Science , Japan, 2.Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA, 3.GNS Science, PO Box 30-368, Lower Hutt 5040, New Zealand, 4.School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington, New Zealand, 5.Disaster Prevention Research Institute, Kyoto University, Uji 611-0011, Japan, 6.Faculty of Sciences, Kyushu University, 2-5643-29 Shin'yama, Shimabara, Nagasaki 855-0843, Japan, 7.International Research Institute of Disaster Science (IRIDeS), Tohoku University, Sendai 980-0845, Japan, 8.Department of Geology, University of Otago, PO Box 56, Dunedin 9054, New Zealand)

Keywords:Slow-earthquake, Tremors

In a previous study, tremors have been detected in Marlborough (New Zealand) region using 6 stations from the national GEONET network during the period 2010-2017. This region hosted the 2016 Mw7.8 Kaikōura earthquake, and lies at the transition between the Alpine Fault and the Hikurangi subduction zone. The previous study detected 40 tremors in this region, however this work was limited by the scarcity of seismic stations and their distance from the tremor area. Here we conduct the same analysis, but in addition to 4 stations from the national network, we added 16 stations from a local campaign network that surrounds the tremor area. This composite network enabled the construction of an enhanced tremor catalogue for the region, composed of around 100 times more tremors than the previous study (4,699 tremors) during the period 2013-2019.



The tremor locations form two clusters, one that has a circular shape, and another one that as an elongated shape in the direction of the major upper-plate dextral (Hope and Clarence) faults of the Marlborough Fault system. The estimated depth of the tremor, as well as the shape of the cluster suggests that these tremors may be on the dextral strike-slip faults and not on the subduction interface although It needs to be confirmed by further study.



We observed that the tremor rate is almost constant before the Mw 7.8 Kaikōura earthquake. After the earthquake, we observed a huge increase in the tremor rate in both the clusters. At the end of our analysis in May 2019, more than 2 years after the earthquake, the tremor-rate has still not recovered to the pre-earthquake rate. This increase in tremor-rate appears unrelated to changes in seismic networks.



We were also able to observe several local migrations of tremors with jumps from one cluster to the other. Most of the migrations take place from the South-West to the North-East. We estimated the migration velocity to be of around ~50km/day.



This new tremor catalog provides unique information on the tectonic context of the Marlborough fault system and may help to better understand interaction of regular earthquakes with tremors.