16:45 〜 17:00
[SSS08-06] 2016年ニュージーランド・カイコウラ地震震源域における応力場の時空間変化（２）
The 2016 Kaikoura earthquake (Mw 7.8) earthquake is one of the most complex earthquakes in the world. In our previous studies (e.g., Okada et al., 2019; Okada et al., 2020, GSNZ), we found seismic low-velocity and high Vp/Vs in and along the focal area of the earthquake. This suggests overpressured fluid promotes the occurrence of the Kaikoura earthquake. Here we consider stress conditions.
We study spatio-temporal change of the stress field due to the 2016 Kaikoura earthquake in the northern area of the South Island of New Zealand. Data from 51 temporary stations and 22 permanent GeoNet stations were used. We determined focal mechanisms, combined with the GeoNet Moment Tensor Solutions, and estimated the stress field in the period from 2013 to 2019 before and after the Kaikoura main shock using stress tensor inversion. We improved the stability of the solution by adopting Vavrycuk (2014)’s method.
If there are many aftershocks on the fault plane of the main shock, the fault plane may be distributed with a bias. Therefore, it is necessary to study the effect of focal mechanisms of aftershocks that occurred off the fault plane of the main shock. So, we attempted to remove the mechanism solution on the fault plane of the main shock using the fault model of Hamling et al. (2017) and the Kagan angle. We found that the stress field did not change significantly independent of the value of the Kagan angle, and we consider that the effect of the aftershocks occurring on the fault plane of the main shock on the result is small.
We carried out the analysis by dividing the hypocenters into several regions. The stress field types are strike-slip types. The maximum horizontal stress direction is about ESE both before and after the Kaikoura earthquake. This would be consistent with shear wave splitting analysis (Graham et al., 2020) which also didn’t show significant coseismic temporal change. This absence of coseismic change of orientations of stress axes suggests large differential stress (sigma1-sigma3) before the Kaikoura earthquake. The stress ratio (R = (sigma1-sigma2)/(sigma1-sigma3)) decreased after the main shock in the northeastern and central regions, and increased in the southwestern region. The coseismic changes in the northeastern and central regions might be interpreted as coseismic stress drops (e.g., Hardebeck and Haukson, 2001).
We considered the possibility that the stress field has a postseismic temporal change after the main shock. We calculated the stress fields in three time windows after the main shock (2016/11/13-2016/11/31, 2016/12/01-2017/5/31, 2017/6/1-2019/12/4), so that the number of events in these three time windows would be approximately the same. In all three windows after the main shock, a stress field of the strike-slip fault type was determined. The value of stress ratio was about 0.8-0.9 in all three windows. This means no significant temporal change after the Kaikoura earthquake through 2019.
The stress field was obtained by dividing the southwestern region into four regions in order to consider spatial change. The stress fields are all strike-slip fault types except in an eastern area where the stress field was intermediate between reverse and strike-slip fault type. This could be interpreted as stress heterogeneity change due to the Kaikoura earthquake (e.g., Michael et al., 1990). This could be also consistent with the co-existence of strike-slip and reverse faults in the North Canterbury Domain (e.g., Ghisetti and Sibson, 2012).
We calculated the slip tendency (Morris et al., 2016) for the fault model (Hamling et al., 2016) using the stress tensor inversion result before the Kaikoura earthquake. High slip tendency is observed at the sub-faults which correspond to the hypocenter and the large slip, and low slip tendency is at the northernmost sub-fault where the slip process of the Kaikoura earthquake stopped.
We conclude that there was no significant temporal change in stress related to the Kaikoura earthquake and pre-stress could explain the slip process of the Kaikoura earthquake.
We study spatio-temporal change of the stress field due to the 2016 Kaikoura earthquake in the northern area of the South Island of New Zealand. Data from 51 temporary stations and 22 permanent GeoNet stations were used. We determined focal mechanisms, combined with the GeoNet Moment Tensor Solutions, and estimated the stress field in the period from 2013 to 2019 before and after the Kaikoura main shock using stress tensor inversion. We improved the stability of the solution by adopting Vavrycuk (2014)’s method.
If there are many aftershocks on the fault plane of the main shock, the fault plane may be distributed with a bias. Therefore, it is necessary to study the effect of focal mechanisms of aftershocks that occurred off the fault plane of the main shock. So, we attempted to remove the mechanism solution on the fault plane of the main shock using the fault model of Hamling et al. (2017) and the Kagan angle. We found that the stress field did not change significantly independent of the value of the Kagan angle, and we consider that the effect of the aftershocks occurring on the fault plane of the main shock on the result is small.
We carried out the analysis by dividing the hypocenters into several regions. The stress field types are strike-slip types. The maximum horizontal stress direction is about ESE both before and after the Kaikoura earthquake. This would be consistent with shear wave splitting analysis (Graham et al., 2020) which also didn’t show significant coseismic temporal change. This absence of coseismic change of orientations of stress axes suggests large differential stress (sigma1-sigma3) before the Kaikoura earthquake. The stress ratio (R = (sigma1-sigma2)/(sigma1-sigma3)) decreased after the main shock in the northeastern and central regions, and increased in the southwestern region. The coseismic changes in the northeastern and central regions might be interpreted as coseismic stress drops (e.g., Hardebeck and Haukson, 2001).
We considered the possibility that the stress field has a postseismic temporal change after the main shock. We calculated the stress fields in three time windows after the main shock (2016/11/13-2016/11/31, 2016/12/01-2017/5/31, 2017/6/1-2019/12/4), so that the number of events in these three time windows would be approximately the same. In all three windows after the main shock, a stress field of the strike-slip fault type was determined. The value of stress ratio was about 0.8-0.9 in all three windows. This means no significant temporal change after the Kaikoura earthquake through 2019.
The stress field was obtained by dividing the southwestern region into four regions in order to consider spatial change. The stress fields are all strike-slip fault types except in an eastern area where the stress field was intermediate between reverse and strike-slip fault type. This could be interpreted as stress heterogeneity change due to the Kaikoura earthquake (e.g., Michael et al., 1990). This could be also consistent with the co-existence of strike-slip and reverse faults in the North Canterbury Domain (e.g., Ghisetti and Sibson, 2012).
We calculated the slip tendency (Morris et al., 2016) for the fault model (Hamling et al., 2016) using the stress tensor inversion result before the Kaikoura earthquake. High slip tendency is observed at the sub-faults which correspond to the hypocenter and the large slip, and low slip tendency is at the northernmost sub-fault where the slip process of the Kaikoura earthquake stopped.
We conclude that there was no significant temporal change in stress related to the Kaikoura earthquake and pre-stress could explain the slip process of the Kaikoura earthquake.