3:30 PM - 4:30 PM
[J05-P-07] Regional stress field inferred from focal mechanisms obtained by dense seismic observation in the northern South Island, New Zealand
The northern South Island and the southernmost North Island of New Zealand lie in the transitional area of subduction of the Pacific plate, with the Pacific plate subducting beneath the Australian plate obliquely from the northeast. Due to the plate subduction, active seismicity in the northern South Island can be seen. For example, a Mw 7.8 earthquake occurred in the Kaikoura region of northern South Island at 11:32 am (UT) on 14 November 2016. In this study, we obtained focal mechanisms of the micro- to moderate-sized earthquakes in order to estimate the spatiotemporal variation of the crustal stress field, including the Kaikoura earthquake.
We analyzed the data acquired by a dense seismic array (see also Okada et al, this meeting) which has been recording over 2 years from 1 April 2013 to April 2015. We determined focal mechanisms using the HASH program (Hardebeck, 2002; 2003). During that time period, there have been two major seismic clusters; the first, consisting of aftershocks of the 1990 Lake Tennyson earthquake, occurred in the center of the northern South Island, while the second, which consisted of aftershocks of the 2013 Cook Strait earthquakes, occurred in the northeast of the northern South Island. For shallow earthquakes, strike-slip type focal mechanisms seem to dominate. P axes were oriented to ~N120E, which is similar to that found in previous studies (Reyners et al., 1997; Balfour et al., 2005; Sibson and Ghisetti, 2011; Townend et al., 2012) and which seems to be consistent with most of GEONET-CMTs of the aftershocks of the 2016 Kaikoura earthquake. T axis were oriented to NE-SW. For intermediate-deep earthquake, normal, strike-slip, reverse faulting seems to be mixed. Most of the P axes were oriented to NE-SW, which is also consistent with previous studies (Reyners et al., 1997; Townend et al., 2012).
We analyzed the data acquired by a dense seismic array (see also Okada et al, this meeting) which has been recording over 2 years from 1 April 2013 to April 2015. We determined focal mechanisms using the HASH program (Hardebeck, 2002; 2003). During that time period, there have been two major seismic clusters; the first, consisting of aftershocks of the 1990 Lake Tennyson earthquake, occurred in the center of the northern South Island, while the second, which consisted of aftershocks of the 2013 Cook Strait earthquakes, occurred in the northeast of the northern South Island. For shallow earthquakes, strike-slip type focal mechanisms seem to dominate. P axes were oriented to ~N120E, which is similar to that found in previous studies (Reyners et al., 1997; Balfour et al., 2005; Sibson and Ghisetti, 2011; Townend et al., 2012) and which seems to be consistent with most of GEONET-CMTs of the aftershocks of the 2016 Kaikoura earthquake. T axis were oriented to NE-SW. For intermediate-deep earthquake, normal, strike-slip, reverse faulting seems to be mixed. Most of the P axes were oriented to NE-SW, which is also consistent with previous studies (Reyners et al., 1997; Townend et al., 2012).