[S08P-02] Stress field in the northeastern part of South Island, New Zealand, and the evaluation of the activity of the 1990 Lake Tennyson earthquake
Introduction
The stress field would control fault activity. Thus, stress-based evaluation of fault is important for understanding the condition of fault activity.
In New Zealand, the Pacific plate is subducted into the Australian plate in the North Island and the northern part of the South Island. The characteristics of fault activity in the northern part of the South Island is differ between the west and east sides of the Alpine Fault.
The northwestern part is under a compressional stress field with ESE-WNW direction. Many reverse faults active in the current tectonic regime are inherited from Late Cretaceous-Paleocene normal faults formed during the opening of the Tasman Sea that were subsequently inverted (e.g., Ghisetti et al., 2014).
In the northeastern part, the stress field with strike slip fault type is dominant with ESE-WNW compressional axis (e.g., Balfour et al., 2005). This corresponds to the relative plate motion of the upper plate of the plate oblique subduction zone. The Marlborough Fault System (MFS) is composed of strike slip fault with strike NE-SW and dip 60° - 90° (e.g., Anderson et al. 1993).
The overall aim of our study is to clarify the conditions that cause fault activities in the northern part of South Island. In our previous study, we evaluated the fault likelihood of slip to stress field using Slip Tendency (ST) method (Morris et al., 1996) in the northwestern part (e.g., Tagami et al., JpGU 2023). The faults which large-sized earthquakes occurred were in favorable to slip to the stress field.
This study focuses on the northeastern part of the South Island. We estimate the stress field in this region and compare it with the northwestern part of the South Island. Also, we focus on the 1990 Lake Tennyson earthquake (Mw 5.9) to compare the ST value and the actual activity fault plane, which was estimated by previous studies.
Data and Methods
We estimate the stress field using the focal mechanisms obtained by temporal and permanent stations (Okada et al., 2019; Matsuno et al., 2022) and the GeoNet moment tensor solutions. The data period is 2003 - 2023. We deploy the stress tensor inversion method developed by Michael (1984, 1987). Then, we evaluate the likelihood of slip by the ST method (Morris et al., 1996). For the fault plane, we use the fault distribution map (Robinson, 2004) and focal mechanisms of the Lake Tennyson earthquake estimated by Anderson et al. (1993).
Results
1. Stress fields
To observe the overall spatial trend, we divided the study area into four areas (Area 1 (northern MFS): -42° - -41°; 173° -174°, Area 2 (Cook Strait): -42° - -41°; 174° - 175°, Area 3 (southwestern MFS): -43° - -42°; 172° - 173°, and Area 4 (southeastern MFS): -43° - -42°; 173° - 174°). Also, considering the effects of the 2013 Cook Strait earthquake (Mw 6.6) and the 2016 Kaikōura earthquake (Mw 7.8), we set three timeframes (A: 2003 - 2013, B: 2013 - 2016, and C: 2016 - 2023).
Focal mechanisms of strike slip fault type are dominant in these areas. Most regions show stress fileds of strike slip type. Each SHmax is in between EW to NWN-ESE (90° - 120°). Our Results is consistent with previous studies (e.g., Balfour et al., 2005; Townend et al., 2012), although Area 2 and Area 4 show different SHmax direction. In Area 2 and Area 4 where the Cook Strait earthquake and the Kaikōura earthquakes occurred, the rotation of SHmax can be confirmed before and after these large events. The results from Michael’s method have large errors. Therefore, we re-estimate the stress field by a different method of Vavryčuk (2014). The results from Vavryčuk’s method are consistent with the results from Michael’s method with smaller errors. For this reason, the results obtained in this study are considered reliable.
2. Slip tendency of the 1990 Lake Tennyson earthquake
This event occurs in Area 3 (southwestern MFS). McGinty et al. (1997) estimated the main fault lineation as ENE with 8 km. We calculate the ST value using the nodal plane estimated by Anderson et al. (1993). For the stress parameter, we use the results of Area 3 estimated by Vavryčuk’s method. The nodal plane with the strike of 55° shows a large ST value of 0.71, and 0.86 in the timeframe A and B, respectively. A large ST value means the fault plane is favorable to slip. If the stress field has not changed significantly since 1990, the Lake Tennyson earthquake seems to have occurred with a favorable slip.
We also estimate the ST value for other fault in Area 3 using Robinson’s fault model. The fault planes with strike (61° - 76°) and vertical dip (90°) show large ST values (0.80 - 0.98) in timeframes A and B. This is consistent with the Lake Tennyson earthquake result. However, a fault plane with a strike of 52° shows a lower ST value (~0.66) in the timeframe B.
The stress field would control fault activity. Thus, stress-based evaluation of fault is important for understanding the condition of fault activity.
In New Zealand, the Pacific plate is subducted into the Australian plate in the North Island and the northern part of the South Island. The characteristics of fault activity in the northern part of the South Island is differ between the west and east sides of the Alpine Fault.
The northwestern part is under a compressional stress field with ESE-WNW direction. Many reverse faults active in the current tectonic regime are inherited from Late Cretaceous-Paleocene normal faults formed during the opening of the Tasman Sea that were subsequently inverted (e.g., Ghisetti et al., 2014).
In the northeastern part, the stress field with strike slip fault type is dominant with ESE-WNW compressional axis (e.g., Balfour et al., 2005). This corresponds to the relative plate motion of the upper plate of the plate oblique subduction zone. The Marlborough Fault System (MFS) is composed of strike slip fault with strike NE-SW and dip 60° - 90° (e.g., Anderson et al. 1993).
The overall aim of our study is to clarify the conditions that cause fault activities in the northern part of South Island. In our previous study, we evaluated the fault likelihood of slip to stress field using Slip Tendency (ST) method (Morris et al., 1996) in the northwestern part (e.g., Tagami et al., JpGU 2023). The faults which large-sized earthquakes occurred were in favorable to slip to the stress field.
This study focuses on the northeastern part of the South Island. We estimate the stress field in this region and compare it with the northwestern part of the South Island. Also, we focus on the 1990 Lake Tennyson earthquake (Mw 5.9) to compare the ST value and the actual activity fault plane, which was estimated by previous studies.
Data and Methods
We estimate the stress field using the focal mechanisms obtained by temporal and permanent stations (Okada et al., 2019; Matsuno et al., 2022) and the GeoNet moment tensor solutions. The data period is 2003 - 2023. We deploy the stress tensor inversion method developed by Michael (1984, 1987). Then, we evaluate the likelihood of slip by the ST method (Morris et al., 1996). For the fault plane, we use the fault distribution map (Robinson, 2004) and focal mechanisms of the Lake Tennyson earthquake estimated by Anderson et al. (1993).
Results
1. Stress fields
To observe the overall spatial trend, we divided the study area into four areas (Area 1 (northern MFS): -42° - -41°; 173° -174°, Area 2 (Cook Strait): -42° - -41°; 174° - 175°, Area 3 (southwestern MFS): -43° - -42°; 172° - 173°, and Area 4 (southeastern MFS): -43° - -42°; 173° - 174°). Also, considering the effects of the 2013 Cook Strait earthquake (Mw 6.6) and the 2016 Kaikōura earthquake (Mw 7.8), we set three timeframes (A: 2003 - 2013, B: 2013 - 2016, and C: 2016 - 2023).
Focal mechanisms of strike slip fault type are dominant in these areas. Most regions show stress fileds of strike slip type. Each SHmax is in between EW to NWN-ESE (90° - 120°). Our Results is consistent with previous studies (e.g., Balfour et al., 2005; Townend et al., 2012), although Area 2 and Area 4 show different SHmax direction. In Area 2 and Area 4 where the Cook Strait earthquake and the Kaikōura earthquakes occurred, the rotation of SHmax can be confirmed before and after these large events. The results from Michael’s method have large errors. Therefore, we re-estimate the stress field by a different method of Vavryčuk (2014). The results from Vavryčuk’s method are consistent with the results from Michael’s method with smaller errors. For this reason, the results obtained in this study are considered reliable.
2. Slip tendency of the 1990 Lake Tennyson earthquake
This event occurs in Area 3 (southwestern MFS). McGinty et al. (1997) estimated the main fault lineation as ENE with 8 km. We calculate the ST value using the nodal plane estimated by Anderson et al. (1993). For the stress parameter, we use the results of Area 3 estimated by Vavryčuk’s method. The nodal plane with the strike of 55° shows a large ST value of 0.71, and 0.86 in the timeframe A and B, respectively. A large ST value means the fault plane is favorable to slip. If the stress field has not changed significantly since 1990, the Lake Tennyson earthquake seems to have occurred with a favorable slip.
We also estimate the ST value for other fault in Area 3 using Robinson’s fault model. The fault planes with strike (61° - 76°) and vertical dip (90°) show large ST values (0.80 - 0.98) in timeframes A and B. This is consistent with the Lake Tennyson earthquake result. However, a fault plane with a strike of 52° shows a lower ST value (~0.66) in the timeframe B.