11:00 AM - 11:15 AM
[SSS06-07] Influence of the 1929 Buller earthquake (Mw 7.3), New Zealand, on four large-magnitude (Mw > 6) aftershocks
Keywords:Inversion tectonic fault, Stress field, New Zealand, Slip tendency, Inland earthquake, Fault
Introduction
The Pacific plate is subduct beneath the Australian plate beneath the North Island and the north part of the South Island, New Zealand. Compressional stress with ESE-WNW direction is dominant in the north-western part of the South Island (Townend et al., 2012). Many old normal faults are distributed on the northwest side of the Alpine Fault which passes through the region; these faults developed due to the extensional stress fields (Ghisetti et al., 2014). Therefore, tectonic inversion, where previously normal faults act as a reverse fault, has been confirmed in this region.
The Mw 7.3 1929 Buller earthquake occurred on the White Creek fault, one of the tectonic inversion faults (e.g., Doser et al., 1999). This earthquake occurred on June 16th, and was followed by four large-magnitude aftershocks (> Mw 6) before July 15th. The large aftershocks did not have any observed surface rupture expression. Unfortunately, not much seismic data is available for the 1920’s.
We evaluated the likelihood of slipping of the fault against the stress field using the slip tendency analysis (Morris et al., 1996). However, we couldn’t clarify the actual activity from these results because of the uncertainty of the actual fault plane.
Therefore, assuming the rake angle could be the same as the maximum shear stress orientation, we consider the possibility of the fault plane of these aftershocks, and evaluate these aftershocks by the Coulomb stress change and the slip tendency analysis.
Methods
We use moment tensor data derived by GNS Science which were determined from routinely operated stations (GeoNet) and focal mechanisms estimated from P-wave initial motions at temporary stations deployed across the region (Okada et al., 2019). For calculating the stress field, we use the stress tensor inversion method developed by Michael (1984, 1987), and we use the Synthetic Slip program (Neves et al., 2009) to calculate the rake angle from the input stress field. To calculate stress change, we use the Coulomb program (Toda et al., 2005; Lin and Stein, 2004), and use the nodal planes of focal mechanisms for the input fault plane parameter estimated by Doser et al. (1999).
Results
Synthetic Slip
We defined the difference between the rake of the fault model (λobs) and calculated rake (λcal) as rake differences (λdif = λcal - λobs).
In the B2, both planes have a small λdif (< 10). In the B3, the eastward dipping plane has a small λdif (< 10). In the B4 and B5, the westward dipping plane has a small λdif (< 10).
Stress Change
We input the fault plane of the Buller mainshock (B1) for the source fault, and the nodal planes of each Buller sequence (B2, B3, B4, and B5) for the receiver fault. We define the fault shape of the B1 by the focal mechanism estimated by Doser et al. (1999), the dip direction of the White Creek fault, the amount of displacement, and the scaling law (Wells & Coppersmith, 1994). We focused on whether the Coulomb stress change on the Buller aftershocks was positive or not.
In B2 and B5, both planes showed positive. In the B3, both planes didn’t show a positive. In the B4, the westward dipping plane showed positive.
Discussion
We will compare the results (Table 1), including the slip tendency analysis results, and consider the fault plane’s possibility
B2
This event occurred between the west-dipping fault and the east-dipping fault. There needs to be a clear distinction from our results. Since the result of B1 caused of stress change was positive, it is possible that the B1 trigger the B2.
B3
Based on the ST value, ST error, and the λdif, the eastward dipping plane may have slipped. However, stress change shows negative, so it is hard to think of the influence of B1.
B4
From the results of λdif and stress change, it is possible that the westward dipping plane slipped. Although the ST value is small, it is possible that B1 could have triggered the B4.
B5
From the λdif, it is possible that the westward dipping plane slipped. In addition, B5 occurs near the fault with a westward dip. The stress change was positive, so the B1 may trigger the activity of B5.
The Pacific plate is subduct beneath the Australian plate beneath the North Island and the north part of the South Island, New Zealand. Compressional stress with ESE-WNW direction is dominant in the north-western part of the South Island (Townend et al., 2012). Many old normal faults are distributed on the northwest side of the Alpine Fault which passes through the region; these faults developed due to the extensional stress fields (Ghisetti et al., 2014). Therefore, tectonic inversion, where previously normal faults act as a reverse fault, has been confirmed in this region.
The Mw 7.3 1929 Buller earthquake occurred on the White Creek fault, one of the tectonic inversion faults (e.g., Doser et al., 1999). This earthquake occurred on June 16th, and was followed by four large-magnitude aftershocks (> Mw 6) before July 15th. The large aftershocks did not have any observed surface rupture expression. Unfortunately, not much seismic data is available for the 1920’s.
We evaluated the likelihood of slipping of the fault against the stress field using the slip tendency analysis (Morris et al., 1996). However, we couldn’t clarify the actual activity from these results because of the uncertainty of the actual fault plane.
Therefore, assuming the rake angle could be the same as the maximum shear stress orientation, we consider the possibility of the fault plane of these aftershocks, and evaluate these aftershocks by the Coulomb stress change and the slip tendency analysis.
Methods
We use moment tensor data derived by GNS Science which were determined from routinely operated stations (GeoNet) and focal mechanisms estimated from P-wave initial motions at temporary stations deployed across the region (Okada et al., 2019). For calculating the stress field, we use the stress tensor inversion method developed by Michael (1984, 1987), and we use the Synthetic Slip program (Neves et al., 2009) to calculate the rake angle from the input stress field. To calculate stress change, we use the Coulomb program (Toda et al., 2005; Lin and Stein, 2004), and use the nodal planes of focal mechanisms for the input fault plane parameter estimated by Doser et al. (1999).
Results
Synthetic Slip
We defined the difference between the rake of the fault model (λobs) and calculated rake (λcal) as rake differences (λdif = λcal - λobs).
In the B2, both planes have a small λdif (< 10). In the B3, the eastward dipping plane has a small λdif (< 10). In the B4 and B5, the westward dipping plane has a small λdif (< 10).
Stress Change
We input the fault plane of the Buller mainshock (B1) for the source fault, and the nodal planes of each Buller sequence (B2, B3, B4, and B5) for the receiver fault. We define the fault shape of the B1 by the focal mechanism estimated by Doser et al. (1999), the dip direction of the White Creek fault, the amount of displacement, and the scaling law (Wells & Coppersmith, 1994). We focused on whether the Coulomb stress change on the Buller aftershocks was positive or not.
In B2 and B5, both planes showed positive. In the B3, both planes didn’t show a positive. In the B4, the westward dipping plane showed positive.
Discussion
We will compare the results (Table 1), including the slip tendency analysis results, and consider the fault plane’s possibility
B2
This event occurred between the west-dipping fault and the east-dipping fault. There needs to be a clear distinction from our results. Since the result of B1 caused of stress change was positive, it is possible that the B1 trigger the B2.
B3
Based on the ST value, ST error, and the λdif, the eastward dipping plane may have slipped. However, stress change shows negative, so it is hard to think of the influence of B1.
B4
From the results of λdif and stress change, it is possible that the westward dipping plane slipped. Although the ST value is small, it is possible that B1 could have triggered the B4.
B5
From the λdif, it is possible that the westward dipping plane slipped. In addition, B5 occurs near the fault with a westward dip. The stress change was positive, so the B1 may trigger the activity of B5.