5:15 PM - 6:45 PM
[SSS05-P01] Fault re-activation of middle to small earthquakes in a tectonic inversion region, northwestern South Island, New Zealand
Keywords:Stress field, Fault activity, New Zealand, tectonic inversion fault
Introduction
In New Zealand, the Pacific plate is subducting beneath the Australian plate in the North Island and the north part of the South Island. Compressional stress field with ESE-WNW direction is dominant in the northwestern part of the South Island (e.g., Townend et al., 2012). In the northwestern part of the South Island, many normal faults were formed during the opening of the Tasman Sea (Ghisetti and Sibson, 2006), and these fault planes are presently active as a reverse fault (tectonic inversion fault) (Ghisetti et al., 2014). Earthquakes occur less frequently in the northwestern part of South Island than in many other parts of New Zealand. However, large earthquakes (e.g., the 1929 Buller earthquake, Mw 7.3; the 1968 Inangahua earthquake, Mw 7.2) have occurred in this area. Therefore, assessing the likelihood of the fault slip in this area is important for understanding seismic activity in this area.
We evaluated the likelihood of the fault slip for large events (e.g., the Buller earthquake; the Inangahua earthquake) using the Slip Tendency (ST) method (Morris et al., 1996) (e.g., Tagami et al., JpGU 2023). The faults (strike N-S) were mostly favorable to slip in the stress field in the northwestern part of South Island. Previous studies had estimated focal mechanisms for most large earthquakes, but the actual fault plane had not been determined. The problem is that the choice of fault plane is arbitrary.
Therefore, in this study, we detect the medium to small earthquake (aftershocks of template earthquake) using matched filter detection methods to try to estimate the trend of fault planes. If the aftershocks distributed like a plane shape, we could assume that the aftershocks occur on fault planes. Thus, the aftershock distribution can help discriminate the fault planes from the two nodal planes of the focal mechanism. We will also compare the estimated fault planes with the ST value.
Data and Methods
We used matched filter detection methods (EQcorrscan; Chamberlain et al., 2018) for detecting the aftershocks. For the template waveform, we used phase arrival data of focal mechanisms determined from the P-wave initial polarity in the Hash program (Hardebeck and Shearer, 2002). Waveform data was obtained from temporary (Okada et al., 2024) and permanent seismic network (GeoNet). The absolute location of the detect events was calculated using NonLinLoc (Lomax et al., 2000). Also, relative location was determined with GrowClust (Trugman and Shearer, 2017) using travel time difference from waveform correlation.
Results
Five earthquakes that occurred in June 2013 and were classified as earthquakes in the GeoNet catalog were used as templates. The aftershocks were detected using EQcorrscan for the June 2013 and we detected 74 earthquakes. The detected earthquakes include those not registered in the GeoNet catalog.
The following three earthquakes were detected with particularly large number of aftershocks.
June 1st 15:38 (M 3.2)
Focal mechanism showed strike-slip fault type.
Aftershock distribution tends to strike NE-SW with approximate vertical dip angle. This is consistent with the nodal plane of the focal mechanism. The nodal plane with strike NE-SW and dip 87° showed ST = 0.451. The ST value smaller than 0.7 is indicating that it was not favorable to slip.
June 12th 07:29 (M 3.3)
Focal mechanism showed reverse fault type.
Aftershock distribution tends to dip eastward. The nodal plane of the eastward dipping plane with dip 37° showed ST=0.821, indicating that this plane is favorable to slip.
June 25th 04:41 (M 3.2)
Focal mechanism showed reverse fault type.
Aftershock distribution tends to strike NE-SW with a westward dipping plane. The westward dipping nodal plane with dip 30° showed ST=0.987, indicating that this plane is favorable to slip.
We were able to identify the fault plane using the aftershock distribution detected by the matched filter method. Also, some of the fault planes were consistent with the ST value results. Hypocenter distribution is likely to be useful for discovering fault planes and evaluating trends in seismic activity in this area.
In New Zealand, the Pacific plate is subducting beneath the Australian plate in the North Island and the north part of the South Island. Compressional stress field with ESE-WNW direction is dominant in the northwestern part of the South Island (e.g., Townend et al., 2012). In the northwestern part of the South Island, many normal faults were formed during the opening of the Tasman Sea (Ghisetti and Sibson, 2006), and these fault planes are presently active as a reverse fault (tectonic inversion fault) (Ghisetti et al., 2014). Earthquakes occur less frequently in the northwestern part of South Island than in many other parts of New Zealand. However, large earthquakes (e.g., the 1929 Buller earthquake, Mw 7.3; the 1968 Inangahua earthquake, Mw 7.2) have occurred in this area. Therefore, assessing the likelihood of the fault slip in this area is important for understanding seismic activity in this area.
We evaluated the likelihood of the fault slip for large events (e.g., the Buller earthquake; the Inangahua earthquake) using the Slip Tendency (ST) method (Morris et al., 1996) (e.g., Tagami et al., JpGU 2023). The faults (strike N-S) were mostly favorable to slip in the stress field in the northwestern part of South Island. Previous studies had estimated focal mechanisms for most large earthquakes, but the actual fault plane had not been determined. The problem is that the choice of fault plane is arbitrary.
Therefore, in this study, we detect the medium to small earthquake (aftershocks of template earthquake) using matched filter detection methods to try to estimate the trend of fault planes. If the aftershocks distributed like a plane shape, we could assume that the aftershocks occur on fault planes. Thus, the aftershock distribution can help discriminate the fault planes from the two nodal planes of the focal mechanism. We will also compare the estimated fault planes with the ST value.
Data and Methods
We used matched filter detection methods (EQcorrscan; Chamberlain et al., 2018) for detecting the aftershocks. For the template waveform, we used phase arrival data of focal mechanisms determined from the P-wave initial polarity in the Hash program (Hardebeck and Shearer, 2002). Waveform data was obtained from temporary (Okada et al., 2024) and permanent seismic network (GeoNet). The absolute location of the detect events was calculated using NonLinLoc (Lomax et al., 2000). Also, relative location was determined with GrowClust (Trugman and Shearer, 2017) using travel time difference from waveform correlation.
Results
Five earthquakes that occurred in June 2013 and were classified as earthquakes in the GeoNet catalog were used as templates. The aftershocks were detected using EQcorrscan for the June 2013 and we detected 74 earthquakes. The detected earthquakes include those not registered in the GeoNet catalog.
The following three earthquakes were detected with particularly large number of aftershocks.
June 1st 15:38 (M 3.2)
Focal mechanism showed strike-slip fault type.
Aftershock distribution tends to strike NE-SW with approximate vertical dip angle. This is consistent with the nodal plane of the focal mechanism. The nodal plane with strike NE-SW and dip 87° showed ST = 0.451. The ST value smaller than 0.7 is indicating that it was not favorable to slip.
June 12th 07:29 (M 3.3)
Focal mechanism showed reverse fault type.
Aftershock distribution tends to dip eastward. The nodal plane of the eastward dipping plane with dip 37° showed ST=0.821, indicating that this plane is favorable to slip.
June 25th 04:41 (M 3.2)
Focal mechanism showed reverse fault type.
Aftershock distribution tends to strike NE-SW with a westward dipping plane. The westward dipping nodal plane with dip 30° showed ST=0.987, indicating that this plane is favorable to slip.
We were able to identify the fault plane using the aftershock distribution detected by the matched filter method. Also, some of the fault planes were consistent with the ST value results. Hypocenter distribution is likely to be useful for discovering fault planes and evaluating trends in seismic activity in this area.