10:45 AM - 11:00 AM
[SSS06-06] Spatiotemporal distribution of regular earthquakes and slow slip events in the Hikurangi subduction zone, New Zealand
Introduction
On and surrounding the Hikurangi subduction zone, beneath North Island, New Zealand, various faulting phenomena including slow slip events (SSEs), tremors, and earthquake swarms have been observed. These phenomena have been found to vary along-strike of the Hikurangi subduction zone. On the northern Hikurangi subduction zone, recent studies suggest that fluid movement from the subducted oceanic crust to the plate boundary or into the Australian plate impacts various seismic activities. In the central Hikurangi, studies suggest that stress changes due to distant earthquakes or deeps SSE may trigger shallow SSEs. It needs to be clarified what the main driving force of SSEs and related earthquakes is, and how the driving force varies along the strike. A fundamental limitation in understanding their mechanisms is the variable quality of earthquake hypocenter locations on and around the Hikurangi subduction zone.
The main objective of this study is to clarify the driving force of SSEs and related earthquakes in the northern and central Hikurangi using smaller earthquakes than those used by Tateiwa et al. (2022, SSJ).
Data and Method
We selected all earthquakes in the GeoNet catalog of M > 2.8 occurring between 2004 and 2007, M > 2.6 occurring between 2008 and 2011, and M > 2.5 occurring between 2012 and 2020 in and around the shallow SSE source region shallower than 100 km for analysis. We relocated 17,359 earthquakes, calculated their focal mechanisms, and then divided them into those occurring within the Australian plate (AUS), within the Pacific plate (PAC), or at the interplate interface (INT), and examined the spatiotemporal distribution of these earthquakes. We also estimated the occurrence timing of SSEs using GNSS data and calculated the lag time between earthquakes and SSEs.
Result and Discussion
PAC earthquakes were distributed widely, while AUS and INT were distributed locally. Many AUS earthquakes in the northern Hikurangi located where extensional areal strain was observed (Dimitrova et al., 2016). We infer that this extension may increase the permeability of the crust, resulting in an increase in fluid-related earthquakes. INT earthquakes occurred at the periphery of the SSE source regions. In and around the SSE source region in the northern Hikurangi, AUS earthquakes were active after SSEs, INT earthquakes were active during SSEs, and PAC earthquakes were slightly active before and during SSEs, which is the same as observed by Tateiwa et al. (2022, SSJ) who used larger earthquakes with magnitudes greater than three. The fluid movement may explain this characteristic of different periods of activation depending on the location of the earthquake. That is, fluid movement from the oceanic crust to the plate boundary or upper plate, as proposed by Nishikawa et al. (2021), may have triggered SSEs, tremors, and AUS or INT earthquakes. In and around the SSE source region in the central Hikurangi, AUS, INT, and PAC are active during SSEs, which is different from the northern Hikurangi. It indicates that the mechanism of SSEs and related earthquakes may also be different from that in the northern Hikurangi.
On and surrounding the Hikurangi subduction zone, beneath North Island, New Zealand, various faulting phenomena including slow slip events (SSEs), tremors, and earthquake swarms have been observed. These phenomena have been found to vary along-strike of the Hikurangi subduction zone. On the northern Hikurangi subduction zone, recent studies suggest that fluid movement from the subducted oceanic crust to the plate boundary or into the Australian plate impacts various seismic activities. In the central Hikurangi, studies suggest that stress changes due to distant earthquakes or deeps SSE may trigger shallow SSEs. It needs to be clarified what the main driving force of SSEs and related earthquakes is, and how the driving force varies along the strike. A fundamental limitation in understanding their mechanisms is the variable quality of earthquake hypocenter locations on and around the Hikurangi subduction zone.
The main objective of this study is to clarify the driving force of SSEs and related earthquakes in the northern and central Hikurangi using smaller earthquakes than those used by Tateiwa et al. (2022, SSJ).
Data and Method
We selected all earthquakes in the GeoNet catalog of M > 2.8 occurring between 2004 and 2007, M > 2.6 occurring between 2008 and 2011, and M > 2.5 occurring between 2012 and 2020 in and around the shallow SSE source region shallower than 100 km for analysis. We relocated 17,359 earthquakes, calculated their focal mechanisms, and then divided them into those occurring within the Australian plate (AUS), within the Pacific plate (PAC), or at the interplate interface (INT), and examined the spatiotemporal distribution of these earthquakes. We also estimated the occurrence timing of SSEs using GNSS data and calculated the lag time between earthquakes and SSEs.
Result and Discussion
PAC earthquakes were distributed widely, while AUS and INT were distributed locally. Many AUS earthquakes in the northern Hikurangi located where extensional areal strain was observed (Dimitrova et al., 2016). We infer that this extension may increase the permeability of the crust, resulting in an increase in fluid-related earthquakes. INT earthquakes occurred at the periphery of the SSE source regions. In and around the SSE source region in the northern Hikurangi, AUS earthquakes were active after SSEs, INT earthquakes were active during SSEs, and PAC earthquakes were slightly active before and during SSEs, which is the same as observed by Tateiwa et al. (2022, SSJ) who used larger earthquakes with magnitudes greater than three. The fluid movement may explain this characteristic of different periods of activation depending on the location of the earthquake. That is, fluid movement from the oceanic crust to the plate boundary or upper plate, as proposed by Nishikawa et al. (2021), may have triggered SSEs, tremors, and AUS or INT earthquakes. In and around the SSE source region in the central Hikurangi, AUS, INT, and PAC are active during SSEs, which is different from the northern Hikurangi. It indicates that the mechanism of SSEs and related earthquakes may also be different from that in the northern Hikurangi.