11:30 AM - 11:45 AM
[SCG39-28] Seismic characteristics of the northern Hikurangi subduction zone revealed by offshore seismic observations
Keywords:Slow slip events, Fluid transfer, High Vp/Vs bodies
Since 2012, we conducted marine seismic observations in the northern part of the Hikurangi subduction zone, where slow slip events have occurred regularly at an interval of 1.5 ~ 2 years. Owing to the shallow subduction of the Pacific Plate beneath the Australian Plate, the structural features of the plate interface have been well identified from seismic reflection sections and magnetic anomaly maps. Such structural features include subducted seamounts and high seismic reflectivity of the plate interface attributable to an abundance of fluid (Bell et al., 2010; Barker et al., 2018).
We started offshore seismic observations by deploying four ocean bottom seismometers (OBSs) for a year-long observation from April 2012 to March 2013. We successfully determined hypocenters of mostly offshore earthquakes and subsurface seismic velocity structures down to ~30 km depth and imaged detailed velocity variations (Mochizuki et al., 2021). We found that the seismicity is mainly distributed around a core of high Vp/Vs bodies within the subducted oceanic crust of ~12 km thickness and we interpreted those high Vp/Vs values representing abundance of fluid that has been produced by dehydration reactions during subduction of the oceanic crust. The seismically highly reflective plate interface is found to be located above such high Vp/Vs bodies, which is consistent with our interpretation. Therefore, the seismicity around the high Vp/Vs bodies coincides with the peripheries of the highly reflective plate interface.
During a 2014-15 observation period and through international collaboration, we captured for the first time offshore tectonic tremor activity accompanying a large SSE (Wallace et al., 2016; Todd et al., 2018). The tremors appear to be activated near the end of the SSE and continued for about two weeks. The envelope correlation method revealed that a total of 120 tremors were only distributed around one of the subducted seamounts.
Another deployment was undertaken between 2018 and 2019, during which a large SSE occurred in the same region as the 2014 SSE event (Woods et al., 2020). We also observed tremor activity over the subducted seamount, and it showed the same temporal relationship with the SSE occurrence as the 2014 activity (Yamashita et al., 2020).
These observations strongly suggest the involvement of cyclic fluid transfer around the plate interface in concordance with the occurrences of SSE and tremors. Indeed, the spatio-temoral distribution of focal mechanisms as well as changing seismic velocity and anisotropy, suggests the systematic changes of fluid pressure within the subducting oceanic crust (Warren-Smith et al., 2019; Zal et al., 2020; Wang et al., in preparation). Therefore, the fluid production rate within the subducting oceanic crust is possibly a major factor that controls the seismic behavior at the northern Hikurangi margin. Rolling ocean bottom seismograph deployments are improving our ability to evaluate seismic potential, by accounting for the influence of slow slip events.
We started offshore seismic observations by deploying four ocean bottom seismometers (OBSs) for a year-long observation from April 2012 to March 2013. We successfully determined hypocenters of mostly offshore earthquakes and subsurface seismic velocity structures down to ~30 km depth and imaged detailed velocity variations (Mochizuki et al., 2021). We found that the seismicity is mainly distributed around a core of high Vp/Vs bodies within the subducted oceanic crust of ~12 km thickness and we interpreted those high Vp/Vs values representing abundance of fluid that has been produced by dehydration reactions during subduction of the oceanic crust. The seismically highly reflective plate interface is found to be located above such high Vp/Vs bodies, which is consistent with our interpretation. Therefore, the seismicity around the high Vp/Vs bodies coincides with the peripheries of the highly reflective plate interface.
During a 2014-15 observation period and through international collaboration, we captured for the first time offshore tectonic tremor activity accompanying a large SSE (Wallace et al., 2016; Todd et al., 2018). The tremors appear to be activated near the end of the SSE and continued for about two weeks. The envelope correlation method revealed that a total of 120 tremors were only distributed around one of the subducted seamounts.
Another deployment was undertaken between 2018 and 2019, during which a large SSE occurred in the same region as the 2014 SSE event (Woods et al., 2020). We also observed tremor activity over the subducted seamount, and it showed the same temporal relationship with the SSE occurrence as the 2014 activity (Yamashita et al., 2020).
These observations strongly suggest the involvement of cyclic fluid transfer around the plate interface in concordance with the occurrences of SSE and tremors. Indeed, the spatio-temoral distribution of focal mechanisms as well as changing seismic velocity and anisotropy, suggests the systematic changes of fluid pressure within the subducting oceanic crust (Warren-Smith et al., 2019; Zal et al., 2020; Wang et al., in preparation). Therefore, the fluid production rate within the subducting oceanic crust is possibly a major factor that controls the seismic behavior at the northern Hikurangi margin. Rolling ocean bottom seismograph deployments are improving our ability to evaluate seismic potential, by accounting for the influence of slow slip events.