Detecting short-term slow slip events using six-years of ocean bottom pressure at the Hikurangi subduction zone offshore New Zealand

Over the last decade, ocean bottom pressure gauges (OBPG) have been deployed in several subduction zones to detect seafloor crustal deformation due to tectonic events such as short-term slow slip events (SSE). However, better methods are required to improve detection of SSEs in OBPG data, because pressure changes from oceanographic variations can make it difficult to identify those tectonic signals within the data. One of the common approaches to address this problem is a reference-station method, which utilizes pressure records from a reference station outside zone of crustal deformation under the assumption that the non-tidal components are common-mode over a large region (e.g., Wallace et al., 2016). Fredrickson et al. (2019) and Inoue et al. (2021) recently demonstrated that high coherence in bottom pressure timeseries was observed between sites in similar water depths offshore Cascadia and New Zealand, respectively; this suggests an effective method is to place reference sites at common isobaths to achieve large reductions in oceanographic noise in OBP timeseries. Here, we implement a method for the detection of SSEs with the reference-station method using both GNSS and OBP records for 6 years between 2014 to 2020 using calculations of AIC and fault models. The AIC (Akaike 1974) is evaluated from the site-pair time series records used in Nishimura et al (2013). We then fit a rectangular fault model on the plate interface in an elastic half space (Okada, 1992) for the possible transients detected only on the OBPG records with a grid-search. These calculations result in 18 possible SSEs detected only in OBPG data between May 2014 and Oct. 2020, five of which coincide with transients detected at onshore GNSS stations (e.g., at September 2014; January 2015, February 2017, and March 2019); The 2014 event coincides with the calculated GNSS transients near the SSE reported in Wallace et al. (2016). However, the other ~10 possible events are not clearly observed at onshore GNSS stations. We compare the detected transient signals and fault models with other tectonic events, such as earthquake swarms, repeaters, and tremors. We find that three of these events not seen on GNSS stations coincide with earthquake swarms. These results suggest that SSEs are occurring offshore that are not clearly detected by the onshore GNSS network.