## 5 Conclusions  
This study investigated the optimal deployment region for offshore tsunami observation instruments in order to reduce coastal hazards for the east coast of Korea in terms of early  
Figure 11: Optimal area for offshore tsunami observation instrument deployment. (**a**) Ocean bottom slope for the higher probability area in the eastern sea of Korea (the black line is the boundary area); (**b**) The product of MLE and probability of tsunami travel time within 1.5 after the earthquake over a flat ocean bottom. The MLE results for 0.1 m maximum tsunami wave height.  
warning. This study hypothesized that the optimal region of offshore instrument deployment for tsunami detection could be determined for far-field tsunami cases based on numerical simulation (COMCOT) by adopting the approaches of previous studies (water depth, warning time, and bottom slope) for near-field tsunami cases [3; 19; 21; 22]. Plus, various tsunami propagations were calculated by numerical modeling (COMCOT) with 39 potential tsunami scenarios. The results were applied to the approaches of previous studies to identify optimal deployment for offshore tsunami observation instruments [3; 9; 21; 22]. The approach in this study considered the higher probability of tsunami detection, frequentist method, with the minimum instrument and the maximum warning time, and bottom slope according to the current situation in Korea, far-field tsunami occurrences. The frequentist method, i.e., MLE, instead of EOF, which was used in the previous studies, was adopted to determine the cost-effectiveness in a large number of tsunami scenario cases. The bottom slope could be considered when bottom-mounted sensors are installed for tsunami detection. Three major steps for the approach are listed below;  
1. Define a high probability area for tsunami detection using numerical modeling based on 39 potential tsunami scenarios.
2. Define the probability area of maximum warning time by considering tsunami travel time, the delay time of detection, data transmission time, and measurement confirmation time.
3. Specify the probability map for the optimal region of offshore tsunami instrument by the product of the probability of tsunami detection and warning time.
4. Consider the bottom slope required for tsunami instrument installation.  
We conclude that the northeast area around Ulleung-do island is the optimal region for installing offshore tsunami observation instruments. The approach in this study successfully identified the optimal area for instrumentation in the case that the tsunami source is far from the territorial sea. This study provides the framework for the guidance of offshore tsunami observation instrument installation in the Korea tsunami warning system.  
**Author Contributions:** Introduction, E.L.; data curation, E.L.; the result, E.L. and T.J.; analysis of results, E.L. and S.S.; writing--original draft preparation, E.L.; writing--review and editing, S.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by Korea Meteorological Institute, grant number KMI2018-02510 and was supported by the Brain Korea 21 plus program through the National Research Foundation (NRF) funded by the Ministry of Education of Korea.  
**Conflicts of Interest:** The authors declare no conflict of interest.