# A Survey on Unmanned Surface Vehicles for Disaster Robotics: Main Challenges and Directions
## 3 USVs for Natural Disasters
### Earthquakes & Tsunamis  
Earthquakes are sudden and shivering tremors that shake the surface of the Earth, with varying intensity and duration. The most common cause of earthquakes is the underground shock of tectonic plates. When earthquakes happen offshore, depending on their intensity, they can cause tsunamis. Characterized by the displacement of large bodies of water, tsunamis can be generated whenever the seafloor undergoes sudden deformations and create vertical displacement of the body of water. Tsunamis and earthquakes are extreme events that can cause vast destruction and damage miles away from their focus. Therefore, early detection is essential for evacuation actions and to reduce the number of fatalities. Tsunami and offshore earthquakes can be detected by monitoring seafloor conditions such as pressure created by crustal friction and off-beat water surface height.  
For tsunamis and earthquakes detection, studies present the usage of USVs with GPS/Acoustic (GPS/A) devices, seafloor pressure sensors, seismometers, and hydrophones. [PERSON] et al. [53; 54] use a USV as a central base of a system for detection of crustal deformation. The system is composed of a GPS/A transducer attached to the USV in the sea surface, a seafloor pressure sensor, wired to the USV, and six GPS/A transponders in the seafloor. [PERSON] et al. [55] present the progress of a four-year project of the Tohoku University. Together with Nagoya University and the Japan Coast Guard, they use a USV as a survey station for crustal deformation detection. The USV is equipped with a GPS/A device for collection of seafloor acoustic transponder measurements around the Japanese Islands.  
[PERSON] et al. [56] use a USV (named MERMAID) equipped with a hydrophone for seismic monitoring. The hydrophone detects acoustic signals generated by seismic waves from Earth's interior. The MERMAID USV was launched near the Indian Ocean triple junction on the 24 th of November 2013 and detected an earthquake swarm that was followed by the main event of magnitude 5.1.  
[PERSON] et al. [57; 58] use a Wave Glider USV as an intermediate station for data transmission between a seismometer on the seafloor and a land station. [PERSON] et al. [59] discuss the usage of Wave Glider as USV for retransmission of seismic data collected from the seafloor DART (Deep-ocean Assessment and Reporting of Tsunami) network through GPS/A. The DART network is maintained by the US National Oceanic and Atmospheric Administration (NOAA). It has 39 DART monitoring stations in its network. Each DART station consists of a seafloor bottom pressure recorder (BPR) with a surface buoy anchored next to it. An acoustic link transmits data and commands between the buoy and the BPR, which collects pressure readings.  
Another way to detect tsunamis is through estimation and comparison of sea-surface height. [PERSON] et al. [60] present a Wave Glider equipped with a geodetic GPS and uses Precise Point Positioning (PPP) for determination of water surface heights in Loch Ness, with a precision of around 0.05 m. PPP uses real-time satellite signals to derive orbital and clock and can be a tool to measure tsunami wave height [54].