English (UK) 
简体中文(中国) In a blog post by Google Global Networking experts Valey Kamalov and Mattia Cantono on Friday, Google unveiled a recent experiment using its subsea cables to detect earthquakes and tsunamis. Google has developed a technique which is capable of dectecting earthquakes and tsunamis over tens of thousands of kilometers away, using current in-service subsea cables and equipment that is present on the vast majority of the world’s existing subsea cable systems.
Google's CEO Sundar Pichai also twittered this exciting achievements.
There’s a long tradition of using optical fiber for sensing applications.
For example, French scientists published the research and experimental results of using submarine optical cables as seismic sensors in the journal Nature Communications in December 2019. French scientists used a submarine cable of 41 kmilometers near the coast of Toulon in southern France to conduct experiments. The 41-kilometer submarine cable was set up as thousands of seismic sensors and successfully detected a magnitude 1.9 earthquake occured over more than 100 kilometers from the submarine cable. And it is reported that the sensitivity of seismic sensors from the submarine cable is quite close to that of coastal seismic stations. It claimed that the submarine cable has become enable people to observe for the first time how the submarine vibrations are generated. These vibrations continuously interact with the earth, allowing geophysicists to detect the structure of the earth. Researchers believe that submarine optical cables can be used in the same way to detect underwater noise produced by ships and cetaceans.
Compared with above research by French scientists, Google's experiment and technique has taken a big step forward.
Most of traditional techniques are effective over distances of up to 100 km, Google has developed a technique that works over tens of thousands of kilometers. And while previous approaches require special sensing fiber and special equipment, Google uses existing fiber to detect disturbances on the seafloor.
Further more, Google's technique relies on equipment that is present on the vast majority of the world’s existing fiber optic systems, so it is broadly applicable.
At present, Google has invested in of a total of 14 international submarine optical cable systems worldwide. The total length of Google's submarine cables is about 120,000 kilometers, which accounts for about 10% of the total length of 1.1 million kilometers of global submarine cable networks.
According to Google news, Google detected earthquakes by using changes in the state of polarization (SOP) produced by optical pulse signals transmitted on the submarine optical fiber. The movement of the seabed caused by the earthquake produces mechanical interference and physical deformation of the optical fiber in the submarine optical cable, and the deformation of the optical fiber causes changes on the state of polarization (SOP) for the optical pulse signal transmitted therein. By monitoring and tracking these changes on the state of polarization seismic activity can be dected.
As early as 2013, Google researchers began to use the terrestrial cable system to conduct research and analyze the SOP changes monitored on the terrestrial cable. However, the environmental factors around the terrestrial cable caused too disruption to detect seismic signatures, so the study was suspended. In 2018, inspired by a paper published in Science, Google researchers began to restudy how to use submarine cables for seismic detection.
In October 2019, Google researchers got an idea to detect earthquakes based on spectral signatures—performing a spectral analysis of Stokes parameters to look at frequencies that are typical of earthquakes.. The Stokes parameters are ones of the important parameters describing the characteristics of the SOP.
At the end of 2019, Google researchers began to monitor the state of polarization (SOP) of some of its submarine cables around the world. In the initial field test, Google researchers observed that the state of polarization (SOP) was very stable after the optical signal was transmitted through a submarine cable system about 10,500 kilometers long. It shows that the seabed was in a quiet state during this period, that is, there was no seismic activity.
On January 28, 2020, Google researchers detected a magnitude 7.7 earthquake that occurred near Jamaica, 1500 km away from the closest point of one of Google's submarine cables used for the experiment.
In the months after the Jamaica earthquake, Google researchers also monitored multiple moderate-strength earthquakes, some of which were closer, and some were even longer.
On March 22, 2020, Google researchers detected a magnitude 6.1 earthquake that occurred in the East Pacific Rise, 2,000 kilometers away from the subsea cable used for the experiment.
On March 28, Google researchers monitored a 4.5-magnitude earthquake that occurred near the coast of the Chilean port city of Valparaiso. The source of the earthquake was only 30 kilometers away from the detecting subsea cable. According to information from SubmarineNetworks.com, Google solely invested the Curie cable system connecting Los Angeles, USA to Valparaiso, Chile, with a total length of about 10,500 kilometers, and landed in Valparaiso, Chile. The Curie cable system is the first private international submarine cable system solely funded by Google. No traditional telecom operator has participated in the investment and construction of the Curie cable system. The Curie cable system was put into operation in November 2019. According to the information disclosed by Google, it can be judged that Google used the Curie cable system to detect the earthquake.
Subsequently, Google and California Institute of Technology conducted joint research. The research shows that Google's technique can not only detect earthquakes caused by tectonic plates, but also detect pressure changes in the ocean itself, which could help predict tsunamis.
Today, most tsunami warning equipment is either on shore or scattered in the ocean. The former cannot give coastal communities enough time to evacuate, while the latter is limited by the speed of traveling waves-the maximum speed of deep sea waves is 800 kilometers per hour, and it takes a long time for the early warning signal to reach the monitoring equipment. On the contrary, when the tsunami warning signal is detected through the submarine cable laid on the seabed near the earthquake epicenter, the warning signal will be transmitted to the monitoring center within a few milliseconds at the speed of light.
In this way, Google's achievement is of great importance.
Although Google’s research and application are only in the early stages, if the more than 1.1 million submarine optical cables that have been laid around the world can be used, it is possible to form a huge and economical global submarine earthquake and tsunami monitoring network, which is a dedicated seismic sensor And the monitoring network provides supplementary information sources to improve the early warning of global earthquakes and tsunamis.
