06/02/2019 16:09

Russian experts’ proposals are submitted to the resolution of Intergovernmental Oceanographic Commission of UNESCO

Category: Общие, Научная жизнь

At the beginning of April 2019 the Twenty-eighth Session of the Intergovernmental Coordination Group for the Pacific Tsunami Warning and Mitigation System (ICG/PTWS-XXVIII) was held in Montelimar (Nicaragua) at which the state and progress of the tsunami warning systems and response measures in the Pacific Ocean were discussed together with interaction coordination issues between the Pacific Coast countries.

The Russian delegation was headed by Vyacheslav M. Shershakov, General Director of the Research and Production Association “Typhoon”. As an expert in the field of satellite geodesy and geodynamics, the chief researcher of the Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences (IPE RAS), Professor of RAS Grigory M. Steblov was included in the delegation of Ministry of Science and Higher Education of the Russian Federation. During the session, he presented a report on Russian theoretical and practical studies on the effective use of GNSS in the near-field zones of the mega-thrust undersea earthquake, in which he offered practical recommendations for these systems implementation to improve the global tsunami warning system, urgent decision-making and alerting.

"Conventionally, there are two large groups of issues discussed: the first group - scientific issues on the monitoring systems and early estimation of the tsunami source parameters, and the second one – organizational issues on response measures after identifying tsunamigenic events (evacuation schemes and maps, public alarm systems and others). As an expert, I participated in the discussion of the first group of issues on monitoring systems and source identification," - Grigory Steblov comments on his report. – “We have reliable estimates how quick and accurate the source parameters can be resolved from the coastal GNSS observations, and that is a critical issue of reliable determination of the tsunami source in order to apply hydrodynamic models afterwards. We’ve shown how it can be calculated – really in a few minutes. We’ve made practical recommendations on how to implement such systems and what determines the delay. The issue of quick response is critical, so it is important to identify what the delay in decision-making consist on, and to have its quantitative characteristics.”

G. Steblov’s presentation

The current systems of monitoring and determination of the tsunami source are based mainly on the extensive, fairly dense seismic monitoring network around the globe, and also the systems include networks of the bottom pressure sensors, which record the passage of tsunami waves, and tide gauges.

The disadvantages of these systems are in recording the passage of the tsunami wave when it has already arrived. Tsunami warning can be made earlier from seismic monitoring, because the seismic wave comes much earlier than the tsunami wave. But the seismic system has also its own limitations: according to modern calculations the reliable tsunami-risk assessment takes about 20 minutes, because in the near-field zone the seismographs are saturated, so the strong motion sensors are needed. To estimate the seismic source parameters of the tsunamigenic event from seismic data, sufficient teleseismic observations should be acquired, that prevents reduction of the evaluation time.

A comprehensive solution to this problem is found by using satellite geodetic observation based on Global Navigation Satellite Systems (GNSS). They are very good in the near-field zone and they are not subject to magnitude saturation giving a good reliable assessment of the earthquake mechanism in 3-5 minutes, that is 30-40 minutes ahead of the tsunami wave arrival. This is the rapidness and accuracy that is required to trigger the warning and to launch the already worked-out response measures.

This issue, raised in G. Steblov’s report, was actively discussed at the session, mainly by American delegation members. "My report compared our approach to the American," - explains Grigory Steblov. – "For the focal parameter estimation they use empirical relations between GNSS-observed displacements and the source slip distribution. We use physical modeling, more accurate and adequate, it allows us to invert the observable motions on the earth's surface in the coastal zone and on the Islands, where the GNSS stations can be installed, into those source slip pattern, from which the energy and dynamic characteristics of tsunami sources are calculated.

The time required for taking decision and alerting consists of three main components: the signal travel time, the delivery time of the measurements to the processing center and the processing time of the measurements.

All three components can be reduced by the following means: the signal travel time - by network densification and installation of the tracking sites closer to the expected seismic events; the delivery time of measurements - by real time data flow enabled; processing time – by pre-calculating the response functions which are the most time consuming operations. The theory of dislocations in an elastic medium allows us to relate the source slip distribution with the surface displacements. First, the inverse problem is solved: the source slip is searched from the surface displacement. As soon as the source slip distribution is found, we solve the direct problem: we calculate the seafloor uplift at the epicenter and the continental slope horizontal motion with its speed. These are the two main components of the energy of the tsunami source.

If this is done upon the data arrival, the time for assessing the focal parameters from GNSS measurements is reduced to 3-5 minutes.

Next, we proposed practical steps required for integration of a GNSS based augmentation to the global Tsunami Warning System. It means the development of regional GNSS systems with enough dense coverage of the coastal zones exposed to tsunamis. It also supposes the development and organization of real time data flow channels to the processing center, the organization of the processing centers themselves, the development of algorithms for inversion of these measurements into the focal parameters of the potentially tsunamigenic earthquakes.

All these statements were actively supported by the appropriate professionals, and were submitted to the resolution of the session in the form of recommendations to the Member States of the Intergovernmental Coordination Group on Cooperation in GNSS." (Excerpt from the resolution)

At the suggestion of Vyacheslav Shershakov, head of Russian delegation, the inclusion of a representative of Schmidt Institute of Physics of the Earth RAS Professor Grigory Steblov into the WG2 Task Team was agreed. He joined the WG2 Task Team: Integrated PTWS Sensor Networks for Tsunami Detection and Characterization.

It should be noted that the system proposed by our scientists was not tested in Russia in real time, because (according to G. Steblov) "we do not function as a facility. The question was discussed sometimes ago – could we implement the proposed algorithms as a facility, not limited to scientific research, but that was mostly the organizational and financial issues." However, retrospective comparisons show good compliance with refined seismic estimates on the scalar seismic moment and the source slip distribution. We can obtain the coseismic displacement estimation in 3-5 minutes (after the Rayleigh wave arrival), not in 25-30 minutes or in a few days, when the finalized data comes, as it happens in current practice. Also, system testing shows adequate response to false alarms (for example, the GNSS did not classify the Okhotsk Earthquake of 2013 as a tsunamigenic). The advantage of using GNSS data and their analysis results would be improved tsunami impact forecasts for the vulnerable coastlines, with the possibility of many lives saved, particularly in the near-field.


Related Links:

Предложения российских экспертов внесены в резолюцию Межправительственной Океанографической Комиссии ЮНЕСКО (this news in russian)

Twenty-eighth Session of the Intergovernmental Coordination Group for the Pacific Tsunami Warning and Mitigation System

ICG/PTWS Adopted Recommendations

UNESCO Intergovernmental Oceanographic Commission 

Russian marine research