Objectives With the development of gravity observation technology, the typhoon tracking research by gravity network composed of gPhone with 1 Hz sampling rate can not only improve the ability of typhoon monitoring, but also distinguish the pre-earthquake anomalous signals related to earthquake hypocenters from typhoon excitation signals, thereby improving the ability of earthquake prediction.
Methods Firstly, the original gravity earth tide data are pre-processed and cross-correlated as the ambient noise cross-correlation function(NCF) matrix from the gravity network in Fujian Province and its adjacent areas. Secondly, by selecting the NCF with signal-to-noise ratio (SNR)≥25 as NCF matrix and calculating the optimal azimuth of the signal source by the normalized background energy flux (NBEF) method, the NCF matrix is sorted by the distance of gravity station pairs and the travel time of the Rayleigh wave cross-correlated signal is confirmed. Assuming that the speed of microseisms is 2.9 km/s in the ocean, and 11 km/s in the land, the normalized value in the location area is calculated by the difference of travel time from a signal source to the gravity station pairs in research region. The minimal normalized value in locating area is the typhoon center or the excited signal source. Meanwhile, the microseisms vertical displacement simulated with the Ardhuin seismic spectra model (ASSM) and the optimal path of MITAG typhoon(2019) has been used to check the reliability of the locating results. Finally, the 7 optimal azimuth estimated strategies have been used for comparison of typhoon orientation, and the influence of the high-speed cross-correlated signal has been discussed to verify the reliability of the location method.
Results When the MITAG typhoon is approaching the gravity network, the locating area can cover the location of the typhoon center. The extreme value region of the microseism vertical displacement simulated by the ASSM model is also consistent with the typhoon path during the period of approaching the gravity network. After eliminating the NCF with high-speed cross-correlated signals, the location area can also cover the typhoon trajectory during the period of turning away from the gravity network.
Conclusions By specifying SNR and distance of gravity station pairs and excluding NCFs with high-speed cross-correlated signal, the Raleigh wave cross-correlated signal travel time locating method can be used to trace the typhoon based on Fujian Province gravity network. The study can provide new data and methods for tracking typhoons by gravity network.
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