The Preliminary Research Of Earthquake Early Warning Based On Downhole Observations

Rapid calculation of magnitude in Earthquake Early Warning is critical, to which the reseach is related has become heated in the field of Earthquake Engineering. When used in magnitude calculation, seismic recordings must be obtained from basement, while the recordings which are not from basement and those from downhole should be corrected to meet the requirement. This paper makes a preliminary research of Earthquake Early Warning based on downhole observations. The core outcome can be summarized as follows.(1) We can conclude the laws on amplitude, frequency and Signal to noise ratio varying between different depth comparing three component acceleration, velocity and displacement observations obtained from borehole and ground surface. The reason that different parameters vary as the depth goes deeper and each parameter of different components differs can be analyzed from the theory of seismic wave propagation.(2) The magnification times of Surface-to-Borehole concerning three component PGA, PGV and PGD can be used to fit the absorption index and impedence one. Comparing the accuracy of each fitting result and curve, we can discover that the magnification times of Surface-to-Borehole related PGA of the vertical component can best fit the influence indexes, which can more accurately reveal the relationship between soil and the transmission of seismic wave.(3) Comparing the results of P save, S wave on amplitude during their travel upwards, we can find similarities and differences between each wave. We can find the magnification times related PGA of P wave are higher than that of S wave, besides, we can confirm the common rule that the peak of S wave obtained at the surface will be magnified twice.(4) We can roughly detect seismic phase of each borehole or surface recording using STA/LTA, then obtain more accurate result according to Akaika Information Criteria (AIC). We can get equation set using three stations'information and the result of seismic phase detection in order to compute the location of hypocenter.(5) Using the relationship between maximum predominant period or the ratio of PGA to PGV and magnitude of borehole observations and surface, we can get a set of parallel lines. Observing the curves, we can discover the predominant period corresponding to the same magnitude becomes larger as seismic wave travels through deeper soil. If there is no station on the surface, we can only use borehole recordings to estimate magnitude considering predominant period and the influence of soil which is represented by the magnification times of Surface-to-Borehole related PGA simultaneously. That is to say, we can utilize maximum predominant period, the magnification times of PGA to fit magnitude and estimate it according to the fitting relationship at last.(6) Using seismic recordings from surface and downhole stations of KIK-net, we can obtain the relationship between maximum predominant period and magnitude both on surface and downhole. Compared similarities and differences between two expressions, the conclusion which is inferred from (5) can be further testified. Using the magnification times of Surface-to-Borehole related PGA to correct the the relationship between maximum predominant period and magnitude, we can calculate the ultimate magnitude.