1. WHY SUCH A STUDY?
1.1 Efforts of the IAU and IAG since 1991
* Although many astrometric stations (astrolabes, PZT) ceased their activities after 1/1/1988. A recommendation of IAU Commission 19 has been adopted during the XXI IAU General Assembly (1991); IAG endorsed this recommendation. It concerns "Application of optical astrometry time and latitude programs".
The Shanghai Observatory, having established and operating an analysis center for optical Earth rotation data, has been invited to continue to collect optical astrometric data in order to investigate the possibility of deriving long term variations in the deflection of the vertical within the reference frame provided by Hipparcos.
This recommendation involves too that IAG undertakes this project.
After the working of the IAG SSG 5.146: "Processing of Optical Polar Motion Data in View of Plumb Line Variations", Chaired by Professor P.Paquet from 1991 to 1995, the IAG SSG 5.175 was established in the fall of 1995.
* There are also reports of some astronomical observatories from time to time, which locate at seismic areas, on the possible relation between their observed abnormal deflection of the vertical prior seismic events. It has been suggested also to search if deflection of the vertical could be a precursor to seismic events.
1.2 In an historical point of view, the topic of the SSG has long been an interesting problem, but remain unsolved, since the establishment of the IAG and IAU
* In 1924, Dr. Kimura, director of the ILS central bureau, reported in his paper: "A change of vertical amounting to 0.18" must have taken place at Mizusawa", he also related it to the possible "Change in internal distribution of mass underneath the Earth" (Bull. Geod. 1925, p. 535-537).
* At the occasion of IAG General Assembly held at Rome in 1954, Dr.
Cecchini, director of the ILS central bureau, specified the "Variation de la verticale locale" as one of the three possible sources of the latitude changes. Based on his report, the IAG demonstrated again the necessity of the determination of the plumb line variations (Bull. Geod. 1955, p.3).
* In 1961, Dr. Yumi, director of the IPMS, used the term of "Non-polar latitude variation", in which plumb line variation is included (Publi. Int. Lat. Obs. Mizusawa 3, No. 2, 55-120,1961). Since then a numerous of papers have been published by his colleagues, including the two papers recently (Astron. Astrophys. Suppl. Ser.86, 251; Astron. Astrophys. Suppl. Ser. 86,95), in which the possible "variation in the vertical" at the IPMS collaborating stations have been discussed.
1.3 The work of the IAG SSG 5.146 (1991-1995)
It has been stated in the final report of the previous IAG SSG 5.146, "Processing of Optical Polar Motion Data in View of Plumb Line Variations", that "It is quite clear now that there is a component, of which the order is 0.01" at interannual time scales, in latitude residuals which is correlated with the SOI in many cases but with different phase lag at different regions in the world". Further investigation on the interannual variations of the plumb line has been recommended.
1.4 The objectives of the IAG SSG 5.175
The efforts of the IAG SSG 5.175 have been concentrated on the
Following points in the past four years:
* Other than the known tidal variations of the plumb line, are there Non-tidal Plumb Line Variations (PLV) on earth? The SSG intends to have evidences as strong as possible;
* Possible interpretation of the PLV;
* Possible use of the PLV in the studying related to the geoid and the earthquakes.
Under the efforts of the members of the SSG, a series of studies have been carried on, which are now described as the following:
2.1 Non-tidal Plumb Line Variations (PLV) and their interpretation in the case of Lijiang Earthquake
The strongest evidence of the existence of PLV on earth obtained by the SSG is the one finished just recently: "Plumb line variations in Yunnan before the Lijiang Earthquake on Feb.3, 1996".
In west Yunnan of China, there is a gravitational network with the size of 300*300 km**2 for which continuous repeated gravimetric measurements have been performed since 1985. The PLV in the whole region have been calculated recently, with which the PLV before and after the Lijiang Earthquake (M=7.0, at 19h 14m, Feb.3, 1996, location: 100.3 (Longitude); 27.2 (Latitude)) have been studied (see Fig.1 and Fig.3).
It is very clear now that PLV with amplitudes in the order of 0.1" emerged even two months before the event of earthquake (see Fig.1), and then disappeared after the earthquake since contrary PLV happened (see Fig.3). In using these PLV data, it is really interested to see the whole process of the deformation of the geoid around Lijiang before and after the earthquake event. Before the event the geoid was going up (see Fig.2). The highest point of the deformed geoid was very near Lijiang, the place where a earthquake of M=7.0 happened two months later; but almost contrary process happened after the earthquake event, the geoid at the same region was coming down (see Fig.4). Thus, the geoid around Lijiang was restoring to the previous situation.
The relationship between the PLV and the Lijiang earthquake has given us a living example in which the existence of the PLV, of which the amplitudes were in the order of 0.1" and the interpretation of the PLV is very clear now. The PLV amounting to 0.1" in this event are caused by the same causes of an earthquake: the inner changing of the earth.
2.2 Interannual PLV found in a comparison between the gravimetric PLV results and the corresponding astronomical observational residuals obtained at the same place
Non-tidal PLV can also be proved by a comparison between the PLV results, derived by gravimetric techniques, and the observational residuals of astronomical techniques at the same location. The astronomical technique is related to the vertical (plumb line) at the observational location but has nothing to do with the non-tidal PLV until now. Due to the remaining star catalog errors in astronomical observations, the comparison is usually done at interannual time scales. From the comparisons made at the three observatories, which are now described in the following, the interannual variations of the plumb line have been studied:
* Direct evidence: comparison done while the gravimetric PLV results have been calculated exactly at the same site where the astronomical instrument install:
- At W-E direction for the case of Beijing Observatory, China (see Fig.5);
- At S-N direction for the case of Jozefoslaw Observatory, Poland (see Astron. Astrophy. Suppl. Ser. 129, 353-355).
* Indirect evidence: the site where the gravimetric PLV results have been calculated is not far away from the astronomical instrument
- At both W-E and S-N directions for the case of Yunnan Observatory of China (see Fig.6).
Fig.6 Comparison between the PLV (S-N) and PLV (W-E) results of the two techniques at interannual time scales in Kunming
The correlation at interannual time scales is quite evident between the results of the two different techniques. There may be a scale difference between the results of the two techniques. It can be explained by the size of the gravimetric network used in deriving the gravimetric PLV results (see Astron. Astrophy. Suppl. Ser. 129, 353-355).
The only common component existing in both the astronomical residuals and the PLV from the gravimetric technique is the PLV itself at the observatory. From the correlationship, including the level of significance and zero phase lag between the two time series derived from different techniques, the existence of the interannual PLV, of which the amplitudes are in the order of 0.02", and the ability of the two techniques in measuring them can be concluded.
2.3 Other works
There are also some works in the past four years, mainly:
- Study of anomalous atmospheric refraction in astronomical time and latitude observations (Li, Z.X. and Wilson, C.)
- Possible PLV caused by atmosphere (Shen, H.P. et al.)
- Possible relationship of PLV with ENSO
All the other works also support that the PLV, which we are now talking about, can not be caused by the sources other than the one we have already explained in the Section 2.1: the inner changing of the earth.
Thus, it has been suggested that the PLV may be used as a new observational quantity in the studying related to the geoid and earthquake.
(1) Other than the known tidal plumb line variations, there are also detectable Non-tidal Plumb Line Variations (PLV). The amplitudes of the PLV are: 0.02" at interannual time scales; 0.02" - 0.05" at shorter time scales; bigger one are possible when a earthquake is going to take place, for example at least 0.1" in the case of Lijiang Earthquake (M=7.0) on Feb. 3,1996;
(2) Both the gravimetric and astronomical techniques are able to measure the PLV;
(3) Non-tidal variations of plumb line may be used as a new observational quantity in the studies related to the Earth, especially to the deformation of geoid and earthquake.
(1) Establish a new SSG: "Deformation of the geoid and earthquakes" in the Section V (Geodynamics) of the IAG;
(2) Or include this special study into an existing Commission of the IAG but with an appropriate name.
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