Investigations on Vertical Crustal Movements in the Venezuelan JL148-8 Andes by Gravimetric Methods Hermann Drewes Escuela de Ingenieria Geodesica, Universidad de Zulia Maracaibo, Venezuela Abstract. A precise gravimetric net- supposed movements in the Venezuelan Andes. work has been installed in the Venezuelan Andes to study eventual gravity changes due to vertical tectonic movements. The 2. Regional gravimetric network design and the measurements of the net- work are described and the accuracy is i'he precise gravimetric network con- estimated. sists of 58 stations and covers all the In the center of the region a local Venezuelan Andes in a length of about 600 gravity network has been reobserved three km between the Colombian border (3an .An- times. The detected variations are dis- tonio) and the Caribbean 3ea (Puerto Ca- cussed. bello).lthas a width of about 100 km snd In order to obtain a genuine state- is formed as it were three parallel pro- ment as far as possible about the signi- files, one situated in the center of the ficance of observed gravity changes, re- Andes and one and one on each side of the quirements for the procedure of monitor- mountains in the lowlands. One reference ing precise gravity networks are pointed station is situated far off the network out. in. Maracaibo (fig. 2). The points are in general BlUitoi of the 1. Introduction first order levelling net of Venezuela, that are concrete monuments with a 1m - The tectonic plate boundary between foundation. Some stations are situated on the Caribbean and the southern part of foundation walls of churches. The net in- the American plate crosses Northwestern cludes 11 stations of the National Gravity Venezuela following the course of the Network of Venezuela. The total range of Venezuelan Andes (fig. 1). Horizontal gravity is 0.85 cm«s~2. movements in this area have been detected In two sites of the investigation area, by geological methods along the Second in Maracaibo and near r.Ce'rida, earth tides Fault (Schubert and Sifontes, 1970), ver- are recorded with an equipment of the tical movements being supposed in connec- Institute of Theoretical Geodesy of the tion with the Andes' uplift. University of Hannover/ Germany (laCoste As the total length of the considered and Romberg model G gravimeter, chart- zone is about 600 km with elevations from recorder) in order to obtain actual para- 100m to 4000m, it would hardly be poss- meters for the earth tide reduction. ible to control vertical movements by use of levelling methods in a short time in- 3. Gravimetric measurements terval. Gravimetric observations, however, are capable of detecting height changes The first gravimetric survey of the net at reobserved points because of the depen- was carried out in February /March 1978 dence of gravity on elevation. This method using two LaCoste and Romberg model G has been used in several extended tectonic gravimeters (no. 401 andno. 405). Totally active regions (e.g. Torge and Drewes, there were performed 280 observations, 1977). each of those, being the mean of three in- dependent readings at one station. The The problem is the conversion of grav- instruments were carried by a station wa- ity variations into height variations, gon. which only can be done by knowledge of the actual vertical gravity gradient along The sequence of point observations had the path of the moving masses. The deter- been planned before by anet optimisation, mination of this value is somewhat pro- the target function for a free net adjust- blematic, but we can measure the local ment being free air gradient as a rough estimation. 10«10~8 m.s~2, In any case we obtain at least a qualita- m (1) tive estimation of vertical movements. (m'p= a/v r.m.s.e. of point gravity values) In conjunction with the present gravity The advantage of the gravimetric sur- network, another net around the Lake of vey is the easy and rapid performance and Karacaibo was observed. This net was in- the almost- invariable accuracy in respect stalled to detect gravity changes in the to the distance betv/een the points. There- oilfields near the. lake, which are due to fore the gravimetric methodwas chosen for the extraction of petroleum and related monitoring the vertical component of the subsidences (Drewes, 1978). By means of the direct connection, a fusion of both nets is possible, covering in this manner Proc. of the 9th GEOP Conference, ^4/j International Symposium on the Applications of • Ceodesv to Gemlvnamirs, October 2-5.1978. Dept. of Geodetic Science Kept. No. 280. The a large-region of the tectonic "Maracaibo- Ohio State Univ.. Columbus. Ohio 43210. -Block". 159 Pig. 1. Global Situation of the Precise Gravimetric Network LAKE OF MARACAIBO LEGEND: STATION OF THE NETWORK STATION OF THE NATIONAL NET USED ROAD CONNECTIONS 20 10 60 60 »• KM Fig.2. PRECISE GRAVIMETRIC NETWORK IN THE VENEZUELAN ANDES 160 4. Evaluation and results The main resulting crux is the determina- tion of the actual scale factor of the After transformation of the readings instruments. The uncertainty of a roughly into approximate mgal-scale (1 mgal= 10"3 determined scale factor from the RGNV is m-s~2) by means of the manufacturer' s tables, and the reduction because of earth m^ = ± 6 .. 7 • 1C"5. tides by a modified Cartwright-Edden pro- cedure with 505 partial tides (V/enzel, Related to the total range of the Andes' 1576), separate free net adjustments for gravity network this oproduces an uncer- each instrument have been calculated.The tainty of 50 .. 60-10-°m.s-2. This exceeds mathematical model for the adjustment of by far the internal precision of the net. parameters is (Drewes, 1978-', formula 14): The r.m.s.e. of the connected point values are also given in table 1. v. = fi± - L^ - r.P± - D.t± I P± (2) 5. Local network Mucubaji (gi=gravity value of station i, Lk=gravity level of instrument at the begin of period In the center of the Andes at Mucubaji k corresponding to readingO.OOO, £=scale a small gravity network was installed to factor, D= drift coefficient, ri=trans- study local variations. The net is situ- formed and reduced reading, ti=time since ated, on both sides of the Bocond Fault, begin of periodk, pi=weight of the obser- which in this region is defined by geolo- vation). gists within ±100 m uncertainty of posi- As a result we obtain the r.m.s.e. of tion, and it is identical with a geode- unit weight for both gravimeters: tic network of horizontal control (Schu- bert and Henneberg, 1975).- Totally there 8 2 m0(401) = ± 13-10~ m.s~ are eight stations, all concrete monument 8 2 with an Imximxim foundation. mQ(405) = ± l3-10~ m.s~ . The mean topographic height of the net is about 3500 m, the mean gravity value As there is no difference between the 9.77360 m-s-2.The total gravity range is precision of the two-instruments, all ob- 18-10-5 m«s-2. Because of this small diff- servations were introduced with an unit erence in gravity the above mentioned pro- weight p=l to a common adjustment, which blem of scale determination does not affect also was calculated as a free net. There- this network. by the instrument no. 401 was fixed in its The first gravimetric observation took scale (Y=l). As a resulting r.m.s.e. of place in September 1977, the second in unit weight (now including also discrepan- January 1978 and. the third in August 1978 . cies between the gravimeters) we obtain The same principle of readings and evalu- 8 2 ation was. used as described in sec. 3 and mQ = ± 15.10~ m-s~ . 4. The results of the free net adjustment are given in table 2, the relative gravity The average root mean square error of variations corresponding to the different the point gravity values is periods are shown in fig. 3- 8 2 In addition to the measurements of the mp '= ± 9-10" m.s~ ,. net, the vertical gradient of gravity has been observed in one reference station. fulfilling the condition (1) of optimisa- The result is tion. The results for the g-values of the adjustment are given in table 1. •jjf = 0.385-10"5 s~2. The connection of the free network with a superior net, e^g. the National Gravity At one side of the fault (north) we Network of Venezuela (-HGWV) - in order to find no significant changes of gravity. get an absolute orientation and scale for The observed variations are always within later comparisons -meets two principal pr£ the r.m.s.e. of determination. In the bleras : southern part, however, gravity changes 1) The RGNV was observed in 1970 and ad- .exceed the threefold r.m.s.e. in several justed within' the Latin American Grav- points. But there is no systematic in it. ity Network (LAGN). In a readjustment, Therefore one should be careful interpre- however, there were found gross errors ting those variations as tectonic move- of observations included in the former ments. A great deal of local influences computation. The old and the new ad- (ground-water etc.) should be considered. justment differ strongly. To filter all the local effects from 2) As the time interval between the mea- regional variations, an analysis of time surement of the RGNV and the present series can be used (e.g.' least squares Andes' Network is 8 years, we cannot prediction filtering). For this reason, suppose constant gravity values.