Investigations on Vertical Crustal Movements in the Venezuelan JL148-8 by Gravimetric Methods Hermann Drewes Escuela de Ingenieria Geodesica, Universidad de , 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. So we however, a greater set of data in different may not connect the present net to the epochs is necessary. The repetitions of gravity values, of the RGNV 1970. gravity observations should therefore be 161 TABLE 1. ' Point Values arid Errors of the .Andes Gravity Network Station Gravity r.m-. s .e. r.m.s ,e . Station . Gravity r.m.s.e. r ,.m.s..e . Name .Value . Free ' : National V •V Name Value . . 'Free National (mgal) Net ' Net (mgal)' Net ' Net Chinita- • 175.053 0.006 ' 0.010 Chachopo -439.728 0.009 0.030 Pto.Cab. 229.299 0.008 0.012 Mucubaji -621.586 0.008 0.039 Moron 220.079 0.009 0.01.3 Las Pie. -237.251 0.008 0.020 La' Pica 196.232 0.009 0.013 Mucuruba -417.334 0.008 0.028 S. Felipe 1-33.279 0.007 0.009 M6rida -248.612 0.010 0.018 . Chivacoa 98.002 0.009 0.011 Lagunil. -162.673 0.009 0.016 Yaritag. 74 . 304 0.009 0.011 S.Cruz -69.474 0.009 0.013 Barquis. 39.610 0.007 0.008 LaPlaya -184.742 0.009 0.017 S.Pablo 19.210 0.009 0.011 LaGrita -223.115 0.010 0.018 Pte.Tor. 86.170 0.009 0.011 te . Aura -347.861 0.010 0.024 Carora 85.950 0.007 0.009 S.Crist. -130.516 0.007 0.014 Sicarig. 63.917 0.009 0.011 S.Anton. -57.097 0.009 0.011 El Empe. 28.146 0.009 0.011 Valencia 62.797 0.009 0.011 Valerita 69.827 0.008 0.010 65.583 0.009 0.011 Mendoza 32.231 0.008 0.010 Tinaqui. 90.555 0.009 0.011 Cajar Se. 9.918 0.010 ' 0.012 S.Carlos 121.069 0.009 0.011 ElVigia -3.727 0.010 0.012 S.Rafael 120.028 0.009 0.011 Cano Am. 4.028 0.010 0.012 Acarigua 92.644 0.007 0.009 La Pria 10.244 0.009 0.012 Ospino 88.145 0.009 0.011 Colon -92.116 0.007 0.013 Guanare 79.762 0.005 0.008 Quibor 12.282 0.009 0.012 Bo cono i. 67.531 0.009 0.011 El Tocu. 26.332 0.009 0.011 66.329 0.006 0.009 Guarico -70.098 0.009 0.014 Barinit . 5.714 0.008 0.011 Biscucuy 28.235 0.007 0.010 Bolivia 44.105 0.009 0.011 Cimarro . 56.06.9 0.009 0.011 Socopo 15.743 0.009 0.011 Bo cono -118.134 0.007 0.014 Capitan. 31.803 0.009 0.011 Sta.Ana -192.994 0.009 0.018 S.Barba. 35.110 0.007 0.010 Valera* -20.503 0.007 0.010 Abe 3 ales 24.822 0.009 0.011 La Puer. -264.184 0.008 0.021 ElPinal -30.645 0 .009 0.012 Gravity Value (IGSN'71) of the Reference Station (Chinita) : 978160.12mgal Scale Factor (IGSN'71) of the Reference Gravimeter (401) : 1.0005*0.0001

GRAVITY VARIATION

* ! S1/78~ 99/77

t 9 8/78 ~9 1/78

120^ 500m

Fig. 3. Gravity Variations in the Gravimetric Network Mucubajl

162 done in a short time interval. TABLE 2. Local Gravity Network Mucubaji 6. Conclusions Station Gravity (/^gal, free net) Name Sept. 77 Jan. 78 Aug. 78 From the analysis of the presentednet- work we learn two principal requisites for Mucu 9 -2676 ± 10 -2679 ± 4 -2678 + 7 the gravimetric control in geodynamics: I.iucu 10 -1226 ± 10 -1226 ± 4 -1222 ± 6 a) The internal precision of a network is Mucu 11 2671 ± 9 2673 ± 3 2675 ± 5 often superposed by a greater uncer- Mucu 12 7474 ± 11 7480 ± 4 7478 ± 7 tainty due to the insufficient deter- Mucu 13 -3192 ± 11 -3171 + 4 -3200 ± 7 mination of the scale factor. So, if Mucu 14 178 ± 14 190 ± 4 152 ± 7 we repeat the observations with another Mucu 15 1787 ± 14 1790 ± 4 1790 ± 7 gravimeter, or if the scale factor of Mucu 16 -10370 ± 9 -10405 ± 4 -10408 i 7 one gravimeter changes with time, we cannot compare the different epochs. The scale factor should therefore be fieferences determined externally with the same (or less) uncertainty as the determi- Drewes, H., Regional Subsidence of the nation of point values: Lake of L-aracaibo as Determined by Re- peated Gravimetric Measurements, Paper pres.to the 8th meeting of the Intern. (3) Gravity Comm., Paris 1978. Drewes, K., Zur Ausgleichung von Gravi- (my= r.m.s.e. of scale factor, mp= a/v meternetzen , Z. f. Vennessunfiswesen. r.m.s.e. of point values in a free net 103.103. , 485-496, 1978". adjustment, rg=gravity range of net) SchubSchuberte , C., and H. G. Henneberg , Geo- Observed gravity variations are often logical and Geodetic Investigations on superposed by local effects. To elimi- the Movements along the Bocon<5 Fault, nate these disturbances we need short Venezuelan Andes, Tectonophysics , 29, period repetitions. Precise gravity 199-207, 1975. networks for geodynamics should there- Schubert, C., and R. S. Sifontes, Bocon<5 fore be reobserved at least as soon as Fault, Venezuelan Andes : Evidence of variations are observable, i.e. the Postglacial Movement, Science, 170, 66- gravity changes increase to the order 69, 1970. . of the uncertainty of determination: Torge, W., and H. Drewes, Gravity Vari- ation with Time in Northern Iceland 1965-1975, J. Geophysics. 4_3_, 771-790, AT £ -r±- Ag (4) 1977. Wenzel, H. G., Zur Genauigkeit von gravi- (AT=time interval of observation, Ag= metrischen Erdgezeitenbeobachtungen , supposed or actual gravity variation) Wiss. Arb. Lehrst. Geod., Phot. u. Kart. Techn. Univ. Hannover, Nr. 67, 1976. Acknowledgement. The presented project is part of the joint investigation of the "Escuelade Ingenierla Geod6sica, Univer- sidad del Zulia" , Maracaibo/Venezuela and the "Direccidn de Cartografla Nacional de Venezuela", , in cooperation with the "Institut fur Theoretische Geodasie der Universitat Hannover", Germany. The computations were performed at the " Centre de Calculo Electr<5nico de la Universidad del Zulia" . The author wants to thank all concerned persons.

163