Comunicación presentada en el XIV International Symposium on Earth Tides (ETS2000), celebrado en Mizusawa, Japón, entre el 28 de agosto y el 1 de septiembre de 2000. Las actas del congreso han sido publicadas como special issue del Journal of the Geodetic Society of Japan.Analyses of tiltmeter data frorn the Geodynarnics Observatory of Lanzarote (Canary Islands,
Spain) are evaluated in order to investigate ground deforrnations. The tiltrneters are operating in two
locations of the Island, with different sensitivity ranges depending on the site conditions. The results
of tidal analysis show a clear influence of the ocean loading and also rneteorological disturbances. Air
pressure and temperature effects are different depending on type, location and orientation of the
tiltrneter, with different rnagnitude of the effect in the high and low frequency bands.Peer reviewe

Arnoso, José

Cai, Weixin

Jiang, Jun

Tan, Shiling

Venedikov, A. P.

Vieira, Ricardo

Vélez, Emilio

Digital.CSIC

ijliJffll~~¡t, m 47 ~, m 1%(2001), -156 - 462 J{Journal of the Geodetic Society of JapanVol. 47, No. 1, (2001), pp. -156 - -162Monitoring Tidal and Non-tidal Tilt Variations in Lanzarote Island (Spain)José Arnoso!", Ricardo Vieira", Emilio Vélezl), Caí Weixin2), Tan Shiling",Jiang Jun2) and Angel Venedikov"1) Instituto de Astronomía y Geodesia (CSIC-UCM), Madrid. Spain2) Institute of Seismology (China Seismological Bureau), Wuhan. P.R. of China.3) Geophysical Institute, Sofia. Bulgaria(Received September 30,2000, Revised December 14,2000, Accepted December 25,2000)AbstraetAnalyses of tiltmeter data frorn the Geodynarnics Observatory of Lanzarote (Canary Islands,Spain) are evaluated in order to investigate ground deforrnations. The tiltrneters are operating in twolocations of the Island, with different sensitivity ranges depending on the site conditions. The resultsof tidal analysis show a clear influence of the ocean loading and also rneteorological disturbances. Airpressure and temperature effects are different depending on type, location and orientation of thetiltrneter, with different rnagnitude of the effect in the high and low frequency bands.1. IntroductionThe role played by tiltmeter gauges is well known in crustal structure and grounddeformation studies (Edge et al., 1981; Mentes et al., 1999). However, continuous observationswith tiltmeters are strongly influenced by local and regional effects, especially the long perioddata (Agnew, 1986). Furthermore, continuous recording with tiltmeters is very sensitive to localatmospheric perturbations such as air pressure and temperature variations, as well as to theinverse barometric effect if the station is close to the ocean. In consequence, it is necessary tocorrect such disturbances in order to investigate ground deformations praduced by other naturalphenomena.In our case, the tiltmeters are placed in Lanzarote (Canary Islands). The island is of volcanicorigin and the last eruption took place at the beginning of Eighteen Century. The instruments areinstalled in two locations of the Geodynamics Laboratory of Lanzarote (Vieira et al., 1991;Amoso et al., 2000) known as Cueva de los Verdes (CV) and Timanfaya National Park (T). Thefirst one is located at the most north-eastem part of the island, in a lava tunnel (Bravo et al.,1964) and the second one is located at southwest part, inside a National Park, just above thelargest geothermal anomaly in the zone (Araña et al., 1984). The tiltmeters considered in thisstudy are operating thanks to cooperation between the Institute of Astronomy and Geodesy ofMadrid (Spain) and the Institute of Seismology of Wuhan (P. R of China) (see Cai Weixin et al.,1994; Cai Weixin et al., 1997; Amoso et al., 1998; Vélez et al., 2001).*E-mail: jose_amoso@rnat.ucm.esMonitoring tidal and non-tidal tilt variations in Lanzarote !sland 4572. Tiltmeters Installed in Cueva de los Verdes (CV)Two long base-line tiltmeters are installed at the station CV. The instruments are a kind ofwater tube type and are placed in the two orthogonal directions of the lava tunnel. The tiltmeterswere installed in May 1992 (WTCE92 is installed in the longitudinal direction of the tunnel,which has an azimuth of 135°.2 N) and December 1994 (WTCE94 is installed in the transversaldirection of the tunnel, which has an azimuth of 45°.2 N). Both water tube tiltmeters are glass-assembled tubes of 1.5 m length, with two end pots with a diameter of 230 rom. The tubes arerefilled with distilled water. The two pots are equipped with a special float, which detects thewater level variations by means of a magnetic sensor (a more detailed description can be foundin Cai Weixin, et al., 1997; Amoso et al., 1998; Vélez et al., 2001). The WTCE92 has 38.2 mlength, with a sensitivity of 0.21 mas (1 mas = 10-3 are seconds) and a range of ±5000 mas. TheWTCE94 has 8m length, with a sensitivity of 0.24 mas and a range of ±5000 mas. Bothinstruments are connected to a Meteodata-256 data logger, with a sample period of 2 secondsand acquisition period of 10 minutes.2.1. Tidal Analysis and Meteorological InfluencesTiltmeters installed at the station CV are dedicated mainly to tidal research. Analyses of tidalrecord s for WTCE92 and WTCE94 are listed in Table 1a and Table 1b, respectively.Table 1. Results of the tidal analysis (observed amplitude A, tidal parameters y, ex and tidal residualvector B, ~) evaluated by the least squares harmonic analysis method at station CV (cp: 29"09'36" N,)..: 13"26'28" W, H: 37m). The Earth response is according to Molodensky for body tides and the tidalpotential used is Tamura (1987). Amplitudes are in mas, and phases, in degree, are local. (a) for theWTCE92 tiltmeter (from 15-05-1992 to 23-10-1999) and (b) for WTCE94 tiltmeter (from 03-12-1994to 23-10-1999).A mse y mse a mse B B(a) Ql 1.05 0.04 1.6476 0.0601 -2.54 2.09 0.61 -4.4Ol 3.51 0.04 1.0541 0.0112 -29.79 0.61 1.90 -66.6Pl 0.55 0.03 0.3532 0.0208 -49.82 3.38 0.84 -150.2Kl 1.33 0.03 0.2846 0.0075 -51.49 1.51 2.80 -158.1N2 7.04 0.06 3.4629 0.0287 -48.72 0.48 6.21 -58.4NU2 1.32 0.06 3.4048 0.1506 -55.69 2.53 1.19 -66.3M2 32.95 0.06 3.1027 0.0053 -62.34 0.10 30.27 -74.7S2 11.23 0.06 2.2720 0.0117 -83.19 0.29 11.34 -100.5K2 3.22 0.07 2.3956 0.0511 -80.33 1.22 3.20 -96.9(b) Ql 0.80 0.08 1.2634 0.1267 10.42 5.75 0.38 22.401 3.53 0.08 1.0640 0.0241 5.92 1.30 1.28 16.5Pl 1.27 0.07 0.8249 0.0424 -14.29 2.94 0.35 -63.9Kl 3.92 0.07 0.8417 0.0158 -19.30 1.07 1.33 -77.5N2 4.23 0.05 2.031~ 0.0227 74.09 0.64 4.08 93.8NU2 0.74 0.05 1.864 0.1213 73.71 3.73 0.71 95.2M2 18.59 0.05 1.708 0.0043 58.57 0.14 16.02 82.0S2 4.97 0.05 0.9824 0.0094 43.52 0.54 3.43 87.8K2 1.54 0.06 1.1199 0.0443 36.61 2.27 0.96 72.3Furthermore, data from these water tubes are also used to study ground deformations. As westated above, meteorological perturbations are clearly present on the observed data (see Figure 1).Thus, air temperature induced tilt and ground deformation produced by variations of local458 José Arnoso et alatmospheric pressure are the main perturbations. By using the DAD program (Amoso, et al.,1997) we have computed a single linear regression coefficient for long term and diurnal tidalbands (see Table 2) to quantify these effects. The program divides the data into intervals withoutoverlapping, where the drift is represented by a polynomial of power k. At the first stage, k is oneand the same for all intervals (k=O or k=1). Thus, the method of least squares is applied directlyon the hourly data (without any pre-filtration) and a zero iteration result is obtained. Next, k isvaried individually for every interval until we get a value of k with lowest mean square deviationof the tidal parameters. In the final approach, the drift is represented by polynomials of differentpower k in every intervalo If we deal with more than one non-tidal channel (air-pressure andtemperature), our model is namely a multi-variate approach, i.e. the regression coefficients areestimated all together.Concerning the LP (periods larger or equal than 15 days) tidal band, as we can see fromTable 2, the temperature effect in WTCE94 is 8 times larger than that on WTCE92. The airpressure effect is almost 2 times larger for the same water tube. With respect to D tidal band(periods less or equal than 1 day), the air pressure effect for WTCE92 is negligible whereas forWTCE94 is obviously significant. On the other side, the air temperature effect is rather similarfor both water tubes.Due to the closeness to the sea from the station (about 1.3 km) we have an air pressure effecton the tilt through the ocean loading, which is affected by air pressure variations. The pressureTable 2. Regression coefficients computed for both diurnal (D) and long period (LP) tidal bands atstation CV. Numerical values are given in mas/O.! degree Celsius and mas/mbar for air temperatureand air pressure, respectivel y.air temperature air pressureD LP D LPWTCE92 1.72 ±O.19 0.36 ±O.03 0.63 ±O.01WTCE94 1.42 ±O.47 2.99 íO.08 1.13 ±O,11 1.24 ±O.02g ::~~r"""""'('~'~:;""""'" .400 t a) .. ,.,., •••••.•.••• 'I I I I I I I03112194 02101/95 01102195 03103195 02104/95 02105195 01/06195i::k.~1\r-t\ ~.. A."-_ •••••960t b~ v VV~ VV-yTI70~ 6356a)I27/01/95984~ 980~ 976b)18,0u~ 17,8.lje)17,6I I I I I........ .....29/01/95 31/01195 02102195 04/02195 0610219519,5u•.• 18,0t 16.5 e)Fig. 1. (Left) a) Residual drift, after removing the tides, for WTCE92 and WTCE94 (dotted line) at station CVduring a period of 6 months. b) and e) represents the air pressure and temperature variations, respectivel y.(Right) Same as left graphic, but for the period of time marked with a dashed circle in left graphic.Monitoring tidal and non-tidal tilt variations in Lanzarote Island 459effect is related with the inverse barometric effect. It causes an elastic deformation of the Earth'ssurface, which in case of tilt is larger than the attraction effect. For WTCE94 this effect is clearlylarger than for WTCE92, for D and LP tidal bands.1050900750ig,600.•.'"tl~ 450~ JOO150150 JOO 450 600 750 900 1050Tnt WTCE92 (mas)Fig. 2. Tracer plot showing the predominant direction of two water tube tiltmeters installed at stationCV (Tidal signal was removed).If we compute the tilt trace (Figure 2), we observe that the main direction is closest toWTCE94 (azimuth: 45°.2 N). This direction is also very close to the coast-line in this area.However, the effect should be larger in the direction perpendicular to the coast-line (Rabbel andZschau, 1985). The reason could be related with the base-line direction of WTCE94,perpendicular to the tunnel axis. Thus, other effects could affect tilt measurements fromWTCE94 as cavity, which are expected to be larger in the direction perpendicular to the tunnelaxis (Harrison, 1976). Although cavity effect is not yet evaluated, further studies are necessary toanswer this question.3. Tiltmeters Installed in Timanfaya National ParkTimanfaya National Park is the area of Lanzarote where the last eruptions took place. Thestation is located near the geothermal anomalies field, inside a special cave named Casa de losCamelleros, about 3 meters below the surface. In this station we have installed a set of water tubetiltmeters (in two orthogonal directions of the cave) and two vertical pendulums. Site conditionsat this station are different from Station CV. Thus, the initial sensitivity of the instrumentsinstalled has been diminished by a factor of 10 due to out of scale problems.The water tubes are of the same kind as installed at the station CV. The tiltmeter installed in1996 (WTCE96) has 4.6m length and has an azimuth of 42°.8 N. At the end of 1999 wasinstalled a new tiltmeter (WTCE99) with a base line of 11.6m and an azimuth of 132°.8 N. Inboth cases the resolution is of 4.5 mas and the range is of ±l00 as. The instruments are placedinside a special gallery, 60 cm below the surface, where several pillars are fixed in order tosupport the tubes.Two vertical pendulums GK-10 were installed in 1996 (Cai Weixin et al., 1997). Bothinstruments are placed in two respective pillars at some 60 cm depth and with the same460 José Arnoso et alorientations as the two water tubes. The dimensions of the pendulums are 170xll0x220 rnm.The pendulums have a resolution of 57 mas, a period of 0.665 seconds and the range is of ±5degrees.AlI the instruments are connected to a Meteodata-1256 data logger, with a sample period of2seconds and acquisition period of 10 minutes (or 1 minute eventually).3.1. Temperature Induced TiltNear the station (about 100 m) are installed several thermometers for high temperaturemeasurements, placed inside a borehole at different depths (Vélez et al., 2001). Figure 3 is anexample of tilt record s with WTCE96 and the vertical pendulum GK-10 set in the azimuth 132°.2from North (hereafter denoted as VPGK-10) at station T, together with the air temperaturevariation inside the station (external temperature and gallery temperature) and the temperaturevariations recorded inside the borehole at 10m (denoted as HIT3) and 20m (denoted as HTT2)depth.-- Verticalpelldrl/um, Al: JJ2~82 N210~•• 14070O+-~~-+~-+-+-~-+~~+-~-r--+12O+-~-r-+-~+-~~-+-~+-~-r~06/0419906/05/99 06/06199 06/07/99 06/08199 06/09/99 06/10/99"~~: (a)~ 20¡ (b) T15t:;± I 1: 1: I '4• tI! th~ p ctWU162 (e)U mI "~ !~ 144 (d) t135 I I I I ; ; "j06/04/99 06/05/99 06/06/99 06/07/99 06/08199 06/09/99 06/10/99Fig.3. (Left) Raw data observed at station T with WTCE96 and VPGK-lO tiltmeters. (Right) (a) Airtemperature variation inside station T; (b) air temperature variation inside the gallery; (e) temperaturerecorded inside the borehole, at 20 m depth; (d) temperature recorded inside the borehole, at lO mdepth.Figure 4 shows the spectral amplitude of the tiltmeters data as well as the spectral amplitudeof the high temperature observations, for diurnal frequencies. From this figure, the solarcomponents are clearly present for both tiltmeters, which coincide with the peaks found in thespectral amplitude of the thermometers. Note that amplitudes of the deepest thermometer HTT2are twice as high as for the shallowest HIT3. At present, we cannot conclude any explanationbut investigations related with the response of the upper crust due to tidal strain as well asinstrumental effects on the electronics (not yet evaluated) will be considered for further studies.During this period, no noticeable changes in the drift have been found for these tiltmeters.The small changes are mostly related with temperature variations. To quantify the temperatureeffect on tilt observations, we have computed a single linear regression coefficient (see Table 3)by using the DAD program (Amoso et al., 1997). The results show, for diurnal frequencies, alarge influence of temperature in the data from vertical pendulum, whereas for water tube only aslight significant coefficient has been found with external temperature variation. It is importantMonitoring tidal and non-tidal tilt variations in Lanzarote Island0,4Q, 0,2 ,S2 IS3 S4~ 0,0 .J••.~"""",_"",l~_.,....¡._ •••.•••••+l,_.••••1"""!1 2 3 4 5 1 2 3 4 5Fig. 4. Spectral arnplitude of (a) vertical pendulum, (b) water tube, (e) therrnorneter HTI3 (10 mdepth) and (d) therrnorneter HTI2 (20 m depth) at station T. The abscissa axis shows the frequency incycIes per day.I ¡ IVPGK-I0 (a)S2S3S4O~~~~~~ __ ~+,~~4 52 3~ 0,60 ~~ S1! 0,48]•• 0,36"E 0,24 ~c. 3E 0,121 S3 S4-e 0,00 -:IIIIII•••••••••••••••~--+-.,....-...,...-.-.-1HTT3 (e)S2461O,12 '--r-r~.,......,~~...--¡-~~r-r~~--"~ 0,10-!.." 0,08'O,; 0,06~ 0,04-< 0,02 IS2 IS3 j4O,00 1II~ ••• .-!••••_,-+_ ,,-+...,...,....,~2 3S1 WTCE96 (b)4 5S1 BTT2 (d)"..'-01)"'O..'O=0,6 ~to note that VPGK-lO exhibits an important direct correlation with HIT3 thennometer, which isinversely correlated with air temperature variations (the regression coefficient obtained is -0.2±O.07 degree Celsius/degree Celsius) for diurnal frequencies. This is not the case with HIT2,installed 10 m deeper than HIT3, which is less influenced by external temperature variations.Table 3. Regression coefficients computed for diurnal and long period frequencies for tiltrnetersWTCE96 and VPGK-IO at station T. Nurnerical values are given in as/degree Celsius,Station Temperature H1T2diurnalVPGK-I0WTCE96long periodVPGK-lOWTCE96-27.40 ±O.8490.23 ±O.016-50.05 ±1.9840.36 ±O.042-1.06 ±O.146-16.86 ±O.421-0.05 ±O.012-34.25 ±O.903GalleryTemperature1.66 ±0.248HTT38.61 ±0.306-0.09 ±O.007In case of long period frequencies, the values found for the linear regression coefficients aremuch higher than for diurnal frequencies. Even, for VPGK-lO a significant coefficient is foundwith the borehole thennometer HIT2, but opposite in sign that for HIT3. For WTCE96 thecoefficients are clearer defined for long period frequencies than for diurnal frequencies. Thus,long period tilt is clearly correlated with seasonal variations of temperature, which are lessnoticeable for deepest thennometer. Also, the vertical pendulum is more sensitive to suchvariations than the water tube tiltmeter.4. ConclusionsWe show in this paper results from tiltmeter observations at two different locations of theGeodynamics Laboratory of Lanzarote. Data from tiltmeters, installed with different sensitivity462 José Arnoso et aland ranges according to the local conditions of observations, are useful to study ocean andmeteorological perturbations. The ground deformations are mainly related with temperature-induced tilt at both locations. In case of station CV, the tilt meters show an important tilt changerelated with the orientation of the lava tunnel. In case of station T, the largest perturbationscorrespond to the short baseline tiltmeter. A preliminary study of the correlation with hightemperature measurements shows larger regression coefficients for long period than for diurnalfrequencies. This correlation is lower for the deepest thermometer. Also, a temperature effect onthe vertical profile of the borehole is reported and will be investigated in further studies.AcknowledgementThis research has been supported by the Spanish Agency of Intemational Cooperation, theSpanish-Chinese Commission of Science and Technology and partially by project AMB99-0824of Spanish CICYT. We also thank the staff of House of Volcanoes and National Park ofTimanfaya. We are grateful to Malte Westerhaus and Shigeru Nakao for their suggestions toimprove the paper.ReferencesAgnew, D. (1986): Strainmeters and Tiltrneters. Rev. Geophys., 24, 579-624.Araña, V., J. Diez-Gil, R Ortiz, J. Yuguero (1984): Convection of Geothermal Fluids in the Timanfaya VolcanicArea (Lanzarote, Canary Islands). Bull. Volcano!., 47, 667-677.Arnoso, 1., C. de Toro, A. P. Venedikov and R Vieira (1997): On the Estimation of the Precision of the Tidal Data.Bull. d'lnfomation Marées Terrestres 127, 9757-9767.Arnoso, J., Cai Weixin, R Vieira, Tan Shiling and EJ. Vélez (1998): Tidal Tilt and Strain Measurements in theGeodynamics Labnoratory of Lanzarote, Earth Tides XIII, 149-156.Arnoso, J., J. Femández, R Vieira and M. Van Ruyrnbeke (2000): A Prelirninary Discussion on Tidal GravityAnomalies and Terrestial Heat Flow in Lanzarote (Canary Islands). Bull, d'Inforrnation Marées Terrestres132,10271-10282.Bravo, T. (1964): El Volcán y el Malpaís de la Corona. La Cueva de los Verdes y los Jameos. Publ, Cabildo Insularde Lanzarote, 30p.Cai Weixin, R Vieira, Tan Shiling and J. Arnoso (1994): The First Results of the Cooperative Research inGeodynamics Between China and Spain at Lanzarote. Crustal Deforrnation and Earthquake, 14, 1-5.Cai Weixin, R Vieira, Tan Shiling, 1. Arnoso, and Gao Weimin (1997): Geodynamics Technique Applied in theForecast of Lanzarote Island in Spain, Earth Res. in China, 13, 97-105.Edge, R J., T. F. Baker and G. Jeffries (1981): Borehole Tilt Measurements: a Periodic Crustal Tilt in a SeismicArea. Tectonophysics, 202, 209-213.Harrison, J. C. (1976): Cavity and Topographic Effects in Tilt and Strain Measurement. J. Geophys. Res., 81, 319-328.Mentes, Gy., P. Varga, H. J. Cumple (1999): Recording of Recent Crustal Movements by Borehole Tiltmeters in theVicinity of a Tectonic Fault. Bull. d'Information Marées Terrestres 131, 10207-10215.Rabbel W. and J. Zschau (1985): Static Deforrnations and Gravity Changes at the Earth's Surface due toAtrnospheric Loading, J. Geophys., 56.81-99.Vélez, EJ., R Vieira, J. Arnoso, Cai Weixin, Jun Jiang and Tan Shiling (2001): Tidal and Nontidal Observations ina Volcanic Active Region, Review of the Cooperation Between Spain and P.R of China in the GeodynamicsLaboratory of Lanzarote, J. Geod. SocoJapan. (this issue)Vieira, R, M. Van Ruymbeke, J. Femández, J. ArnosoO and C. Toro (1991): The lanzarote underground laboratory,Cahiers du Centre Europeen de Geodynamique et de Seismologie, 4, 71-86.

Monitoring tidal and non-tidal tilt variations in Lanzarote Island (Spain)

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