Physics of the Earth and Planetary Interiors 108Ž. 1998 129±141 Short and long baseline tiltmeter measurements on axial seamount, Juan de Fuca Ridge Maya Tolstoy ), Steven Constable 1, John Orcutt 2, Hubert Staudigel 3, Frank K. Wyatt 4, Gregory Anderson 5 Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, La Jolla, CA 92093-0225, USA Received 17 December 1996; revised 1 September 1997; accepted 3 September 1997 Abstract Long-term observations of seismic activity and ground deformation at mid-ocean ridges and submarine volcanoes are required for an understanding of the spatial and temporal characteristics of magma transport and intrusion. To make precise records of tilt on the seafloor we have installed short baseline tiltmeters in six ocean bottom seismometersŽ. TILT-OBS and developed a long baselineŽ. 100±500 m two-fluid tiltmeter Ž. LBT . In the TILT-OBS, the seismometer platform is levelled to better than 18 after deployment. The tiltmeter consists of a pair of electrolytic bubble sensors mounted on a secondary levelling stage on the seismometer platform. The levelling stage uses two motor-driven micrometers on a triangular mounting plate to bring the sensors to null. The sensitivity of these tiltmeters is 0.05 mrad, at a dynamic range of 0.2 mrad. A long baseline instrument was developed to achieve a better spatial average of deformation. Most approaches used on land to measure stable long baseline tilt cannot be applied to a submarine instrument, but tiltmeters in which the pressure of a fluid in tubes is measured are amenable to installation on the seafloor. The development resulted in a device that is essentially a center-pressure instrument folded back on itself, with fluids of different densities in the two tubes. During July to September 1994, these instruments were deployed on Axial Seamount, on the Juan de Fuca Ridge off Washington state, for a test of their relative performance on volcanic terrain, yielding 9 weeks of continuous dataŽ seismic, tilt, and temperature. from five TILT-OBS and one long baseline instrument. Drift on all instruments was of the order of 1 mradrday, with higher frequency variations of order 5±10 mrad. Initial drift on the TILT-OBS is shown to be associated with platform settling rather than with the sensor or its mounting. High frequency noise is coherent across instruments and tidal in character, and we conclude that tidal currents moving the sensors are responsible. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Short and long baseline tiltmeters; Axial seamount ) Corresponding author. Lamont-Doherty Earth Observatory, 1. Introduction Route 9W, Palisades, NY 10964, USA. Fax: q1-914-365-3181; e-mail: [email protected] Mid-Ocean RidgeŽ. MOR volcanoes are volumet- 1 Fax: q1-619-534 2902; e-mail: [email protected] rically the most important type of volcano on earth, 2 Fax: q1-619-534 2902; e-mail: [email protected] 3 Fax: q1-619-534 2902; e-mail: [email protected] yet our understanding of MOR volcanoes is still in 4 Fax: q1-619-534-2902; e-mail: [email protected] its infancy when compared to the extent of knowl- 5 Fax: q1-619-534-2902. edge acquired on sub-aerially exposed active volca- 0031-9201r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S0031-9201Ž. 98 00091-0 130 M. Tolstoy et al.rPhysics of the Earth and Planetary Interiors 108() 1998 129±141 noes like KilauearHawaii, or KraflarIceland. Most they fail to provide unique interpretations of magma of what we have learned about the dynamics and chamber geometries, and they are not suited for intrusive geometries of sub-aerial volcanoes comes studies of intrusive dynamics. from detailed monitoring programs carried out con- In Fig. 1Ž. from Decker, 1987 , we show the tilt tinuously over decades of diverse volcanic and intru- record of the Uwekahuna vault at the Hawaii Vol- sive activityŽ. e.g., Decker, 1987 . The most success- cano ObservatoryŽ. HVO located on the Kilauea ful observatory programs on sub-aerial volcanoes caldera rim. This 30-year record typically displays involve seismic monitoring and deformation, neither steady rises over several months with sudden drops. of which has been systematically applied to MOR This behavior is interpreted as slow inflation of a volcanoes over significant time periods. We need shallow magma chamber and catastrophic deflation, similar long-term monitoring of MOR volcanoes be- usually by eruption. The apparent increase in `noise' fore we can understand fully their emplacement and in the January 1984 to July 1985 record is caused by chemical fractionation behavior, or the mechanisms approximately monthly 10±20 mrad cycles with of chemical ridge segmentation. steady tilt increases and sudden drops, perfectly Experience from sub-aerial volcanoes shows that mimicking the 21 eruption cycles observed in this detailed monitoring programs provide independent time period at Puu Oo on the Kilauea East Rift, 20 constraints on the geometry, sizes, and dynamics of km SE of Uwekahuna vault. These observations diverse magma reservoirs and feeders. At Kilauea clearly show that the timing of eruptive and intrusive volcano, for example, interpretation of petrological events is well reflected in the measurement of tilt. data was effectively guided by knowledge of the The intrusive geometry of sheeted dikes and the geometry and interconnectedness of feeder systems dynamics of their emplacement at MORs is probably Ž.e.g., Wolfe et al., 1987 . At MORs, however, few one of the major open structural problems in marine monitoring data are available, and the chemical and geology. Currently, there is no consensus about the petrological data themselves have to be used to infer direction of sheeted dike emplacement. Sheeted dikes the physics of the magma supply systemsŽ e.g., are traditionally thought to intrude verticallyŽ e.g., Langmuir et al., 1986. The latter approach is typi- Cann, 1970; Kidd and Cann, 1974; Kidd, 1977. , cally non-unique and necessarily results in highly reflecting the passive upwelling of magma as a direct speculative models. Even though studies of ophio- consequence of ocean floor spreading. However, lites do provide important structural constraints, in there are several arguments which may be advanced particular, the dominance of intrusive rocks, so far in favor of lateral dike emplacement at mid-ocean Fig. 1. Radial tilt at the Uwekahuna vault near the Hawaii Volcanic Observatory situated on Kilauea. Rapid deflation events are associated with a pressure drop in the magma chamber associated in turn with eruptions. Slow replenishment from depth produces the steady tilt Ž.Decker, 1987 . M. Tolstoy et al.rPhysics of the Earth and Planetary Interiors 108() 1998 129±141 131 ridges. In particular, theoretical considerations and in Several approaches have been used in the devel- situ observations on sub-aerial volcanoes in Iceland opment of submarine short baseline tiltmetersŽ. SBT . and HawaiiŽ e.g., Sigurdsson and Sparks, 1978; Sig- Sakata and ShimadaŽ. 1984 and Shimamura and urdsson, 1987; Rubin and Pollard, 1987; Okamura et KanazawaŽ. 1988 developed tools that were em- al., 1988. suggest that lateral propagation of sheeted placed as fixtures in relatively shallow water. A dikes may occur in at least some MOR settings. number of other parallel efforts to develop re-de- Subsequent to predictions by Baragar et al.Ž. 1987 ployable SBTs are underway. Duennebier and Harris and SigurdssonŽ. 1987 , recent studies of sheeted dike Ž.Duennebier and Harris, 1990 of HIG reported a emplacement processes in the Troodos ophiolite shallow water deployment of a tiltmeter in an OBS Ž.Varga et al., 1991; Staudigel et al., 1992 showed and FoxŽ. Fox, 1990a of NOAA reported that he is that dike intrusive directions are certainly not exclu- developing tiltmeters for a volcanic event identifica- sively vertically upward, and horizontal directions tion system. StakesŽ. personal communication of are common. Radial outward intrusion of sheeted MBARI is developing a borehole tiltmeter, and Chave dikes into the merging rift zones of MOR volcanoes Ž.personal communication of WHOI is developing a cannot be distinguished from vertically upward intru- short-baseline instrument. Our efforts in SBT devel- sion any other way than through monitoring of the opment are discussed by Staudigel et al.Ž. 1991 , intrusive behavior of submarine volcanoes. Willoughby et al.Ž. 1993 and Wyatt et al. Ž. 1996 , The deformation of the host rock in response to and LBT development is summarized by Anderson magma intrusion may be brittle or elastic, with or et al.Ž. 1997 . While it is relatively simple to equip without the release of significant seismic energy. The any seafloor instrument, such as a magnetometer or recent ability to use military hydrophone arrays ocean bottom seismometerŽ. OBS , with some form Ž.SOSUS has already proven extremely valuable for of inclinometer, our efforts have been to develop a our understanding of MOR tectonic behaviorŽ e.g., tiltmeter with drift rates which are small compared Fox et al., 1993; Fox, 1995. However, this only with the expected volcanic signals. Volcanic defla- provides information on deformation associated with tion events, seen prior to and during eruptions and a high level of seismicity. It is still not known how during re-distribution of magma between reservoirs, important aseismic deformation and magma transport have tilt rates which exceed tens of microradians per are in MOR processes. day, but inflation events can be as low as a few microradians per month. Our goal, therefore, is to attain this level Ž.mradrmonth of stability, appropri- ate for volcanic monitoring, and we are very close to 2. Ocean bottom tiltmeters for submarine volcano this. While these values are still orders of magnitude monitoring larger than non-volcanic tectonic tilt observed by long baseline tiltmeters on land, they are orders of Among the instruments used for surface deforma- magnitude smaller than have so far been observed on tion measurements, only a few can be adapted to the seafloor.