Wayne C. Crawford

Discovery of a magma chamber and faults beneath a Mid-Atlantic Ridge hydrothermal field

Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers. The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast-spreading centres, but it has been difficult to image such magma chambers beneath slow-spreading centres, owing to rough seafloor topography and associated seafloor scattering. In the absence of any images of magma chambers or of subsurface near-axis faults, it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow-spreading ridges. Here we report the presence of a crustal magma chamber beneath the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge. The reflection from the top of the magma chamber, centred beneath the Lucky Strike volcano and hydrothermal field, is approximately 3 km beneath the sea floor, 3–4 km wide and extends up to 7 km along-axis. We suggest that this magma chamber provides the heat for the active hydrothermal vent field above it. We also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward-dipping faults cutting through the volcanic edifice, suggesting continuous interactions between tectonic and magmatic processes.

Infragravity Waves in the Deep Ocean

Energetic pressure fluctuations at periods longer than 30 s are a ubiquitous feature of pressure spectra from instruments sited on the deep seafloor in both the Atlantic and the Pacific oceans. We show these pressure fluctuations are caused by freely propagating ocean surface waves. The waves are generated in the near shore region along the entire coastline of an ocean basin through nonlinear transfer of energy from short-period waves. This view contrasts with some earlier work, which described these long-period pressure fluctuations as trapped waves tied to groups of short waves. We have constructed a model based on the average energy in the short (wind driven and swell) wave band along the North Atlantic coast to predict the energy in the long wave band at a site in the Atlantic. Maximum likelihood wave number-frequency spectra calculated on data from an 11 element array in the North Pacific confirm that the long wave energy is confined to wave numbers corresponding to the surface gravity wave dispersion relation. We have used the wave number spectra to isolate particular regions of the Pacific Ocean which are sources of long wave energy. Energetic short-period waves are incident on the coastline in these regions. Long waves are detected at the army which originate in the Gulf of Alaska, the northwestern Pacific, and at the southern tip of South America.

Constraints on melt in the lower crust and Moho at the East Pacific Rise, 9°48′N, using seafloor compliance measurements

Seafloor compliance measurements across the East Pacific Rise at 9°48′N reveal low shear velocities throughout the crust and at the crust-mantle boundary, with the lowest shear velocities centered beneath the rise axis. The compliance method uses the seafloor deformation under the loading of long wavelength ocean waves to probe the oceanic crust. The shape of the compliance function as a function of frequency is primarily controlled by regions of low shear velocity within the crust. At 9°48′N, the shear velocity is less than 20 m/s in the shallow on-axis melt lens located 1.4 km beneath the seafloor, demonstrating that the melt lens at this site is fully melt rather than a connected crystal mush. The compliance data also require a second on-axis melt lens 5.4 ± 1 km beneath the seafloor, with shear velocities slower than 50 m/s. This “deep” melt lens may be created by melt pooling at a permeability or density barrier at the crust-mantle interface. The shear velocity in the lower crust between the two melt lenses averages 1.7 km/s, indicating 2.5–18% melt. Melt persists in the lower crust to at least 10 km off-axis, where the top of the lower crustal low-velocity zone is approximately 4 km beneath the seafloor. In seismic layer 2B, the ratio of shear to compressional velocity increases from 0.41 on-axis to 0.58 by 10 km off-axis, indicating that there are abundant thin cracks in the sheeted dikes on-axis and that these cracks close away from the rise axis. High on-axis porosity in layer 2B may allow hydrothermal circulation down to near the shallow melt lens.

Automated identification, location, and volume estimation of rockfalls at Piton de la Fournaise volcano

Since the collapse of the Dolomieu crater floor at Piton de la Fournaise Volcano (la Reunion) in 2007, hundreds of seismic signals generated by rockfalls have been recorded daily at the Observatoire Volcanologique du Piton de la Fournaise (OVPF). To study rockfall activity over a long period of time, automated methods are required to process the available continuous seismic records. We present a set of automated methods designed to identify, locate, and estimate the volume of rockfalls from their seismic signals. The method used to automatically discriminate seismic signals generated by rockfalls from other common events recorded at OVPF is based on fuzzy sets and has a success rate of 92%. A kurtosis-based automated picking method makes it possible to precisely pick the onset time and the final time of the rockfall-generated seismic signals. We present methods to determine rockfall locations based on these accurate pickings and a surface-wave propagation model computed for each station using a Fast Marching Method. These methods have successfully located directly observed rockfalls with an accuracy of about 100 m. They also make it possible to compute the seismic energy generated by rockfalls, which is then used to retrieve their volume. The methods developed were applied to a data set of 12,422 rockfalls that occurred over a period extending from the collapse of the Dolomieu crater floor in April 2007 to the end of the UnderVolc project in May 2011 to identify the most hazardous areas of the Piton de la Fournaise volcano summit.

Comparing the role of absolute sea-level rise and vertical tectonic motions in coastal flooding, Torres Islands (Vanuatu)

Since the late 1990s, rising sea levels around the Torres Islands (north Vanuatu, southwest Pacific) have caused strong local and international concern. In 2002–2004, a village was displaced due to increasing sea incursions, and in 2005 a United Nations Environment Programme press release referred to the displaced village as perhaps the world’s first climate change “refugees.” We show here that vertical motions of the Torres Islands themselves dominate the apparent sea-level rise observed on the islands. From 1997 to 2009, the absolute sea level rose by 150 + /-20 mm. But GPS data reveal that the islands subsided by 117 + /-30 mm over the same time period, almost doubling the apparent gradual sea-level rise. Moreover, large earthquakes that occurred just before and after this period caused several hundreds of mm of sudden vertical motion, generating larger apparent sea-level changes than those observed during the entire intervening period. Our results show that vertical ground motions must be accounted for when evaluating sea-level change hazards in active tectonic regions. These data are needed to help communities and governments understand environmental changes and make the best decisions for their future.

A new sea-floor gravimeter

A new reservoir management application uses precise time‐lapse gravity measurements on the sea floor to detect seawater infiltration in offshore natural gas fields during production. Reservoir models for the North Sea Troll field predict gravity changes as large as 0.060 mGal within a 3–5‐year period. We have constructed and deployed a new instrument—the ROVDOG (Remotely Operated Vehicle‐deployed Deep‐Ocean Gravimeter) system—for this application. Because the measurements must be relocated accurately (within 3 cm), we required a gravimeter which could be handled by an ROV and placed atop sea‐floor benchmarks. We have built an instrument based upon the Scintrex CG‐3M land gravimeter. Motorized gimbals level the gravimeter sensor within a watertight pressure case. Precision quartz pressure gauges provide depth information. A shipboard operator remotely controls the instrument and monitors the data. The system error budget considers both instrumental and field measurement uncertainties.The instrument prototy...

Upper crustal velocity structure beneath the central Lucky Strike Segment from seismic refraction measurements

We present a three-dimensional velocity model of the upper crust around the central volcano of the Lucky Strike Segment, Mid-Atlantic Ridge. The model, constructed from a 3-D array of air gun shots (37.5 m spacing along line and 100 m between lines) to ocean bottom seismometers fired during a 3-D seismic reflection survey, shows an off-axis velocity increase (∼1 km/s), a low-velocity region within the median valley, and a low-velocity anomaly underneath the Lucky Strike volcano. Our observations indicate a porosity decrease of 1%–9% (corresponding to a velocity increase of ∼0.5–1 km/s) over a distance of 8 km from the ridge axis (∼0.7 Ma) and a porosity decrease of 4%–11% (corresponding to a velocity increase of ∼2 km/s) between a depth of 0.5 and 1.75 km below seafloor. A sinusoidal variation in the traveltime residuals indicates the presence of azimuthal anisotropy with cracks aligned approximately along the ridge axis. We favor an interpretation in which upper crustal porosities are created by a combination of magmatic accretion (lava–sheeted dike boundary) and active extension (faults, fractures, and fissures). The porosity variation with depth probably depends on pore space collapse, hydrothermal alteration, and a change of stress accommodation. The off-axis porosities are possibly influenced by both hydrothermal precipitation and the aging of the crust.

MOISE: A Prototype Multiparameter Ocean-Bottom Station

Multiple geophysical datasets were recorded during the international cooperative pilot experiment, Monterey Bay Ocean Bottom International Seismic Experiment (MOISE). This experiment, conducted from June to September 1997, demonstrated the feasibility of installing, operating, and recovering different geophysical sensors (seismometers, electromagnetometers and environmental sensors). The seismic noise level was stable throughout the experiment. The noise level was comparable to a high noise model for periods below 15 sec and showed strong diurnal variations at longer periods. We demonstrate that these diurnal variations can be removed from the vertical component by subtracting the effect of the horizontal components, decreasing the vertical noise level by up to 40 db. We investigate possible coherence between long-period seismic, electromagnetic, and environmental data. The coherence between the vertical seismic signal and pressure and current speed is close to unity between 2 × 10-5 and 10-4 Hz. In particular, there is a peak of coherence at 2.3 × 10-5 Hz (12 hr), which is a consequence of tidal effects. No significant high coherence is observed with the vertical magnetic field. The MOISE experiment demonstrates that permanent broadband seismic and geophysical observatories can now be installed on the seafloor. It also illustrates the importance of installing various kinds of geophysical sensors in order to increase the signal-to-noise ratio of seismic data, validating the concept of multiparameter ocean-bottom stations. Manuscript received 9 March 2000.

A Second Look at Low-Frequency Marine Vertical Seismometer Data Quality at the OSN-1 Site off Hawaii for Seafloor, Buried, and Borehole Emplacements

We improve marine low-frequency (1–100 mHz) vertical seismometer data by subtracting noise generated by tilting under fluid flow and by seafloor deformation under ocean-surface gravity waves. We model the noise from the coherency and transfer functions between the vertical channel and other data channels that are more sensitive to the noise sources: the horizontal seismometer components for tilting and a differential pressure gauge for ocean waves. We subtract noise from three adjacent seafloor broadband seismometer stations at the OSN-1 deep-ocean test site: one sitting on the seafloor, another buried 1 m deep in sediments, and the third clamped in a borehole 248 m beneath the seafloor. Seafloor currents generate the seafloor sensor tilt noise, whereas tidally driven fluid pumping generates the borehole sensor tilt noise. Subtracting the tilt noise reduces the vertical channel noise levels by 35–40 dB between 1 and 60 mHz on the seafloor sensor and by 15–20 dB between 1 and 10 mHz on the borehole sensor. Subtracting the ocean-wave noise further reduces the noise level on all instruments by 5–15 dB between 4 and 20 mHz. After subtracting tilt and ocean-wave noise, the seafloor vertical channel is 5–10 dB quieter than the buried sensor vertical channel at frequencies below 30 mHz. The corrected borehole vertical channel has a similar noise level to the seafloor and buried sites above 10 mHz, but noise increases rapidly at lower frequencies, probably because of vertical strumming under tidally driven fluid flow.

Spatial distribution and temporal evolution of crustal melt distribution beneath the East Pacific Rise at 9°–10°N inferred from 3‐D seafloor compliance modeling

Determining the melt distribution in oceanic crust at mid-ocean ridges is critical to understanding how magma is transported and emplaced in the crust. Seafloor compliance—deformation under ocean wave forcing—is primarily sensitive to regions of low shear velocity in the crust, making it a useful tool to probe melt distribution. Analysis of compliance data collected at East Pacific Rise between 9° and 10°N through 3-D numerical modeling reveals strong along-axis variations in the lower crustal shear velocities, as well as temporal variation of crustal shear velocity near 9°48′N between measurements spanning 8 years. Compliance measured across the rise axis at 9°48′N and 9°33′N suggest a deep crustal low-velocity zone beneath the ridge axis, with a low Vs/Vp ratio consistent with melt in low aspect ratio cracks or sills. Changes in compliance measured at 9°48′N between years 1999 and 2007 suggest that the melt fraction in the axial crust decreased during this interval, perhaps following the 2005–2006 seafloor eruption. This temporal variability provides direct evidence for short-term variations of the magmatic system at a fast spreading ridge.