Abstract. From the marine refraction data recorded on five instruments during the Clipperton Area Seismic Survey to Investigate Compensation (CLASSIC) experiment in 1994 we construct a compressional velocity model for a 108 km long profile across the Clipperton transform. We apply a new seismic tomography code that alternates between ray tracing and linearized inversions to find a smooth seismic velocity model that fits the observed refraction travel times. The solution to the forward ray-tracing problem is a hybrid of the graph (or shortest path) method and a ray-bending method. The inversion is performed with least squares penalties on the data misfit and first derivatives of the seismic structure. Starting with a one-dimensional compressional velocity model for oceanic crust, the misfit in the normalized travel time residuals is reduced by 96%, decreasing the median travel time residual from 110 to 25 ms. The compressional velocity structure of the Clipperton transform is characterized by anomalously low velocities, about 1.0 km/s lower than average, beneath the median ridge and parallel troughs of the transform domain. The low compressional velocities can be explained by an increased porosity due to fracturing of the oceanic crust. We found crustal thicknesses of 5.6-5.9 km under the transform fault to produce the best fit of the Prop phase arrivals and Pg/Pn crossovers. Since the crust is not thin beneath the transform parallel troughs and the velocity anomaly is not confined to the median ridge, we find uplift by serpentinite diapirs unlikely as an explanation for the relief of the median ridge. A median ridge that is the result of brittle deformation due to compression across the transform domain is, however, compatible with our results. The upper crust is thicker to the north of the transform than to the south, which is likely a consequence of the contrast in temperature structure of these two spreading segments.

Abstract. A three-dimensional (3-D) seismic refraction study of the Clipperton transform fault, northern East Pacific Rise, reveals anomalously low compressional velocities from the seafloor to the Moho. We attribute this low-velocity anomaly to intensive brittle deformation, caused by transpression across this active strike-slip plate boundary. The seismic velocity structure south of the Clipperton transform appears unaffected by these tectonic forces, but to the north, seismic velocities are reduced over 10 km outside the zone of sheared seafloor. This contrast in seismic velocity structure corresponds well with the differences in mid-ocean ridge morphology across the Clipperton transform. We conclude that the amount of fracturing of the upper crust, which largely controls seismic velocity variations, is strongly dependent on the shallow temperature structure at the ridge axis. Intermittent supply of magma to the shallow crust north of the Clipperton transform allows seawater to penetrate deeper, and the cooler crust is brittle to a greater depth than south of the transform, where a steady state magma lens is known to exist. The crustal thickness averages 5.7 km, only slightly thinner than normal for oceanic crust, and variations in Moho depth in excess of-0.3 km are not required by our data. The absence of large crustal thickness variations and the general similarity in seismic structure imply that a steady state magma lens is not required to form normal East Pacific Rise type crust. Perhaps a significant portion of the lower crust is accreted in situ from a patchwork of short-lived gabbro sills or from ductile flow from a basal magma chamber as has been postulated in some recent ophiolite studies.

In 1982 we undertook a seismic refraction experiment, known as the MAGMA expedition, to examine the detailed structure of the East Pacific Rise near 12°50′N. This segment of the rise, where the full spreading rate is about 110 mm/yr, is near the projected trace of the O'Gorman fracture zone and is the site of an overlapping spreading center as well as “black smoker” hydrothermal activity. In this paper we describe the analysis of a subset of the travel time data collected during the MAGMA expedition, namely the data from profiles which were oriented normal to the rise axis. These profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross‐sectional structure of the rise. We have modeled these data using a two‐dimensional ray‐tracing program. We have found that the seismic velocities in the young oceanic crust are rather high, with velocity gradients of 4.0–5.5 s−1 in the uppermost crust. The highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0.1 Ma. This decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads. The decrease in velocity does not appear to penetrate deeper than about 0.5 km and may reverse itself as hydrothermal alteration fills the pores and cracks. The fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits, a result consistent with the high temperatures and discrete nature of the vents. Our best model includes a magma chamber some 4 km wide and extending from the Moho to within about 1.1 km of the seafloor. This magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the East Pacific Rise. These discrepancies probably arise because the magma chamber under the East Pacific Rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis.

In July and August 1980, an array of five ocean bottom seismographs was deployed within 3 km of the 350°C hydrothermal vents at the Rivera submersible experiment (RISE) site at 21°N, on the East Pacific Rise. Two of these instruments were placed within 600 m of the vents, using a transponder navigation network. The array detected four basic types of events. The first type consisted of local, very small microearthquakes. Locations obtained for 11 of these events place three within 1 km of the vents, with the others elsewhere along the rise crest. They appear to originate either from movement on the faults in the area or from the hydrothermal system beneath this area. A study of the S‐P times of this type indicates a maximum hypocentral depth of 2–3 km, implying a similar limit to the depth of hydrothermal circulation and brittle fracturing in the vicinity of the vents. The second type of event found consisted of emergent earthquakes that have many of the characteristics of volcanic harmonic tremor. The frequency of these events falls in the 1–5 Hz range and are similar in appearance to those seen at Mount St. Helens prior to and during its May 1980 eruption. They may be either hydrothermal or volcanic in origin. The third type of event produced a very monochromatic, high‐frequency seismogram, with the energy concentrated at 20 Hz. These events also appear to have a local origin. The particle motion of these earthquakes shows almost pure Stonely wave propagation, and they appear to originate from a regularly oscillating dilatational source. The nature of the source of event types two and three is clearly not the usual sort of motion on a fault and is still not understood. The fourth type of earthquake recordings were regional events, apparently from the nearby Rivera and Tamayo fracture zones The low‐frequency acoustic noise in the water was 16–64 times higher within 300 m of the vents than it was 2 km away. This implies that the black smoker vents may be generating a large acoustical disturbance as the 350°C water pours into the ocean. The observations are among the first of seismicity on moderate to fast spreading ridges, and many of the events appear to be related to the hydrothermal activity found there.

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