A microearthquake survey at the junction of the East Pacific Rise and the Wilkes (9 S) fracture zone

Lilwall, R. C.; Francis, T. J. G.; Porter, I. T.

doi:10.1111/j.1365-246x.1981.tb05962.x

Cited by 14 publications

(8 citation statements)

References 21 publications

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“…This apparent depth difference is supported by the waveforms ob- [Francis et al, 1978;Rowlett, 1981;Rowlett and Forsyth, 1983]. Previous studies along East Pacific Rise transforms have not had the hypocentral resolution needed to constrain the lateral extent of the seismic zone at the intersection [Reid, 1976;Lilwall et al, 1981 ].…”

Section: Effect Of Lateral Heterogeneity In Velocitymentioning

confidence: 75%

Earthquakes in the Orozco Transform Zone: Seismicity, source mechanisms, and tectonics

Tréhu¹,

Solomon²

1983

J. Geophys. Res.

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As part of the Rivera Ocean Seismic Experiment, a network of ocean bottom seismometers and hydrophones was deployed in order to determine the seismic characteristics of the Orozco transform fault in the central eastern Pacific. We present hypocentral locations and source mechanisms for 70 earthquakes recorded by this network. All epicenters are within the transform region of the Orozco Fracture Zone and clearly delineate the active plate boundary. About half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough that forms the northern boundary of the transform zone (region 1). Most focal depths for these events are very shallow, within 4 km of the seafloor; several well‐determined focal depths, however, are as great as 7 km. No shallowing of seismic activity is observed as the rise‐transform intersection is approached; to the contrary, the deepest events are within 10 km of the intersection. First motion polarities for most of the earthquakes in region 1 are compatible with right‐lateral strike slip faulting along a nearly vertical plane, striking parallel to the spreading direction. Another zone of activity is observed in the central part of the transform (region 2). The apparent horizontal and vertical distribution of activity in this region is more scattered than in the first, and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism. Pronounced lateral variations in crustal velocity structure are indicated for the transform region from refraction data and measurements of wave propagation directions. The effect of this lateral heterogeneity on hypocenters and fault plane solutions is evaluated by tracing rays through a three‐dimensional velocity grid. While findings for events in region 1 are not significantly affected, in region 2, epicentral mislocations of up to 10 km and azimuthal deflections of up to 45° may result from assuming a laterally homogeneous velocity structure. When corrected for the effects of lateral heterogeneity, the epicenters and fault plane solutions for earthquakes in region 2 are compatible with predominantly normal faulting along a topographic trough trending NW–SE; the focal depths, however, are poorly constrained. These results suggest an en echelon spreading center or leaky transform regime in the central transform region.

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Section: Effect Of Lateral Heterogeneity In Velocitymentioning

confidence: 75%

Earthquakes in the Orozco Transform Zone: Seismicity, source mechanisms, and tectonics

Tréhu¹,

Solomon²

1983

J. Geophys. Res.

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“…The presence of a high-temperature, black-smoker hydrothermal vent field atop the along-axis high at 26øN indicates that the crustal thermal structure probably plays a major role in determining the spatial distribution of hypocenters and thus the extent of brittle failure. Along the faster spreading East Pacific Rise, the general aseismicity and shallow depths for the few locatable microearthquakes [Lilwall et al, 1981;Riedesel et al, 1982], together with the presence of a ridge-axis zone of low velocities [Derrick et al, 1987;Burnett et al, 1989;Toomey et al, 1990], constitute evidence for only a thin brittle zone (less than 2-3 km thick) overlying the inferred axial magma chamber. The microearthquake results from 26øN show some qualitative resemblance to the East Pacific Rise, including the lack of seismic activity beneath the hydrothermal field and the relatively shallow earthquake activity beneath the along-axis high.…”

Section: Addition B Values For Earthquakes Beneath the Along-axis Dementioning

confidence: 99%

Microearthquake Characteristics of a Mid‐Ocean Ridge along‐axis high

Kong¹,

Solomon²,

Purdy³

1992

J. Geophys. Res.

183

147

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We report results from a 3‐week microearthquake survey of the segment of the Mid‐Atlantic Ridge axis near 26°N. The segment is centered on an along‐axis median valley bathymetric high that includes the site of the TAG hydrothermal field. The seismic network, consisting of seven ocean bottom hydrophones and two ocean bottom seismometers, spanned the median valley inner floor and eastern valley wall. Hypocenters were determined for 189 earthquakes, with good resolution of focal depth obtained for 105 events. Almost all events occurred at depths between 3 and 7 km beneath the seafloor, with earthquakes occurring at shallower depths (less than 4 km) beneath the along‐axis high. No events were detected in the immediate vicinity of the hydrothermal field. The along‐axis high is the site of a midcrustal low‐velocity zone, significant attenuation of P wave energy, and high b values; the low‐velocity volume extends about 10 km south of the high to the vicinity of volcano within the axial neovolcanic zone. Fault plane solutions indicate high‐angle (or very low angle) normal faulting beneath the along‐axis high and the base of the adjacent western wall, reverse faulting beneath the axial volcano, and a more conventional normal‐faulting geometry for earthquakes beneath the eastern wall. The distribution of seismicity and the diversity of faulting styles suggest a spatially variable tectonic state for the ridge segment at 26°N. These variations are likely a signature of along‐axis differences in thermal structure and state of stress. We suggest that the low‐velocity volume beneath the along‐axis high is the site of a relatively recent crustal injection of magma. Continued cooling of the now largely solid but still hot intrusion, and associated thermal stress and fracturing in the immediately surrounding crust, can account generally for the distribution of areas of most intense earthquake activity, the diversity of observed faulting mechanisms, and the presence of the high‐temperature vent field. These results are supportive of the spreading cell model for segmentation of magmatism and thermal structure along a slowly spreading ridge.

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“…This inactivity contrasts dramatically with the teleseismic and local observations along the Mid-Atlantic Ridge (e.g . Forsyth 1975;Weidner & Aki 1973;Lilwall et al 1981). Riedesel et al (1982) have recently exploited the growing body of knowledge of spreading-centre fault lengths obtained by detailed surveys on slow-and fast-spreading ridges to place upper bounds on the depth extent of the earthquake fault planes.…”

Section: Microearthquakes On the East Pacific Rise At 21~mentioning

confidence: 94%

“…Microearthquake surveys conducted near the rise crest have been no more successful than the teleseismic studies in detecting events which clearly lie along the spreading axis (Reid et al 1977;Lilwall et at. 1981;Prothero & Reid 1982).…”

Section: Microearthquakes On the East Pacific Rise At 21~mentioning

confidence: 98%

Evolution of the ocean crust: results from recent seismic experiments

Orcutt¹,

Burnett

McClain

1984

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We present results from recent analyses of seismic refraction and sea-floor microseismicity studies in the Pacific and Atlantic oceans which lend support to the hypothesis that processes responsible for the construction of ophiolite suites are similar to phenomena extant at midocean ridges. Seismicity at fast-spreading ridges is characterized by very low magnitude (0-1) and shallow (~2-3 km) microearthquakes and long-term oscillations or harmonic tremor. A detailed seismic-refraction experiment on a fast-spreading portion of the East Pacific Rise supports the hypothesis of the existence of a crustal magma chamber. Analyses of these data indicate that the chamber is largely unperturbed by the presence of conjugate spreading centres and the inverted deltashaped zone of partial melt is characterized by a half width in excess of 6 km. Finally, a new approach for obtaining upper-crustal velocities when applied to data characterized by the presence of shear waves provides several counter-examples to the hypothesis that the shallow crust evolves with time.During the past few years, marine seismology has proved to be an effective tool in studying the detailed elastic structure of the oceanic crust. Concomitant advances in the quality and quantity of physical properties' data from ophiolite suites has permitted detailed comparisons between these allochthonous terranes and the oceanic lithosphere. A properly testable 'ophiolite hypothesis' for the genesis, evolution and structure of the oceanic crust has arisen from this work. We present two new data sets which, in general, support this hypothesis. These consist of oceanbottom seismograph seismicity and refraction studies of the East Pacific Rise which require a substantial crustal magma chamber. This chamber, by implication, is responsible for the bulk of the fractionation processes which construct a vertically heterogeneous crustal column. Finally, a recently developed method for accurately determining the compressional velocity of the shallow crust casts some doubt on the evolutionary systematics attributed to 'Layer 2A' and the oceanic crust in general.

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A microearthquake survey at the junction of the East Pacific Rise and the Wilkes (9 S) fracture zone

Cited by 14 publications

References 21 publications

Earthquakes in the Orozco Transform Zone: Seismicity, source mechanisms, and tectonics

Earthquakes in the Orozco Transform Zone: Seismicity, source mechanisms, and tectonics

Microearthquake Characteristics of a Mid‐Ocean Ridge along‐axis high

Evolution of the ocean crust: results from recent seismic experiments

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