Mechanism of earthquakes and nature of faulting on the mid‐oceanic ridges

The mechanisms of 17 earthquakes on the mid-oceanic ridges and their continental extensions were investigated using data from the World-Wide Standardized Seismograph Network of the U. S. Coast and Geodetic Survey and from other long-period seismograph instruments. Mechanism solutions of high precision can now be obtained for a large number of earthquakes with magnitudes as small as 6 in many areas of the world. Less than 1% of the data used in this study are inconsistent with a quadrant distribution of first motions of the phases P and PKP; in many previous investigations 15 to 20% of the data were often inconsistent with the published solutions. Ten of the earthquakes that were studied occurred on fracture zones that intersect the crest of the mid-oceanic ridge. The mechanism of each of the shocks that is located on a fracture zone is characterized by a predominance of strike-slip motion on a steeply dipping plane; the strike of one of the nodal planes for P waves is nearly coincident with the strike of the fracture zone. The sense of strike-slip motion in each of the ten solutions is in agreement with that predicted for transform faults; it is opposite to that expected for a simple offset of the ridge crest along the various fracture zones. The spatial distribution of earthquakes along fracture zones also seems to rule out the hypothesis of simple offset. Two well documented solutions for earthquakes that are located on the mid-Atlantic ridge but that do not appear to be located on fracture zones are characterized by a predominance of normal faulting. The mechanisms of four earthquakes on extensions of the mid-oceanic ridge system—one near northern Siberia and three in East Africa—are also characterized by a predominance of normal faulting. The inferred axes of maximum tension for these six events are approximately perpendicular to the strike of the mid-oceanic ridge system. The results are in agreement with hypotheses of sea-floor growth at the crest of the mid-oceanic ridge system.

[1]  F. J. Vine,et al.  Spreading of the Ocean Floor: New Evidence , 1966, Science.

[2]  W. C. Pitman,et al.  Magnetic Anomalies over the Pacific-Antarctic Ridge , 1966, Science.

[3]  J. Wiseman.  St Paul Rocks and the Problem of the Upper Mantle , 1966 .

[4]  K. Bloomfield.  A Major East-north-east Dislocation Zone in Central Malawi , 1966, Nature.

[5]  C. R. Allen,et al.  Parkfield earthquakes of June 27-29, 1966, Monterey and San Luis Obispo Counties, California—Preliminary report , 1966 .

[6]  R. Cooke.  Some Seismological Features of the North Macquarie Ridge , 1966, Nature.

[7]  L. Sykes.  The seismicity and deep structure of Island arcs , 1966 .

[8]  Xavier Le Pichon,et al.  Magnetic anomalies over the Reykjanes Ridge , 1966 .

[9]  R. Stefánsson.  Methods of focal mechanism studies with application to two atlantic earthquakes , 1966 .

[10]  J. Cann,et al.  A discussion concerning the floor of the northwest Indian Ocean - An area on the crest of the Carlsberg Ridge: petrology and magnetic survey , 1966, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[11]  L. Sykes,et al.  Relationship of hypocenters of earthquakes to the geology of Alaska , 1966 .

[12]  G. Bollinger,et al.  THE S WAVE PROJECT FOR FOCAL MECHANISM STUDIES. THE ALASKA EARTHQUAKE SEQUENCE OF 1964. , 1966 .

[13]  A. Scheidegger.  Tectonics of the arctic seismic belt in the light of fault-plane solutions of earthquakes , 1966 .

[14]  H. W. Menard.  Fracture zones and offsets of the East Pacific Rise , 1966 .

[15]  X. Pichon,et al.  East Pacific Rise: The Magnetic Pattern and the Fracture Zones. , 1965, Science.

[16]  M. Tharp,et al.  Tectonic fabric of the Atlantic and Indian oceans and continental drift , 1965, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[17]  Wilson Jt,et al.  Transform Faults, Oceanic Ridges, and Magnetic Anomalies Southwest of Vancouver Island. , 1965 .

[18]  J. Wilson.  Transform Faults, Oceanic Ridges, and Magnetic Anomalies Southwest of Vancouver Island , 1965, Science.

[19]  J. Tuzo Wilson,et al.  Magnetic Anomalies over a Young Oceanic Ridge off Vancouver Island , 1965, Science.

[20]  M. Ewing,et al.  The seismicity of the Caribbean region , 1965 .

[21]  J. Brodie,et al.  Morphology of North Macquarie Ridge , 1965, Nature.

[22]  C. R. Allen,et al.  Relationship between seismicity and geologic structure in the Southern California region , 1965 .

[23]  J. Wilson,et al.  A New Class of Faults and their Bearing on Continental Drift , 1965, Nature.

[24]  L. Sykes.  The seismicity of the Arctic , 1965 .

[25]  J. C. Savage.  The effect of rupture velocity upon seismic first motions , 1965 .

[26]  H. W. Menard.  SEA FLOOR RELIEF AND MANTLE CONVECTION , 1965 .

[27]  R. W. Bemmelen.  The evolution of the atlantic mega-undation : Causing the American continental drift , 1965 .

[28]  M. Landisman,et al.  The seismicity of East Africa, the Gulf of Aden and the Arabian and Red Seas , 1964 .

[29]  G. A. Bollinger,et al.  The S-wave project for focal mechanism studies, earthquakes of 1963 , 1964 .

[30]  A. Ritsema.  Some reliable fault plane solutions , 1964 .

[31]  D. C. Krause.  Guinea Fracture Zone in the Equatorial Atlantic , 1964, Science.

[32]  M. Talwani.  A review of marine geophysics , 1964 .

[33]  W. Stauder.  A comparison of multiple solutions of focal mechanisms , 1964 .

[34]  M. Tharp,et al.  Chain and romanche fracture zones , 1964 .

[35]  M. Tharp,et al.  The Vema fracture zone in the equatorial Atlantic , 1964 .

[36]  L. Sykes.  Seismicity of the South Pacific ocean , 1963 .

[37]  J. Wilson,et al.  Hypothesis of Earth's Behaviour , 1963, Nature.

[38]  R. Adams.  Source characteristics of some deep New Zealand earthquakes , 1963 .

[39]  S. H. Quon,et al.  Rocks of Northern Part of Mid-Atlantic Ridge , 1963 .

[40]  C. Laj,et al.  Magnetic Anomalies Over Oceanic Ridges , 1963, Nature.

[41]  V. Vacquier.  Chapter 5 - Magnetic Evidence for Horizontal Displacements in the Floor of the Pacific Ocean , 1962 .

[42]  H. Honda,et al.  Earthquake Mechanism and Seismic Waves , 1962 .

[43]  H. H. Hess.  The history of ocean basins , 1962 .

[44]  R. Dietz,et al.  Continent and Ocean Basin Evolution by Spreading of the Sea Floor , 1961, Nature.

[45]  R. L. Fisher.  Middle America Trench: Topography and Structure , 1961 .

[46]  L. M. Balakina,et al.  Study of stresses and ruptures in earthquake foci with the help of dislocation theory , 1961 .

[47]  G. Lensen.  Principal horizontal stress directions as an aid to the study of crustal deformation , 1961 .

[48]  W. Stauder.  The Alaska earthquake of July 10, 1958: Seismic studies , 1960 .

[49]  M. Ewing,et al.  The Floors of the Oceans I. The North Atlantic , 1959 .

[50]  E. Berg,et al.  Seismological studies of the Western Rift Valley of Africa , 1958 .

[51]  E. Berg,et al.  Direction of faulting from first-motion studies , 1958 .

[52]  W. Adams,et al.  A study of the inconsistent observations in the fault-plane project , 1958 .

[53]  C. Romney.  Seismic waves from the Dixie Valley-Fairview Peak earthquakes , 1957 .

[54]  J. Hodgson.  NATURE OF FAULTING IN LARGE EARTHQUAKES , 1957 .

[55]  H. W. Menard.  DEFORMATION OF THE NORTHEASTERN PACIFIC BASIN AND THE WEST COAST OF NORTH AMERICA , 1955 .

[56]  J. H. Hodgson,et al.  Tables of extended distances for PKP and PcP , 1954 .

[57]  R. S. Storey,et al.  Tables extending byerly's fault-plane techniques to earthquakes of any focal depth , 1953 .

[58]  K. A. Davies.  The Uganda Section of the Western Rift , 1951, Geological Magazine.

[59]  B. Heezen,et al.  Submarine Topography in the North Atlantic , 1951 .

[60]  M. Ewing,et al.  NORTH ATLANTIC HYDROGRAPHY AND THE MID-ATLANTIC RIDGE , 1949 .

[61]  S. J. Shand.  Rocks of the Mid-Atlantic Ridge , 1949, The Journal of Geology.

[62]  C. E. Tilley.  The dunite-mylonites of Saint Paul's Rocks (Atlantic) , 1947 .

[63]  D. Carder.  Influence of reservoir‐loading on earthquake‐activity in the Boulder dam area , 1945 .