DOCSLIB.ORG

Seismotectonics of Western Canada from Regional Moment Tensor Analysis

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Seismotectonics of Western Canada from Regional Moment Tensor Analysis Seismotectonics of Western Canada From Regional Moment Tensor Analysis Johannes Peter Ristau University of Victoria 2004 Seismotectonics of Western Canada From Regional Moment Tensor Analysis Johannes Peter Ristau B.Sc., University of Manitoba, 1995 M.Sc., University of Manitoba, 1999 A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY in the School of Earth and Ocean Sciences @ Johannes Peter Ristau, 2004 University of Victoria All rights reserved. This dissertation may not be reproduced in whole or in part, by photocopying or other means, without permission of the author. Supervisor: Dr. Garry C. Rogers Abstract Moment tensor analysis of regional earthquakes (distances .v< 1000 km) in western Canada is now possible due to the installation of more than 40 three-component broad- band seismometers in western Canada and adjacent regions. In this study, regional moment tensor (RMT) analysis using robust waveform fitting techniques are employed to routinely caIcuIate source mechanisms, moments, and depths of earthquakes with M 2~ 4.0 in and near western Canada. This has resulted in about 10 times as many solutions per year for this region than have been calculated with teleseismic methods which are limited to earthquakes about M > 5.0. To date, more than 380 RMT solutions have been calculated in this study for west- ern Canada and adjacent regions for the years 1995-2004. These solutions provide new insights into a number of tectonic problems in western Canada. Local magnitudes (ML) have been calibrated with moment magnitudes (M,) providing a more consistent estimate of the magnitude of an earthquake. This is particularly important in the offshore region of British Columbia where RMT analysis shows that ML is underestimated by 0.3-0.7 mag- nitude units compared with M,, depending on the amount of oceanic crust present in the source-receiver travel path. This has important consequences for seismic hazard analysis and tectonic studies. Focal mechanisms from RMT solutions are also used to constrain the motions of the Explorer plate, a small oceanic plate off the coast of British Columbia. Rotation poles are calculated by leaving Pacific/Explorer motion unconstrained, and by constraining Pacific/Explorer motion using moment release rates along the Pacific/Explorer boundary. The Pacific/Explorer rotation rate decreases by a factor of 2 if Pacific/Explorer motion is constrained. This changes the convergence direction of the Explorer plate rel- ative to the North America plate from NE-SW in the unconstrained case to N-S in the constrained case. This suggests that Explorer plate motion cannot be modeled with a single rotation pole and cannot be treated as a rigid plate. The Explorer plate is likely un- dergoing intense internal deformation. The strain tensor for the Explorer plate, calculated from RMT solutions, gives a strain rate of 7.8 x yr-l in a N-S direction. Comparing moment (M,) with ML values for the northern and southern Canadian Cordillera demonstrates there is a 1:l relationship between M,, which is derived from M,, and ML. Stress tensors for the Canadian Cordillera give a NE-SW compressive stress direc- tion (gl) for most of western Canada. The northern Canadian Cordillera shows a change in a1 from E-W to N-S to NE-SW from south to north. Principal compressive strain and stress directions for the northern Canadian Cordillera have similar orientations suggesting that the earthquakes occur on faults which are favourably oriented for failure. In southern British Columbia the compressive stress regime is N-S and RMT data suggests that the N-S stress regime may extend through to the eastern Canadian Cordillera. RMT analysis will provide valuable data in the future to map the stress field in southern British Columbia. Stress tensor analysis of moment tensor and first motion solutions in the Queen Charlotte Islands region results in a local a1 azimuth of 20" which gives an angle of N 45" to the northern segment of the Queen Charlotte fault. This may suggest that the northern Queen Charlotte fault has a higher frictional strength than the San Andreas fault where angles of up to 80" are observed between a1 and the fault strike. Moment tensor solutions in the Glacier Bay region show a change in P axis orientation from N-S to E-W which could indicate that the stress field is influenced by post-glacial rebound. Contents Abstract 11.. Contents iv . List of Figures vlll.. List of Tables xviii List of Symbols xix List of Acronyms xx Acknowledgements xxi Dedication xxiii 1 Introduction 1 2 Tectonic Setting 10 2.1 Introduction . 10 2.2 Juan de Fuca Plate . 10 2.3 Pacific-North America-Juan de Fuca Triple Junction . 13 2.4 British Columbia Interior . 16 2.5 Yukon and Northwest Territories . 16 3 Seismic Moment Tensor Theory and Method 19 3.1 Introduction. 19 3.2 Seismic Moment Tensor Theory . 21 3.3 Preparing the Observed Waveforms . 26 3.4 Green's Functions . 28 3.4.1 .Calculating Green's Functions . 28 3.4.2 Earth Models ............................... 30 3.5 Intermediate Steps ................................ 33 3.6 Inversion ...................................... 34 3.7 Moment Tensor Solutions ............................ 35 4 Moment Magnitude . Local Magnitude Calibration 36 4.1 Introduction .................................... 36 4.2 The Local Magnitude Scale ........................... 37 4.3 Moment Magnitude . Local Magnitude Calibration .............. 38 4.3.1 Previous Studies ............................. 38 4.3.2 Canadian Cordillera Earthquakes .................... 38 4.3.3 Offshore Earthquakes .......................... 39 4.4 Summary ..................................... 47 5 Explorer Region Tectonics 48 5.1 Introduction .................................... 48 5.2 Calculating Rotation Poles ........................... 49 5.3 Unconstrained Pacific/Explorer Motion .................... 50 5.4 Constrained Pacific/Explorer Motion ...................... 54 5.4.1 Slip Rates From Recurrence Relations ................. 54 5.4.2 Constrained Rotation Poles ....................... 55 5.5 Explorer Plate Strain ............................... 60 5.5.1 The Strain Tensor ............................ 60 5.5.2 Explorer Plate Strain Rates ....................... 63 5.6 Summary ..................................... 67 6 Northern Canadian Cordillera Tectonics 70 6.1 Introduction .................................... 70 6.2 The Stress Tensor ................................ 73 6.3 Earthquake Focal Mechanisms, Stress. and Strain ............... 75 6.3.1 Focal Mechanisms ............................ 75 6.3.2 Stress Orientations ............................ 77 6.3.3 Strain Orientations ............................ 82 6.3.4 Comparison of Stress and Strain Orientations ............. 85 6.4 Summary ..................................... 89 7 Southern Canadian Cordillera and Vancouver Island/Puget Sound Tec- tonics 9 1 7.1 Introduction ....................................91 7.2 Southern Canadian Cordillera Seismic Activity ................ 92 7.3 CrustalStresses .................................. 95 7.3.1 Orientation of Principal Horizontal Stresses .............. 98 7.3.2 Stress Tensor Analysis ..........................101 7.4 Vancouver Island/Puget Sound Region ..................... 104 7.4.1 Crustal and In-Slab Stress Fields ....................104 7.4.2 Magnitude Comparisons ......................... 105 7.5 Summary .....................................111 8 Queen Charlotte Islands and Glacier Bay Region 114 8.1 Queen Charlotte Islands Region ......................... 114 8.1.1 Introduction ...............................114 8.1.2 Focal Mechanisms ............................116 8.1.3 Stress Analysis ..............................118 8.2 Glacier Bay Region ................................127 8.2.1 Introduction ...............................127 8.2.2 Post-Glacial Rebound And Seismicity .................127 8.3 Summary .....................................129 9 Summary 132 9.1 Introduction ....................................132 9.2 Moment Magnitude . Local Magnitude Calibration .............. 132 9.3 Explorer Region Tectonics ............................133 9.4 Northern Canadian Cordillera Tectonics ....................134 9.5 Southern Canadian Cordillera Tectonics .................... 135 9.6 Queen Charlotte Islands and Glacier Bay Region ............... 137 References 138 A Moment Tensor Solutions 151 B Waveform Fits 166 B.l 10 April 2001. 09:36 UT. Revere-Dellwood-Wilson Fault ...........166 B.2 20 May 2001. 10:04 UT. Revere-Dellwood-Wilson Fault ........... 166 B.3 14 July 1998. 01.49 UT. Sovanco Fracture Zone ................170 B.4 14 September 2001. 04:45 UT. Nootka Fault Zone ...............170 B.5 9 March 2001. 07:lO UT. Mackenzie Mountains. NWT ............ 170 B.6 9 March 2001. 19:02 UT. Mackenzie Mountains. NWT ............ 174 B.7 29 March 2001. 20:26 UT. Mackenzie Mountains. NWT ........... 174 B.8 10 June 2001. 13:19 UT. Puget Sound. Washington .............. 174 C Moment Tensor Solution Comparisons 178 C.l Green's Function Comparison ..........................178 C.2 Moment Tensor Solutions Using One Or Two Stations ............ 180 C.3 Moment Tensor Solutions For Older Events ..................183 C.3.1 10 November 2001, 20:20 UT. Sovanco Fracture Zone ......... 183 C.3.2 4 April 2002. 04:29 UT. Revere-Dellwood-Wilson Fault ........ 186 C.3.3 5 September 2002. 11:29 UT. Queen Charlotte Islands ........ 186 C.3.4 14 February 2002. 04:33 UT. Mackenzie Mountains. NWT ...... 186 C.3.5
Load more

Recommended publications
  • Canada's Active Western Margin
    Geoscience Canada. Volume 3. Nunber 4 269 in the area of Washington and Oregon seemed necessary geometrically. This suggestion was reiterated by Tobin and Sykes (1968) in their study of the seismicity ofthe regionanddevelopedin established the presence and basic bathymetric trench at the foot of the Canada's Active configuration of an active spreading continental slope, the absence of a ocean ridge system ofl the west coast of clearly defined eastward dipping Beniofl Western Margin - Vancwver Island. The theory of the earthquake zone and the apparently geometrical mwement of rigid quiescent state of present volcanism The Case for lithospheric plates on a spherical earth have all contributed toward a (introducedas the 'paving stone' theory) widespreaa uncertalnfy as to whether Subduction was tirst ~ro~osedand tasted bv subddctlon 1s cbrrently taking place ~c~enziiand Parker (1967) using data (e.8.. Crosson. 1972: ~rivast&a,1973; R. P. Riddihough and R. D. Hyndnan from the N. Pacific and in the global Stacey. 1973). Past subduction, fran Victoria Geophysical Observatory, extension of the theory by Morgan several tens of m.y. ago until at leastone Earth Physics Branch, (1 968) this reglon again played a major or two m y. ago. IS more generally Department Energy, Mines and role Morgan alsoconcluded that a small accepted ana it seems to us that the Resources, block eitof theJuandeFuca ridge (the case for contemporary subduction, in a 5071 W. Saanich Rd. Juan de Fuca plate. Fig. 2) was moving large part, must beargued upona Victoria B. C. independently of the main Pacific plate. continuation into the present of the The possibility that theJuan de Fuca processes shown to be active in the Summary plate is now underthrustingNorth recent geological past.
    [Show full text]
  • Washington Division of Geology and Earth Resources Open File Report
    l 122 EARTHQUAKES AND SEISMOLOGY - LEGAL ASPECTS OPEN FILE REPORT 92-2 EARTHQUAKES AND Ludwin, R. S.; Malone, S. D.; Crosson, R. EARTHQUAKES AND SEISMOLOGY - LEGAL S.; Qamar, A. I., 1991, Washington SEISMOLOGY - 1946 EVENT ASPECTS eanhquak:es, 1985. Clague, J. J., 1989, Research on eanh- Ludwin, R. S.; Qamar, A. I., 1991, Reeval­ Perkins, J. B.; Moy, Kenneth, 1989, Llabil­ quak:e-induced ground failures in south­ uation of the 19th century Washington ity of local government for earthquake western British Columbia [abstract). and Oregon eanhquake catalog using hazards and losses-A guide to the law Evans, S. G., 1989, The 1946 Mount Colo­ original accounts-The moderate sized and its impacts in the States of Califor­ nel Foster rock avalanches and auoci­ earthquake of May l, 1882 [abstract). nia, Alaska, Utah, and Washington; ated displacement wave, Vancouver Is­ Final repon. Maley, Richard, 1986, Strong motion accel­ land, British Columbia. erograph stations in Oregon and Wash­ Hasegawa, H. S.; Rogers, G. C., 1978, EARTHQUAKES AND ington (April 1986). Appendix C Quantification of the magnitude 7.3, SEISMOLOGY - NETWORKS Malone, S. D., 1991, The HAWK seismic British Columbia earthquake of June 23, AND CATALOGS data acquisition and analysis system 1946. [abstract). Berg, J. W., Jr.; Baker, C. D., 1963, Oregon Hodgson, E. A., 1946, British Columbia eanhquak:es, 1841 through 1958 [ab­ Milne, W. G., 1953, Seismological investi­ earthquake, June 23, 1946. gations in British Columbia (abstract). stract). Hodgson, J. H.; Milne, W. G., 1951, Direc­ Chan, W.W., 1988, Network and array anal­ Munro, P. S.; Halliday, R. J.; Shannon, W.
    [Show full text]
  • Cambridge University Press 978-1-108-44568-9 — Active Faults of the World Robert Yeats Index More Information
    Cambridge University Press 978-1-108-44568-9 — Active Faults of the World Robert Yeats Index More Information Index Abancay Deflection, 201, 204–206, 223 Allmendinger, R. W., 206 Abant, Turkey, earthquake of 1957 Ms 7.0, 286 allochthonous terranes, 26 Abdrakhmatov, K. Y., 381, 383 Alpine fault, New Zealand, 482, 486, 489–490, 493 Abercrombie, R. E., 461, 464 Alps, 245, 249 Abers, G. A., 475–477 Alquist-Priolo Act, California, 75 Abidin, H. Z., 464 Altay Range, 384–387 Abiz, Iran, fault, 318 Alteriis, G., 251 Acambay graben, Mexico, 182 Altiplano Plateau, 190, 191, 200, 204, 205, 222 Acambay, Mexico, earthquake of 1912 Ms 6.7, 181 Altunel, E., 305, 322 Accra, Ghana, earthquake of 1939 M 6.4, 235 Altyn Tagh fault, 336, 355, 358, 360, 362, 364–366, accreted terrane, 3 378 Acocella, V., 234 Alvarado, P., 210, 214 active fault front, 408 Álvarez-Marrón, J. M., 219 Adamek, S., 170 Amaziahu, Dead Sea, fault, 297 Adams, J., 52, 66, 71–73, 87, 494 Ambraseys, N. N., 226, 229–231, 234, 259, 264, 275, Adria, 249, 250 277, 286, 288–290, 292, 296, 300, 301, 311, 321, Afar Triangle and triple junction, 226, 227, 231–233, 328, 334, 339, 341, 352, 353 237 Ammon, C. J., 464 Afghan (Helmand) block, 318 Amuri, New Zealand, earthquake of 1888 Mw 7–7.3, 486 Agadir, Morocco, earthquake of 1960 Ms 5.9, 243 Amurian Plate, 389, 399 Age of Enlightenment, 239 Anatolia Plate, 263, 268, 292, 293 Agua Blanca fault, Baja California, 107 Ancash, Peru, earthquake of 1946 M 6.3 to 6.9, 201 Aguilera, J., vii, 79, 138, 189 Ancón fault, Venezuela, 166 Airy, G.
    [Show full text]
  • Geology 111 • Discovering Planet Earth • Steven Earle • 2010
    H1) Earthquakes The plates that make up the earth's lithosphere are constantly in motion. The rate of motion is a few centimetres per year, or approximately 0.1 mm per day (about as fast as your fingernails grow). This does not mean, however, that the rocks present at the places where plates meet (e.g., convergent boundaries and transform faults) are constantly sliding past each other. Under some circumstances they do, but in most cases, particularly in the upper part of the crust, the friction between rocks at a boundary is great enough so that the two plates are locked together. As the plates themselves continue to move, deformation takes place in the rocks close to the locked boundary and strain builds up in the deformed rocks. This strain, or elastic deformation, represents potential energy stored within the rocks in the vicinity of the boundary between two plates. Eventually the strain will become so great that the friction and rock-strength that is preventing movement between the plates will be overcome, the rocks will break and the plates will suddenly slide past each other - producing an earthquake [see Fig. 10.4]. A huge amount of energy will suddenly be released, and will radiate away from the location of the earthquake in the form of deformation waves within the surrounding rock. S-waves (shear waves), and P-waves (compression waves) are known as body waves as they travel through the rock. As soon as this happens, much of the strain that had built up along the fault zone will be released1.
    [Show full text]
  • Mantle Flow Through the Northern Cordilleran Slab Window Revealed by Volcanic Geochemistry
    Downloaded from geology.gsapubs.org on February 23, 2011 Mantle fl ow through the Northern Cordilleran slab window revealed by volcanic geochemistry Derek J. Thorkelson*, Julianne K. Madsen, and Christa L. Sluggett Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada ABSTRACT 180°W 135°W 90°W 45°W 0° The Northern Cordilleran slab window formed beneath west- ern Canada concurrently with the opening of the Californian slab N 60°N window beneath the southwestern United States, beginning in Late North Oligocene–Miocene time. A database of 3530 analyses from Miocene– American Holocene volcanoes along a 3500-km-long transect, from the north- Juan Vancouver Northern de ern Cascade Arc to the Aleutian Arc, was used to investigate mantle Cordilleran Fuca conditions in the Northern Cordilleran slab window. Using geochemi- Caribbean 30°N Californian Mexico Eurasian cal ratios sensitive to tectonic affi nity, such as Nb/Zr, we show that City and typical volcanic arc compositions in the Cascade and Aleutian sys- Central African American Cocos tems (derived from subduction-hydrated mantle) are separated by an Pacific 0° extensive volcanic fi eld with intraplate compositions (derived from La Paz relatively anhydrous mantle). This chemically defi ned region of intra- South Nazca American plate volcanism is spatially coincident with a geophysical model of 30°S the Northern Cordilleran slab window. We suggest that opening of Santiago the slab window triggered upwelling of anhydrous mantle and dis- Patagonian placement of the hydrous mantle wedge, which had developed during extensive early Cenozoic arc and backarc volcanism in western Can- Scotia Antarctic Antarctic 60°S ada.
    [Show full text]
  • Recent Movements of the Juan De Fuca Plate System
    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. B8, PAGES 6980-6994, AUGUST 10, 1984 Recent Movements of the Juan de Fuca Plate System ROBIN RIDDIHOUGH! Earth PhysicsBranch, Pacific GeoscienceCentre, Departmentof Energy, Mines and Resources Sidney,British Columbia Analysis of the magnetic anomalies of the Juan de Fuca plate system allows instantaneouspoles of rotation relative to the Pacific plate to be calculatedfrom 7 Ma to the present.By combiningthese with global solutions for Pacific/America and "absolute" (relative to hot spot) motions, a plate motion sequencecan be constructed.This sequenceshows that both absolute motions and motions relative to America are characterizedby slower velocitieswhere younger and more buoyant material enters the convergencezone: "pivoting subduction."The resistanceprovided by the youngestportion of the Juan de Fuca plate apparently resulted in its detachmentat 4 Ma as the independentExplorer plate. In relation to the hot spot framework, this plate almost immediately began to rotate clockwisearound a pole close to itself such that its translational movement into the mantle virtually ceased.After 4 Ma the remainder of the Juan de Fuca plate adjusted its motion in responseto the fact that the youngest material entering the subductionzone was now to the south. Differencesin seismicityand recent uplift betweennorthern and southernVancouver Island may reflect a distinction in tectonicstyle betweenthe "normal" subductionof the Juan de Fuca plate to the south and a complex "underplating"occurring as the Explorer plate is overriddenby the continent.The history of the Explorer plate may exemplifythe conditionsunder which the self-drivingforces of small subductingplates are overcomeby the influenceof larger, adjacent plates. The recent rapid migration of the absolutepole of rotation of the Juan de Fuca plate toward the plate suggeststhat it, too, may be nearingthis condition.
    [Show full text]
  • Tidally Controlled Gas Bubble Emissions: a Comprehensive 10.1002/2016GC006528 Study Using Long-Term Monitoring Data from the NEPTUNE
    PUBLICATIONS Geochemistry, Geophysics, Geosystems RESEARCH ARTICLE Tidally controlled gas bubble emissions: A comprehensive 10.1002/2016GC006528 study using long-term monitoring data from the NEPTUNE Key Points: cabled observatory offshore Vancouver Island Continuous monitoring of transient gas bubble emissions over 1 year was Miriam Romer€ 1, Michael Riedel2, Martin Scherwath3, Martin Heesemann3, and George D. Spence4 enabled by the NEPTUNE cabled observatory 1 Flare onsets were triggered and MARUM-Center for Marine Environmental Sciences and Department of Geosciences, University of Bremen, Germany, controlled by tidally induced bottom 2GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, 3Ocean Networks Canada, University of Victoria, British pressure changes Columbia, Canada, 4School of Earth and Ocean Sciences, University of Victoria, British Columbia, Canada Decreasing pressure allowing gas to migrate and shifting the CH4 solubility in pore waters leads to ebullition Abstract Long-term monitoring over 1 year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore Vancouver Island, British Columbia. Data from the North East Pacific Supporting Information: Time series Underwater Networked Experiment observatory operated by Ocean Networks Canada were Figure S1 used. The site is equipped with a 260 kHz Imagenex sonar collecting hourly data, conductivity-temperature- Table S1 Movie S1 depth sensors, bottom pressure recorders, current meter, and an ocean bottom seismograph. This enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity. Correspondence to: Three periods of gas emission activity were observed: (a) short activity phases of few hours lasting several M. Romer,€ months, (b) alternating activity and inactivity of up to several day-long phases each, and (c) a period of sev- [email protected] eral weeks of permanent activity.
    [Show full text]
  • Forwards Geology,Seismology & Geotechnical
    - c. x . @ , , C PUBLIC DOCUMENT ROOM , , , , UNITED STATES //- - / #,% O[ ~ !" o NUCLEAR REGULATORY COMMISSION 2 9 . 3 ; ,E WASHINGTON, D. C. 20555 @@ %...../ g e October 3, 1979 . a Valentine B. Deale, Esq., Chairman Dr. Frank F. Hooper Atomic Safety and Licensing Board School of Natural Resources 1001 Connecti:ut Avenue, N.W. University of Michigan Washington, DC 20036 Ann Arbor, MI 48109 Mr. Gustave A. Linenberger Atomic Safety and Licensing Board U.S. Nuclear Regulatory Commission Washington, DC 20555 In the Matter of Puget Sound Power & Light Company,~et al. (Skagit Nuclear Power Project, Units 1 and 2) ~~ - , Docket Nos. STN 50-522 and STN 50-523 Gentlemen: Enclosed is the NRC Staff's geology, seismology and geotechnical engineering review report which will be incorporated as Section 2.5 of the Final Supple- ment to the Staff's Safety Evaluation Report (SER). This report will also constitute the Staff's direct testimony on geology-seismology issues in this proceeding. The Staff's witnesses will consist of H. Lefevre, S. Wastler, Dr. J. Kelleher, and Dr. R. Regan. Also enclosed is the supplemental testi- many of Dr. Kelleher which addresses the Board request for testimony concerning the methods used by the Staff to estimate strong ground motion. The U.S. Geological Survey has assisted the NRC Staff in the review of the Skagit site. The U.S.G.S.'s review reports of February 23, 1978 and Sep- tember 17, 1979 (these reports have been previously distributed to the Board and parties), are attached as Appendix D to the Staff's report.
    [Show full text]
  • Seismic Structure of the Endeavour Segment, Juan De Fuca Ridge: Correlations with Seismicity and Hydrothermal Activity E
    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B02401, doi:10.1029/2005JB004210, 2007 Click Here for Full Article Seismic structure of the Endeavour Segment, Juan de Fuca Ridge: Correlations with seismicity and hydrothermal activity E. M. Van Ark,1 R. S. Detrick,2 J. P. Canales,2 S. M. Carbotte,3 A. J. Harding,4 G. M. Kent,4 M. R. Nedimovic,3 W. S. D. Wilcock,5 J. B. Diebold,3 and J. M. Babcock4 Received 9 December 2005; revised 23 June 2006; accepted 21 September 2006; published 3 February 2007. [1] Multichannel seismic reflection data collected in July 2002 at the Endeavour Segment, Juan de Fuca Ridge, show a midcrustal reflector underlying all of the known high-temperature hydrothermal vent fields in this area. On the basis of the character and geometry of this reflection, its similarity to events at other spreading centers, and its polarity, we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface. The Endeavour magma chamber reflector is found under the central, topographically shallow section of the segment at two-way traveltime (TWTT) values of 0.9–1.4 s (2.1–3.3 km) below the seafloor. It extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends. On cross-axis lines the axial magma chamber (AMC) reflector is only 0.4–1.2 km wide and appears to dip 8–36° to the east. While a magma chamber underlies all known Endeavour high-temperature hydrothermal vent fields, AMC depth is not a dominant factor in determining vent fluid properties.
    [Show full text]
  • The Gorda Deformation Zone
    The Gorda Deformation Zone Miles Bodmer University of Oregon Goals • Better understand deformation of the Gorda Plate • Look at deformation at a range of depths (crust – lithosphere - asthenosphere) • Highlight some of my research What Makes The Gorda Interesting? • The Gorda is the oceanic plate outboard of southern Cascadia subduction zone • Southern Cascadia is where large megathrust earthquakes are thought to nucleate • 1/3 of the plate configuration that makes up the Mendocino triple junction • Seismically active Modified from Byrnes et al. (2017) Tectonic History • ~10 Ma Pacific plate motion changes • Ridges begin to reorganize • ~5 Ma the Blanco transform develops • ~4 MA Explorer plate breaks off • The Mendocino transform and Gorda ridge fail to reorient Atwater and Stock, 1998 Magnetic Anomalies • Clear bending of anomalies in Gorda • Juan de Fuca shows signs of reorganization • Formation of new segments • Ridge rotation • Pacific side near Gorda does not show similar signs of reorganization • Variable spreading rates Wilson (2002) Gorda Is Stagnating • Gorda motion becomes increasingly independent in the last 3 Ma • At 0.5 Ma the Pacific controls Gorda motion Riddihough (1984) Seismicity Throughout The Plate 1964 – 1980 Star: M 5.7 (Jay Patton http://earthjay.com/) Wilson (1989) Historic seismicity (USGS www.earthquake.usgs.gov) Large Events In The Plate Correlate With Bathymetry Chaytor (http://activetectonics.coas.oregonstate.edu/gorda.htm) Deformation Accommodated By Left-lateral Faults • Bathymetry clearly shows ridges associated with faulting • Ridges appear smoothly deformed, kinked, or undeformed depending on the region • Moment tensors show dominantly left-lateral strike-slip motion with some normal faulting • Faults show clear regions of deformation Chaytor et al.
    [Show full text]
  • The Earth's Lithosphere-Documentary
    See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/310021377 The Earth's Lithosphere-Documentary Presentation · November 2011 CITATIONS READS 0 1,973 1 author: A. Balasubramanian University of Mysore 348 PUBLICATIONS 315 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Indian Social Sceince Congress-Trends in Earth Science Research View project Numerical Modelling for Prediction and Control of Saltwater Encroachment in the Coastal Aquifers of Tuticorin, Tamil Nadu View project All content following this page was uploaded by A. Balasubramanian on 13 November 2016. The user has requested enhancement of the downloaded file. THE EARTH’S LITHOSPHERE- Documentary By Prof. A. Balasubramanian University of Mysore 19-11-2011 Introduction Earth’s environmental segments include Atmosphere, Hydrosphere, lithosphere, and biosphere. Lithosphere is the basic solid sphere of the planet earth. It is the sphere of hard rock masses. The land we live in is on this lithosphere only. All other spheres are attached to this lithosphere due to earth’s gravity. Lithosphere is a massive and hard solid substratum holding the semisolid, liquid, biotic and gaseous molecules and masses surrounding it. All geomorphic processes happen on this sphere. It is the sphere where all natural resources are existing. It links the cyclic processes of atmosphere, hydrosphere, and biosphere. Lithosphere also acts as the basic route for all biogeochemical activities. For all geographic studies, a basic understanding of the lithosphere is needed. In this lesson, the following aspects are included: 1. The Earth’s Interior. 2.
    [Show full text]
  • Not My Fault: When It Comes to Cascadia, Not All Parts Are Equal
    There were many areas of agreement. The central or Juan de Fuca region offshore of Oregon and Washington is the only part that is behaving like a classic plate with earthquakes concentrated on the boundaries and a quake- Not My Fault: When it comes to free interior. The Gorda and Explorer regions are riddled Cascadia, not all parts are equal with intraplate earthquakes. Since 2000, over 200 Lori Dengler/For the Times-Standard earthquakes of magnitude 4 and larger have occurred Posted June 27, 2021 within the Explorer plate, including nine in the M6 range. https://www.times-standard.com/2021/06/26/lori-dengler- The Gorda plate is likewise earthquake prone. More than when-it-comes-to-cascadia-not-all-parts-are-equal/ 130 M4 and larger earthquakes have occurred with the plate’s interior including a 7.2 and three in the M6 range At some point in the next 15 million years, the Cascadia since 2000. Two 5.9s were recorded only three weeks ago subduction zone (CSZ) will no long pose an earthquake far offshore of the CA-OR border. In contrast, only nine threat to Northern California and the Pacific Northwest. M≥4 intraplate quakes were recorded within the central That’s not a long time geologically speaking, but from a Juan de Fuca plate in this same time period. human perspective it means that in the foreseeable future, the CSZ threat is real. The rate of intraplate earthquake activity in the Gorda and Explorer interiors have led some scientists to argue they The CSZ marks the convergent boundary between the Juan shouldn’t be called ‘plates’ and that ‘deformation zone’ is de Fuca plate system and the North American plate.
    [Show full text]