Our results indicate considerable variation in moment release and hence in fault coupling along the strike of the three Eltanin transforms. . We used a box from 54S to 58S and 140W to 120W to identify earthquakes that might be located on the Eltanin transform system. Their length, relative simplicity, 80 mm yr-1, long-term slip rate and the lack of known shocks of Mw > 7 make them a good laboratory for studying quasi-periodic recurrence of earthquakes on a timescale of years to a few decades. Fracture zone evolution is investigated using dynamic models that allow the fault zones to freely slip. For the four seismically active fault segments in Table 1 we obtain similar downdip widths of seismic faulting of about 5 km assuming coupling at the full plate rate. The new results that include surface wave data are not very different from the earlier results. Aseismic parts of the Heezen and Tharp fracture zones can be traced far to the northwest and southeast. Distances in kilometres same as in Fig. M.S. That segment may well not be a single isolated an asperity as that in Fig. Kent What causes the formation of offsets in the mid-ocean ridge system? At least one seamount chain, the New England Seamounts, lies in the northwestern Atlantic. They represent a good laboratory for examining fault segments that slip aseismically, others that are well coupled, and the short recurrence times of earthquakes. Decide for yourself which test is easiest to use, but dont forget the preliminary test. "zone of fracture" in Chinese : , ; ; ,; ; "fracture system" in Chinese : ; "amsterdam fracture zone" in Chinese : [1], Lower North Atlantic Deep Water (LNADW) is found around 3,6004,000 metres (11,80013,100ft) below sea level and flows from the Greenland and Norwegian seas; it brings high salinity, oxygen, and freon concentrations towards the equator. "zone" in Chinese : n. 1. () 2. 3. . 8) contains six events of Mw= 5.89 since 1971. Ryan They are the scars on the seafloor basalt that are created by offsets in the mid-ocean ridge. [5], The hydrothermal vents of the MAR support many life forms. If we assume that all of the true depths of these earthquakes are shallow, what size error or bias should we expect in estimates of Mo? Red and green symbols denote strike-slip faulting. Events of Mw > 5.9 account for 70 per cent of the moment in Fig. Intermediate-period surface waves have been used in the GCMT analysis since 2004, and often are very useful for constraining solutions for earthquakes smaller than Mw= 5.5. There are about 300 fracture zones, with an average north-south separation of 55 kilometres (34 mi): two for each degree of latitude. . What causes the formation of offsets in the mid-ocean ridge system? 3391km (2107mi) WSW of Ushuaia, Argentina They conclude that the Blanco and Gofar transforms, which were the target of their study, appear to be comprised of multiple patches on a single fault segment that repeatedly fail in earthquakes that rupture each of those patches. Haiti is situated on a transform boundary also known as a strike Stewart The linearity of the CMT locations along the shorter Hollister transform (Fig. 3315km (2060mi) WSW of Punta Arenas, Chile One 30-42-km-long segment of the Heezen transform that appears to be an isolated well-coupled asperity has ruptured in eight earthquakes of Mw 5.9-6.1 quasi-periodically with a coefficient of variation of 0.26 every 4.0 1.0 yr. Other well-coupled fault segments, which were sites with earthquakes up to Mw 6.39 and fewer events since 1976, have average repeat times of about 7-24 yr. Smith (1993) obtained two normal-faulting mechanisms and computed depths of faulting from waveform matching. Search for other works by this author on: Earthquake slip on oceanic transform faults, Global CMT analysis of moderate earthquakes, Mw S.M. R.L. Finally we are left with what accounts for transform segments that rupture with high seismic coupling and those with low coupling. How that may vary along strike and affect seismic coupling is unknown. Symbols, axes and origin of coordinates same as Fig. [1] The two major faults in the Eltanin Fracture Zone are the Heezen Fault and the Tharp Fault. The two dotted lines in Fig. Our determinations of Mw typically use observations at more stations than those of Ms. Those for Mw also use Love and Rayleigh waves. In the basin on the eastern side of the MAR the Walvis Ridge blocks the northward passage. Tolstoy It and some other segments of the three Eltanin transforms appear to be well coupled seismically. 4. His relocations are shown in Figs 4-6 in green. The trench has been formed by the actions of the Romanche Fracture Zone, a portion of which is an active transform boundary offsetting sections of the Mid-Atlantic Ridge. When Sykes showed his results to M. Ewing, the latter arranged with NSF (US National Science Foundation) for the Research Vessel Eltanin to map the region. (2005), McGuire (2008) and Boettcher & McGuire (2009) find that some fault segments are characterized by a predominance of aseismic slip while others are fully coupled. The centroid epicentre and time did not shift appreciably in these different inversions, nor did the geometry of the moment tensor change appreciably. McGuire (2008) examined 16 pairs of Mw= 5.5 earthquakes that occurred along approximately the same segments of transform faults on the East Pacific Rise to the south and north of the equator. A large iceberg interfered with one scientific mapping (Lonsdale 1994). For LNADW and AABW, the Romanche and Chain Fracture Zones (just south of the equator) are the only deep passages in the MAR where interbasin exchange is possible. Added to Thesaurus in 1989. Our calculations for all events have the advantage of having been corrected for lateral heterogeneity with the same model. Enter your email address to follow this blog and receive notifications of new posts by email. Individual ridge segments trend, how- ever, up to a few tens of degrees from the smooth trend. 5 and the bathymetric expression of it in Fig. Three normal-faulting events were found with T-axes nearly perpendicular to the Heezen transform (Fig. We found that Mo evaluated at 10-km depth is 15 per cent smaller than that calculated at the best-fitting centroid depth (25.7 km). 2. D.W. 2. Eltanin Fracture Zone. The segment of the Heezen transform between 29 and 71 km (Fig. Since the number and azimuthal distribution of the global long-period stations used in computing the CMT solutions increased dramatically since 1976, especially in the 1990s, we choose to use digital data from as many stations as possible in our analyses rather than from merely the few that recorded most of the events. plates. The surface wave catalogue (Ekstram 2006) starts in 1990, and the magnitude that is associated with events in that catalogue is usually very close to the Mw obtained in a CMT analysis. Their surface wave magnitudes, Ms, are plotted. et al. If those short spreading centres are very young, they may not have had enough time to become oriented perpendicular to the transform. The segment of the Hollister transform between -635 and -650 km (Fig. Dziewonski Dziewonski Cumulative moment release is shown in Fig. According to the normal scenario for the opening of the South Atlantic, it is spreading at a rate of 1.75cm/year (0.69in/year) and began forming about 50 Ma. 5) are characterized by shocks of Mw= 5.9 but about half of its length is not. The excitation of the long-period waveforms used in the CMT analysis does not vary rapidly as a function of source depth for shallow-focus earthquakes. Cumulative seismic moment release from 1976 to 2010 along a 42-km section of Heezen transform indicated in Fig. Figs 5 and 6 show five other normal-faulting mechanisms adjacent to but not on the Tharp and Hollister transforms. The CMT centroid depths for events along the Eltanin system are not accurate enough to define the depths or range of seismic faulting at the level of several to 20 km. 7 are the best fit to that model. Recurrence of events along nearly the same fault segment is much more difficult to determine for transform faults on land, such as the San Andreas, where large to great earthquakes typically have repeat times of 100 yr or longer and instrumental data and exact dates for more than one earthquake along a fault segment are rare or non-existent. (2010) that describes the relative motion of the Pacific and Antarctic plates. Wolfe et al. Use the facts stated above when they apply. The San Andreas fault is part of a fault zone known as a transform Topographic Map of Eltanin Fracture Zone. It bisects the Mid-Atlantic Ridge (MAR) just north of the equator at the narrowest part of the Atlantic between Brazil and West Africa, extending from 2N to 2S and from 16W to 20W. . C.J. The new result is well constrained by the surface wave data, which were not included in the original analysis. Phipps Morgan & Forsyth (1988), Shen & Forsyth (1992), Behn (2007), Gregg et al. North of and parallel to the fracture zone is a transverse ridge which is particularly prominent over hundreds of kilometres east and west of the MAB of the South Atlantic. Thus, the use of the CMT methodology insured more complete coverage of earthquakes in this very remote area of the Earth. Lonsdale (1994) concludes that a zone of extension north of the Heezen transform developed following a Pliocene change in relative plate motion. That view was strengthen when Menard reviewed the manuscript and provided an unpublished bathymetric map that included more data in the far southeast Pacific. fault zone where the two blocks/plates on either side move side by 4). Sykes Lin In contrast, normal-faulting events of Mw 5-6.5 are common along slower spreading ridges such as those in the Atlantic. 9 for two sequences of earthquakes along the Tharp transform. 3 and studied separately. -Different segments of the mid-ocean ridge system spreading at different rates. Distances in kilometres same as in Fig. F. The island is very long and narrow. 8. The three sets of large events have a repeat time of 13 yr and a COV of 0.13. This transform migration was preceded by a process 2517 Ma during which the Miocene shallow-water platform mentioned above reached sea level as the transverse ridge was first uplifted, then deformed, and finally buried under the sea. G. The relationship between the two events that occurred on the same day with computed CMTs only 5 km apart is unknown. . When the event of Mw 6.3 on 1969 August 18 (Stewart & Okal 1983) is included, the repeat time of the three largest shocks is 19 yr with a COV of 0.37. It is therefore unlikely that the event selection for 19902009 missed any earthquake with a moment magnitude greater than 5.5. E.A. 4, between -25 km and 0 km). His events along what we identify as a well-coupled zone between 29 and 59 km in Fig. Ekstram Given the uncertainty of its CMT location, we think it is reasonably associated with a previous rupture of the same asperity as in Figs 4 and 7. To the north of the Eltanin Fracture Zone, a wider compositional diversity seems to occur, including FeTi-basaltsand trachy andesite, than that observed from within and to the south of this fracture zone and on the Chile Rise. This is a consequence of the continuity of displacement eigenfunctions for surface waves across discontinuities, and the dependence of the excitation on these displacements. It seems unlikely that our CMT location errors would change much over the length of the Tharp transform. For the eight fault segments examined in Figs 7-10 some systematic behaviour can be seen among Mws of the largest earthquakes, their average repeat times, T, and the numbers of large events. The steep slope of about -2.9 for Mw > 6 is unusual. Others related faults include the Vacquier Transform Fault, the Menard Transform Fault, and the Udintsev Fault.[3]. The average displacement per earthquake is 0.32 m assuming full coupling and an average repeat time of 4 yr. Figs 8-10 show cumulative moment release throughout 2010 for seven additional segments of the three Eltanin faults. Two 20-km segments of the Hollister transform (Figs 3 and 6) are the sites of several earthquakes of Mw= 5.9 whereas only single shocks of that size are found along its western half. Eltanin is an island located in the South Pacific Ocean, following the contours of the Eltanin Fracture Zone. P. The Romanche Fracture Zone offsets the Mid-Atlantic ridge by 900 km (560 mi), making it the largest equatorial fracture zone in the Atlantic. https://session.masteringgeology.com/problemAsset/3004355/4/MGEO_3004355_PartD.jpg. According to the normal scenario for the opening of the South Atlantic, it is spreading at a rate of 1.75 cm/year (0.69 in/year) and began forming about 50 Ma. Watts We did not use the ISC catalogue for events after 1999. Bohnenstiehl transform plate boundaries learning geology. We chose one of the Eltanin earthquake of (2009 May 7, Mw= 5.6) for which the CMT centroid depth is 25.7 km, and calculated the CMT at various fixed depths, inverting as usual for all the other parameters. Ryan and M. Tolstoy for helpful discussions. The fault segment between -665 and -680 km (green symbols in Fig. 4. Wolfe et al. Forsyth Okal A major purpose of our work is to examine the degree of repeatability (or lack thereof) of major earthquakes along various segments of the three Eltanin transforms. Wilson R.G. Examine the figure above. . Map view of CMT locations of earthquakes along Hollister transform fault from 1976 to 2010. Approximate slopes are denoted by dashed lines. LOUISVILLE RIDGE AND ELTANIN FRACTURE ZONE 3053 west to isolated peaks superimposed on ridge segments in the southeast. Also, there was no discontinuous change in the calculated Mo when the CMT was evaluated above the Moho (which in PREM is at 24.4 km) or below it. Strike-slip faulting with moment magnitudes Mw up to 6.4 characterizes most of these events. For example, the caridean shrimp Alvinocaris muricola inhabits cold seeps in both the Gulf of Mexico and the Gulf of Guinea, which clearly suggests a gene flow across the Atlantic, but the amount of data is very limited and dispersal pathways are not fully understood. 4. They obtained a depth in the range 8-14 km below the seafloor for the 1978 earthquake off the Heezen fault, and approximately 10 km below the seafloor for the 1989 earthquake off the Tharp transform. 4 are more tightly clustered in map view. Hence, it and the two larger events occurred along an off-transform feature. Challenger Fracture Zone. Eltanin Fracture Zone System is located in the Undersea Features Area of the Country of Undersea Features. The CMT locations for the Heezen and Hollister transforms and the eastern end of the Tharp are biased to the south of the bathymetric expression of the transforms (Fig. The Eltanin Transform Fault and Fracture Zone is a series of six or seven dextral transform faults along ~800 km of the Pacific-Antarctic Ridge that offset it ~1600 km. We find or confirm two anomalous characteristics: the . Montesi In contrast, VanDeMark (2006) used few events, a double-difference methodology and a uniform velocity model for Rayleigh waves in calculating revised earthquake locations along the Eltanin transforms. Wolfe et al. In the Atlantic Ocean most fracture zones originate from the Mid-Atlantic Ridge, which runs from north to south, and are therefore west to east oriented in general. west coast of the USA through California. Seabeam and SeaMARC II swath-mapping surveys, with ancillary magnetic and gravity profiling, describe the fast-slipping (84-86 mm year1) 380 km-long Heezen transform (56 S) and the 145 km-long Raitt transform (54 S), together with the youngest parts of their rise-flank fracture zones. At faster separating parts of the Pacific-Antarctic boundary, and on most of the rest of the EPR, fracture zone structure is mainly inherited from the variable arrangement of volcanic ridges and tectonic uplifts at the risecrest intersections, rather than from structures formed at the transform valley. We find one segment of the Heezen transform has ruptured quasi-periodically since 1976 in eight shocks of Mw 5.9-6.1 with an average repeat time of 4 yr. We recomputed centroid moment tensor (CMT) solutions for 190 earthquakes along and close to the three transforms of the Eltanin system from 1976 to 2010. The eastern part of the transverse ridge, however, consists of a thick sequence of stratified material called the Romanche Sedimentary Sequence (RSS). It is one the the major earthquake zones in The present application of this method points to a recent change in Pacific plate motion, and. The series includes two events of 6.17 and 6.18 and one of less certain Mw of 6.5 (Stewart & Okal 1983) on 1971 April 4 that are much larger than the other three. The greater number of stations in the last 20 years permitted CMT solutions to be obtained for a large number of smaller earthquakes, some as small as Mw 5.15. Wolfe R.P. Knowledge gained from them and other oceanic transform faults may aid in understanding the long-term behaviour of transform faults on land and those at subduction zones where large-to-great earthquakes recur with repeat times of 100 yr or more. FRACTURE ZONE. Their remoteness and rough seas, however, puts a premium on studying them at large distances using seismic waves and various satellite technologies. Broader Terms: East Pacific Pacific Ocean South Pacific Southeast Pacific Excluding it does not change the average repeat time for events of Mw > 5.9; it increases the SD and COV slightly. We used a 97-km length of the Hollister that includes all large shocks (Fig. The North Anatolian Fault is formed along the Anatolian plate Of the 84 earthquakes from 1976 to 1989, we rejected seven CMT solutions. The Romanche and Chain Fracture Zones creates a huge gap in the MAR and can act as a "Subsea Berlin Wall" segregating the North Atlantic communities from those in other oceans. Hence, we suspect that the events at the western end of the Tharp transform are anomalous in some sense. R. D.R. The continuity in smooth trend of the ridge and the Eltanin Fracture Zone suggests some relationship between them. Major differences in seismic coupling along strike are not in accord with common thermal models of plate cooling but instead are attributed to varying degrees of metamorphism, rock type and effective normal stress and possibly to the presence of short intratransform spreading centres. H.W. Please see the related question for more information. GE was partially supported by NSF grant EAR-09-24694. A Seabeam reconnaissance of the 400 km-long fast-slipping (88 mm yr-1) Heezen transform fault zone and the 55 km-long spreading center that links it to Tharp transform defined and bathymetrically described several types of ridges built by tectonic uplift and volcanic construction. Spreading Centre. Rupture of two or more nearby asperities in shocks as large as Mw 6.4 can occur but is rare as ascertained from the steep slope of the frequency-magnitude relationship in Fig. Eltanin Fracture Zone System: IHO-IOC GEBCO Gazetteer of Undersea Feature Names : English: Eltanin Fracture Zone: US BGN Advisory Committee on Undersea Features (ACUF) PlaceType: Fracture Zone : Latitude: 54 8' 16.8" S (-54.137999) Longitude: 127 32' 7.3" W (-127.53537) Min. The Atlantis Fracture Zone (30 N) is one of the smallest transform faults along the Mid-Atlantic Ridge with a spatial offset of 70 km and an age offset of ~ 6 Ma. The segment of the Heezen from 179 to 200 km has the next smallest T, 7.1 yr, Mw 5.89-6.5 and six events. M.D. 11 for Mw > 6. Cumulative seismic moment release 1971 to 2010 along two segments of Tharp transform as indicated in Fig. Which letter is pointing to a transform fault? From the scatter of locations perpendicular to the Eltanin transforms, the standard deviation of locations would be around 15 km, after allowing for a constant regional bias. Similarly, earthquake recurrence is more difficult to examine for other long oceanic transform faults, such as those in the equatorial Atlantic, that have considerably smaller long-term slip rates. Their 1000 km length is greater than that of any other oceanic transforms at fast-spreading plate boundaries. T.H. C. Our work using much greater numbers of stations and earthquakes confirms the general findings of Stewart and Okal about the abundance of aseismic slip and the presence of asperities. florida a jurassic transform plate boundary. The waveforms were edited using the standard Global CMT (GCMT) automated waveform editor, and inverted using the standard Global CMT computer codes using corrections for 3-D earth structure in the calculation of synthetic seismograms (Ekstram . A break in the smooth trend of the ridge at latitude 37.5S has been recognized in both bathymetric and altimetric data. The presence of large events just to its west and the variety of Mws, however, suggest that it does not always rupture as a single asperity. They proposed that the Eltanin transforms consisted of small, well-separated asperities that rupture in earthquakes. 4), to constrain their source depths. Locations of earthquakes with CMT solutions (red) characterized by a predominance of strike-slip faulting along the Heezen, Tharp and Hollister transform faults of the Eltanin system from 1976 to 2010. Maternovskaya The principal fracture zones are identified, as are segments of the PAR. Atwater 7 for events of Mw > 5.55 (blue) and Mw > 5.9 (red). M. Mw is systematically higher than mb for strike-slip events along the three transforms, on average about 0.5 units. J.H. The CMT locations for the Tharp transform are rotated several degrees with respect to both the computed transform direction in Fig. We choose to use a greater number of stations as time proceeded rather than use a few common stations as he did. We show in detail, however, that some segments of the three transforms are aseismic at the Mw > 5.4 level whereas others are more strongly coupled. 11 displays the log of the cumulative number of events per 20 yr as a function of decreasing Mw for the periods 1976-1990 and 1991-2010. The 30- to 42-km length that we used for the repeating events along the Heezen transform in Fig. Instead, we have chosen not to use models of the depth of seismic faulting in estimating coupling but to calculate downdip width from a segments length along strike, moment release rate and relative plate rate assuming' full seismic coupling. The mechanism in the GCMT catalogue indicates normal faulting, while the reanalysis resulted in a strike-slip mechanism. Our calculations of moment magnitude Mw are compared with determinations of the short-period magnitude mb in Fig. To address part of this question, we performed the following experiment. 2006). C.G. If the 1981 shock were mislocated at least 12 km eastwards, the length of the repeating segment would be about 30 km. If the anomalous trend is real, having one group of events (east or west) off the primary transform then almost becomes a necessary interpretation. The western part of the transverse ridge consists of fragments of uplifted oceanic crust and upper mantle. In that simple model, earthquakes should extend to greater depths and presumably be larger near the centres of transforms, which is clearly not the case for the Heezen and Tharp faults. We determined a few CMT mechanisms for which PDE or ISC did not compute hypocentres, for which we relied on detections and locations based on intermediate-period surface waves (Ekstram 2006). Pacific Ocean depth map showing the Heezen Fault and the Tharp Fault as #17. E.A. Turkey. The point source depth of 10.4 km places the 2007 earthquake 6-7 km below the ocean floor. How are fracture zones like the Eltanin created? 7; several were the sites of two or three earthquakes of Mw > 6.15. . Shen Enter the email address you signed up with and we'll email you a reset link. Mw determined in this study compared with the surface wave magnitude Ms as computed by either ISC or PDE for periods between about 17 and 22 s. Mw is determined at longer periods. These results suggest that asperities responsible for earthquake release at the Mw > 6 levels vary in size along strike but not much in downdip width. Although magmatism was more or less continuous on the ridge during 28-60 Ma, it probably occurred on crust . The relatively small size of that earthquake, its early date and the off-transform locations of other normal-faulting events lead us conclude it more likely occurred just north of the Heezen transform. Deep water flow through the trench is from west to east with a rate of 3.6 Sverdrups (million m/s) of 1.57C (34.83F) water. The efforts that we made, which are described in Appendix A, resulted in a number of additional solutions, especially for the period 1976-1991 and for Mw 5.4-6.0 (see Fig. right. . Dziewonski (1993) attribute it to one of those small spreading centres. Gregg 4786 km SE of Avarua, Cook Islands / pop: 13,400 / local time: 01:42:15.1 2017-06-15, http://www.emsc-csem.org/Earthquake/earthquake.php?id=524555, Distances: 4676 km SE of Wellington, New Zealand / pop: 381,900 / local time: 06:09:44.3 2016-08-19 C1. 10) was the site of events of Mw 6.39, 6.03 and 6.36 in 1984.4, 1997.4 and 2005.98, respectively. (1993) and Lonsdale (1994) concluded that the event of 1984 August 16 (Mw 5.3) was located along a bathymetric trough to the north of the Heezen transform. The normal-faulting event of 2001 August 6 is larger than that of any of the strike-slip events along the three Eltanin transforms. It is formed by a transform boundary. Seafloor magnetic anomalies suggest formation. Distances in kilometres same as in Fig. This is an improvement over past formulations where bathymetric offsets were imposed kinematica. Red triangle indicates CMT solution for shock of 1975 April 19. Gordon The plate boundary is about 1,200 kilometers long along the Since the depth range (Table 1) of faulting in strike-slip events along the Eltanin is very small, those earthquakes may be occurring just in the crust, only in the uppermost oceanic mantle or a combination of the two. McGuire They find that aftershock zones of nearby pairs of moderate-size earthquakes along fast-moving transforms were separated by small spreading centres. For the Eltanin events, this bias is unlikely to be larger than 15 per cent, even for the events with the deepest calculated centroids. This partitioning of slip likely results from changes during the last few million years in the Euler pole for relative motion between the Antarctic and Pacific plates. . 5) and the 42-km section of the Heezen (Figs 4 and 7) are well isolated from other large shocks along those transforms we took the scatter in locations along them as a measure of L. Our knowledge of L is limited otherwise by the paucity of aftershocks of Mw > 5-5.5. Four segments of the Tharp transform (Fig. NASA Technical Reports Server (NTRS) Kyte, Frank T.; Gersonde, Rainer; Kuhn, Gerhard. Atwater S.C.