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Earthquake Slip on Oceanic Transform Faults

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Earthquake Slip on Oceanic Transform Faults letters to nature 28. Shackleton, N. J., Berger, A. & Peltier, W. R. An alternative astronomical calibration of the lower These depths indicate that the maximum depth of brittle failure is Pleistocene timescale based on ODP site 677. Trans. R. Soc. Edinb. Earth Sci. 81, 251±261 (1990). at a temperature of ,600 8C in oceanic lithosphere. We ®nd that 29. Maasch, K. A. Statistical detection of the mid-Pleistocene transition. Clim. Dyn. 2, 133±143 (1988). 30. Chambers, J., Cleveland, W., Kleiner, B. & Tukey, P. Graphical Methods for Data Analysis 395 the body waves from the Romanche 1994 earthquake can be well (Chapman & Hall/CRC, Boca Raton, 1983). modelled with relatively deep slip on a single fault, and we use Supplementary information is available on Nature's World-Wide Web site the mechanism and depth of this earthquake to recalculate its (http://www.nature.com) or as paper copy from the London editorial of®ce of Nature. source spectrum. The previously reported slow precursor can be explained as an artefact of uncertainties in the assumed model Acknowledgements parameters. We thank the BDP Leg IV members from Russia, America and Japan who worked to drill The depth extent of seismic slip in shallow oceanic earthquakes is the BDP98 sediment cores, and J. Laskar, A. Berger and M.-F. Loutre for providing the poorly known, limiting our understanding of how oceanic trans- calculated results on insolation. We also thank colleagues in the hydro-geomorphological form faults move, and how the lithospheric composition affects the laboratory at Kanazawa University and the geomagnetic laboratory of Toyama University earthquake rupture process. Estimates of seismic coupling deter- for their support in this research. mined from the ratio of seismic slip to plate motion by summing the Correspondence and requests for materials should be addressed to K.K. earthquake moments are crucially dependent on the width of the (e-mail: [email protected]). fault assumed to slip in earthquakes. Oceanic crust is only ,6km thick. Theoretical strength pro®les predict that seismic slip should extend into the upper mantle, unlike continents, where it is restricted to the upper crust6. ................................................................. The international agencies locating earthquakes typically report the depth of earthquakes on oceanic ridges and transforms as 10 or Earthquake slip on oceanic transform 15 km. More accurate depths and mechanisms of oceanic earth- quakes have been obtained by modelling long period (LP) body faults waves7±9. The depths of oceanic intraplate earthquakes increase with lithospheric age, implying that temperature controls the depth of Rachel E. Abercrombie & GoÈran EkstroÈm the brittle±ductile transition7. The LP data were inadequate to resolve similar trends in the depths of earthquakes along the Department of Earth and Planetary Sciences, Harvard University, relatively young transform faults. Few transform fault studies have 20 Oxford Street, Cambridge, Massachusetts 02138, USA used ocean-bottom seismometers, and most were unable to resolve .............................................................................................................................................. hypocentres accurately10. Oceanic transform faults are one of the main types of plate Very slow slip occurs in some earthquakes11, but the observations boundary, but the manner in which they slip remains poorly of oceanic transform earthquakes3±5 remain controversial. This is understood. Early studies suggested that relatively slow earth- because the hypothesized slow component is often a small fraction quake rupture might be common1,2; moreover, it has been of the total moment and because other authors have failed to reported that very slow slip precedes some oceanic transform con®rm the proposed slow rupture during the largest earthquake earthquakes, including the 1994 Romanche earthquake3±5. The considered12,13. The shape of the long-period spectrum, used to presence of such detectable precursors would have obvious impli- identify possible slow rupture, is strongly dependent on the Earth cations for earthquake prediction. Here we model broadband structure and source parameters (including depth) assumed in the seismograms of body waves to obtain well-resolved depths and modelling14. More accurate measurements of these parameters are rupture mechanisms for 14 earthquakes on the Romanche and therefore necessary. Chain transform faults in the equatorial Atlantic Ocean. We found Modern broadband data have signi®cantly higher resolution than that earthquakes on the longer Romanche transform are system- the LP data. We use these data to compare earthquakes on two atically deeper than those on the neighbouring Chain transform. contrasting transform faults on the equatorial Mid-Atlantic Ridge. 336˚ 338˚ 340˚ 342˚ 344˚ 346˚ 348˚ 2˚ 2˚ 7/90 12/92 7/90 2/93 8/94 3/98 3/94 5/95 8/90 0˚ 2/96 2/96 0˚ 2/96 8/92 4/98 Romanche –2˚ Chain –2˚ 336˚ 338˚ 340˚ 342˚ 344˚ 346˚ 348˚ –8000 –6000 –4000 –2000 0 Metres Figure 1 Map of the equatorial Mid-Atlantic Ridge, showing the Romanche and Chain on the Romanche transform all dip to the south at ,808, and those on the Chain transform faults and also the bathymetry26,27. The transform valleys are dark blue and the transform all dip to the north at ,738. The centroid-moment tensor (CMT) mechanisms interconnecting ridges are pale blue. The focal mechanisms obtained in the broadband are not accurate enough to resolve these differences. The active faults thus seem planar, body-wave modelling are joined to their National Earthquake Information Center but the reason for their opposite sense of dip is unknown. hypocentre locations (red circles), with radius proportional to magnitude. The earthquakes 74 © 2001 Macmillan Magazines Ltd NATURE | VOL 410 | 1 MARCH 2001 | www.nature.com letters to nature We compare earthquakes on the Romanche transform, which is one since 1990 (refs 15, 16) (Figs 1 and 2). Most of these earthquakes are of the longest on the mid-ocean ridge system and offsets 50-Myr well recorded, and depths and mechanisms are well constrained. lithosphere, with events on the neighbouring Chain transform, The mechanisms of the earthquakes on each transform are which is shorter and offsets younger lithosphere (17 Myr) (Fig. 1). extremely similar. They all have strikes within 28 of the strike of We use broadband body waves to determine focal mechanisms and the transforms (,808), implying that the transform faults are depths for the 14 largest earthquakes in the Harvard CMT catalogue planar. Using these well-constrained focal mechanisms, we deter- mine the distribution of slip in the largest, 1994 Romanche, earth- quake (Fig. 3). This earthquake has previously been reported to have a complex rupture, including a slow precursor4. Strong eastward directivity is clear in the waveforms. The earthquake had a small a PAB 8.84 5/95P amplitude onset lasting about 12 s (subevent A (ref. 4), which is not HRV 4.54 Romanche (P) 5/95 P 17 3/98PAB 1.55 unusual for a large earthquake . The main slip occurred in two large P subevents to the east of the hypocentre. Their centroid depth agrees SSPA 3/98 0.49 PAB 1.07 P 2/93P well with that obtained from the body-wave modelling (12 km; all depths quoted are below the sea ¯oor) and seismic slip extends to SJG 5/95 7.95 LSZ 10.3 P 5/95P 5/95: 16 km, Mw6.8 TSUM 2.27 3/98P 3/98: 20 km, Mw6.1 PAB 20 s 2/93: 21 km, M 6.3 w HRV ECH b Chain (P) PAB CCM HRV 4.51 2/96a 8.60 2/96b P P SSB PAB 10.8 2/96b P SJG 1.35 ANMO 4/98 P TAM TSUM 2/96a SJG 8.98 4/98 .1.88 SJG P P OBN 2/96b SJG 7.04 TSUM P 2/96a P 10.7 4/98: 8 km, M 6.1 100 s KIV w TSUM 2/96a: 8 km, Mw6.5 2/96b P 13.5 20 s 2/96b: 10 km, Mw6.6 NNA c LBTB SH waves Romanche 5/95 PAB 9.72 LPAZ Romanche S BOSA 3/98 KMBO 3/98 PAB 2.39 S 2.30 S BDFB PAB SUR 2/93 S 3.28 Chain Chain 2/96a KMBO 22.2 S 4/98 PAB 2.40 S 0 KMBO 2/96b S 22.8 20 s 2/96a PAB 13.2 S 10 2/96b PAB 27.4 S 20 Depth (km) 30 Figure 2 Comparison of the waveforms from six earthquakes, three on each of the 0 40 80 120 Romanche and Chain transforms. The observed displacement broadband waveforms Distance along strike, E (km) (solid lines, 20 samples per second), are corrected for the instrument response and ®ltered between 1 and 150 s. The dashed lines are the synthetics. The stations are chosen to allow comparison between earthquakes on each transform and between transforms. 0 0.5 1.0 1.5 2.0 The dates, depths and magnitudes are given. The amplitudes of the seismograms (mm) Slip (m) are given at the top right. The arrows mark the P picks; the vertical lines indicate the duration used in the inversion. a, b, P waves are shown from earthquakes on the Figure 3 Inversion for slip distribution in the 14 March 1994 Romanche earthquake Romanche transform (a) and the Chain transform (b). All six mechanisms are shown, and (Mw 7.1). We use the mechanism obtained in the body-wave modelling (strike 828, dip those of the 1995 and February 1996b earthquakes are shaded. c, SH waves from all six 758, rake 1798) to invert for slip on a ®nite fault plane. We use broadband displacement earthquakes. The SH mechanisms of the Romanche earthquakes are shown in black, and P waves, low-pass-®ltered at 1 Hz. We use an oceanic structure and invert for slip on a those on Chain in grey.
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