Segmentation of Transform Systems on the East Pacific Rise
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Segmentation of transform systems on the East Paci®c Rise: Implications for earthquake processes at fast-slipping oceanic transform faults Patricia M. Gregg Massachusetts Institute of Technology/WHOI Joint Program in Oceanography, Woods Hole, Massachusetts 02543, USA Jian Lin Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA Deborah K. Smith ABSTRACT Per®t et al., 1996) indicate that ITSCs are Seven of the eight transform systems along the equatorial East Paci®c Rise from 128 N magmatically active, implying that the regions to 158 S have undergone extension due to reorientation of plate motions and have been beneath them are hotter, and thus the litho- segmented into two or more strike-slip fault strands offset by intratransform spreading spheric plate is thinner than the surrounding centers (ITSCs). Earthquakes recorded along these transform systems both teleseismically domains. To explore the effect of segmenta- and hydroacoustically suggest that segmentation geometry plays an important role in how tion on the transform fault thermal structure, slip is accommodated at oceanic transforms. Results of thermal calculations suggest that we use a half-space steady-state lithospheric the thickness of the brittle layer of a segmented transform fault could be signi®cantly cooling model (McKenzie, 1969; Abercrom- reduced by the thermal effect of ITSCs. Consequently, the potential rupture area, and bie and Ekstrom, 2001). The temperature thus maximum seismic moment, is decreased. Using Coulomb static stress models, we within the crust and mantle, T, is de®ned as T 5 k 21/2 illustrate that long ITSCs will prohibit static stress interaction between transform seg- Tmerf [y(2 t) ], where Tm is the mantle ments and limit the maximum possible magnitude of earthquakes on a given transform temperature at depth, assumed to be 1300 8C; k system. Furthermore, transform earthquakes may have the potential to trigger seismicity y is the depth; is the thermal diffusivity, 26 2 21 on normal faults ¯anking ITSCs. assumed to be 10 m s ; and t is the age of the lithosphere obtained by dividing distance Keywords: seismology, earthquake stress triggering, Siqueiros transform fault, transform faults, from the ridge axis by half the spreading rate. East Paci®c Rise, Clipperton transform fault. INTRODUCTION servations of earthquakes recorded on East Segmented transform systems are com- Paci®c Rise transform faults indicate that seg- posed of several fault strands offset by short mentation is an important factor in¯uencing ridges or rifts referred to as intratransform rupture of large earthquakes at oceanic trans- spreading centers (ITSCs) (Menard and At- forms. While it has been shown that segmen- water, 1969; Searle, 1983; Pockalny et al., tation and fault steps play an important role in 1997), where active sea¯oor spreading and controlling the earthquake behavior of conti- crustal accretion are occurring (Fornari et al., nental strike-slip faults (e.g., Harris and Day, 1989; Hekinian et al., 1992; Per®t et al., 1993), the in¯uence of segmentation and 1996). Along the equatorial East Paci®c Rise ITSCs on earthquake processes at an oceanic between 158 S and 128 N (Fig. 1), the Siquei- transform system has not been studied in ros, Quebrada, Discovery, Gofar, Yaquina, detail. Wilkes, and Garrett transform systems have In this paper, we use teleseismically and hy- all undergone transtension due to changes in droacoustically recorded seismicity data from plate motions, and each of these transforms is the equatorial East Paci®c Rise and Coulomb segmented by at least one ITSC (Searle, 1983; static stress models to explore the effect of Fornari et al., 1989; Lonsdale, 1989; Goff et ITSCs on static stress interaction between al., 1993; Pockalny et al., 1997). The Clip- transform fault segments. We investigate perton transform system has undergone sev- whether adjacent fault segments can behave eral periods of transpression (Pockalny, 1997), independently of one another, and how the in- and is the only unsegmented transform system teraction between segments depends on their along the equatorial East Paci®c Rise. offset distance. The global de®ciency of seismic moment release on oceanic transform systems has led TRANSFORM SEGMENTATION researchers to hypothesize that a signi®cant Segmentation of the transtensional trans- portion of oceanic transform slip is accom- form systems at the equatorial East Paci®c Figure 1. Regional map of the equatorial modated aseismically (e.g., Boettcher and Jor- Rise has resulted in individual strike-slip fault East Paci®c Rise showing large transform dan, 2004). However, global seismicity studies strands with lengths of 18±89 km, with an av- and nontransform offsets. Segmentation ge- have yet to consider the prevalence of trans- erage of ;37 km. The ITSCs separating the ometry is included based on previous geo- form fault segmentation. Dziak et al. (1991) fault strands have lengths of 5±20 km, with logical mapping of the transform systems ; (e.g., Fornari et al., 1989; Lonsdale, 1989). observed that earthquake sizes generally cor- an average length of 11 km. Fresh lavas col- Inset: Regional map showing location of the relate with the lengths of individual fault seg- lected from the ITSCs within the Siqueiros full array of NOAA Paci®c Marine Environ- ments at the Blanco transform fault. Our ob- and Garrett transforms (Hekinian et al., 1992; mental Laboratory hydrophones. q 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; April 2006; v. 34; no. 4; p. 289±292; doi: 10.1130/G22212.1; 7 ®gures. 289 Figure 2. Comparison of estimated areas of Figure 4. Schematic models showing the ge- brittle lithosphere using a one-dimensional, ometry of two transform segments bisected steady-state lithosphere cooling model by a single ITSC of variable length, L. The (McKenzie, 1969) for the Clipperton (B) and source earthquake is located on the bottom Siqueiros (C) transforms. A: The 90 km Clip- right transform segment with its left edge perton transform system (X±X') and the 150 located at a distance, d, from the ITSC- km Siqueiros transform system (Y±Y'), transform intersection. The source earth- which is broken into ®ve major segments quake is assumed to be a strike-slip event S1, S2, S3, S4, and S5 separated by four on a vertical plane parallel to the transform ITSCs SA, SB, SC, and SD (Fornari et al., segment. A: A scenario in which the receiv- 1989). B: Calculated area of brittle litho- er fault is a strike-slip fault located on the sphere for temperatures <600 8C (shaded re- top left transform segment, which is as- gion) for the Clipperton transform. C: Com- sumed to have the same dip, strike, and parison of the calculated areas of brittle Figure 3. A: The predicted maximum mo- rake as the source earthquake. B: A scenar- lithosphere for the Siqueiros transform for ment magnitude, M , of earthquakes W io in which the receiver fault is a normal a model of unsegmented geometry (area (curved lines) for a given transform segment fault located along the ITSC, which is as- above the dotted line) versus a model con- area and a constant slip of 0.1, 0.3, and 1.0 sumed to have a dip of 608 and is parallel to sisting of ®ve individual segments offset by m. The calculations assume that the earth- the ITSC. steady-state ITSCs (shaded area). quake ruptures the entire transform seg- ment. Black dots mark the observed maxi- mum MW recorded on the transform Figure 2 compares the calculated areas of segments of the equatorial East Paci®c lihood of rupturing multiple transform seg- 5 brittle deformation, de®ned as regions with Rise. The rightmost data point, MW 6.6, ments during a single earthquake. According calculated temperatures #600 8C, for the ge- corresponds to the Clipperton transform. B: to Coulomb failure criteria, when an earth- The predicted maximum M assuming a ometries of the Clipperton and Siqueiros W quake occurs on a source fault, changes in constant stress drop of transform earth- Ds transform systems. The calculated area under quakes of 1, 10, and 100 bar. Coulomb failure stress ( f) on a receiver 8 Ds 5Dt 1m93Ds the 600 C isotherm for the Clipperton trans- fault are expressed as f s n, 2 2 Dt Ds form fault is 326 km , compared to 710 km where s and n are changes in shear and for a model of a single unsegmented fault with to a speci®c transform segment using hydro- normal stresses, on the receiver fault, and m9 the cumulative length of the Siqueiros trans- acoustically recorded earthquakes, which have is the apparent friction coef®cient adjusted for form system. When the actual segmentation smaller location errors (,6 km) (Fox et al., the pore pressure effect (King et al., 1994). geometry of the Siqueiros transform system is 2001). The maximum earthquake magnitudes We consider a simple geometry in which considered, however, the integrated area of the observed on each of the transform fault seg- two adjacent transform segments are offset by calculated brittle deformation region is de- ments at the equatorial East Paci®c Rise from an ITSC of variable length, L, for two scenar- creased by ;60% to 277 km2. 1996 to 2001 are plotted in Figure 3. Assum- ios assuming the receiver faults are either Seismic moment (Mo), which re¯ects the ing the complete rupture of a given individual strike-slip events along the adjacent transform energy released by an earthquake, is a func- fault segment, we can estimate the amount of segment (Fig. 4A) or normal-faulting events tion of the rupture area of the fault. Speci®- slip or the stress drop for a given earthquake. located along the ITSC (Fig.