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PERSPECTIVES 15 M The fates of stars. In this Hertzsprung-Russel diagram, the main diagonal line de- 10,000 notes “main sequence” stars, which, like the Sun, burn hydrogen in their cores. Lines Luminosity (solar units) 9M moving away from the main sequence are followed by stars after they have ex- hausted their hydrogen fuel supply. After a brief period of helium burning, most stars 100 eventually reach the white dwarf cooling track—the last part of this evolution for 3M δ Scuti t Hot roApp stars of less than ~8 solar masses (M ). Asteroseismology has produced insights in- 2M subdwarfs to the interior and evolution of a growing variety of stars (see shaded areas). The lat- Sun and 1 1M solarlike est stars to yield some of their secrets are the β-Cepheids (red area)—massive main- White dwarf sequence stars that are destined to become supernovae. cooling sequence 0.8 M 0.01 and stellar models, they infer may seem like a long time, yet to the uni- that the interior of the star ro- verse it is but the blink of an eye. Thanks to tates at different rates at dif- the observers who collected data for future 200 100 60 30 20 10 6 3 ferent depths, and that the tur- analysis since the 1980s, the next few years Surface temperature (1000 K) bulent process of convection should see a rapid growth in the seismolo- in its central nuclear furnace gy of HD 129929 and other massive stars With more than 90% of all stars sharing drives mixing of material beyond the “classi- as the data continue to accumulate. fates similar to that of the Sun, more mas- cal” limits of the convective core. Both of sive stars (which will eventually explode as these results had been hinted at by earlier References and Notes supernovae) escaped this detailed scruti- studies of the evolution of massive stars, but 1. C. Aerts et al., Science 300, 1926 (2003); published online 29 May 2003 (10.1126/science1084993). ny—but not anymore. Aerts et al. (1) shed new light on these 2. D. Gough, Science 291, 2325 (2001). Aerts et al. monitored the β-Cephei star processes. 3. T. Bedding et al., Astrophys. J. Lett. 549, 105 (2001). HD 129929 (with a mass of 9.5 solar mass- For their work to succeed, Aerts et al. 4. A. Landolt, Astrophys. J. 153, 151 (1968). es) for more than 21 years. Earlier studies required precision photometry, which was 5. R. E. Nather et al., Astrophys. J. 361, 309 (1990). 6. S. D. Kawaler, in Variable Stars as Essential Astrophysical identified some oscillation modes of this made possible by advances in instrumenta- Tools, C. Ibanoglu, Ed. (NATO Science Series C, Kluwer, star, but gaps in data sampling precluded un- tion in the second half of the 20th century. Dordrecht, Netherlands), vol. 544, p. 511 (2000). ambiguous assignments. Aerts et al. now But no instrument can remove the final 7. M. Breger, Baltic Astronomy 9, 149 (2000). show that HD 129929 is indeed a multiperi- constraint—time. Rewards for these past 8. D. W. Kurtz, Astrophys. Space Sci. 284, 29 (2003). 9. D. Kilkenny et al., Mon. Not. R. Astron. Soc. 285, 640 odic star. They resolve six independent oscil- advances in instrumentation are now final- (1997). lation modes in the star. From these periods ly being realized. For us humans, 20 years 10. S. Charpinet et al., Astrophys. J. Lett. 471, L103 (1996). GEOPHYSICS Quantification of the stress caused by the deep, slow earthquakes requires knowledge Slow But Not Quite Silent of the precise location and amount of the slow slip. Herein lies a problem. Static sur- Timothy I. Melbourne and Frank H. Webb face deformation from deep faulting pro- vides only a blurry image of creep at depth. aults at subduction zones—regions Cascadia subduction zone are not silent. Moreover, the vertical deformation that is F where one tectonic plate dives beneath another—generate the world’s largest earthquakes, which rapidly release strain Their geodetic deformation signature corre- lates with a characteristic seismic tremor that bears the telltale signature of forced flu- most useful for locating the creep is the least resolvable with GPS. As a result, stress drops have remained largely unconstrained, and over large areas of id flow. This correlation opens up a more the loading of the seismogenic zone by slow Enhanced online at the plate interface. facile avenue for studying slow earthquakes. earthquakes has not been well quantified. www.sciencemag.org/cgi/ In recent years, a Isolated reports of slow earthquakes Such was the state of affairs until last content/full/300/5627/1886 much slower form of have been around for decades (2). But un- year, when Obara discovered nonvolcanic strain release has til a few years ago, the geophysical net- tremor associated with subduction of the been detected in many subduction zones works needed to resolve subtle signatures Philippine sea plate beneath southeast Japan throughout the world. It involves episodes of slow earthquakes did not exist. It took (5). With the ultralow noise, bore-hole Hi- of fault slip that resemble conventional the deployment of dense global positioning Net array, Obara was able to detect long-pe- earthquakes, except that faulting occurs system (GPS) arrays around the world in riod seismic tremor at levels that on any slowly, often lasting weeks or months. the 1990s for transient slow faulting to be conventional network would have gone un- Such sluggish faulting should not by it- recognized as a widespread and fundamen- noticed or been attributed to anthropogenic self produce shaking at frequencies or inten- tal phenomenon. Japan, with its state-of- or other nontectonic sources. The signals sities that can be detected with seismome- the-art arrays of seismic and geodetic in- Obara recognized were previously only ters. Hence, “slow earthquakes” were held strumentation, has led the way in identify- found within active volcanoes, where they to be seismically quiet, or aseismic. But on ing transient slow faulting events (3, 4). are generated by flow-induced resonance in page 1942 of this issue, Rogers and Dragert A common characteristic of slow earth- magma-carrying conduits (6). Obara’s (1) show that slow earthquakes in the quakes in subduction zones is that they are tremor, however, appeared to come from deep. They occur along the deeper reaches deep regions, at depths of at least 35 km, and of the plate interface, below the seismo- well away from any known volcanic source. T. I. Melbourne is in the Department of Geological genic region that breaks every few hundred Like their volcanic cousins, the signals Sciences, Central Washington University, Ellensburg, WA 98926, USA. E-mail: email@example.com F. years to produce great earthquakes. Like described by Obara are emergent, that is, they H. Webb is in the Jet Propulsion Laboratory, California tickling the dragon’s belly, the slow fault- mostly lack any isolated seismic P or S waves Institute of Technology, Pasadena, CA 91125, USA. ing stress loads the seismogenic regions. that can be used to locate their origin. : 1886 20 JUNE 2003 VOL 300 SCIENCE www.sciencemag.org PERSPECTIVES Interseismic deformation from WILL ures at Earth’s surface. With nearly 2000 subduction of the Juan de Fuca new geophysical instruments coming on- plate. The deformation vectors line with EarthScope (10), the future reverse themselves for 2 to 6 HOLB promises better seismic locations, energy weeks every 14.5 ± 1 months WSLR estimates, and source mechanisms, as well during slow earthquakes. Tremor NANO CHWK DRAO as tighter constraints on along-strike prop- correlated with the vector re- UCLU SEDR agation of tremor and slip. versals is detected to the north NEAH ALBH It may therefore be only a matter of of the Olympic Peninsula. uc a SEAT time before the initiation of regular earth- eF n PABH quakes is itself tied definitively to fault n d otio a m SATS LIND Through cross-correlation of Ju fluid flow, an idea that has been around for ar ica JRO1 their filtered signal en- /ye mer FTS1 years. If this idea is proven to be correct, it mm h A K KELS GWEN GOBS velopes, however, Obara was 34 Nort CHZZ probably applies to faults beyond those at able to estimate that their subduction zones. Free-flowing brine has hypocenters fall along the NEWP CORV REDM been detected in faults at depths below 10 35- to 40-km depth contour km in the deepest boreholes on Earth (11). within the subducting Phil- DDSN Like many other aspects of earthquake CABL BURN ippine sea plate. At precise- MDMT physics, discoveries first made in subduc- ly this depth, the water-re- tion-zone faults may prove to be applicable 51 th A No MODB SHLD mm me r leasing dehydration from P S PTSG YBHB to all active faults—particularly those on /ye rica basalt to eclogite is ex- TRND which many of our cities are built. ar mo pected to occur (7). It thus Pa tio CME1 ci f n seems likely that the QUIN References ic tremor originates from 10 mm/year 1. G. Rogers, H. Dragert, Science 300, 1942 (2003); pub- lished online 8 May 2003 (10.1126/science. the forced flow of fluids 25 mm/year 1084783). that are released near the 200 km 2. J. Beavan, E. Hauksson, S. R. McNutt, R. Bilham, K. H. plate interface during Jacob, Science 222, 322 (1983). nism on how the fluid flow enables slow 3. I. Kawasaki et al., J. Phys. Earth 43, 105 (1995). metamorphic dehydration. But how is the 4. H. Hirose et al., Geophys. Res. Lett. 26, 3237 (1999). tremor related to slow earthquakes? slip remains unclear, but may prove as sim- 5. K. Obara, Science 296, 1679 (2002). Obara’s data show clearly that tremor ple as hydraulic pressure unclamping the 6. K. Aki, M. Fehler, S. Das, J. Volcanol. Geotherm. Res. 2, occurs in regions of known slow earth- fault walls that sandwich the fluid. 259 (1977). 7. S. M. Peacock, K. Wang, Science 286, 937 (1999). quakes, but is absent in areas where no slow The correspondence established by 8. B. Julian, Science 296, 1625 (2002). earthquakes have been detected. However, Rogers and Dragert (1) provides an impor- 9. M. M. Miller, T. I. Melbourne, D. J. Johnson, W. Q. he did not show that tremor and slow earth- tant new tool with which to study the slow Sumner, Science 295, 2423 (2002). quakes occur simultaneously. As Julian has earthquake process. Tremor can potentially 10. J. H. Whitcomb, Seismol. Res. Lett. 71, 251 (2000). 11. E. Huenges, W. Kessels, J. Kueck, in International Union pointed out (8), the Cascadia subduction be used to locate slow slip at depth more of Geodesy and Geophysics; XXI General Assembly zone off the western coast of North precisely than can static deformation meas- (Univ. of Colorado, Boulder, 1995), vol. 21, p. 145. America, with its periodic and predictable slow earthquakes (see the figure) (9), is ide- E C O L O G Y A N D E VO L U T I O N al for addressing the relation between slow earthquakes and Obara-type tremor. After detailed analysis of 10 years of seismic recordings from Vancouver Island, Rogers Desperately Seeking Similarity and Dragert now conclude not only that Janis L. Dickinson and Walter D. Koenig slow earthquakes and tremor are highly correlated, but that one is the hallmark of ver since W. D. Hamilton pointed out cial to the evolution of helping behavior. the other. Cascadia slow earthquakes are not silent; rather, they are accompanied by tremor that is notably absent when slow E that cooperation is facilitated by genet- ic relatedness (1), “kin selection” has held a central place in the study of social be- The importance of kinship relative to other more direct benefits of group living and co- operation is the subject of much debate. Two faulting is not occurring. havior. Although most cooperative societies studies on pages 1947 and 1949 of this issue The slow earthquakes in the Cascadia comprise close relatives (2), there has al- (3, 4) shed new light on this problem. subduction zone, and by extension else- ways been a caveat to the logical conclusion In support of kin selection, a preponder- where around the world, thus seem to be that kin selection is the driving force in their ance of cooperative species have groups moderated by fluid flow in or near the plate evolution. Consider cooperative breeders consisting of close relatives. One particular- interface. As in southwest Japan, the such as the meerkat (Suricata suricatta) that ly notable experiment in a British bird Cascadia tremor peaks between 1 and 5 Hz, have “helpers” providing care to young that species showed that returning helpers pre- persists for days to weeks, migrates tens of are not their own. Although helpers are usu- ferred to assist relatives over unrelated pairs km horizontally along the fault plane, and ally a breeder’s offspring from prior years, at closer nests (5). Critics argue that cooper- appears to both trigger and be triggered by close genetic relatedness between the giver ation among relatives arises as a side effect adjacent conventional earthquakes. The and recipient of aid is not necessarily cru- of delayed dispersal, which causes off- tremor is not caused by near-simultaneous spring to remain near kin. This viewpoint is slip of large regions, as in conventional advocated in recent reviews highlighting The authors are at the Hastings Reservation and earthquakes, but probably by brine resonat- Museum of Vertebrate Zoology, University of gains for helpers independent of aiding rel- ing the walls of the conduits through which California, Berkeley, CA 93924, USA. E-mail: sialia@ atives (6, 7) and severe competition that re- it episodically bursts. The precise mecha- uclink.berkeley.edu, firstname.lastname@example.org duces or eliminates kin-selected benefits www.sciencemag.org SCIENCE VOL 300 20 JUNE 2003 1887
"Slow But Not Quite Silent"