Slow But Not Quite Silent

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Science  20 Jun 2003:
Vol. 300, Issue 5627, pp. 1886-1887
DOI: 10.1126/science.1086163

Faults at subduction zones—regions where one tectonic plate [HN1] dives beneath another—generate the world's largest earthquakes, which rapidly release strain [HN2] over large areas of the plate interface. In recent years, a much slower form of strain release has been detected in many subduction zones throughout the world. It involves episodes of fault slip that resemble conventional earthquakes, except that faulting occurs slowly, often lasting weeks or months.

Such sluggish faulting should not by itself produce shaking at frequencies or intensities that can be detected with seismometers. Hence, “slow earthquakes” [HN3] were held to be seismically quiet, or aseismic. But on page 1942 of this issue, Rogers and Dragert (1) [HN4] show that slow earthquakes in the Cascadia subduction zone [HN5] are not silent. Their geodetic deformation signature correlates with a characteristic seismic tremor that bears the telltale signature of forced fluid flow. This correlation opens up a more facile avenue for studying slow earthquakes.

Isolated reports of slow earthquakes have been around for decades (2). But until a few years ago, the geophysical networks needed to resolve subtle signatures of slow earthquakes did not exist. It took the deployment of dense global positioning system (GPS) [HN6] arrays around the world in the 1990s for transient slow faulting to be recognized as a widespread and fundamental phenomenon. Japan, with its state-of-the-art arrays of seismic and geodetic instrumentation, has led the way in identifying transient slow faulting events (3, 4).

A common characteristic of slow earthquakes in subduction zones is that they are deep. They occur along the deeper reaches of the plate interface, below the seismogenic region that breaks every few hundred years to produce great earthquakes. Like tickling the dragon's belly, the slow faulting stress [HN7] loads the seismogenic regions. Quantification of the stress caused by the deep, slow earthquakes requires knowledge of the precise location and amount of the slow slip. Herein lies a problem. Static surface deformation from deep faulting provides only a blurry image of creep [HN8] at depth. Moreover, the vertical deformation that is most useful for locating the creep is the least resolvable with GPS. As a result, stress drops have remained largely unconstrained, and the loading of the seismogenic zone by slow earthquakes has not been well quantified.

Such was the state of affairs until last year, when Obara [HN9] discovered nonvolcanic tremor associated with subduction of the Philippine sea plate [HN10] beneath southeast Japan (5). With the ultralow noise, bore-hole Hi-Net array [HN11], Obara was able to detect long-period seismic tremor at levels that on any conventional network would have gone unnoticed or been attributed to anthropogenic or other nontectonic sources. The signals Obara recognized were previously only found within active volcanoes [HN12], where they are generated by flow-induced resonance in magma-carrying conduits (6) [HN13]. Obara's tremor, however, appeared to come from deep regions, at depths of at least 35 km, and well away from any known volcanic source.

Like their volcanic cousins, the signals described by Obara are emergent, that is, they mostly lack any isolated seismic P or S waves [HN14] that can be used to locate their origin. Through cross-correlation of their filtered signal envelopes, however, Obara was able to estimate that their hypocenters fall along the 35- to 40-km depth contour within the subducting Philippine sea plate. At precisely this depth, the water-releasing dehydration from basalt to eclogite [HN15] is expected to occur (7). It thus seems likely that the tremor originates from the forced flow of fluids that are released near the plate interface during metamorphic dehydration. But how is the tremor related to slow earthquakes?

Obara's data show clearly that tremor occurs in regions of known slow earthquakes, but is absent in areas where no slow earthquakes have been detected. However, he did not show that tremor and slow earthquakes occur simultaneously. As Julian has pointed out (8) [HN16], the Cascadia subduction zone off the western coast of North America, with its periodic and predictable slow earthquakes (see the figure) (9), is ideal for addressing the relation between slow earthquakes and Obara-type tremor. After detailed analysis of 10 years of seismic recordings from Vancouver Island, Rogers and Dragert now conclude not only that slow earthquakes and tremor are highly correlated, but that one is the hallmark of the other. Cascadia slow earthquakes are not silent; rather, they are accompanied by tremor that is notably absent when slow faulting is not occurring.

Interseismic deformation from subduction of the Juan de Fuca plate.

The deformation vectors reverse themselves for 2 to 6 weeks every 14.5 ± 1 months during slow earthquakes. Tremor correlated with the vector reversals is detected to the north of the Olympic Peninsula.

The slow earthquakes in the Cascadia subduction zone, and by extension elsewhere around the world, thus seem to be moderated by fluid flow in or near the plate interface. As in southwest Japan, the Cascadia tremor peaks between 1 and 5 Hz, persists for days to weeks, migrates tens of km horizontally along the fault plane, and appears to both trigger and be triggered by adjacent conventional earthquakes. The tremor is not caused by near-simultaneous slip of large regions, as in conventional earthquakes, but probably by brine resonating the walls of the conduits through which it episodically bursts. The precise mechanism on how the fluid flow enables slow slip remains unclear, but may prove as simple as hydraulic pressure unclamping the fault walls that sandwich the fluid.

The correspondence established by Rogers and Dragert (1) provides an important new tool with which to study the slow earthquake process. Tremor can potentially be used to locate slow slip at depth more precisely than can static deformation measures at Earth's surface. With nearly 2000 new geophysical instruments coming online with EarthScope (10) [HN17], the future promises better seismic locations, energy estimates, and source mechanisms, as well as tighter constraints on along-strike propagation of tremor and slip.

It may therefore be only a matter of time before the initiation of regular earthquakes is itself tied definitively to fault fluid flow, an idea that has been around for years. If this idea is proven to be correct, it probably applies to faults beyond those at subduction zones. Free-flowing brine has been detected in faults at depths below 10 km in the deepest boreholes on Earth (11). Like many other aspects of earthquake physics, discoveries first made in subduction-zone faults may prove to be applicable to all active faults—particularly those on which many of our cities are built.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

Dictionaries and Glossaries

A glossary of geological terms is offered by GeologyLink.

An earthquake image glossary is provided by the Earthquake Hazards Program of the U.S. Geological Survey (USGS).

A glossary of earthquake and other geological terms is provided by B. Weaver, School of Geology and Geophysics, University of Oklahoma.

Web Collections, References, and Resource Lists

The Open Directory Project provides links to Earth sciences Internet resources.

Academic Info provides links to geology resources. A section of earthquake resources is included.

Geology Central, a database of geological Web sites, is maintained by R. Robinson, Geology Program, Santa Monica College, CA.

Geo-Guide, a collection of links to Internet geology resources, is provided by the Göttingen State and University Library, Germany.

The Yahoo! Science Directory includes a collection of seismology links.

GeologyLink is an educational resource center provided by Houghton Mifflin. A collection of earthquake Internet resources is included.

Seismosurfing the Internet for Earthquake Data is a collection of links to Internet resources maintained by S. Malone, Department of Earth and Space Sciences, University of Washington.

Online Texts and Lecture Notes

The USGS Earthquake Hazards Program provides information on worldwide earthquake activity, earthquake science, and earthquake hazard reduction.

Exploring Our Dynamic Planet is an educational interactive map presentation by the Digital Library for Earth System Education at the University Corporation for Atmospheric Research (UCAR).

L. Fichter, Department of Geology and Environmental Science, James Madison University, offers a collection of geology references and tutorials. A plate tectonic primer is included.

The Dynamic Earth Web site, provided as a teaching resource by the School of Earth Sciences, University of Leeds, UK, offers presentations with Internet links on the dynamic Earth and tectonics.

Understanding Earthquakes is a presentation by the Institute for Crustal Studies, University of California, Santa Barbara.

G. Anderson, USGS Pasadena Office, makes available lecture notes for a course on Earth sciences.

J. Louie, Seismological Laboratory, University of Nevada, provides lecture notes and other resources about earthquakes.

H. Maher, Department of Geography and Geology, University of Nebraska, provides lecture notes for a course on plate tectonics.

T. Lay, Department of Earth Sciences, University of California, Santa Cruz, offers lecture notes for a course on Earth catastrophes.

C. Ammon, Department of Geophysics, Pennsylvania State University, makes available lecture notes for an earthquake course.

P. Davis, Department of Earth and Space Sciences, University of California, Los Angeles, provides lecture notes for a course on earthquakes.

S. Nelson, Department of Earth and Environmental Sciences, Tulane University, offers lecture notes on earthquakes for a physical geology course.

S. Dutch, University of Wisconsin, Green Bay, offers a lecture notes on plate tectonics and faults and earthquakes for a physical geology course. Lecture notes for a course on crustal movements are also provided.

General Reports and Articles

This Dynamic Earth: The Story of Plate Tectonics by W. J. Kious and R. Tilling is made available on the Web by the USGS.

Living on an Active Earth: Perspectives on Earthquake Science is a 2002 report made available on the Web by the National Academies Press.

Numbered Hypernotes

1. Tectonic plates and subduction zones. A map showing the locations of the major tectonic plates of the world is provided by the USGS Eastern Region office. The University of Leed's Dynamic Earth Web site offers introductions to tectonic plates and subduction zones. The Plate Tectonics Web site provides an introduction to tectonic plates and a presentation on subduction zones. The plate tectonics presentation by the USGS Geology in the Parks Program includes a section on convergent plate boundaries and subduction zones. W. Leeman, Department of Earth Science, Rice University, offers a presentation on subduction zones. C. Lithgow-Bertelloni, Department of Geological Sciences, University of Michigan, provides lecture notes on convergent margins and subduction zones for a geology course. J. Revenaugh, Department of Earth Sciences, University of California, Santa Cruz, offers lecture notes on plate tectonics for a course on geologic principles. S. Stein, Department of Geological Sciences, Northwestern University, provides slide lecture notes for a course on plate tectonics.

2. Strain is defined in the USGS earthquake glossary.

3. Slow earthquakes. The 21 October 1983 issue of Science had a report by J. Beavan, E. Hauksson, S. R. McNutt, R. Bilham, and K. H. Jacob titled “Tilt and seismicity changes in the Shumagin seismic gap” (2) about an inferred slow slip event (access to the JSTOR archive of Science articles is available at The Southern California Earthquake Center makes available an April 2000 article by M. Forrest about slow earthquakes. The November 2002 issue of Discover had an article by K. Wright about slow earthquakes titled “The silent type.” The 25 May 2001 issue of Science had a report by H. Dragert, K. Wang, and T. James titled “A silent slip event on the deeper Cascadia subduction interface” and a Perspective by W. Thatcher titled “Silent slip on the Cascadia subduction interface.” The 29 March 2002 issue had a Brevia report by M. M. Miller, T. Melbourne, D. Johnson, and W. Sumner titled “Periodic slow earthquakes from the Cascadia subduction zone” (9) and a News of the Week article by E. Stokstad titled “Deep quakes slow but very steady.” The Geodesy Laboratory & PANGA Data Analysis Facility at Central Washington University provides a presentation about this study. V. Kostoglodov, Instituto de Geofísica, Universidad Nacional Autònoma de México, offers a research presentation titled “New large, silent earthquake in the seismic gap of Guerrero, Mexico.”

4. G. Rogers is in the National Earthquake Hazards Program and H. Dragert is in the Geodynamics Program at the Pacific Geoscience Centre, Geological Survey of Canada (GSC), Sidney, British Columbia. The GSC's Pacific Geoscience Centre offers a presentation on observing tremor and slip in the Cascadia subduction zone.

5. Cascadia subduction zone. For a course on plate tectonics, S. Stein provides a map locating the Cascadia subduction zone and a diagram of the geological processes occurring there. The Pacific Northwest Seismograph Network at the University of Washington offers a presentation about the Cascadia subduction zone and a feature titled “Samples of webicorders and spectrograms for Feb-Mar, 2003 deep tremor period.” The GSC's Pacific Geoscience Centre offers presentations on earthquakes and plate tectonics in western Canada and on the measurement of subduction zone deformation. The 1999 NEPTUNE Seismology and Geodynamics Working Group Report, made available by the Marine Geophysics Research Group, University of Washington, includes a section on the seismic potential of the Cascadia subduction zone.

6. Global positioning system (GPS). The UK Centre for the Observation and Modelling of Earthquakes and Tectonics provides an introduction to the use of GPS. The Southern California Integrated GPS Network offers an educational module with sections on plate tectonics, earthquakes, and GPS. The International GPS Service provides information about the densification program and links to GPS resources. The GSC's Pacific Geoscience Centre offers a presentation titled “The Western Canada Deformation Array: Measuring crustal motions in coastal British Columbia with continuous GPS” and provides links to crustal dynamics/GPS Internet resources. UNAVCO, a consortium of universities and laboratories joined to promote the use of GPS, offers a presentation on GPS Earth science applications. The UCAR UNAVCO Facility is the primary operational activity of UNAVCO and exists to support research investigators in their use of GPS technology for Earth sciences research.

7. Stress is defined in the USGS earthquake glossary. C. Ammon includes a section on stress and strain in lecture notes on faults and faulting for an earthquake course. C. Lithgow-Bertelloni provides lecture notes on stress and strain for a geology course.

8. Creep is defined in the USGS earthquake glossary.

9. K. Obara is in the National Research Institute for Earth Science and Disaster Prevention (NIED), Ibaraki, Japan. The 31 May 2002 issue of Science had a report by K. Obara titled “Nonvolcanic deep tremor associated with subduction in southwest Japan” (5) and a Perspective by B. Julian titled “Seismological detection of slab metamorphism” (8). S. Earle, Geology Department, Malaspina University College, Nanaimo, BC, Canada, provides an article about Obara's research.

10. Philippine sea plate. A presentation on volcanoes by the Faculty of Science, Kyushu University, Japan, includes a section on the Philippine sea plate. Volcano World provides information on the Philippine plate and tectonics of Japan. T. Seno, Earthquake Research Institute, University of Tokyo, makes available the abstract and figures for an article titled “Why the Philippine sea plate moves as it does”; an animation of the Philippine sea plate reconstruction is also provided.

11. A Hi-Net (High Sensitivity Seismograph Network) Web site (in Japanese) is provided by NIED. The NIED Seismic Page, a portal to Japanese seismic data, provides access to Hi-Net data. The U.S. National Earthquake Information Center offers information (with links) about seismicity in Japan.

12. Volcanic seismic tremors. Tremor is defined in the University of Oklahoma glossary of earthquake and other geological terms. The USGS earthquake glossary defines harmonic tremor. The presentation on earthquakes and seismicity provided by the USGS Cascades Volcano Observatory has a section on harmonic tremor. The Vents Program at the Pacific Marine Environmental Laboratory offers a presentation on volcanic tremors. New Mexico Tech's Mt. Erebus Geophysics Web site offers a presentation on volcanic seismic tremor.

13. Magma and magma conduits are defined and illustrated in the photo glossary of volcano terms.

14. Seismic waves. P wave and S wave are defined by the USGS earthquake glossary. R. Mellors, Department of Geological Sciences, San Diego State University, offers a presentation on seismic waves and earthquakes. C. Ammon provides lecture notes on waves, seismograms, and seismometers for an earthquake course. D. Schmidt, Department of Physics, University of Alberta, offers lecture notes on earthquakes and seismicity for a geophysics course.

15. Basalt and eclogite are defined in the Britannica Concise Encyclopedia. The USGS photo glossary of volcano terms has an entry for basalt. L. Fichter's Igneous Rocks Web site has information about basalt and the Metamorphic Rocks Web site includes an entry for eclogite. The 29 October 1999 issue of Science had a report by S. Peacock and K. Wang titled “Seismic consequences of warm versus cool subduction metamorphism: Examples from southwest and northeast Japan” (7).

16. B. Julian is in the Earthquake Hazards Program (Northern California), USGS, Western Region. A Perspective by B. Julian titled “Seismological detection of slab metamorphism” appeared in the 31 May 2002 issue with K. Obara's report.

17. The EarthScope Web site provides an overview of the project, publications and other resources, and links to educational presentations. Review of EarthScope Integrated Science is a 2001 report made available on the Web by the National Academies Press.

18. T. I. Melbourne is in the Department of Geological Sciences, Central Washington University, Ellensburg, WA.

19. F. H. Webb is at the Jet Propulsion Laboratory, California Institute of Technology.


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