DOCSLIB.ORG

DOI: 10.1126/Science.285.5431.1231 , 1231 (1999); 285 Science Miaki

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
DOI: 10.1126/Science.285.5431.1231 , 1231 (1999); 285 Science Miaki Normal-Mode and Free-Air Gravity Constraints on Lateral Variations in Velocity and Density of Earth's Mantle Miaki Ishii and Jeroen Tromp Science 285, 1231 (1999); DOI: 10.1126/science.285.5431.1231 This copy is for your personal, non-commercial use only. If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of August 27, 2012 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: on August 27, 2012 http://www.sciencemag.org/content/285/5431/1231.full.html This article cites 23 articles, 1 of which can be accessed free: http://www.sciencemag.org/content/285/5431/1231.full.html#ref-list-1 This article has been cited by 155 article(s) on the ISI Web of Science This article has been cited by 13 articles hosted by HighWire Press; see: http://www.sciencemag.org/content/285/5431/1231.full.html#related-urls www.sciencemag.org This article appears in the following subject collections: Geochemistry, Geophysics http://www.sciencemag.org/cgi/collection/geochem_phys Downloaded from Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 1999 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS. R ESEARCH A RTICLES ficient quality. There are density models based on current and past plate motions in which the Normal-Mode and Free-Air sources of heterogeneity are cold subducted slabs (5), and an upper mantle density model, Gravity Constraints on Lateral containing strong high-degree components, has been determined with surface wave data (6). Variations in Velocity and A density model of the mantle is im- portant to geophysics because mantle-flow calculations cannot be performed without Density of Earth’s Mantle it (7, 8). However, a whole-mantle den- Miaki Ishii* and Jeroen Tromp sity model is usually obtained by scaling an S velocity model, which is justifiable With the use of a large collection of free-oscillation data and additional con- if the heterogeneity has a single cause straints imposed by the free-air gravity anomaly, lateral variations in shear that affects both, for example, variations in velocity, compressional velocity, and density within the mantle; dynamic to- temperature. pography on the free surface; and topography on the 660-km discontinuity and In recent years, mainly as a result of the 9 the core-mantle boundary were determined. The velocity models are consistent June 1994 Bolivia earthquake, the quantity and with existing models based on travel-time and waveform inversions. In the quality of normal-mode, or free-oscillation, lowermost mantle, near the core-mantle boundary, denser than average ma- data have improved substantially (9), allowing terial is found beneath regions of upwellings centered on the Pacific Ocean and us to constrain the long-wavelength structure of Africa that are characterized by slow shear velocities. These anomalies suggest Earth. We used this data set, combined with the existence of compositional heterogeneity near the core-mantle boundary. further constraints imposed by Earth’s gravity Seismologists have produced numerous shear develop a whole-mantle density model because Department of Earth and Planetary Sciences, Harvard (S) and compressional (P) velocity models of short-period body waves are not directly sensi- University, Cambridge, Massachusetts 02138, USA. the mantle based mainly on body-wave data tive to density and long-period normal-mode *To whom correspondence should be addressed. E- (1–4). Unfortunately, they have been unable to observations have been too scarce and of insuf- mail: [email protected] on August 27, 2012 A 0 Fig. 1. Comparison of the BC S part of SPRD6 and SKS12WM13. (A) Relative perturbations in S velocity at (from the top) 100 (Ϯ 4.5%) 600 (Ϯ 1.0%), 1300 (Ϯ 1.0%), 1800 (Ϯ 0.8%), 2300 (Ϯ 1.0%), 1000 and 2850 (Ϯ 3.0%) km, where the values in paren- theses indicate the scale www.sciencemag.org for the maps at each depth. The S part of mod- Depth (km) el SPRD6 is shown on the left, and model 2000 SKS12WM13 (1) is shown on the right. Blue colors indicate regions of higher than average velocities, and red colors indicate re- Downloaded from gions of slower than aver- age velocities. Coastlines 3000 have been superimposed 0.0 1.0 2.0 0.0 0.5 1.0 for reference. Because the RMS amplitude (%) Correlation maximum harmonic de- gree in SPRD6 is 6, we truncated degree 12 model SKS12WM13 at the same degree. (B) Root-mean- square (RMS) amplitudes of the S part of SPRD6 and SKS12WM13 as a function of depth. The red line is SPRD6, and the black line is SKS12WM13. (C) Correlation between the S model of SPRD6 and SKS12WM13 as a function of depth. For this number of model parameters, two maps are correlated at the 95% confidence level if the correlation is greater than 0.24. www.sciencemag.org SCIENCE VOL 285 20 AUGUST 1999 1231 R ESEARCH A RTICLES ␦␤ ␤ t ␦␣ ␣ t field, to determine lateral variations in S veloc- velocity ( / )s, P velocity ( / )s, density mantle flow, which depend on lateral variations ␦␳ ␳ t ity, P velocity, and density within the mantle, in ( / )s, and topography on discontinuities in density and the relative viscosity of the layers ␦ t addition to dynamic topography on the free ( h/h)s by (12) above and below the discontinuity. Frequently, surface and topography on the 660-km discon- the objective of mantle-flow calculations is to a tinuity (660) and the core-mantle boundary t t t find a viscosity profile and velocity-to-density c ϭ ͵ ͓͑␦␤/␤͒ K␤ ϩ ͑␦␣/␣͒ K␣ (CMB). s s s scaling that reconcile a given velocity model b with the observed gravity anomaly. Here, Theory ϩ ͑␦␳ ␳͒t ϩ ͸͑␦ ͒t we determined lateral variations in density / s K␳]dr h/h s Kd (2) The sensitivity of a normal mode to lateral d and boundary topography simultaneously; heterogeneity is usually visualized in the form The integration over radius r is from the CMB this circumvents the need for a viscosity of a splitting function (10), which represents a with radius b (3480 km) to Earth’s surface with model when fitting the gravity anomaly. local radial average of Earth’s three-dimension- radius a (6371 km), and the summation is over The spherical harmonic coefficients of the t al (3D) heterogeneity. Each mode has its own all discontinuities d. The sensitivity kernels K␤, free-air gravity anomaly, f s (15), are lin- unique splitting function, which at high degrees K␣, K␳, and Kd are given in terms of the eigen- early related to those describing lateral ␦␳ ␳ t corresponds to a surface-wave phase velocity functions of the modes that define the splitting variations in density, ( / ) s, and boundary ␦ t map. An isolated mode of degree l “sees” even- function (12). The number of unknown param- topography, ( h/h) s,by(16) degree structure up to degree 2l, and some eters can be reduced by assuming that lateral clusters of modes have sensitivity to Earth’s variations in P velocity and density are propor- a t ϭ ͑␦␳ ␳͒t Ј ϩ ͸͑␦ ͒t Ј f ͵ / K␳dr h/h K (3) odd-degree heterogeneity (11). A normal-mode tional to lateral variations in S velocity. In the s s s d d splitting function ␴ depends on colatitude ␪ and past, this has been the common approach (12, b ␾ ␦ ␣ ␦ Ј Ј longitude and may be expanded in spherical 13), with typical scaling values of ln / ln The kernels K␳ and Kd describe the sensitiv- t ␤ϭ ␦ ␳ ␦ ␤ϭ harmonics Ys of degree s and order t as (12) 0.55 and ln / ln 0.2 to 0.4 (8, 14). ity of the gravity anomaly to 3D density and s Lateral variations in density, combined with boundary topography, respectively. ␴͑␪ ␾͒ ϭ ͸ ͸ t t ͑␪ ␾͒ , csY s , (1) flow-induced topography on discontinuities, s ϭ 0 tϭϪs determine Earth’s gravity anomaly. Undula- Velocity and Density Models t The splitting-function coefficients cs are lin- tions on boundaries are determined by the den- The splitting functions were corrected for the early related to 3D relative variations in S sity contrast and radial stresses imposed by effects of Earth’s crust with the recent crustal on August 27, 2012 A 0 Fig. 2. Comparison of the P part of SPRD6 and BC P16B30. (A) Relative perturbations in P ve- locity at six discrete depths throughout the mantle, as in Fig. 1. The scale for the maps at 1000 each depth is (from the top) Ϯ2.5, Ϯ0.8, Ϯ0.5, Ϯ0.5, Ϯ0.5, www.sciencemag.org and Ϯ0.7%. The P part of model SPRD6 Depth (km) is shown on the left, and model P16B30 2000 (3), truncated at de- gree 6, is shown on the right. The color scheme is the same as in Fig. 1. (B) RMS am- Downloaded from plitudes of the P part of SPRD6 and P16B30 3000 as a function of depth. 0.0 0.5 1.0 1.5 0.0 0.5 1.0 The red line is SPRD6, RMS amplitude (%) Correlation and the black line is P16B30.
Load more

Recommended publications
  • Adam Marian Dziewoński Was Spread Upon the Permanent Records of the Faculty
    At a meeting of the FACULTY OF ARTS AND SCIENCES on November 7, 2017, the following tribute to the life and service of the late Adam Marian Dziewoński was spread upon the permanent records of the Faculty. ADAM MARIAN DZIEWOŃSKI BORN: November 15, 1936 DIED: March 1, 2016 Adam Marian Dziewoński was born in Lwów, Poland (now Lviv, Ukraine) and received his M.S. in 1960 from the University of Warsaw and his Doctor of Technical Sciences in 1965 from the Academy of Mines and Metallurgy in Krakow. He moved to the United States in 1965 as a research associate at the Graduate Research Center of the Southwest in Dallas and became an assistant professor at University of Texas at Dallas in 1969. Adam moved to Harvard University as an associate professor in 1972, became a full professor in 1976, and was named the Frank B. Baird, Jr., Professor of Science in 1994. He was the chair of the Department of Geological Sciences between 1982 and 1986 (Department of Earth and Planetary Sciences thereafter). He retired in 2009 but remained actively engaged in research until his death. Adam was born in a year of seismological significance, the year in which the Earth’s inner core was discovered. As if fated, Adam was involved in all subsequent major discoveries about this deepest layer of Earth. With James Freeman Gilbert, Adam provided the first direct evidence that the inner core is solid, settling the debate over its nature. He continued his work on the inner core with his colleagues at Harvard and went on to show that it is anisotropic, rotates at a slightly different rate than the mantle, and has another layer at its center.
    [Show full text]
  • Normal-Mode and Free-Air Gravity Constraints on Lateral Variations In
    R ESEARCH A RTICLES ficient quality. There are density models based on current and past plate motions in which the Normal-Mode and Free-Air sources of heterogeneity are cold subducted slabs (5), and an upper mantle density model, Gravity Constraints on Lateral containing strong high-degree components, has been determined with surface wave data (6). Variations in Velocity and A density model of the mantle is im- portant to geophysics because mantle-flow calculations cannot be performed without Density of Earth’s Mantle it (7, 8). However, a whole-mantle den- Miaki Ishii* and Jeroen Tromp sity model is usually obtained by scaling an S velocity model, which is justifiable With the use of a large collection of free-oscillation data and additional con- if the heterogeneity has a single cause straints imposed by the free-air gravity anomaly, lateral variations in shear that affects both, for example, variations in velocity, compressional velocity, and density within the mantle; dynamic to- temperature. pography on the free surface; and topography on the 660-km discontinuity and In recent years, mainly as a result of the 9 the core-mantle boundary were determined. The velocity models are consistent June 1994 Bolivia earthquake, the quantity and with existing models based on travel-time and waveform inversions. In the quality of normal-mode, or free-oscillation, lowermost mantle, near the core-mantle boundary, denser than average ma- data have improved substantially (9), allowing terial is found beneath regions of upwellings centered on the Pacific Ocean and us to constrain the long-wavelength structure of Africa that are characterized by slow shear velocities.
    [Show full text]
  • Joint Inversion of Normal Mode and Body Wave Data for Inner Core Anisotropy 2
    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B12, 2380, doi:10.1029/2001JB000713, 2002 Joint inversion of normal mode and body wave data for inner core anisotropy 2. Possible complexities Miaki Ishii and Adam M. Dziewon´ski Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA Jeroen Tromp Seismological Laboratory, California Institute of Technology, Pasadena, California, USA Go¨ran Ekstro¨m Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA Received 19 June 2001; revised 10 June 2002; accepted 8 July 2002; published 31 December 2002. [1] In this study we focus on inner core sensitive body wave data to investigate lateral and radial heterogeneity within the inner core. Normal mode data are used to constrain a global model and to monitor if complexities introduced by body wave data are compatible with mode measurements. In particular, we investigate the possibilities of an isotropic layer near the inner core boundary and large-scale variations in anisotropy such as hemispheric dependence proposed in studies based upon differential travel time data. Travel time data from distances between 130° and 140° require anisotropy near the inner core boundary. This evidence is supported by differential travel time data based upon diffracted waves, contrasting the previous inferences of isotropy at the surface of the inner core. Our experiments also show that variations at a hemispheric scale are not necessary and that the sources of apparent hemispheric differences can be localized. A comparison of differential and absolute travel time data suggests that differences in inferred inner core anisotropy models arise mainly from a few anomalous paths.
    [Show full text]
  • For Immediate Release Contact: Nan Broadbent 408-431-9885
    For Immediate Release Contact: Nan Broadbent 408-431-9885, [email protected] Miaki Ishii honored with Richter Early Career Award Harvard professor recognized by Seismological Society of America SAN FRANCISCO -- Since her first forays into studying earthquakes, Miaki Ishii has successfully tackled seismological problems that would likely intimidate even the most seasoned researchers. In her young career, Ishii, an assistant professor of earth and planetary science at Harvard University, has made two groundbreaking discoveries in geophysics that have fostered intense debate and subsequent research that has changed the understanding of deep Earth seismology. For her work, the Seismological Society of America (SSA) will honor the 33-year-old Ishii with its Charles F. Richter Early Career Award, which honors outstanding contributions to the goals of the Society by a member early in her or his career. She will receive the award at a special ceremony at the SSA annual meeting on April 8 in Monterey, CA. Since she entered Harvard to begin her doctoral work in the late 1990s, Ishii has shown a knack for answering big questions. Shortly after her arrival at Harvard, she researched the driving force behind plate tectonics -- lateral variations in mantle density. The research inferred that the slowest parts of the lowermost mantle are denser than average, rather than lighter as most had assumed. The findings flew in the face of the long-held theory of a homogenized mantle and generated significant subsequent research and debate. Recent research is beginning to confirm Ishii’s observations. Her second groundbreaking find built on her previous study with Harvard’s Adam Dziewonski.
    [Show full text]
  • 15 March 2016 Magazine
    VOL. 97 NO. 6 15 MAR 2016 WHERE CURIOSITY HAS TAKEN US Building Better Climate Models No Tolerance for Sexual Harassment Cheers, Yawns for Planet Nine Have an Idea or Topic to Present at the Fall Meeting? Submit a Session Proposal Deadline: 20 April, 11:59 P.M. EDT You must be current in your 2016 membership dues in order to propose a session. fallmeeting.agu.org Earth & Space Science News Contents 15 MARCH 2016 PROJECT UPDATE VOLUME 97, ISSUE 6 22 Better Tools to Build Better Climate Models A Department of Energy collaboration aims to make climate model development faster and more efficient by creating a prototype of a system for testing model components. OPINION 9 Steps to Building a No-Tolerance Culture for Sexual Harassment AGU can lead the way in building an environment where sexual and other types of harassment have no place. 16 AGU NEWS COVER 2017 Fall Meeting 26 to Go to New Orleans, 2018 to Washington, D. C. Where Curiosity Has Taken Us AGU opts to relocate its 26,000-plus- attendee meeting from San Francisco for The Curiosity rover, one of NASA’s flagship missions, analyzes 2 years to avoid detrimental impacts from Martian geology, geochemistry, climatology, and radiation to assess extensive construction at the convention whether Mars could have supported microbial life. center. Earth & Space Science News Eos.org // 1 Contents DEPARTMENTS Editor in Chief Barbara T. Richman: AGU, Washington, D. C., USA; eos_ [email protected] Editors Christina M. S. Cohen Wendy S. Gordon Carol A. Stein California Institute Ecologia Consulting, Department of Earth and of Technology, Pasadena, Austin, Texas, USA; Environmental Sciences, Calif., USA; wendy@ecologiaconsulting University of Illinois at cohen@srl .caltech.edu .com Chicago, Chicago, Ill., USA; [email protected] José D.
    [Show full text]
  • SPRING 2016 TIMES Earthharvard UNIVERSITY DEPARTMENT&Planetary of EARTH and PLANETARY SCIENCES
    SPRING 2016 TIMES EarthHARVARD UNIVERSITY DEPARTMENT&Planetary OF EARTH AND PLANETARY SCIENCES R/V Knorr at sunset in the Atlantic. This vessel was used during a recent five-week MARPEX cruise to collect samples from the Mid-Atlantic Ridge. Earthquakes, movies, sailing: Freshman Seminars not CEDRIC HAMELIN typical Harvard fare 4 Exploring Earth’s Final Frontier What mid-ocean ridges reveal about tectonics and magmatism BY JOCELYN FUENTES, THIRD-YEAR GRADUATE STUDENT Three siblings reflect on hen most people think of the ocean, tonic boundaries, ocean ridges are an essential part 13family traditions—and EPS they imagine a vast expanse of calm of the Earth system where the seafloor is constantly blue water with little thought for what being created through magmatism and faulting. liesW below, or they think of the water that fills the Since the first map of the ocean floor was cre- ocean basins. For many ocean scientists, however, ated, in 1977 by Marie Tharp and Bruce Heezen, what is most interesting lies beneath the water on many advances have been made in the field of the seafloor. Two-thirds of the geology of the marine geology and geophysics. In particular, our planet is there, obscured by the water above. The ability to map and image the seafloor has improved Rock Hard Bodies play most prominent feature of the seafloor is the greatly. Unfortunately, these techniques generally Meanhouse Effect in first mid-ocean ridge system, one of Earth’s most active require a ship, which is costly and time-consuming. department Earth19 Bowl tectonic, volcanic, and biologic features.
    [Show full text]
  • Final Technical Report-Cover Page
    Final Technical Report Award Number G10AP00086 Sponsors Name: Seismological Society of America Conference Title: Seismological Society of America’s 2010 Annual Meeting in Portland, Oregon Conference Dates: April 21‐23, 2010 Summary of Program: This year’s SSA Annual Meeting included 282 oral presentations and 258 poster presentations distributed among 31 sessions. The meeting was attended by 557 participants from 26 countries. The meeting was preceded by the annual board meeting and committee meetings. Selected results of those meetings were shared with the membership at the annual luncheon, the first day of the meeting. Special presentations were given at the two other luncheons. The Joyner Lecture, a special lecture focusing on the boundary of seismology and engineering, was presented on Thursday night. The attached PDF contains the following: ‐ The program schedule (pages 2‐32) ‐ The meeting report including the annual luncheon minutes, the board minutes, awards presentations, and treasurer’s report (pages 33‐62) ‐ A list of registrants (Pages 63‐73) ‐ All the meeting abstracts (Pages 74‐187) SSA Annual meeting program SSA 2010 Annual Meeting Announcement Seismological Society of America Technical Sessions 21–23 April 2010 (Wednesday–Friday) Portland, Oregon IMPORTANT DATES Meeting Pre-registration Deadline 19 March 2010 Hotel Reservation Cut-Off 29 March 2010 Online Registration Cut-Off 9 April 2010 PROGRAM COMMITTEE TECHNICAL PROGRAM This year’s technical program committee is composed of co- Advances in Seismic Hazard Mapping chairs Seth Moran (USGS–Cascades Volcano Observatory) Recently much progress has been made in developing infor- and Nick Beeler (USGS–Vancouver), Ivan Wong (Seismic mative regional seismic hazard maps that incorporate local Hazards Group, Oakland CA), Ray Weldon (University of as well as regional geologic and geotechnical data.
    [Show full text]
  • Earthscope Usarray Facility Final Report
    EarthScope USArray Facility Final Report Award # EAR-0733069 Award Title Collaborative Research: EarthScope-Facility Operations and Maintenance (USArray) Award Term October 1, 2008 – September 30, 2013 Report Prepared on December 30, 2014 Report Prepared for National Science Foundation 4201 Wilson Boulevard Arlington, VA 22230 Report Prepared by Incorporated Research Institutions for Seismology 1200 New York Avenue, NW, Suite 400 Washington, DC 20005 Introduction The USArray component of the National Science Foundation’s EarthScope program facility consists of four major observatory components: (1) a Reference Network of permanent seismic stations; (2) a Transportable Array (TA) of ~400 seismic stations; (3) a Flexible Array (FA) pool of seismic instruments available to Principal Investigators (PIs); and (4) a Magnetotelluric (MT) observatory with permanently and temporarily deployable instruments. USArray also includes comprehensive data management and siting outreach efforts. USArray efforts during FY09–FY13 continue and expand facility activities established under the previous MREFC and O&M awards (EAR-0323309 and EAR-0323311, respectively) and continued under the “Seismological Facilities for the Advancement of Geoscience and EarthScope” award (EAR-1261681). Quarterly and annual reports submitted to the National Science Foundation throughout the award document USArray progress and accomplishments. This present report provides an overview of programmatic activities conducted under EAR- 0733069. Facility Information The Incorporated Research Institutions for Seismology (IRIS) operates USArray. Under the governance structure in place during the O&M award, USArray management received community input via the IRIS Board of Directors, the USArray Advisory Committee, and the Transportable Array and Electromagnetic Working Groups (Figure 1). The organizational structure of the USArray has been designed to leverage the experience and facilities that already exist within IRIS.
    [Show full text]