Table Of ContentSeismogenic and
Tsunamigenic Processes in
Shallow Subduction Zones
Edited by
Jeanne Sauber
Renata Dmowska
Springer Basel AG
Reprint from Pure and Applied Geophysics
(pAGEOPH), Volume 154 (1999), No. 3/4
Editors:
Jeanne Sauber Renata Dmowska
Laboratory for Terrestrial Physics Harvard University
NASA's Goddard Space Flight Center Division of Engineering and Applied
Greenbelt, MD 20771 Sciences
USA Cambridge, MA 02138
USA
A CIP catalogue record for this book is available from the Library of Congress, Washington
D.C., USA
Deutsche Bibliothek Cataloging-in-Publication Data
Seismogenic and tsunamigenic processes in shallow subduction zones / ed. by Jeanne Sauber;
Renata Dmowska. -Basel ; Boston; Berlin : Birkhiiuser 1999
(pageoph topica1 volumes)
ISBN 978-3-7643-6146-4 ISBN 978-3-0348-8679-6 (eBook)
DOI 10.1007/978-3-0348-8679-6
This work is subject to copyright. All rights are reserved, whether the whole or part of the mate
rial is concerned, specifically the rights of translation, reprinting, re-use of iIlustrations, recita
tion, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. For
any kind of use, permission of the copyright owner must be obtained.
© 1999 Springer Basel AG
Originally published by Birkhăuser Verlag Basel in 1999
Printed .on acid-free paper produced from chlorine-free pulp
ISBN 978-3-7643-6146-4
987654321
Contents
405 Introduction: Seismogenic and Tsunamigenic Processes in Shallow Subduc
tion Zones
J. Sauber, R. Dmowska
409 Dynamic Stress Drop of Recent Earthquakes: Variations within Subduction
Zones
L. J. Ruff
433 Comparison of Depth Dependent Fault Zone Properties in the Japan Trench
and Middle America Trench
S. L. Bilek, T. Lay
457 Changes in Earthquake Source Properties across a Shallow Subduction
Zone: Kamchatka Peninsula
V. M. Zobin
467 Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones
K. Satake, Y. Tanioka
485 Local Tsunamis and Distributed Slip at the Source
E. L. Geist, R. Dmowska
513 Geologic Setting, Field Survey and Modeling of the Chimbote, Northern
Peru, Tsunami of 21 February 1996
J. Bourgeois, C. Petroff, H. Yeh, V. Titov, C. E. Synolakis, B. Benson, J.
Kuroiwa, J. Lander, E. Norabuena
541 Asperity Distribution of the 1952 Great Kamchatka Earthquake and its
Relation to Future Earthquake Potential in Kamchatka
J. M. Johnson, K. Sa take
555 The October 4, 1994 Shikotan (Kuril Islands) Tsunamigenic Earthquake: An
Open Problem on the Source Mechanism
A. Piatanesi, P. Heinrich, S. Tinti
575 Relation between the Subducting Plate and Seismicity Associated with the
Great 1964 Alaska Earthquake
R. von Huene, D. Klaeschen, J. Fruehn
593 Seismicity of the Prince William Sound Region for over Thirty Years
Following the 1964 Great Alaskan Earthquake
D. l. Doser, A. M. Veilleux, M. Velasquez
633 Historical Seismicity and Seismotectonic Context of the Great 1979 Yapen
and 1996 Biak, Irian Jaya Earthquakes
E. A. Okal
677 Rupture Process of the 1995 Antofagasta Subduction Earthquake (Mw=8.1)
D. L. Cario, T. Lay, C. J. Ammon, J. Zhang
709 GPS-derived Deformation of the Central Andes Including the 1995 An
tofagasta Mw = 8.0 Earthquake
J. Klotz, D. Angermann, G. W. Michel, R. Porth, C. Reigber, J. Reinking, J.
Viramonte, R. Perdomo, V. H. Rios, S. Barrientos, R. Barriga, O. Cifuentes
731 Source Characteristics of the 12 November 1996 Mw 7.7 Peru Subduction
Zone Earthquake
J. L. Swenson, S. L. Beck
753 Seismic Subduction of the Nazca Ridge as Shown by the 1996-97 Peru
Earthquakes
W. Spence, C. Mendoza, E. R. Engdahl, G. L. Choy, E. Norabuena
© BirkMuser Verlag, Basel, 1999
Pure appl. geophys. 154 (1999) 405-407 I
0033-4553/99/040405-03 $ 1.50 + 0.20/0 Pure and Applied Geophysics
Introduction
Seismogenic and Tsunamigenic Processes in Shallow Subduction
Zones
J. SAUBER and R. DMOWSKA
Earthquakes in shallow subduction zones account for the ~reatest part of
seismic energy release in the Earth and often cause significant damage; in some
cases they are accompanied by devastating tsunamis. Understanding the physics of
seismogenic and tsunamigenic processes in such zones continues to be a challenge
as well as a focus of ongoing research. In particular, questions that are being
addressed include:
What are the mechanisms underlying higher slip in some areas (asperity
distributions)? Are these mechanisms stable in space and time? Is the slip distribu
tion in consecutive large/great earthquakes similar or different to the previous ones
in the same place?
How much of the coseismic slip in large earthquakes occurs on the plate
interface and how much on faults within the overriding plate?
What is the role of roughness in the subducting oceanic plate and/or the amount
of subducting sediments for the earthquake dynamics? What is the importance of
structural features in the downgoing slab? What is the role of fluids trapped in the
seismogenic zone?
Are there any systematic differences between earthquakes which occur close to
the trench and the deeper, interplate events? What are the characteristics of
tsunamigenic earthquake sources? Could we predict in advance, only from the
tectonic features of a subduction segment, if it is capable of generating a tsunami
genic earthquake?
What are the stress interactions between adjacent subduction earthquakes? How
do these large/great subduction events modulate the seismicity in the upper plate
and outer-rise area following the main event?
What controls the type and location of post-seismic slip? How prevalent is
afterslip along the down-dip extension of the coseismic rupture plane versus
post-seismic viscous relaxation of the asthenosphere?
406 Introduction Pure appJ. geophys.,
What can we learn from current GPS measurements regarding the strength and
distribution of coupling along the main interplate interface? Could it be used to
predict slip distribution in future earthquakes along that subduction segment?
Some of these questions are addressed in this topical issue.
Systematic, depth-dependent variations in earthquake source properties across a
shallow subduction zone are investigated by Ruff, Bilek and Lay, Zobin, and
Satake and Tanioka. The last two authors concentrate on tsunami generation of
subduction earthquakes and systematic differences between tsunamigenic (interplate
or intraplate) and tsunami earthquakes, in which most of the moment release
occurs in a narrow region near the trench.
The influence of nonhomogeneities in earthquake slip on local tsunamis is
discussed by Geist and Dmowska. Bourgeois et al. investigate and model the local
tsunami caused by the Chimbote, northern Peru earthquake of 21 February, 1996.
Tsunami inversion leading to slip distribution of the 1952 great Kamchatka
earthquake is presented by Johnson and Satake, followed by the analysis of the
20th century seismicity in that area which aims to determine the relationship
between the asperities of the 1952 event and the large earthquakes of the Kam
chatka subduction zone.
The difficulties in applying solely tsunami data to infer source parameters of an
earthquake are illustrated by Piatanesi et al. in the example of the October 4, 1994
Shikotan earthquake.
Von Huene et at. use high resolution bathymetry and detailed seismic profiles
to evaluate the influence of subducted topographic features and the amount
of subducted sediment on the slip distribution in the great Alaska 1964 earth
quake.
Moderate seismicity in the region of Prince William Sound for over thirty years
following the 1964 great Alaska earthquake is analyzed by Doser et al. in relation
to the slip distribution of this event. Another study of historical as well as modern
seismicity follows, for the northwestern part of Irian Jaya, Indonesia, in which Okal
presents relocations of over 220 earthquakes in the context of the great 1979 Yapen
and 1996 Biak earthquakes.
A new seismic study of the rupture process of the Mw = S.l 1995 Antofagasta
(northern Chile) earthquake is presented by Carlo et al. and compared with
previous inversions. The 1995 event is significant both as the first great thrust event
observed in the region and for its possible interactions with other portions of the
interplate contact zone.
Results of the GPS study of the same but broader area, performed in 1993, 1994
and 1995, and presented by Klotz et al., follow. The analysis considers three
different deformation processes: interseismic accumulation of elastic strain due to
subduction coupling, coseismic strain release during the Antofagasta earthquake
and crustal shortening in the Sub-Andes. The study illustrates that the inter seismic
accumulation of elastic deformation requires full locking of the subduction inter-
Vol. 154, 1999 Introduction 407
face. Geodetically derived slip distribution of the Antofagasta earthquake is in good
agreement with previous seismic inversions.
The last two papers, by Swenson and Beck, and Spence et aI., discuss the central
Peru subduction zone and, in particular, seismic subduction of the Nazca Ridge, as
evidenced by the 12 November 1996 Mw = 7.7 Peru earthquake. The papers offer
detailed inversions of the 1996 event as well as a complimentary view of seismotec
tonics of the area.
The editors are grateful to the following scientists for providing critical,
thoughtful, and sometimes timely reviews: S. Beck, T. Brocher, W.-P. Chen, D.
Christensen, S. Cohen, T. Dixon, G. Ekstrom, E. L. Geist, J. Johnson, H.
Kanamori, A. McGarr, S. Nishenko, E. A. Okal, L. Ruff, K. Satake, J. Savage, T.
Seno, S. Schwartz, M. Simons, S. Stein, W. Spence, H.-K. Thio, J. Vidale.
Jeanne Sauber
Laboratory for Terrestrial Physics
NASA's Goddard Space Flight Center
Greenbelt, MD 20771, U.S.A.
and
Renata Dmowska
Harvard University
Division of Engineering and Applied Sciences
Cambridge, MA 02138, U.S.A.
To access this journal online:
http://www.birkhauser.ch
© Birkhll.user Verlag. Basel. 1999
Pure appl. geophys. 154 (1999) 409-431 I
0033-4553/99/040409-23 $ 1.50 + 0.20/0 Pure and Applied Geophysics
Dynamic Stress Drop of Recent Earthquakes: Variations within
Subduction Zones
LARRY J. RUFFl
Abstract-Stress drop is a fundamental parameter of earthquakes, but it is difficult to obtain
reliable stress drop estimates for most earthquakes. Static stress drop estimates require knowledge of the
seismic moment and fault area. Dynamic stress drop estimates are based entirely upon the observed
source time functions. Based on analytical formulas that I derive for the crack and slip-pulse rupture
models, the amplitude and time of the initial peak in source time functions can be inverted for dynamic
stress drop. For multiple event earthquakes, this method only gives the dynamic stress drop of the first
event. The Michigan STF catalog provides a uniform data base for all large earthquakes that have
occurred in the past four years. Dynamic stress drops are calculated for the nearly 200 events in this
catalog, and the resultant estimates scatter between 0.1 and 100 MPa. There is some coherent tectonic
signal within this scatter. In the Sanriku (Japan) and Mexico subduction zones, underthrusting
earthquakes that occur at the up-dip and down-dip edges of the seismogenic zone have correspondingly
low and high values of stress drop. A speculative picture of the stress state of subduction zones emerges
from these results. A previous study found that the absolute value of shear stress linearly increases down
the seismogenic interface to a value of about 50 MPa at the down-dip edge. In this study, the dynamic
stress drop of earthquakes at the up-dip edge is about 0.2 MPa, while large earthquakes at the down-dip
edge of the seismogenic plate interface have dynamic stress drops of up to 5 MPa. These results imply
that: (I) large earthquakes only reduce the shear stress on the plate interface by a small fraction of the
absolute level; and thus (2) most of the earthquake energy is partitioned into friction at the plate
interface.
Key words: Stress drop, rupture, seismogenic zone, source time functions, subduction, friction.
1. Introduction
Earthquakes reduce stress over most of the fault area, hence stress drop is a
fundamental parameter of earthquakes. Unfortunately, it is difficult to reliably
estimate stress drop; thus it is determined only in special studies of particular
earthquakes. This lack of uniform treatment of earthquakes can be excused because
one of the key tenets of seismology is that stress drop is "approximately constant"
for earthquakes of all types and sizes (KANAMORI and ANDERSON, 1975; SCHOLZ,
1 Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, U.S.A.
410 Larry 1. Ruff Pure appl. geophys.,
1990). In detail, "approximately constant" means that stress drop estimates typi
cally fall in the range of 1 to 100 bars (0.1 to 10 MPa), with an occasional report
of much higher values. Given the fundamental importance of stress drop to
earthquake physics, seismologists must make progress on two fronts: (1) systemati
cally estimate stress drop for all seismicity above some magnitude threshold; and (2)
provide more reliable and consistent stress drop estimates such that we can extract
information from within the factor of one hundred variation in current estimates.
In this paper, I show that the dynamic stress drop for the initial rupture process can
be reliably determined from source time functions, and I show results for large
earthquakes that occurred in the past four years. These preliminary results do not
reduce the scatter in stress drop estimates, but we do see some structure within the
"approximately constant" stress drop. In particular, I focus on underthrusting
earthquakes in subduction zones and find some evidence for systematic variation in
the dynamic stress drop between events at the down-dip and up-dip edges of the
seismogenic zone.
2. Static Stress Drop
Static stress drop is the simplest measure of the overall reduction in shear stress
due to slip on the fault zone. It is the difference between the average shear stress on
the fault zone before and after the earthquake (Fig. 1). Since the stress drop of real
earthquakes varies across the fault area, the overall static stress drop is a slip
weighted average of the spatially variable stress drop. Seismologists typically use
simple constant stress drop models to estimate earthquake stress drops. Regardless
of the details of fault geometry and slip distribution, the basic formula for stress
drop is:
where D is the average slip over the faulted area (A), L is the characteristic length
of the fault area, f1 is the elastic shear modulus, and c is a geometric constant that
is close to one if L is properly chosen. Since seismic moment (Mo) for most large
earthquakes can be reliably determined from seismic waves, rewrite the above
equation as:
DA Mo
/J..(J = CII-LA= cL-A' (1)
st f'"
This formula shows that we need three quantities to calculate stress drop: a
measurement of the seismic moment, some estimate of the fault area (A), and then
some appropriate choice for the characteristic fault dimension. While the choice of
L presents an interpretational problem, it is the estimation of A that presents
