Shallow subduction zone earthquakes and their tsunamigenic potential

SUMMARY We have examined the source spectra of all shallow subduction zone earthquakes from 1992 to 1996 with moment magnitude 7.0 or greater, as well as some other interesting events, in the period range 1‐20 s, by computing moment rate functions of teleseismic P waves. After comparing the source spectral characteristics of ‘tsunami earthquakes’ (earthquakes that are followed by tsunamis greater than would be expected from their moment magnitude) with regular events, we identified a subclass of this group: ‘slow tsunami earthquakes’. This subclass consists of the 1992 Nicaragua, the 1994 Java and the February 1996 Peru earthquakes. We found that these events have an anomalously low energy release in the 1‐20 s frequency band with respect to their moment magnitude, although their spectral drop-oV is comparable to those of the other earthquakes. From an investigation of the centroid and body wave locations, it appears that most earthquakes in this study conformed to a simple model in which the earthquake nucleates in a zone of compacted and dehydrated sediments and ruptures up-dip until the stable sliding friction regime of unconsolidated sediments stops the propagation. Sedimentstarved trenches, e.g. near Jalisco, can produce very shallow slip, because the fault material supports unstable sliding. The slow tsunami earthquakes also ruptured up-dip; however, their centroid is located unusually close to the trench axis. The subduction zones in which these events occurred all have a small accretionary prism and a thin layer of subducting sediment. Ocean surveys show that in these regions the ocean floor close to the trench is highly faulted. We suggest that the horst-and-graben structure of a rough subducting oceanic plate will cause contact zones with the overriding plate, making shallow earthquake nucleation and up-dip propagation to the ocean floor possible. The rupture partly propagates in sediments, making the earthquake source process slow. Two factors have to be considered in the high tsunami-generating potential of these events. First, the slip propagates to shallow depths in low-rigidity material, causing great deformation and displacement of a large volume of water. Second, the measured seismic moment may not represent the true earthquake displacement, because the elastic constants of the source region are not taken into account in the standard CMT determination.

[1]  H. Kanamori.  Mechanism of tsunami earthquakes , 1972 .

[2]  L. Ruff,et al.  What controls the lateral variation of large earthquake occurrence along the Japan Trench? , 1997 .

[3]  M. Gutscher,et al.  Development of the accretionary prism along Peru and material flux after subduction of Nazca Ridge , 1996 .

[4]  R. Huene,et al.  OBSERVATIONS AT CONVERGENT MARGINS CONCERNING SEDIMENT SUBDUCTION, SUBDUCTION EROSION, AND THE GROWTH , 1991 .

[5]  M. Yamano,et al.  The seismogenic zone of subduction thrust faults , 1997 .

[6]  L. Sykes,et al.  Loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones , 1988 .

[7]  K. Imanishi,et al.  Three-stage rupture process of the 28 December 1994 Sanriku-Oki Earthquake , 1996 .

[8]  Walter H. F. Smith,et al.  Global Sea Floor Topography from Satellite Altimetry and Ship Depth Soundings , 1997 .

[9]  David M. Boore,et al.  Average body-wave radiation coefficients , 1984 .

[10]  E. Okal.  Seismic parameters controlling far-field tsunami amplitudes: A review , 1988 .

[11]  K. Satake,et al.  Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones , 1999 .

[12]  Masayuki Kikuchi,et al.  The 1992 Nicaragua earthquake: a slow tsunami earthquake associated with subducted sediments , 1993, Nature.

[13]  Fumihiko Imamura,et al.  ESTIMATE OF THE TSUNAMI SOURCE OF THE 1992 NICARAGUAN EARTHQUAKE FROM TSUNAMI DATA , 1993 .

[14]  Comparison of Depth Dependent Fault Zone Properties in the Japan Trench and Middle America Trench , 1999 .

[15]  L. Ruff,et al.  The Sanriku-Oki, Japan, Earthquake of December 28, 1994 (Mw 7.7): Rupture of a different asperity from a previous earthquake , 1996 .

[16]  Steven N. Ward,et al.  On tsunami nucleation II. An instantaneous modulated line source , 1982 .

[17]  P. Ihmlé.  Frequency-dependent relocation of the 1992 Nicaragua slow earthquake: an empirical Green's function approach , 1996 .

[18]  W. Nakayama,et al.  Slip history of the 1994 Sanriku-Haruka-Oki, Japan, earthquake deduced from strong-motion data , 1997, Bulletin of the Seismological Society of America.

[19]  E. Engdahl,et al.  Global teleseismic earthquake relocation with improved travel times and procedures for depth determination , 1998, Bulletin of the Seismological Society of America.

[20]  C. Scholz.  Earthquakes and friction laws , 1998, Nature.

[21]  John Milsom,et al.  Subduction of seamounts at the Java Trench: a view with long-range sidescan sonar , 1990 .

[22]  C. Synolakis,et al.  Geologic Setting, Field Survey and Modeling of the Chimbote, Northern Peru, Tsunami of 21 February 1996 , 1999 .

[23]  Fumihiko Imamura,et al.  Source characteristics of the Nicaraguan Tsunami Earthquake of September 2, 1992 , 1993 .

[24]  J. Hartog,et al.  Directivity analysis of the December 28, 1994 Sanriku‐Oki Earthquake (Mw=7.7), Japan , 1996 .

[25]  W. D. Stuart,et al.  Cyclic stressing and seismicity at strongly coupled subduction zones , 1996 .

[26]  Kenji Satake,et al.  Linear and nonlinear computations of the 1992 Nicaragua earthquake tsunami , 1995 .

[27]  J. Rice,et al.  Seismicity and deformation at convergent margins due to heterogeneous coupling , 1996 .

[28]  Emile A. Okal,et al.  Teleseismic estimates of radiated seismic energy: The E/M 0 discriminant for tsunami earthquakes , 1998 .

[29]  Sunarjo,et al.  Damage to coastal villages due to the 1992 Flores Island earthquake tsunami , 1995 .

[30]  D. Masson,et al.  Recent tectonics around the island of Timor, eastern Indonesia , 1991 .

[31]  T. Hilde.  Sediment subduction versus accretion around the Pacific , 1983 .

[32]  Y. Fukao.  Tsunami earthquakes and subduction processes near deep-sea trenches , 1979 .

[33]  K. Satake.  Mechanism of the 1992 Nicaragua Tsunami Earthquake , 1994 .

[34]  H. Kanamori,et al.  Fault parameters and tsunami excitation of the May 23, 1989, MacQuarie Ridge Earthquake , 1990 .

[35]  K. Crook,et al.  Sediment slump likely caused 1998 Papua New Guinea tsunami , 1999 .

[36]  J. Pacheco,et al.  The October 9, 1995 Colima‐Jalisco, Mexico Earthquake (Mw 8): An aftershock study and a comparison of this earthquake with those of 1932 , 1997 .

[37]  K. Satake,et al.  Modeling the seismic source and tsunami generation of the December 12, 1992 Flores Island, Indonesia, earthquake , 1995 .

[38]  S. Tinti,et al.  The slip distribution of the 1992 Nicaragua Earthquake from tsunami run‐up data , 1996 .