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Comparison of Coulomb Shear Stress Changes from the Feb. 17, 1996 Biak Mw = 8.2 Event and a Subsequent Seismic Inversion

 
M. A. J. Taylor1,2, R. Dmowska1 and J. R. Rice1

1 Department of Earth and Planetary Sciences and Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA

2 Now at Bullard Laboratories, University of Cambridge, Cambridge, CB3 0EZ, UK

 

Abstract

The February 17, 1996 (Mw = 8.2) Biak earthquake ruptured at least 270 km along the New Guinea trench. The event was a thrust in a zone of very oblique subduction with estimated relative plate motion of 13 cm/yr The main event was followed by a number of events in the upper-plate, the two largest of which were the Mw = 6.4 (Feb. 18) and 6.5 (Feb. 17) events that occurred SW of the mainshock within 2 days.

Coseismic changes in Coulomb shear stress have been shown to correlate well with changes in seismicity following great earthquakes for both transform faults in continental regions and, in our work, for dip-slip faults at subduction zones [Dmowska et al., 1996, Taylor et al., JGR, in press, 1998]. These studies almost invariably adopt the approach of applying the inferred slip from one or more inversions of the main event to a 3D elastic half-space dislocation model to calculate the resulting Coulomb shear stress changes. There was no such inversion available immediately after the Biak, 1996 event, and so we took the alternative approach of using the spatial distribution of post-mainshock upper-plate seismicity to infer information about the distribution of slip in the main event. Our 3D subduction models with highly heterogeneous slip along-strike reveal distinct, characteristic patterns for the distribution of stresses in the upper-plate. The shear stress on arc-parallel strike-slip faults separates into two lobes, one of increased and the other decreased coseismic stress change. The extensional stress changes resolved onto normal faults with trace inclined at moderate to large angles to the trench likewise form two lobes of increased and decreased change. Based on this pattern and the positions and mechanisms of the first two upper-plate events (Feb. 17 and 18), the area of highest seismic slip was placed in the first week after the main event and confirmed by the subsequent year of seismicity [Taylor et al., 1998].

A subsequent inversion for the event by Kikuchi [1998] using the subevent deconvolution method of Kikuchi and Kanamori [1991] reveals a two phase source-time function. The rupture propagates roughly NW with the initial phase (Mw = 7.3, 19 s) followed by a second (Mw = 8.1, 32s) in which most of the moment is released. A comparison between this inversion and the position of highest slip inferred from coseismic stress changes shows they are essentially co-incident. The remarkable correlation between these results provides grounds for confidence in our method of approximately placing the position of highest moment release in a main event.
 
 


Introduction

 
 

 

Use of subsequent seismicity and Coulomb failure concept to constrain location of high-slip asperity in a great subduction earthquake

 

Figure 1: Tectonic map of Irian Jaya showing fault plane solutions, faults (solid lines) and inferred faults (dashed lines). Long arrows near enclosed regions show expected motion of Pacific relative to Australia and the short arrows show the slip vector azimuths from summing moment tensors of all earthquakes in boxes. From Puntodewo et al. [1994].
 

 
 Figure 2: Back-arc seismicity in Irian Jaya, Indonesia, following the Feb. 17, 1996, Mw = 8.2 earthquake (Feb. 1996 - July 1997).
 
 

Coulomb shear stress on fault:

Oblique slip on the thrust interface produces extensional and shear stress changes in the upper plate. Overall stress change is governed by change in "Coulomb Shear Stress" :
 
 
f = coefficient of friction
 
 
For oblique slip in the same sense as for Irian Jaya:
 
 
 
is the change in left lateral Coulomb shear stress, .

 

3D FEM model:

For large (M > 8) subduction zone earthquakes - distribution of slip along strike highly non uniform - isolated asperity regions of highest slip.

Use simple 3D finite element model of generic subduction zone [Dmowska et al., 1996, Taylor et al., 1998] (Figure 3) to calculate the extensional and shear stress changes due to oblique slip on an asperity (dark shaded region).

Slip, magnitude 'D', applied on asperity on thrust interface and modeled as "freely slipping" elsewhere (slip calculated to assure zero net coseismic change in shear stress).

Parameters in model:

= dip angle of subducting slab (~ 11° for Irian Jaya)

= angle of oblique slip from trench-normal (~ -13° for Irian Jaya)

W = down-dip width of seismogenic thrust interface (e.g. 100 km)

D = magnitude of imposed slip on asperity (e.g. 3 m)

= shear modulus (e.g. 30 GPa)

Calculate coseismic left-lateral Coulomb shear stress  and extensional stress  changes consistent with two largest (Feb. 17 and 18) upper-plate events (Figure 2):
 

Figure 3: 3D finite element model with oblique slip, heterogeneous along-strike: finite element mesh and distribution of slip on thrust interface with an asperity [after Dmowska et al., 1996].
 
 
 
 Figure 4: Coseismic change in left-lateral Coulomb shear stress  on arc-parallel faults for Irian Jaya-like parameters,  = 11°,  = -13°, and for f = 0.4.
 

 
Figure 5: Coseismic change in extensional normal stress  on faults oriented at 30° anti-clockwise from trench for Irian Jaya-like parameters,  = 11°,  = -13°.
 
 

Inferred position of highest slip:

From pattern of coseismic increased  (back and to left of asperity - Figure 4) and  (back and to right of asperity - Figure 5) infer position of region of highest moment release in Biak event:

Although most of upper-plate seismicity exhibits distinctly non double-couple solutions, still easily divisible into distinct groups relative to centroid of main event (Figure 2):

This pattern is exactly co-incident with that of Figures 4 and 5 if highest slip occurred on an asperity between two groups. Therefore suggest position of such an asperity - hatched elliptical region in Figure 2.

 

Comparison of seismic inversion with inferred slip distribution

 

At time of analysis of the Biak earthquake no available inversion of the source-time function or distribution of slip from the main event. However, subsequently, an analysis by Kikuchi [1998] was carried out using the subevent deconvolution method of Kikuchi and Kanamori, [1991].

Figure 6 shows details of inversion:

Figure 7 shows result of re-orienting and scaling the slip distribution from the inversion (Figure 6) and super-imposing it onto Figure 2. If assume rupture propagates parallel to trench (roughly NW) -
 
main phase from the inversion lies directly above the region inferred from the 3D finite element modeling results.
 
- Remarkable result. Both results are also consistent with position of CMT solution.
 

 
Figure 6: Inversion of the Feb. 17 1996, Biak earthquake from M. Kikuchi.  
 
Figure 7: Back-arc seismicity in Irian Jaya, Indonesia, following the Feb. 17, 1996, Mw = 8.2 earthquake (Feb. 1996 - July 1997), showing super-imposed solution for slip from Kikuchi inversion.
 
 

Conclusions

 
 

References:

Dmowska, R., G. Zheng, and J. R. Rice, Seismicity and deformation at convergent margins due to heterogeneous coupling, J. Geophys. Res., 101, 3015-3029, 1996.

Kikuchi, M., Inversion of the February 17 1992 Biak Earthquake, details available from ftp://ftp.eri.u-tokyo.ac.jp/pub/reports/YCU_News/Y51/txt.51, fig.2.ps, 1998.

Kikuchi, M. and H. Kanamori, Inversion of complex body waves, III, Bull. Seism. Soc. Am., 81, 2335-2350, 1991.

Puntodewo, S. S. O., R. McCaffrey, E. Calais, Y. Bock, J. Rais, C. Subarya, R. Poewariardi, C. Stevens, J. Genrich, Fauzi, P. Zwick, and S. Wdowinski, GPS measurements of crustal deformation within the Pacific-Australia plate boundary zone in Irian Jaya, Indonesia, Tectonophys., 237, 141-153, 1994.

Taylor, M. A. J., R. Dmowska, and J. R. Rice, Upper plate stressing and seismicity in the subduction earthquake cycle, J. Geophys. Res., 103, 24523-24542, 1998.
 



   
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