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Crustal Deformation and Fault Mechanics

 
    Crustal Deformation and Fault Mechanics

 

 

 

Subsurface Magamtic Processes in the Western Galapagos Islands from High-Resolution SAR Interferometry Deformation Measurements

In this project we used InSAR to study deformation and subsurface magmatic processes in the Galapagos. While these volcanoes are among the most active in the world, prior to our study there had been virtually no deformation measurements there. We found that nearly all of the volcanoes on the islands of Fernandina and Isabella were actively deforming. Accurate displacement maps obtained from ERS-1 and ERS-2 InSAR satellites show that uplift rates on these two islands range from a few mm/yr to over 0.9 m/yr. Most importantly, we found that the deformation rates and even patterns changed significantly over time (Fig. 1).

Line of sight displacements at Sierra Negra volcano

Figure 1. Line of sight displacements on Sierra Negra, from interferograms spanning 7 years from 1992 to 1999. Each color-cycle is 5 cm range change between the ground and the satellite. The first and third time periods show inflation of the caldera, while the second period shows evidence of “trap door” faulting. Inset: estimated change in thickness of best fitting sill (Amelung et a.l, 2000).

Sierra Negra volcano exhibits the most varied and interesting behavior. Peak uplift rates there ranged from 0.32 m/yr (1992-97), to 0.9 m/yr (1997-98), and 0.65 m/yr (1998-99). Using data from both ascending and descending orbits, we were able to determine both the vertical and one horizontal component displacement. The ratio of maximum horizontal to vertical displacement is ~0.3, consistent with a sill, but inconsistent with a spherical (Mogi) source (Yun et al., 2004). Amelung et al. (2000) showed that a sill at a depth of ~2 km with spatially variable opening could fit the data during the inflationary periods (1992-97 and 1998-99) quite well (Fig. 2a). In Yun et al. (2004) we explored a new inversion approach, in which the deformation is caused by a uniformly pressurized sill whose plan outline is unknown. In this case, we invert for the geometry of the sill and the magma pressure; the sill opening is uniquely determined by its shape and the pressure boundary condition. This more physically motivated model fits the data equally well (Fig. 2b).

This does not, however, prove that the source of deformation is a thin sill. If the radius of the sill is large compared to the depth, the surface deformation is dominated by displacement of the sill’s upper surface. The surface deformation is thus insensitive to the sides and bottom of a shallow, flat-topped magma body. Using boundary element methods we demonstrated that flat-topped diapirs produce almost identical surface deformation to sills, as long as the depth is small compared to the radius.

Sill model for deformation at Sierra Negra volcano

Figure 2. (a) Best-fitting sill with spatially varying opening distribution (Amelung et al., 2000). One fringe in the interferogram equals 5 cm LOS displacement. (b) Opening distribution from a uniformly pressurized sill. In this inversion the plan shape of the sill is adjusted to fit the data.

The deformation of Sierra Negra volcano from 1997-98 (Fig. 1, middle) is radically different from the oblate pattern observed from 1992-97 and since 1998. The 1997-98 interferogram shows maximum uplift near the southern edge of the caldera, and discontinuities in phase near a topographic ridge known to mark an intra-caldera fault system. We interpreted these data as indicating 1.2 meters of trap door faulting on the pre-existing fault system (Amelung et al., 2000). There is no evidence of an earthquake of this magnitude (Mw 5.7) implying that slip occurred slowly without significant seismic radiation. We were able to model this deformation reasonably well using simple elastic dislocations to represent the sill opening and the fault slip (Fig. 3). Note that the fault uplift follows the topography, indicating the ridge was built by repeated slip events on the fault.

Trap door faulting model for deformation at Sierra Negra volcano

Figure 3. (a) Modeled deformation during the period of trap door faulting (compare to Fig. 1b). The model includes a horizontal sill with uniform opening (large rectangle) and slip on normal faults bounding the elevated inner caldera floor. The four smaller rectangles show the surface projection of the normal faults; thick line marks the upper edge. (b) Residuals. (c) Comparison between data and model prediction along the profile A-A' and the intra-caldera surface topography.

Former Ph.D. student Sjonni Jónsson was able to accompany Dennis Geist (U. Idaho) to Sierra Negra and search for evidence of recent faulting. Sjonni found evidence for recent fault offset at a number of localities and mapped the faults using handheld GPS. Slip appears to have been distributed across a number of parallel fault strands. Summing the displacement along north-south transects yields cumulative offsets of 0.5 to 1.0 meters (Jonsson et al, 2004 in press). The largest offsets were observed in the field at the same location as the maximum line-of-sight displacement determined by InSAR. While the distributed nature of the faultingand the rugged surface topography limited the accuracy of the field measurements, the ground truth data are consistent with the inferences made from the space-based measurements.

References

Amelung F., S. Jónsson, H. Zebker, and P. Segall, Widespread uplift and trap door faulting on Galapagos volcanoes, Nature, 407, 993-996, 2000.

Jónsson, S., H. Zebker, P. Cervelli, P. Segall, H. Garbeil, P. Mouginis-Mark, and S. Rowland, A shallow-dipping dike fed the 1995 Flank Eruption at Fernandina Volcano, Galapagos, observed by Satellite Radar Interferometry, Geophys. Res. Lett., 26, 1077-1080, 1999.

Jónsson, S., Modeling volcano and earthquake deformation from satellite radar interferometric observations, Ph.D. thesis, Stanford University, 164pp., 2002.

Yun, S., P. Segall, and H. Zebker, Constraints on Magma Chamber Geometry at Sierra Negra Volcano, Galápagos Islands, Based on InSAR Observations, J. Volc. Geotherm. Res., in press, 2004.

 

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