Potential fault slip areas are still left in Nepal?

(Guest post by Tomokazu Kobayashi)

SAR satellite observation

A satellite-based observation has a strong point in that spatially comprehensive ground information can be grasped at once even in a vast region. A synthetic aperture radar interferometry (InSAR) is an application technique of SAR observation by which we can remotely measure ground deformation with centimeter-level accuracy with meter-order high spatial resolution. In response to successive launches of new SAR satellites in 2014; ALOS-2 and Sentinel-1A, the InSAR has been utilized as a powerful geodetic tool.

The 2015 Gorkha earthquake, Nepal

The Indian plate underthrusts beneath the Eurasia plate nearly northward, which has made the Himalaya mountains grow. The plate motion has also accumulated seismic strain energy on the plate interface along the Himalayan belt, and the strain has been historically released as large earthquakes. In such a seismic belt, a devastating earthquake with a moment magnitude (Mw) of 7.8, called Gorkha earthquake, struck central Nepal on April 25, 2015, left about 9,000 people dead.

Kobayashi et al. (2015) has successfully detected widely distributed ground displacements for the earthquake by applying an InSAR method to ALOS-2 data (Fig. 1a). A major displacement area extends with a length of about 160 km in the east-west direction, and the most concentrated crustal deformation with ground displacement exceeding 1 m is located 20–30 km east from Kathmandu. It suggests that the accumulated strain has been released by a large slip on the plate interface under the ground.

eps_crustaldeformation
Figure. 1 (a) InSAR image showing the crustal deformation associated with the main shock and the largest aftershock (Mw7.3). (b) Slip distribution on the interface between the Indian and the Eurasia plates.

Unruptured area means a possibility of future earthquake?

It is vital to investigate the slip amount and its spatial distribution to evaluate the balance between the strain accumulation and release. The analysis result shows that a nearly pure reverse fault motion occurred with a slip amount of approximately 6 m at maximum (Fig. 1b). The rupture, however, did not propagate to the subsurface, suggesting that it is likely that there still remains a potential to release the accumulated strain. Also, a remarkable point is that there is a clear slip deficit area with a radius of ~10 km just west side of the largest aftershock (Mw 7.3) (Fig. 1b). This area is presumably subjected to a strong shear stress, and thus there remains a possibility to produce a fault slip equivalent to Mw ~7.0.

We have not had any evidences of whether the slip would occur in future and the slip occurs as either coseismic or aseismic style. A successive geodetic monitoring will be helpful in understanding of how these slip deficits are released on the plate interface.

eps_schematic_gorkha
Figure. 2 Schematic view for the 2015 Gorkha earthquake interpreted from the InSAR analysis. A top layer represents the spatial pattern of ground deformation. The lower part shows a simplified diagram of the rupture/un-ruptured areas for the 2015 Gorkha earthquake.

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