The Mw 7.9 2015 Nepal earthquake happened at 06:11 UTC (11h41 Nepal time) on Saturday 25 April 2015 at the northwest of the Nepalese capital of Kathmandu.

The figure below shows a preliminary W phase solution computed with a selection of "low noise" stations showing the best signal to noise ratios. This solution yields a moment magnitude Mw=7.9 with a quasi pure double-couple mechanism showing a low dip toward the north. The optimum W phase centroid location is 27.77°N, 85.26°E at a depth of 25 km and the best double couple is [Strike=287°, Dip=5°, Rake=97°]. The relative location of the USGS hypocenter and W-phase centroid sugest a rupture propagation toward the south-east. The centroid delay of 32 sec indicates that the total rupture duration is about 1 min.

Comparison between data (black) and predictions (red) is available here.

W phase CMT solution obtained for the 2015 Nepal earthquake. Yellow star is the USGS hypocenter. Yellow circle is the W-phase centroid location.

W phase is a very long period phase (mainly 100s-1000s) arriving at the same time as the P wave. When the displacement seismograms are filtered between 100s and 1000s, it is conspicuous between the P wave and the surface waves. Because of its long period nature and because it precedes the large amplitude surface-wave arrivals, the W phase can provide rapid and reliable estimates of the overall source parameters of large events. This contrasts with other source inversion methods which usually take several hours to determine the first order attributes of a great earthquake (i.e. Mw>8.5) even in a well instrumented region.

Articles

Kanamori, H., 1993. W Phase, Geophys. Res. Lett., 20, 1691-1694.

Kanamori, H. & Rivera, L., 2008. Source inversion of W phase: speeding tsunami warning, Geophys. J. Int., 175, 222-238.

Hayes, G., Rivera, L. & Kanamori, H., 2009. Source inversion of the W phase: real-time implementation and extension to low magnitudes, Seismol. Res. Let., 3, 800-805.

Duputel, Z., Rivera, L., Kanamori & H., Hayes, G., 2012. W-phase fast source inversion for moderate to large earhquakes (1990 - 2010), Geophysical Journal International, 189, 1125-1147

W phase solutions

USGS W phase solution

Data sources

IRIS Data Management System, and specifically the IRIS Data Management Center, were used for access to waveform and metadata required in this study (http://www.iris.edu).

Global Seismographic Network (GSN) is a cooperative scientific facility operated jointly by the Incorporated Research Institutions for Seismology (IRIS), the United States Geological Survey (USGS), and the National Science Foundation (NSF). (http://www.iris.edu/hq/programs/gsn).

Geoscope operated by the Institut de Physique du Globe de Paris (IPGP) and the Ecole et Observatoire des Sciences de la Terre (EOST) (http://geoscope.ipgp.fr).