Aftershock Distribution and the Mainshock’s Fault Plane by
the MJHD Method: Application to the Western Sichuan, China,
Earthquake on April 20, 20132013/04/22
HURUKAWA Nobuo
IISEE, Building Research Institute (BRI), Japan
Earthquake Information (USGS)
Origin Time: April 20, 2013 at 00:02:47 UTC
Hypocenter: 30.284°N, 102.956°E, 12.3 km (depth)
Magnitude: M = 6.6
Data: ‘Latest Earthquakes in the World - Past 7 days’ by the US Geological Survey
Events Relocated: Mainshock and aftershocks until 15h, April 21
Method: Modified Joint Hypocenter Determination (MJHD) by Hurukawa and Imoto
Results: Length of aftershock area: ~30 km
Fault plane: Nodal plane striking NNE-SSW dipping WNWComment: We compared the aftershock distribution with nodal planes by global CMT,USGS CMT, and USGS W-Phase. A nodal plane (Strike: 198°, Dip: 33°)
by USGS CMT fits to the aftershock distribution best among them. The
fault plane is located ~50 km SSW of the fault plane of the 2008 Sichuan
earthquake (Mw 7.9).
2013/04/20 WESTERN SICHUAN, CHINA
Mw 6.631°00’
AC30°30’
Magnitude
4D30°00’

567DepthB015304529°30’

Depth (km)102°30’ 103°00’ 103°30’ 104°00’
0 A
102030405060-100

B-50

050
Distance (km)C100-100

D-50050
Distance (km)0102030405060100

Figure 1. Hypocenters relocated by the MJHD method. USGS CMT solution of themainshock is also shown. Epicentral distribution and two vertical cross sections along
A-B and C-D lines, which are perpendicular to strikes of the two nodal planes of themainshock, are shown. Two nodal planes are shown by lines in cross sections. Thenodal plane corresponding to the fault plane is shown by a thick solid line.

2013/04/20 WESTERN SICHUAN, CHINA
Mw 6.631°00’
AC30°30’
Magnitude
4567D30°00’

Depth

B

015304529°30’

Depth (km)102°30’ 103°00’ 103°30’ 104°00’
0 A
102030405060-100B-50

050Distance (km)C100-100D-50

050
Distance (km)0102030405060

100

Figure 2. Hypocenters located by the USGS. Two nodal planes of the mainshock arealso shown by solid lines in cross sections.

Figure 3. Stations used in relocation.

2008 05 12 SICHUAN, CHINA
ACMw 7.9

2008 SICHUAN EQ

D5CD2678Magnitude2013 SICHUAN EQ
B0204060

Depth (km)Depth0 ABCD05050100100150

150-200-1000100
Distance (km)200-200-100
0100
Distance (km)200Figure 4. The 2008 Sichuan earthquake and its immediate aftershocks (Coloredsymbols: IISEE's aftershock distributions and corresponding fault planes). Black circles
denote epicenters of the 2013 Sichuan earthquake and its immediate aftershocks.
References
Hurukawa, N., Quick aftershock relocation of the 1994 Shikotan earthquake and its
fault planes, Geophys. Res. Lett., 22, 3159-3162, 1995.
Hurukawa, N. and Imoto, M., 1990, Fine structure of an underground boundary between
the Philippine Sea and Pacific plates beneath the Kanto district, Japan, Zisin (J.
Seismol. Soc. Jpn), 43, 413-429 (in Japanese with English abstract).
Hurukawa, N. and M. Imoto, Subducting oceanic crusts of the Philippine Sea andPacific plates and weak-zone-normal compression in the Kanto district, Japan,Geophys. J. Int., 109, 639-652, 1992.
Cf. IISEE's aftershock distributions and corresponding fault planes: 1976 – 2009, Mw ≥ 7.0
http://iisee.kenken.go.jp/cgi-bin/eqcatalog.newv6/mjhdcatalog_eng.cgi