Seismicity and Seismotectonics of Nepal and its Neighbouring Regions

Harihar PaudyalTribhuvan University

NEPAL

H N Singh, V P singhBanaras Hindu University

Varanasi

(after (after PandayPanday et al., 1999)et al., 1999)

Clock tower and Tri-Chandra College, Kathmandu

Reconstruction in same fashionAfter 1934 event

Kathmandu: vulnerable mega city

Kathmandu: vulnerable mega city

In Brief: 1978 First seismic station at PhulchowkiNow 20 Short Period and 10 GPS Stations operation from 1994

Seismic stations in Nepal

Seismicity map of Nepal with epicenter M>2 for the period of 1995 -1998 (by DMG, Kathmandu).

Tectonics elements of Nepal and adjoining regions

Seismicity map for m≥4 (1963-2004)

Variation of number of events with focal depth

140

120

100

80

60

40

20

0 50 100 400 450

Number of events

Dep

th (k

m)

Variation of focal depth (Km) with Variation of focal depth (Km) with Longitude and LatitudeLongitude and Latitude

Samples of FPS calculated in this study

More samples …

Seismotectonic map with fault plane solutions of 25 events (1970-2004) calculated in this study.

Variation for orientation of fault plane with depth

Stress Pattern in Two Prominent Seismogenic Sources in Nepal region

• Two seismic zones:• zone A (260 - 280 N, 850 – 890 E) in south eastern Nepal

and adjoining areas and

• zone B (28.50 - 310 N, 780 – 820 E) in western Nepal and vicinity, have been delineated using available historical records as well as current seismic events in Nepal and the adjoining regions.

• Data used• Focal mechanism solution of earthquakes occurred in the

region have been collected from published literature and 16 new focal mechanism solution for the period 1970 –2004 with M 4.7 to 5.6 have been calculated using P –wave first motion data

Seismo-tectonic map of Nepal and its adjoining regions, Central Himalaya. Two zones A and B are delineated using spatial distribution of earthquakes that have occurred in this region from 1964 to 2004.

TS65106253004.727.485.9527.7713:14:28.71/3/200419

TS744015236524.588.1727.2122:41:13.06

12/2/200118

TS680803204.83387.2627.4902:03:55.912/8/199717

TS59300301465.2785.2027.9820:02:14.221/31/199716

TS7430191684.83386.7627.4911:08:19.05

12/30/199615

TS6678162064.931.788.8027.6017:41:17.59/25/199614

TS7813010263524.987.827.8516:30:57.64/26/199613

ZH325261574.9*7088.1427.919:52:45.912/21/199112

CMT71352162055.2*1885.7627.3909:11:0.810/29/198811

CMT4169402076.9*3586.6426.5223:09:15.98/20/198810

TS65148243145.627.588.3627.1919:10:10.37/29/19889

NB35731217261788.827.419:00:4511/19/19808

CMT1184782435*2487.5726.2916:29:12.46/19/19797

TS4869341975.21985.9327.8213:53:50.710/4/19786

TS59358261444.925.587.0427.4301:35:51.24/24/19755

MT477431875.42086.0027.6614:16:01.3/24/19744

TS642512111559685.727.619:30:14.52/26/19703

CH571331816.12387.8527.3813:22:24.11/12/19652

SG5827524468.4**2086.526.508:43:181/15/19341

Ref.T-Pl

T-Az

P-Pl

P-Az

M Depthkm

Long.(oE)

Lat.(oN)

Origin time

DateSl. Nos.

Focal mechanism solutions of the earthquakes in zone A, south eastern Nepal and its vicinity

Ref: TS:This Study, CH:Chandra (1978), SG: Singh and Gupta (1980), CMT: Centroid Moment Tesor solution, NB: Ni and Barazangi (1984), ZH: Zhu and Helberger (1996).

CMT3966105.6*1581.2530.1314:36:8.56/4/200239

CMT7046182025.4*1581.4928.9108:53:59.411/27/200138

CMT4816421925.5*1581.7529.6107:31:57.111/27/200137

CMT5227382036.6*1579.2130.3819:05:18.13/28/199936

CMT6242272065.6*1580.2929.4308:47:31.61/5/199735

TS600202455.256.481.9028.9322:07:45.36/2/199234

CMT5714322076.8*1578.2430.2221:23:21.610/19/199133

CMT5379332315.5*1578.1930.4822:03:12.47/16/198632

TS78340111605.12180.5429.8415:46:25.92/19/198431

TS63330241215.13381.9229.4617:22:43.45/15/198130

CMT6619241996.6*22.380.9529.4214:58:51.47/29/198029

CMT709201885.5*1081.1128.9612:23:15.97/29/198028

CMT1527063315.1*1581.5930.114:38:58.46/22/198027

CMT5216381945.8*1680.3229.5822:59:14.75/20/197926

TS24122663025.112.381.9429.2104:44:33.19/6/197525

TS3533718805.32581.5729.2401:51:48.42/12/197024

CH841061905.81580.7929.6220:52:16.312/16/196623

CH1511751915.65379.3928.6702:15:298/15/196622

CH719182126.13780.8329.6210:41:08.66/27/196621

CH6219282075.85080.4629.9600:46:02.69/26/196420

Ref.T-Pl

T-Az

P-Pl

P-AzMDepth

KmLong.(oE)

Lat.(oN)Origin timeDateSl.

Nos.

Focal mechanism solutions of the earthquakes in zone B, western Nepal and its vicinity

Composite plot of P- and T- axes for Eastern Nepal (Zone A) for 19 events and Western Nepal (Zone B) for 20 events.

• 1. The western and eastern portion of Nepal region shows a compressive stress regime. Such information could not be obtained for Central Nepal due to lack of seismic data.

• 2. The greatest compressive stress direction in inferred to be NE – SW to NNE - SSW in western and eastern Nepal region. The direction of tensile stress is found nearly horizontal E – W in eastern Nepal where as NW – SE in western Nepal. This also suggest that the direction of extensional force changes from east west in eastern Nepal to approximately NW – SE in the western region which are separated approximately by 750 km in NW – SE direction.

• 3. Our study supports existing idea of approximately east west mass movement to the north of MCT.

The stress pattern study shows:

Seismogenic sources in Central Himalaya

• Based on inter event times of main shocks in the four seismogenic sources of Central Himalayan region; the time and magnitude predictable relationships has been established as:

logTt = 0.25 Mmin + 0.03 Mp – 0.43log m0 + 10.05 (1)• with a correlation coefficient of 0.92 and standard deviation of 0.19.

Similarly the values of parameters of (4) were calculated by the use of all available information from Table 1, and the following formula is derived:

Mf = - 0.48 Mmin + 1.19Mp + 1.13 log m0 – 26.27, (2)

• This equation has multi-correlation coefficient, R=1.13 and standard deviation 0.74.The positive correlation between the repeat time and the magnitude of the preceding main shock in (X) infers that thetime-predictable model hold in the area under study.

Long-term prediction of the next shallow main shock

The conditional probability, P, may be calculated, for main shock with M ≥Mmin during the next ∆t years (from present), when the previous such event with magnitude Mp occurred t years ago (from now), by using the following relation (Papazachos and Papaioannou, 1993):

⎟⎠⎞

⎜⎝⎛−

⎟⎠⎞

⎜⎝⎛−⎟

⎠⎞

⎜⎝⎛

=∆

σ

σσ1

12

1)(

XF

XFXFtP

wherewhere

( )⎟⎟⎠

⎞⎜⎜⎝

⎛=

∆+=

tt TtX

TttX log,log 12

In above equation, F is the complementary cumulative value of the normal distribution with mean equal to zero and standard deviation σ

.

Expected magnitude, Mf, and the corresponding probabilities, P10, P20 and P30, for the occurrence of large (Mmin ≥5.5) shallow shocks during the Periods 2006-2015, 2006-2025 and 2006-2035 in the Nepal and adjoining Central-Himalaya (NCH).

----------------------------------------------------------------------------------------------Name ofSeismogenic P10 P20 P30 Mmin Mp tp log m0Sources Mf±0.09 (Year of Mp)

------------------------------------------------------------------------------------------------NCH-1 5.3 0.59 0.94 0.99 5.5 5.5 2003 24.46

NCH-2 6.9 0.90 0.99 1.00 5.7 6.6 1988 24.85

NCH-3 6.1 0.84 0.98 1.00 5.9 6.0 1998 24.83

NCH-4 6.4 0.96 1.00 1.00 5.8 5.8 2002 25.31-----------------------------------------------------------------------------------------------

Acknowledgements

Tribhuvan University, Nepal

University Grant Commission, Nepal

Dr. Daya Shanker, Assoc. Prof., IIT Roorkee, India.

The Abdus Salam ICTP, Trieste, Italy

Thank you