Civil Insight: A Technical Magazine Volume 3 | Page 54
Shrestha P. et al.
Civil Insight (2019) 51-56
magnitude earthquake is called return period of earthquake exceeding that magnitude. When the logarithm
of ɉ of Southern California earthquakes was plotted against earthquake magnitude, a linear relationship
was observed and the resulting Gutenberg-Richter law for earthquake recurrence was expressed as the
following equation (Kramer, 2007).
ɉ ൌȂ ȗ
ሺሻ
where,
ɉ = average number of earthquakes per annum with magnitude greater than or equal to
ͳͲ = mean yearly number of earthquake of magnitude greater than zero
= relative likelihood of large and small earthquakes
ߚ = 2.303
In general, Gutenberg-Richter (G-R) parameters, and ǡ are obtained from the least square regression
analysis; the catalog must be complete for all the magnitudes. The completeness analysis was carried out
using the method proposed by Kijko and Smit (2012). The estimation of ߚ-value is given as,
ߚൌቀ
భ
ఉ భ
ିଵ
ఉ మ ڮ ఉ ೞ ቁ
మ
ೞ
ሺͺሻ
where,
ߚ ൌ
ͳ
݊
ǡ ݎ ൌ ǡ ݊ ൌ σ݊ ǡ
݉
ഥ െ ݉
݊
݅ ൌ ͳǡʹǡ ǥ ݏ
݊ = number of earthquakes greater than the completeness magnitude (݉ ) for a sub-catalog
݉
ഥ = average magnitude
= number of sub-catalogs
Once the ߚ-value is known, the mean seismic activity rate ߣሺ݉ ሻ is obtained as,
ߣሺ݉ ሻ ൌ σ ೞ
సభ ௧ ௫ൣିఉ൫ ି ൯൧
ሺͻሻ
where ݐ = duration of the catalog and ݉ = minimum magnitude considered.
3) Results and Discussions
3.1) Earthquake Declustering
After converting all the earthquake magnitudes into moment magnitude , earthquake declustering was
carried out using dynamic window algorithm presented in Table 1. The raw data before aftershock removal
in the earthquake catalog consisted of around 1175 events, as shown in Fig. 3. After declustering, the data
were reduced to 809, as presented in Fig 4.
Fig. 3. Earthquake data around Nepal (raw data before declustering)
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