The b-value and fractal dimension of local seismicity around Koyna Dam(India)

Earthquakes began to occur in Koyna region （India） soon after the filling of Koyna Dam in 1962. In the present study, three datasets 1964-1993, 1993-1995, and 1996-1997 are analyzed to study the b-value and fractal dimension. The b-value is calculated using the Gutenberg- Richter relationship and fractal dimension Dcorr. using correlation integral method. The estimated b-value and Dcorr. of this region before 1993 are found to be in good agreement with previously reported studies. In the sub- sequent years after 1995, the b-value shows an increase. The estimated b-values of this region are found within the limits of global average. Also, the pattern of spatial clustering of earthquakes show increase in clustering and migration along the three zones called North-East Zone, South-East Zone （SEZ）, and Warna Seismic Zone. The earthquake events having depth ≤5 km are largely confined to SEZ. After 1993, the Dcorr. shows decrease, implying that earth- quake activity gets clustered. This seismic clustering could be helpful for earthquake forecasting.

Software to Estimate Earthquake Spectral and Source Parameters

A software (EQK_SRC_PARA) has been developed to estimate spectral parameters of earthquake source spectrum, namely: low frequency displacement spectral level (Ω0), corner frequency above which spectrum decays with a rate of 2 (fc), the cut-off frequency above which the spectrum again decays (fmax) and the rate of decay above fmax (N). A Brune’s source model [1,2] that yield a fall-off of 2 beyond corner frequency is considered with high cut-off frequency factor presented by Boore [3] that fits well for frequencies greater than fmax. The software EQK_SRC_PARA is written in MATLAB and uses input data in Sesame ASCII Format (SAF) format. The obtained spectral parameters have been used to estimate source parameters (e.g., seismic moment, source dimension and stress drop etc.) and to develop scaling laws for the study region. The cut-off frequency “fmax” can also be studied and interpreted to confirm about its origin.

A cautionary note on decadal sea level pressure predictions from GCMs

A comparison of sea level pressure(SLP)trends in a subset of seven Coupled Model Intercomparison Project(CMIP)phase 5 general circulation models(GCM),namely decadal simulations with CCSM4,CanCM4,MPI-ESM-LR,FGOALS-g2,MIROC4h,MIROC5,and MRICGCM3,to their CMIP3 counterparts reveals an unrealistically strong forecast skill in CMIP3 models for trend predictions for 2001e2011 when using the 1979e2000 period to train the forecast.Boreal-winter SLP trends over five high-,mid-,and low-latitude zones were calculated over the 1979e2000 initialization period for each ensemble member and then ranked based on their performance relative to HadSLP2 observations.The same method is used to rank the ensemble members during the 2001e2011 period.In CMIP3,17 out of 38 ensemble members retain their rank in the 2001e2011 hindcast period and 3 retain the neighboring rank.However,numbers are much lower in more recent CMIP5 decadal predictions over the similar 2001e2010 period when using the 1981e2000 period as initialization with the same number of ensembles.Different periods were used for CMIP3 and CMIP5 because although the 1979e2000 initialization is widely used for CMIP3,CMIP5 decadal predictions are only available for 30-year periods.The conclusion to consider the forecast skill in CMIP3 predictions during 2001e2011 as unrealistic is corroborated by comparisons to earlier periods from the 1960s to the 1980s in both CMIP3 and CMIP5 simulations.Thus,although the 2001e2011 CMIP3 predictions show statistically significant forecast skill,this skill should be treated as a spurious result that is unlikely to be reproduced by newer more accurate GCMs.