- Ground-motion model is:
where PGA is in g, for central Himalayan earthquakes b1 = -4.135, b2 = 0.647, b3 = -0.00142, b4 = -0.753
and σ = 0.59 and for non-subduction earthquakes in N.E. India b1 = -3.443, b2 = 0.706, b3 = 0,
b4 = -0.828 and σ = 0.44 (coefficients of other equations not given here because they are for a particular
- Data from strong-motion accelerographs (SMA) and converted from structural response recorders (SRR),
which consist of six seismoscopes with natural periods 0.40, 0.75 and 1.25s and damping levels 5 and 10%.
Conversion achieved by deriving spectral amplification factors (ratio of response ordinate and PGA) using
SMA recordings close to SRR, checking that these factors were independent of distance. The mean of
the six estimates of PGA (from the six spectral ordinates) from each SRR are then used as PGA values.
Check quality of such PGA values through statistical comparisons and discard those few which appear
- Data split into four categories for which derive separate equations:
- Central Himalayan earthquakes (thrust): (32 SMA records, 117 SRR records), 3 earthquakes with
5.5 ≤ M ≤ 7.0, focal depths 10 ≤ h ≤ 33km and 2 ≤ R ≤ 322km.
- Non-subduction earthquakes in NE India (thrust): (43 SMA records, 0 SRR records), 3 earthquakes
with 5.2 ≤ M ≤ 5.9, focal depths 33 ≤ h ≤ 49km and 6 ≤ R ≤ 243km.
- Subduction earthquakes in NE India: (33 SMA records, 104 SRR records), 1 earthquake with M = 7.3,
focal depth h = 90km and 39 ≤ R ≤ 772km.
- Bihar-Nepal earthquake in Indo-Gangetic plains (strike-slip): (0 SMA records, 38 SRR records), 1
earthquake with M = 6.8, focal depth h = 57km and 42 ≤ R ≤ 337km.
- Limited details of fault ruptures so use epicentral distance.
- Use epicentral locations which give best correlation between distance and PGA.
- Find PGA not well predicted by earlier equations.
- Simple model and regression method because of limited data.
- Remove one PGA value from category b equation because significantly affecting equation and because
epicentral location only approximate.
- Constrain b3 for category b equation to zero because otherwise positive.
- Category c originally contained another earthquake (14 SMA records, M = 6.1, 200 ≤ d ≤ 320km) but
gave very small b2 so exclude it.
- Equations for category c and category d have b2 equal to zero because only one earthquake.
- Find considerable differences between predicted PGA in different regions.
- Note lack of data hence use equations only as first approximation.