TY - JOUR
AU - GHOSH, UDAYAN
PY - 2002/10/01
Y2 - 2024/04/18
TI - Variation of Vp/Vs ratios for granitic layers and weekly average number of earthquakes in NE India
JF - MAUSAM
JA - MAUSAM
VL - 53
IS - 4
SE - Research Papers
DO - 10.54302/mausam.v53i4.1666
UR - https://mausamjournal.imd.gov.in/index.php/MAUSAM/article/view/1666
SP - 515-528
AB - <p><em>V<sub>p</sub></em>/<em>V<sub>s</sub></em> ratios for the granitic layers in Shillong Plateau and the partially overlapping Tezpur seismic area, have been calculated from Wadati diagrams drawn on the basis of seismic phase data.</p><p> </p><p>From Shillong plateau the average of 23 readings for some months of 1979 for <em>V<sub>p</sub></em>/<em>V<sub>s</sub> </em>is found to be 1.71.</p><p> </p><p>For Tezpur area the average of 29 readings of <em>V<sub>p</sub></em>/<em>V<sub>s</sub></em> for 1991-92 is found to be 1.73, but for 1995-96, the average <em>V<sub>p</sub></em>/<em>V<sub>s</sub></em> from 49 readings is found to be 1.68. The overall average from 78 readings for this area is 1.70. The low <em>V<sub>p</sub></em>/<em>V<sub>s</sub></em> for 1995-97 seems to be precursory.</p><p> </p><p>For the granitic layer, taking <em>V<sub>p </sub></em>= 5.92 kms/s and <em>V<sub>p</sub></em>/<em>V<sub>s </sub></em>= 1.70. We get <em>V<sub>s</sub></em>=3.48 km/s for NE India.</p><p> </p><p>Values of <em>V<sub>p</sub></em>/<em>V<sub>s</sub></em> ratios and number of shocks per day are plotted against time and are shown to undergo sudden lowering before many M³4.2 earthquakes. <em>V<sub>p</sub></em>/<em>V<sub>s</sub></em> is generally lowered below 1.60 in such cases.</p><p> </p><p>In Shillong plateau the number of shocks per day for 1979 is found to be three times the number in the adjoining Tezpur area, for 1991-97.</p>
ER -