A new approach for preparation of quantitative seismicity maps as applied to Alpide Belt-Sunda Arc and adjoining areas

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Abstract
A new method is developed for preparation of quantitative seismicity maps of an area. In this method, the slope b of the cumlative regression curve log N = A − bM, is supposed to be variable from one region to another and not constant as assumed by Riznichenko (1958) in his seismostatistical method. Under such a premise of variable b, the seismicity at any place is defined as log of the area under the curve representing incremental number of earthquakes versus magnitude on linear graph paper, within the magnitude range M = M to M = Mmax. The log of the total area under such a curve from magnitude M = 0 to M = Mmax will represent A value on the cumulative regression curve, and the same can also be used as a measure of seismicity level in any region. Worldwide relations are established between N(occ) the cumulative number of earthquakes occurring in an area with magnitude 4 and above and those actually detected N(det). This enables one to define the ratio between log N(det) and log N(occ) as the detectability level D of an area. The constants A and b of the cumulative regression curves for various regions of the Earth (normalized to 2° × 2° grid area and 14-year period of earthquake observation) are found to be related to each other by the equation A = 6.35b − 1.41. Making use of this relation and the detectability level D, b and the normalized A values for every 2° × 2° grid area as at the equator can be calculated. These A or b values are then used to draw the seismicity maps for any region of the Earth. Using this new approach, a quantitative seismicity map based on A values has been prepared for the entire Alpide Belt-Sunda Arc and adjoining areas. This seismicity map has not only brought out the regions of high and low seismicity in great detail, but, at the same time, it also shows the trends of these highs and lows very clearly. This map is then compared with regional seismic zoning maps prepared by other workers covering small portions of this seismic belt and the two pictures are seen to show very good agreement both qualitatively as well as quantitatively.

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