a-Value and b-Value as Basic Calculation Seismicity Index, Repeated Period, and Vulnerability of Earthquakes

a-Value and b-Value can be used to basic calculation to get seismicity index, repeated period, and vulnerability of earthquakes in a region. Yogyakarta is a region of Indonesia that often earthquakes are happened, caused by an active fault it has, Opak fault. And also because it is flanked by Java Megathrust and an active volcano. Earthquake phenomenon is very detrimental to life and many casualties if there is no preparedness to deal with it. This research aims to get the calculation results from seismicity index, repeated period, and vulnerability of earthquakes to indicate areas prone of earthquakes in Yogyakarta. The method used is a matrix inverse calculation, Gutenberg-Richter’s (1956) least square equation, to obtain a-value and b-value. This research uses USGS and BMKG earthquake catalog data and classifies it based on the region of Yogyakarta in geodetic coordinates ie 7.493°-11.523°S and 109.841°-110.940°E. The results of this research is got a-value and b-value calculations in Yogyakarta are found a = 4.0866 and b = 0.6528. And from the calculation results obtained that the seismicity index, repeated period, and earthquake vulnerability that occurs is Mw ≥ 5 in Yogyakarta.