The Potential of b-value Variations as Earthquake Precursors for Small and Large Events

Abstract: The potential of variations of b-values in the G-R relation, logN=a-bM as earthquake precursors for small events (rockbursts) in Zinkgruvan mine, Sweden and for tectonic (large) earthquakes in the Andaman-Sumatra region were investigated. The temporal frequency-magnitude distribution, b(t), of rockbursts in Zinkgruvan mine was examined using high quality data recorded during the period November 1996 to April 2004 with magnitude ranges from Mw= -2.4 to 2.6. A sliding time-window was applied to compute b-values. The windows contain 50 events and were shifted with steps of 5 events. The results indicated that b-values significantly drop preceding rockbursts of magnitude Mw≥1.6.Temporal and spatial variations of b-values were also examined for tectonic earthquakes, magnitude Mw≥4.1, in the Andaman-Sumatra region. Earthquake data from the ISC, IDC, NEIC and HVRD earthquake catalogs for a period from 01/01/1995 to 12/26/2004 were used for analysis. Spatial variations of b were calculated from circular areas containing 50 events, with nodes on a 0.5° x 0.5° grid. The analysis shows that b(t) estimates using data from different catalogs are comparable and that large earthquakes are preceded by a drop in b(t) of about 0.3~1. The distribution of stress deduced from b-value mapping shows that large earthquakes occurred in the high stress, i.e. low b-value, areas.Aftershock sequences of the Mw=9, December 26, 2004 and the Mw=8.7, March 28, 2005 shocks were investigated by using the same methods. Results from aftershock sequences show similar behaviour as for the large and presumed independent main events.The observed variations of b-values with time and in space support the hypothesis that b-values have a precursory potential. The method can be used for a wide range of earthquake magnitude, from microearthquakes (Mw<3) to giant tectonic shocks (Mw~9) and for both of independent shocks and aftershocks.