Investigations of the b-value and its variations on possible earthquake prediction in the North-South China Seismic Belt

Abstract

The Gutenberg-Richter b-value is studied extensively by researchers as a possible earthquake precursor. In this thesis, two different approaches to compute the b-value for the purpose of earthquake prediction are investigated and discussed. A new methodology, the b-value ratio method, inspired by a 1988 paper by Morgan et al., is also introduced in this thesis as a variation of b-value. To calculate the b-value ratio, the event catalog has to be separated into a group of larger events and a group of smaller events with a change-point magnitude, which leads to two b-values for the catalog, b-value of the smaller events, b-low, and the b-value of the larger events, b-high. The b-value ratio is then obtained by dividing b-high by b-low. Both b-value and b-value ratio methods are applied to a set of earthquakes occurring between 1983 and 2015 in the North-South China Seismic Belt. The dataset contains 4454 events for M >/= 3.6. Within this dataset, there is the catastrophic 2008 M = 7.9 Wenchuan earthquake. The b-value time series are computed in two different ways, the time-based method and the event-based method. Moving windows and overlapping windows are used in both ways. Our results calculated with the event-based method show an initial increase in b-value followed by a constant-slope decrease prior to the 2008 Wenchuan event. After the 2008 large earthquake occurred, the b-value bounces back to about 1.0 and starts to decrease again. The b-value ratio shows a completely reversed trend. Both b-value and b-value ratio in this case could be used as post-prediction precursors of the 2008 M = 7.9 Wenchuan earthquake. Analysis of b-value versus depth in the North-South China Seismic Belt region shows a monotonic decrease in b-value between 8km and 13km depth, which reflects an increase in differential stress in the upper crust. It is observed that b-value increases between 13 km and 22 km depth and decreases below 22 km depth. These observations correspond to the changes in the stress regimes and indicates the inverse relationship between b-value and differential stress in the crust.