Correlation between Fractal of Spatial Distribution of Aftershock and Active Fault in Sumatra Island and Surrounding

• Main Author: Setiawan, Bahari
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/43992
• Institution: Institut Teknologi Bandung

Sumatra tectonic is an oblique convergence that generates slip segmentations. These segmentations are dip-slip in the fore-arc (subduction zone) and strike-slip in the arc which is Sumatra Fault Zone (SFZ) as a dextral fault. Seismicity and active fault, in particular, fractal studies, in Sumatra has been carried out many times by another researcher, but only a few kinds of research have specifically learned about this active fault. The earthquake itself is an event that has an undoubtedly complex pattern, but this can be quantified by using the fractal concept. Earthquake catalogue and active fault data were obtained from PuSGeN (Centre of National Earthquake Study). These active faults are defined as a fault that has slipped or earthquake less than 10,000 years. This catalogue has a magnitude above 4.5 Mw and starts from 1900 to 2016. In this study, aftershocks are defined as earthquakes that occur after the main shock and have a smaller magnitude. The aftershocks analyzed are the earthquakes that are not related to the Benioff Zone or subduction zone. Then, 15 earthquakes were identified in this research and followed by aftershock swarm. The fractal of spatial epicenter distribution of aftershock in 15 main shocks was estimated by using the two-point correlation integral. We estimated the fractal dimension of active fault in the aftershock region by using The Box-Counting Method. Fractal of the spatial distribution of aftershock (D2) values varies from 1.03 ± 0.03 to 1.68 ± 0.08. The fractal of active fault is found in range 0.95 ± 0.03 to 1.16 ± 0.01. The positive correlation is obtained in this study using least-squared regression and three trends are identified. The first trend defines as the formula D2 = (0.72 ± 0.02)D0 + (0.91 ± 0.02). In contrast, higher gradient was found and its equation is D2 = (1.882 ± 0.022)D0 - (0.634 ± 0.234). The last correlation is D2 = (0.951 ± 0.153)D0 + (0.038 ± 0.170). The first and third correlation, the gradient is relatively similar, in 0.05 different. However, the intercept, in fact, shows a clear difference. The similarity and difference are speculated as a similar mechanism happens in the aftershock, but there is a significantly dissimilar in b-value, segment differences and the quantity of aftershock. It also approves that between active fault and aftershock spatial distribution follows the self-organized critical or scale invariance. In the second correlation, the higher gradient is caused by an earthquake doublet mechanism. A higher rate of aftershock is responsible for the higher gradient. In this research, two earthquake events did not follow the major three trends. Those earthquakes are interpreted as occurs in different mechanisms to generate aftershock that was not related to an active fault.