Rapid Rupture Directivity Determination of Moderate Dip-Slip Earthquakes with the Reduced Finite Source Approximation

Xiaohui He, & Sidao Ni

Published August 15, 2016, SCEC Contribution #6724, 2016 SCEC Annual Meeting Poster #211

Moderate (M5.5-7) dip-slip earthquakes are frequent in both interplate and intraplate regions, and some of them could cause severe damage to buildings and even casualties. For an earthquake with up-dip rupture, the hanging wall region usually experiences enhanced strong ground motion significantly, and may suffer more damage. Determining the fault plane is thus essential for rapid hazard assessment, and rupture directivity analysis based on seismic waveforms is an effective method to resolve the ruptured plane as well as the rupture direction.

For strike-slip earthquakes which usually feature horizontal rupture propagation, rupture directivity could be determined via the spatial difference between hypocenter and centroid. But this technique is usually not feasible for dip-slip earthquakes with along dip rupture due to the large uncertainty of hypocenter depth in routine location process. For dip-slip earthquakes, aftershock distribution and finite fault inversion are commonly used to resolve the rupture directivity. In regions with dense seismic network, the spatial distribution of early aftershocks and the relative focal depths for mainshock and aftershocks may indicate the ruptured plane and the rupture direction, respectively. However, the strike of the two nodal planes are approximately same for dip-slip earthquakes, thus accurate focal depths for aftershocks are required to distinguish the fault plane from the auxiliary plane, which are difficult to obtain when the network is sparse. Slip distribution and rupture process can be derived in finite fault inversion, but the rapid application to moderate earthquakes is limited due to the low resolution of teleseismic waves or sparse local seismic network.

Instead, the teleseismic P wave can be split into the down-going direct P-wave and up-going depth phases (pP and sP). When the earthquake ruptures along dip, there is opposite rupture directivity effects on direct P-wave and depth phases. For instance, up-dip rupture will result in smaller amplitude and longer duration for direct P-wave, larger amplitude and shorter duration for pP and sP. Furthermore, it usually takes less than 15 minutes for teleseismic P wave to travel, therefore, teleseismic P wave can be used in rapid rupture directivity analysis. We proposed to calculate the reduced finite source (point source with rupture directivity) synthetics for teleseismic P wave, and determine the rupture directivity via waveform fitting. We verified the effectiveness of this method with forward tests, and investigated its robustness against station selection, uncertainties in point source parameters and uncertainties in finite source parameters. Then, we applied this method to the 2011 Mw5.8 Virginia earthquake and the 2008 Mw6.0 Nevada earthquake, and obtained consistent results with previous studies, suggesting our method is effective in rapid rupture directivity analysis for moderate dip-slip earthquakes.

Key Words
rupture directivity, dip slip earthquake, moderate earthquake

Citation
He, X., & Ni, S. (2016, 08). Rapid Rupture Directivity Determination of Moderate Dip-Slip Earthquakes with the Reduced Finite Source Approximation. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology