Characteristics of Earthquakes Induced by Shallow and Deep Crustal Dike Intrusions

Abstract:

Shallow dike intrusions may be accompanied by fault slip above the dikes, complicating inverse and forward models of non-unique geodetic data. The diverse volcano-tectonic and low-frequency earthquakes accompanying the 2005 – 2010 Dabbahu-Manda Hararo large-volume dike intrusions, some of which caused 3 m fault displacements at the surface, provide an opportunity to differentiate between brittle failure accompanying lateral dike migration and shallow fault displacements above the dike. We apply the frequency index (FI) method of Buurman and West (2010) to characterize the spectra of swarm earthquakes from 6 of the dikes. These earthquakes often have broad spectra with multiple peaks, making the usual peak frequency classification method unreliable. Our results show a general bi-modal character with high FI earthquakes associated with deeper dike segments (top > 3 km subsurface) and low FI earthquakes with shallow dikes, indicating that shallow dikes result in earthquakes with more low-frequency content and larger amplitude surface waves. The bi-modal character is clearer in FI determined from full waveforms than from P-waves, which implies path effects are partially responsible for the enhanced surface waves. Low FI earthquakes are more common during dike emplacement, suggesting interactions between the dike and faults may lead to lower frequencies and surface waves. Taken together, likely source processes for low FI earthquakes are shallow hypocenters (< 3 km) possibly with surface rupture, slow rupture velocities, and interactions with dike fluids. Source time functions remain elusive due to natural damping of high frequency content and lower than necessary sampling rate. Our results suggest a continuum of spectral responses, implying either that impulsive volcano-tectonic earthquakes and the unusual, emergent earthquakes have similar source processes or that spectral analyses, such as FI, cannot distinguish different source processes.