Further evidence for the compound nature of slow earthquakes: The Prince Edward Island earthquake of April 28, 1997

Abstract

Seismograms of the April 28, 1997, Prince Edward island transform fault earthquake (Mw 6.8) show clear evidence for the superposition of two types of rupture, a normal earthquake and a slow component which primarily radiated energy at low frequencies. We combined low-frequency spectral data with P waveform data to invert for a source time function that satisfied data from 0.001 to 2.5 Hz. The results show that the earthquake started with a slow event which began ∼15 s before the fast mainshock and was part, of a slow earthquake which lasted over 30 s. The smoothness of the initial event is inferred from the low amplitude of high-frequency energy at regional stations during the initial portion of the P wave arrival. Directivity analysis shows that the two components of the rupture occurred on different faults. The slow event occurred on the main transform, and the fast event occurred on a parallel fault 40 km to the east. A similar compound sequence, involving both slow and fast components and rupture on multiple faults, occurred on the Romanche transform in 1994. These and other results for slow earthquakes suggest that many large events on oceanic transform faults are compound events. Copyright 2000 by the American Geophysical Union.