Ground-Motion Prediction Equations of Intermediate-Depth Earthquakes in the Hellenic Arc, Southern Aegean Subduction Area

Abstract

A response-spectra database is compiled of hundreds of seismic records from intermediate-depth earthquakes (earthquakes whose foci are located between 45 to 300 km from the earth's surface) with moment magnitudes of M 4.5-6.7 that occurred in the South Aegean subduction zone. The database consists of high-quality data from both acceleration-sensor and broadband velocity-sensor instruments. The database is much larger than previous databases used in the development of past em- pirical regressions enabling the determination of various parameters of ground-motion attenuation not previously examined. New variables accounting for the highly com- plex propagation of seismic waves in the Greek subduction zone are introduced based on the hypocentral depth and the location of the event, as these factors control the effects of the back-arc low-velocity/low-Q mantle wedge on the seismic-wave propa- gation. The derived results show a strong dependence of the recorded ground motions on both hypocentral depth and distance, which leads to the classification of the dataset into three depth-hypocentral distance categories. Ground motions from in-slab earth- quakes, especially with hypocentral depthsh� > 100 km, are amplified for along-arc stations, an expected effect of channeled waves through the high-velocity slab. The ground motions are also strongly attenuated in the back-arc region, due to the low-Q mantle wedge, which are almost independent of the recording hypocentral distance. In contrast, for shallower in-slab events (60 km <h< 100 km), the corresponding dif- ferentiation of seismic motion for along-arc and back-arc stations is observed beyond a specific critical distance range. Moreover, for longer periods, both along-arc ampli- fication and back-arc anelastic-attenuation factors strongly diminish, suggesting that the longer wavelengths of seismic waves are not affected by the complex geophysical structure, resulting in more similar ground motions for both back-arc and along-arc stations. Finally, results for interface events ( h< 45 km) occurring along the outer Hellenic arc suggest their wave propagation is not affected by the presence of the low-velocity/low-QS mantle wedge, but is mainly controlled by the differences of the anelastic attenuation between the Mediterranean and Aegean lithospheres.