Intraplate and interplate earthquakes in Chilean subduction zone: A theoretical and observational comparison

Abstract

During the last decade, efforts to improve our knowledge of great Chilean earthquakes have shown that not all of the major destructive events have occurred in the contact between the Nazca and south American plates (interplate earthquakes). Waveform analysis of records from the Ms = 8.0, 1950 Antofagasta and the Ms = 7.8, 1939 Chillán earthquakes have shown that these large events are tensional, rupturing along nearly vertical, intermediate depth, fault planes within the subducting slab (intraplate events). Previous studies found that other earthquakes in Chile, like Santiago 1945 (Ms = 7.1), La Ligua 1965 (Ms = 7.1), Tal-tal 1965 (Ms = 6.9), Tocopilla 1970 (Ms = 6.5), and Tarapacá 2005 (Mw = 7.8) were also of tensional type. In the present work, we analyze theoretical and observational evidence comparing interplate and intraplate earthquakes. We found clear differences in the source characteristics between these two kinds of events, with intermediate depth, intraplate earthquakes presenting larger corner frequencies and greater seismic energy release than interplate events (for a given seismic moment). This is also reflected in the higher averaged apparent stress drop for intraplate earthquakes ( σ a ∼ 90 bar) compared to interplate events ( σ a ∼ 30 bar). From theoretical computations, we found that the rupture velocity has a minor effect on the resulting displacements; while directivity and changes in the static stress drop produced notable (and similar) results. We believe that the differences found in the data might be due to changes in the static stress drop, the effect of directivity, or both. Regardless of the cause of the observed differences in the apparent stress drop, these results should be taken into consideration into the assessment of the Seismic Hazard in Chile.