Regional variation in the number of aftershocks (mb ≧ 5) of large, subduction-zone earthquakes (Mw ≧ 7.0)

Abstract

It is reasonable to expect that the number of aftershocks following a large earthquake should reflect the degree of heterogeneity on and near the fault plane. To investigate this question, we plot N , the number of aftershocks with mb ≧ 5.0 occurring within the next 30 days after large, subduction-zone, thrust-faulting earthquakes ( Mw ≧ 7.0), versus the magnitude ( Mw ) of the main shock. Only earthquakes that occurred after the installation of the World-Wide Standard Seismographic Network stations are considered. Because the detection threshold of aftershocks in many regions is greater than mb = 5.0, N is obtained by fitting the data to Gutenberg and Richter's relation (log N = a − b · mb ). For the same Mw , N varies by a factor of about 40 in different regions. N is less than expected in the eastern Pacific (including Alaska) and is greater than expected in the western Pacific. Relative lack and excess of N correlates well with regions of supposedly strong and weak coupling, respectively. While small and large N also show some correlation with the expansion rate of aftershock area with time, the variation in N is much larger than would be expected from the mere increase of aftershock areas. In some cases, it appears that the number of aftershocks, N , for earthquakes of similar Mw , may also reflect the complexity and enrichment of the P -wave amplitude spectra from 1 to 10 sec. For example, the Valparaiso, Chile, earthquake of 1985 ( Mw = 8.0) generated N = 46 earthquakes, whereas for the Michoacan, Mexico, event of 1985 ( Mw = 8.1), N is only equal to 5. Furthermore, for the Valparaiso earthquake, P waveforms are much more complex, and the spectra are more energetic at 1 to 10 sec as compared to the 1985 Michoacan event.