Using regional geophysical databases to facilitate the identification of surface reflections on P seismograms

Abstract

The relative amplitude method allows the focal mechanism of an earthquake to be determined using the relative amplitudes of P and the surface reflections pP and sP observed on P seismograms recorded at long range. Regional structural information is helpful when interpreting P seismograms in terms of P, pP, sP and other arrivals such as near-source S-to-P conversions. Here we re-interpret array seismograms from the 1972 January 12 Gulf of Suez earthquake and demonstrate that many of the regionally dependent parameters, required by the method, can be obtained not only from conventional published sources, but also from geophysical databases that can be easily accessed via the internet. For example, one interpretation of the P seismogram recorded by the seismometer array at Gauribidanur, India (GBA), from the Gulf of Suez earthquake is that the first large-amplitude arrival after P is pP, followed by sP. However, modelling shows that this interpretation requires an anomalously high Poisson's ratio in the above-source structure. After careful consideration of plausible arrival times and relative amplitudes of P, pP and sP at GBA and on seismograms recorded by two other arrays, our preferred model has a focal mechanism with fs=105°, d=85°, ?=-165° and a source depth of 7.8 km in a Poisson crust 23 km thick. In our preferred model, the first large-amplitude arrival after P at GBA is not pP, but an S-to-P conversion at the Moho (independent studies suggest the Moho is about 25 km deep in the epicentral region). Our example demonstrates that the relative amplitudes of P and surface reflections have the potential to identify seismic disturbances that are too deep to be possible violations of the Comprehensive Nuclear Test Ban Treaty (CTBT). The availability of data from seismometer arrays (within the International Monitoring System) and easily accessible geophysical databases containing regional structural parameters will allow the relative amplitude method to be used to help verify the CTBT.