Seismic Energy Radiated by Earthquakes in the North-Central Region of the Gulf of California, Mexico

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ABSTRACT We investigate the radiated seismic energy of P and S waves in the north-central region (27.5–31.5° N) of the Gulf of California (GoC), Mexico, from 83 earthquakes recorded at stations of the Broadband Seismological Network (RESBAN) of the GoC, Mexico, and the Mexican National Seismological Service for the period 2015–2021. The earthquakes occurred within the limits of transform and normal faults at depths between 10 and 20 km and had moderate magnitudes (3.4<Mw<5.7). We calculated Fourier acceleration spectra from P and S waves, corrected the spectral records for source–receiver path effects, and separated site and source functions using a spectral inversion technique. We corrected the source functions for attenuation effects near the source, and then we calculated the radiated energy by integrating the square of the source velocity spectrum (P and S wave) for each earthquake. We found that on average, 91% of the radiated energy corresponds to S waves. The average total energy (7.1172×109 N·m) of the analyzed events is less than two orders of magnitude of that from earthquakes in Mexican subduction zones. Unlike other results, the apparent stress shows an ostensible inverse correlation with the seismic moment. The apparent stress from the northern to the central regions of the GoC varies between 0.0005 and 0.4 MPa. We observed that earthquakes with lower and higher apparent stress tend to concentrate in regions with normal and strike-slip faulting stress regimes, respectively. To explain the spatial variation of apparent stress, we estimated the coefficient of friction and compare our results with focal mechanisms, stress regime, and other geophysical and seismological studies. The variations of apparent stress could be influenced by lateral heterogeneities of the physical properties of the crustal rocks, the friction on faults, the heat flow, and the presence of fluids.