Abstract
We interpret seismic activity in the active spreading centers of the Salton Trough to indicate 1) a magmatic intrusion in the lower crust beneath the active Brawly, Cerro Prieto, Imperial, Elsinore, and San Jacinto fault systems; and 2) fluids in the upper crust that have been released from that magmatic body. The absence of a magmatic body and fluids at the location of fossil spreading centers along the Sand Hill and Algodones faults ndicated by little or no seismic activity in those areas. We show several lines of evidence to point out that both melt and fluids related to the seismic activity. In particular, receiver function analysis, Vp/Vs ratios, and tomographic data reveal low velocity zones coincide with the location of the active spreading centers. High Vp/Vs ratios and low velocity zones in the lower crust and upper mantle attributed to melt inclusion, while low Vp/Vs ratios in the upper crust are attributed water inclusions. Frequency-mag- nitude distributions characterized by high b-values in southern California; high b-values have also been associated with crustal fluids. A crustal scale model developed from the receiver functions, gravity, and magnetic data supports the existence of a magmatic intrusion within about 20 km of the surface southwest of the Salton Sea, that intrusion extends for 70 km in a SW-NE direction.
Highlights
Mantle fluids have been interpreted to be associated with the San Andreas fault system in central and southern California [1]
We examine the relationship between fluids and seismicity in active spreading centers (Brawly, Cerro Prieto, Imperial, Elsinore, and San Jacinto) of the Salton Trough and compare those areas with areas of little or no earthquake activity in nearby fossil spreading centers (Sand Hill and Algodones)
Seismic activity is restricted to the active spreading centers of the Brawly, Cerro Prieto, Imperial, Elsinore, and San Jacinto fault zones, these are strike-slip faults that extend from 15 km (Brawley fault) to 210 km (San Jacinto fault zone) in the active spreading centers of the Salton Trough rift (Figure 2)
Summary
Mantle fluids have been interpreted to be associated with the San Andreas fault system in central and southern California [1]. One of the few studies to point out that high pore pressure would be necessary to trigger such lower crustal events has been that of [6], who studied lower crustal earthquakes in the East African Rift and found no difference in rupture characteristics between upper and lower crustal events (based on mb-Mw relation). This suggests that similar fluid triggering processes may occur throughout the crust as proposed by [5]. That intrusion extends for 70 km in SW-NE direction
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