Determination of Focal Depths of Earthquakes in the Mid-Oceanic Ridges from Amplitude Spectra of Surface Waves
MASSACHUSETTS INST OF TECH CAMBRIDGE
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A method based on the normal mode theory for surface waves excited by a slip dislocation in a multilayered clastic medium is successfully developed tor determining the focal depths of remote earthquakes with known fault plane solutions. After examining various possible sources of uncertainty and testing on several earthquakes, the method is si-own to be dependable for studying earthquakes with magnitude mb 6. Since the amplitude spectra bf surface waves arc used in this method, not only the focal depth but also the seismic moment of the equivalent double couple system of an earthquake can be determined. 32 earthquakes with known fault plane solutions and in the three major mid-oceanic ridge systems of the world are studied by this method. Our results can be surmmarized as the following 1 All strike-slip earthquakes from the east Pacific rise the Gulf of California, the San Andreas fault, the Mendocino fracture zone, the Blanco fracture zone and the Queen Charlotte Island fault occurred at depths within 10 km excluding the water depth. This similarity of focal depth distribution strongly supports Wilsons 1965a,b ideas that the San Andreas fault is a transform fault, and that the east Pacific rise extends through the tectonic features mentioned and terminate at the Queen CharloCAtc island fault. All the strike-slip earthquakes from the mid-Atlantic and the mid-In dian ocean ridges were also characterized by extremely shallow focal depths. 2 All 16 dip-slip earthquakes studied in this paper occurred systematically at greater depths --- from about 30 to 65 km. This evidence suggests that the thickness of the lithosphere beneath the central ridges is at least 65 km. 3 By using Akis 1967 scaling law for seismic spectrum the observed seismic moments are translated to the surface wave magnitude Ms and compared with the body wave magnitude mb.
- Physical and Dynamic Oceanography
- Theoretical Mathematics
- Atomic and Molecular Physics and Spectroscopy