A second shallow earthquake occurs at depth in crust with

PHYSICS · Multi-part · Fermi University Physics · Earth, Climate & Energy Physics · Seismology, tectonics and geodynamics · Seismology, tectonics and geodynamics

The Solid Earth, Ch The earthquake source: moment tensor, radiation and spectra

Problem

A second shallow earthquake occurs at depth h=18 kmh = 18\,\mathrm{km} in crust with vP=6.3 km s−1v_P = 6.3\,\mathrm{km\,s^{-1}}. Four P arrivals give. Data: station 1, azimuth 10∘10^\circ, p=5.20 s deg−1p=5.20\,\mathrm{s\,deg^{-1}}, up (compression); station 2, azimuth 100∘100^\circ, p=9.60 s deg−1p=9.60\,\mathrm{s\,deg^{-1}}, down (dilatation); station 3, azimuth 190∘190^\circ, p=5.85 s deg−1p=5.85\,\mathrm{s\,deg^{-1}}, up (compression); station 4, azimuth 280∘280^\circ, p=9.05 s deg−1p=9.05\,\mathrm{s\,deg^{-1}}, down (dilatation). ((a)) Find the takeoff angle at the source for each station, using sin⁡ih=pvP/rh\sin i_h = pv_P/r_h with rhr_h the source radius taken as 6371 km6371\,\mathrm{km} minus the depth, and pp converted to s rad−1\mathrm{s\,rad^{-1}} using 1 s deg−1=57.30 s rad−11\,\mathrm{s\,deg^{-1}} = 57.30\,\mathrm{s\,rad^{-1}}. ((b)) Taking the simplest fit, vertical nodal planes placed midway between stations of opposite polarity, state their strikes and the azimuths of the P and T axes, and say whether this is a strike-slip, thrust or normal mechanism. Say what range of strikes the four stations would also allow.

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