On the excitation of free oscillations on the Moon

Since the Moon’s outer shells are very inhomogeneous, a global, spherically symmetric model of its interior structure is difficult to construct by using only seismic body-wave data. We study the diagnostic capabilities of the free-oscillation method. The sources of the largest moonquakes are located in the outer 200-km thick layer. Their seismic moments reach 10²² dyn. cm. Current seismometers are capable of detecting ground accelerations of ～10^?8 cm s^?2. Taking M ₀≈10²² dyn. cm, we show that torsional modes _n T _l with l≥7 and n=0 (l is the degree of oscillation, and n is the overtone number) can be detected. These modes contain information about the outer layers to a depth of ≈500 km and can allow a global model to be constructed for the outer, most inhomogeneous layers of the satellite. The largest moonquakes excite spheroidal modes _n S _l much worse, and it is unlikely that they can be detected with current instruments. We provide detailed information on the excitation of free oscillations for various moonquake focal mechanisms and focal depths.