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On the Nature of an Unusual Wave Train Observed in the Northern Tien Shan Region

Joint Institute of Physics of the Earth
Russian Academy of Sciences
Kamo Str. 8a
Talgar, Almaty Region
Kazakhstan, 483310
cse{at}mail.kz
(Y.F.K.)

We describe an unusual wave group that propagates in the Northern Tien Shan region along the border between mountain ranges and the Chu and Ili depressions to the north. This wave group, which we call S_L, stands out against the short-period coda on the recordings of local earthquakes and quarry explosions (at distances up to 100 km). The time t_m needed for the S_L group to reach maximum amplitude varies between 98 and 131 sec, and at the same time, the t_m value does not depend on epicentral distance. At distances up to 30 km, the S_L group has a very narrow spectrum (T = 1.5–1.8 sec), but when the epicentral distance increases, the predominant period also increases up to approximately 2.3 sec (at 100 km). Characteristics of the wave group are significantly different for the Zaili fault zone than for the regions to the west of 76° E and to the east of 79° E. In the Zaili fault region, the S_L group is characterized by great variability of velocity, amplitude, and polarization parameters even for nearby paths. The S_L group maintains an amplitude within 0.7 of its maximum for 4–36 sec depending on the path. The polarization of the S_L group is in the vertical plane in the Zaili fault-zone region and in the horizontal plane in other regions. Particle motion is elliptical or almost circular. We suggest that the S_L group consists of shear waves propagating in a subvertical wave guide and reflected from a thin upper mantle layer.

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