Scenario of the January 1997 West Bohemia earthquake swarm

In order to learn more about the nature of the dynamic processes taking pla ce in the West Bohemia/Vogtland earthquake swarm region, we investigated th e temporal and spatial variations of the source mechanisms of the January 1 997 swarm beneath Novy Kostel (NKC). Visual analyses of WEBNET seismograms of over 800 events revealed that a specific feature of this swarm was the o ccurrence of eight classes of multiplet events. The result of single-source , absolute moment tensor inversion of the P and SH peak amplitudes of a sub set of 70 events representing all multiplet classes indicated that eight st atistically significant types of mechanisms occurred during the swarm. Two of them, types A and B in our denotation, comprised all M-L greater than or equal to 1.3 events and predominated in the swarm. Type A were pure strike -slip mechanisms or strike-slip mechanisms containing a small normal compon ent, with a nearly pure double-couple source. For class B events, oblique-t hrust faulting and nan-double-couple components significant at a fairly hig h confidence level were typical. Type A events predominated in the southern subcluster of the swarm, whereas most of type B events occurred in the sub cluster northwards from NKC. This indicates that two major seismogenic plan es were active during the swarm. The swarm essentially developed in four ph ases. in the first, type A events prevailed and the southern plane was acti ve: during the second, characterised by the occurrence of both type A and B events (the former in the southern, the latter predominantly in the northe rn subcluster), the activity of the swarm culminated; in the third and four th, the occurrence of type B events in the northern plane predominated, and only weak single events occurred southwards from NKC. Mechanisms of types AB, C. D, E, F and G, which were typical for M(L)less than or equal to1.2 e vents, occurred randomly throughout the swarm. Type AB events were identifi ed in both the southern and northern clusters, type C, E, F and G mechanism s only southwards from NKC. Type D events exhibited a large scatter of hypo centres which fell in neither the southern nor the northern cluster. Focal mechanisms like those reported in this study and with analogous temporal an d spatial variations were observed by other authors already fifteen years a go in the 1985/86 earthquake swarm and may, therefore, be typical for the r egion under study.