Indications for different types of brittle failure due to active coal mining using waveform similarities of induced seismic events

摘要

Longwall mining activity in the Ruhr coal mining district leads tomining-induced seismicity. For detailed studies the seismicity of a singlelongwall panel beneath the town of Hamm-Herringen in the eastern Ruhr areawas monitored between June 2006 and July 2007 with a dense temporary networkof 15 seismic stations. More than 7000 seismic events with magnitudes between–1.7 ≤ M_L ≤ 2.0 were detected and localized in this period.Most of the events occurred in the vicinity of the moving longwall face.In order to find possible differences in the brittle failure types of theseevents an association of the events to distinct clusters is performed basedon their waveform characteristics. This task is carried out using a newclustering algorithm utilizing a network similarity matrix which is createdby combining all available 3-component single station similarity matrices.The resultant network matrix is then sorted with respect to the similarity ofits rows leading to a sorted matrix immediately indicating the clustering ofthe event catalogue. Finally, clusters of similar events are extracted byvisual inspection.This approach results in the identification of several large clusters whichare distinct with respect to their spatial and temporal characteristics aswell as their frequency magnitude distributions. Comparable clusters are alsofound with a conventional single linkage approach, however, the new routineseems to be able to associate more events to specific clusters withoutmerging the clusters.The nine largest observed clusters can be tentatively divided into threedifferent groups that indicate different types of brittle failure. The firstgroup consists of the two largest clusters which constitute more than half ofall recorded events. Results of a relative relocation using cross-correlationdata suggest that these events are confined to the extent of the mined outlongwall and cluster close to the edges of the active longwall at the depthof active mining. These events occur in lockstep with the longwall advanceand exhibit a high b value of the Gutenberg–Richter relation (GR) of about1.5 to 2.5 and consist of small magnitude events. Thus, these eventsrepresent the immediate energy release adjacent to the mined out area.The second group consists of clusters located either slightly above or belowthe depth of active mining and occurring at the current position of thelongwall face within the confines of the longwall. They consist of generallystronger events and do not follow GR. This activity might be linked to thefailure of more competent layers above and below the mined out seam resultingin larger magnitude events.Finally, one cluster represents seismic activity with a rather low b valuebelow 1 and events located partly towards the north of the longwall whichare delayed with respect to the advance of the longwall face. These eventsare interpreted as brittle failure on pre-existing tectonic structuresreactivated by the mining activity.