Review of Survey activities 2011: Testing of an automatic earthquake detection method on data from Station Nord, Greenland

Abstract

Earthquakes are continuously monitored by a global network of several thousand seismic stations equipped with highly sensitive digital seismometers. The Geological Survey of Denmark and Greenland (GEUS) takes part in it by operating five seismic stations in Denmark and 18 in Greenland, some of the latter in collaboration with international partners. There are two main ways of detecting earthquakes from digital recordings of seismometers: (1) by a manual review of the data by an expert in processing seismic earthquake signals and (2) by an automatic method that uses a computerised algorithm to analyse the recordings. Since the beginning of earthquake recording, earthquake detection at GEUS has been based on the manual review method. There are several reasons why an automatic detection procedure has not yet been implemented at GEUS: (1) historically, the staff at GEUS have conducted high-quality manual detection of earthquakes, based on a long tradition of manual seismogram analysis (Lehmann 1954), (2) the ambient noise level in Denmark is generally too high for small local earthquakes to be detected automatically and (3) in Greenland, the distance between the seismometers is too long for automatic methods. Previous tests on GEUS data showed that automatic detection using the so-called standard method resulted in a very high number of false detections, and the effort needed to distinguish real earthquake signals from noise signals was much greater than that needed in the manual method. In addition, the automatic method detected fewer earthquakes than the manual analysis. Therefore, new automatic methods are needed to extract real earthquake signals from the background noise. In this article, we present results from testing a newly developed automatic detection method based on analysis of the frequency content of seismic signals. The aim of the study was to investigate whether the automated method can be used in Greenland or whether the manual procedure is still superior. The new method was tested on seismic data from Station Nord, which was selected because it is located in a region with many earthquakes, and because there are no nearby seismograph stations to support the measurements. The closest station is located at Danmarkshavn c. 540 km to the south. Therefore, the majority of earthquakes that occur in this region are only registered at Station Nord. Earthquakes in the Station Nord region Station Nord is located in eastern North Greenland, in a region where a major tectonic factor is the spreading that occurs along the rift zones in the northern North Atlantic and the Arctic Ocean (Døssing et al. 2010), and which gives rise to high seismic activity (Fig. 1). Another tectonic factor is postglacial isostatic rebound that was the source of three major earthquakes in 1971, 1987 and 1993 (Chung 2002), with magnitudes of 5.1, 5.5 and 5.4 on the Richter scale. Apart from the seismicity observed at the rift zone, most of the earthquakes in the region occur to the south and west of Station Nord (Fig. 1; Gregersen 1982). The earthquake hazard in the region is low, but represents the highest in Greenland (Voss et al. 2007). The hazard was illustrated by a strong tremor felt at Station Nord on 30 August 2005, caused by a magnitude 4.2 earthquake with an epicentre only 20 km away.