Kouvola earthquake swarm - using a cross-correlator to find very small events and cluster them
Authors: Luhta Tuija , Komminaho Kari , Oinonen Kati , Tiira Timo , Uski Marja , Veikkolainen Toni , Vuorinen Tommi 
Published: March 2020

&lt;p&gt;&lt;span&gt;Kouvola area, a part of the Vyborg rapakivi batholith in southeastern Finland, has been experiencing an intraplate earthquake swarm since December 2011&lt;/span&gt;&lt;span&gt;. T&lt;/span&gt;&lt;span&gt;he events hav&lt;/span&gt;&lt;span&gt;e&lt;/span&gt;&lt;span&gt; magnitudes ranging from M&lt;/span&gt;&lt;sub&gt;&lt;span&gt;L&lt;/span&gt;&lt;/sub&gt;&lt;span&gt; -1.2 to 2.8&lt;/span&gt;&lt;span&gt; and they happen in the uppermost two kilometers of the crust. &lt;/span&gt;&lt;span&gt;The &lt;/span&gt;&lt;span&gt;Vyborg &lt;/span&gt;&lt;span&gt;batholith has a long history of earthquake swarms&lt;/span&gt;&lt;span&gt; with macroseismic data from 1751 onwards and the first &lt;/span&gt;&lt;span&gt;instrumentally &lt;/span&gt;&lt;span&gt;recorded swarm &lt;/span&gt;&lt;span&gt;in &lt;/span&gt;&lt;span&gt;2003-2004. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Inspired by the ongoing activity&lt;/span&gt;&lt;span&gt;,&lt;/span&gt;&lt;span&gt; Institute of Seismology&lt;/span&gt;&lt;span&gt; of&lt;/span&gt;&lt;span&gt; University of Helsinki&lt;/span&gt;&lt;span&gt; (ISUH)&lt;/span&gt;&lt;span&gt; has &lt;/span&gt;&lt;span&gt;installed&lt;/span&gt; &lt;span&gt;temporary&lt;/span&gt; &lt;span&gt;seismic station&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; in &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;area &lt;/span&gt;&lt;span&gt;to &lt;/span&gt;&lt;span&gt;complement&lt;/span&gt; &lt;span&gt;seismic station&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; of the Finnish National Seismic network&lt;/span&gt;&lt;span&gt; (FNSN)&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;The detection t&lt;/span&gt;&lt;span&gt;h&lt;/span&gt;&lt;span&gt;reshold of FNSN is &lt;/span&gt;&lt;span&gt;M&lt;/span&gt;&lt;sub&gt;&lt;span&gt;L&lt;/span&gt;&lt;/sub&gt;&lt;span&gt;1.0, not sufficiently low to catch the small&lt;/span&gt;&lt;span&gt;est &lt;/span&gt;&lt;span&gt;earthquakes of the swarm.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Several ta&lt;/span&gt;&lt;span&gt;i&lt;/span&gt;&lt;span&gt;lored cross-correlator&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; have been developed at the ISUH to &lt;/span&gt;&lt;span&gt;lower the event detection threshold&lt;/span&gt;&lt;span&gt;. These can&lt;/span&gt; &lt;span&gt;be used to &lt;/span&gt;&lt;span&gt;detect even very small seismic events&lt;/span&gt;&lt;span&gt; well below the current FNSN detection t&lt;/span&gt;&lt;span&gt;h&lt;/span&gt;&lt;span&gt;reshold&lt;/span&gt;&lt;span&gt;. The method is especially well suited &lt;/span&gt;&lt;span&gt;to s&lt;/span&gt;&lt;span&gt;warm events, which &lt;/span&gt;&lt;span&gt;generate&lt;/span&gt; &lt;span&gt;nearly identical &lt;/span&gt;&lt;span&gt;signal&lt;/span&gt;&lt;span&gt;s&lt;/span&gt;&lt;span&gt; due to their common origin. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;O&lt;/span&gt;&lt;span&gt;nly the &lt;/span&gt;&lt;span&gt;largest&lt;/span&gt;&lt;span&gt; events &lt;/span&gt;&lt;span&gt;of the swarm &lt;/span&gt;&lt;span&gt;can be used to calculate focal mechanisms&lt;/span&gt; &lt;span&gt;or other event parameters reliably. &lt;/span&gt;&lt;span&gt;One approach to &lt;/span&gt;&lt;span&gt;use all data is w&lt;/span&gt;&lt;span&gt;aveform clustering&lt;/span&gt;&lt;span&gt;.&lt;/span&gt; &lt;span&gt;E&lt;/span&gt;&lt;span&gt;vent groups with identical signal&lt;/span&gt;&lt;span&gt; can be formed&lt;/span&gt;&lt;span&gt;, allowing e.g. calculation of&lt;/span&gt; &lt;span&gt;composite focal mechanisms for each &lt;/span&gt;&lt;span&gt;event &lt;/span&gt;&lt;span&gt;cluster&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;/p&gt;