Kouvola earthquake swarm - using a cross-correlator to find very small events and cluster them

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