It's configured in '\config\log4net.config' and set to 'Info' by default:

<?xml version="1.0"?>
<log4net>
  <root>
    <priority value="Info"/> <!-- Over here -->
    <appender-ref ref="AsynchronousLog4NetAppender" />
  </root>


  <!--To Change the way logging works for certain namespaces, insert a new logger like: -->
  <!--
    <logger name="Umbraco.Core">
        <level value="WARN" />
    </logger>-->

  <appender name="AsynchronousLog4NetAppender" type="Umbraco.Core.Logging.AsynchronousRollingFileAppender, Umbraco.Core">
    <file value="App_Data\Logs\UmbracoTraceLog.txt" />
    <lockingModel type="log4net.Appender.FileAppender+MinimalLock" />
    <appendToFile value="true" />
    <rollingStyle value="Date" />
    <maximumFileSize value="5MB" />
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %-5level %logger - %message%newline" />
    </layout>
    <encoding value="utf-8" />
  </appender>
</log4net>