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<oai_dc:dc xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/ http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
  <dc:contributor>J. A. Waller</dc:contributor>
  <dc:contributor>Roger D. Borcherdt</dc:contributor>
  <dc:contributor>E. Cranswick</dc:contributor>
  <dc:contributor>Edward G. Jensen</dc:contributor>
  <dc:contributor>J. Van Schaak</dc:contributor>
  <dc:creator>P. E. Malin</dc:creator>
  <dc:date>1988</dc:date>
  <dc:description>&lt;p&gt;Direct evidence of site distortion of&amp;nbsp;&lt;i&gt;P&lt;/i&gt;- and&amp;nbsp;&lt;i&gt;S&lt;/i&gt;-wave microearthquake source spectra at Oroville, California, is presented. The data were gathered by placing vertical and three-component seismometers at 90 m intervals in a 500 m borehole through the Cleveland Hill normal fault, on which the 1975,&amp;nbsp;&lt;i&gt;M&lt;sub&gt;L&lt;/sub&gt;&lt;/i&gt;&amp;nbsp;= 5.7 Oroville earthquake took place. High-pressure, hydraulic locking mechanisms were used to firmly lock the seismometer packages against the borehole wall. Digital, event-triggering GEOS recorders were used to receive the data. Some 30 seismic events, including 12 microearthquakes, were recorded during the 4 months the seismometers were deployed.&lt;/p&gt;&lt;p&gt;By comparing the velocity spectra of microearthquake waves at the different depths, it can be seen that scattering and attenuation in the shallow crust around the borehole dominantly affect high-frequency&amp;nbsp;&lt;i&gt;S&lt;/i&gt;&amp;nbsp;waves. Above 15 Hz, the value of the apparent&amp;nbsp;&lt;i&gt;S&lt;/i&gt;-wave quality factor,&amp;nbsp;&lt;i&gt;Q&lt;sub&gt;as&lt;/sub&gt;&lt;/i&gt;, for the upper 500 m of crust at this site is 9. One feature of this low&amp;nbsp;&lt;i&gt;Q&lt;sub&gt;as&lt;/sub&gt;&lt;/i&gt;&amp;nbsp;is a gross difference between&amp;nbsp;&lt;i&gt;S&lt;/i&gt;-wave “corner” frequencies observed at ground level and 500 m downhole. For example, the uphole and downhole&amp;nbsp;&lt;i&gt;S&lt;/i&gt;-wave corner frequencies for the&amp;nbsp;&lt;i&gt;M&lt;sub&gt;coda&lt;/sub&gt;&lt;/i&gt;&amp;nbsp;= 0.4 microearthquake of Julian Day 259 differ by a factor of 2 or more. Low quality factors and depth-dependent corner frequencies were also observed for&amp;nbsp;&lt;i&gt;P&lt;/i&gt;&amp;nbsp;waves, but these data are less definitive due to lower signal-to-noise ratios.&lt;/p&gt;&lt;p&gt;Based on their three-component particle motions and polarizations, the direct&amp;nbsp;&lt;i&gt;S&lt;/i&gt;&amp;nbsp;waves of the microearthquakes appear to be composed of two phases, which have similar amplitudes but different apparent velocities. It is possible that the velocity differences are a result of anisotropy in the underlying rock.&lt;/p&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.1785/BSSA0780020401</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>Seismological Society of America</dc:publisher>
  <dc:title>Vertical seismic profiling of Oroville microearthquakes: Velocity spectra and particle motion as a function of depth</dc:title>
  <dc:type>article</dc:type>
</oai_dc:dc>