2012 IRIS Workshop

Focal Mechanisms & Thermal Fluctuations of Seismometers

Alberto Lopez-Venegas: University of Puerto Rico, Mayaguez, Dwight Williams: Morehouse College/ University of Michigan

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Seismological networks need quick and efficient ways to obtain information related to seismic events for the purposes of seismic activity monitoring, risk assessment, and scientific knowledge among others. A simple routine to obtain a focal mechanisms, the geometry of the fault, based on first motions was developed and implemented for data analysts routine operations at PRSN. The new tool provides a quick way to assess the probable faulting mechanism that occurred while performing the interactive earthquake location procedure. The focal mechanism is generated on-the-fly when data analysts pick P wave arrivals onsets and motions. An in-house PRSN utility is employed to obtain the double couple representation and later plotted using GMT's psmeca utility. In addition, the noise related to thermal fluctuations inside seismic vaults was addressed. Seismometers and seismic sites can be extremely noisy and give skewed readings due to fluctuations in temperature. Seismologists have used different insulation techniques to reduce the amount of unwanted noise that a seismometers experience due to these thermal changes with items such as styrofoam, fiber glass, etc. PRSN traditionally uses Styrofoam boxes to cover their seismic sensors. The deficiency of these techniques in the Caribbean and especially Puerto Rico is that these thermal fluctuations still happen because of the intense sun and humidity. A proper study comparing this method to IRIS Transportable Array standard operating procedure employing sand burial of the sensor has never been done. Two Guralps CMG-3T's connected to RefTek's 150 digitizers were used at PRSN's MPR site seismic vault to compare the two types of insulation. Two temperature loggers were placed along each seismic sensor for a period of one week to observe how much thermal fluctuations occur in each insulation method and then compared its capability for noise reduction due to thermal fluctuations. With only a single degree Celsius fluctuation inside

Keywords: focal_mechanism, seismometers, thermal

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