Laserfiche WebLink
• <br />• <br />OBSERVATIONAL METHODOLOGY <br />Identification of Seismic Events <br />Events are regarded as seismic in origin and logged by the triggered recording system if they <br />appear on five or more stations with sufficient amplitude, impulsive onset, and appropriate time <br />differences from station to station. Subnet triggering conditions are also used to ensure that <br />significant events do not escape the triggering condition. Using the triggered system, more than <br />4000 data files were recorded during the quarter. Events are selected for investigation by <br />inspection of a summary record for several stations. These continuously-recorded records, called <br />helicorder records, summarize 12 hours of recording onto one page of data for inspection. Once <br />an event is determined to have sufficient ground motion throughout the network for location, the <br />triggered files are used for event classification. <br />Event Classification <br />P-wave arrival times are picked by amplitude or frequency changes. Events without more than <br />seven arrival times were catalogued for ground-motion statistical purposes only; events with eight <br />or more arrival times are input to the location process. The location accuracy improves with the <br />number of arrivals; the shape of this network (two five-station subnets) requires several arrivals in <br />the'other' subnet to produce a consistent and accurate location. Three additional arrivals in the <br />'other' subnet are usually sufficient to produce the consistency required. <br />Velocity Model <br />The largest contributor to reliable location of seismic events is the velocity model utilized in the <br />hypocenter determinations. The model is typically defined for compressional wave arrivals and <br />may take several different forms depending upon the location algorithm utilized. In this report, we <br />use alayered-earth model as input to the location program (HYPOINVERSE-2000, 2002). Using a <br />layered-earth model, velocities are constant over certain vertical intervals and change at discrete <br />horizontal boundaries. The travel path for this model is a series of line segments with abrupt <br />emergence angle changes at velocity boundaries. <br />Table 3 Preliminary Velocity Model <br />Layer Top(km) <br />0.00 <br />0.07 <br />0.27 <br />0.37 <br />1.50 <br />2.50 <br />6.00 <br />Velocity in km/sec <br />2.21 <br />2.73 <br />3.01 <br />3.18 <br />3.96 <br />4.40 <br />6.00 <br />Velocity in ft/sec <br />7470 <br />9227 <br />10174 <br />10748 <br />13385 <br />14872 <br />20280 <br />• <br />