Spherical Subset of Events

Description

Selects events within a specified spherical selection criteria (i.e. a given radius from a hypocentral location), from a given catalogue. Note that subset.sphere is deprecated; it conflicts with the generic function subset. Change to subsetsphere; it has the same arguments.

Usage

subsetsphere(x, centrelat=0, centrelong=0, centredepth=0, minradius=0,
             maxradius=Inf, minmag=-Inf, maxmag=Inf, minday=-Inf,
             maxday=Inf, radius=6371, report.count=TRUE, na.rm=TRUE, ...)

#   Deprecated Usage:
subset.sphere(x, centrelat=0, centrelong=0, centredepth=0, minradius=0,
              maxradius=Inf, minmag=-Inf, maxmag=Inf, minday=-Inf,
              maxday=Inf, radius=6371, report.count=TRUE, na.rm=TRUE, ...)

Arguments

Details

The subsetting is based on the catalogue variables latitude, longitude, depth, magnitude and time. The default settings for each of these variables is to take all events. The function only works through each of the “min” and “max” values for magnitude and time included in your function call. Those arguments not explicitly listed will have no effect. For each event in the correct magnitude-time window, its distance from the centre of the sphere is calculated. A logical vector is constructed with the same length as the number of events in the catalogue, and indicates whether a given event satisfies the subsetting criteria. From this, the vector of indices of the required events is created.

Historical events often have missing values, particularly the magnitude and depth. Consider the situation where minmag=6. The logical vector within the function will contain TRUE's and FALSE's for those events that satisfy or not satisfy this condition, respectively. However, for those historical events with a missing magnitude or depth (NA), the elements in the logical vector will also be NA. The argument na.rm determines whether these events should be removed (TRUE) or included (FALSE) in the subset. The default setting is to remove such events, though in the context of large historical events, it would be more sensible to include them, i.e. na.rm=FALSE.

The arguments minday and maxday represent the boundary points of the time interval, and can represent fractions of days. For example, 00:00 hrs on 1 Jan 1970 is represented as minday=0, whereas 18:00 hrs on 2 Jan 1970 is represented as minday=1.75. Similarly, 18:32:24 hrs on 30 Sept 1990 could be specified as minday = julian(9,30,1990) + 18/24 + 32/(24*60) + 24/(24*60*60). Note that maxday = julian(1,1,1990) represents the point 00:00 hrs on 1 Jan 1990, and therefore will not include events on 1 Jan 1990 after midnight.

Value

Returns an object of type list with class "subset". All optional arguments above, except radius, are included as elements within the list. Other components within the list are:

See Also

subsetcircle, subsetpolygon, subsetrect, print.subset, summary.subset

Examples

#   Select events from the NZ55 catalogue with hypocentre no more
#   than 50kms from Wellington, but at least 20kms from Wellington.
data(NZ55)

b <- subsetsphere(NZ55, centrelat=-41.3, centrelong=174.8, 
                   maxradius=100, minradius=20, minmag=2)
summary(b)