principles and applications of microearthquake networks

224 9. Discussion
denser station coverage for a portion of the network area at a time. This
allows the permanent stations to monitor the general seismicity of the
entire area, and the temporary stations to fill in the details.
The average station spacing 5 in a microearthquake network is related
to the network areaA and the number of stations N by 5 = (A/N)”‘. Since
reducing ( by a factor of 2 requires increasingN by a factor of 4, it is not
practical to have small station spacing over a large area. Let us consider
the problem of monitoring an area of 100 km x 100 km with a network of
100 stations. If the stations are permanent and distributed evenly over the
area, then the average station spacing is 10 km, which is not adequate for
some detailed microearthquake studies. An alternative plan is to equip the
network with 75 permanent stations and a mobile array of 25 temporary
stations. The average spacing for the permanent stations is increased to
1 I .6 km, but the network location capability is not degraded significantly.
If we use the mobile array to supplement the permanent stations and to
occupy one-tenth of the network area at a time, we will achieve an effective
station spacing of 5.6 km. Thus, we can carry out detailed microearthquake
studies without increasing the total number of stations. In
practice, such a combination of permanent and mobile stations has not
yet been fully exploited.
There are two major difficulties in operating a microearthquake network
on a continuing basis: (1) obtaining adequate financial support and management
year after year, and (2) maintaining high standards of quality in
network operations, especially with respect to data processing and analysis.
A microearthquake network can become unmanageable if the tasks to
achieve the desired goals are not clearly defined and performed. For example,
if the scientific goal is to map the geometry of active faults in a
given area, then it is not necessary to operate the network continuously in
time. Furthermore, because the fault geometry can be defined by welllocated
earthquakes, it is not necessary to save all the waveform data. The
main tasks then are to operate a microearthquake network that surrounds
the target area, and to collect first P-arrival times and first P-motion data
for precise earthquake locations and fault-plane solutions. It is not easy to
strike the proper balance between the desire to collect, process, and analyze
as much data as possible, and the ability to perform these tasks with
high standards of quality.
It is tempting to rely on partial or total automation in the routine operation
of a microearthquake network in order to reduce the staff and the
cost. However, development and implementation of automatic procedures
requires substantial amounts of resources and some lead time. It is
easy to overlook this fact and thereby end up with disappointing results.