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Trends and
Recent Developments
Throughout its history, PASSCAL has evolved and program
emphases have changed in response to the demands of
science and the scientific community. In this section,
we explore some of this evolution, its impact on operational
procedures and budget structure, and anticipate
future directions.
As initially conceived in 1984, PASSCAL was a basic community
instrument resource facility, and acquiring and
maintaining hardware were the primary activities. As the
program has evolved, there has been increasing emphasis
on training, field services, and software support. All of these
activities place high demand on human resources, which
has in turn increased pressure on balancing budgets to
include both growth of the instrument pool and attendant
expanded services.
As IRIS completed the fourth five-year cooperative
agreement with NSF (2001–2006), the PASSCAL facility
approached the initial targets set in 1984 in terms of numbers
of instruments and channels. In recent years, the budget
profile for PASSCAL has shifted from growth of the pool
through acquisition of new instruments to sustaining the
pool through replacement of aging and damaged equipment.
Unlike the USArray project where the focus of study lies
within the North American continent, the PASSCAL
program provides instruments for worldwide investigations.
Most PI’ using the facility are funded by national organizations
such as NSF and DOE to conduct studies driven by
global tectonics. In particular, the majority of broadband and
active-source (TEXAN) experiments have been conducted
outside the US (Figure 36). This has been a consistent trend
since the beginning of the program. In 2007, for example,
out of a total of 18 broadband deployments, 11 were conducted
overseas. In contrast, experiments using short period
equipment have remained predominantly within the United
States (Figure 36). Short period equipment is mainly used for
regional or local seismicity studies often augmenting existing
networks. All PASSCAL equipment types combined, the
distribution of experiment are evenly split between foreign
and domestic locations.
Usage Trends
Demand for instruments from the user community has
exceeded the available resources. The PASSCAL pool has
grown over the years to a complement of over 1000 digital
recording systems (Table 1). What has changed is the
character of the typical experiment. Through time,
experiments have evolved to deploy larger numbers of
instruments, reflecting the scientific need for higherresolution
studies, and longer durations, reflecting the
higher data return through capturing more earthquakes
(Figure 36). Experiments using multiple instrumentation
types have also increased.
The average number of stations deployed in a typical broadband
experiment now exceeds 30 (Figure 37a), and several
deployments have been fielded in recent years that have
exceeded 75. Instruments used for controlled-source studies
(primarily the single-channel TEXANs) have also grown with
the available pool now in excess of 2600 stations (including
USArray equipment, Table 1). Interestingly, the number of
broadband experiment starts has remained relatively level
at around 10 experiments per year (Figure 37b). Another
important trend observed in passive-source recording is the
duration of an average experiment, which has increased gradually
to around 2.5 years from approximately 1 year in the
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