Abstract
Continuous seismic threshold monitoring is a technique that has been
developed over the past several years to assess the upper magnitude
limit of possible seismic events that might have occurred in a geographical
target area. The method provides continuous time monitoring at a
given confidence level, and can be applied in a site-specific, regional
or global context. In this paper (Part 1) and a companion paper (Part
2) we address the problem of optimizing the site-specific approach
in order to achieve the highest possible automatic monitoring capability
of particularly interesting areas. The present paper addresses the
application of the method to cases where a regional monitoring network
is available. We have in particular analyzed events from the region
around the Novaya Zemlya nuclear test site to develop a set of optimized
processing parameters for the arrays SPITS, ARCES, FINES, and NORES.
From analysis of the calibration events we have derived values for
beam-forming steering delays, filter bands, short-term average (STA)
lengths, phase travel times (P and S waves), and amplitude-magnitude
relationships for each array. By using these parameters for threshold
monitoring of the Novaya Zemlya testing area, we obtain a monitoring
capability varying between mb 2.0 and 2.5 during normal noise conditions.
The advantage of using a network, rather than a single station or
array, for monitoring purposes becomes particularly evident during
intervals with high global seismic activity (aftershock sequences),
high seismic noise levels (wind, water waves, ice cracks) or station
outages. For the time period November-December 1997, all time intervals
with network magnitude thresholds exceeding mb 2.5 were visually
analyzed, and we found that all of these threshold peaks could be
explained by teleseismic, regional, or local signals from events
outside the Novaya Zemlya testing area. We could therefore conclude
within the confidence level provided by the method, that no seismic
event of magnitude exceeding 2.5 occurred at the Novaya Zemlya test
site during this two-month time interval. As an example of particular
interest in a monitoring context, we apply optimized threshold processing
of the SPITS array for a time interval around 16 August 1997 mb 3.5
event in the Kara Sea. We show that this processing enables us to
detect a second, smaller event from the same site (mb 2.6), occurring
about 4 hours later. This second event was not defined automatically
by standard processing.
Users
Please
log in to take part in the discussion (add own reviews or comments).