Variations with time in the detection thresholds of P for seismological stations reporting to the ISC |
| |
| Authors: | Email author" target="_blank">J?B?YoungEmail author A?Douglas P?D?Marshall |
| |
| Institution: | (1) AWE Blacknest, Brimpton, Reading, RG7 4RS, United Kingdom |
| |
| Abstract: | To determine maximum-likelihood magnitudes for a given seismological network requires estimates of the detection thresholds
of short-period P for the individual stations. Ideally the detection threshold of a station with fixed instrumentation and
operating procedures would be constant. However, some stations reporting to the International Seismological Centre (ISC) show
systematic variations with time. Other stations are unreliable or report amplitudes (A) and periods (T) only intermittently. Ways are therefore needed for choosing stations that have a consistent performance over many years.
Two rough measures are described here that can be used to look at variation in the apparent detection thresholds with time:
one is the number of A/T observations (at distances greater than 20∘) and the other
, for the P detections associated each month by the ISC;
being the average
. Some of the factors that control detection thresholds and clearly revealed by these rough measures are: array stations have
lower thresholds than conventional observatories; installation of automatic detectors can lower the threshold; and annual
variations in noise level are reflected in annual variations in detection thresholds. For some stations an apparent lowering
of the threshold appears not to be due to changes at the station, but is a consequence of the lowering of the ISC reporting
thresholds in the region around the station. For stations of the Swiss seismological network there is a slow rise in the threshold
from the mid-1970s to 1992 which is difficult to explain but seems to be due to a gradual changes in the detail with which
the seismograms are analysed. |
| |
| Keywords: | detection thresholds maximum-likelihood magnitudes |
| 本文献已被 SpringerLink 等数据库收录！ |
