Article,
A threshold-based earthquake early warning using dense accelerometer networks
Geophysical Journal International, 183 (2): 963--974 (November 2010)
DOI: 10.1111/j.1365-246X.2010.04765.x
Abstract
Most earthquake early warning systems (EEWS) developed so far are
conceived as either 'regional' (network-based) or 'on-site' (stand-alone)
systems. The recent implementation of nationwide, high dynamic range,
dense accelerometer arrays makes now available, potentially in real
time, unsaturated waveforms of moderate-to-large magnitude earthquakes
recorded at very short epicentral distances (<10-20 km). This would
allow for a drastic increase of the early warning lead-time, for
example, the time between the alert notification and the arrival
time of potentially destructive waves at a given target site. By
analysing strong motion data from modern accelerograph networks in
Japan, Taiwan and Italy, we propose an integrated regional/on-site
early warning method, which can be used in the very first seconds
after a moderate-to-large earthquake to map the most probable damaged
zones. The method is based on the real-time measurement of the period
(tau\_c) and peak displacement (Pd) parameters at stations located
at increasing distances from the earthquake epicentre. The recorded
values of early warning parameters are compared to threshold values,
which are set for a minimum magnitude 6 and instrumental intensity
VII, according to the empirical regression analyses of strong motion
data. At each recording site the alert level is assigned based on
a decisional table with four alert levels defined upon critical values
of the parameters Pd and tau\_c, which are set according to the error
bounds estimated on the derived prediction equations. Given a real
time, evolutionary estimation of earthquake location from first P
arrivals, the method furnishes an estimation of the extent of potential
damage zone as inferred from continuously updated averages of the
period parameter and from mapping of the alert levels determined
at the near-source accelerometer stations. The off-line application
of the method to strong motion records of the Mw 6.3, 2009 Central
Italy earthquake shows a very consistent match between the rapidly
predicted (within a few seconds from the first recorded P wave) and
observed damage zone, the latter being mapped from detailed macroseismic
surveys a few days after the event. The proposed approach is suitable
for Italy, where, during the last two decades, a dense network of
wide dynamic-range accelerometer arrays has been deployed by the
Department of Civil Protection (DPC), the Istituto Nazionale di Geofisica
e Vulcanologia (INGV) and other regional research agencies.
