Real-time testing of the on-site warning algorithm in southern
California and its performance during the July 29 2008 M
w
5.4 Chino
Hills earthquake
M. Bo ̈se,
1
E. Hauksson,
1
K. Solanki,
1
H. Kanamori,
1
and T. H. Heaton
1
Received 17 October 2008; revised 26 November 2008; accepted 10 December 2008; published 5 February 2009.
[
1
] The real-time performance of the
t
c
-
P
d
on-site early
warning algorithm currently is being tested within the
California Integrated Seismic Network (CISN). Since
January 2007, the algorithm has detected 58 local
earthquakes in southern California and Baja with moment
magnitudes of 3.0
M
w
5.4. Combined with newly
derived station corrections the algorithm allowed for rapid
determination of moment magnitudes and Modified
Mercalli Intensity (MMI) with uncertainties of ±0.5 and
±0.7 units, respectively. The majority of reporting delays
ranged from 9 to 16 s. The largest event, the July 29 2008
M
w
5.4 Chino Hills earthquake, triggered a total of 60 CISN
stations in epicentral distances of up to 250 km. Magnitude
predictions at these stations ranged from M
w
4.4 to M
w
6.5
with a median of M
w
5.6. The closest station would have
provided up to 6 s warning at Los Angeles City Hall,
located 50 km to the west-northwest of Chino Hills.
Citation:
Bo ̈ se, M., E. Hauksson, K. Solanki, H. Kanamori, and
T. H. Heaton (2009), Real-time testing of the on-site warning
algorithm in southern California and its performance during the
July 29 2008 M
w
5.4 Chino Hills earthquake,
Geophys. Res. Lett.
,
36
, L00B03, doi:10.1029/2008GL036366.
1. Introduction
[
2
] The purpose of earthquake early warning (EEW) is to
provide real-time information about earthquakes to distant
sites before the seismic S or surface waves arrive. Because
warning times usually are extremely short, EEW systems
must recognize the severity of expected ground motions
within seconds after the P-wave arrival at the EEW sensors.
If warnings can be issued in a timely manner, suitable
actions for damage mitigation can be initiated and executed
[e.g.,
Goltz
, 2002].
[
3
] Currently, the performance of the
t
c
-
P
d
on-site
warning algorithm [
Kanamori
, 2005;
Wu and Kanamori
,
2005a, 2005b;
Wu et al.
, 2007;
Wu and Kanamori
, 2008a,
2008b] is being real-time tested within the California
Integrated Seismic Network (CISN) [
Hauksson et al.
,
2006]. The algorithm is based on single sensor observa-
tions using two parameters: period parameter
t
c
and high-
pass filtered displacement amplitude
P
d
. Both parameters
are determined from the vertical components of velocity
and/or displacement data,
_
u
and
u
, using the first t
0
=3s
of P-waveforms. The period parameter
t
c
,
computed by
t
c
¼
2
p
=
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Z
t
0
0
_
u
2
t
ðÞ
dt
=
Z
t
0
0
u
2
t
ðÞ
dt
s
;
approximately represents the P-wave pulse width [
Wu et al.
,
2008b]. Previous studies have determined empirical rela-
tionships between
t
c
and the moment magnitudes M
w
, and
between
P
d
and the peak ground velocities (PGV) at the
sites of observation [
Kanamori
, 2005;
Wu and Kanamori
,
2005a, 2005b].
Wu et al.
[2007] established corresponding
relationships for earthquakes in southern California using
seismic off-line data. Observed and estimated values of
PGV can be transformed into Modified Mercalli Intensity
(MMI) scale using empirical relationships developed by
Wald et al.
[1999].
[
4
] For the real-time testing, we have implemented the
t
c
-
P
d
algorithm in an UNIX environment, using existing
software components developed by the California Institute
of Technology (Caltech), the U.S. Geological Survey
(USGS), and UC Berkeley, that are built on software
systems developed for the CISN and the Advanced National
Seismic System (ANSS). The processing steps are as
follows [
Solanki et al.
, 2007]: (1) retrieve velocity data
from the CISN; (2) set the baseline to 0 by using average
values continuously determined from the real-time data
streams in intervals of 60 s, and apply gain correction;
(3) convert velocity to displacement data by recursive
integration; apply high-pass Butterworth filter (>0.075 Hz);
(4) calculate
t
c
and
P
d
from the initial 3 s of waveform data;
(5) keep only triggers with
t
c
-
P
d
combinations that are
characteristic of a local earthquake [
Bo ̈se et al.
, 2009]: for a
local earthquake with period
t
c
in a rupture-to-site distance
r
,
r
min
r
r
max
, we expect
P
d
,min
P
d
P
d,
max
.
Bo ̈se et
al.
[2009] determined displacement amplitudes
P
d
,min
and
P
d
,max
from empirical attenuation relations for earthquakes
in southern California with
r
min
= 1 km and
r
max
= 100 km.
To avoid false alerts, we currently require the triggering of
at least 3 stations before an earthquake is processed.
[
5
] To improve the accuracy of M
w
and PGV estimates,
we refined in this study the
t
c
-M
w
and
P
d
-PGV relations by
Wu et al.
[2007] with new station corrections. We deter-
mined these factors from the median of residuals from
(1) 431 off-line estimates of M
w
during 27 earthquakes
(4.0
M
w
7.3) [
Wu et al.
, 2007], and from (2) 257 real-
time estimates of M
w
during 58 earthquakes (3.0
M
w
5.4)
as analyzed in this paper. Correction factors were deter-
mined and applied in this study only for stations for which
at least 2 records were available.
GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L00B03, doi:10.1029/2008GL036366, 2009
Click
Here
for
Full
A
rticl
e
1
Seismological Laboratory, California Institute of Technology,
Pasadena, California, USA.
Copyright 2009 by the American Geophysical Union.
0094-8276/09/2008GL036366$05.00
L00B03
1of5
[
6
] Based on the performance between January 2007 and
September 2008, we want to analyze in this paper (1) the
real-time applicability of the
t
c
-
P
d
on-site warning algo-
rithm for earthquakes in southern California, and (2) the
suitability of the current CISN instrumentation and telem-
etry for issuing EEW. Of course, small and moderate-sized
earthquakes (M
w
< 6.0) as analyzed in this paper usually
will not cause damage and therefore do not require EEW. To
gain experience more quickly and to have working algo-
rithms when large earthquakes occur, we use the more
frequent small events for testing and calibrating of EEW
algorithms and systems [
Bo ̈se et al.
, 2009].
2. Data
[
7
] The
t
c
-
P
d
on-site warning algorithm at Caltech cur-
rently processes the waveform data from about 160 broad-
band stations of the California Integrated (CI) and Anza
(AZ) networks deployed in southern California [
Bo ̈se et al.
,
2009], including stations with 80 and 100 samples per
second. Since January 2007, the
t
c
-
P
d
algorithm has pro-
cessed 58 local earthquakes in southern California and Baja
with 3.0
M
w
5.4 with a total of 257 triggers. The July
29th, 2008 M
w
5.4 Chino Hills mainshock in the eastern Los
Angeles Basin (33.95
°
N,
117.76
°
W, 14.7 km depth) was
the largest earthquake to occur in the greater Los Angeles
metropolitan area since the M
w
6.7 Northridge earthquake in
1994. The event was widely felt across southern California,
but caused only minor damage [
Hauksson et al.
, 2008]. The
Chino Hills earthquake sequence produced 97 estimates of
M
w
and PGV values by the
t
c
-
P
d
algorithm: 60 during the
M
w
5.4 mainshock (Table 1) and between 8 and 15 during
the three largest aftershocks (M
w
2.8, M
w
3.8, and M
w
3.6).
[
8
] The current EEW system at Caltech uses broadband
observations only. To avoid the clipping of wave amplitudes
during large magnitude earthquakes and earthquakes at
close distances, we recently started to integrate strong
motion sensors in addition. To test the performance of the
t
c
-
P
d
algorithm for this type of data, we also processed in
this study, in an off-line mode, acceleration records of the
Chino Hills mainshock from 11 CISN stations at epicentral
distances of
D
30 km.
3. Results
[
9
] Both M
w
and PGV values of the 58 local earthquakes
were estimated automatically by our software from the
observed
t
c
and
P
d
values using the relations proposed by
Wu et al.
[2007]. The real-time estimated parameters corre-
late well with the corresponding values reported by CISN
(Figures 1a and 1c). However, the scatter is often quite
large, sometimes with outliers of as much as two magnitude
units. The median values, taken for each earthquake over all
available M
w
estimates, usually show a slight overestima-
tion of M
w
by 0.3 units. The uncertainties in the predictions
of magnitude (Figure 1a) and the logarithmic values of PGV
(Figure 1c) are ±0.6 and ±0.3 units, respectively. The latter
is equivalent to an uncertainty of ±0.75 MMI intensity units
[
Wald et al.
, 1999].
[
10
] The newly derived station corrections lead to signif-
icant improvement of the predictions, but some outliers still
remain (Figures 1b and 1d); the majority of them are
associated with events with small magnitudes (M
w
< 4.5)
and large epicentral distances (
D
> 150 km), i.e. are caused
by poor signal-to-noise ratios. The station corrections re-
duce the errors in magnitude and intensity estimates to ±0.5
and ±0.7 units, respectively (Figures 1b and 1d). In general,
the M
w
and PGV values estimated from the off-line pro-
cessed strong motion data agree well with the real-time
processed broadband observations of the Chino Hills earth-
Table 1.
Performance of the On-Site Warning Algorithm at Caltech During 9 Local Earthquakes (M
w
> 4.5) in Southern California and
Baja
a
Origin
Time (PST)
Latitude and
Longitude (deg)
M
w
Number of
Reports
Time of
First Report
First
Reporting
Station
Estimated M
w
at
First Reporting
Station
Median and
Scattering in M
w
Over All Reports
2008-7-29
33.95
5.4
60
11:42:25
CI.PSR.
b
5.6
5.6
11:42:15
117.76
(10 s after O.T.)
(
D
= 16 km)
4.4–6.5
Chino Hills
2008-2-8
32.36
5.1
5
23:12:30
CI.DRE
c
5.4
5.4
23:12:04
115.28
(26 s after O.T.)
(
D
= 52 km)
5.1–5.9
2008-2-11
32.33
5.1
2
10:29:57
CI.DRE
c
5.3
5.0
10:29:30
115.26
(27 s after O.T.)
(
D
= 56 km)
4.8–5.3
2008-2-19
32.43
5.0
5
14:41:52
CI.DRE
c
4.8
5.0
14:41:29
115.31
(23 s after O.T.)
(
D
= 43 km)
4.7–6.0
2008-2-11
32.45
5.0
6
20:33:02
CI.DRE
c
4.7
5.0
20:32:39
115.32
(23 s after O.T.)
(
D
= 41 km)
4.4–5.4
2008-2-19
32.43
4.8
4
17:29:18
CI.DRE
c
4.0
4.9
17:28:55
115.31
(23 s after O.T.)
(
D
= 44 km)
4.0–5.5
2008-2-22
32.42
4.8
1
11:31:41
CI.DRE
c
4.6
4.6
11:31:18
115.29
(23 s after O.T.)
(
D
= 45 km)
2007-9-2
33.73
4.7
27
10:29:34
CI.CHN
d
3.9
4.6
10:29:14
117.45
(20 s after O.T.)
(
D
= 35 km)
3.2–5.6
2007-8-9
34.30
4.6
26
00:59:03
CI.RIN
d
4.6
4.6
00:58:49
118.62
(14 s after O.T.)
(
D
= 13 km)
3.9–5.9
a
Magnitude estimates are station corrected. O.T. is origin time,
D
epicentral distance.
b
Q330; direct ethernet radio link to Caltech.
c
Q4120; frame relay.
d
Q4120; Virtual Private Network (VPN) over microwave.
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quake (triangles in Figures 1a–1d), but show more scatter,
possibly due to poor signal-to-noise ratios. Note that the
strong motion data in Figures 1a–1d are not station corrected.
[
11
] The largest outlier in Figures 1a and 1b is observed
for a M
w
4.0 earthquake which occurred on March 30, 2007
at six kilometers east of Coso Junction, an area known for
the frequent occurrence of earthquake swarms: the magni-
tude of this event was overestimated by 1.5 units.
Bo ̈se et al.
[2009] suspected that this overestimation might have been
caused by a small foreshock which occurred 48 s before the
mainshock resulting in a high background noise level at the
majority of close EEW stations before and during the arrival
of the seismic P phase from the mainshock. Although such
foreshocks are relatively rare, the real-time identification of
such events will pose a major challenge in the future
developments of EEW systems [
Bo ̈se et al.
, 2009].
[
12
] The real-time performance of the
t
c
-
P
d
on-site
warning algorithm for all analyzed earthquakes with M
w
>
4.5 is summarized in Table 1. The errors in the station
corrected estimates of M
w
at the first reporting station reach
from 0.0 to 0.8 units. Six of the events occurred within an
earthquake swarm near the Cerro Prieto Geothermal field at
the U.S./Mexican border, which started
20 miles southeast
of Calexico with a M
w
5.1 earthquake on February 8, 2008
(http://www.scsn.org/2008bajaaddendum.html). Note that
the first reporting station CI.DRE is located at approximately
40 to 55 km north, i.e. relatively far away from the
epicenters of the large events in the swarm. The first
magnitude and PGV estimates thus were not available until
23 s after origin time (O.T.).
[
13
] The first magnitude prediction of the M
w
5.4 Chino
Hills mainshock was available 10 s after O.T. (M
w
5.6 at
station CI.PSR). Subsequent (independent) estimates based
on data from other stations ranged from M
w
4.4 to M
w
6.5
with a median value of M
w
5.6(Table1).TheMMI
intensities determined from PGV [
Wald et al.
, 1999] were
slightly underestimated by 0.2 ± 0.8 units. The largest
prediction errors occurred in the western part of the Los
Angeles basin where seismic wave amplitudes were strong-
ly amplified due to basin effects (Figures 2a and 2b).
Because the current database is insufficient for the determi-
nation of correction terms for many of the stations shown in
Figure 2b, the PGV estimates in the map are not station
corrected.
[
14
] Neglecting the telemetry and processing delays of
the current system, each single estimate in Figure 2b could
have been made available within 3 s after the P-wave arrival
(This is the time required by the
t
c
-
P
d
algorithm.). The
entire map in Figure 2b was available in less than 1 minute
after O.T.. For comparison the first automatically generated
CISN ShakeMap [
Wald et al.
, 1999] of the Chino Hills
earthquake was released about 12 minutes after O.T.
[
Hauksson et al.
, 2008].
[
15
] To illustrate the effect of EEW delays in more detail,
we have drawn in Figure 2 circles centered at the epicenter
of the M
w
5.4 Chino Hills earthquake. These circles show
Figure 1.
Estimated vs. observed source and ground motion parameters of 58 local earthquakes (3.0
M
w
5.4) in
southern California and Baja: (a and b) moment magnitudes M
w
and (c and d) peak ground velocities (PGV). Estimates in
Figures 1b and 1d include correction factors, which are applied to stations with observations during at least two
earthquakes. Crosses show real-time processed data, triangles off-line processed strong motion data of the July 29 2008
M
w
5.4 Chino Hills earthquake. The median M
w
values in Figures 1a and 1b, taken over the magnitude estimates of each
earthquake, are shown by squares. Values along the dashed lines show a perfect correlation.
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the predicted locations of the S-wave arrivals at 10 s, 20 s,
30 s and 50 s after O.T., assuming a constant shear wave
velocity of 3.2 km/s. Depending on the time, at which a
warning or estimate is delivered, only sites outside of the
corresponding circle can obtain a warning before the S wave
arrives. This delay depends on (a) the P-wave arrival time at
the reporting EEW sensor, which is controlled by the station
density, and (b) the so-called
warning delay
between P-wave
arrival and the reporting of parameters and/or warnings. The
warning delays include delays caused by station equipment
(in particular by the type of datalogger), by telemetry of
waveform data to the central processing facility at Caltech,
3 s waveform data required by the
t
c
-
P
d
algorithm, and
processing delays. In an operational system, additional delays
will be caused by the transmission of warnings to users.
[
16
] The current CISN configuration provided a first esti-
mate 10 s after O.T. of the Chino Hills mainshock (Table 1).
In an operational EEW system, only sites outside of the
smallest circle in Figure 2 with a radius of 30 km could have
obtained a warning. As an example, at Los Angeles City
Hall, located 50 km to the west-northwest of Chino Hills,
the current system would have provided up to 6 s warning
before S-wave arrival. For comparison the first M
L
estimate
(M
L
5.6) by CISN was automatically released
80 s after
O.T., an up-dated estimate (M
L
5.8) around 60 s later. The
automatic moment tensor and M
w
estimate (M
w
5.4) were
available
10 min after O.T. [
Hauksson et al.
, 2008].
[
17
] From January to September 2008, the testing system
at Caltech recorded around 50,000 triggers, which were
mostly not caused by local earthquakes and not processed
by the EEW algorithm. Fortuitously, these triggers provide
data for comprehensive statistics of warning delays. The
majority of warning delays range from 9 to 16 s (Figure 3).
The older generation Refteks in the Anza network use both
microwave and internet based telemetry. While older gen-
erations of CISN dataloggers (Q4120 and Q730) transmit
3 s of demultiplexed miniSEED waveform packets, the
new Q330 dataloggers transmit data packets of 1 s multi-
plexed waveform data. We have developed the capability of
capturing and analyzing these 1 s packets and therewith
were able to reduce the warning delays by several seconds.
4. Discussion and Conclusions
[
18
] Between January 2007 and September 2008, 58 local
earthquakes with 3.0
M
w
5.4 were real-time processed
by the EEW software at Caltech, including the July 29 2008
M
w
5.4 Chino Hills earthquake as the largest event. The
performance during these events demonstrates the real-time
applicability of the
t
c
-
P
d
algorithm.
[
19
] A scatter in the real-time predicted values of M
w
for
both on- and off-line processed earthquakes (Figure 1a) is
unavoidable, because
t
c
is affected by many factors, includ-
ing source (radiation patterns, directivity, and stress drop),
propagation and site effects. Poor signal-to-noise ratios pose
an additional problem for small magnitude events (M
w
< 4.5)
and events at large epicentral distances (
D
> 150 km). The
scatter can be reduced by the application of station correc-
Figure 2.
Distribution of (a) observed and (b) predicted values of peak ground velocity (PGV) at 60 CISN stations
triggered by the EEW software during the July 29 2008 M
w
5.4 Chino Hills earthquake. Neglecting telemetry and
processing delays, each single estimate in Figure 2b could have been made available within 3 s after P-wave arrival. Circles
show S-wave arrivals at 10 s, 20 s, 30 s and 50 s after origin time (O.T.). The current system provided a first estimate of M
w
and PGV 10 s after O.T., i.e. in an operational EEW system, only sites outside of the smallest circle could have obtained a
warning using the current CISN stations and equipment.
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tions (Figures 1b and 1d). The current database includes
station corrections only at a small subset of CISN stations,
but we plan to up-date the station corrections with data from
future earthquakes.
[
20
] At present, we are working on the implementation of
the remote processing sites for the BK, NP, NC, and PG
networks operated by UC Berkeley and USGS Menlo Park
in northern California. These processing sites will analyze
available local waveform data and provide
t
c
-
P
d
values as
well as M
w
and PGV estimates, i.e. more data for algorithm
testing will become available. Based on this data, further
tests of the
t
c
-M
w
and
P
d
-PGV relations for earthquakes in
California will be undertaken.
[
21
] The future inclusion of strong motion channels in the
EEW system at Caltech will help avoid clipping of wave
amplitudes during strong and close earthquakes and will
help increase the required station density for EEW. As
shown in this paper, M
w
and PGV estimates obtained from
strong motion records with
D
30 km are in good agree-
ment with broadband observations.
[
22
] The current test system at Caltech uses a 3 s long
time window to determine the EEW parameters
t
c
and
P
d
.
Previous studies have shown that it should be possible to
recognize from this time window if M
w
6.5 or M
w
>6.5
[
Kanamori
, 2005]. We chose the 3 s length as a compromise
between recognizing a large magnitude earthquake and
issuing estimates and warnings as soon as possible. In the
future, additional research needs to be undertaken to deter-
mine the optimum length of the window, including both
moderate and large magnitude earthquakes [
Bo ̈seetal.
, 2009].
[
23
] The performance results of the EEW software at
Caltech reveal technological limitations of the current CISN
instrumentation and telemetry for issuing early warnings.
The majority of warning delays based on the CISN infra-
structure range from 9 to 16 s (Figure 3). The speed of
delivery mainly depends on the type of datalogger and
telemetry path. To eliminate delays caused by the telemetry
of waveform data, we recently started to implement the
t
c
-
P
d
algorithm software on SLATE Field Processors. SLATE
receives data from a Q330 datalogger on-site, computes
t
c
and
P
d
, and transmits M
w
and PGVestimates to Caltech as a
short notification message. Warning delays are expected to
decrease significantly compared to the current centralized
processing of waveform data at Caltech. In the future, such
station processors can also transmit warnings to local users
directly.
[
24
]
Acknowledgments.
We thank two anonymous reviewers for
their positive comments on an earlier version of our manuscript. This work
is funded through contract 06HQAG0149 from USGS/ANSS to the
California Institute of Technology (Caltech). The Southern California
Seismic Network (SCSN) and the Southern California Earthquake Data
Center (SCEDC) are funded through contracts with USGS/ANSS, the
California Office of Emergency Services (OES), and the Southern
California Earthquake Center (SCEC). This is contribution 10010 of the
Seismological Laboratory, Geological and Planetary Sciences at Caltech.
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M. Bo
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(mboese@caltech.edu)
Figure 3.
Warning delays between P-wave arrivals at the
CISN stations and the reporting of M
w
and PGV values at
Caltech. The delays include times required for the
processing and communication of the waveform data, i.e.
delays caused by station equipment (in particular types of
dataloggers, see legend), the telemetry, 3 s required by the
t
c
-
P
d
algorithm, and the centralized waveform processing at
Caltech. The histogram shows triggers obtained between
Jan. and Sept. 2008.
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