Preliminary Report:
Mexico City UrEDAS detects Colima earthquake on 02:06:35, January 22, 2003
System and Data Research, Co., Ltd.
http://www.sdr.co.jp

1. Introduction
System and Data Research, Co., Ltd., SDR, has installed UrEDAS in Mexico City as a cooperationof centro de instrumentación y registro sísmico, CIRES. CIRES has observing earthquakes aroundAcapulco and warned to Mexico City. UrEDAS earthquake information is sent to UrEDAS Centerat CIRES headquarter via radio transmission and received by the system of CIRES immediately.
CIRES server soon provides the information via Internet. Mexico City UrEDAS had started
observation at October 30, 1999. Because the former site had high traffic noise, the system wasmoved to another strong motion station #78 on hard ground at December 14, 2001. After this thenetwork connection has been established and more than 100 earthquakes, mainly locates at theGuerrero, are detected and informed.
As a result of observation, at the time of P wave detection, the hypocentral distance tends to beestimated far and also underestimating magnitude about one is confirmed. At the time of S wave
detection, the result of estimation is almost correct. The system will be adjusted by the result ofobservation.

2. Detect Situation
Earthquake parameters of the Colima earthquake are following (USGS).
Time:

Magnitude:
Location:
Depth:

Wed Jan 22 02:06:35 2003 (UTC)
Wed Jan 21 20:06:35 2003 (local time)
Wed Jan 22 11:06:35 2003 (JST)Mw7.8
18.807N, 103.886W (400km West from Mexico City)
5 km

E-mail information for this earthquake was delivered at around 11:08:30 (JST) for a server in Japan.
The result of detection is following.
Detect time:
Estimation with P waveEstimation with S wave

Wed Jan 22 02:07:51 2003 (UTC)
Received time at CIRES
Wed Jan 22 02:07:56 2003 (UTC)
Magnitude
M6.8Azimuth
280 (deg.)
Hypocentral distance
232 kmDepth
134 km(Between Mexico City and Colima)
Received time at CIRES
Wed Jan 22 02:08:53 2003 (UTC)
Hypocentral distance
442 kmDepth
255 km(60 km North East from Colima)

Around 20:07:50, the observation station of UNAM near Mexico City detected earthquake motion.
Because of more than 400 km hypocentral distance, p-s time, duration from the P wave detection of1/4

UrEDAS to large earthquake motion, seems more than one minute. See following UrEDASwaveform.
UrEDAS says that, at the time of the preliminary tremor detection, occurrence of M6.8 earthquake
hundreds km west was estimated. And after one minutes, at the time of the large earthquake
motion detection, the location became clear to the near of Colima.
Mexico City UrEDAS is on the way of adjustment and not tuned to detect only for large events.
However, from the detect situation of this earthquake, it is clear to be effective to decrease thehazard. Of course, although suitable employment is required, while correcting the parameters
immediately from now on and raising presumed accuracy, it is considered that the employment
method wants to talk with CIRES.

3. Waveforms
Following figures show the waveforms of Mexico City UrEDAS with the order of UD, NS and EW
in 1/1000 cm/sec. Time scale in seconds is counted from 02:07:00 (UTC) and may include error ofabout 0.5 sec in maximum. The lines around 51 sec and 105 sec indicate P wave detection and S
wave detection, respectively.
These waveforms show the first preliminary motion is relatively small for four seconds and after this,large P wave motion was arrived. UrEDAS detected this preliminary P wave motion and sent thefirst information after three seconds. And just after S wave detection, secondary information wasalso sent. The system at CIRES received these information one second later. This S wave
detection was 10 seconds earlier than the real S wave arrival.
The time with large motion was 40 seconds or more, and it shows this earthquake was a big event,too. A local person at the relatively hard site in the city felt the vibration continued more than twominutes.

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Waveform of the 02:07, January 22, 2003 earthquake observed by Mexico City UrEDAS

Short period velocity waveform (1 Hz – 10 Hz)
200100
0-100-200
400
200
0-200-400
400
200
0-200-400

050

100

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200

250

150

200

250

Long period velocity waveform (0.1 Hz – 10 Hz)
2000
1000
0-1000-2000
2000
1000
0-1000-2000
2000
1000
0-1000
-2000

050

100

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Enlarged short period velocity waveform (1 Hz – 10 Hz)
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10-1-22
10-1-22
10-1-24041

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(end)

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