M4.3 and M4.1, January 13, 2001: Web Service Report


A pair of events, M4.3 and M4.1 occurred on the evening of Saturday, January 13, 2001. These events were centered in the northeast San Fernando Valley area of Los Angeles. They were felt by many people, and provided a good real test of the USGS Pasadena Office and Trinet web service capabilities.

The Internet is fast becoming the medium of choice for timely distribution of earthquake information to the public. The USGS Pasadena Field Office and the Trinet project provide earthquake information for Southern California. As is usually the case, these events caused large increases in the traffic to our web servers. The typical pattern shows a large spike which peaks about 10 minutes after the event, followed by an exponential decay in the traffic rate.

USGS Server Trinet Server
Figure 1
The first event occurred at 18:26 PST. It was apparently felt over a wide populated area. The traffic on our web server began increasing almost immediately. This can be seen in Figure 1. These graphs show the overall traffic on the USGS and Trinet web servers. The peak recorded after the first event was 90 hits/sec on the USGS server, and 25 hits/sec on Trinet. This peak was starting to decay when the second event occurred at 18:50 PST. The second event's traffic peaked at 134 hits/sec on the USGS server, and 53 hits/sec on Trinet. Shortly after, the Trinet server experienced another surge after the URL for the Shake Map was shown on TV. This peak reached 233 hits/sec, and is the largest traffic surge we have recorded as of this writing. The brown line in the graph is the ratio of cache hits to total activity on the Squid server, which is the reverse-proxy server in front of the main web server.


Figure 2
The main pages that people come to see after an earthquake are the Recent Earthquakes and Community Internet Intensity Maps pages. Recent Earthquakes shows a near-real-time map of California with all events for the past week plotted. The CIIM pages show maps of perceived shaking felt by the public, and include an online questionnaire for people to fill out to report their experiences. A detailed look at the overall traffic for two hours around the events is show in Figure 2. The double peak can be seen clearly here.


Figure 3
Figure 3 shows traffic for just the Recent Earthquakes pages. The overall pattern looks about the same. The main thing to note is that this traffic amounted to nearly half of the total activity on the USGS site.


Figure 4
Figure 4 shows the calls made to ciim_update.pl. This is the CGI script that is called after a visitor has filled out the online questionnaire. It writes a summary of their input for later processing and provides them with an estimated Mercalli Intensity for their location. There are two features of note in this graph. First is that the peak activity was on the order of 84 questionnaires per minute. This shows that the new server is performing well. Before we got the new server, there were problems with processing questionnaires after the September 3 Yountville earthquake. The new server did not have any apparent problems dealing with the load after this event, even though the load was nearly the same. In particular, the 'submitted' and 'processed' lines on the graph match exactly, except for one questionnaire submitted at 19:02. This request failed with a 500 Error, which was most likely due to faulty input. The second feature to note in this graph is that the peak for the second event was delayed. For some reason, the CIIM map and questionnaire for the second event was not created until about 30 minutes after the event. This is the reason the second peak is delayed.

Total shake map activity
Shake Map page views
Figure 5
Another popular page is the Shake Map. This is served off of the Trinet web server. Figure 5 shows activity for the Shake Map pages after the events. The top graph shows all HTTP activity for the /shake/ directory tree, and the bottom graph shows just page views for the two events on Saturday. The interesting feature here is the third peak on the first graph. The first two peaks correspond to the traffic peaks after each of the two events. The third peak came about two minutes after KCBS news showed the shake map and URL on TV. After this, the Trinet web server recorded 233 hits/second for several minutes. This was the largest traffic spike to date on our web servers. The data rate on the Trinet Squid server was about 6Mb/sec at the peak, and the server CPU was about 50% idle. This corresponds well with our experiences in testing, which indicated that a single K6-2 Squid server should be able to handle on the order of 400 hits/sec.


Overall, it appears that the current web server configuration performed well under load. The lessons from the Yountville earthquake helped us to successfully keep up with the load this time. Still, the continued growth of the Internet means we will probably have to further increase our capacity in the future.


Stan Schwarz
Honeywell Technical Services
Southern California Seismic Network Contract
Pasadena, California
17 January, 2001