Earthquake early warning systems are algorithms designed to detect the initial P-waves from an earthquake, rapidly estimate the magnitude of the event, and predict subsequent ground shaking in the surrounding regions. Earthquake Alarm Systems, or ElarmS, is one early warning algorithm that uses a network of seismic stations to hone in on the size and location of the earthquake. Averaging the magnitude estimates from multiple stations improves the accuracy of the estimate. ElarmS has been tested on multiple Northern and Southern California datasets, and now automatically processes streaming seismic data across California. In order to improve the robustness of the methodology, we test it on a dataset of large-magnitude events from Japan's Kyoshin Net (K-NET) strong-motion seismic network.
K-NET consists of 1,000 digital strong-motion seismometers spaced at approximately 25km intervals throughout Japan. Each station is capable of recording acceleration up to 2000 . Our K-NET dataset contains 84 earthquakes occurring within 100km of K-Net stations between September 1996 and June 2008 (Figure 2.61). The local magnitudes range from 4.0 to 8.0, the largest being the 26 September 2003 Tokachi-Oki event. Forty-three of the events are of magnitude 6.0 or greater.
ElarmS in California assumes a fixed depth of 8km and estimates the epicentral distance with a two-dimensional grid search. In the Japanese subduction zone this is inappropriate. For the Japanese dataset we create a three-dimensional grid and calculate the IASP91 P-S travel time for seismic waves originating at each point of the grid. We then compare the calculated travel times to the observed P-S travel times at each station to find the best estimate of hypocentral location. Data is used only from stations within 100km of the hypocenter.
We also use a least squares fit to calculate a local relationship between magnitude and peak displacement of log10() = -4.02 + 0.66*M, corrected for epicentral distance, compared to log10() = -3.77 + 0.73*M for California (Wurman et al, 2007) (Figure 2.62b). The relations have comparable slopes for Japan and Northern California, but Japan displays lower observed values, implying greater attenuation in the region.
We further consider the effect of different quantities of data by limiting the number of stations used for magnitude estimates. Figure 2.63 shows the average error in the ElarmS estimated magnitude using only the single closest station to the epicenter, the two closest stations, three closest, etc. The dashed lines show the error in the magnitude estimate using only or . The solid line is the error using both and .
The combined and estimate has an average error of less than 0.6 magnitude units using only one station for each event, and that error drops lower with the addition of more stations. by itself produces an average error of less than 0.5 magnitude units for all numbers of stations. by itself produces an error that is higher than that of , but still less than one magnitude unit when using more than one station. Previous studies have shown that the ElarmS magnitude estimates are more robust for large events when and estimates are combined (Wurman et al, 2007), although we have not yet verified this for the Japanese dataset.
Olson, E.L., and R.M. Allen, The deterministic nature of earthquake rupture, Nature, 438, 212-215, 2005.
Wurman, G., R.M. Allen and P. Lombard, Toward earthquake early warning in northern California, J. Geophys. Res., 112 (B08311), 2007.
Berkeley Seismological Laboratory
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