Back Projection of the Rupture for the

2006 West Java Earthquake

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The July 17, 2006 earthquake (Mw7.7) off the southern coast of West Java, Indonesia has been recognized as a tsunami earthquake. The levels of high frequency radiation were relatively low for the large moment of the event. Because the shaking was only felt weakly or not at all in the coastal areas of southern Java, people did not leave the beaches immediately following the earthquake, which resulted in over 600 deaths from the large tsunami. This back projection analysis shows the slow rupture speed for this tsunami earthquake.

Fig. 1. White star and black circles show the locations of the mainshock and aftershocks, respectively, of the West Java earthquake. Roman numerals show the level of intensity at location on Java. Black stars show locations of other significant events in the region.

Data

P waveforms from about 700 Hi-Net stations in Japan were used in a back projection analyses of the rupture for the earthquake. The stations in Japan are at a distance of about 52 to 70 degrees from the earthquake. The data are low-passed at 0.2 hz.

Fig. 2. Hi-net data for the P-wave recorded in Japan. The waveforms have been aligned on the first arrival.

Method

The back-projection method determines tests a grid of points to determine which is the best location for the source of seismic radiation in each designated time window of the P wave.The initial arrival of the first time window was assumed to come from the grid point corresponding to the earthquake hypocenter. For each subsequent time window (itim), the data were stacked assuming a source at each grid point (igrid) using the equation,

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where s(igrid, itim) is the stack amplitude and t(ista, ipt) are time series for each station, ista., that has time points, ipt.. Relative time shifts for each time series were calculated using the theoretical travel times from the station to the grid point, using the IASPEI91 model. The grid of the 240 tested source locations for each time window is shown below.

Figure 3. Grid of source locations tested for each time window.

Results

The results in the animation below show the rupture progresses from the epicenter in the southwest toward the northeast. The rupture velocity appears to be variable with an average speed of about 1 km/second.

Figure 4. Animation of the rupture propagation.

Fig. 5. The position of the rupture front as a function of time. The different lines are for different length time windows in the analysis. The dotted line shows a rupture propagation of 1 km/sec.

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