Preliminary report on crustal deformation surveys and tsunami measurements caused by the July 17, 2006 South off Java Island Earthquake and Tsunami, Indonesia

Teruyuki Kato,Hasanuddin Z Abidin,Agustan Agustan,Takeo Ito

doi:10.1186/bf03352046

Abstract

Abstract A large earthquake (Mw=7.7) along a plate boundary occurred in the south of Java Island on July 17, 2006, and caused a significant tsunami. We made GPS observations and tsunami heights measurements during the period from July 24 to August 1, 2006. The earthquake seems to be due to an interplate low angle reverse faulting, though there might be a possibility of high angle faulting within the subducting lithosphere. Crustal deformation distribution due to the earthquake, aided by tsunami heights measurements, might clarify which would be the case. We occupied 29 sites by GPS in the area of southern Java encompassing the area from 107.8 E to 109.50 E. These sites were occupied once before the earthquake. However, we were not able to detect significant coseismic displacements. The obtained displacements, most of which span several years, show ESE direction in ITRF2000 frame. This represents the direction of Sunda block motion. The tsunami heights measured at 11 sites were 6–7 m along the southern coast of Java and indicate that the observed heights are systematically higher than those estimated from numerical simulations that are based on seismic data analysis. This might suggest that fault offsets might have been larger—nearly double—than those estimated using seismic analysis. These results lead us to an idea that the rupture was very slow. If this is the case, the earthquake might have been a “tsunami earthquake” that is similar to the one that occurred on June 2, 1994 in the east of the present earthquake.

Highlights

Indonesian Islands are located near the Equator spanning from 95 E to 140 E and 8 S to 8 N
Recent study using GPS suggests that central to western part of Indonesian Islands are on the Sunda block that moves to ESE in ITRF2000 frame, whereas the eastern part of Indonesia is under a complicated tectonic regime (Bock et al, 2003; see Fig. 2)
GPS results shown in this study suggested that the motion of the Java island is ESE and do not show any influence of the plate convergence of the Australian plate that subducts underneath the Java Island (see Fig. 6(b))

Summary

Introduction

Indonesian Islands are located near the Equator spanning from 95 E to 140 E and 8 S to 8 N. In July 2006, another earthquake occurred in about 200 km south of Java nearby the Java trench as shown in Fig. 3 and generated significant tsunami. Mw7.7, Harvard global CMT) occurred about 200 km south of western Java Island (see Fig. 3). We made GPS measurements along the south coast of Pangandaran and the surrounding region, where large tsunami damage was recorded (Fig. 4). SD (mm/yr) EW NS 22.5 8.1 22.2 8.2 13.2 11.1 22.3 12.6 49.6 34.3 25.9 15.7 75.2 53.9 and Fig. 6(a) showing that the velocity vectors shown in this study is mostly similar to the previous results by Bock et al (2003), suggesting that the obtained velocity field in this study seems to reflect interseismic block motion of Sunda block. Neither of these results predicts observed tsunami heights of about 6–7 meters

Was the earthquake “Tsunami earthquake”?

Conclusion and Remarks