Complex source process of the 17 August 1999 Izmit, Turkey, earthquake

Broadband teleseismic P-wave data at epicentral distances of 30degrees-90degrees are inverted to retrieve the complex source process of the 17 August 1999 Izmit earthquake. A comparison of the waveforms of the mainshock with those of the aftershocks suggests a spatially complex rupture process having a duration of more than 45 sec. Inversion and modelino, of broadband P waveforms are performed in this time window, incorporating a priori field observations of coseismic displacements. Both single-point and multiple-point source representations are determined. We found that the rupture process can be divided into at least three steps. In the first step, rupture initiated from the hypocenter and extended bilaterally along the E-W-striking fault with an almost pure right-lateral strike-slip motion. The eastern portion of the rupture front passed through the Sapanca Lake and stopped around Akyazi, whereas the western portion stopped near Hersek. This step lasted for 22-23 sec, releasing a total moment of 1.43 X 10(20) N m. The dislocation was concentrated briefly at two areas: one near Golcuk and the other between the Sapanca Lake and Akyazi. Slip in these two areas was concentrated in the upper 10 km of the crust. After 1-2 sec, a second source started, possibly initiating on an underwater fault west of Hersek. Although its rupture lasted for almost 20 sec, its moment release was very low, just 0.17 X 10(20) N m. Another 8-9 sec later, the easternmost fault segment between Akyazi and the area west of Duzce was triggered, with a rupture lasting for more than 15 sec, producing little energy in the short-period band, with slip concentrated in a zone extending vertically 25 km to the surface, and with a moment of 0.61 X 10(20) N m.