Low Free-Field Accelerations of the 1999 Kocaeli Earthquake?

Abstract

The August 17, 1999 Kocaeli earthquake ruptured the earth's surface along 145 km and produced a magnitude of M w ¼ 7.4. As expected for such an event Modified Mercalli intensities of typically IX and X in the vicinity of the fault were determined. Yet the observed accelerations at the five near-fault sites remained amazingly small with horizontal PGA values of 0.14 g to 0.4 g. We attempt to resolve the enigma by modeling surface ground motion with a finite-difference algorithm, utilizing two different rupture and slip histories derived from the strong-motion observations and 'translate' the computed horizontal motion in intensity values. We can show that (a) in a given simple crustal velocity model different slip distributions result in significantly different ground motion distributions in the vicinity of the fault even though both slip distributions fit the observed accelerometer data quite well. (b) Both slip distributions 'project' high ground motion into areas adjacent to the fault where no accelerometer was located. (c) Both slip distributions are not fully compatible with observed intensity observations around the fault, although this could be partly attributed to the lack of knowledge regarding to the crustal structure. In the light of our results it would thus be misleading if the few strong-motion observations around the Kocaeli earthquake fault were taken as typical or representative for the entire area and for potential future events.

Investigation of 3-D basin structures in the İzmit Bay area (Turkey) by single-station microtremor and gravimetric methods

“…A limited number of strong motion stations, situated mostly on rock or thin sedimentary cover in the Bay area, in the near field (distances less than 10 km) revealed low levels of peak ground accelerations (<0.4 g) associated with the İzmit earthquake (Bouchon et al . 2002; Miksat et al . 2005).…”

“…The lack of strong motion observations on the thick sedimentary cover of the basins present in this region does not preclude the occurrence of high level of strong ground motions from this or any other large future earthquake. In fact, the 3‐D strong motion modelling study of Miksat et al . (2005) concludes that their modelling projects high levels of strong motions on to the sedimentary areas in the bay area, where no instrumental observations were available.…”

“…The thickness and velocity of soft sedimentary layers overlying the bedrock are considered among the most critical parameters that influence strong motions during large earthquakes in such geological structures (Field et al 2000;Wills et al 2000). A limited number of strong motion stations, situated mostly on rock or thin sedimentary cover in the Bay area, in the near field (distances less than 10 km) revealed low levels of peak ground accelerations (<0.4 g) associated with theİzmit earthquake (Bouchon et al 2002;Miksat et al 2005). The lack of strong motion observations on the thick sedimentary cover of the basins present in this region does not preclude the occurrence of high level of strong ground motions from this or any other large future earthquake.…”

Simulating three-dimensional seismograms in 2.5-dimensional structures by combining two-dimensional finite difference modelling and ray tracing

“…The simulations are carried out by applying the 3‐D FD code of Olsen (1994) which is frequently used for earthquake wave propagation simulations (e.g. Olsen 2000; Gottschämmer et al 2002; Miksat et al 2005; Oth et al 2007). The code is of fourth order in space and second order in time.…”

“…In seismology, wave propagation for earthquakes is simulated in order to understand the ground motion of past earthquakes and to estimate the threat of future earthquakes (e.g. Olsen 2000; Graves & Wald 2004; Furumura & Kennett 2005; Goto et al 2005; Miksat et al 2005). In exploration seismology, the FD method is used for reservoir modelling and survey planning (e.g.…”