A seismological study of normal faulting in the Demirci, Ala ehir and Gediz earthquakes of 1969-70 in western Turkey: implications for the nature and geometry of deformation in the continental crust

Abstract

In this study, seismological techniques are combined with surface observations to investigate the faulting associated with three large earthquakes in western Turkey. All involved normal faulting that nucleated at 6-10 km depth with dips in the range 30-50". The two largest earthquakes, at Alajehir (1969.3.28) and Gediz (1970.3.28), were clearly multiple events and their seismograms indicate that at least two discrete subevents were involved in producing the observed surface faulting. In addition, their seismograms contain later, longer-period signals that are likely to represent source, not structure or propagation, complexities. These later signals can be modelled by subevents with long time functions on almost flat detachment-type faults.As a result of these observations, we propose a model for the deformation of the lower crust, in which brittle failure of the top part occurs when high strain rates are imposed during an earthquake that ruptures right through the upper, brittle crust. Under these special circumstances, seismic motion occurs on discrete faults in the lower crust, which otherwise normally deforms by distributed creep. In the case of the normal faults studied here, motion in the uppermost lower crust takes place on shallow dipping faults that are downward continuations of the steeper faults that break to the surface. The faults thus have an overall listric geometry, flattening into a weak zone below the brittle layer at a depth that is probably dependent on the termperature gradient. This interpretation explains why detachment-type mechanisms are not seen in first motion fault plane solutions of normal faulting earthquakes, and suggests an origin for the Metamorphic Core Complexes seen in the Basin and Range Province, which probably represent flat lower crustal faults, analogous to those postulated at Alajehir and Gediz, that have been uplifted to the surface.

The role of pre-existing thrust faults and topography on the styles of extension in the Gran Sasso range (central Italy)

“…Below this depth, it is generally considered that the high-angle normal faults (rupturing during seismic events) assume shallower dips rooting into the ductile lower crust (Eydogan and Jackson, 1985;Rigo et al, 1996). This work shows that a major shallowing in normal fault geometry is possible in the upper crust as a result of the reactivation of pre-existing thrust faults.…”

Kinematic analysis and palaeoseismology of the Edremit Fault Zone: evidence for past earthquakes in the southern branch of the North Anatolian Fault Zone, Biga Peninsula, NW Turkey

“…Accurate determination of focal depths using the joint hypocentre location techniques used by Jackson and Fitch (1979) and the modelling of the long-period body-waves by Eyidoğan and Jackson (1985) indicate that the major earthquakes in the region are not deeper than 10-15 km (Eyi̇doǧan, 1988). These shallow earthquakes and their seismic magnitudes, scanned from many catalogues and researches (close to 500) with 4 ≤ M events that occurred in the Marmara region from 1900 to 2015, are illustrated in Figure 5(a).…”

Low‐angle normal fault earthquakes and triggering

“…The last two (triggered ?) subevents were modeled with long source duration (8 and 14 s) on LANFs (2 ø and 10 ø) at depths of 10 km [Eyidogan and Jackson, 1985]. The low-angle nodal planes were preferred (also for the Gediz event below) because their moment releases were sufficiently large that surface breaks would be expected for steep planes, and because rupture velocity for steep planes would be abnormally slow (<1 km/s) given the long rupture duration.…”