Static stress changes induced by the 1924 Pasinler (M = 6.8) and 1983 Horasan-Narman (M = 6.8) earthquakes, Northeastern Turkey

The 1924 Pasinler & 1983 Horasan-Narman earthquakes which struck the Erzurum region occurred on the NE–SW-trending Horasan fault zone about 60 km east of Erzurum basin. The inversion of teleseismic seismograms, the aftershock pattern and the surface faulting of the 30 October 1983 ( M _s = 6.8) Horasan-Narman earthquake indicate that it had dominantly left-lateral motion. One moderately sized aftershock occurred 8 h after the main event and two others a year later on the NE extension of the fault zone. The aftershock distribution dominantly overlapped with the Horasan fault zone, and the aftershocks also migrated from south-west to north-east within the year following the mainshock. The results obtained from modelling of static stress changes caused by the 1983 earthquake are consistent with the spatial distribution of aftershocks. Macroseismic observations of the 1924 earthquake ( M _s = 6.8) indicated that this event occurred on the SW extension of the Horasan fault zone. Static stress modelling of the 1924 earthquake, by using the same input parameters of the 1983 event, has shown that its occurrence increased the stress in the region of the 1983 rupture zone. The static stress changes caused both by the 1924 and the 1983 earthquakes has increased the failure stress at the NE and SW extensions of the Horasan fault zone and in Narman area. Furthermore, the stress has decreased in the vicinity of the Erzurum fault zone, east of the city of Erzurum, the largest city in eastern Turkey, and in the populated Sarikamis area. This might delay the occurrence of a future probable damaging earthquake in these areas.     

Paleoseismology of the Gazikoy-Saros segment of the North Anatolia fault, northwestern Turkey: Comparison of the historical and paleoseismic records, implications of regional seismic hazard, and models of earthquake recurrence

We excavated five trenches across the North Anatolia fault zone (NAFZ)along the Ganos fault (Gazikoy-Saros segment), which last produced surfacerupture in 1912, near Kavakkoy where the fault enters the Gulf of Saros. The trenches exposed faulted sediments in a flood-plain environment withabundant detrital charcoal and scattered land-snail shells. Twenty-tworadiocarbon dates place constraints on the ages of the exposed sediments,which range from less than a few hundred years to about 6000 years inage. In two closely spaced trenches, we identified five discrete earthquakeevent horizons in the upper 2.5 m of stratigraphy based on abruptupward termination of shear zones, folding, fissuring, and abruptstratigraphic thickening, four of which may corresponded to historicallyrecorded large regional earthquakes. The earliest of the identified eventsoccurs below an unconformity and dates to about 4 ka B.P. The morerecent four events all occurred within the past 1000–1200 years and maycorrespond to large earthquakes in A.D. 824, ca 1354, 1509, 1766 and1912 (Ambraseys and Finkel, 1987, 1991, 1995). In another trench,we identified at least two events that have occurred during the past 500years and probably correspond to the large events of 1766 and 1912. These observations support an average return period of about 250–300years for the Gazikoy-Saros segment of the NAFZ. They also suggest thatthis segment, which is bound both to the east and west by large releasingstepovers, behaves in a quasi-periodic fashion, at least for the past severalsurface ruptures.Most of the 23 mm/yr of dextral shear between Anatolia and Europeobserved by GPS occurs on the North Anatolian fault. We use18 mm/yr and the 250–300 year recurrence rate, as determined fromour trenching and the historical record, to suggest that each of theearthquakes observed in our trenches produced several meters of slip,consistent with their inferred sizes from the extent of historical damage. Considering that Istanbul has not suffered a large nearby event in theMarmara Sea since 1766, we suggest that about 4 m of strain hasaccumulated across faults in the Marmara during these past centuries. Thisis similar to the average slip in many of the large earthquakes on the NorthAnatolian fault this century. If released seismically, this could result in anearthquake in the M 7.2–M 7.6 range, similar to the August and November,1999 earthquakes east of the Marmara Sea.     

Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering

10 M ≥ 6.7 earthquakes ruptured 1000 km of the North Anatolian fault (Turkey) during 1939–1992, providing an unsurpassed opportunity to study how one large shock sets up the next. We use the mapped surface slip and fault geometry to infer the transfer of stress throughout the sequence. Calculations of the change in Coulomb failure stress reveal that nine out of 10 ruptures were brought closer to failure by the preceding shocks, typically by 1–10 bar, equivalent to 3–30 years of secular stressing. We translate the calculated stress changes into earthquake probability gains using an earthquake-nucleation constitutive relation, which includes both permanent and transient effects of the sudden stress changes. The transient effects of the stress changes dominate during the mean 10 yr period between triggering and subsequent rupturing shocks in the Anatolia sequence. The stress changes result in an average three-fold gain in the net earthquake probability during the decade after each event. Stress is calculated to be high today at several isolated sites along the fault. During the next 30 years, we estimate a 15 per cent probability of a M ≥ 6.7 earthquake east of the major eastern centre of Ercinzan, and a 12 per cent probability for a large event south of the major western port city of Izmit. Such stress-based probability calculations may thus be useful to assess and update earthquake hazards elsewhere.     

Palaeoseismicity of the Dinar fault, SW Turkey

The NW–SE-trending Dinar fault is an active normal fault upon which the 1 October 1995 earthquake ( M  = 6.1) occurred. The 1995 earthquake resulted in a c. 10-km-long surface rupture with the south side down-thrown by 50 cm. Investigations of two trench sites perpendicular to the 1995 rupture suggest at least two prior large earthquakes in historical times. Radiocarbon dates and historical records constrain the age of events between 1500 bc and ad 53, event 2 possibly coinciding with the earthquake that damaged Dinar (the ancient city of Apamea Kibotos) in c. 80 bc and event 1 around 1500 bc. Surface displacements determined for events 1 and 2, compared to the 1995 surface faulting, indicate that M > 6.8 earthquakes were associated with each rupture. Using the total displacement in trenches, a slip rate of about 1 mm yr⁻¹ can be estimated for the Dinar fault. Observations suggest that the return period for large earthquakes in the Dinar area is about 1500–2000 years.     

Tectonic interpretations of enigmatic structures in the North Anatolian fault zone

Two geometrically distinct groups of syn-sedimentary and post-depositional mesofaults and joints cut Neogene-Quaternary sediments in basins situated along the convex-northwards arc of the North Anatolian fault zone between Çerkes and Erbaa. One group comprises second-order fractures interpreted as having developed during episodes of right-lateral shear along the fault zone, while the morphologically identical fractures in the other group have been interpreted as secondary products of left-lateral shear; thus apparently implying one or more former episodes of eastwards motion of the Anatolian scholle. Because such a reversal of motion would be counter to the well-established westward escape of Anatolia the structures have been called anomalous or incompatible.Alternative hypotheses which have been advanced to explain the development of the anomalous mesofractures include: localized reversals related to displacements of rigid blocks acting as buttresses within basins; selective operation of intra-pull-apart strike-slip faults; stress release; the coincidence of the present western sector of the fault with an older left-lateral fault zone; and the influence of a North Turkish neotectonic stress regime.     

Opposed shear senses inferred from neotectonic mesofracture systems in the North Anatolian fault zone

The 1200-km long North Anatolian fault zone is a right-lateral, intracontinental transform boundary which was initiated in the Late Neogene. Sediments of Pliocene to Holocene age in basins between Cerkes and Erbaa, within the convex-northwards arc of the fault zone, are deformed by syn-sedimentary and post-depositional mesoscopic faults and joints. The mesofractures, which strike obliquely to the fault zone, include reverse faults, normal faults, normal shear joints, conjugate vertical joints and strike-slip faults. Each type of structure occurs in two geometrical groups, one comprises four systems of fractures, the other is made up of five systems. The directions of secondary compression and/or extension inferred from the first group of mesofractures, which are restricted to sediments of Pliocene to Early Pleistocene age, are interpreted as being related to left-lateral shear along the North Anatolian fault zone. The directions of compression and/or extension inferred from the second group of mesofractures, which cut sediments of Pliocene to late Holocene age, were generated during right-lateral shear.The presence of the second group of mesofractures is understandable because they are related to the shear sense which operates at the present-day, but those interpreted as being related to left-lateral shear are more puzzling: their development implies one or more reversals of the dominant sense of displacement. Several tentative models to explain such reversals are proposed, including regional and local influences, the latter related to mechanical constraints and/or the effects of other fault systems.     

The 1 October 1995 Dinar earthquake, SW Turkey

The Dinar earthquake (M_s= 6.1, USGS-PDE) of 1 October 1995 occurred on the NWSE-trending Dinar Fault. The earthquake is associated with a 10-km-long surface rupture with predominantly normal faulting. The mainshock was preceded by a series of foreshocks that started 6 days before the mainshock and included two M_d = 4.5 events. The mainshock source mechanism derived from the inversion of broad-band P waves revealed that two sub-events occurred on a NW-SE trending normal fault with a small strike-slip component. According to the source model estimated in this study, the first rupture started at a depth of about 8 km and reached to a depth of about 12 km propagating north-west. The total seismic moment found from the inversion of P waveforms is 2.0 times 10 ¹⁸ Nm. The seismic moment of the second sub-event was about four times larger than the first one. Field observations, GPS measurements and slip vector obtained from the inversion of broad-band P waveforms suggest that the NW-SE trending Dinar Fault is due to the internal deformation of SW Anatolia moving south-westwards.