Ömer Alptekin

Spatial variations of the fractal properties of seismicity in the Anatolian fault zones

Abstract The Anatolian fault zones are seismically active strike-slip fault zones transcending the Anatolian plate in E-W and N-S directions. We investigate the spatial variations of seismicity along these zones in an attempt to investigate fault complexity along strike, quantified by the Gutenberg-Richter b -value and the fractal (correlation) dimension of earthquake epicentres, using the maximum likelihood method and the correlation integral, respectively. The investigation covers instrumentally recorded carthquakes of magnitude M > 4.5 occurring between 1900 and 1992. We find systematic spatial variations which may be related to structural or mechanical variability along strike. In particular the large change in strike at the northern apex of the North Anatolian Fault Zone is associated with the highest correlation dimension and lowest b -value for seismicity this century. The correlation dimension and b -value show a negative correlation with respect to each other, similar to results reported in other regional studies of Japan and southern California. This statistical correlation is stronger when more objective seismic zoning is carried out (based on number of events) rather than more subjective seismotectonic zoning in common use in seismic hazard analysis.

Temporal variations in the fractal properties of seismicity in the North Anatolian Fault Zone between 31°E and 41°E

We investigate the nature of temporal variations in the statistical properties of seismicity associated with the North Anatolian Fault Zone between longitudes 31°–41°E during the instrumental period 1900–1992. Temporal variations in the seismicb value and the fractal (correlation) dimensionDc of earthquake epicenters are examined for earthquakes of magnitudeMS≥4.5, using sliding windows of 100 consecutive events.b varies temporally between 0.6 and 1.0, andDc between 0.6 and 1.4, both representing significant fluctuations above the errors in measurement technique. A strong negative correlation (r=−0.85) is observed betweenb andDc, consistent with previous observation of seismicity in Japan and southern California. Major events early in this century (MS≥7) are associated with lowb and highDc, respectively consistent with greater stress intensity and greater spatial clustering of epicenters—both implying a greater degree of stress concentration at this time.

Failure stress change caused by the 1992 Erzincan Earthquake (Ms=6.8)

We calculated Coulomb failure stress change caused by the March 13, 1992 Erzincan, Turkey, earthquake, and explored the relationship between failure stress and the aftershock distribution which includes the Pulumur earthquake (Ms=5.8) that occurred two days later. One of the most significant features of the Erzincan earthquake was the location of aftershocks, which did not correspond with either the eastern segment of the North Anatolian fault zone or the Ovacik fault. This feature can be explained by mapping the failure stress due to the Erzincan earthquake. The map revealed that there is a significant correlation between the aftershock distribution and the areas where static stress was raised by ≥0.3 bar. The 1992 Erzincan earthquake raised the Coulomb failure stress about 1.4 bar at the site of the Pulumur event. This stress rise and optimum orientation of the Pulumur fault favoured its occurrence.

Effect of Aftershocks on Earthquake Hazard Estimation: An Example from the North Anatolian Fault Zone

In order to investigate the effect of aftershocks on earthquake hazard estimation, earthquake hazard parameters (λm, b and Mmax) have been estimated by the maximum likelihood method from the main shocks catalogue and the raw earthquakes catalogue for the North Anatolian Fault Zone (NAFZ). The main shocks catalogue has been compiled from the raw earthquake catalogue by eliminating the aftershocks using the window method. The raw earthquake catalogue consisted of instrumentally detected earthquakes between 1900 and 1992, and historical earthquakes that occurred between 1000–1900. For the events of the mainshock catalogue the Poisson process is valid and for the raw earthquake catalogue it does not fit. The paper demonstrates differences in the hazard outputs if on one hand the main catalogues and on the other hand the raw catalogue is used. The maximum likelihood method which allows the use of the mixed earthquake catalogue containing incomplete (historical) and complete (instrumental) earthquake data is used to determine the earthquake hazard parameters. The maximum regional magnitude (Mmax, the seismic activity rate (λm), the mean return period (R) and the b value of the magnitude-frequency relation have been estimated for the 24°–31° E, 31°–41° E, 41°–45° E sections of the North Anatolian Fault Zone from the raw earthquake catalogue and the main shocks catalogue. Our results indicate that inclusion of aftershocks changes the b value and the seismic activity rate λm depending on the proportion of aftershocks in a region while it does not significantly effect the value of the maximum regional magnitude since it is related to the maximum observed magnitude. These changes in the earthquake hazard parameters caused the return periods to be over- and underestimated for smaller and larger events, respectively.

A detailed source study of the Orta (Çankırı) earthquake of June 6, 2000 (MS = 6.1): An intraplate earthquake in central Anatolia

The devastating İzmit and Düzce earthquakes were followed by theOrta intra-plate earthquake (MS = 6.1) occurred in the central Anatolianblock on June 6, 2000. The focal mechanism, aftershock distribution andthe field studies (Emre et al., 2000) suggest a movement on a 21-km longDodurga fault striking nearly N-S where the sense of motion is left-lateralstrike-slip with considerable amount of normal component. We applied theconstrained linear finite-fault inversion method of Hartzell and Heaton(1983) to the teleseismic P and SH waveforms to derive a coseismic slipdistribution model for the earthquake. Time windows approach is appliedallowing variable rise times and rupture velocities. The source-rise timefunction is discretized into consecutive time intervals that stand for slipcontribution of individual subfaults. Although no clear surface ruptureswere associated with the earthquake, the resulting slip model suggestscoseismic slip in the order of several tens of centimetres. Our coseismicslip distribution model identifies two slip patches with the followingmaximum slip values: (1) the larger one (42 cm) is located to the southof the hypocenter at depth range of 4–8 km and (2) the smaller one(31 cm) is located just above and north of the hypocenter. Theslip-model yield a seismic moment of 1.0 × 1018 Nm, most of whichis released from the rupture over the depth of 8 km.