Revised for GJI, February 2000

The major asperities of the 1999 Mw=7.4 Izmit earthquake, defined by the microseismicity of the two decades before it

Ali Osman Öncel1 and Max Wyss2

1Department of Geophysical Engineering, University of İstanbul , Avcılar-Istanbul 34850, Turkey, 90-212-5938237/1037; oncel@hotmail.com.

2Geophysical Institute, University of Alaska, Fairbanks, AK 99775, United States, 907-474-5529; max@giseis.alaska.edu.

SUMMARY

We compare the rupture location of the Mw7.4 Izmit earthquake to the local seismic hazard estimated by the technique of mapping local recurrence time, TL, based on the microseismicity. After correcting for a magnitude shift in 1990, the declustered earthquake catalogue, produced by the University of Istanbul for the Marmara sea region, is homogeneous for Md³ 2.9 during 1983-1999. We mapped TL in the area bounded by 40°-41° latitude and 27.6°-30.5° longitude. TL is the probabilistic estimate of recurrence time, calculated from the a- and b-values of the frequency-magnitude relation of the seismicity within a radius of 20 km from every point on a grid with 5 km spacing. TL varies strongly as a function of space, since a- and b-values also vary strongly. In our interpretation, the 5% to 20% of locations with the shortest recurrence times map major asperities. In the Marmara region, we mapped four anomalies of short TL, covering together about 12% of the total area. They are centred near 40.25°/29.4°, 40.8°/28.3°, 40.75°/28.8° and 40.7°/29.8°. The last two of these coincide with the western end of the rupture and epicentre location of the Izmit earthquake, respectively. Thus, we suggest that the major asperity of this rupture and a point, past which it could not propagate, were mapped out by the background seismicity during years before the event as locations that produced more large microearthquakes than average, and hence showed anomalously short TL. The TL method does not contain information about when earthquakes are expected, and the absolute values of the recurrence time could be inaccurate. The method only specifies the most likely locations of main shocks. Since the method is new, it will have to be tested in many cases and areas before its reliability can be assessed.

 

INTRODUCTION

Recently, the technique of mapping local recurrence time (TL(M)) was successfully used to estimate locations of asperities, based on the earthquakes within r=const of the location in question (5£ r£ 20 km) (Wiemer and Wyss, 1997; Wyss et al., 1999; Zuniga and Wyss, 2000). TL can be estimated from the parameters of log-linear scaling:

 

log N= a - b M (1)

 

where N is the number of earthquakes with magnitude M and larger over the observation period dT, by

 

TL(M) = dT/10(a-bM) (2)

 

Wiemer and Wyss (1997) argued that asperities are the only segments of faults that may contain information about the frequency of ruptures in main shocks, because they control the time of rupture. For this reason, the recurrence time should be mapped as a local parameter. Bulk values of recurrence time, derived from asperities plus passive fault segments that slide along with the asperity when the latter ruptures, are incorrect, according to this hypothesis.

Size scaling parameters of seismicity (a and b) in the space and time domains can be affected by several physical factors, such as material heterogeneity and applied shear stress level (Mogi, 1962; Scholz, 1968; Wyss, 1973; Urbancic et al., 1992) or thermal gradient (Warren, 1970). However, only recently has it become clear that b also varies strongly depending upon clustering degree of seismicity and fracture (e.g., Öncel et al., 1995, 1999) over distances of a few kilometres (Ogata and Katsura, 1993; Wiemer and Benoit, 1996; Wiemer and McNutt, 1997; Wiemer and Wyss, 1997), although the average b-value in large volumes tends to show constant values near 1 (Frohlich and Davis, 1993; Kagan and Jackson, 1991).

The creeping part of the San Andreas fault north of Parkfield shows anomalously high b-values (Amelung and King, 1997; Wiemer and Wyss, 1997), whereas the segment beneath Middle Mountain, which is known as an asperity, shows the lowest b-values in the 150 km long fault segment including it (Wiemer and Wyss, 1997). In addition the asperities of the Morgan Hill M6 earthquake (Wiemer and Wyss, 1997) and the Anza seismic gap in the San Jacinto fault zone (Wyss et al., 2000) are locations of anomalously low b-values. These anomalies become even more pronounced in maps of TL, where they constitute about 10% of the areas mapped, and where TL is substantially shorter than in other fault segments.

Thus we further investigate in this paper the possibility that local changes of TL(M) are related to the rate of activity (a parameter) and stress (b parameter) and may be used to map asperities. We define asperities as segments of fault planes resisting faulting more than its surrounding, as it is generally used in models for earthquake rupture (e.g. Wyss and Brune, 1967; Lay et al., 1982; Aki, 1984). The bulk a- and b-values of the main shock rupture areas lead often to over-estimates of the recurrence time for main shocks, whereas the asperities are found to exhibit the lowest values of b (b» 0.5) and the lowest TL which agree better with the historically observed recurrence times (Wiemer and Wyss, 1997; Öncel and Alptekin, 1999).

The most important aspect of our mapping of TL is the contrast between the less than 10% of the area that show anomalously short recurrence times and the rest. The absolute values of the recurrence time is only of secondary interest, because it may not be very reliable at this stage of our understanding of the problem. The model for the short TL anomalies that we have in mind is the following. Fault segments with anomalously short TL are relatively active segments in which the mean magnitude resulting from fractures is larger than along the rest of the fault. Thus these segments are singled out as having different physical properties, and we propose they are asperities. These asperities control the time of rupture and surrounding segments of lower strength rupture along with the asperities relatively passively. As strong spots, the asperities can inhibit rupture propagation when they are not highly loaded with stress, but once they are loaded, their rupture leads to a major earthquake. The rupture of such a major earthquake is most likely to initiate within an asperity, but it can also happen that a rupture that is initiated outside but near the asperity can propagate into the asperity and trigger the event. In our hypothesis, the asperities do not contain information on the maximum size to which a rupture involving it may grow. The minimum magnitude is given by the area of the asperity itself as the earthquake that would result if the rupture stops at the edge of the asperity. However, the maximum magnitude is unknown, because the rupture can reach another asperity and beyond this one yet another one that is loaded with stress.

Based on this model, we expect that a major asperity should be located at or near the epicentre (rupture initiation) of large earthquakes. Also, large ruptures likely, but not necessarily, may end at asperities. Finally, one or several additional asperities may be located within a multiple-event-rupture of a large earthquake. Although a great earthquake might be generated by connecting a large number of moderate size asperities, it is probable that the maximum size of asperities scales in most cases approximately with the largest main shock a fault is capable of. In a preliminary investigation we found that the ratio of the lengths of a main shock and its major asperity is about 3 to 4 (Wyss et al., 1997). Thus, we estimate that the size of major asperities in the case of the Izmit earthquake may be about 25 km. To map such features crisply, it would be desirable to have a data density that would allow sampling with radii of 10 km. However, mapping TL anomalies corresponding to the expected asperity size using larger radii for sampling (r=20 km, as we are force to use here) is still possible, although not optimal.

The seismicity of the Marmara sea region is strongly related to the tectonic characteristics of the North Anatolian Fault Zone (NAFZ) (Crampin and Üçer, 1975, Öncel and Alptekin, 1995). This region is located in a transition zone between Eurasia and NW Anatolia (Straub and Kahle, 1995). Details of the fault geometry associated with large destructive earthquakes along the NAFZ and their relationship to spatial and temporal patterns of seismic clustering are not well understood (Ambraseys and Finkel, 1991). The comprehensive evaluation of spatial attributes of earthquake clustering in this region may define the evolution of asperities and provide important information relevant to earthquake hazard assessment for the cities such as Istanbul in the Marmara Sea Region. The main purposes of this paper are to 1) evaluate the size scaling parameters of seismicity (a and b) in the Marmara sea region and 2) to determine local variations of earthquake recurrence correlating in some way to locations of asperities that might point to potential rupture initiation or termination in future large earthquakes.

 

DATA

The raw earthquake catalogue was compiled by the Kandilli Observatory and the Earthquake Research Institute (KOERI), augmented by the Turkish national network and by local seismic networks, with good station coverage particularly since 1970 (Öncel et al., 1995). The map of seismicity in Figure 1 indicates that the regional seismicity is controlled by strong clusters along the Marmara sea region; especially at the location where the recent Izmit earthquake occurred in the eastern part of the region. The frequency-magnitude relation in Figure 2 shows that the data are complete for the events of Md³ 2.9.

After declustering, using the algorithm of Reasenberg (1985) programmed in ZMAP (Wiemer, 1996), and correcting for a magnitude shift in 1990, the cumulative number of earthquakes reported in the catalogue for the Marmara sea region (http://www.angelfire.com/al/geophysics) (Öncel and Alptekin, 1999) shows a constant slope with time (Figure 3). We interpret this to mean that the reporting was homogeneous since 1983, if the magnitude correction is applied.

 

MAPPING b-VALUES AND LOCAL RECURRENCE TIME

For mapping b and TL we used spatial subdivisions along the Marmara Sea (Figure 1) consisting of fixed cylindrical volumes with radius r equal to 20 km and height h equal to 40 km. The centres of the cylindrical volumes are positioned at nodes with 5 km spacing throughout the region. The parameters a and b are then estimated from the events with M³ Mc for each volume by the gridding technique introduced by Wiemer (1996). Mapping of TL(M) from the resulting matrix of a- and b-values can be computed from equation (2) for a given magnitude of the expected main shock. All of the computations are programmed in ZMAP, a computer code introduced by Wiemer (1996) and expanded continuously since by this author.

The b-value map, resulting from the procedure outlined above for the Marmara sea in NW Turkey, is shown in Figure 4a. One can see that, on average, b=1.3, but locally the values range from 0.9 to 1.6. Clearly, a strong heterogeneity of b-values exists in the region. The a value map in Figure 4b shows the varying activity in the region.

The map of local recurrence time (Figure 4c) calculated by equation (2) must be based on a main shock magnitude on the same scale as the earthquake catalog. This is the MD scale, on which the Izmit Mw=7.4 earthquake measures MD=6.7 according to the Kandilli observatory.

The TL map shows similar patterns as the b-value map, but it sharpens the contrasts. We identify four areas of anomalously low TL. (1) Area I, a segment along the Izmit bay, between 29.5° and 30.0° longitude, which is located adjacent to the moment centroid epicenter. In this area, TL is estimated as about 1000 to 1500 years. (2) Area II, a segment of strongly anomalous TL, coincides with the westernmost end of the 1999 Izmit rupture. (3) Area III, located at longitude 28.3° along the northern branch of the north Anatolian fault, is a weaker anomaly with estimates of about 500 to 1500 years for TL. (4) Area IV is located between longitude 29.0° and 29.5° and has the shortest TL estimated of about 500 years. It coincides with the largest earthquake in the catalog during the 17 years before the Izmit earthquake of August 1999.

 

DISCUSSION AND CONCLUSION

Recently, the question has been discussed if the background seismicity during inter-main shock times could provide clues about the location of past and future main shocks (Wyss et al., 1999). The occurrence of the Mw7.4 Izmit earthquake allows a test if the mapping of anomalous areas by the TL–technique can identify asperities that may play a key role in ruptures along the NAFZ. The Marmara sea region is one of the seismically best monitored parts of Turkey, thus, the data allow relatively detailed mapping of TL. During the period of the modern earthquake catalogue (1983-1999), no earthquakes with Md³ 5.6 occurred, and hence no such events, nor their aftershock sequences contaminate the data on b- and a-values. Thus, the data set available is adequate to determine if any parts of the 1999 Izmit rupture could have been identified as special fault segments before the event, based on the last two decades of seismicity.

We interpret four areas with significantly low b-value and anomalously short local recurrence times along the Marmara Sea Region as volumes with relatively high stress regimes, that is, as asperities. Area I in Figure 4c corresponds to the central part of the 1999 rupture zone and includes at its eastern edge the epicentral location of the moment tensor solution of the USGS. This suggests that the rupture initiated at the edge of an asperity recognizable by the TL method.

The length of the 1999 aftershock zone in Figure 4c is about 220 km estimated from the distribution of aftershocks with magnitude of Md>3, compiled by the Kandilli Observatory. The aftershocks extend from about 29° to 31° , with a strongly active cluster at its western end, at the edge of the anomalous area II (Figure 4c). The most pronounced part of this TL-anomaly is located west of the end of the aftershocks. We interpret this as indicating that here the rupture stopped at an asperity, centred at 28.8° , that did not break though.

The low b-values responsible for this anomaly, as well as for the others discussed in the following, are not due to incomplete reporting of small events. Not only did we map the minimum magnitude of completeness and cut the catalogue at a conservatively high level of Mc=2.9, but the ZMAP program also estimates Mc for every sample in constructing the b- and TL-maps, using only the data above the local Mc. As a final measure of quality control, we visually check the FMDs of all areas showing anomalies, to be sure that the program’s estimates of b and Mc agree with the estimates an observer would obtain manually.

The importance of area II is also underscored by the very local increase of the Coulomb stress change due to a nearby earthquake in 1963 (Plate 1d of Nalbant et al., 1998). In addition, the M4.4 earthquake of October 21, 1999 (square in Figures 4), with its five aftershocks, indicates that stresses may be high in area II. The event’s magnitude was small but was felt strongly in the Marmara region and especially in Istanbul. It follows that a future rupture along the NAFZ may be likely to initiate at 28.8° E, and propagate westward from there.

The data are too sparse for estimates of TL in the segment between 28.4° and 28.8° E (Figure 4c). Several historic main shocks occurred within this low-seismicity segment. Further west than 28.4° we recognize another anomalous volume for TL, area III (Figure 4c). This anomaly is located in a segment without historic main shock epicentres. Nevertheless, we propose that the anomaly III may play a significant role in future ruptures. Of course, the representation of a historic earthquake of M7 class by a single point is inadequate to discuss its possible relationship to the potential asperity segments we are trying to map. Thus, we cannot say what role area III may have played in ruptures in the past.

The TL-anomaly in area IV is the only one we found off the northern strand of the NAFZ. It is the most pronounced anomaly of short recurrence times and the largest earthquake in the data set (a M5.5 event in 1983) is located within it (Figure 4c). Also, this volume is currently seismically quiet at a highly significant statistical confidence level. However, both of these observations, the short TL and the quiescence, could be due to the 1983 moderate magnitude earthquake. This earthquake may have increased the seismicity for a few years after it and now it returned to a low level that may be normal. Although the location near 40.15° /29.33° attracts attention, we are hesitant to interpret this as an indication that a main shock is to be expected, because the observed recent changes can be interpreted as a consequence of the 1983 M5.5 earthquake.

We have shown that mapping the b-value, together with estimates of local recurrence time in the Marmara sea region identified the segment of the NAFZ near the centroid location and the western end of the Izmit M7.4 earthquake. This identification is based on the roughly two decades of seismicity before the 1999 Izmit event and had no input from the aftershock sequence. Therefore, we suggest that the mapping of TL provides an important clue for the identification of fault segments which may initiate or stop future large ruptures.

Given the fact that the migration of large earthquakes along the NAFZ from east to west is well established (Ambraseys, 1970; Toksöz et al, 1979, Barka, 1992, Armijo et al, 1999), and given that stress coupling is observed between the previous instrumental events in the region (Nalbant et al., 1998), we suggest that the areas II and III with anomalously short TL are likely locations for future main shocks, or one rupture that may connect to two locations. If the latter should happen then the rupture length would be about 100 km, reaching from about longitude 28° to 29° along the NAFZ in the Marmara sea.

Since the method of using local recurrence time to map asperities is not extensively tested yet, our conjectures about the possibility of future earthquakes should be viewed with caution. Also, we have no way of knowing when future events in the asperities II, III and IV should be expected.

ACKNOWLEDGMENTS

We thank S. Wiemer for making available his computer code ZMAP, P. Bernard as well as S. Gross for helpful critical reviews and Ian Main for helpful discussion. This work was supported by the Research Fund of the Istanbul University under the project number1038/250897 and AIST in Japan, as well as NSF grant EAR 9902717 and the Wadati foundation at the University of Alaska Fairbanks.

 

REFERENCES

Aki, K., 1984. Asperities, barriers, characteristic earthquakes and strong motion prediction, J. Geophys. Res., 89, 5867-5872.

Ambraseys, N.N. and C.F. Finkel, 1991. Long-term seismicity of Istanbul and the Marmara Sea Region, Terra Nova, 3, 527-539.

Ambraseys, N. N., 1970. Some characteristic features of the North Anatolian Fault Zone, Tectonophys ., 9, 143-165.

Amelung, F., and G. King, 1997. Earthquake scaling laws for creeping and non-creeping faults, Geophys. Res. Lett., 24, 507-510.

Armijo, R., B. Meyer, A. Hubert, and A. Barka, 1999. Westwards Propagation of the North Anatolian Fault into the Northern Aegean: Timing and kinematics, Geology, 27, 3, 267-270.

Barka, A., 1992. The North Anatolian fault zone, Annales Tectonicae, 6, 164-195.

Crampin, S. and S.B. Üçer, 1975. The seismicity of the Marmara Sea Region of Turkey, Geophys. J. R. Astr. Soc., 40, 269-288.

Frohlich, C., and S. Davis, 1993. Teleseismic b-Values: Or, Much Ado about 1.0, J. Geophys. Res., 98, 631-644.

Kagan, Y.Y., and D.D. Jackson, 1991. Long-term earthquake clustering, Geophys. J. Int., 104, 117-133.

Lay, T., H. Kanamori, and L. Ruff, 1982. The asperity model and the nature of large subduction zone earthquakes, Earthq. Pred. Res., 1, 3-72.

Mogi, K., 1962. Magnitude-frequency relation for elastic shocks accompanying fractures of various materials and some related problems in earthquakes, Bull. Earthq. Res. Inst., Univ. of Tokyo, 40, 831-853.

Nalbant, S., Hubert, A., King, G., 1998. Stress coupling between earthquakes in northwest Turkey and the north Agean sea, J. Geophys. Res., 103, 24,469-24,486.

Ogata, Y., and K. Katsura, 1993. Analysis of temporal and spatial heterogeneity of magnitude frequency distribution inferred from earthquake catalogues, Geophys. J. Int., 113, 727-738.

Öncel, A.O., Ö. Alptekin, 1999. Microseismicity and Seismic Hazard of Marmara Region, Project number 1038/250897, founded by Istanbul University.

Öncel, A.O., I. Main, Ö. Alptekin, P. Cowie, 1996a. Spatial variations of the fractal properties of seismicity in the Anatolian Fault Zones, Tectonophys., 257, 189-202.

Öncel, A.O., I. Main, Ö. Alptekin, P. Cowie, 1996b. Temporal variations of the fractal properties of seismicity in the north Anatolian fault zone between 31°E and 41°E, Pure and Appl. Geophys. 146, 147-159.

Öncel, A.O., Ö. Alptekin, I. Main, 1995. Temporal variations of the fractal properties of seismicity in the western part of the north Anatolian fault zone: possible artefacts due to improvements in station coverage, Nonlinear Processes in Geophysics, 2, 147-157.

Öncel, A.O., Alptekin, Ö., 1995. Earthquake risk in Marmara Sea, Proceedings of Turkish National Union of Geodesy and Geophysics.General Assembly, 3, 981-989.

Öncel, A.O., H.T. Wilson, O. Nishizawa, 1999. Size scaling relationships in the active fault networks of Japan and their correlation with Gutenberg-Richter b values, J. Geophys. Res., submitted.

Öncel, A.O., Ö. Alptekin, 1999. Effect of aftershocks on earthquake hazard estimation: An example from the north Anatolian fault zone, Natural Hazards, 19, 1-11.

Reasenberg, P.A., 1985. Second-order moment of Central California Seismicity, J. Geophys. Res., 90, 5479-5495.

Scholz, C.H., 1968. The Frequency-Magnitude Relation of Microfracturing in Rock and its Relation to Earthquakes, Bull. Seism. Soc. Am., 58, 399-415.

Straub, C and Kahle, H., 1995. Active crustal deformation in the Marmara Sea Region, NW Anatolia, inferred from GPS measurements, Geophy. Res. Lett., 22, 2533-2536.

Toksöz, M.N., A.F., Shakal, A.J. Michael, 1979. Space-time migration of earthquakes along the North Anatolian fault zone and seismicity gabs, Pageoph, 117, 1258-1270.

Urbancic, T.I., C.I. Trifu, J.M. Long, and R.P. Young, 1992. Space-time correlations of b-values with stress release, Pageoph, 139, 449-462.

Warren, N.W., and G.V. Latham, 1970. An Experimental Study of Thermally Induced Microfracturing and its Relation to Volcanic Seismicity, J. Geophys. Res, 75, 4455-4464.

Wiemer, S., 1996. Analysis of seismicity: New techniques and case studies, Dissertation thesis, University of Alaska, Fairbanks, Alaska.

Wiemer, S., and J. Benoit, 1996. Mapping the b-value anomaly at 100 km depth in the Alaska and New Zealand subduction zones, Geophys. Res. Letts., 23, 1557-1560.

Wiemer, S., and S. McNutt, 1997. Variations in frequency-magnitude distribution with depth in two volcanic areas: Mount St. Helens, Washington, and Mt. Spurr, Alaska, Geophys. Res. Letts., 24, 189-192.

Wiemer, S., S.R. McNutt, and M. Wyss, 1998. Temporal and three-dimensional spatial analysis of the frequency-magnitude distribution near Long Valley caldera, California, Geophys. J. Int., 134, 409 - 421.

Wyss, M., and J.N. Brune, 1967. The Alaska earthquake of 28 March 1964: A complex multiple rupture, Bull. Seismol. Soc. Am., 81, 1231-1254.

Wiemer, S., and M. Wyss, 1997. Mapping the frequency-magnitude distribution in asperities: An improved technique to calculate recurrence times?, J. Geophys. Res., 102, 15115-15128.

Wyss, M., 1973. Towards a physical understanding of the earthquake frequency distribution, Geophys. J. R. Astron. Soc., 31, 341-359.

Wyss, M., and S. Wiemer, 1997. Two current seismic quiescences within 40 km of Tokyo, Geophys. J. Int., 128, 459-473.

Wyss, M., D. Schorlemmer, and S. Wiemer, 2000. Mapping asperities by minima of local recurrence time: The San Jacinto-Elsinore fault zones, J. Geophys. Res., 103, in press.

Zuniga, R., and M. Wyss, 2000. Most and Least Likely Locations of Large to Great Earthquakes Along the Pacific Coast of Mexico, Estimated from Local Recurrence Times Based on b-values, J. Geophys. Res., 103, in press.

FIGURE CAPTIONS

Figure 1: Epicenter map of the Marmara sea region for earthquakes with M ³ 2.9 between 1983 and 1998.

Figure 2: Cumulative numbers of earthquakes, excluding clusters, as a function of magnitude for the Marmara sea region during the period 1983-1998 (MD³ 2.9.) The minimum magnitude of completeness is 2.9.

Figure 3: Cumulative numbers of earthquakes with magnitude of MD³ 2.9 as a function of time for Marmara sea region, corrected for a magnitude shift of dM=0.3 in 1990. The relatively constant and smooth slope suggests that this catalogue can be accepted as homogeneous.

Figure 4: (a) Map of the local b-values in the Marmara sea region and the source volume of the MS=7.4 Izmit earthquake of August 1999 (star). The node spacing of the grid was 5 km, the radius for sampling earthquakes was 20 km and the minimum number of earthquakes required for an estimate was 50. (b) Map of a-values with parameters the same as in (a). Small circles mark the epicentres of 1999 aftershocks, large circles indicate epicentres of historic earthquakes (Ambraseys and Finkel, 1991) for the periods and with the magnitudes indicated in the legend. (c) Map of local recurrence times, TL, estimated probabilistically for an MS=7.4 (equal to MD=6.7) main shock, using the b- and a-values from Figures 4a and 4b, respectively. Four areas of anomalously short TL are labelled.