Rapid Source Parameter Determination of the
August 17, 1999, Izmit Earthquake at GFZ Potsdam
Günter Bock
| GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany . |
Introduction -
Data Collection -
Source Duration and Focal Depth
Inversion of Source Mechanism -
References
Introduction
In this note, I report about the source parameter determination of the Izmit eartquake as it was done at GFZ in the morning of August 17, 1999. The results were distributed via electronic mail to customers of the European-Mediterranean Seismological Centre (EMSC). Two messages were disseminated. The first one at 08:08 UTC was based only on data from 5 stations, for the second one posted at 10:17 UTC, about 10 hours after the event, data from 12 stations could be used. In the following I shall briefly describe the procedure, the problems that arose on this day and present seismogram examples that illustrate various parameters of the solution.
Fig. 1. Distribution of stations available in GEOFON Spyder® online data pool on the morning after the August 17, 1999, Izmit eartquake whose epicentral location is depicted by the red star.
Data Collection
The rapid source parameter determination at GFZ makes use of waveforms that are made availble in the global Spyder®, usually within a few hours after the event. Within Europe Spyder® data is available from GEOFON and ORFEUSAutomatic data retrieval by the Spyder® system is triggered by alarm messages that are received from NEIC. As part of testing a regional Spyder® system set up for eartquakes in the European-Mediterranean area, several alarm messages of the Turkey event were received from EMSC in addition to the one of NEIC. As a result, the Spyder® retrieval system got stalled and had to be restarted manually in the morning of August 17. So we faced the problem in the morning of August 17 that only relatively few data had become available for a source parameter inversion, and most of these data were from stations at regional distances. Most data from teleseismic distances (D ³ 30°) started to flow into the online datapool in the afternoon beginning at 17:31 UTC.
The stations for which data had become available are depicted in Fig. 1. Most of the stations lie in the European area. Stations that were not suitable for the source mechanism inversion are plotted as red dots, however, some of these stations had useful vertical-component waveforms that were used to constrain focal depth and/or source duration.
Source Duration and Focal Depth
As a first step in the inversion an attempt is made to constrain focal depth and source duration. This is done by visual inspection of broadband waveforms. Examples are shown in Figs 2-3.
2. Comparison of observed P wave displacement seismogram (blue) observed at WET (D = 14.76°) with synthetics for a source duration of 20 s and a variety of focal depths. The red synthetic is for h=15 km.
Shown in Fig. 2 is the vertical-component displacement seismogram of the P wave recorded at the GRSN station Wettzell (WET). It illustrates the way of how focal depth is constrained by interactive interpretation of depth phases. The interpretation is shown by the picks of P and pP phases in the bottom trace. Comparison of the observed blue waveform with synthetics calculated with the reflectivity method for a range of focal depths is shown in the middle and upper part of Fig. 2. The red trace is the synthetic for a focal depth of 15 km which was later adopted in the source mechanism inversion. The evaluation of depth phases for all stations gave a mean depth of 17 km.
Fig. 3 illustrates the way of how an estimate of source duration is obtained. This again is done interactively on P wave displacement seismograms. The duration of the P wave pulse is taken as a measure for the source duration. We use the formula given by Brüstle and Müller (1983) which approximates the moment release of a point source by a half-sinusoid from 0 to its final value M0. The far-field displacement is proportional to the time derivative of this source-time function which has the form of a half cosine function. This is a very simple model of a source-time function which does not account for any complexities that may arise from multiple sources. Also, directivity effects are not considered. The approach is very subjective, but the interpreter can be supported by displaying reflectivity synthetics for a variety of source durations and comparing them with the observed seismogram as shown in Fig. 3. In the example shown the measured P wave duration was 16.5 s; the average from all observations was 18 s.
Fig. 3. Comparison of P wave displacement seismogram recorded at PAB (San Pablo, Spain, D = 26.14°) with synthetics calculated for 10 km focal depth and a variety of source durations. The »20 sec« synthetic (in red) is a good approximation for the Izmit earthquake.
Inversion for Source Mechanism
A non-linear grid search algorithm is used which minimizes the difference between observed and theoretical P/S amplitude ratios. The method has been described at greater detail by Bock (1993) and Bock et al. (1994). It resembles in many ways the relative amplitude method described by Pearce (1977). A brief account of the method can be found at the GFZ web site. The peak-to-peak amplitude of the P wave is measured on the vertical component and that of the S wave on the vertical, radial- and tangential-horizontal components. Amplitudes are measured over a full wavelength so that the estimate may contain besides P also pP and sP in case of shallow events. The time windows over which peak-to-peak P and S wave amplitudes were measured are indicated for station PAB in Fig. 4.
Fig. 4. Example of picking P and S wave amplitudes.
The P/S amplitude ratios are the entry parameters for the grid search algorithm. Observed amplitude ratios are compared with synthetic ratios obtained with the reflectivity method. Reflectivity synthetics are stored for distances up to 80° and 5 km steps in focal depth. This explains the fact that focal depths adopted in the inversion are rounded to the nearest 5 km interval. First, a rough search is conducted in 10° interval for strike, dip and slip of a double couple source. For the Izmit earthquake and other events in the EMSC area the results of the inversion are published at a GFZ web page. This page displays a table of events with links to the files containing the detailed description of source parameters. For reasons outlined at the beginning of this article, the list of stations used in the source mechanism inversion contains only stations at regional distances. This may explain the relatively large value for the moment magnitude as compared to Harvard (7.5) and USGS (7.4). It also illustrates the need to compare observed amplitude ratios with reflectivity synthetics to account for wave propagation through the upper mantle.
Observed and synthetic waveforms are compared in Fig. 5. The station distribution is far from ideal as data are from one quadrant in azimuth only. Despite the fact that there are discrepancies between synthetics and observations, the overall P/S amplitude ratios are well matched by the proposed solution. The overall error of the focal mechanism is estimated to be about 15° for strike, dip and rake based on the distribution of the misfit function. Later modelling of broadband waveforms using the dataset distributed by IRIS provided better constraints on the source-time function and spatial extent of the Izmit event (Bock et al., 1999). In particular, we believe that the main event was followed by two more subevents to the east of rupture onset. These two events show up in Fig. 3 at about 30 s and 42 s relative to the centroid time of the main event.
References
Bock, G., 1993. The Woods Reef (New South Wales) earthquake of 14 November 1990: Focal mechanism derived from amplitude ratios and synthetic seismograms, Australian Journal of Earth Sciences, 40, 360-376.Bock, G., Hanka, W. and Kind, R., 1994. EMSC rapid source parameter determination, EMSC Newsletter, 6, 2-4.
Bock, G., Tibi, R., Baumbach, M., Grosser, H., Milkereit, C., Kind, R. And Zschau, J., 1999. Rupture process of the Great Izmit (Turkey) earthquake of August 17, 1999, Invited poster, AGU Fall meeting, December 1999.
Pearce, R. G., 1977. Fault plane solutions using relative amplitudes of P and pP, Geophys. J. R. Astr. Soc., 50, 381-394.