Using DIAdem to Register Earthquakes at the Geological Survey of Northrhine-Westphalia

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"We chose DIAdem because DIAdem manages very large amounts of data very comfortably and we can do all the follow-up processing in one single program."

- Dr. Rolf Pelzing, geoinfo@gd.nrw.de

The Challenge:
The Geological Survey of Northrhine-Westphalia operates a seismic network of 13 stations in the Lower Rhine and neighboring areas. The registered data is sent per ISDN and DSL to the main office in Krefeld, where the data is examined and evaluated. For these large amounts of data an efficient software system is needed that provides all the steps involved in processing the data.

The Solution:
DIAdem meets all of the above requirements perfectly. One can view and select data in DIAdem VIEW, process the signals in DIAdem ANALYSIS, and display results quickly and easily in DIAdem REPORT. Scripts are used at all stages of the evaluation. Special seismological interpretation methods are integrated via DLLs.

Author(s):
Dr. Rolf Pelzing - geoinfo@gd.nrw.de

General

On the morning of July 22, 2002, many people in the Rhineland were jolted awake by an earthquake that reached 4.8 on the Richter scale. The epicenter was located near Alsdorf, which is north-east of Aachen. This earthquake came as no surprise to seismologists, because the Lower Rhine area is one of the most seismically active regions in Central Europe. A violent earthquake had already hit the area on April 13, 1992, reaching 5.9 on the Richter scale - the most severe earthquake to occur in over 250 years. More than 20 people were injured, and damage to property in the border region of Germany and the Netherlands amounted to 150 million euros. The vibration amplitudes during this quake were ten times higher than those recorded on July 22, 2002.

Many earthquakes have caused damage in this area over the past centuries. Earthquake damage was reported in Aachen as early as the times of Charlemagne. Based on descriptions of the damage that occurred, the earthquakes are estimated to have reached up to 6.5 on the Richter scale. For a more detailed examination of the seismicity in the Lower Rhine region and an improved classification of endangered areas, the Geological Survey of Northrhine-Westphalia established a seismic network in 1980, that currently consists of 13 stations. Over 1000 natural earthquakes have been recorded, most of which were too weak to be noticed by the local population. However, they show that in particular the south-west of the Lower Rhine area is seismically active and that serious earthquakes are to be expected in the future (Figure1).

Recording System

The actual measurement devices used at the stations are seismometers. The ground vibrations generate a relative movement between a magnet that is attached to a spring and a coil that is attached to the outer casing of the seismometer. This induces an electric voltage in the coil, which is proportional to the velocity of the ground movement. This information can then be used in DIAdem to calculate ground displacement and ground acceleration. Each seismic station has three seismometers that register vertical vibrations, north-south vibrations, and east-west vibrations. The seismometers at the stations in the center of the Lower Rhine area are mounted in boreholes at depths of up to 400 meters. The measurement signals are amplified by a factor of two hundred and transmitted to the surface over cables. The stations outside the Lower Rhine area are set up at the surface on solid rock and therefore require only a few meters of signal cable and no amplification. In both cases, the measurement signals are digitalized by hardware specially designed for earthquake registration. The signals are digitalized at a 24-bit resolution and a rate of 200 cycles per second, and then recorded on a computer hard drive in a format (I32) that is compatible to DIAdem. At stations that have particularly low seismic noise, only signals that exceed a specific trigger level are registered. The data is recorded with a 20-second delay to avoid missing the start of the signal. At stations that have a high degree of seismic noise, the signals are recorded continuously in two-minute blocks from which the relevant time ranges are later extracted.

The data is transmitted either daily via ISDN or continuously via DSL, from the stations to the main office in Krefeld, where it is evaluated. The DIAdem software system plays a key role here. In the first step of the evaluation, the data is visualized in DIAdem VIEW, allowing to distinguish between natural earthquake signals - from the Lower Rhine region and from other areas in the world - and artificial signals such as detonations, which are then discarded. Within a few minutes DIAdem REPORT converts a seismological registration into a publication-quality graphic. Figure 2 shows the seismogram of the earthquake in Alsdorf on July 22, 2002, as it appeared processed in DIAdem shortly after the event.

Processing Seismograms with DIAdem

Data that requires further processing may be filtered, either to eliminate distortion or to highlight frequency ranges that reflect key information. Correlating earthquake signals from various stations and correlating signals from several quakes at one station, produces important information about the position of the hypocenters and the time history of the seismicity. The frequency spectra of various wave groups are used to determine the time history of the rupture process at the seismic sources. DIAdem ANALYSIS executes all these evaluation steps quickly and easily, which is a huge improvement on the old system of having to use separate programs for each step, making it unnecessary to convert the data into different formats to match the various programs. Routine process steps such as filtering are saved as scripts. More extensive evaluations such as determining the magnitude values on the Richter scale and locating the seismic source using the records from the various stations, are executed using special programs developed mainly with Digital Fortran and Delphi, which can be integrated in DIAdem as DLLs. Figure 3 shows one example - the calculation of the seismic moment and from this the size of the rupture area, displacement, and moment magnitude. The moment magnitude is a new measure for the size of an earthquake. It is gradually replacing the Richter scale because it is physically better substantiated. Minor differences may occur between the two scales (0.1 units in this case).

Until now no reliable method has been developed for predicting earthquakes. No preliminary seismic activity was recorded before the earthquakes on April 13, 1992, and July 22, 2002. Nevertheless, earthquake registration is useful because we now have digital data that can be used for engineering seismological purposes. When buildings and technical plants are designed - especially if there is a secondary risk for the population from damages due to earthquakes (dams, chemical plants) - this data is used as vibration input signals for computer models of the buildings or plants, either as time series or as response spectra. Therefore, when earthquake data is registered and processed, it is important to use not only high-quality hardware, but high-quality software as well, to ensure fast, reliable interactive data processing. DIAdem, with its host of signal analysis and graphic display options, meets these requirements perfectly and is second to none on the software package market.

Summary

DIAdem is perfectly suited for the evaluation of the data registered at the seismic stations of the Geological Survey of Northrhine-Westphalia, because the program provides the entire range of data processing steps involved. The data is examined for relevant seismic events in DIAdem VIEW, the signals are analyzed in DIAdem ANALYSIS, and the results are graphically displayed in DIAdem REPORT. Special seismological evaluation methods can be integrated as DLLs. Scripts can be used to execute all the steps quickly and easily. In view of the large amounts of data acquired at the stations, being able to run all the required processing steps with one program is an enormous benefit.

Author Information:
Dr. Rolf Pelzing
geoinfo@gd.nrw.de
De-Greiff-Straße 195
D-47803 Krefeld
Germany

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