Hans-Peter Harjes

Injection‐induced earthquakes and crustal stress at 9 km depth at the KTB deep drilling site, Germany

A fluid injection-induced seismicity experiment was conducted in the German Continental Deep Drilling Program (KTB) main borehole at 9.1 km depth (in situ temperature of 260°C) to extend knowledge about stress magnitudes and brittle faulting to depths and temperatures approaching the brittle-ductile transition. Almost 400 microearthquakes were induced at an average depth of 8.8 km by injection of KBr/KCl brine into a ∼70 m open hole section near the bottom of the borehole. Although most focal plane mechanisms were poorly constrained due to the very small size of the induced earthquakes, several different clusters of microearthquakes with distinct mechanisms were defined. Most of the microearthquakes for which focal plane mechanisms were determined were strike-slip events with a NNW trending P axis, essentially parallel to the direction of maximum horizontal compression observed in the borehole. The largest induced earthquake, M 1.2, occurred 18 hours after injection was started. This event was a strike-slip/reverse faulting event which also had a NNW trending P axis. Utilization of a precise relative location technique indicates that many of microearthquakes occurred relatively far (50–100 m) from the well bore. Modeling of the pore pressure disturbance caused by injection suggests that many of the earthquakes were induced by extremely small pore pressure perturbations (<1 MPa) less than 1% greater than the ambient, approximately hydrostatic pore pressure at depth. Thus it is apparent that there are critically stressed, permeable fault zones in the crust, even at great depth and temperature. A frictional analysis of the focal plane mechanisms of the induced microearthquakes indicates that fault slip is consistent with the stress magnitudes and orientations determined in situ at depths to 7.7 km in the borehole and relatively high coefficients of friction (∼0.6–0.7) reported by Brudy et al. [this issue]. This and the observation that very small pore pressure perturbations were able to trigger seismicity appear to confirm the hypothesis that “Byerlees law” (i.e., that differential stresses in situ are limited by the frictional strength of well-oriented, preexisting faults) is valid to great crustal depth and that the crust is in brittle failure equilibrium at depths and temperatures approaching the brittle-ductile transition, even in this relatively stable intraplate area.

Lithospheric structure of the Aegean obtained from P and S receiver functions

Combined P and S receiver functions from seismograms of teleseismic events recorded at 65 temporary and permanent stations in the Aegean region are used to map the geometry of the subducted African and the overriding Aegean plates. We image the Moho of the subducting African plate at depths ranging from 40 km beneath southern Crete and the western Peloponnesus to 160 km beneath the volcanic arc and 220 km beneath northern Greece. However, the dip of the Moho of the subducting African plate is shallower beneath the Peloponnesus than beneath Crete and Rhodes and flattens out beneath the northern Aegean. Observed P-to-S conversions at stations located in the forearc indicate a reversed velocity contrast at the Moho boundary of the Aegean plate, whereas this boundary is observed as a normal velocity contrast by the S-to-P conversions. Our modeling suggests that the presence of a large amount of serpentinite (more than 30%) in the forearc mantle wedge, which generally occurs in the subduction zones, may be the reason for the reverse sign of the P-to-S conversion coefficient. Moho depths for the Aegean plate show that the southern part of the Aegean (crustal thickness of 20–22 km) has been strongly influenced by extension, while the northern Aegean Sea, which at present undergoes the highest crustal deformation, shows a relatively thicker crust (25–28 km). This may imply a recent initiation of the present kinematics in the Aegean. Western Greece (crustal thickness of 32–40 km) is unaffected by the recent extension but underwent crustal thickening during the Hellenides Mountains building event. The depths of the Aegean Moho beneath the margin of the Peloponnesus and Crete (25–28 and 25–33 km, respectively) show that these areas are also likely to be affected by the Aegean extension, even though the Cyclades (crustal thickness of 26–30 km) were not significantly involved in this episode. The Aegean lithosphere-asthenosphere boundary (LAB) mapped with S receiver functions is about 150 km deep beneath mainland Greece, whereas the LAB of the subducted African plate dips from 100 km beneath Crete and the southern Aegean Sea to about 225 km under the volcanic arc. This implies a thickness of 60–65 km for the subducted African lithosphere, suggesting that the Aegean lithosphere was not significantly affected by the extensional process associated with the exhumation of metamorphic core complexes in the Cyclades.

Probing the Crust to 9-km Depth: Fluid-Injection Experiments and Induced Seismicity at the KTB Superdeep Drilling Hole, Germany

A 60-day, long-term fluid-injection experiment was performed at the 9.1-km-deep Kontinentale Tiefbohrung, Germany (KTB), borehole. About 4000 m3 of water were injected into the well head to induce seismicity near the open-hole section at 9-km depth. Because of several leaks in the borehole casing (unknown before), seismicity occurred at distinct depth levels between 3-km and 9-km depth. Two events occurred at 10-km and 15-km depth. The combination of a temporary, 40-element, three-component surface network of seismometers and a three-component downhole sonde at 3.8-km depth in the nearby pilot hole enabled us to determine absolute hypocenter locations by using a velocity model that was calibrated from several downhole shots at depths of 5.4 km and 8.5 km. Of a total of 2799 induced events, hypocenter locations were obtained for 237 events having good signal-to-noise ratio at surface stations. The spatiotemporal distribution of hypocenters at each depth level exhibits complex structures extending several hundred meters from the injection points, with strong spatial and temporal clustering. Regions that were seismically active at a certain time often showed reduced or no activity at later times, indicating local shear-stress relaxation. A similar “memory” effect (Kaiser effect) is observed by comparing hypocenter locations of the present experiment with those obtained for a previous injection experiment at KTB. The limitation of hypocentral depths to 9.1 km for events near the borehole suggests changes in rheological properties of the upper crust and thus supports a transition from the regime of brittle failure to ductile deformation at this depth. Large fluid-level changes observed in the nearby pilot hole demonstrate that fluid flow occurs over distances greater than 1.5 km and that major flow zones are not mapped by the induced seismicity. This might also explain the occurence of isolated events at greater distances and depths. Brittle failure at depths greater than 10 km indicates the existence of critically stressed fractures even at temperature over 300°C.

Earthquake Cluster: What Can We Learn from Waveform Similarity?

Abstract The seismograms of earthquakes, which have closely spaced hypocenters, tend to be similar due to the similarity of the Green’s functions characterizing the source-receiver paths. Based on the λ /4 criterion, it is frequently argued that similar earthquakes may represent repeated slip of the same patch of a fault. Because of the phenomenological nature of waveform similarity, such interpretations are strongly dependent on seismic signal characteristics and on the way, the waveform similarity is defined. In this article, we use two-dimensional synthetic wave-field simulations in lateral heterogeneous media to investigate how the waveform similarity of closely spaced hypocenters changes with interevent separation. We analyze the influence of correlation window length, signal frequency bandwidth, and source-receiver geometry on the waveform similarity and discuss under which conditions the λ /4 criterion can be applied to the synthetic data set. With the correlation window length defined as 2.8 times the travel-time difference between the S - and P -phase onsets, we find a correlation threshold value of 0.95 independent of the signal frequency bandwidth. We use the same threshold value for two field data examples that are similar to the synthetic data in frequency content and waveform complexity, and we discuss the implications of the λ /4 criterion. For three microearthquakes occurring during a fluid-injection experiment at the German deep drilling site (Kontinentale Tiefbohrung [KTB]), the interevent separation constrained by the λ /4 criterion is sufficient to identify these events as a sequence of repeating earthquakes in the sense that at least a fraction of the source area experienced repeated slip. For a second data example of four natural (micro-) earthquakes occurring near the island of Crete, the λ /4 criterion does not sufficiently constrain the hypocenter location to identify these events as repeating earthquakes due to the lack of high-frequency information.

Fault plane solutions of microearthquakes and small events in the Hellenic arc

Abstract The Hellenic arc is the most active seismic region of the transition zone between the African and Eurasian plates. In this rapidly deforming region, the physical state of the crust and upper mantle may vary laterally and vertically. However, a detailed 3D model of the region is lacking. The reliable determination of fault plane solutions is crucial if such a model is to be produced. On this basis, fault plane solutions have been determined from recent microearthquakes and small events to assess their reliability for use in tectonic interpretation. A search algorithm on first motion data and amplitude ratios and also a waveform-matching scheme were employed on data from our temporal network, operated on the western part of the island of Crete. There, a network of 47 three-component digital stations with an aperture of 60 km and an average station spacing of 5 km was operated in 1997. The majority of the shallow microearthquakes under Crete for which fault plane solutions were determined have a near horizontal T -axis oriented essentially E–W. Differences between well determined fault plane solutions from the search algorithm and the waveform-matching technique were significant (strike up to 45°, dip up to 10°, and slip up to 50°). Second, data of small events within the Hellenic subduction zone were obtained from recently installed broadband stations at local and regional distances and inverted for a deviatoric seismic moment tensor. Available solutions from other sources such as the Harvard group, EMSC, or SED show drastic differences in part. Owing to still limited azimuthal station coverage, the resulting fault plane solutions of small events have generally not been well constrained. The observed large variation of fault plane solutions for micoearthquakes and small events poses a caveat to tectonic interpretation in case of using data from a few stations only.

Automatic seismogram analysis at GERESS

Abstract The German Experimental Seismic System (GERESS) array extends the Scandinavian regional array network, that is NORESS, ARCESS, and FINESA, into Central Europe. Data from GERESS are transmitted via 64-kbit telephone lines to Ruhr-University Bochum and to NORSAR. At Bochum, an automatic online seismogram analysis has been in operation since January 1991. Between April and July 1991, an average of 17 local and regional events have been localized each day. Parameter lists with information on detected seismic phases and locations are available for users (X.25 or telephone modem) in near real-time. The automatic GERESS localizations generally can be used as fair starting solutions for rapid epicenter determination by regional data centers, for example European Mediterranean Seismological Center (EMSC), although some mislocalizations by the automatic routine have been detected for mixed events and small signal to noise ratios. After 1 yr of full operation, GERESS has evolved as the most sensitive station in Central Europe.

IMAGING OF CRUSTAL STRUCTURES FROM VERTICAL ARRAY MEASUREMENTS

Abstract Walk-away or offset Vertical Seismic Profiling (VSP), a technique widely used in oil and gas exploration, was carried out in crystalline rock at the KTB (Kontinentales Tief-Bohrprogramm) drill site in Germany. The walk-away VSP experiment comprised a shot profile of 10 km length at the surface across the well and a vertical receiver array in the borehole between 3210 m and 3560 m depth with geophone intervals of 25 m. To identify reflected/refracted seismic energy from crustal structures the ‘Controlled Directional Reception’ (CDR) method was used. The key element is the vertical geophone array which allows the determination of the vertical slowness with great accuracy due to the excellent coherence of waveforms. Slant stack processing and subsequent directional migration produced a clear image of mid-crustal reflectors at 10 to 12 km depth. Additionally, a number of dipping faults were detected in the upper crust, that can be connected to surface geology. As the walk-away VSP experiment was part of an integrated seismic measurement program, the results were compared with those from 2D and 3D surface surveys and borehole measurements. This led us to conclude that hydraulic fracture zones play an important role in the reflectivity of crystalline rocks at KTB.

Towards a Global Seismic Monitoring System — Lessons Learned from Geneva Experiments

This lecture summarizes results from technical experiments conducted by the GSE (Group of Scientific Experts) in Geneva over the past 15 years. It shows the interrelation between conceptual design of an international seismic data exchange system and technical developments in instrumentation and communication channels. Although the GSE-experiments have given valuable experience in many technical aspects of a seismological monitoring system, a full scale test still awaits the existence of a calibrated station network.

Adaptive Processing of Digital Broadband Seismic Data

The main purpose of this paper is to develop a step-wise approach to recursive parameter estimation for time-invariant autoregressive moving average (ARMA) models used to track slowly time variant-seismic noise. In these steps computational complexity is balanced against estimation accuracy. By updating with every new data sample, the recursions are well adapted to on-line implementation. They are designed to be insensitive to spurious additive glitches in the data. Assuming that the ARMA parameters vary slowly with time, the estimated parameters contain information about the long time behavior of the modeled process compared with the time duration of additive transient signals. In seismological applications these transients are thought to be earthquake signals. The estimated ARMA parameters are used a) for the design of robust prediction error filters with arbitrary prediction distance to reduce the microseismic noise while passing the earthquake signal widely undisturbed, and b) for automatic detection of earthquake signals. A three-step scheme for the detection of weak earthquake signals is developed: The first step is to clean the data from glitches (for example data transmission errors) by replacing these with predicted values. The second step involves conventional recursive bandpass filtering to focus upon relevant frequency bands. In the third step a detection variable is computed from the difference of time consecutive ARMA parameter vectors for the bandpass filtered traces.

Erdbeobachtung aus dem Weltraum

Es gibt wohl kaum einen Deutschen, der mit dem Begriff „Trabbi“ keine emotionalen Erinnerungen verbindet. Fur viele Menschen im Osten bedeutete der kleine Zweitakter lange Jahre ein Stuck Freiheit. Im Westen galt das Auto dagegen als stinkender Zeuge langst uberholter Fahrzeugtechnik. Als dann aber am 9. November 1989 der Eiserne Vorhang fiel, wurde das Fahrzeug uber Nacht zum Symbol der bevorstehenden Wiedervereinigung der beiden deutschen Staaten. Was anschliesend mit diesen Autos aus Zwickau angestellt wurde, ist inzwischen Legende. Trabbis wurden im Tal des Todes, in der Sahara und in Alaska gesichtet, sie dienten Kunstlern als Objekt, wurden zu Liebhaberpreisen gehandelt und fehlen heute in keinem Automuseum. Ein ausergewohnlicher „Trabbi“ liegt einer Wissenschaftlergruppe aus Potsdam jedoch ganz besonders am Herzen. Die Geodaten mogen ihn so sehr, das sie ihn regelmasig vom Telegrafenberg in der brandenburgischen Landeshauptstadt aus mit Laserstrahlen beleuchten.