Jochen Braunmiller

Regional moment tensor determination in the European-Mediterranean area — initial results

The broadband seismic network in the European–Mediterranean area provides high-quality data. We invert these regional three-component data for the source parameters of moderate-to-strong earthquakes in the entire European–Mediterranean area. Regional seismograms have a good signal-to-noise ratio even for moderate-sized events that are too small for teleseismic analysis. The magnitude threshold for source parameter determination can, thus, be significantly lowered. The threshold depends on the average event–station distances. Within dense broadband networks, we analyze MW≈3.0 earthquakes. In areas far from broadband seismic stations, the lower bound is MW≈4.5–4.8, still considerably lower than the teleseismic analysis threshold (about MW≈5.0–5.3). For larger events, we perform rapid moment tensor analysis using near-real-time data; solutions are posted within hours after event occurrence. In a second step, we merge near-real-time and later available data to obtain a regional moment tensor catalog of moderate-to-large earthquakes for the entire European–Mediterranean area. Within less than 1 year, we have analyzed 67 earthquakes ranging in size from MW=2.9 to 7.5. The solutions cover the seismically active areas of the European–Mediterranean area. Particularly important are solutions for slowly deforming regions where large earthquakes, that could be analyzed with teleseismic data, occur infrequently. The solutions are reliable: for events with independent source parameter estimates, the agreement is generally high. The solutions are robust: variations in epicentral parameters, source depth, or exact choice of stations do not affect source parameter estimates strongly. The moment magnitudes provide a unified estimate of earthquake size for the European–Mediterranean area. We perform regression analyses to link our moment magnitudes with local, body, and surface wave magnitudes.

Homogeneous Moment-Magnitude Calibration in Switzerland

An earthquake catalog containing a uniform size estimate is important for long-term seismic hazard assessment in regions of low-to-moderate seismicity. During the update of the Earthquake Catalog of Switzerland (ecos), we performed regression analyses to convert all earthquake size information in ecos to physically meaningful moment magnitude M w. For 34 events in and near Switzerland, we determined seismic moment (thus M w) by regional waveform inversion. Independent M w estimates for the same events do not exist; however, M w from European-Mediterranean events, obtained in the same way, agree with M w from Harvard CMT solutions. All other size estimates, M L, M D, m b, M S, and intensities, are calibrated relative to these 34 events. Teleseismic M S and m b from international data centers are directly regressed against M w. Most observations in ecos consist of local magnitudes ( M L, M D) and intensities. For local magnitudes, we first calibrated the Swiss Seismological Service’s M L. Then we calibrated magnitudes from observatories in neighboring countries (France, Germany, Italy) using only events in the border region (e.g., France–Switzerland). Modern instrumental records exist only since the mid-1970s. We calibrated the macroseismic dataset, which represents by far the largest period in the catalog, by determining surface wave magnitude M S for stronger twentieth century Swiss earthquakes from analog seismograms. These M S, which were converted to M w, connect intensities and M w. After calibration, all 20,300 events in ecos have a unified M w, including a class-type uncertainty estimate based on the original magnitude scale. ecos covers the period 250–2001, from 44° N to 51° N and 4° E to 13° E. The largest event in ecos is the 1356 M w 6.9 Basle earthquake.

Geometry of continental normal faults: Seismological constraints

Teleseismic body waves from large earthquakes are used to study the downdip geometry of continental normal faults in the Aegean. Waveform modeling techniques together with rigorous statistical tests are applied to put firm bounds on the amount of downdip curvature of these faults and the role of coseismic slip on a basal detachment. Synthetic modeling shows that good azimuthal station coverage and inclusion of SH waves are necessary to resolve fault curvature. The data indicate ruptures of the Aegean events occurred on planar faults extending across the entire brittle portion of the crust. No seismogenic low-angle detachment faulting at the base of the upper crust was detected for these events. Decoupling of the brittle upper crust from the plastic lower crust probably occurs aseismically in a ductile fashion.

Probable Low-Angle Thrust Earthquakes on the Juan de Fuca–North America Plate Boundary

In 2004, two clusters of earthquakes occurred in the central part of the Cascadia forearc, which displays several characteristics indicative of along-strike and downdip variations in plate coupling. Moment tensor analysis for the main shock in each cluster indicates that both events, which had magnitudes of 4.9 and 4.8, were compatible with low-angle thrust motion on fault planes dipping 6°–15° to the east, consistent with the plate boundary dip of ∼12°. By tracing rays through a high-resolution two-dimensional crustal velocity model to match arrival times of secondary arrivals (pP and PmP), we estimate that the source depth was 9–11 km for the M4.9 event and 15–17 km for the M4.8 event. We conclude that these earthquakes probably represent seismogenic thrust motion on the nominally locked or transitional part of the Cascadia megathrust, updip of where episodic tremor and slip (ETS) have been documented. Continued high-resolution observations of seismicity and improved crustal models are needed to confirm whether apparent temporal correlations between ETS and continued seismic activity in these clusters indicate stress transfer along the megathrust.

The 1993 Klamath Falls, Oregon, earthquake sequence: Source mechanisms from regional data

We use regional broadband seismograms to obtain seismic moment-tensor solutions of the two September 20, 1993, Mw=6, Klamath Falls, Oregon earthquakes, their foreshock and largest aftershocks (My>3.5). Several sub-groups with internally consistent solutions indicate activity on several fault segments and faults. From the estimated moment-tensors and depths of the main shocks and from the aftershock distribution we deduce that both main shocks occurred on an east-dipping normal fault, pos- sibly related to the Lake of the Woods fault system. Rotation of T-axes between the two main shocks is consistent with the two dominant trends of the aftershocks and mapped faults. We pro- pose that a change in fault strike acted as temporary barrier sepa- rating the rupture of the main shocks. Empirical Greens function analysis shows that the first main event had a longer rupture du- ration (half-duration 1.7 s) than the second (1.2 s). In December, vigorous shallow activity commenced near Klamath Lakes west- ern shore, 5-10 km east of the primary aftershock zone. It ap- pears a Mw=5.5 aftershock occurring the day before, though within the primary aftershock zone, triggered the activity.

Seismic anisotropy beneath the Juan de Fuca plate system: Evidence for heterogeneous mantle flow

Here we use SKS shear wave splitting observations from ocean-bottom seismometer data to infer patterns of mantle deformation beneath the Juan de Fuca plate and its adjoining boundaries. Our results indicate that the asthenosphere beneath the Juan de Fuca plate responds largely to absolute plate motion with an anisotropic layer developing rapidly near the ridge and persisting into the subduction zone. Geographically restricted deviations from this pattern indicate the presence of secondary processes. At discrete plate boundaries, such as the Blanco transform fault, seismic anisotropy is attributed to relative plate motion within a narrow zone (<50 km). Beneath the deforming southern Gorda plate region—a diffuse plate boundary—splitting observations similarly suggest deformation dominated by relative motion between the rigid Juan de Fuca and Pacific plates but distributed over a broad zone (∼200 km). Our results are inconsistent with toroidal flow around the southern edge of the subducting slab due to rollback, as suggested by onshore studies. Instead, reorganization of upper mantle flow associated with plate fragmentation seems to dominate the anisotropic signature of southern Cascadia.

Seismic Moment from Regional Surface-Wave Amplitudes: Applications to Digital and Analog Seismograms

Accurate, consistent earthquake size estimates are fundamental for seismic hazard evaluation. In central Europe, seismic activity is low and long-term seismicity, available as intensities from written historical records, has to be included for meaningful assessments. We determined seismic moments M 0 of 25 stronger twentieth-century events in Switzerland from surface-wave amplitude measurements. These M 0 can be used to calibrate intensity-moment relations applicable to preinstrumental data. We derived the amplitude-moment relation using digital data from 18 earthquakes in and near Switzerland where independent M 0 estimates exist. The surface-wave amplitudes were measured at empirically determined distance varying reference periods T Δ . For amplitudes measured at T Δ , the distance attenuation term of the surface-wave magnitude relation S (Δ) = log ( A / T ) max + 1.66 log Δ is independent of distance. For log M 0 = M S + C E , we get log M 0 = S (Δ) + 14.90. Uncertainties of ±0.3 for the 14.90-constant correspond to a factor of 2 M 0 uncertainty, which was verified with independent data. Our relation allows fast, direct M 0 determination for current earthquakes, and after recalibration of the constant, the relation can be applied anywhere. We applied our relation to analog seismograms from early-instrumental earthquakes in Switzerland that were collected from several European observatories. Amplitude measurements from scans were performed at large amplifications and corrected for differences between T Δ and actual measurement periods. The resulting magnitudes range from M w = 4.6 to 5.8 for the largest earthquake in Switzerland during the twentieth century. Uncertainties for the early-instrumental events are on the order of 0.4 magnitude units. Online material : Moment-tensor analysis of 14 recent earthquakes.

Mantle Transition Zone Thickness in the Central South‐American Subduction Zone

We used receiver functions to determine lateral variations in mantle transition zone thickness and sharpness of the 410- and 660-km discontinuities in the presence of subducting lithosphere. The mantle beneath the central Andes of South America provides an ideal study site owing to its long-lived subduction history and the availability of broadband seismic data from the dense BANJO/SEDA temporary networks and the permanent station LPAZ. For LPAZ, we analyzed 26 earthquakes between 1993-2003 and stacked the depth-migrated receiver functions. For temporary stations operating for only about one year (1994-1995), station stacks were not robust. We thus stacked receiver functions for close-by stations forming five groups that span the subduction zone from west to east, each containing 12 to 25 events. We found signal significant at the 2σ level for several station groups from P to S conversions that originate near 520- and 850-900 km depth, but most prominently from the 410- and 660-km discontinuities. For the latter, the P to S converted signal is clear in stacks for western groups and LPAZ, lack of coherent signal for two eastern groups is possibly due to incoherent stacking and does not necessitate the absence of converted energy. The thickness of the mantle transition zone increases progressively from a near-normal 255 km at the Pacific coast to about 295 km beneath station LPAZ in the Eastern Cordillera. Beneath LPAZ, the 410-km discontinuity appears elevated by nearly 40 km, thus thickening the transition zone. We compared signal amplitudes from receiver function stacks calculated at different low-pass frequencies to study frequency dependence and possibly associated discontinuity sharpness of the P to S converted signals. We found that both the 410- and 660-km discontinuities exhibit amplitude increase with decreasing frequency. Synthetic receiver function calculations for discontinuity topography mimicking observed topography show that the observed steep topography can adequately explain the observed frequency dependence. The large transition zone thickness beneath LPAZ can be explained by a ∼300 K cooler slab or a mantle saturated with water (∼1 wt %).

Mantle dynamics beneath the discrete and diffuse plate boundaries of the Juan de Fuca plate: Results from Cascadia Initiative body wave tomography

We use the delay times of teleseismic S phases recorded by ocean bottom seismometers during the plate-scale Cascadia Initiative community experiment to constrain the heterogeneity of seismic velocity structure beneath young oceanic lithosphere. Our study area covers the entire Juan de Fuca (JdF) and Gorda plates, from their creation at the JdF and Gorda Ridges to their subduction beneath the North American continent, and the entire length of the Blanco transform fault. The range of the observed Vs anomalies requires variations in the melt fraction of the asthenosphere. The data require that low Vs anomalies extend to depths of at least 200 km, which is within the carbonatite melting regime. In the upper 200 km of the mantle, Vs increases rapidly to the east of the JdF Ridge, while there is no clear relationship with the age of the lithosphere in the Gorda region. The distribution of melt is asymmetric about both the JdF and Gorda Ridges. Dynamic upwelling – due to the buoyancy of the mantle – and accompanying downwelling can explain the rapid decrease in melt fraction to the east of the JdF Ridge, the asymmetry about the JdF Ridge, and the sinuous pattern of upwelling near the Blanco transform fault. Finally, mantle flow beneath the diffuse Gorda and Explorer plate boundaries is distinct from that beneath the discrete plate boundary of the JdF Ridge. In particular, shear between the Pacific and JdF plates appears to dominate mantle deformation over seafloor spreading beneath the Gorda Ridge.

Intraplate Earthquakes in Europe—Source Parameters from Regional Moment Tensor Analysis

Plate tectonics provides a highly successful framework to describe a wide range of geological observations invoking the motion of lithospheric plates. In its simplest form the plates are rigid and earthquakes are confined to boundaries where plates move relative to each other.