Torsten Dahm

Structure of the seismogenic zone of the southcentral Chilean margin revealed by local earthquake traveltime tomography

We use traveltime data of local earthquakes and controlled sources observed by a large, temporary, amphibious seismic network to reveal the anatomy of the southcentral Chilean subduction zone (37–39°S) between the trench and the magmatic arc. At this location the giant 1960 earthquake (M = 9.5) nucleated and ruptured almost 1000 km of the subduction megathrust. For the three-dimensional tomographic inversion we used 17,148 P wave and 10,049 S wave arrival time readings from 439 local earthquakes and 94 shots. The resolution of the tomographic images was explored by analyzing the model resolution matrix and conducting extensive numerical tests. The downgoing lithosphere is delineated by high seismic P wave velocities. High vp/vs ratio in the subducting slab reflects hydrated oceanic crust and serpentinized uppermost oceanic mantle. The subducting oceanic crust can be traced down to a depth of 80 km, as indicated by a low velocity channel. The continental crust extends to approximately a 50-km depth near the intersection with the subducting plate. This suggests a wide contact zone between continental and oceanic crust of about 150 km, potentially supporting the development of large asperities. Eastward the crustal thickness decreases again to a minimum of about a 30-km depth. Relatively low vp/vs at the base of the forearc does not support a large-scale serpentinization of the mantle wedge. Offshore, low vp and high vp/vs reflect young, fluid-saturated sediments of forearc basins and the accretionary prism.

Gradual caldera collapse at Bárdarbunga volcano, Iceland, regulated by lateral magma outflow

Driven to collapse Volcanic eruptions occur frequently, but only rarely are they large enough to cause the top of the mountain to collapse and form a caldera. Gudmundsson et al. used a variety of geophysical tools to monitor the caldera formation that accompanied the 2014 Bárdarbunga volcanic eruption in Iceland. The volcanic edifice became unstable as magma from beneath Bárdarbunga spilled out into the nearby Holuhraun lava field. The timing of the gradual collapse revealed that it is the eruption that drives caldera formation and not the other way around. Science, this issue p. 262 Magma flow from under the Bárdarbunga volcano drove caldera collapse during the 2014 eruption. INTRODUCTION The Bárdarbunga caldera volcano in central Iceland collapsed from August 2014 to February 2015 during the largest eruption in Europe since 1784. An ice-filled subsidence bowl, 110 square kilometers (km2) in area and up to 65 meters (m) deep developed, while magma drained laterally for 48 km along a subterranean path and erupted as a major lava flow northeast of the volcano. Our data provide unprecedented insight into the workings of a collapsing caldera. RATIONALE Collapses of caldera volcanoes are, fortunately, not very frequent, because they are often associated with very large volcanic eruptions. On the other hand, the rarity of caldera collapses limits insight into this major geological hazard. Since the formation of Katmai caldera in 1912, during the 20th century’s largest eruption, only five caldera collapses are known to have occurred before that at Bárdarbunga. We used aircraft-based altimetry, satellite photogrammetry, radar interferometry, ground-based GPS, evolution of seismicity, radio-echo soundings of ice thickness, ice flow modeling, and geobarometry to describe and analyze the evolving subsidence geometry, its underlying cause, the amount of magma erupted, the geometry of the subsurface caldera ring faults, and the moment tensor solutions of the collapse-related earthquakes. RESULTS After initial lateral withdrawal of magma for some days though a magma-filled fracture propagating through Earth’s upper crust, preexisting ring faults under the volcano were reactivated over the period 20 to 24 August, marking the onset of collapse. On 31 August, the eruption started, and it terminated when the collapse stopped, having produced 1.5 km of basaltic lava. The subsidence of the caldera declined with time in a near-exponential manner, in phase with the lava flow rate. The volume of the subsidence bowl was about 1.8 km3. Using radio-echo soundings, we find that the subglacial bedrock surface after the collapse is down-sagged, with no indications of steep fault escarpments. Using geobarometry, we determined the depth of magma reservoir to be ~12 km, and modeling of geodetic observations gives a similar result. High-precision earthquake locations and moment tensor analysis of the remarkable magnitude M5 earthquake series are consistent with steeply dipping ring faults. Statistical analysis of seismicity reveals communication over tens of kilometers between the caldera and the dike. CONCLUSION We conclude that interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path explain the gradual near-exponential decline of both the collapse rate and the intensity of the 180-day-long eruption. By combining our various data sets, we show that the onset of collapse was caused by outflow of magma from underneath the caldera when 12 to 20% of the total magma intruded and erupted had flowed from the magma reservoir. However, the continued subsidence was driven by a feedback between the pressure of the piston-like block overlying the reservoir and the 48-km-long magma outflow path. Our data provide better constraints on caldera mechanisms than previously available, demonstrating what caused the onset and how both the roof overburden and the flow path properties regulate the collapse. The Bárdarbunga caldera and the lateral magma flow path to the Holuhraun eruption site. (A) Aerial view of the ice-filled Bárdarbunga caldera on 24 October 2014, view from the north. (B) The effusive eruption in Holuhraun, about 40 km to the northeast of the caldera

Discrimination of induced seismicity by full moment tensor inversion and decomposition

Human activities, including operations related to mining and reservoir exploitation, may induce seismicity and pose a risk for population and infrastructures. While different observations are commonly used to assess the origin of earthquakes, there is a lack of rules and methods for the discrimination between natural and induced seismicity. The inversion and decomposition of the full moment tensor and the observation of relevant deviation from a pure double couple (DC) model may be an indicator for induced seismicity. We establish here a common procedure to analyse a set of natural and induced events of similar magnitude, which occurred in Germany and neighbouring regions. The procedure is based on an inversion method and on a consistent velocity model and recording network. Induced seismicity is recorded during different mining and/or reservoir exploitations. Moment tensors are inverted using a multi-step inversion approach. This method, which was successfully applied in previous studies at regional and teleseismic distances, is further developed here to account for full moment tensor analysis. We first find a best DC solution and then perform a full moment tensor inversion, fitting full waveforms amplitude spectra at regional distances. The moment tensor solution is decomposed into DC, compensated linear vector dipole and isotropic terms. The discrimination problem is then investigated through the evaluation of distributions of non-DC source components for natural and induced data sets. Results illustrate the potential of the inversion and discrimination approach. Additional detailed analyses are carried out for the two most significant induced earthquakes, and rupture models are compared with the full moment tensor solutions.

Automated moment tensor inversion to estimate source mechanisms of hydraulically induced micro-seismicity in salt rock

Abstract Two methods for an automated moment tensor inversion are investigated. The first is a single-source inversion using amplitude spectra of observed body-wave phases and theoretical Green functions, and the second a relative moment tensor inversion using spectral moments of body-wave phases. An optional general-dislocation-source constraint is introduced to stabilize the inversions for sparse data. Both methods are successfully tested on synthetic data and are applied to hydraulically induced micro-seismicity in salt rock. The estimated moment tensors of 187 events show similar radiation patterns with a major double-couple component and a small or zero isotropic component. We explain the double-couple component with failure on horizontal, preexisting planes of weakness within the process zone of circular tension-fractures

Seismicity and geometry of the south Chilean subduction zone (41.5°S-43.5°S) : Implications for controlling parameters

In 2005 an amphibious seismic network was deployed on the Chilean forearc between 41.75°S and 43.25°S. 364 local events were observed in a 11-month period. A subset of the P and S arrival times were inverted for hypocentral coordinates, 1-D velocity structure and station delays. Main seismic activity occurred predominantly in a belt parallel to the coast of Chiloe Island in a depth range of 12–30 km presumably related to the plate interface. The 30° inclination of the shallow part of the Wadati-Benioff zone is similar to observations further north indicating that oceanic plate age is not controlling the subduction angle of the shallower part for the Chilean subduction zone. The down-dip termination of abundant intermediate depth seismicity at approximately 70 km depth seems to be related to the young age (and high temperature) of the oceanic plate. Crustal seismicity is associated with the Liquine-Ofqui fault zone and active volcanoes.

COMPARISON OF ABSOLUTE AND RELATIVE MOMENT TENSOR SOLUTIONS FOR THE JANUARY 1997 WEST BOHEMIA EARTHQUAKE SWARM

Moment tensor solutions for 70 clustered events of the 1997 West Bohemia microearthquake swarm, as calculated by two different methods, are given. The first method is a single-event, absolute moment tensor inversion which inverts body-wave peak amplitudes using synthetic Green functions. The second method is a multiple-event, relative method for which Green functions are “reduced” to 2 geometrical angles of rays at the sources. Both methods yield similar moment tensors, which can be divided into at least two or three different classes of focal solutions, indicating that, during the swarm activity, different planes of weakness were active. The major source component of most events is a double couple. However, the deviations from the double-couple mechanisms seem to be systematic for some classes of solutions. Error analysis was based on transforming the estimate of the standard deviation of amplitudes extracting from the seismograms into confidence regions of the absolute moment tensor. They show that the non-DC components are significant at a fairly high confidence level.

Seismic Broadband Ocean-Bottom Data and Noise Observed with Free-Fall Stations: Experiences from Long-Term Deployments in the North Atlantic and the Tyrrhenian Sea

In a comparative study of two long-term deployments we characterize the seismic noise on the seafloor in the North Atlantic south of Iceland and in the Tyrrhenian Sea north of Sicily. We estimate the teleseismic body-wave detection threshold to be approximately magnitude 6.0 at frequencies below the microseismic band (f 0.1 Hz) on vertical components at the quietest sites in both regions. At the microseismic peak (0.25 Hz) in the North Atlantic, the minimum magnitudes for events to be recorded most of the time are Mw 7.0 for the Tyrrhenian Sea deployment and above 8 for the North Atlantic deployment. By correlating seismic noise and oceanic waveheight amplitudes we are able to find the major generation areas of microseismic noise in the North Atlantic. Although the high noise of secondary microseisms at 0.24 Hz is generated far away from the ocean-bottom stations at three near-coastal regions, the microseismic noise at about 1 Hz is generated directly at the stations. We present a technique to estimate the noise generation areas prior to future deployment by using noise at nearby land stations. The ambient low-frequency noise below 0.1 Hz occurs mainly on horizontal components and is probably induced by seafloor-current-induced tilt. The power spectral density of this noise varies by a factor of up to 10,000 between different stations and deployment sites, indicating in some cases wobbling deployments, possible problems of frame weakness, and a possible higher noise sensitivity of external packs to seafloor currents. Cross-coupling between horizontal and vertical channel noise is strong at some of our stations, demonstrating that the leveling mechanics can be further improved to reduce vertical channel noise.

The 2004 Mw 4.4 Rotenburg, Northern Germany, Earthquake and Its Possible Relationship with Gas Recovery

We study the 20 October 2004 M w 4.4 Rotenburg (Wumme)/Neuenkirchen earthquake, located in a previously aseismic region in the northern German sedimentary basin. We constrain the source parameter by using different techniques. A possible relationship between this event, the regional tectonic setting, and local gas recovery is investigated. Different waveform inversion and modeling approaches constrain the depth of the mainshock between 5 and 7 km. The source mechanism was oblique normal faulting on planes striking roughly north–south. An inversion for kinematic rupture parameters indicates a unilateral rupture propagation toward the north, consistent with the higher macroseismic intensities found toward the north in the region of Hamburg compared with those at a similar distance toward the south in the region of Hannover. Relocations of the mainshock and three of the largest aftershocks indicate that these events occurred within a few kilometers of three major gas fields and at depth close to gas production intervals. Comparison with seismicity triggered in the northern Netherlands by depletion of similar gas reservoirs in a similar tectonic environment suggests that the M w 4.4 Rotenburg event may be related to gas recovery. Online material: Focal mechanism and waveform fit.

Current challenges in monitoring, discrimination, and management of induced seismicity related to underground industrial activities: A European perspective

Due to the deep socioeconomic implications, induced seismicity is a timely and increasingly relevant topic of interest for the general public. Cases of induced seismicity have a global distribution and involve a large number of industrial operations, with many documented cases from as far back to the beginning of the twentieth century. However, the sparse and fragmented documentation available makes it difficult to have a clear picture on our understanding of the physical phenomenon and consequently in our ability to mitigate the risk associated with induced seismicity. This review presents a unified and concise summary of the still open questions related to monitoring, discrimination, and management of induced seismicity in the European context and, when possible, provides potential answers. We further discuss selected critical European cases of induced seismicity, which led to the suspension or reduction of the related industrial activities.

Recommendation for the discrimination of human-related and natural seismicity

Various techniques are utilized by the seismological community, extractive industries, energy and geoengineering companies to identify earthquake nucleation processes in close proximity to engineering operation points. These operations may comprise fluid extraction or injections, artificial water reservoir impoundments, open pit and deep mining, deep geothermal power generations or carbon sequestration. In this letter to the editor, we outline several lines of investigation that we suggest to follow to address the discrimination problem between natural seismicity and seismic events induced or triggered by geoengineering activities. These suggestions have been developed by a group of experts during several meetings and workshops, and we feel that their publication as a summary report is helpful for the geoscientific community. Specific investigation procedures and discrimination approaches, on which our recommendations are based, are also published in this Special Issue (SI) of Journal of Seismology.