Stephan Husen

Evidence for gas and magmatic sources beneath the Yellowstone volcanic field from seismic tomographic imaging

The 3-D P-wave velocity and P- to S-wave velocity ratio structure of the Yellowstone volcanic field, Wyoming, has been determined from local earthquake tomography using new data from the permanent Yellowstone seismic network. We selected 3374 local earthquakes between 1995 and 2001 to invert for the 3-D P-wave velocity (Vp) and P-wave to S-wave velocity ratio (Vp/Vs) structure. Vp anomalies of small size (15×15 km) are reliably imaged in the northwestern part of the model outside the Yellowstone caldera; inside the caldera only Vp anomalies of large size extending over several grid nodes are reliably imaged. The Vp/Vs solution is generally poorer due to the low number of S–P arrival times. Only the northwestern part of the model is resolved with confidence; the Vp/Vs solution also suffers from strong vertical and horizontal velocity smearing. The tomographic images confirm the existence of a low Vp-body beneath the Yellowstone caldera at depths greater than 8 km, possibly representing hot, crystallizing magma. The most striking result of our study is a volume of anomalously low Vp and Vp/Vs in the northwestern part of the Yellowstone volcanic field at shallow depths of <2.0 km. Theoretical calculations of changes in P- to S-wave velocity ratios indicate that these anomalies can be interpreted as porous, gas-filled rock. The close spatial correlation of the observed anomalies and the occurrence of the largest earthquake swarm in historic time in Yellowstone, 1985, suggest that the gas may have originated as part of magmatic fluids released by crystallization of magma beneath the Yellowstone caldera.

Changes in geyser eruption behavior and remotely triggered seismicity in Yellowstone National Park produced by the 2002 M 7.9 Denali fault earthquake, Alaska

Following the 2002 M 7.9 Denali fault earthquake, clear changes in geyser activity and a series of local earthquake swarms were observed in the Yellowstone National Park area, despite the large distance of 3100 km from the epicenter. Several geysers altered their eruption frequency within hours after the arrival of large-amplitude surface waves from the Denali fault earthquake. In addition, earthquake swarms occurred close to major geyser basins. These swarms were unusual compared to past seismicity in that they occurred simultaneously at different geyser basins. We interpret these observations as being induced by dynamic stresses associated with the arrival of large-amplitude surface waves. We suggest that in a hydrothermal system dynamic stresses can locally alter permeability by unclogging existing fractures, thereby changing geyser activity. Furthermore, we suggest that earthquakes were triggered by the redistribution of hydrothermal fluids and locally increased pore pressures. Although changes in geyser activity and earthquake triggering have been documented elsewhere, here we present evidence for changes in a hydrothermal system induced by a large-magnitude event at a great distance, and evidence for the important role hydrothermal systems play in remotely triggering seismicity.

Local earthquake tomography of shallow subduction in north Chile: A combined onshore and offshore study

Selected travel time data from the aftershock series of the great Antofagasta earthquake (M w =8.0) have been inverted simultaneously for both hypocenter locations and three-dimensional V P and V P / V S struc- ture. The data were collected with a dense 44-station seismic network including ocean bottom hydrophones. We performed a series of inversions with increasing complexity: 1-D, 2-D, and 3-D. This approach was cho- sen to account for the heterogeneous seismicity distribution and to obtain a smooth regional model in areas of low resolution. Special efforts were made to assess the solution quality including standard resolution esti- mates and tests with synthetic travel times. The subducted plate is imaged between 20 and 50 km in depth as an eastward dipping high- V P feature. High V P / V S ratios within the oceanic crust possibly indicate elevated fluid content. Underplating of material eroded close to the trench is found beneath the Mejillones Peninsula. The lower crust of the overlying plate is characterized by an average V P of 6.8-6.9 km/s and an average to low V P / V S ratio. Large areas of anomalously high V P are found in the lower crust south of the city of Antofagasta; they are interpreted as remants of magmatic intrusions. A zone of high V P / V S ratios is found within the rup- ture area of the Antofagasta main shock, just above the subducted slab. Its location within the region of high- est stress release from the main shock suggests that the main shock rupture causes the high V P / V S ratio. The high V P / V S ratio could indicate postseismic fluid migration from the subducted oceanic crust into the overly- ing lower crust.

Postseismic fluid flow after the large subduction earthquake of Antofagasta, Chile

We interpret the time evolution of high P-wave to S-wave velocity ( V p/ V s) ratios after the large Antofagasta subduction earthquake (M w = 8.0) as due to postseismic fluid flow into the overriding plate. We suggest that accumulation of high stress forms a permeability barrier along the plate interface, capturing the fluids in the subducting plate. This seal is broken only by large subduction earthquakes that allow the fluids to rapidly migrate into the overlying plate. Postseismic fluid flow implies a relatively high permeability of the overlying lower continental crust, which we estimate to be 10 −16 to 10 −17 m 2 .

Model parametrization in seismic tomography: a choice of consequence for the solution quality

Abstract To better assess quality of three-dimensional (3-D) tomographic images and to better define possible improvements to tomographic inversion procedures, one must consider not only data quality and numerical precision of forward and inverse solvers but also appropriateness of model parametrization and display of results. The quality of the forward solution, in particular, strongly depends on parametrization of the velocity field and is of great importance both for calculation of travel times and partial derivatives that characterize the inverse problem. To achieve a quality in model parametrization appropriate to high-precision forward and inverse algorithms and to high-quality data, we propose a three-grid approach encompassing a seismic, a forward, and an inversion grid. The seismic grid is set up in such a way that it may appropriately account for the highest resolution capability (i.e. optimal data) in the data set and that the 3-D velocity structure is adequately represented to the smallest resolvable detail apriori known to exist in real earth structure. Generally, the seismic grid is of uneven grid spacing and it provides the basis for later display and interpretation. The numerical grid allows a numerically stable computation of travel times and partial derivatives. Its specifications are defined by the individual forward solver and it might vary for different numerical techniques. The inversion grid is based on the seismic grid but must be large enough to guarantee uniform and fair resolution in most areas. For optimal data sets the inversion grid may eventually equal the seismic grid but in reality, the spacing of this grid will depend on the illumination qualities of our data set (ray sampling) and on the maximum matrix size we can invert for. The use of the three-grid approach in seismic tomography allows to adequately and evenly account for characteristics of forward and inverse solution algorithms, apriori knowledge of earth’s structure, and resolution capability of available data set. This results in possibly more accurate and certainly in more reliable tomographic images since the inversion process may be well-tuned to the particular application and since the three-grid approach allows better assessment of solution quality.

Remotely Triggered Seismicity in the Yellowstone National Park Region by the 2002 Mw 7.9 Denali Fault Earthquake, Alaska

Coincident with the arrival of low-frequency, large-amplitude surface waves of the Mw 7.9 Denali fault earthquake (DFE), an abrupt increase in seismicity was observed in the Yellowstone National Park region, despite the large epicentral distance of 3100 km. Within the first 24 hr following the DFE mainshock, we located more than 250 earthquakes, which occurred throughout the entire Yellowstone Na- tional Park region. The elevated seismicity rate continued for about 30 days and followed a modified Omori law decay with a P value of 1.02 ! 0.07. For a declus- tered earthquake catalog, the seismicity following the 2002 DFE uniquely stands out with a significance of 30r. The increase in seismicity occurred over all magnitude bands. In general, we observed that seismicity following the DFE outlined the spatial pattern of past seismicity routinely observed in the Yellowstone National Parkregion. However, we found significant differences in triggered seismicity inside and outside the caldera. Earthquakes inside the Yellowstone caldera occurred preferentially as clusters close to major hydrothermal systems, were of larger magnitude, and seis- micity decayed more rapidly. This suggests that either different trigger mechanisms were operating inside and outside the caldera or that the crust responded differently to the same trigger mechanism depending on its different mechanicalstate.Compared with other sites that experienced remote earthquake triggering following the 2002 DFE, Yellowstone showed the most vigorous earthquake activity. We attribute this to strong directivity effects of the DFE, which caused relatively large peak dynamic stresses (0.16-0.22 MPa) in Yellowstone, and to the volcanic nature of Yellowstone.

Microseismic investigation of an unstable mountain slope in the Swiss Alps

[1] Risks associated with unstable rocky slopes are growing as a result of climate change and rapid expansions of human habitats and critical infrastructure in mountainous regions. To improve our understanding of mountain slope instability, we developed a microseismic monitoring system that operates autonomously in remote areas afflicted by harsh weather. Our microseismic system comprising 12 three-component geophones was deployed across ∼60,000 m 2 of rugged crystalline terrain above a huge (30 million m 3 ) recent rockfall in the Swiss Alps. During its 31-month lifetime, signals from 223 microearthquakes with approximate moment magnitudes ranging from -2 to 0 were recorded. Determining the hypocenters was challenging for several reasons: (1) P wave velocities were highly heterogeneous, varying abruptly from 3.8 km/s. (2) First-break picks were either inaccurate or lacking for some microearthquakes. (3) There were no reliable S wave picks. (4) Numerous microearthquakes occurred just outside the network boundaries. These issues were addressed by using a three-dimensional (3-D) P wave velocity model of the mountain slope determined from refraction tomography in a nonlinear inversion for hypocenter parameters and their probability density functions. Recordings from geophones at different altitudes and in boreholes constrained microearthquake depth estimates. Most microearthquakes were concentrated within 50-100 m of the surface in two zones, one that followed the recent rockslide scarp and one that spanned the volume of highest fracture zone/fault density. These two active zones delineated a mass of rock that according to geodetic measurements has moved toward the scarp at 1-2 cm/yr.

Probabilistic Earthquake Relocation in Three-Dimensional Velocity Models for the Yellowstone National Park Region, Wyoming

Recorded seismicity for the Yellowstone National Park region, comprising 25,267 earthquakes from November 1972 to December 2002, has been relocated using three-dimensional velocity models and probabilistic earthquake location. In addition, new coda magnitudes for earthquakes between 1984 and 2002 were computed by using an improved coda magnitude equation. Three-dimensional velocity models for earthquake location were computed by inverting subsets of high quality data of three different periods, 1973-1981, 1984-1994, and 1995-2002, for hypocenter locations and seismic velocities. Earthquakes were relocated by using a nonlinear, probabilistic solution to the earthquake location problem. Fully nonlinear location uncertainties included in the probabilistic solution allow a better and more reliable classification of earthquake locations into four quality classes. Earthquake locations show an improvement in location accuracy with time, which we attribute to improved network geometry and more precise timing of arrival times. No large systematic shifts of the relocated earthquake locations are observed, except a systematic shift of _2 km to greater depth. The new relocated earthquake locations show tighter clustering of epicenters and focal depths when compared with original earthquake locations. The most intense seismicity in terms of number of earthquakes and cumulative seismic moment release in the Yellowstone National Park region occurs northwest of the Yellowstone caldera between Hebgen Lake and the northern rim of the caldera. Seismicity within the Yellowstone caldera is diffuse, and shallow individual clusters of earthquakes can be associated with major hydrothermal areas.

Tomography from 26 years of seismicity revealing that the spatial extent of the Yellowstone crustal magma reservoir extends well beyond the Yellowstone caldera

The Yellowstone volcanic field has experienced three of Earths most explosive volcanic eruptions in the last 2.1 Ma. The most recent eruption occurred 0.64 Ma forming the 60 km long Yellowstone caldera. We have compiled earthquake data from the Yellowstone Seismic Network from 1984 to 2011 and tomographically imaged the three-dimensional P wave velocity (Vp) structure of the Yellowstone volcanic system. The resulting model reveals a large, low Vp body, interpreted to be the crustal magma reservoir that has fueled Yellowstones youthful volcanism. Our imaged magma body is 90 km long, 5–17 km deep, and 2.5 times larger than previously imaged. The magma body extends ~15 km NE of the caldera and correlates with the location of the largest negative gravity anomaly, a −80 mGal gravity low. This new seismic image provides important constraints on the dynamics of the Yellowstone magma system and its potential for future volcanic eruptions and earthquakes.

Automatic S-Wave Picker for Local Earthquake Tomography

High-resolution seismic tomography at local and regional scales requires large and consistent sets of arrival-time data. Algorithms combining accurate pick- ing with an automated quality classification can be used for repicking waveforms and compiling large arrival-time data sets suitable for tomographic inversion. S-wave velocities represent a key parameter for petrological interpretation, improved hypocenter determination, as well as for seismic hazard models. In our approach, we combine three commonly used phase detection and picking methods in a robust S-wave picking procedure. Information from the different techniques provides an in situ estimate of timing uncertainty and of the reliability of the automatic phase identification. Automatic picks are compared against manually picked reference picks of selected earthquakes in the Alpine region. The average accuracy of automatic picks and their classification is comparable with the reference picks, although a higher number of picks is downgraded to lower quality classes by the automatic picker. In the production-mode, we apply the picker to a data set of 552 earthquakes in the Alps recorded at epicentral distances ≤150 km. The resulting data set includes about 2500 S phases with an upper error bound of 0.27 sec. Online Material: Details on the proposed automatic S-wave picking algorithm.