We have developed an efficient method to determine high-resolution hypocenter locations over large distances. The location method incorporates ordinary absolute travel-time measurements and/or cross-correlation P-and S-wave differential travel-time measurements. Residuals between observed and theoretical travel-time differences (or double-differences) are minimized for pairs of earthquakes at each station while linking together all observed event-station pairs. A least-squares solu- tion is found by iteratively adjusting the vector difference between hypocentral pairs. The double-difference algorithm minimizes errors due to unmodeled velocity struc- ture without the use of station corrections. Because catalog and cross-correlation data are combined into one system of equations, interevent distances within multiplets are determined to the accuracy of the cross-correlation data, while the relative lo- cations between multiplets and uncorrelated events are simultaneously determined to the accuracy of the absolute travel-time data. Statistical resampling methods are used to estimate data accuracy and location errors. Uncertainties in double-difference locations are improved by more than an order of magnitude compared to catalog locations. The algorithm is tested, and its performance is demonstrated on two clus- ters of earthquakes located on the northern Hayward fault, California. There it col- lapses the diffuse catalog locations into sharp images of seismicity and reveals hor- izontal lineations of hypocenters that define the narrow regions on the fault where stress is released by brittle failure.

/pdf/a-double-difference-earthquake-location-algorithm-method-and-13ttz2fahg.pdf

A Double-Difference Earthquake Location Algorithm: Method and Application to the Northern Hayward Fault, California

The inverse problem of three-dimensional (3-D) local earthquake tomography is formulated as a linear approximation to a nonlinear function. Thus the solutions obtained and the reliability estimates depend on the initial reference model. Inappropriate models may result in artifacts of significant amplitude. Here, we advocate the application of the same inversion formalism to determine hypocenters and one-dimensional (1-D) velocity model parameters, including station corrections, as the first step in the 3-D modeling process. We call the resulting velocity model the minimum 1-D model. For test purposes, a synthetic data set based on the velocity structure of the San Andreas fault zone in central California was constructed. Two sets of 3-D tomographic P velocity results were calculated with identical travel time data and identical inversion parameters. One used an initial 1-D model selected from a priori knowledge of average crustal velocities, and the other used the minimum 1-D model. Where the data well resolve the structure, the 3-D image obtained with the minimum 1-D model is much closer to the true model than the one obtained with the a priori reference model. In zones of poor resolution, there are fewer artifacts in the 3-D image based on the minimum 1-D model. Although major characteristics of the 3-D velocity structure are present in both images, proper interpretation of the results obtained with the a priori 1-D model is seriously compromised by artifacts that distort the image and that go undetected by either resolution or covariance diagnostics.

/pdf/initial-reference-models-in-local-earthquake-tomography-1uwqgwr2xs.pdf

Initial reference models in local earthquake tomography

[1] Since their development in the 1960s, seismic arrays have given a new impulse to seismology. Recordings from many uniform seismometers in a well-defined, closely spaced configuration produce high-quality and homogeneous data sets, which can be used to study the Earth's structure in great detail. Apart from an improvement of the signal-to-noise ratio due to the simple summation of the individual array recordings, seismological arrays can be used in many different ways to study the fine-scale structure of the Earth's interior. They have helped to study such different structures as the interior of volcanos, continental crust and lithosphere, global variations of seismic velocities in the mantle, the core-mantle boundary and the structure of the inner core. For this purpose many different, specialized array techniques have been developed and applied to an increasing number of high-quality array data sets. Most array methods use the ability of seismic arrays to measure the vector velocity of an incident wave front, i.e., slowness and back azimuth. This information can be used to distinguish between different seismic phases, separate waves from different seismic events and improve the signal-to-noise ratio by stacking with respect to the varying slowness of different phases. The vector velocity information of scattered or reflected phases can be used to determine the region of the Earth from whence the seismic energy comes and with what structures it interacted. Therefore seismic arrays are perfectly suited to study the small-scale structure and variations of the material properties of the Earth. In this review we will give an introduction to various array techniques which have been developed since the 1960s. For each of these array techniques we give the basic mathematical equations and show examples of applications. The advantages and disadvantages and the appropriate applications and restrictions of the techniques will also be discussed. The main methods discussed are the beam-forming method, which forms the basis for several other methods, different slant stacking techniques, and frequency–wave number analysis. Finally, some methods used in exploration geophysics that have been adopted for global seismology are introduced. This is followed by a description of temporary and permanent arrays installed in the past, as well as existing arrays and seismic networks. We highlight their purposes and discuss briefly the advantages and disadvantages of different array configurations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000RG000100

Array seismology: Methods and applications

We have developed a double-difference (DD) seismic tomography method that makes use of both absolute and relative arrival times. By reducing systematic errors using the more accurate relative arrival times, the method produces an improved velocity model. Simultaneously, it yields event locations of a quality equivalent to those of the DD location method, while avoiding simplifying assumptions of that method. We test this method on a synthetic dataset and find that it produces a more accurate velocity model and event locations than standard tomography. We also test this method on a Hayward fault, California, earthquake dataset spanning 1984–1998. The earthquakes relocated by this method collapse to a thin line along the fault trace, consistent with previous results. The DD velocity model has sharper velocity contrasts near the source region than the standard tomography model.

Double-Difference Tomography: The Method and Its Application to the Hayward Fault, California

Unconventional oil and gas production provides a rapidly growing energy source; however, high-production states in the United States, such as Oklahoma, face sharply rising numbers of earthquakes. Subsurface pressure data required to unequivocally link earthquakes to wastewater injection are rarely accessible. Here we use seismicity and hydrogeological models to show that fluid migration from high-rate disposal wells in Oklahoma is potentially responsible for the largest swarm. Earthquake hypocenters occur within disposal formations and upper basement, between 2- and 5-kilometer depth. The modeled fluid pressure perturbation propagates throughout the same depth range and tracks earthquakes to distances of 35 kilometers, with a triggering threshold of ~0.07 megapascals. Although thousands of disposal wells operate aseismically, four of the highest-rate wells are capable of inducing 20% of 2008 to 2013 central U.S. seismicity.

https://science.sciencemag.org/content/sci/345/6195/448.full.pdf

Sharp increase in central Oklahoma seismicity since 2008 induced by massive wastewater injection

An automatic detection algorithm has been developed which is capable of time P -phase arrivals of both local and teleseismic earthquakes, but rejects noise bursts and transient events. For each signal trace, the envelope function is calculated and passed through a nonlinear amplifier. The resulting signal is then subjected to a statistical analysis to yield arrival time, first motion, and a measure of reliability to be placed on the P -arrival pick. An incorporated dynamic threshold lets the algorithm become very sensitive; thus, even weak signals are timed precisely. During an extended performance evaluation on a data set comprising 789 P phases of local events and 1857 P phases of teleseismic events picked by an analyst, the automatic picker selected 66 per cent of the local phases and 90 per cent of the teleseismic phases. The accuracy of the automatic picks was “ideal” (i.e., could not be improved by the analyst) for 60 per cent of the local events and 63 per cent of the teleseismic events.

An automatic phase picker for local and teleseismic events

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

/pdf/program-velest-user-s-guide-short-introduction-1a7jzq1ozp.pdf

Program VELEST USER'S GUIDE - Short Introduction

During the summers of 1982 and 1983, field work was completed on 10 new seismic refraction profiles in the Long Valley-Mono Craters region located within the technically active zone between the eastern Sierra Nevada and the Basin and Range province. The four profiles presented here sample the upper 7–10 km of the crust within the western half of Long Valley cladera, the Mono craters ring fracture system, and the “normal” crust north and northeast of Long Valley. The entire area shares a common crystalline basement consisting primarily of Mesozoic granitic rocks. At a depth of 2 km beneath the topographic surface, the P wave velocity is uniformly 5.6 km/s beneath which the velocity increases with a gradient of about 0.1 s−1. Clear secondary arrivals observed on a profile crossing the west part of the caldera support evidence from 1972 seismic refraction profiles for a reflecting boundary at a depth of 7–8 km beneath the west margin of the resurgent dome, which may represent the top of a magma chamber. Structural details resolved in the upper 2 km of the crust lead to the following conclusions: (1) P wave velocities in the crystalline basement at depths shallower than 2 km vary from 3.6 to 5.0 km/s. This variation reflects differences in the degree of fracturing of the upper portion of the basement. (2) An apparently homogeneous basement in the upper 7–10 km beneath the Mono Craters ring fracture system indicates that any extensive magma chamber must be at least 10 km deep there. (3) The downdropped crystalline basement beneath the western half of Long Valley caldera dips genty (5°–10°) to the northeast. Apparent offsets along steeply dipping caldera-bounding faults are 1.0–1.7 km along the northern, western, and southern sides of the caldera with largest offsets in the north. (4) Rocks forming the caldera fill fall into essentially three P wave velocity groups. From the surface downward these velocity groups are 100–400 of 1.2–1.8 km/s material corresponding to unconsolidated fluvial or lacustrine deposits or highly fractured rhyolite flows, 200–400 m of 2.8–3.1 km/s material corresponding to sparsely jointed postcaldera rhyolite, rhyodacite and basalt flows, and approximately 1000 m of 3.9–4.4 km/s material corresponding to deposits of Bishop Tuff.

Urs Kradolfer

Papers

Initial reference models in local earthquake tomography

An automatic phase picker for local and teleseismic events

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

Program VELEST USER'S GUIDE - Short Introduction

Constraints on the upper crustal structure of the Long Valley‐Mono Craters Volcanic Complex, eastern California, from seismic refraction measurements