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

Cross-correlation of 1 month of ambient seismic noise recorded at USArray stations in California yields hundreds of short-period surface-wave group-speed measurements on interstation paths. We used these measurements to construct tomographic images of the principal geological units of California, with low-speed anomalies corresponding to the main sedimentary basins and high-speed anomalies corresponding to the igneous cores of the major mountain ranges. This method can improve the resolution and fidelity of crustal images obtained from surface-wave analyses.

/pdf/high-resolution-surface-wave-tomography-from-ambient-seismic-2rxarxopq3.pdf

High-resolution surface-wave tomography from ambient seismic noise.

Convective removal of lower lithosphere beneath the Tibetan Plateau can account for a rapid increase in the mean elevation of the Tibetan Plateau of 1000 m or more in a few million years. Such uplift seems to be required by abrupt tectonic and environmental changes in Asia and the Indian Ocean in late Cenozoic time. The composition of basaltic volcanism in northern Tibet, which apparently began at about 13 Ma, implies melting of lithosphere, not asthenosphere. The most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere. The initiation of normal faulting in Tibet at about 8 (± 3) Ma suggests that the plateau underwent an appreciable increase in elevation at that time. An increase due solely to the isostatic response to crustal thickening caused by India's penetration into Eurasia should have been slow and could not have triggered normal faulting. Another process, such as removal of relatively cold, dense lower lithosphere, must have caused a supplemental uplift of the surface. Folding and faulting of the Indo-Australian plate south of India, the most prominent oceanic intraplate deformation on Earth, began between about 7.5 and 8 Ma and indicates an increased north-south compressional stress within the Indo-Australian plate. A Tibetan uplift of only 1000 m, if the result of removal of lower lithosphere, should have increased the compressional stress that the plateau applies to India and that resists India's northward movement, from an amount too small to fold oceanic lithosphere, to one sufficient to do so. The climate of the equatorial Indian Ocean and southern Asia changed at about 6–9 Ma: monsoonal winds apparently strengthened, northern Pakistan became more arid, but weathering of rock in the eastern Himalaya apparently increased. Because of its high altitude and lateral extent, the Tibetan Plateau provides a heat source at midlatitudes that should oppose classical (symmetric) Hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons. For the simple case of axisymmetric heating (no dependence on longitude) of an atmosphere without dissipation, theoretical analyses by Hou, Lindzen, and Plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal (monsoonlike) circulation than such a source centered on the equator, but only if heating exceeds a threshold whose level increases with the latitude of the heat source. Because heating of the atmosphere over Tibet should increase monotonically with elevation of the plateau, a modest uplift (1000–2500 m) of Tibet, already of substantial extent and height, might have been sufficient to exceed a threshold necessary for a strong monsoon. The virtual simultaneity of these phenomena suggests that uplift was rapid: approximately 1000 m to 2500 m in a few million years. Moreover, nearly simultaneously with the late Miocene strengthening of the monsoon, the calcite compensation depth in the oceans dropped, plants using the relatively efficient C4 pathway for photosynthesis evolved rapidly, and atmospheric CO2 seems to have decreased, suggesting causal relationships and positive feedbacks among these phenomena. Both a supplemental uplift of the Himalaya, the southern edge of Tibet, and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the Himalaya that, in turn, enhanced extraction of CO2 from the atmosphere. Thus these correlations offer some support for links between plateau uplift, a downdrawing of CO2 from the atmosphere, and global climate change, as proposed by Raymo, Ruddiman, and Froehlich. Mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts, but might also play an important role in altering regional and global climates.

Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon

Seismic tomography models of the three-dimensional upper mantle velocity structure of the Mediterranean-Carpathian region provide a better understanding of the lithospheric processes governing its geodynamical evolution. Slab detachment, in particular lateral migration of this process along the plate boundary, is a key element in the lithospheric dynamics of the region during the last 20 to 30 million years. It strongly affects arc and trench migration, and causes along-strike variations in vertical motions, stress fields, and magmatism. In a terminal-stage subduction zone, involving collision and suturing, slab detachment is the natural last stage in the gravitational settling of subducted lithosphere.

http://www.mantleplumes.org/WebDocuments/Wortel2000.pdf

Subduction and Slab Detachment in the Mediterranean-Carpathian Region

Slab breakoff: A model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens

An edited version of this paper was published by the American Geophysical Union. Copyright 2000, AGU.
See also:
http://www.agu.org/pubs/crossref/2000/2000JB900024.shtml;
http://atlas.geo.cornell.edu/morocco/publications/calvert2000.htm

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900024

Geodynamic evolution of the lithosphere and upper mantle beneath the Alboran region of the western Mediterranean: Constraints from travel time tomography

The arrival times of compressional (P) and shear (S) waves from approximately 580 microearthquakes recorded by a temporary array in the Pamir-Hindu Kush region in central Asia are used to deduce one- and three-dimensional velocity structures of this region. The results for one-dimensional structures imply that the Moho is at 70±5 depth. Also, there is a velocity reversal near 160 km depth, which is inferred to be the beginning of the low velocity zone. This reversal continues to depths of approximately 230 km. Below 230 km, velocities are somewhat higher than those of normal mantle at similar depths (9.3 km/s versus 8.4 km/s for P waves). The outstanding feature of the results for three-dimensional velocity structures is a broad (>40 km), centrally located region with 8 to 10% lower velocities than those in the surrounding regions. This low velocity region envelopes the seismic zone at depths between 70 and 150 km. The region may actually extend beyond these depths, but the results for shallower and deeper structure lack sufficient resolution to decide. Several tests, using both hypothetical and real data, were performed to estimate the reliability of the three-dimensional solutions. The results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks, although one must be cautious of the effects of averaging in interpreting the solution. The low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust. Therefore, while subduction has occurred in the Pamir-Hindu Kush, the results of the three-dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km.

Appendices are available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C., 20009. Document J81-013; $1.00. Payment must accompany order.

Velocity structure of the Pamir-Hindu Kush Region: Possible evidence of subducted crust

Digital recordings of microearthquake codas from shallow and intermediate depth earthquakes in the Hindu Kush region of Afghanistan were used to determine the attenuation factors of the S -wave coda ( Qc ) and primary S waves ( Qβ ). An anomalously rapid decay of the coda shortly after the S -wave arrival, observed also in a study of coda in central Asia by Rautian and Khalturin (1978), seems to be due primarily to depth-dependent variations in Qc . In particular, we deduce the average Qc in the crust and uppermost mantle (<100-km depth) is approximately four times lower than the deeper mantle (<400-km depth) over a wide frequency range (0.4 to 24 Hz). Further, while Qc generally increases with frequency at any depth, the degree of frequency dependence of Qc depends on depth. Except at the highest frequency studied here (∼48 Hz), the magnitude of Qc at a particular frequency increases with depth while its frequency dependence decreases. For similar depths, determinations of Qβ and Qc agree, suggesting a common wave composition and attenuation mechanism for S waves and codas. Comparison of these determinations of Qc in Afghanistan with those in other parts of the world shows that the degree of frequency dependence of Qc correlates with the expected regional heterogeneity. Such a correlation supports the prejudice that Qc is primarily influenced by scattering and suggests that tectonic processes such as folding and faulting are instrumental in creating scattering environments.

Estimates of Q in central Asia as a function of frequency and depth using the coda of locally recorded earthquakes

Well-constrained hypocenters (latitude, longitude, depth, and origin time) are required for nearly all studies that use earthquake data. We have examined the theoretical basis behind some of the widely accepted “rules of thumb” for obtaining accurate hypocenter estimates that pertain to the use of S phases and illustrate, in a variety of ways, why and when these “rules” are applicable. Results of experiments done for this study show that epicentral estimates (latitude and longitude) are typically far more robust with respect to data inadequacies; therefore, only examples illustrating the relationship between S phase arrival time data and focal depth and origin time estimates are presented. Most methods used to determine earthquake hypocenters are based on iterative, linearized, least-squares algorithms. Standard errors associated with hypocenter parameters are calculated assuming the data errors may be correctly described by a Gaussian distribution. We examine the influence of S -phase arrival time data on such algorithms by using the program HYPOINVERSE with synthetic datasets. Least-squares hypocenter determination algorithms have several shortcomings: solutions may be highly dependent on starting hypocenters, linearization and the assumption that data errors follow a Gaussian distribution may not be appropriate, and depth/origin time trade-offs are not readily apparent. These shortcomings can lead to biased hypocenter estimates and standard errors that do not always represent the true error. To illustrate the constraint provided by S -phase data on hypocenters determined without some of these potential problems, we also show examples of hypocenter estimates derived using a probabilistic approach that does not require linearization. We conclude that a correctly timed S phase recorded within about 1.4 focal depth9s distance from the epicenter can be a powerful constraint on focal depth. Furthermore, we demonstrate that even a single incorrectly timed S phase can result in depth estimates and associated measures of uncertainty that are significantly incorrect.

The effect of S-wave arrival times on the accuracy of hypocenter estimation

Arrival times of compressional (P) and shear (S) waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the western and central Tien Shan are used to determine one- and three-dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt. The best fit one-dimensional structures suggest that the average depth of the Mohorovicic discontinuity in this area is 50 km. The three-dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region. A 7 km-thick wedge of sediment beneath the Chu Depression is outlined at depth by a south dipping plane of seismic activity, suggesting the presence of an active decollemont. These low velocities extend continuously to the southeast toward Issyk-Kul, suggesting a structural relationship between the two. However, rather than being consumed, it appears that Issyk-Kul is overthrusting the surrounding ranges. The low-velocity sediments in the Fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity. Velocities at midcrustal depths generally are lower beneath the central Tien Shan than beneath the western Tien Shan. This pattern becomes more evident in the uppermost mantle, with P velocity contrasts of as much as 10% across a boundary that corresponds roughly to the geographical position of the Talasso-Fergana fault. The low velocities beneath the central Tien Shan exceed 150 km depth but do not appear to be deeper than 300 km depth. There is no evidence for a lithospheric root beneath this part of the range; rather, the low velocities imply the presence of a positive buoyancy force uplifting the mountains. Evidence that this low-velocity region existed before the collision suggests that the Tien Shan may not owe its rejuvenation simply to its location at the northern edge of a strong Tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere.

Steven W. Roecker

Papers

Geodynamic evolution of the lithosphere and upper mantle beneath the Alboran region of the western Mediterranean: Constraints from travel time tomography

Velocity structure of the Pamir-Hindu Kush Region: Possible evidence of subducted crust

Estimates of Q in central Asia as a function of frequency and depth using the coda of locally recorded earthquakes

The effect of S-wave arrival times on the accuracy of hypocenter estimation

Three‐dimensional elastic wave velocity structure of the western and central Tien Shan