Thomas M. Hearn

Anisotropic Pn tomography in the western United States

Pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy. To investigate uppermost mantle anisotropy, a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy, and it was applied to Pn travel time data from the western United States. Results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations. A detailed resolution study examined the trade-off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage. Pn anisotropy beneath the western United States has maximum amplitudes of ±0.3 km/s (7.5%) when resolved on a length scales of around 3°. The orientations of Pn anisotropy often correlate well with those inferred from shear-wave splitting results. This correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions. The orientation of the Pn anisotropy can be correlated with some of the tectonic processes which occur within the western United States. For example, the fast direction of Pn anisotropy parallels the northeast direction of subduction of the Juan de Fuca Plate beneath the northwest Pacific coast, suggesting that there the anisotropy is related to subduction-driven deformation. The fast direction of Pn anisotropy also parallels the strike of the San Andreas Fault system in central California, indicating that shear strains along the plate boundary may be responsible for the anisotropy there. Within the Great Basin, the Pn anisotropy varies substantially, and both partial melting and small-scale convection within the uppermost mantle could be responsible for these anisotropy variations.

Evidence from Earthquake Data for a Partially Molten Crustal Layer in Southern Tibet

Earthquake data collected by the INDEPTH-II Passive-Source Experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern Tibet. North of the Zangbo suture, beneath the southern Lhasa block, a midcrustal low-velocity zone is revealed by inversion of receiver functions, Rayleigh-wave phase velocities, and modeling of the radial component of teleseismic P-waveforms. Conversely, to the south beneath the Tethyan Himalaya, no low-velocity zone was observed. The presence of the midcrustal low-velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern Yadong-Gulu rift and possibly much of southern Tibet.

Seismic polarization anisotropy beneath the central Tibetan Plateau

SKS and SKKS shear waves recorded on the INDEPTH III seismic array deployed in central Tibet during 1998–1999 have been analyzed for the direction and extent of seismic polarization anisotropy. The 400-km-long NNW trending array extended south to north, from the central Lhasa terrane, across the Karakoram-Jiali fault system and Banggong-Nujiang suture to the central Qiangtang terrane. Substantial splitting with delay times from 1 to 2 s, and fast directions varying from E-W to NE-SW, was observed for stations in the Qiangtang terrane and northernmost Lhasa terrane. No detectable splitting was observed for stations located farther south in the central Lhasa terrane. The change in shear wave splitting characteristics occurs at 32°N, approximately coincident with the transcurrent Karakoram-Jiali fault system but ∼40 km south of the surface trace of the Banggong-Nujiang suture. This location is also near the southernmost edge of a region of high Sn attenuation and low upper mantle velocities found in previous studies. The transition between no measured splitting and strong anisotropy (2.2 s delay time) is exceptionally sharp (≤15 km), suggesting a large crustal contribution to the measured splitting. The E-W to NE-SW fast directions are broadly similar to the fast directions observed farther east along the Yadong-Golmud highway, suggesting that no large-scale change in anisotropic properties occurs in the east-west direction. However, in detail, fast directions and delay times vary over lateral distances of ∼100 km in both the N-S and E-W direction by as much as 40° and 0.5–1 s, respectively. The onset of measurable splitting at 32°N most likely marks the northern limit of the underthrusting Indian lithosphere, which is characterized by negligible polarization anisotropy. Taken in conjunction with decades of geophysical and geological observations in Tibet, the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern Tibet is being squeezed and sheared between the advancing Indian lithosphere to the south and the Tsaidam and Tarim lithospheres to the north and west, resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability. In northern Tibet, crustal deformation clearly follows this large-scale deformation pattern.

Rayleigh wave phase velocity maps of Tibet and the surrounding regions from ambient seismic noise tomography

Ambient noise tomography is applied to the significant data resources now available across Tibet and surrounding regions to produce Rayleigh wave phase speed maps at periods between 6 and 50 s. Data resources include the permanent Federation of Digital Seismographic Networks, five temporary U.S. Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiments in and around Tibet, and Chinese provincial networks surrounding Tibet from 2003 to 2009, totaling ∼600 stations and ∼150,000 interstation paths. With such a heterogeneous data set, data quality control is of utmost importance. We apply conservative data quality control criteria to accept between ∼5000 and ∼45,000 measurements as a function of period, which produce a lateral resolution between 100 and 200 km across most of the Tibetan Plateau and adjacent regions to the east. Misfits to the accepted measurements among PASSCAL stations and among Chinese stations are similar, with a standard deviation of ∼1.7 s, which indicates that the final dispersion measurements from Chinese and PASSCAL stations are of similar quality. Phase velocities across the Tibetan Plateau are lower, on average, than those in the surrounding nonbasin regions. Phase velocities in northern Tibet are lower than those in southern Tibet, perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained. At short periods ( 20 s), very high velocities are imaged in the Tarim Basin, the Ordos Block, and the Sichuan Basin. These phase velocity dispersion maps provide information needed to construct a 3-D shear velocity model of the crust across the Tibetan Plateau and surrounding regions.

Uppermost mantle velocities and anisotropy beneath Europe

Pn data collected within southern Europe and the Mediterranean are used to tomographically image variations in both seismic velocity and seismic anisotropy. Seismic anisotropy is an essential part of the inversion, and without it, several low velocity features within the uppermost mantle could not be properly imaged. The technically active mantle of southern Europe has much lower seismic velocities (7.6–8.1 km/s) than the more stable mantle of the sub-African plate of the Adriatic sea (8.3 km/s). However, the most dramatic features within Europes uppermost mantle relate to the Apennine, Dinaride, and Hellenide arcs. These arcs all have extremely low ( 5%) amounts of arc-parallel anisotropy. The Tyrrhenian and Aegean back arc regions also show low velocities (7.7–7.9 km/s) but less anisotropy. The same may be true for the Pannonian Basin, but the tomography has poorer resolution there. A model explaining these observations focuses on subducted water metasomatizing the mantle wedge. The addition of water causes melting, creates arc volcanism, lowers the seismic velocity, and enhances the formation of anisotropy due to preferential olivine orientation. For collisional arcs of the northern Mediterranean, arc-parallel anisotropy has formed in response to compression across them and extension along them. Within back arc regions, water is no longer a major factor. Instead, convection associated with subduction and back arc extension controls the anisotropy.

Lithospheric assembly and modification of the SE Canadian Shield: Abitibi-Grenville teleseismic experiment

This paper presents the results of a joint Lithoprobe-Incorporated Research Institutions for Seismology (IRIS)/Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) teleseismic experiment that investigates portions of the Grenville and Superior Provinces of the Canadian Shield along the Quebec-Ontario border. Data from a 600-km-long, N-S array of 28 broadband seismographs deployed between May and October 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the Canadian National Seismograph Network and the Southern Ontario Seismic Network. Relative delay times of P and S waves from 123 and 40 teleseismic events, respectively, have been inverted for velocity perturbations in the upper mantle and reveal a low-velocity, NW-SE striking corridor that crosses the southern portion of the line at latitude 46°N and lies between 50 and 300 km depth. Multievent S K S-splitting results yield an average delay time of 0.57±0.22 s and a direction of fast polarization of N93°E±18°, which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event. Subtle variations in splitting parameters over the low-velocity corridor may suggest an associated disruption in mantle fabric. Profiling of radial receiver functions reveals large and abrupt variations in Moho topography, specifically, a gradual thickening in crust from 40 to 45 km between latitudes 45°N and 46°N, which is followed by an abrupt thinning to 35 km at 46.6°N, some 65 km southeast of the Grenville Front. This structure is interpreted as a subduction suture extending the full length of the Front and punctuating a major pre-Grenvillian (Archean-Proterozoic) episode of lithospheric assembly in the southeast Canadian Shield. The low-velocity mantle corridor, by contrast, is better explained as the extension of the Monteregian-White Mountain-New England seamount hotspot track below the craton and is here postulated to represent interaction of the Great Meteor plume with zones of weakness within the craton developed during earlier rifting episodes.

Tomography of the western United States from regional arrival times

First arrival times from regional distances (200–1200 km) in the western United States extracted from the International Seismological Centre (ISC) data set were inverted in a tomographic study to map the laterally varying Pn velocity structure of the uppermost mantle (i.e., the mantle lid) and to estimate the crustal static delay at each seismograph station. Synthetic data were used to evaluate resolution. Results correlate well with major tectonic features. We find low uppermost mantle velocities (V 7.9 km/s). These low apparent Pn velocities are primarily due to the effects of isostasy, hotspot volcanism, and crustal extension. We interpret the low apparent Pn velocities beneath the Sierra Nevada (7.6–7.7 km/s) to be the result of time delays introduced by ray paths tunneling through the deep crustal root. The Yellowstone hotspot, migrating along the Snake River Plain, has left low velocity (<7.6–7.8 km/s), hot upper mantle in its wake. Lower Pn velocities (∼7.8 km/s) beneath the extensional Basin and Range are also presumably due to hotter uppermost mantle. Based on station static delays for the Basin and Range, we infer that Moho depths there do not vary significantly from 30 km. Normal continental Pn velocities (∼8.0 km/s) as well as thicker crust (40–50 km) underlying the Rocky Mountains, Wyoming Basin Province, Colorado Plateau, and the northern Columbia Plateau indicate the presence of relatively cool uppermost mantle beneath these regions.

Compressional velocity structure and anisotropy in the uppermost mantle beneath Italy and surrounding regions

Travel times of about 39,000 Pn arrivals recorded from regional earthquakes by the Italian Telemetered Seismic Network and by stations of nearby countries are inverted to image lateral variations of seismic velocity and anisotropy at subcrustal depth in Italy and surrounding regions. This method allows simultaneous imaging of variations of Pn velocity and anisotropy, as well as crustal thickness variations. The Po plain, the Adriatic Sea, and the Ionian Sea have normal to high Pn velocities. In contrast, lower velocities (7.9-8.0 kin/s) are imaged in Italy beneath the western Alps, the northern Apennines, and eastern Sicily and nearby Calabria, as well as in northern Albania and beneath the Pannonian basin. Low Pn velocities beneath the northern Apennines correlate with present-day extension and may have resulted from thermal anomalies in the uppermost mantle possibly due to delamination processes. Low velocities are consistent with the high-attenuation zone inferred in the uppermost mantle beneath the internal Apennine units and the Tyrrhenian margin of the peninsula by Mele et al. (1996, 1997). On the contrary, low velocities beneath the western Alps may be an apparent effect due to the abrupt thickening of the crustal roots. Pn velocity is anisotropic in the study area with a maximum amplitude of + 0.2 km/s. The largest anisotropic velocity anomalies are observed along the major arc structures of Italy, i.e., the northern Apennines and the Calabrian Arc, indicating that these features are controlled by uppermost mantle processes. The anisotropy anomaly along the Calabrian Arc extends as far as Albania but ends abruptly north of this area, suggesting that a lithospheric discontinuity is present along the northern Albanian border.

Seismic azimuthal anisotropy beneath the Pakistan Himalayas

Teleseismic S, SKS and SKKS data, collected from a temporary broadband array across the Himalayan front in Pakistan, are analyzed for shear-wave splitting parameters. The SKS and SKKS phases have ray paths originating from both the South Pacific and Colombia which have azimuths approximately 40° apart with respect to the Pakistan array. If significant seismic azimuthal anisotropy is present we should observe splitting associated with one of these ray paths. No evidence was seen for any shear-wave splitting beneath any of the stations in the array. Teleseismic S waves were also used in order to provide better azimuthal coverage for the shear-wave splitting measurements. We were able to correct for any source-side anisotropy when needed. No receiver-side splitting was observed in any of the S wave data. The lack of shear-wave splitting beneath the Pakistan array indicates that there is no appreciable large-scale azimuthal anisotropy beneath this part of the Himalayas. Therefore, if there is any significant strain in the upper mantle beneath this area, it must either be vertically oriented, or, if horizontal, vertically vary in such a way that the integrated effect on S wave splitting is null.

ML Amplitude Tomography in North China

We have selected 10,899 M L amplitude readings from 1732 events re- corded by 91 stations, as reported in the Annual Bulletin of Chinese Earthquakes (ABCE), and have used tomographic imaging to estimate the lateral variations of the quality factor Q0 (Q at 1 Hz) within the crust of Northern China. Estimated Q0 values vary from 115 to 715 with an average of 415. Q0 values are consistent with tectonic and topographic structure in Eastern China. Q0 is low in the active tectonic regions having many faults, such as the Shanxi and Yinchuan Grabens, Bohai Bay, and Tanlu Fault Zone, and is high in the stable Ordos Craton. Q0 values are low in several topographically low-lying areas, such as the North China, Taikang-Hefei, Jianghan, Subei-Yellow Sea, and Songliao basins, whereas it is high in mountainous and uplift regions exhibiting surface expressions of crystalline basement rocks: the Yinshan, Yanshan, Taihang, Qinlin, Dabie and Wuyi Mountains, and Luxi and Jiaoliao Uplifts. Quality-factor estimates are also consistent with Pn- and Sn-velocity patterns. High- velocity values, in general, correspond with high Q0 and low-velocity values with low Q0. This is consistent with a common temperature influence in the crust and uppermost mantle.