The Central Andes is a key global location to study the enigmatic relation between volcanism and plutonism because it has been the site of large ignim briteforming eruptions during the past several million years and currently hosts the world's largest zone of silicic partial melt in the form of the Alti plano Puna Magma (or Mush) Body (APMB) and the Southern Puna Magma Body (SPMB). In this themed issue, results from the recently completed PLUTONS project are synthesized. This project focused an interdisciplinary study on two regions of largescale surface uplift that have been found to represent ongoing movement of magmatic fluids in the middle to upper crust. The loca tions are Uturuncu in Bolivia near the center of the APMB and Lazufre on the Chile Argentina border, on the edge of the SPMB. These studies use a suite of geological, geochemical, geophysical (seismology, gravity, surface defor ma tion, and electromagnetic methods), petrological, and geomorphological techniques with numerical modeling to infer the subsurface distribution, quantity, and movements of magmatic fluids, as well as the past history of eruptions. Both Uturuncu and Lazufre show separate geophysical anomalies in the upper, middle, and lower crust (e.g., low seismic velocity, low resistiv ity, etc.) indicating multiple distinct reservoirs of magma and/or hydrothermal fluids with different physical properties. The characteristics of the geophysical anomalies differ somewhat depending on the technique used-reflecting the different sensitivity of each method to subsurface melt (or fluid) of different compositions, connectivity, and volatile content and highlight the need for integrated, multidisciplinary studies. While the PLUTONS project has led to significant progress, many unresolved issues remain and new questions have been raised.

S U M M A R YMicroseisms are a continuous source of seismic signal which mainly consist of Rayleigh waves but are known to contain some other types of seismic waves. We developed a simple processing procedure for single-station three-component seismic data which allows us to select Rayleighwave dominated portions. Application of this procedure to data from Southern California led us to confirm that excitation of the secondary-peak microseisms (the predominant energy at about 0.15 Hz) occurs in coastal regions; the source directions, viewed at each station, do not change very much throughout the year. We also discovered that the ratio of the horizontal to vertical amplitudes in Rayleigh waves, termed H/Z in this paper, is shown to have seasonal variations. The H/Z estimates typically reach their maximum in winter and their minimum in summer. Seasonal variations are observed at most stations but peak-to-peak amplitudes of seasonal variations vary greatly from station to station, ranging between 0 and 40 per cent. Two hypotheses were examined to explain this phenomenon; the first hypothesis is that it is caused by seasonal changes in seismic velocities in a layer between the surface and the groundwater level. The second hypothesis is that it is caused by seasonal changes in relative excitation of higher modes compared to dominant fundamental-mode Rayleigh waves. The first hypothesis is not likely, because predicted amplitudes of seasonal variations in H/Z are too small to explain observed variations. The second hypothesis seems quite plausible if source regions move seasonally among regions with different ocean depths. The technique developed in this paper, however, is not sufficient to answer this question conclusively.

S U M M A R YS-wave velocity in the shallow crust is an important controlling parameter for ground motion amplification. It is a key parameter for prediction of ground motion and thus for earthquake hazard mitigation in general. Shallow S-wave velocity structure has often been obtained from phase velocity of microtremors and microseisms. We present a new approach which directly inverts the ratio of horizontal to vertical amplitudes of microseisms which is referred to as the HZ ratio in this paper. Our approach consists of (i) isolation of Rayleigh-wave dominant portions in seismograms by using the 90-degree phase shift between horizontal and vertical component, (ii) measurement of the HZ ratios as a function of frequency and (iii) iterative inversion for S-wave velocity structure in the crust. Depth sensitivity kernels of the HZ ratios for density, P-wave velocity and S-wave velocity are derived by a numerical method. Examples of the application of this method to data in southern California show that the HZ ratio data require modification of the standard seismic velocity model in this region (Southern California Earthquake Center Community Velocity Model 3.0 or SCEC CVM) in the upper 10-15 km. The derived models show amplification of incident SH waves by factors of 2-8, relative to the prediction by the SCEC CVM. The proposed approach has promise for applications to other regions in the world because microseisms are observed everywhere in the world. The method may become even more powerful if it is combined with phase velocity data.

S U M M A R YDirect detection of Rayleigh-wave azimuthal anisotropy is reported by applying an array analysis to broad-band seismic data in Southern California, USA. Our approach has excellent resolution for frequencies between 30 and 60 mHz and good resolution between 10 and 30 mHz. Limitation from array size limits accuracy below 10 mHz and complicated wave propagation effects lead to difficulty above 60 mHz. Between 30 and 60 mHz, azimuthal anisotropy of Rayleigh-wave phase velocity is detected cleanly. Phase velocity results show that the 2θ components dominate the 4θ components. Strength of anisotropy hovers around 1 per cent for the 2θ components with standard error (1σ ) of about 0.4 per cent. Fast axes orientations show stable orientation in the direction 110 • clockwise from north and its opposite direction 290 • . Depth sensitivity kernels for P-wave and S-wave anisotropy indicate that anisotropy is confined to depths in the upper 100 km and may even be to shallower depths. We speculate that the fast axes alignment may be associated with a strong shearing process over the thickness of lithosphere due to obliquely collisional motions between the Pacific Plate and the North America Plate in the region.

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