摘要:

Tomographic inversion is applied to delay times from aftershocks of the 1986 ML5.9 North Palm Springs (NPS) earthquake to image 3‐D velocity variations within the northern Coachella Valley. P‐wave arrival times from 1074 earthquakes, with depths ranging from 3 to 20 km, were used as sources recorded by 12 portable and 4 permanent stations. Preliminary results show well‐defined high‐ and low‐velocity anomalies (2–7%) that correlate with the rupture distribution of the 1986 mainshock. At depths less than 8 km, a low‐velocity anomaly predominates between the two NE‐dipping Banning and Mission Creek faults. From 8 to 12 km in depth, where the NPS mainshock and most of the aftershocks occur, a high‐velocity anomaly is observed. This high‐velocity feature is interpreted as imaging the asperity responsible for the 1986 rupture; and suggests that velocity information may help to define important elements, such as asperities, that control fault rupture, and thus, may help to predict the location and si

ISSN:0094-8276    

DOI:10.1029/91GL03037

年代:1992

数据来源: WILEY

摘要:

The analysis of teleseismic travel time residuals recorded on the Larderello Seismic Network of central Italy has revealed a sharp low velocity zone (LVZ) in the center of the geothermal production area. Residuals from 101 teleseismic events are inverted to produce an image of a region of anomalously low velocities which we have interpreted to be a series of intrusive magmatic bodies. The top of this anomalous region is constrained to be below 6 km in depth from local earthquake seismicity patterns and extends to a depth of greater than 20 km. The LVZ broadens with increasing depth and may extend into the upper mantle. The observed travel time residual anomaly correlates spatially with observed gravity lows, regions of high heat flow, and the shallowing of a dominant upper crustal reflection horizon. This body has elevated the temperatures in the region to over 350° C at 2 km and appears to be the origin of the geothermal energy

ISSN:0094-8276    

DOI:10.1029/91GL01182

年代:1992

数据来源: WILEY

摘要:

Continuous measurements of vertical subsurface geoelectric fields in the frequency range dc – 0.6 Hz and 1 – 3 kHz (VLF) are reported. Anomalous field variations of the ULF band are observed a few days prior to, and a month after, a minor volcanic eruption of Mt. Mihara. The variations displayed essentially distorted pulses, rising sharply initially, gradually decreasing, and then suddenly falling off. The electric field variations also seem to extend to the VLF band. It is felt that the anomalous electric field variations were caused by electrokinetic phenomena induced by variations of hydrothermal circulation around the crater, or by variations of crack density, due to volcanic activ

ISSN:0094-8276    

DOI:10.1029/91GL02822

年代:1992

数据来源: WILEY

摘要:

We analyze over 1300 km of high resolution along‐axis gravity profiles at ridges with half‐spreading rates ranging from 1.2 to 5.5 cm/yr. The results show consistently higher along‐axis gradients of mantle Bouguer anomaly at the slow‐spreading Mid‐Atlantic Ridge (MAR) (0.3–1.2 mgal/km) than at the intermediate‐ to fast‐spreading Cocos‐Nazca Ridge and East Pacific Rise (EPR) (0.1–0.2 mgal/km). The regional peak‐to‐trough amplitude of mantle Bouguer anomaly is also greater along the MAR (30–60 mgal) than the Cocos‐Nazca Ridge and the EPR (10–20 mgal). With increasing spreading rate, the regional peak‐to‐trough amplitude of axial seafloor depth decreases from 1000–1700 m to 200–700 m. 3‐D numerical experiments suggest that mantle contributions to the gravity can be significant only near large‐offset transforms. At the more commonly observed non‐transform offsets, gravity anomalies will reflect crustal thickness variations.The along‐axis gravity data thus indicate that the amplitude of along‐axis crustal thickness variation decreases with increasing spreading rate. We propose that this spreading rate dependent crustal accretion style may originate in the mantle: finite‐amplitude mantle upwelling is intrinsically plume‐like (3‐D) beneath a slow‐spreading ridge but more sheet‐like (2‐D) beneath a fast‐spreading ridge. Such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate. Small amplitude 3‐D upwellings may occur at a fast‐spreading ridge, but their effects on crustal thickness variations will be significantly reduced by along‐axis melt flows along a persistent low‐viscosity crustal magma chamber. In contrast, the large crustal thickness variations due to 3‐D mantle upwellings will be maintained at a slow‐spreading ridge because less

ISSN:0094-8276    

DOI:10.1029/91GL03041

年代:1992

数据来源: WILEY

摘要:

The secondary ion mass spectrometry (SIMS) was applied to measure the trace element abundances in majorite and the ultramafic liquid equilibrating at high pressure and temperature for the first time, and was proved to be useful for the high pressure experimental petrology. The analysis by SIMS was made to determine the abundance of 15 lithophile elements (Li, Be, B, Sc, Ti, V, Cr, Mn, Y, Zr, La, Sm, Gd, Yb, and Hf), one siderophile (Co), and two chalcophile elements (Cu, Zn). The electron microprobe (EPMA) analysis was also made to obtain the major element abundance. Some lithophile elements (Al, Be, and V) are enriched in majorite, whereas the lithophile elements (Li, B, Na, K, Ca, Ti, Zr, Y, La, Sm, Gd, and Hf), and siderophile and chalcophile elements (Co, Cu, Zn) are favored in the liquid. These results give important constraint for the genesis of komatiites and chemical heterogeneity in the Archean mantle.

ISSN:0094-8276    

DOI:10.1029/91GL02824

年代:1992

数据来源: WILEY

摘要:

The Magsat data have yielded a global cartography of long‐wavelength anomalies of lithospheric origin. The magnetizations needed to model these anomalies are much higher than those measured on samples by laboratory experiments. An enhancement of both induced and viscous magnetization at high temperature has often been put forward to account for the “missing magnetization”. Experimental measures of viscous magnetization Mvat high temperature are presented for oceanic deep crustal‐ and mantle‐rocks. At room temperature, Mvhereafter defined in experiments as the magnetization acquired after 400 s in a weak field, is negligible in comparison with the room temperature induced magnetization Mk0. At higher temperatures, between 200°C and 450°C, Mvrepresents about 20% of Mk0and is further enhanced near 500°C, ranging from 25 to 130% of Mk0. An extrapolation of these results to geological times shows that the viscous magnetization acquired since the last reversal of the Earth magnetic field may be of the order of magnitude of the induced magnetization Mk0, above 200°C and possibly predominant near 500°C. However, a magnetic uppermost mantle is still needed to explain the magnetizations of oceanic lithosphere derived fr

ISSN:0094-8276    

DOI:10.1029/91GL02796

年代:1992

数据来源: WILEY

摘要:

The validity of concept that impulsive changes in the volume flux of material rising from the base of the mantle via plumes can cause rapid changes in the heat flux from the core and in the behavior of the geomagnetic field is investigated. It is found that the natural time scale of variations in the heat flux from the core is about 109years, a time scale far longer than that associated with the long‐term changes in the frequency of reversals of the magnetic field; following an impulsive change in plume flux, the heat flux from the core changes on a time scale of several billion years, i.e., on a time scale of the age of the earth. A harmonic variation in the volume flux up plumes results in a similar harmonic variation in the heat flux from the core, but with a phase lag of π/4 and a greatly diminished amplitude. The lack of correlation between mantle plume flux and core heat flux is due to the long natural time scale of the lower mantle and the large thermal inertia of the D″ layer (compared with the heat flux from the core). This calls into question the currently fashionable idea of a direct and immediate correlation between plume volume flux and heat flux from the

ISSN:0094-8276    

DOI:10.1029/91GL02902

年代:1992

数据来源: WILEY

摘要:

As the atmosphere, ocean, and sea ice components of global climate models are made increasingly interactive, systematic errors or biases in one component can adversely affect the other model components. The fidelity of the component interactions is especially important in the polar regions, where many atmospheric General Circulation Models (GCMs) project an amplified climatic response to increasing concentrations of greenhouse gases. In comparing the Arctic performance of five atmospheric GCMs (GFDL, GISS, NCAR, OSU, and UKMO), we illustrate key differences in the fields most relevant to sea ice/ocean forcing: surface air temperature and sea level pressure (surface wind stress). While the amplitude of the seasonal cycle of simulated air temperature is generally realistic, biases of up to 5–10°C relative to observations are apparent over much of the Arctic. The simulated sea‐level pressure pattern varies widely from model to model, and in some cases is incompatible with the observed wind‐forcing of sea ice from the Arctic Basin to the North Atlantic via Fram Strait. The implications that these differences have for transports of salinity are signi

ISSN:0094-8276    

DOI:10.1029/91GL03004

年代:1992

数据来源: WILEY

摘要:

An impactor/concentrator/microdensitometer (ICM) instrument system has been constructed and calibrated. This system is sufficiently sensitive to measure the black (carbon soot) component of Antarctic aerosol with a sampling time of four hours. The impactor concentrator was exposed to Antarctic air at Ross Island in September 1987. Microdensitometer analysis of the collected specimens indicates that the maximum black aerosol concentration was observed concurrently with the arrival of the warmest air accompanying a cyclonic storm. This is similar to the concurrence of continental radon and lead isotopes with warm advection, measured on the Antarctic coast by Polian et al. (1986). It is possible that continental soot can be transported to the Antarctic coast several times each year by this mechanism.

ISSN:0094-8276    

DOI:10.1029/91GL02912

年代:1992

数据来源: WILEY

摘要:

Recent satellite measurements of total atmospheric ozone were analyzed to deduce the changes in biologically active ultraviolet (UV) radiation reaching the Earth's surface from 1979 to 1989. The calculated increases are on average substantially larger than earlier estimates, particularly at mid and high latitudes of both hemispheres.

ISSN:0094-8276    

DOI:10.1029/91GL02954

年代:1992

数据来源: WILEY