摘要:

The June 15, 1896 Sanriku earthquake generated devastating tsunamis with the maximum run‐up of 25 m and caused the worst tsunami disaster in the history of Japan, despite its moderate surface wave magnitude (Ms=7.2) and weak seismic intensity. This is a typical tsunami earthquake, which generates anomalously larger tsunamis than expected from its seismic waves. Previously proposed mechanisms of tsunami earthquakes include submarine slumping and slow rupture in the accretionary wedge or in the subducted sediments. In this paper, we estimate the fault parameters of the 1896 tsunami earthquake by numerically computing the tsunami and comparing the waveforms with those recorded at three tide gauge stations in Japan. The result indicates that the tsunami source is very close to the Japan trench and the fault strike is parallel to the trench axis. The fault width is about 50 km, suggesting that the tsunami earthquake is a slow rupture in the subducted sediments beneath the accretionary wedg

ISSN:0094-8276    

DOI:10.1029/96GL01479

年代:1996

数据来源: WILEY

摘要:

The double‐array (event‐ and receiver‐array) stacking is applied to the P‐wave data from 6 Fiji Is. events recorded by a local network of short‐period seismographs in Japan to investigate Vpstructure in the lowermost mantle. A signal which may be interpreted as the reflection from a discontinuity at 170 km above the CMB (core‐mantle boundary) is observed between P and PcP in the stacked record. As compared with the receiver‐array stacking with a small number of stations applied to a single event, the double‐array stacking is effective to detect the wea

ISSN:0094-8276    

DOI:10.1029/96GL01564

年代:1996

数据来源: WILEY

摘要:

A positive anomaly in the frequency‐magnitude distribution (b‐value) is detected at approximately 90–100 km depth in two subduction zones. The b‐value in this anomalous zone is 40% higher than in adjacent volumes. We use regional catalogs with a magnitude of completeness of 2.6 (Central Alaska and Cook Inlet), 2.0 (Shumagin Islands), and 3.5 (New Zealand). To resolve the b‐value as a function of space in more detail, we project all earthquakes onto planes perpendicular to the strike of the slab. These cross‐section views of the b‐value distribution locate the high b‐value anomalies at a depth of 90–100 km, on the upper surface of the Wadati‐Benioff Zone. At these depths, slab dehydration may increase pore pressure, thus lowering the effective stress and increasing the b‐value. Increased pore pressure would also lower the liquidus in the overlying asthenosphere, giving rise to the volcanism that occurs dir

ISSN:0094-8276    

DOI:10.1029/96GL01233

年代:1996

数据来源: WILEY

摘要:

We calculated Coulomb failure stress change caused by the March 13, 1992 Erzincan, Turkey, earthquake, and explored the relationship between failure stress and the aftershock distribution which includes the Pülümür earthquake (Ms=5.8) that occurred two days later. One of the most significant features of the Erzincan earthquake was the location of aftershocks, which did not correspond with either the eastern segment of the North Anatolian fault zone or the Ovacik fault. This feature can be explained by mapping the failure stress due to the Erzincan earthquake. The map revealed that there is a significant correlation between the aftershock distribution and the areas where static stress was raised by ≥0.3 bar. The 1992 Erzincan earthquake raised the Coulomb failure stress about 1.4 bar at the site of the Pülümür event. This stress rise and optimum orientation of the Pülümür fault favoured its

ISSN:0094-8276    

DOI:10.1029/96GL01323

年代:1996

数据来源: WILEY

摘要:

As part of the UNAM drilling program at the Chicxulub structure, two 700 m deep continuously cored boreholes were completed between April and July, 1995. The Peto UNAM‐6 and Tekax UNAM‐7 drilling sites are ∼150 km and 125 km, respectively, SSE of Chicxulub Puerto, near the crater's center. Core samples from both sites show a sequence of post‐crater carbonates on top of a thick impact breccia pile covering the disturbed Mesozoic platform rocks. At UNAM‐7, two impact breccia units were encountered: (1) an upper breccia, mean magnetic susceptibility is high (∼55 × 10−6SI units), indicating a large component of silicate basement has been incorporated into this breccia, and (2) an evaporite‐rich, low susceptibility impact breccia similar in character to the evaporite‐rich breccias observed at the PEMEX drill sites further out. The upper breccia was encountered at ∼226 m below the surface and is ∼125 m thick; the lower breccia is immediately subjacent and is>240 m thick. This two‐breccia sequence is typical of the suevite‐Bunte breccia sequence found within other well preserved impact craters. The suevitic upper unit is not present at UNAM‐6. Instead, a>240 m thick evaporite‐rich breccia unit, similar to the lower breccia at UNAM‐7, was encountered at a depth of ∼280 m. The absence of an upper breccia equivalent at UNAM‐6 suggests some portion of the breccia sequence has been removed by erosion. This is consistent with interpretations that place the high‐standing crater rim at 130–150 km from the center. Consequently, the stratigraphic observations and magnetic susceptibiity records on the upper and lower breccias (depth and thickness)

ISSN:0094-8276    

DOI:10.1029/96GL01566

年代:1996

数据来源: WILEY

摘要:

A set of geophysical measurements and observations was undertaken to investigate the nature of the crust beneath the epicentral region of the deadly Mw 6.1 Latur earthquake of September 30, 1993. With an estimated focal depth of 2.6 km and the associated well defined but subtle surface ruptures, it is a rare stable continental region (SCR) earthquake with surface rupture. The focal depth of 69 out of 73 well located aftershocks is less than 5.5 km. Broad band (10³–10−3Hz) magnetotelluric (MT) soundings reveal the presence of an anomalously high conductivity zone at a shallow depth range of 6–10 km. Consistent with this result is the observation of a Pc phase, lagging behind the Pg phase by about 0.6 to 0.8 sec in the aftershock seismograms indicating a low velocity layer (LVL) at 7 to 10 km depth. A Bouguer gravity low of 5 m.gal, nearly coincident with this feature, is also observed. Above evidences indicate that the focal zone of the Latur earthquake sequence is limited to depths of about 5 to 6 km in the upper crust by an underlying low‐velocity and high conductivity layer. We interpret this high conductive, low velocity layer as a fluid filled fractured rock matrix. The inferred stress regime, including due to uplift of the Deccan Plateau, triggered by erosion of basalt cover is likely to be confined mostly in the upper part of the crust. Existence of a low velocity, high conductivity fluid filled layer will enhance stress concentration in the uppermost brittle part of the crust causing mechanical

ISSN:0094-8276    

DOI:10.1029/96GL01032

年代:1996

数据来源: WILEY

摘要:

Whether the high seismic‐velocity anomalies in the upper mantle under California represent cold downwelling mantle flow or fragments of the subducted Farallon plate has important implications for mantle dynamics related to the tectonic history of western North America. We investigate the former possibility by simulating the time‐dependent thermal evolution in the asthenosphere beneath California, in the wake of the migrating Mendocino triple junction (MTJ), using a two‐dimensional model that represents a transect parallel to the direction of the MTJ migration. For viscosity within the typical range for the asthenosphere beneath active tectonic regions (1×1018– 2×1019Pa s), our model predicts vigorous small‐scale convection characterized by periodic formation of cold downwelling flows near the southern edge of the migrating Gorda plate. Within the relevant time frame, the predicted convective thermal structures are comparable to the major mantle features shown by seis

ISSN:0094-8276    

DOI:10.1029/96GL00727

年代:1996

数据来源: WILEY

摘要:

A mathematical model is used to investigate whether radon degassing from groundwater may contribute to indoor radon levels. Specifically, the transport of radon in the soil‐gas phase from the groundwater‐soil gas interface to under‐pressurized dwellings is modelled. The question whether radon in groundwater may contribute to indoor radon levels arises from observed high radon concentrations in groundwater, and recent findings that advection in the gas phase may be an important transport mechanism for radon into slightly under‐pressurized dwellings. Most previous radon transport investigations did not consider groundwater as a potential source for contributing to indoor radon. The mathematical model includes a method to directly calculate indoor radon concentrations and an equivalent continuum approach to represent cracks in concrete foundations. The results of the simulations indicate that radon, which partitions from groundwater to the soil gas, may be advectively transported by the gas phase to slightly underpressurized dwellings in relatively permeable soils such that indoor radon concentrations may exceed 148 Bq/m³, which is the action limit impose

ISSN:0094-8276    

DOI:10.1029/96GL01433

年代:1996

数据来源: WILEY

摘要:

We present the first mercury measurements made in African waters utilizing ultraclean collection and state‐of‐the‐art analytical techniques. Concentrations of total mercury (HgT) in streams entering Lake Naivasha, Kenya are similar to values obtained from runoff water in the Northern Hemis

ISSN:0094-8276    

DOI:10.1029/96GL01434

年代:1996

数据来源: WILEY

摘要:

Limestone masonry from monuments dating from 300AD to 1940, remagnetized partly and progressively with age, parallel to the present Earth's magnetic field. The remagnetization was probably due to viscous remanent magnetization (VRM). Néel's theory predicts that natural VRM acquired at low temperature (TA) over a long time (tA, disappears at a higher temperature (TD) in a short period (tD) in the laboratory. I calibrated TDfor the postconstruction remagnetization against the ages (tD) of masonry used in well‐dated, historic buildings in Eastern England. Empirically, tA= m.(TD)brelates acquisition age (tAyears) to demagnetization temperature (TD°C). Lincoln limestone remagnetizes more quickly ( m = 1.298 × 10−6and b = 3.598 over 1800 years) than Speeton Chalk ( m = 4298 × 10−6and b = 1.973 over 350 years). Magnetite with an effective pseudo‐single domain or small multidomain size carries th

ISSN:0094-8276    

DOI:10.1029/96GL01575

年代:1996

数据来源: WILEY