Max Wyss

Estimating maximum expectable magnitude of earthquakes from fault dimensions

The evaluation of seismic risk at locations where sensitive man-made structures are planned depends critically on a correct estimate of the maximum expectable earthquake magnitude, M max, in that region. By assuming that the longest fault (or fault unit) with length L max could break in a single earthquake, one estimates M max from L max on the basis of a magnitude versus source-length relation, which is derived empirically. The maximum expectable ground accelerations are then estimated from M max. I propose that a more accurate estimate of M max can be obtained by determining the maximum expected rupture area, A max, and using the magnitude-area relation M = log A + 4.15 (valid for M > 5.6). A max can be obtained from the product of L max times the expected fault width. The latter can probably be estimated more accurately than L max on the basis of tectonic analysis and microearthquakes studies. The M max estimates derived from rupture area give more accurate results than the estimates based on rupture length alone, because narrow faults produce less powerful earthquakes than do wide faults of the same length.

Precursory variation of seismicity rate in the Assam area, India

The seismicity data from 1825 to the present for the Assam (north-eastern India) region show that seismicity rates there deviate from normal before and after major earthquakes. Along this 1,000-km-long section of a plate boundary, all shocks with magnitude M > 6.6 were preceded and sometimes followed by periods of significant seismic quiescence. No major earthquakes occurred without an associated seismic quiescence, and no such quiescence occurred at times other than before or after a major event. The most remarkable periods of quiescence lasted about 28 and 30 y before the two great (M = 8.7) Assam earthquakes of 1897 and 1950. Other periods of anomalously low seismicity preceded main shocks of magnitudes 6.7 (in 1950 and 1975), 7.8 (in 1869), and 7.7 (in 1947), with durations of 6, 8, 23, and 17 y, respectively. These durations fit (with approximately the scatter of the original data) a published relation between precursor time and magnitude. Since these changes of seismicity rate were observed at the edges of and within the Assam gap, defined by the 1897 and 1950 great earthquakes, it is likely that a future major or great earthquake in this gap will be preceded by seismic quiescence. Whether amorexa0» preparatory phase for an earthquake has begun in the Assam gap cannot be stated for certain because of the changing earthquake-detection capability in the area and because of poor location accuracy. 4 figures.«xa0less

Seismic Quiescence Precursory to a Past and a Future Kurile Island Earthquake

A systematic search was made for seismicity rate changes in the segment of the Kurile island arc from 45°N to 53°N by studying the cumulative seismicity of shallow (h≤100 km) earthquakes within 11 overlapping volumes of radius 100 km for the time period 1960 through beginning of 1978. We found that in most parts of this island arc and most of the time the seismicity rate as obtained from the NOAA catalogue and not excluding any events is fairly constant except for increased seismicity in the mid 1960s in the southern portion due to the great 1963 mainshock there, and for seismicity quiescence during part of the time period studied within two well defined sections of the arc. The first of these is a volume of 100 km radius around a 1973 (Ms=7.3) mainshock within which the seismicity rate was demonstrated at the 99% confidence level to have been lower by 50% during 2100 days (5.75 years) before this mainshock. The second volume of seismic quiescence coincides with the 400 km long north Kuriles gap. In this gap the seismicity rate is shown (at the 99% confidence level) to be lower by 50% from 1967 to present (1978), in comparison with the rate within the gap befor 1967, as well as with the rate surrounding the gap. We propose that the anomalously low seismicity rate within the Kuriles gap is a precursor to a great earthquake, the occurrence time of which was estimated by the following preliminary relation between precursory quiescence time and source dimensionT=190L0.545. We predict that an earthquake with source length of 200–400 km (M>8) will occur along the north Kurile island arc between latitude 45.5°N and 49.2°N at a time between now and 1994.

Magnetism of Rocks and Volumetric Strain in Uniaxial Failure Tests

SummaryThe relation between remanent magnetization and volumetric strain for gabbro samples stressed in uniaxial compression inside a near zero-field μ-metal shield has been examined. For samples with an induced IRM parallel to the axis of compression, remanent magnetization decreased linearly up to the onset of dilatancy. As increased stress produced additional dilatancy, the variation of remanent magnetization became nonlinear, and the stress dependence continually decreased until the rock failed. Stress cycling with the peak stress augmented for each cycle produced a continuous decrease in the zero stress value of the IRM although an appreciable amount of recovery was observed during unloading. When the sample was loaded in constant stress increments after the onset of dilatancy and held for several minutes at each level, time-dependent variations in remanent magnetization coincided with time-dependent increases in inelastic volumetric strain. In general as the inelastic creep rate increases, the rate of change in remanent magnetization increases. These results suggest that dilatancy related effects of the intensity of rock magnetization should be observed in magnetic rocks in epicentral regions prior to earthquakes and may serve as both long- and short-term precursors.

Regular intervals between hawaiian earthquakes: implications for predicting the next event.

During the years 1941 through 1983 five earthquake mainshocks of moderate magnitude occurred at regular intervals of 10.5 � 1.5 years within a 6-kilometer radius in Hawaii. It is proposed that these Kaoiki earthquakes will continue to occur at regular intervals because the strain accumulation rate and the strained volume remain constant. With appropriate instrumentation, it may be possible to refine predictions of subsequent Kaoiki earthquakes.

Comparison of a complex rupture model with the precursor asperities of the 1975 HawaiiMs-7.2 earthquake

A simplified multiple source model was constructed for the 1975 HawaiiMs=7.2 earthquake by matching synthetic signals with three component accelerograms at two stations located approximately 45 km from the epicenter. Six major subevents were identified and located approximately. The signals of these are larger by factors of 1.4 to 3.2 than that of theML=5.9 foreshock which occurred 70 minutes before the main rupture and also triggered the SAM-1 recorders at the two stations. Dividing the rupture length (40 km) by the duration of strong ground shaking (∼ 50 sec) an, average rupture velocity of 0.8 km/sec (about 25% of S-velocity) is obtained. Thus it is likely that the rupture stopped between subevents. The approximate epicenters of the 6 major subevents, and of the foreshock, support the hypothesis that they were located in high stress asperities which rupture during the main shock, except for the last events which is interpreted as a stopping phase generated at a barrier. These asperities have been previously defined on the basis of differences in the precursor pattern before the mainshock. Thus, it appears that both the details of the precursors and of the main rupture depended critically on the heterogeneous tress distribution in the source volume. This suggests that main rupture initiation points and locations of high rupture accelerations may be identified before the mainshock occurs, based on precursor anomaly patterns. A satisfactory match of synthetic signals with the observations could be obtained only if the aximuth of the fault plane of subevents was rotated from N60°E to N90°E and back to N30°E. These orientations are approximately parallel to the nearest Kilauea rift segments. Hence the slip directions and greatest principal stresses were oriented perpendicular to the rifts everywhere. From this analysis and other work, it is concluded that this fault surface consisted of three types of segments with different strength: hard asperities (radius ≈ 5 km), soft but ‘brittle’ segments between the asperities (radius ≈ 5 km), and a ‘viscous’ half (10×40 km) which slipped during the mainshock, but where microearthquakes and aftershocks are not common.

The detection and interpretation of hydrogen in fault gases

Hydrogen gas can be released by chemical and mechanical changes in crustal rocks. Once released, it is highly mobile, buoyant, and almost insoluble in groundwater. A fault system may act as a conduit, allowing hydrogen to accumulate in soil gases near a surface expression. Since hydrogen is scarce in ambient air, its presence at elevated levels in soil gases may be a tool for fault mapping. In order to evaluate this tool, we surveyed eleven different faults by measuring the concentration of hydrogen and methane in 2 to 21 soil-gas samples that were collected near each of them. The sense of motion at four of those faults is normal (western United States, Greece), at five it is strike-slip or dip-slip (California, Colorado, Japan), and at two it is thrusting (California). At four of these faults (Hebgen Lake, Yellowstone, Yamasaki, Burro Mountain) maximum concentrations of hydrogen ranged from 80 ppm to 70% and methane from 300 ppm to 5%. All other sites showed ambient levels of both gases, except for one sample taken at Mt. Borah, Idaho, that was 2% methane. From this preliminary study it is not clear whether the presence of hydrogen is correlated uniquely to the location of faults or whether it occurs randomly. The conditions required to produced and accumulate hydrogen are also not clear. Excess hydrogen may well be produced by different mechanisms in different geological regimes. For example, if ferrous hydroxide is present in local rocks, it may react to produce hydrogen. Detailed and extensive studies are needed to clarify the connection between hydrogen and tectonic faulting.