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1.
This paper describes a new method, single-link cluster analysis (SLC), to evaluate percursory quiescence for shallow earthquakes in sixteen subduction zones, using data from the ISC catalog. To define quiescent regions, we divided the catalog into time intervals with a durationT, overlapping byT/2. We considered all earthquakes having magnitudes larger than some magnitudeM
min, lying within a specified distance of a great circle which is approximately coincident with the trench near a subduction zone. Within each time interval we connected or linked all earthquakes lying within some cutoff distanced of one another. We then projected all these links onto the great circle, and defined a region to be quiescent if it was not covered by the projection of any links. For this study,T was two years,M
min wasm
b
=4.9, and we variedd from 100 to 400 km. We defined an earthquake as following quiescence if it occurred within two years following, and within 75 km of a quiescent zone as defined above. The primary conclusion of this study was that earthquakes with surface wave magnitudes 7.2 and greater were about 5–15% more likely to follow quiescence than were the smaller background earthquakes withm
b
>-4.9. A chi-squared analysis shows that this result is significant at the 99% level. In contrast, earthquakes with surface wave magnitude of 6.7 to 7.1 were no more likely to follow quiescence than were background earthquakes. Of sixteen individual regions, Central America, Japan, and Peru-Chile were the only regions where large earthquakes were more likely to occur following quiescence than were background earthquakes. For a cutoff link length of 300 km, only in Central America was the difference between large earthquakes and background earthquakes significant at the 95% level of significance. For a cutoff link length of 250 km, the significance level exceeded 95% only in Japan. The SLC method is an objective, quantitative method for evaluating large data catalogs, or for monitoring quiescence in regions where quiescence is conjectured to precede large earthquakes. 相似文献
2.
The arrival time difference for the AB branch of PKP from deep Tonga earthquakes is constant over years with a standard deviation
of ±0.05 seconds at seismographs located 10 to 50 km from each other. If published travel time curves are used to calculate
the relative residuals of PKP the standard deviation from the constant mean is improved by approximately 0.01 seconds for
AB branch data. For the BC branch, standard deviations of relative travel times of ±0.06 seconds are reduced to less than
±0.05 seconds by calculating relative residuals. We conclude that changes of crustal transit time forP-waves could be resolved, based on careful PKP arrival time measurement at two or more neighboring stations if the changes
exceed 0.05 sec and last for more than one year. The conditions for achieving this result are that PKP from Tonga earthquakes
is clearly recorded, and that time-keeping is accurate. The data on which these conclusions are based were obtained from the
Graefenberg seismograph array, which is located in West Gemany and consists of 13 stations separated by distances of 10 km
to 100 km. We propose that relative arrival times of PKP from Tonga could be used in the Mediterranean - Middle East area
to search for precursory travel time changes before large earthquakes. 相似文献
3.
The Stone Canyon earthquake sequence started during August 1982 and lasted for about four months. It contained four mainshocks withM
L
4, each with an aftershock zone about 4 km long. These mainshocks, progressing from southeast to northwest, ruptured a segment of the fault approximately 20 km long leaving two gaps, which were later filled by theM
L
=4.6 mainshocks of January 14, and May 31, 1986. The equivalent magnitude of the sequence isM
L
=5.0.Precursory seismic quiescence could be identified in: (1) the northernmost 10 km of the aftershock zone which contained three of the mainshocks; and (2) the southern gap in the aftershock zone. The fault segment containing the first mainshock and its aftershocks did not show quiescence. This pattern of precursory quiescence is very similar to two cases in Hawaii where the rupture initiation points of the mainshocks (M
S
=7.2 and 6.6, respectively) were located in volumes of constant seismicity rate, surrounded by volumes with pronounced precursory quiescence.The precursory quiescence before the August 1982 Stone Canyon earthquakes lasted for 76 weeks, amounted to a reduction in rate of about 60%, and could be recognized without any false alarms. That is, the anomaly was unique within the 60 km study segment of the fault and in the years 1975 through August 1982. Eighteen foreshocks occurred between July 27 and August 7, 1982. We conclude that the August 1982 mainshocks could have been predicted, based on seismic quiescence and foreshocks. 相似文献
4.
In seismological literature, there exist two competing theories (the so-calledW model andL model) treating earthquake scaling relations between mean slip and rupture dimension and between seismic moment and rupture dimension. The core of arguments differentiating the two theories is whether the mean slip should scale with the rupture width or with the rupture length for large earthquakes. In this paper, we apply the elastic theory of dislocation to clarify the controversy. Several static dislocation models are used to simulate strike-slip earthquakes. Our results show that the mean slip scales linearly with the rupture width for small earthquakes with a rupture length smaller than the thickness of the seismogenic layer. However, for large earthquakes with a rupture length larger than the thickness of the seismogenic layer, our models show a more complicated scaling relation between mean slip and rupture dimension. When the rupture length is smaller than a cross-over length, the mean slip scales nearly linearly with the rupture length. When the rupture length is larger than a cross-over length, the mean slip approaches asymptotically a constant value and scales approximately with the rupture width. The cross-over length is a function of the rupture width and is about 75 km for earthquakes with a saturated rupture width of 15 km. We compare our theoretical predictions with observed source parameters of some large strike-slip earthquakes, and they match up well. Our results also suggest that when large earthquakes have a fixed aspect ratio of rupture length to rupture width (which seems to be the case for most subduction earthquakes) the mean slip scales with the rupture dimension in the same way as small earthquakes. 相似文献
5.
G. M. Reimer 《Pure and Applied Geophysics》1984,122(2-4):369-375
The observations of short-term decreases in helium soil-gas concentrations along the San Andreas Fault in central California have been correlated with subsequent earthquake activity. The area of study is elliptical in shape with radii approximately 160×80 km, centered near San Benito, and with the major axis parallel to the Fault. For 83 percent of theM>4 earthquakes in this area a helium decrease preceded seismic activity by 1.5 to 6.5 weeks. There were several earthquakes without a decrease and several decreases without a corresponding earthquake. Owing to complex and unresolved interaction of many geophysical and geochemical parameters, no suitable model is yet developed to explain the observations. 相似文献
6.
Cumulative seismic strain release, during the period from 1964 through 1972, along the East Pacific and Chile rise edges of the Nazca plate is related to cumulative strain release of shallow earthquakes along the South American sinking edge: periods of high strain release along the rises alternated systematically with periods of high strain release along the sink.Volcanic activity (number of volcanoes in eruption per year between 1900 and 1968) migrates from Central to South America at some 900 km/year. High volcanic activity in Central America usually precedes large-magnitude earthquakes in Chile by several years, and would appear to be dynamically interrelated with the alternating strain release along the Nazca plate edges. 相似文献
7.
Seismic data collected at four volcanoes in Central America during 1973 and 1974 indicate three sources of seismicity: regional
earthquakes with hypocentral distances greater than 80 km, earthquakes within 40 km of each volcano, and seismic activity
originating at the volcanoes due to eruptive processes. Regional earthquakes generated by the underthrusting and subduction
of the Cocos Plate beneath the Caribbean Plate are the most prominent seismic feature in Central America. Earthquakes in the
vicinity of the volcanoes occur on faults that appear to be related to volcano formation. Faulting near Fuego and Pacaya volcanoes
in Guatemala is more complex due to motion on a major E-W striking transform plate boundary 40 km north of the volcanoes.
Volcanic activity produces different kinds of seismic signatures. Shallow tectonic or A-type events originate on nearby faults
and occur both singly and in swarms. There are typically from 0 to 6 A-type events per day withb value of about 1.3. At very shallow depths beneath Pacaya, Izalco, and San Cristobal large numbers of low-frequency or B-type
events are recorded with predominant frequencies between 2.5 and 4.5 Hz and withb values of 1.7 to 2.9. The relative number of B-type events appears to be related to the eruptive states of the volcanoes;
the more active volcanoes have higher levels of seismicity. At Fuego Volcano, however, low-frequency events have unusually
long codas and appear to be similar to tremor. High-amplitude volcanic tremor is recorded at Fuego, Pacaya, and San Cristobal
during eruptive periods. Large explosion earthquakes at Fuego are well recorded at five stations and yield information on
near-surface seismic wave velocities (α=3.0±0.2 km/sec.). 相似文献
8.
Result of the algorithm of earthquake prediction, published in 1982, is examined in this paper. The algorithm is based on the hypothesis of long-range interaction between strong and moderate earthquakes in a region. It has been applied to the prediction of earthquakes withM6.4 in Southern California for the time interval 1932–1979. The retrospective results were as follows: 9 out of 10 strong earthquakes were predicted with average spatial accuracy of 58 km and average delay time (the time interval between a strong earthquake and its best precursor) 9.4 years varying from 0.8 to 27.9 years. During the time interval following the period studied in that publication, namely in 1980–1988, four earthquakes occurred in the region which had a magnitude ofM6.4 at least in one of the catalogs: Caltech or NOAA. Three earthquakes—Coalinga of May, 1983, Chalfant Valley of July, 1985 and Superstition Hills of November, 1987—were successfully predicted by the published algorithm.The missed event is a couple of two Mammoth Lake earthquakes of May, 1980 which we consider as one event due to their time-space closeness. This event occurred near the northern boundary of the region, and it also would have been predicted if we had moved the northern boundary from 38°N to the 39°N; the precision of the prediction in this case would be 30 km.The average area declared by the algorithm as the area of increased probability of strong earthquake, e.g., the area within 111-km distance of all long-range aftershocks currently present on the map of the region during 1980–1988 is equal to 47% of the total area of the region if the latter is measured in accordance with the density distribution of earthquakes in California, approximated by the catalog of earthquakes withM5. In geometrical terms it is approximately equal to 17% of the total area.Thus the result of the real time test shows a 1.6 times increase of the occurrence ofC-events in the alarmed area relative to the normal rate of seismicity. Due to the small size of the sample, it is of course, beyond the statistically significant value. We adjust the parameters of the algorithm in accordance with the new material and publish them here for further real-time testing. 相似文献
9.
Most large earthquakes of magnitude 6.0 in California during 1852–1987 appear to show a southeast-to-northwest tendency of epicenter migration. This finding is consistent with earlier findings ofSavage (1971) for a relatively few large earthquakes along the west coast of North America, and ofWood andAllen (1973) for smaller events along the San Andreas fault in central California. The average speed of migration is approximately 130 km/yr, which is within the range of speeds observed for other major seismic zones in the world. The epicenter migration in California may be the result of some small but broad-scaled episodic strain changes associated with creep waves induced by magma injections at the East Pacific Rise and propagating northwestwardly along a broad transform boundary between the Pacific and North American plates at subseismogenic depths as proposed bySavage (1971). 相似文献
10.
The slip distribution and seismic moment of the 2010 and 1960 Chilean earthquakes were estimated from tsunami and coastal geodetic data. These two earthquakes generated transoceanic tsunamis, and the waveforms were recorded around the Pacific Ocean. In addition, coseismic coastal uplift and subsidence were measured around the source areas. For the 27 February 2010 Maule earthquake, inversion of the tsunami waveforms recorded at nearby coastal tide gauge and Deep Ocean Assessment and Reporting of Tsunamis (DART) stations combined with coastal geodetic data suggest two asperities: a northern one beneath the coast of Constitucion and a southern one around the Arauco Peninsula. The total fault length is approximately 400 km with seismic moment of 1.7 × 1022 Nm (Mw 8.8). The offshore DART tsunami waveforms require fault slips beneath the coasts, but the exact locations are better estimated by coastal geodetic data. The 22 May 1960 earthquake produced very large, ~30 m, slip off Valdivia. Joint inversion of tsunami waveforms, at tide gauge stations in South America, with coastal geodetic and leveling data shows total fault length of ~800 km and seismic moment of 7.2 × 1022 Nm (Mw 9.2). The seismic moment estimated from tsunami or joint inversion is similar to previous estimates from geodetic data, but much smaller than the results from seismic data analysis. 相似文献
11.
We develop stochastic approaches to determine the potential for tsunami generation from earthquakes by combining two interrelated
time series, one for the earthquake events, and another for the tsunami events. Conditional probabilities for the occurrence
of tsunamis as a function of time are calculated by assuming that the inter-arrival times of the past events are lognormally
distributed and by taking into account the time of occurrence of the last event in the time series. An alternative approach
is based on the total probabilitiy theorem. Then, the probability for the tsunami occurrence equals the product of the ratio,
r (= tsunami generating earthquakes/total number of earthquakes) by the conditional probability for the occurrence of the next
earthquake in the zone. The probabilities obtained by the total probability theorem are bounded upwards by the ratio r and, therefore, they are not comparable with the conditional probabilities. The two methods were successfully tested in three
characteristic seismic zones of the Pacific Ocean: South America, Kuril-Kamchatka and Japan. For time intervals of about 20
years and over the probabilities exceed 0.50 in the three zones. It has been found that the results depend on the approach
applied. In fact, the conditional probabilities of tsunami occurrence in Japan are slightly higher than in the South America
region and in Kuril-Kamchatka they are clearly lower than in South America. Probabilities calculated by the total probability
theorem are systematically higher in South America than in Japan while in Kuril-Kamchatka they are significantly lower than
in Japan. The stochastic techniques tested in this paper are promising for the tsunami potential assessment in other tsunamigenic
regions of the world. 相似文献
12.
Ran Hong-liu 《地震学报(英文版)》2004,17(6):671-677
In recent years, some researchers have studied the paleoearthquake along the Haiyuan fault and revealed a lot of paleoearthquake
events. All available information allows more reliable analysis of earthquake recurrence interval and earthquake rupture patterns
along the Haiyuan fault. Based on this paleoseismological information, the recurrence probability and magnitude distribution
for M≥6.7 earthquakes in future 100 years along the Haiyuan fault can be obtained through weighted computation by using Poisson
and Brownian passage time models and considering different rupture patterns. The result shows that the recurrence probability
of M
S≥6.7 earthquakes is about 0.035 in future 100 years along the Haiyuan fault.
Foundation item: Joint Seismological Science Foundation of China (103034) and Major Research “Research on Assessment of Seismic Safety” from
China Earthquake Administration during the tenth Five-year Plan. 相似文献
13.
A method was developed to obtain from a signal station the spatial and temporal distribution ofV
p
/V
s
ratios before earthquakes of magnitude>6. It was shown thatV
p
/V
s
values strongly depend upon the relative positions of the stations, the future large earthquake and the foci of the smaller earthquakes used forV
p
/V
s
determination. The appearance of a zone of anomalousV
p
/V
s
values with linear dimensions of the order of 100 km was noted at least 4 years before a deep earthquake of magnitude 7. Similar size anomalous zones were detected one year before some magnitude 6 earthquakes.
V
p
/V
s
values decreased by a small but distinct amount during this time. Additionally, local short term minima inV
p
/V
s
ratios were observed some months before the major event. The epicenters of the large earthquakes were located within the 100 km size zone where the gradients of theV
p
/V
s
field were largest. 相似文献
14.
段星北 《地震学报(英文版)》1997,10(6):731-741
ＧｅｏｇｒａｐｈｉｃａｌｄｉｓｔｒｉｂｕｔｉｏｎｏｆｈｙｐｏｃｅｎｔｒａｌｄｅｐｔｈｓｏｆＣｈｉｎｅｓｅｅａｒｔｈｑｕａｋｅｓＸＩＮＧ－ＢＥＩＤＵＡＮ（段星北）ＩｎｓｔｉｔｕｔｅｏｆＧｅｏｐｈｙｓｉｃａｌ，ＳｔａｔｅＳｅｉｓｍｏｌｏｇｉｃａｌＢｕｒ... 相似文献
15.
S. R. Steele 《Pure and Applied Geophysics》1984,122(2-4):353-368
Anomalous soil-radon activity, including several spike-like surges over periods of 5, 2, and 2 1/2 months, and a year-long declining trend, preceded the most significant earthquakes of the central mid-continental region of North America during 1981 and early 1984. The 5-month period of erratic weekly radon activity, February–June 1981, was followed by a tremor of magnitudeM=4.0, 40 km from the monitoring site in the New Madrid seismic zone. An unusual earthquake swarm in central Arkansas, 160 km from the New Madrid seismic zone and 230 km from the monitoring site in the seismic zone, began in January 1982, shortly after a year-long declining trend in anomalous radon emanation. Earthquakes of magnitudes 4.5, 4.1, and 4.0 occurred at the swarm's outset in early 1982, fitting a pattern anticipated for the New Madrid seismic zone on the basis of the radon activity. Two periods of coincident peak radon emanation have since been observed in the Arkansas and New Madrid seismic regions, as have synchronous seismic pulses for the two separate areas.Two more recent periods of highly erratic soil-radon emanation, March–May 1983 and November–January 1984, were followed by a 4.3 earthquake in southwestern Illinois on 15 may 1983, and 3.5–3.6 tremors and swarm activity in the New Madrid seismic zone in late January and mid-February 1984. Prior to the 4.3 event, radon peaked at three widely separated monitoring sites 1–3 weeks before the tremor at distances of 120, 225, and 320 km from the epicentral region, the station at 225 km, in the New Madrid seismic zone, recording the longest period of anomalous radon activity. As for the recent 3.5–3.6 tremors of 1984, seismic activity of this magnitude had been anticipated for January or February on the basis of the amplitude of the November radon anomaly.These observations provide further evidence of (a) the existence of soil-radon anomalies precursory to the large earthquakes in this intraplate region, (b) the utility of such anomalies in anticipating events of small to moderate magnitudes for the region, and (c) the occurrence of regional-scale strain events prior to some of the larger mid-continental earthquakes.A very recent radon anomaly, the strongest yet to be detected in the seven years of monitoring in the mid-continental region, occurred in the New Madrid seismic zone from mid-February through mid-June 1984. A 4.0 earthquake occurred one month after a peak in the radon activity. The amplitude and duration of the anomaly suggest that a significant change in the state of stress or strain may have occurred in the mid-continental region during 1984. 相似文献
16.
Water levels have been monitored in wells along the San Jacinto fault zone since 1977. The three largest earthquakes to occur within 30 km of the segment of the San Jacinto fault zone being monitored with continuous recorders showed magnitudesM of 4.5, 4.8, and 5.5. Two wells in Borrego Valley, 31 to 32 km southeast of theM=5.5 earthquake on 25 February 1980, showed anomalous spikes recording a probable strain event 88 hours before the earthquake. Two other wells 12 km northwest of the epicenter showed no water-level anomalies. No water-level anomalies preceded theM=4.8 earthquake near Anza on 15 June 1982. Anomalous water-level fluctuations occurred in a well near Ocotillo Wells, 13 km northeast of theM=4.5 earthquake on 22 March 1982, 19 to 23 days prior to the earthquake. Similar fluctuations in other wells have not been followed by sizable earthquakes. A simultaneous drop in water level occurred in four wells on 8 September 1982; this possible strain event was not associated with a sizable earthquake. The presumed strain events occur only in wells that show earth tides and may have been the result of creep on strands of the San Jacinto fault zone. Although water-level anomalies have occurred in only one or two wells prior to two out of three moderate (M=4.5–5.5) earthquakes, the simultaneous drop in water level on 8 September 1982 and the spikes in two wells before theM=5.5 earthquake on 25 February 1980 suggest that wells responsive to earth tides may detect strain events. 相似文献
17.
Earlier attenuation studies for the South American continent indicate that for Sn and Lg waves there is low attenuation in the shield region east of the Andes, whereas in the west of South America, with some exceptions, there is high attenuation. Those studies, however, were nonquantitative. In this study theQ
0 (1-Hz values) for Lg waves for South America are presented, based on a scattering model ofAki (1969) as extended byHerrmann (1980) for the coda waves of shallow local and near-regional earthquakes. The results of the codaQ method are compared with those obtained byNuttli's method (1973). TheseQ
0 values are in good agreement with the apparentQ
0 of Lg waves obtained by the latter method. The data were obtained from over 100 local and regional earthquakes recorded by 12 WWSSN stations throughout continental South America. They provided a range of frequencies from 0.4 to 1.4 Hz, for which the frequency dependence ofQ was investigated by assumingQ=Q
0(f/f
0). The observed data indicate that the tectonic region of western South America is characterized by lowQ
0 and a large frequency-dependent factor , the values ranging from 150 to 350 and 0.4 to 0.7, respectively. TheQ
0 values increase in the shield region east of the Andes, but frequency dependence decreases. The average crustalQ
0 for north and central Argentina ranges from 420 to 580, and ranges from 0.2 to 0.3. TheQ
0 is larger in the Brazil region, ranging from 580 to 980, and varies from 0.0 to 0.2. In the lower-attenuation region of eastern South America higher values of attenuation correlate with greater thickness of the sedimentary layers. 相似文献
18.
Two large shallow earthquakes occurred in 1942 along the South American subduction zone inclose proximity to subducting oceanic ridges: The 14 May event occurred near the subducting Carnegie ridge off the coast of Ecuador, and the 24 August event occurred off the coast of southwestern Peru near the southern flank of the subducting Nazca ridge. Source parameters for these for these two historic events have been determined using long-periodP waveforms,P-wave first motions, intensities and local tsunami data.We have analyzed theP waves for these two earthquakes to constrain the focal mechanism, depth, source complexity and seismic moment. Modeling of theP waveform for both events yields a range of acceptable focal mechanisms and depths, all of which are consistent with underthrusting of the Nazca plate beneath the South American plate. The source time function for the 1942 Ecuador event has one simple pulse of moment release with a duration of 22 suconds, suggesting that most of the moment release occurred near the epicenter. The seismic moment determined from theP waves is 6–8×1020N·m, corresponding ot a moment magnitude of 7.8–7.9. The reported location of the maximum intensities (IX) for this event is south of the main shock epicenter. The relocated aftershcks are in an area that is approximately 200 km by 90 km (elongated parallel to the trench) with the majority of aftershocks north of the epicenter. In contrast, the 1942 Peru event has a much longer duration and higher degree of complexity than the Ecuador earthquake, suggesting a heterogeneous rupture. Seismic moment is released in three distinct pulses over approximately 74 seconds; the largest moment release occurs 32 seconds after rupture initiation. the seismic moment as determined from theP waves for the 1942 Peru event is 10–25×1020N·m, corresponding to a moment magnitude of 7.9–8.2. Aftershock locations reported by the ISS occur over a broad area surrounding the main shock. The reported locations of the maximum intensities (IX) are concentrated south of the epicenter, suggesting that at least part of the rupture was to the south.We have also examined great historic earthquakes along the Colombia-Ecuador and Peru segments of the South American subduction zone. We find that the size and rupture length of the underthrusting earthquakes vary between successive earthquake cycles. This suggests that the segmentation of the plate boundary as defined by earthquakes this century is not constant. 相似文献
19.
本文研究了1970——1979年华北地区11个中等强度地震前区域微震时-空分布图象.发现大部分地震前1——2年,在其周围70——120公里范围内(简称震兆区),微震活动出现活化——寂静图象.同时,震前微震活动似有一个逐渐扩散到日趋收缩直至发震的过程.利用泊松分布检验活化、寂静阶段地震次数发生的概率,其结果是活化阶段绝大多数地震的概率小于5%.而寂静阶段只有近半数地震的概率小于5%.另外,发现大多数地震前震兆区的累加频度曲线的速率有一个加速——减小的过程,而外围区的速率基本保持稳定.最后,给出典型的异常图象模式并予以初步解释. 相似文献
20.
We examined the hypothesis that minima in local recurrence time, TL, or equivalently maxima in local probability, PL, may map asperities in the Kanto and Tokai areas of Japan, where the earthquake catalog of the National Research Institute for Earth Science and Disaster Prevention (NIED) is complete at the M=1.5 (M1.5) level. We mapped TL (PL) based on the a- and b-values of the nearest earthquakes within 20 km of every node of a grid spaced 0.01° for M7 target events. Only earthquakes within the top 33 km were used. The b-values increase strongly with depth, in several areas. Therefore, some of the TL (PL) anomalies are not revealed if data from the entire crustal seismogenic zone are mixed. Thus, we mapped TL (PL) separately for the top 15 km and the rest of the depth range, as well as for the entire seismogenic crust. The resulting TL- and PL-maps show that approximately 12% of the total area shows anomalously short recurrence times. Out of six shallow target events with M≥6.5 and which occurred since 1890, five are located within the anomalous areas with TL <450 years. We interpret this to mean that areas with anomalously short TL map asperities, which are more likely than other areas to generate future target events. The probability that this result is due to chance is vanishingly small. The great Kanto rupture of 1923 appears to have initiated in the most significant asperity we mapped in the study area. One anomaly is located in the northeastern part of the area of the proposed future rupture of the Tokai earthquake, and another one at its southwestern corner. The absolute values of TL calculated are uncertain because they depend on the size of the volume used for the calculation. 相似文献