The rupture process and tectonic implications of the great 1964 Prince William Sound earthquake

We have determined the rupture history of the March 28, 1964, Prince Williams Sound earthquake (M _w=9.2) from long-period WWSSNP-wave seismograms. Source time functions determined from the long-periodP waves indicate two major pulses of moment release. The first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture. The second smaller pulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds. Because of the large size of this event and thus a deficiency of on-scale, digitizableP-wave seismograms, it is impossible to uniquely invert for the location of moment release. However, if we assume a rupture direction based on the aftershock distribution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault. The first moment pulse most likely initiated near the epicenter at the northeastern down-dip edge of the aftershock area and then spread over the fault surface in a semi-circular fashion until the full width of the fault was activated. The rupture then extended toward the southwest approximately 300 km (Ruff andKanamori, 1983). The second moment pulse was located in the vicinity of Kodiak Island, starting at 500 km southwest of the epicenter and extending to about 600 km. Although the aftershock area extends southwest past the second moment pulse by at least 100 km, the moment release remained low. We interpret the 1964 Prince William Sound earthquake as a multiple asperity rupture with a very large dominant asperity in the epicentral region and a second major, but smaller, asperity in the Kodiak Island region.The zone that ruptured in the 1964 earthquake is segmented into two regions corresponding to the two regions of concentrated moment release. Historical earthquake data suggest that these segments behaved independently during previous events. The Kodiak Island region appears to rupture more frequently with previous events occurring in 1900, 1854, 1844, and 1792. In contrast, the Prince William Sound region has much longer recurrence intervals on the order of 400–1000 years.     

P in the shadow zone of the earth's core—Part I

Summary Regional variations have been indicated in the slope of theP travel-time curve in the shadow zone of the earth's core. Further study is needed since the uncertainties of the slope are large, especially for the observations from North American stations. There is no significant difference between themean slope of theP travel-time curve in the 95°102.9 range and those obtained byJeffreys, andJeffreys andBullen. However, there is a significant difference between themean slope in the 103° to 135° range as obtained in this study, and those obtained byJeffreys andBullen, and in a later revision byJeffreys. Themean travel-time curve ofP in the shadow zone of the earth's core should be lowered. A trial travel-time table is given with amean slope of 4.41 sec/deg. This table is in close agreement with the times obtained byGutenberg andRichter, and with the trial travel-times ofLehmann. Under the assumption of diffraction the longitudinal wave velocity has been determined to be 13.7 km/sec at the core-mantle boundary.This paper was presented at the Annual Meeting of the Seismological Society of America Reno, Nevada, 1966.     

Volcano-seismic crisis in Montserrat,West Indies, 1966–67

The Soufriere Hills volcano in the south-eastern part of Montserrat erupted pyroclast flows as recently as A. D. 1646 ± 54 years and must therefore be considered dormant, not extinct. The highly destructive nature of pyroclast flow eruptions makes it imperative that such activity should be predicted and, if the threat of eruption becomes sufficiently large, the population should be moved to a sale distance from the volcano. Sharp increases in seismic and solfataric activity occurred in 1966 and these events indicated the abnormally high risk of an eruption in the near future. A network of four short period seismographs was established in the island in May 1966 and between this date and the end of 1967, 723 local earthquakes were recorded, of which 32 were reported felt in the island. Hypocentres were determined for 189 of these earthquakes, and most of these lay in a WNW to ESE belt beneath the Soufriere Hills, at depths of less than 15 km. The average rate of seismic energy release decreased with time throughout the series but there was a strong seasonal variation with maxima in May and November–December of each year. The average depth of the earthquakes decreased from 5.2 km in April and May 1966 to a minimum of 2.8 km from July to September 1966. After September the mean focal depths increased again and by the end of the crisis in November 1967 the mean depth was 9.7 km. Measurements carried out using water-tube tiltmeters showed that the region 2–3 km south-east of the Soufriere Hills was doming upwards until January 1967, then subsided between January–March 1967 and finally rose again at a slower rate between March and September 1967. Heat flow from Galway’s Soufriere which was 3 × 10⁵ cal/sec in 1954 increased to a maximum of 2 × 10⁵ cal/sec in October 1966, then declined to 5 × 10⁵ cal/sec in September 1967. The earthquake series was not the aftershock sequence of any major tectonic earthquake, and only two hypocentres were determined at depths greater than 15 km. It is concluded that magma was intruded into the upper crust beneath the Soufriere Hills volcano and that the earthquakes and other phenomena resulted from the upward migration of this magma.     

Intermediate term earthquake prediction in the area of Greece: Application of the algorithm M8

The 3 strongest earthquakes,M7.0, which have occurred since 1973 in the area of Greece were preceded by a specific increase of the earthquake activity in the lower magnitude range. This activation is depicted by algorithm M8. This algorithm of intermediate term earthquake prediction was originally designed for diagnosis by Times of Increased Probability (TIPs) of the strongest earthquake,M8.0 worldwide (Keilis-Borok andKossobokov, 1984). At present the algorithm is retrospectively tested for smaller magnitudes in different seismic regions (Keilis-Borok andKossobokov, 1986, 1988). A TIP refers to a time period of 5 years and an area whose linear size is proportional and several times larger than that of the incipient earthquake source. Altogether the TIPs diagnosed by the algorithm M8 in the area of Greece occupy less than 20% and the Times of Expectation (TEs) about 10% of the total space-time domain considered. Also there is a current TIP for the southeastern Aegean sea and 1988–1992. It may specify the long-term prediction given inWyss andBaer (1981a,b).The results of this study are further evidence favoring applicability of algorithm M8 in diverse seismotectonic environment and magnitude ranges and support indirectly the hypothesis of self-similarity of the earthquake activity. It also implies the possibility of intermediate term prediction of the strongest earthquakes in the area of Greece.     

Estimates of vertical crustal movements along the coast of Greece,based on mean sea level data

Mean annual sea level (MASL) data for 25 Greek stations were analyzed for the time period 1969–1982. The data from 4 of these were unacceptably poor, and the record of 3 stations showed unexplained step functions that were interpreted as errors. Relative MASL between stations showed crustal stability at 10 of the 18 useful stations. The standard deviation from the long-term average of these stations was ±1.8 cm. We conclude that if station records are carefully kept in this area crustal movements in excess of 5 cm can be detected by relative MASL. A comparison of MASL data with gravity changes measured in the Peloponnese and Central Greece suggests that vertical movements occurred along a gradient equal to or larger than the free air gradient. We conclude that the gravity network should be reoccupied frequently such that the non-tectonic effects to be determined from the probable observed gravity changes, and the tectonic vertical movements may be better understood. A co-seismic subsidence of about 5 cm is inferred to have taken place near Korinth during the 1981,M _s =6.8, earthquake, which occurred 20 km N of this tide gauge (Posidonia). During 2.5 years before the 1968 Thessaloniki,M _s =6.6, earthquake, sea level was lower than average suggesting possible crustal elevation of 3.6 cm at about 28 km epicentral distance. Because of the small amplitude of this change we are not certain that it represents crustal uplift. At station Myrina (on Limnos) a strong and consistent trend of subsidence accumulated a 15 cm change between 1975 and 1980. Chios showed a trend of emergence (total accumulation about +5 cm), while Volos showed a trend of subsidence (approximately ?5 cm total). Kefalinia appears to have subsided about 10 cm during the data period. The six stations along the Hellenic arc plate boundary showed nearly constant MASL, suggesting that crustal stability existed there during the last 14 years. We conclude that MASL data in Greece can be useful for understanding tectonic processes, especially if these data are gathered carefully and at numerous locations, and are cross-correlated to high precision repeat gravity measurements, and geodetic releveling. Also, MASL data on active volcanic islands have excellent potential for detecting uplift before future eruptions.     

长江口以东海域MS6.1地震前的地倾斜异常

１９９６年１１月９日长江口以东海域发生ＭＳ６．１地震,震前距震中２９０ｋｍ的湖州地倾斜台观测到中期异常和短临异常,距震中２４０ｋｍ的宁波地倾斜台也观测到短临异常．这里简单介绍了地倾斜异常的演化过程以及地震三要素的预测．震前我们进行了中短期预报,取得较好效果,但没有提交短临预报意见,这主要是由于短临异常出现较晚和台站资料报送不及时所致．     

Discussion on the feature of strong earthquake: Orderly distribution in time,space and intensity before the Western Kunlun Mountain Pass <Emphasis Type="Italic">M</Emphasis>=8.1 earthquake

In the paper, the feature of strong earthquake orderly distribution in time, space and intensity before the Western Kunlun Mountain Pass M=8.1 earthquake is preliminarily studied. The modulation and triggering factors such as the earth rotation, earth tides are analyzed. The results show that: the giant earthquakes with the magnitude more than 8 occurred about every 24 years and the earthquakes with the magnitude more than 7 about every 7 years in Chinese mainland. The Western Kunlun Mountain M=8.1 earthquake exactly occurred at the expected time; The spatial distance show approximately the same distances between each two swarms. The earth rotation, earth tide, sun tide and sun magnetic field have played a role of modulation and triggering in the intensity. At last, the conditions for earthquake generation and occurrence are also discussed.     

河北应变固体潮汐参数及震例研究

选取河北省5个形变台站的观测资料,利用Venedikov调和分析法计算河北省境内M_S4.0以上的潮汐因子、相位等参数,发现应变潮汐因子对地震都有多则半年,少则几周的异常期。宽城台对河北省境内发生M_S4.0以上的地震中,震中距较其他台站最近(震中距最远为350km、最近为100km),资料较好,故在综合分析河北应变台站的基础上,重点选取宽城台的应变潮汐因子对河北2005—2015年地震案例进行简要分析,发现该台对地震的对应率达到60%,虚报率40%,漏报率极低只有14%,前兆异常可信度较高,可作为前兆异常的参考。     

On a possible origin of the anomalous effects that were observed during the May 24, 2013 earthquake in the Sea of Okhotsk

The deep-focus Sea of Okhotsk earthquake that occurred on May 24, 2013 (h = 630 km, M _w = 8.3) was accompanied by anomalous effects that were unknown previously. A combined analysis of published data concerning the source rupture evolution and some features of the deep structure provided an explanation of some anomalous effects, such as the large number of aftershocks and the low level of ground shaking in the epicentral area. However, GPS observations revealed high coseismic vertical displacements in the area. The seafloor uplift in the Sea of Okhotsk and the adjacent coasts was 3–12 mm, peaking at the approximate center of the sea, while Kamchatka and the North Kuril Islands subsided by 3–18 mm, peaking at the Apacha station 190 km east of the earthquake epicenter. These maximum estimates are 1.2–1.8 times the analogous values (10 mm) for the Chile mega-earthquake of May 20, 1960 (M _w ~ 9.5). It is known that the large distances at which ground shaking is felt during deep-focus earthquakes are due to the fact that the body waves travel through the high-Q lower mantle. However, this does not explain the paradox of the present earthquake in the Sea of Okhotsk, viz., a constant intensity of shaking (two grades) in the range of epicentral distances between 1300 and 9500 km. The explanation requires consideration of the earth’s free oscillations excited by the earthquake.     

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 (h100 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 (M _s =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=190L ^0.545. 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.     

Coseismic slip in the 1964 Prince William Sound earthquake: A new geodetic inversion

The 1964 Prince William Sound earthquake (March 28, 1964;M _w =9.2) caused crustal deformation over an area of approximately 140,000 km² in south central Alaska. In this study geodetic and geologic measurements of this surface deformation were inverted for the slip distribution on the 1964 rupture surface. Previous seismologic, geologic, and geodetic studies of this region were used to constrain the geometry of the fault surface. In the Kodiak Island region, 28 rectangular planes (50 by 50 km each) oriented 218°N, with a dip varying from 8^o nearest the Aleutian trench to 9^o below Kodiak Island, define the rupture surface. In the Prince William Sound region 39 planes with variable dimensions (40 by 50 km near the trench, 64 by 50 km inland) and orientation (218°N in the west and 270°N in the east) were used to approximate the complex faulting. Prior information was introduced to constrain offshore dip-slip values, the strike-slip component, and slip variation between adjacent planes. Our results suggest a variable dip-slip component with local slip maximums occurring near Montague Island (up to 30 m), further to the east near Kayak Island (up to 14 m), and trenchward of the northeast segment of Kodiak Island (up to 17m). A single fault plane dipping 30°NW, corresponding to the Patton Bay fault, with a slip value of 8 m modeled the localized but large uplift on Montague Island. The moment calculated on the basis of our geodetically derived slip model of 5.0×10²⁹ dyne cm is 30% less than the seismic moment of 7.5×10²⁹ dyne cm calculated from long-period surface waves (Kanamori, 1970) but is close to the seismic moment of 5.9×10²⁹ dyne cm obtained byKikuchi andFukao (1987).     

The earthquake of February 3, 2015, near Sumy,Ukraine: Source parameters and focal mechanism

The parameters of the earthquake that took place February 3, 2015, near the city of Sumy, Ukraine, were calculated from an analysis of records obtained by both Russian and Ukrainian seismic stations (Poltava, Skvira, Nikolaev, Dneropetrovsk, and Desna). The calculated hypocenter depth of 54 km was verified by several approaches: isolation of deep PP, SP phases from the records of remote stations and solution of the kinematic problem for the Poltava station. The focal mechanism as shear with a complex fault component was determined by the first arrivals of P-waves. The data on the azimuthal travel-time curve confirm the focal mechanism. We have calculated the earthquake parameters; they are as follows: length gap L₁ = 8.08 km, L₂ = 6.68 km, a destruction rate of C = 2 km/s. We have obtained the dynamic parameters of the event. The calculated fault length (L = 5.46 km) within the accuracy limits of the method coincides with the early result obtained by the azimuthal travel-time curve. On the basis of these results, we suggest that elastic energy release and formation of the dislocations in the earthquake source occurred on a smooth, prefractured fault (σ_r > 0). Association of the hypocenter with the tectonic node of the northern marginal fault of the Dnieper–Donets graben and northern branch of Kryvyi Rih–Kremenchuk suture confirm this. Here, we observe a considerable Moho depth, structural alteration, and high gradients of the temperature and magnetic and electric rock properties in the lower Earth’s crust and upper mantle. These circumstances are favorable for the earthquake occurring here.     

The optimum Bayesian probability procedure and the prediction of strong earthquakes felt in Mexico city

Bayes' theorem has possible application to earthquake prediction because it can be used to represent the dependence of the inter-arrival time (T) of thenext event on magnitude (M) of thepreceding earthquake (Ferraes, 1975;Bufe et al., 1977;Shimazaki andNakata, 1980;Sykes andQuittmeyer, 1981). First, we derive the basic formulas, assuming that the earthquake process behaves as a Poisson process. Under this assumption the likelihood probabilities are determined by the Poisson distribution (Ferraes, 1985) after which we introduce the conjugate family of Gamma prior distributions. Finally, to maximize the posterior Bayesian probabilityP(/M) we use calculus and introduce the analytical condition .Subsequently we estimate the occurrence of the next future large earthquake to be felt in Mexico City. Given the probabilistic model, the prediction is obtained from the data set that include all events withM7.5 felt in Mexico City from 1900 to 1985. These earthquakes occur in the Middle-America trench, along Mexico, but are felt in Mexico City. To see the full significance of the analysis, we give the result using two models: (1) The Poisson-Gamma, and (2) The Poisson-Exponential (a special case of the Gamma).Using the Poisson-Gamma model, the next expected event will occur in the next time interval =2.564 years from the last event (occurred on September 19, 1985) or equivalently, the expected event will occur approximately in April, 1988.Using the Poisson-Exponential model, the next expected damaging earthquake will occur in the next time interval =2.381 years from the last event, or equivalently in January, 1988.It should be noted that very strong agreement exists between the two predicted occurrence times, using both models.     

Evidence of earthquakes triggered by the tidal force of the sun and the moon

Studies by many scientists show that Hebei, China is an area with strong correlation between the tidal force and the occurrences of major earthquakes, the Xingtai earthquake of 1966, the Hejian earthquake of 1967 and the Tangshan earthquake of 1976 were triggered by the tidal force, in this paper the study on the common characteristics of their occurrence times confirms these facts. The computed times of maximum horizontal of the semi diurnal solid tide tidal force show that the occurrence times of the above mentioned earthquakes were close to the times of maximum horizontal tidal force of the semi diurnal solid tide at new moon or full moon. The Longyao earthquake of M=6.8, the Ningjin earthquake of M=7.2 and the Hejian earthquake of M=6.3 occurred tens of minutes after the maximum horizontal tidal force of the semi diurnal solid tides, and the Tangshan earthquake of M=7.8 occurred 16 minutes before the maximum horizontal tidal force. The tidal forces were directed to the west. This is their temporal characteristic. It is generally accepted that the 1969 Bohai earthquake of M=7.4 and the 1975 Haicheng earthquake were not triggered by the tidal force. These events did not show such characteristics. The temporal characteristics of the earthquakes indicate that the occurrences of these events were not random, but were controlled by the tidal force from the sun and the moon, and triggered by the tidal force. These facts agree with the triggering mechanism of the tidal force, are evidences of earthquakes triggered by tidal force.     

Large thrust earthquakes and volcanic eruptions

Forty-eight hours after the occurrence of the May 22, 1960 (M _W =9.5) Chile earthquake, Puyehue volcano initiated its eruptive activity. The closeness in space and time of both phenomena provides us with a unique opportunity to examine the possible causal relationship between the sudden strain change and the mechanism of the eruption. From the slip distribution of the 1960 event (Barrientos andWard, 1990) and a static propagator technique, which allows for variable slip faults in vertically heterogeneous media, I calculate the strain field and its depth dependence in the region beneath the volcano. The presented semi-analytical formalism can be applied to any two-dimensional dipping fault. Calculations show extension at the surface of the order of 40 strain, in agreement with what was observed in triangulation networks in the central valley about 50 km oceanward from the line of volcanoes. The amplitude of the strain field beneath the volcano is uniform up to a depth of 20 km and decreases downward. The sudden extension of the region is thought to be the main factor in facilitating the eruption of the volcano. It is postulated that strain beneath the volcano triggered the eruption of the Puyehue-Cordón Caulle volcanic system because it was in a mature stage of its eruptive cycle and there was lack of eruptive activity in other volcanoes located along the 1960 rupture region in the immediate period following the earthquake.     

常熟台倾斜观测资料的异常特征分析

在研究常熟台水管仪与垂直摆多年倾斜观测资料的基础上,利用潮汐因子调和分析、Nakai拟合检验、小波分析等方法对台站周边约250km范围内发生的高邮-宝应M_S 4.9及如东近海M_S 3.8等地震前的异常信息进行了提取,对数据异常分析方法进行评判,旨在为以后地震前及时发现地震提供参考。     

GPS测定的四川九寨沟MW6.6地震同震形变

2017年8月8日四川阿坝州九寨沟发生M_W6.6地震,震源机制解显示该地震为左旋走滑型地震。对震中周围的GPS连续站观测资料进行处理,获得高频GPS动态形变和静态同震水平位移。震中100km范围内四川松潘和甘肃武都站观测到1 Hz动态形变。距离震中约69km的松潘站观测的同震水平位移为7.4mm。根据少量的GPS静态同震位移反演的同震破裂模型显示本次地震的最大滑动量为376mm,地震矩为7.25×1018 N·m,等效矩震级为M_W6.6。正演计算的同震三维形变场显示本次地震的最大水平位移可达4~5cm,垂直位移呈四象限分布,最大可达1.5cm,区域内10个流动GPS站可观测到同震形变。     

Case 24 reevaluation of anomalous vertical crustal movement associated with the 1964 Niigata, Japan, earthquake

It is widely acknowledged that the 1964 Niigata, Japan, earthquake is associated with the preseismic anomalous crustal movement detected by repetition of precise levelings, while some doubts have been raised on the validity of reported precursory movement. Validity of the crustal movement is tested by an analysis of tidal data. When we are able to deduce crustal movement referring to the determined mean sea level, we can discuss the absolute crustal movement. Tidal data along the Japan Sea coast of northeast Japan are analyzed by the method developed inTsumura (1963, 1970) for the period from 1955 to 1986.The final results at tidal station, Nezugaseki, indicate clearly the steady-state movement during 1955–1958, 4 cm of abnormal upheaval from 1959–1964, 20 cm of coseismic abrupt subsidence, and 4 cm of postseismic rapid subsidence, and finally very gradual subsidence. Thus the present results support the idea of the typical pattern of the seismic crustal movement, including the precursory movement.     

按震兆共迁法以平凉远台地电前兆异常回顾性讨论2017年九寨沟大震的预测

2017年九寨沟7级大震前平凉台上的短临地电阻率异常甚为明显,平凉距九寨沟震中360 km。如此远的震中距离只能用"震兆共迁法"预测九寨沟大震震中。