Foreshocks and earthquake prediction

Results of a statistical investigation of the magnitude and time distributions of foreshocks in the area of Greece are reported. Further evidence is presented that the parameter b, in the frequency—magnitude relation, has a smaller value before than after the main shock, and that the time distribution of foreshocks follows a statistical law similar to that followed by aftershocks. The difference in magnitude between the main shock and the largest foreshock seems to be independent of the magnitude of the main shock. The average of this difference has been found equal to about two magnitude units. The significance of these results to the problem of statistical prediction of earthquakes is noted.     

Recent evaluation of the assessment seismic hazards for Nuweiba, Gulf of Aqaba

The Gulf of Aqaba is considered seismically as one of the most active zones of the Dead Sea Transform region. The main shock of the 1995 Gulf of Aqaba earthquake sequence is considered as the largest shock in the (surface wave magnitude Ms?=?7.2) since the sixteenth century. The present study is a trial to detect the probabilistic seismic hazard analysis (PSHA) for Nuweiba site. Data used for this study was a combination of both historical and recent instrumental data. Results of the hazard assessment, expressed as in the worst case scenario, reveal that Nuweiba is exposed to the occurrence of a maximum credible earthquake of magnitude $ m_{{\max }} ~ = ~7.4 \pm 0.31 $ , at hypocentral distance of 15.6?±?10 km. For structure with the return period of 100 years, with a 90% probability of exceedance, the maximum expected earthquake magnitude (M_L) is 5.9 in this lifetime. The possibility of the maximum peak ground acceleration at the Nuweiba site is 0.41 g. Results of the hazard assessment can be used as an input data to assess the seismic risk for site of interest.     

The pyrenean earthquake of february 29, 1980: An example of complex faulting

An M_b = 5.1 earthquake occurred on February, 29, 1980, in the Western Pyrenees near Arudy (France). The telemetred network of the I.P.G.P., operating in this area since 1978, allowed a good localization of this earthquake (43°4.21′N, 0°24.59′W, depth 4km). During the two years preceding this earthquake the seismic activity exhibited a gradual decrease. A foreshock of magnitude 1.6 was recorded three hours before the main shock.A temporary network set up in the epicentral area a few days after the main shock permitted a precise study of the aftershock sequence. Fifty fault-plane solutions for earthquakes ranging from magnitude 1.5-4 were obtained. A complex pattern of faulting was revealed, with both strike-slip and normal faulting. However, a regional tectonic stress tensor can be proposed from a detailed investigation of the aftershock sequence. This stress tensor is in agreement with previous results in this area.     

Statistical parameters of Bhuj earthquake sequence of January 26th, 2001

An intraplate earthquake of magnitude (M _c) 6.9 (Anon 2001a) struck Bhuj and the adjoining region of Kachchh in Gujarat on January 26th, 2001 at about 0316 hrs (GMT) and was followed by a number of aftershocks. The epicentre of this earthquake was located at 23.4‡N and 70.28‡E close to the Kachchh mainland fault. The intensity observed around the epicenter was X on the MSK scale. A study of 531 aftershocks, in the magnitude range of 3.0–5.7, recorded at Vadodara Seismological Observatory till March 31st, 2001 has been carried out and various statistical parameters calculated. The total energy released during the study period is calculated to be 8.2 × 10¹⁴ joule. Sudden occurrence of the main shock without any foreshock in the same tectonic system is a unique feature of this sequence. Theb- value (0.86), value of M₀-M₁ (1.2), high M₁/M₀ (0.89) and high value of the decay constanth (0.91), all support the tectonic origin of the present study.     

On the possibilities of premonitory swarms for three sequences of earthquakes of the Burma—Szechwan region

Short term spatial and temporal variations in seismicity prior to the three sequences of earthquakes of m_b 5.8 of the Burma—Szechwan region are studied. Six years (1971–1976) of ISC seismicity data, as reported in the Regional Catalogue of Earthquakes, are considered. During the period, six earthquakes of body wave magnitude m_b 5.8 occurred in four sequences. Of these, three sequences are preceded by swarm activity in the epicentral regions. Evison (1977b) suggested that the swarm before the sequences of large shocks is a possible long-term precursor. He derived the conclusion by analyzing earthquakes in New Zealand and California. The analysis of the seismicity data for the region under investigation supports Evison's view and suggests that a relation between swarms and sequences of large events exists. The precursory time period (i.e. the time from beginning of the swarm to the main shock) for the Szechwan earthquakes of m_b = 5.9 (Feb. 6, 1973) and m_b = 5.8 (May 10, 1974) and the Burma earthquake of m_b = 6.2 (Aug. 12, 1976) are 305, 317 and 440 days, respectively.     

On the validity of time-predictable model for earthquake generation in north-east India

North-east India is seismically very active and has experienced many widelydistributed shallow, large earthquakes. Earthquake generation model for the region was studied using seismicity data [(1906–1984) prepared by National Geophysical Data Centre (NGDC), Boulder Colorado, USA]. For establishing statistical relations surface wave magnitudes (M _s≥5·5) have been considered. In the region four seismogenic sources have been identified which show the occurrences of atleast three earthquakes of magnitude 5·5≤M _s≤7·5 giving two repeat times. It is observed that the time interval between the two consecutive main shock depends on the preceding main shock magnitude (M _p) and not on the following main shock magnitude (M _f) revealing the validity of time predictable model for the region. Linear relation between logarithm of repeat time (T) and preceding main shock magnitude (M _p) is established in the form of logT=cM _p+a. The values ofc anda are estimated to be 0–36 and 1–23, respectively. The relation may be used for seismic hazard evaluation in the region.     

Estimation of causative fault parameters of the Rudbar earthquake of June 20, 1990 from near field SH-wave data

We analyze the strong motion accelerograms recorded for the large (M_S=7.7, M_W=7.3, m_b=6.4) Rudbar earthquake of June 20, 1990. The earthquake had a complex source process. We have identified the imprints of rupture of three localized asperities on the major causative fault on the accelerograms. These asperities are interpreted to correspond to (i) the main shock that initiated the rupture process and was located in the domino block between the Kabateh and Zard Goli faults, (ii) a foreshock that occurred about 10 s earlier in the Kabateh fault and (iii) a later shock, on the western end of the Baklor fault, which terminated the bilateral rupture process at the western end. We estimate the strike, dip and slip of these causative sub-event rupture planes using the SH spectral amplitudes, based on a point source representation of sub-events and a non-linear least square formulation for inversion of the amplitude data. The results of our inversion of the near field data are comparable to other studies based on teleseismic data.     

Maximum Likelihood Estimation of Seismic Hazard for Sweden

The maximum magnitude, the activity rate, and the Gutenberg-Richterb parameter as earthquake hazard parameters, have been evaluated for Sweden. The maximum likelihood method permits the combination of historical and instrumental data. The catalog used consists of 1100 earthquakes in the time interval 1375–1989. The extreme part of the catalog contains only the strongest historical earthquakes, whereas the complete part is divided into several subcatalogs, each assumed complete above a specified threshold magnitude. The uncertainty in magnitude determination was taken into account. For southern Sweden, the calculations giveb-values of 1.04 (0.05) for the whole area south of 60° N and 0.98 (0.06) for a subregion of enhanced seismicity in the Lake Vänern area. For the whole area north of 60° N, theb-value is 1.35 (0.06) and for the seismicity zone along the Gulf of Bothnia 1.26 (0.06). The number of annually expected earthquakes with magnitude equal to or larger than 2.4 [M_L(UPP) or M_M(UPP)] is 1.8 for the whole southern Sweden, 1.3 for the Lake Vänern region, 3.7 for northern Sweden, and 2.4 for the region along the Gulf of Bothnia. The maximum expected regional magnitude is calculated to 4.9 (0.5) for a time span of 615 years for southern Sweden and the Lake Vänern subregion, and 4.3 (0.5) for a time span of 331 years for northern Sweden and the Gulf of Bothnia subregion. However, several historical earthquakes with magnitude above 5 in nearby areas of Norway indicate that the seismic potential may be higher.     

Seismic hazard of Egypt

Earthquake hazard parameters such as maximum expected magnitude,M _max, annual activity rate,, andb value of the Gutenberg-Richter relation have been evaluated for two regions of Egypt. The applied maximum likelihood method permits the combination of both historical and instrumental data. The catalogue used covers earthquakes with magnitude 3 from the time interval 320–1987. The uncertainties in magnitude estimates and threshold of completeness were taken into account. The hazard parameter determination is performed for two study areas. The first area, Gulf of Suez, has higher seismicity level than the second, all other active zones in Egypt.b-values of 1.2 ± 0.1 and 1.0 ± 0.1 are obtained for the two areas, respectively. The number of annually expected earthquakes with magnitude 3 is much larger in the Gulf of Suez, 39 ± 2 than in the other areas, 6.1 ± 0.5. The maximum expected magnitude is calculated to be 6.5 ± 0.4 for a time span of 209 years for the Gulf of Suez and 6.1 ± 0.3 for a time span of 1667 years for the remaining active areas in Egypt. Respective periods of 10 and 20 years were reported for earthquakes of magnitude 5.0 for the two subareas.     

Field-seismic investigation of the August 1975 Oroville, California, earthquake sequence

Several thousand aftershocks of the August 1, 1975 Oroville, California, earthquake (M_L = 5.7) were recorded by an 8-station field-seismic network. Focal coordinates of 104 of these events were fitted by least-squares to a plane striking N07°W and dipping 59°W; the strike (but not the dip) of this plane is in good agreement with that (N09°W) obtained from a fault-plane solution for a large foreshock 8 sec before the main shock, and it agrees fairly well with the trend (N15°W) of structural lineaments in the vicinity of Lake Oroville. The surface trace of the plane of foci passes through the Oroville Dam, as well as through surface cracking 12 km south of the dam. The main shock occurred 7 years after the filling of Lake Oroville, but only a month after the most rapid filling since 1968. The rate of aftershock occurrence during the first month decayed approximately as1/t. Event duration was measured for more than 2,000 aftershocks during August and September; average log-duration, taken over samples of 100 events, decreased gradually during this period. Close-in spectra obtained from strong-motion recordings of several of the larger aftershocks have corner frequencies that are quite high compared to other western U.S. earthquakes of similar magnitude. The Oroville earthquakes had several features in common with another Sierra Nevada earthquake sequence, near Truckee, California, in September, 1966.     

Bayesian analysis of marked stress release models for time-dependent hazard assessment in the western Gulf of Corinth

We consider point processes defined on the space–time domain which model physical processes characterized qualitatively by the gradual increase over time in some energy until a threshold is reached, after which, an event causing the loss of energy occurs. The risk function will, therefore, increase piecewise with sudden drops in correspondence to each event. This kind of behaviour is described by Reid's theory of elastic rebound in the earthquake generating process where the quantity that is accumulated is the strain energy or stress due to the relative movement of tectonic plates. The complexity and the intrinsic randomness of the phenomenon call for probabilistic models; in particular the stochastic translation of Reid's theory is given by stress release models. In this article we use such models to assess the time-dependent seismic hazard of the seismogenic zone of the Corinthos Gulf. For each event we consider the occurrence time and the magnitude, which is modelled by a probability distribution depending on the stress level present in the region at any instant. Hence we are dealing here with a marked point process. We perform the Bayesian analysis of this model by applying the stochastic simulation methods based on the generation of Markov chains, the so called Markov chain Monte Carlo (MCMC) methods, which allow one to reconcile the model's complexity with the computational burden of the inferential procedure. Stress release and Poisson models are compared on the basis of the Bayes factor.     

Seismotectonic and seismological aspects of the Mostaganem (Western Algeria) May 22, 2014 (Mw 4.9) seismic event

On Thursday, 22 of May 2014, at 6 h 22 min 0.3.3 s (GMT?+?1) a moderate-sized earthquake struck the Mostaganem, Western Algeria, region. The main shock, recorded by many international and national seismological stations, was preceded by a foreshock, 3 hours before, on May 22, 2014 (Ml?=?4.1) at 3 h 57 min 41.4 s and followed by four well-felt aftershocks (M?>?3.0) that lasted about 1 year. The main shock did not cause loss of lives but serious panic among the population was reported. The main shock, however, caused cracks in walls and roofs, sometimes destroyed, the old non-engineered and precarious adobe dweller corresponding to I₀?=?VI–VII (Msk scale). We used accelerograph records to (i) determine the epicenter location (longitude?=?0.3537 E, latitude?=?35.8598 N, (ii) perform waveforms inversion to calculate the earthquake parameters. The obtained results are, respectively, the seismic moment (M₀)?=?2.71 E + 16, the Mw?=?4.9 and the focal depth?=?6 km. The obtained focal mechanism solution shows reverse faulting with small right lateral component with the following nodal plans: NP1, strike?=?193.5, dip?=?49.5, slip?=?57.6 and NP2, strike?=?57.8, dip?=?50, slip?=?122.1. On the other hand, the seismotectonic framework of the Dahra area exhibits a serie of NE-SW trending “en echelon” faulted folds that may be active as suggested by this study.     

Sequential processes in a landslide hazard at a slate quarry in Okayama,Japan

The 12 March 2001 landslide at a slate quarry in Okayama, Japan killed three workers. Composite studies based on field surveys of the landslide slope, interviews with local residents and quarry workers, and inspections of hydrological and seismological data have been used to clarify the causes of this slide and its movements. The results indicate that the landslide was enabled firstly by the steepness of the slope, which had been undercut by river; secondly, the structure was that of a dip-slope that was prone to deep-seated slides along bedding planes; thirdly, numerous joints and faults were present. Surprisingly, rainfall, earthquakes, and explosions do not appear to have played any role in the triggering of this slide. The interviews demonstrated that the frequency of precursory failures increased over a period of several hours before the 12 March 2001 landslide. Inspection of the seismograph records and the eyewitness evidence both indicate that the main part of the landslide consisted of two phases of slope failure within 23 s. After the slide, the frequency of the failures gradually decreased with time over a period of several days. Three new terms are proposed for landslides: foreslide, mainslide, and afterslide, following the terms foreshock, main shock, and aftershock used in seismology.     

Present Kinematic Regime and Recent Seismicity of Gulf Suez,Egypt

In this study we computed recent seismicity and present kinematic regime in the northern and middle zones of Gulf of Suez as inferred from moment tensor settlings and focal mechanism of local earthquakes that happened in this region. On 18 and 22 of July, 2014 two moderate size earthquakes of local magnitudes 4.2 and 4.1 struck the northern zone of Gulf of Suez near Suez City. These events are instrumentally recorded by Egyptian National Seismic Network (ENSN). The earthquakes have been felt at Suez City and greater Cairo metropolitan zone while no losses were reported. The source mechanism and source parameters of the calculated events were considered by the near-source waveform data listed at very broadband stations of ENSN and supported by the P-wave polarity data of short period stations. The new settling method and software used deem the action of the source time function, which has been ignored in most of the program series of the moment tensor settling analysis with near source seismograms. The obtained results from settling technique indicate that the estimated seismic moments of both earthquakes are 0.6621E + 15 and 0.4447E + 15 Nm conforming to a moment magnitude Mw 3.8 and 3.7 respectively. The fault plan settlings obtained from both settling technique and polarity of first-arrival indicate the dominance of normal faulting. We also evaluated the stress field in north and middle zones of Gulf of Suez using a multiple inverse method. The prime strain axis shows that the deformation is taken up mainly as stretching in the E–W and NE–SW direction.     

Seismic hazard monitoring in Kamchatka and its applications to the M = 6.6 earthquake of 6 October 1987

The earthquake of 6 October 1987 (M = 6.6), which occurred near the Shipunsky Cape, Kamchatka, was the largest crustal event in the vicinity of the main city of Kamchatka — Petropavlovsk-Kamchatsky — during the last three decades. It was followed by numerous aftershocks. This earthquake allowed us to test the effectiveness of the seismic hazard monitoring in Kamchatka, including the seismological, geodetic and hydrogeochemical surveys. The seismic survey provided the location and source nature of the main shock and aftershocks and the seismic environment of the main shock. The geodetic and hydrogeochemical surveys have yielded data on the response to earthquakes of the Earth's surface deformations, water level, and chemical elements concentration in the underground water. As a result, the following data were obtained:

u

The earthquake of 6 October had a seismic moment 4–10 E18 Nm, thrust type of faulting and the source volume of 20 × 20 × 10 km³. The maximum intensity was VI–VII (MSK-64 scale) and maximum acceleration 88 cm/s².

Before this event, a relative increase in the number of the upper mantle (depth more than 100 km) moderate magnitude earthquakes during 5 years and a one-year period of seismic quiescence for small shallow earthquakes, were recognized. Significant anomalies in HCO₃ and H₃BO₃ concentrations in the underground waters were observed in the wells a week before the main shock.

     

Estimation of earthquake hazard parameters for incomplete and uncertain data files

The maximum likelihood estimation of earthquake hazard parameters (maximum regional magnitudem _max, activity rate λ, and theb parameter in the Gutenberg-Richter distribution) is extended to the cases of incomplete and uncertain data. The method accepts mixed data containing only large (extreme) events and a variable quality of complete data with different threshold magnitude values. Uncertainty of earthquake magnitude is specified by two values, the lower and upper magnitude limits. It is assumed that such an interval contains the real unknown magnitude. The proposed approach allows the combination of different quality catalog parts, e.g. those where the assignment of magnitude is questionable and those with magnitudes precisely determined. As an illustration of the method, the seismic hazard analysis for western Norway and adjacent sea area (4–8°E, 58–64°N) is presented on the basis of the strongest earthquakes felt during the period 1831–1889 and three complete catalog parts, covering the period 1890–1987.     

Seismicity and seismotectonics of the Gulf of Aqaba region

The historical seismicity of the last ten centuries and the instrumental data that occurred in the Gulf of Aqaba region during the period 1982–2008 are evaluated. It is found that 12 historical earthquakes have occurred with average recurrence periods 70–90 and 333–500 years for M?≥?6.0 and 7.0, respectively. Those with M?≤?6.5 appear to be incomplete and require further investigation. More than 98 % of the instrumental data has occurred in the form of swarms and sequences. The first have released about 32 % of the total energy and are most likely related to subsurface volcanic activities. Their epicentral distribution indicates that all regional faults of the gulf area are active in the present, but with clear concentration within the area bound by latitudes 28.2°–29.8° and longitudes 34.4°–35.2°. Regional strike-slip faults of the northern two basins appear to be as twice active as the normal, or more. An appreciable level of seismic hazard is envisaged as the “a” value is 6.0–6.2 while the “b” value shows a temporal variation, mostly in the range 0.8–1.05. More than 95 % of the seismic energy was released from earthquakes shallower than 22 km. This indicates a brittle upper crust and a ductile lower crust and upper mantle. Tectonic movements at depths?>?22 km appear to be aseismic. The epicentral distribution of the five swarms indicates that the lengths of the causative faults varied in the range 45–70 km. The maximum expected magnitude is Mw?=?6.8–7.2. This implies a seismic slip rate of about 0.54–0.8 Cm/year and some 20–30 % of aseismic tectonic movements. This and the sequence nature of the seismicity of this region result in a noticeable hazard reduction. Combining the seismicity data of the Gulf of Aqaba region with other geophysical, geological, tectonic, and environmental data, clearly indicate that the seismicity of this region is as old as the initiation of the gulf itself. No apparent southward or northward migration of activity is observed.     

Surface fault traces and fault plane solutions of the May–June 1978 major shocks in the Thessaloniki area, Greece.

It is shown that the foci of the recent earthquakes in the Thessaloniki area of northern Greece are located in an arcuate seismic zone which is associated with the Serbomacedonian geologic zone. Three main lines of fracture have been observed in the epicentral area after the May–June 1978 earthquakes. Field and macroseismic observations as well as fault plane solutions for the main shock and for the largest foreshock show that both earthquakes are due to a strike slip sinistral motion with a small reverse component on a steeply dipping and trending southeast-northwest fault.     

Seismicity of Sinai Peninsula, Egypt

The Sinai Peninsula has a triangular shape between the African and Arabian Plates and is bounded from the western and eastern borders by the Gulf of Suez and Gulf of Aqaba–Dead Sea rift systems, respectively. It is affected by strong and destructive earthquakes (e.g., March 31, 1969 and November 22, 1995) and moderate earthquakes (m _b?>?5) throughout its history. After the installation of the Egyptian National Seismic Network (ENSN), a great number of earthquakes has been recorded within and around Sinai. Consequently, the seismogenic source zones and seismotectonic behavior can be clearly identified. Available data, including both historical and instrumental (1900–1997), have been collected from national and international data centers. While the data from 1998 till December 2007 are gathered from ENSN bulletins. The seismogenic source zones that might affect Sinai Peninsula are defined more precisely in this work depending on the distribution of earthquakes, seismicity rate (a value), b value, and fault plane solution of the major earthquakes. In addition, the type of faults prevailed and characterized these zones. It is concluded that the Gulf of Aqaba zone–Dead Sea transform zone, Gulf of Suez rift zone, Cairo–Suez District zone, and Eastern Mediterranean dislocation zone represent the major effective zones for Sinai. Furthermore, there are two local seismic zones passing through Sinai contributing to the earthquake activities of Sinai, these are the Negev shear zone and Central Sinai fault (Themed fault) zone. The source parameters, a and b values, and the maximum expected moment magnitude have been determined for each of these zones. These results will contribute to a great extent in the seismic hazard assessment and risk mitigation studies for Sinai Peninsula to protect the developmental projects.