Earthquake Of February Research Articles

Enduring Lessons and Opportunities Lost from the San Fernando Earthquake of February 9, 1971

Twenty-five years have passed since the San Fernando earthquake of February 9, 1971. The paper reviews the lessons learned and not learned from this notable event. Most of the major lessons were reported within a few weeks of the earthquake by a panel appointed by the National Academies of Sciences and of Engineering. In this paper, the status of each of the eighteen general lessons cited by the panel is reviewed, plus two additional lessons selected from other studies of the earthquake. The lessons learned ranged broadly and concerned measures needed to reduce future earthquake hazards, as well as recommended scientific and engineering efforts. Although all of the lessons learned were not heeded, the San Fernando earthquake represented a turning point in public awareness and in actions taken to reduce earthquake hazard. Recent earthquakes have shown, however, that much remains to be done.

The earthquake of 27thFebrary 1768 in Lower Austria: repair costs and seismic intensity in Wr.Neustadt

During the Age of Enlightenment the importance of administration increased. It left its traces in the official inquiries after natural disasters. For financial and other reasons authorities found it important to record the dam- age done to buildings by earthquakes. This background reflects the official inquiries after the damaging earthquake of 27th February I 768 in Lower Austria, which includes an important additional piece of information: a record of the respective damage costs il1 floril1s per house in Wr. Neustadt. In this study an attempt of quantification is made to relate the damage costs to the respective damage class and intensity according to the EMS- 92 scale. The earthquake intensity for Wr. Neustadt as a whole is estimated to be I = VII EMS-92. This result contrasts with older publications, where the intensity VII- VIII or VIII has been assessed.

Structural Damage, Ground Failure, and Hydrologic Effects of the Magnitude (Mw) 5.9 Draney Peak, Idaho, Earthquake of February 3, 1994

INTRODUCTION A magnitude ( Mw ) 5.9 earthquake occurred at 09:05 (GMT) at 42° 44.42′ N. and 111° 8.93′ W. (Arabasz et al. , 1994) in the mountains of southeastern Idaho near the Idaho-Wyoming border, 100 km ESE of Pocatello, Idaho, and 18 km W of Afton, Wyoming (Figure 1). The earthquake, which was located near the center of the active part of the 1500-km-long Intermountain Seismic Belt, was preceded by foreshocks as large as M 4.7 (Nava et al. , 1994), and had an estimated focal depth of 8 km (U.S. Geological Survey, 1994). The main shock was felt in the immediate area and was reported felt by residents as far west as Pocatello, Idaho, and as far south as Moab, Utah, and Grand Junction, Colorado (Figure 2). Only minor damage, such as broken glassware, cracked plaster, and minor structural damage were reported by residents of Afton and the immediate epicentral area...

Earthquake prediction, societal implications

“If I were a brilliant scientist, I would be working on earthquake prediction.” This is a statement from a Los Angeles radio talk show I heard just after the Northridge earthquake of January 17, 1994. Five weeks later, at a monthly meeting of the Southern California Earthquake Center (SCEC), where more than two hundred scientists and engineers gathered to exchange notes on the earthquake, a distinguished French geologist who works on earthquake faults in China envied me for working now in southern California. This place is like northeastern China 20 years ago, when high seismicity and research activities led to the successful prediction of the Haicheng earthquake of February 4, 1975 with magnitude 7.3. A difficult question still haunting us [Aki, 1989] is whether the Haicheng prediction was founded on the physical reality of precursory phenomena or on the wishful thinking of observers subjected to the political pressure which encouraged precursor reporting. It is, however, true that a successful life‐saving prediction like the Haicheng prediction can only be carried out by the coordinated efforts of decision makers and physical scientists.

Late quaternary activity of the Laguna Salada fault in northern Baja California, Mexico

Faulted alluvial fans and bajadas along the central Laguna Salada fault zone in northernBajaCaliforniarecordarecurrent history of oblique-slip Holocene earthquakes.Alluvialsurfaces,whichrangefrom late Pleistocene to historic in age have been progressively displaced along the base of the crystalline rangefront, as well as along more basinward fault splays in alluvium. The recurrence interval determined from displacedalluvialdepositsthataredatedby soil profile development is in the range of 1‐2 ka, with a corresponding right lateral slip rate of ;2‐3 mm/yr, similar to that of thesouthernElsinorefaultinsouthernCalifornia. The most recent event along the fault zone is probably the widely felt earthquake of February 23, 1892. At least 22 km ofthefaultzonerupturedduringthisevent, along both an oblique-dextral section of the northwest-strikingLagunaSaladafaultand

The Aljibes half-graben—Active extension at the boundary between the trans-Mexican volcanic belt and the Basin and Range Province, Mexico

The Aljibes half-graben is located 140 km west-northwest of Mexico City and is characterized by several parallel, ≥10-km-long, south-dipping, and east-striking normal faults. Late Miocene basalt flows with 40 Ar/ 39 Ar ages of 7.7 ± 0.1 Ma and 7.1 ± 0.5 Ma are displaced along these faults. With reference to this basalt, the composite throw and heave of the half-graben are 480 m and 270 m, respectively, and the relative extension is 5%, whereas the vertical slip rate along the faults of the Aljibes half-graben after the deposition of the basalt is ≥0.07 mm/yr. The Aljibes half-graben is located at the northern margin of the trans-Mexican volcanic belt and perpendicularly intersects northstriking normal faults of the Basin and Range Province. The relative sequence of activity of the two stress fields and fault systems cannot be resolved in the study area, and the two systems may have been active simultaneously. We explain this structural configuration by horizontal migration of the boundary between the two stress provinces with time, which requires intermittent permutations between the intermediate and least principal stresses σ 2 and σ 3 . This is supported by the low-stress ratio Φ = (σ 2 − σ 3 /(σ 1 − σ 3 ) obtained from modeling the stress tensor associated with the slip along the faults of the Aljibes half-graben. The orientations of the fault planes and slip vectors of the Aljibes half-graben are practically identical to those of the nearby Acambay graben and the Venta de Bravo fault, which caused the Acambay earthquake of November 19, 1912 (M S = 6.9), and the Maravatio earthquake of February 22, 1979 (m b = 5.3). Furthermore, the Aljibes half-graben forms the extrapolated western continuation of the northern master fault of the Mezquital graben, which ruptured partly in the Cardonal earthquake of March 26, 1976 (m b = 5.3). This suggests the Aljibes half-graben may also be seismically active, although no historical earthquake is known to have occurred along this structure. Further evidence for the likely seismic activity of the Aljibes half-graben are two east- to eastnortheast–trending epicenter alignments between the Aljibes halfgraben and the Mezquital graben, and structural indication for stickslip behavior. Based on the structure of the Aljibes half-graben, we believe the maximum credible earthquake that may occur along the faults of the Aljibes half-graben is on the order of 2 × 10 27 dyne cm ≥ M 0 ≥ 6 × 10 25 dyne cm and 7.5 ≥ M w ≥ 6.5.

Bering Sea Earthquake of February 21, 1991: Active faulting along the Bering Shelf Edge

On February 21, 1991, an MS = 6.8 shallow earthquake occurred in the eastern Bering Sea in an area which previously has shown no significant seismic activity. The earthquake was located on the Bering Shelf close to the shelf edge, near one of the elongate basins (Zhemchug Basin) that follow the outer shelf. To better understand the causes of this unusual event, we relocated the earthquake and its aftershocks, determined its source mechanism and depth, and examined multichannel seismic observations of structures near the epicenter. The event was relocated from regional and teleseismic P and S arrival times using a nonlinear inverse technique, as were the 19 previous well‐recorded Bering Sea earthquakes in global catalogs (1964–1987). The 1991 mainshock epicenter was located on the western flank of Zhemchug Basin. The only four previous events with small (<1°) location errors relocated at the outer shelf near the Pribilof Islands. The focal mechanism for the 1991 event, determined by inversion of teleseismic, broadband body waves and centroid‐moment tensor data, shows oblique normal faulting with the T‐axis oriented north‐south. One nodal plane dips steeply to the northeast and strikes parallel to the basin axis, subparallel to faults that bound Zhemchug Basin. This earthquake may be due to slip on one of these normal faults, several of which are seen to offset young strata on a multichannel seismic line 30 km south of the earthquake. It has previously been suggested that the large canyons that cut the Bering Shelf and the adjacent outer‐shelf basins are fault controlled, but this event and its aftershocks provide the first strong evidence to support the hypothesis that these structures are currently active and produce earthquakes. Seismic activity along the Bering Shelf edge may provide a good explanation for felt reports of several large events in the 1800s that have so far been difficult to associate with interplate activity along the Aleutian trench or with any other seismically active structure.

横ずれ断層地震による津波の励起

We succeeded in explaining a small tsunami on a vertical displacement field of a pure strike-slip fault. The Izu-Oshima Kinkai Earthquake (M=6.5) of February 20, 1990, being accompanied with a small tsunami, was modeled as a pure strike-slip fault on a focal mechanism solution and aftershock area. The sinistral strike slip fault with a plane of 15×12km2, dip of almost 90° and strike of N0°E was assumed at the epicenter.A numerical experiment on the vertical displacement field reproduced the observed tsunami fairly well. The computation was carried out on a linear framework using a mesh of 1km, time step of 1 sec and an additional fine mesh of 0.25km in the vicinity of tide stations. The assumed dislocation of 1m explained the observed amplitude at the nearest tide station. Comparison of waveforms between model and four observations supports the model.Furthermore, a spectral analysis was attempted to reveal predominant frequencies of the observed tsunami. The observed predominant frequencies were explained from wave lengthes characteristic to the pure stlike-slip fault. A strong azimuth dependence is one of the characteristic properties of the pure strike slip tsunami. An unexplainable predominant frequency was attributed to the reflected wave from the Izu Oshima Island.

The Yecheng, Xinjiang, China, earthquake of February 14, 1980

In this study, we have checked the location and focal depth of the Yecheng earthquake (m b = 6.0, maximum intensity VII) of February 14, 1980. The result shows that this is an intermediate event with a focal depth of 90 km. The microepicenter is located at 36. 4°N, 76, 9°E, while the macroepicenter is at 37.3°N, 76.9°E, 90 km to the north of the microepicenter. This is the first destructive intermediate event in China which led to a damage as severe as of intensity VII. The focal mechanism of the event is determined to be of thrust type. Combined with the analysis of seismological and geological data in surrounding area, the possible relation between the event and plate movement has been discussed. The result in this paper indicates that in some particular place, the destructive effect of intermediate event should be considered in seismic hazard assessment.

A discussion on the amplitude ratios of PP and P waves for earthquakes with different focal mechanisms

Using the WKBJ approximation method we calculate the synthetic teleseismograms of P and PP waves to match the observed ones of six large Chinese earthquakes with known focal mechanisms: Tibet earthquake of July 14, 1973; Haicheng earthquake of February 4, 1975; Songpan earthquakes of August 16, 1976, August 21, 1976 and August 23, 1976 and Nignhe earthquake of November 15, 1976. The focal mechanism of the Tibet earthquake is discussed to examine the technique used in the calculation. We note that the amplitude ratios of PP and P waves (A PP/A P) have different characteristics for dip—slip events and strike—slip events within certain epicentral distances. We calculate the synthetic teleseismograms of P and PP waves for the strike—slip and dip—slip events with fault angles of 330°, 240° and 0°, focal depths of 8 km, 17 km and 24 km, at the assumed station with an azimuth of 310° and epicentral distances from 40°; to 80°. The diagrams of maximum amplitude ratios of PP and P waves (A PP/A P) versus distances are given. The possibility to use the (A PP/A P) values to give an approximate estimation for the focal mechanism type is discussed. This work may be useful for determining the focal mechanism type for those earthquakes which have only few records such as the Chinese earthquakes from the 1930s to 1960s.

Identification of Linear Structures Using Discrete‐Time Filters

Most of the previous studies considered structural system identification in the continuous‐time domain. The discrete‐time approach to the problem is more natural since all the recordings are in the discrete‐time form. This study presents a discrete‐time method for system identification by using discrete‐time linear filters. The method itself is well known in electrical and systems engineering fields, and therefore is not new. The objective in the paper is to present the method by emphasising its relation to the more familiar continuous‐domain modal analysis approach that is widely used in structural engineering. In addition to the method, some practical but important problems are also discussed in the paper, such as the processing of data, the selection and validation of models in the identification, and the detection of soil‐structure interaction. As an example, a 12‐story building was identified by using recordings from the magnitude 6.4, San Fernando, California earthquake of February 9, 1971.

The effects of the November 1, 1755 “Lisbon” earthquake in Morocco

The “Lisbon” earthquake of November 1, 1755, the effects of which have been described at length in the Iberian Peninsula, was likewise strongly felt in Morocco, especially on the Atlantic coast, which was laid waste not only through the direct agency of seismic waves, but also through that of a formidable tsunami. The study of these effects in Morocco, based on a critical analysis of European and Arab sources entailing, whenever possible, a consultation of the original sources, has lead to the hypothesis that two distinct destructive earthquakes would have followed the main shock of November 1; one, on November 18, 1755, was probably an aftershock of the November 1 event and originated in the Gorringe Bank; the other, on November 27, 1755, was located in the Meknes area and is supposed to have destroyed that town. The values of intensities interpreted from the observations of the November 1 event have been compared with the values derived from the attenuation laws obtained from the recent earthquake of February 28, 1969 originating at the same source, the Moroccan isoseismals of which are well established. Extrapolated isoseismals for the effects of the November 1, 1755 event in Morocco as derived from this study are then assigned

Seismological study of the Kliuchevskoi flank eruption of 1983 (Kamchatka)

A flank eruption, called the Predskazanny eruption, occurred at an altitude of 2875 m on the east flank of the Kliuchevskoi volcano from March 8 to June 27, 1983. The eruption was largely effusive, with a constant chemical composition of the lavas (calc-alkaline aluminium basalts). Their volume is estimated at 0.11 km 3 . The seismic precursors of the Predskazanny eruption was concentrated beneath the new eruption center, first at depths of between 10 and 15 km (earthquake of February 28, K s = 8.3, M = 3.5) and then at depths between 5 km b.s.l. and 3 km a.s.l. (earthquake swarm of March 1–7). Volcanic tremor during the earthquake swarm of March 1–7 was absent. With the approach of an eruption, the depth of earthquake foci in the swarm decreased. An analysis of the seismicity implies that the magma was apparently ascending in the newly formed channel from a depth of at least 5 km. The feeding area of the Predskazanny eruption was apparently connected with the major magma channel of Kliuchevskoi at depths between 5 and 10 km, but not deeper than 15 km.

The Lake Keowee, South Carolina Earthquakes of February through July 1986

Abstract In the early morning of 13 February 1986, an earthquake with a duration magnitude (MD) of 3.2 rumbled through northwestern South Carolina. The event was centered near Lake Keowee in Oconee County in a region of prior low level seismicity. Approximately eighty aftershocks with magnitudes ranging from −1.0 to 2.0 were recorded during the next six days. The locations of five aftershocks were accurately determined, utilizing data from portable seismographs deployed in the epicentral area. Depths of the two earthquakes with a location quality of B or better were between 3 and 4 km. First motion focal mechanism solutions for the mainshock suggest oblique slip along a plane striking northeast or northwest. The P axis was oriented northeast-southwest in support of the directions obtained from mechanisms of other local earthquakes and from direct measurements of the maximum horizontal stress in the regions. A second mainshock (MD = 2.8) occurred in the vicinity of Lake Keowee on 11 June 1986 and was followed by over sixty earthquakes during the next five weeks. Focal mechanism solutions from first motion data obtained for the mainshock resemble those of the 13 February event and suggest oblique slip along a northeast or northwest striking plane. Depths of the best located aftershocks were approximately 1 km. Two tests were applied to the data to assess the reliability of the depth estimates. These involve the determination that the plot of RMS travel time residual versus fixed solution depth exhibits a single, sharp RMS minimum at the depth obtained from a free solution (depth uniqueness) and that the final free solution depth is not dependent upon the choice of starting depth (depth stability). Free solution depths obtained for the majority of the better located aftershocks were found to be unique and stable at depths between 1 and 4 km. A northeast trending anomaly is prominent in the geophysical data for this area. This anomaly is interpreted to result from an abrupt, lateral change in lithology along a shallow, northeast striking plane. The earthquakes do not appear to be associated with this feature. Instead, these earthquakes appear to be associated with a shallow body and may represent slip along northeast or northwest striking joints. The proximity of these earthquakes to Lake Keowee suggests the possibility of reservoir triggering. No correlation between seismicity and reservoir level is evident prior to the February events. Rapid fluctuations in water level did precede the events in June and July, providing possible triggering mechanisms.

The Whittier Narrows, California Earthquake of October 1, 1987—Emergency Response

The Whittier Narrows earthquake of October 1, 1987 was the most significant seismic event to occur in southern California since the San Fernando earthquake of February 1971. As such, the 5.9 magnitude temblor provided a basis for evaluating the level of preparedness in at least a portion of the Los Angeles metropolitan region. The Whittier Narrows event provides a hint of the progress that has been made in local, state and private sector preparedness in the last six years as well as suggesting areas that need additional effort.

Hypothesis for a submarine landslide and cohesionless sediment flows resulting from a 17th century earthquake-triggered landslide in Quebec, Canada

Textural isotopic and microfossil data from two gravity cores obtained in Saguenay Fjord, Quebec, suggest that a distinctive sandy clay bed was deposited as the result of a major landslide in the Saguenay River basin. Pb-210 dating of the cores indicate that the bed is of similar age to the magnitude 7 earthquake of February 5, 1663. The slide involved sensitive marine clays and may have occurred in two stages. Slide sediments carried into the Saguenay River channel were probably reworked and subsequently transported down the Fjord basin as two distinct cohesionless mass flows. Fine clay laminae that overlie the older mass flow bed record the modulation of depositional processes by tidal currents for several weeks after this event.

Small Amplitude Vibration Measurements of Buildings Undamaged, Damaged, and Repaired after Earthquakes

Ambient vibrations provided the means to measure dynamic properties of reinforced concrete undamaged buildings, damaged and repaired after the destructive earthquakes of February 24, 1981, in the major area of Athens and in Central Greece. Each fundamental period is given as function of the number of stories, the height of the building, the dimensions of its plan and the percentage of shear walls. The shape of the deformation of the vertical centerline and the associated percentage of critical damping resulted also from the measurements, which in addition proved that for the damaged buildings the vertical centerline presents a discontinuity at the level where the damages are concentrated. For the repaired buildings this line becomes smooth.

Re‐Examination of Slide of Lower San Fernando Dam

A major slide occurred in the upstream slope of the Lower San Fernando Dam immediately following the earthquake of February 9, 1971. Previous analyses of this slide have been based on evaluation of seismic shear stresses and the results of cyclic triaxial tests. These previous analyses fail to explain several key features of the slide. The re‐analysis presented herein, which is based on the concepts of liquefaction, steady state strength, and flow slide stability analysis, and on the data obtained during the previous analyses, is consistent with the principal features of the observed slide. The estimated in situ undrained steady state strength is substantially lower than the undrained strength measured in conventional laboratory tests due to the sample densification that normally occurs during sampling, handling, setup, and consolidation of loose sandy soils. An interpretation of the limited data on undrained strength available for the San Fernando Dam indicates that the soils that liquefied had very low ...

The Earthquake in Algeria in 1980

On October 10, 1980, the town of El-Asnam (130,000 inhabitants) and villages within a 50 km radius (229,000 inhabitants) were 80% destroyed by an earthquake of intensity 7.5 on the Richter scale (Figure l). The first unofficial estimates reported 50,000 dead and 100,000 injured. These numbers were probably based on data from the Agadir earthquake of February 29, 1960, when 12,000 out of the 30,000 inhabitants were killed. The official figures now list for the El-Asnam earthquake 2,600 dead and 8,250 injured. That was nevertheless a sufficiently heavy toll to fully justify the volume of relief that came from practically all over the world. The relief of Baden-Wurttenberg was organized by the German Air Rescue Guard of Stuttgart, in collaboration with the German Red Cross in Karlsruhe, as well as with physicians from Karlsruhe, Stuttgart and Tubingen. In a very short time the necessary aircraft (Merlin IV/Metro and BAC 1–11) for the transport of material and personnel from the German Headquarters were ready for takeoff.

The Catalan earthquake of February 2, 1428

The effects of the Catalan earthquake of February 2, 1428, between 8 h and 9 h local time, have been studied by reevaluating the information gathered by Fontseré and Iglesies (1971). MSK-intensities have been assigned to 45 localities to obtain an isoseismal map. A maximum intensity of IX has been deduced. The most probable location is the area around the village of Queralps (42.4° N 2.2° E) at a depth of about 8 km. The attenuation seems to be similar to that found for earthquakes in Central Europe, which considerably differs from that in southern Spain.