Observations of the Coyote Lake, California earthquake sequence of August 6, 1979

abstract At 1705 UTC on August 6, 1979, a strong earthquake (ML = 5.9) occurred along the Calaveras fault zone south of Coyote Lake about 110 km southeast of San Francisco. This strong earthquake had an aftershock sequence of 31 events (2.4 ≦ ML ≦ 4.4) during August 1979. No foreshocks (ML ≧ 1.5) were observed in the 3 months prior to the main shock. The local magnitude (ML = 5.9) and the seismic moment (Mo = 6 × 1024 dyne-cm from the SH pulse) for the main shock were determined from the 100 × torsion and 3-component ultra-long period seismographs located at Berkeley. Local magnitudes are determined for the aftershocks from the maximum trace amplitudes on the Wood-Anderson torsion seismograms recorded at Berkeley (Δ ≊ 110 km). Temporal and spatial characteristics of the aftershock sequence are presented and discussed. Some key observations are: (1) the first six aftershocks (ML ≧ 2.4) proceed along the fault zone progressively to the south of the main shock; (2) all of the aftershocks (ML ≧ 2.4) to the south of the largest aftershock (ML = 4.4) have a different focal mechanism than the aftershocks to the north; (3) no aftershocks (ML ≧ 2.4) were observed significantly to the north of the main shock for the first 5 days of the sequence; and (4) the b-value (0.70 ± 0.17) for the aftershock sequence is not significantly different from the average b-value (0.88 ± 0.08) calculated for the Calaveras fault zone from 16 yr of data.

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The Tejon Pass earthquake of October 22, 1916

Bulletin of the Seismological Society of America ◽

10.1785/bssa0070020051 ◽

1917 ◽

Vol 7 (2) ◽

pp. 51-60

Author(s):

John Casper Branner

Keyword(s):

Los Angeles ◽

San Francisco ◽

Fault Zone ◽

Fault Line ◽

The South ◽

Epicentral Area ◽

The North ◽

San Andreas ◽

North Side ◽

To Come

Summary The area over which the shock was felt by persons at rest was 27,000 square miles or more, extending from Fresno on the north to San Diego on the south, and from Mojave to the coast. The epicenter seems to have been near the summit of the Tejon Pass, where the intensity reached VII or a little more, of the Rossi-Forel scale. At many places the shock was preceded by a pronounced roar like thunder or a high wind. Wherever the direction of the sound was noted it appeared to come from the epicentral area. The region is too thinly populated and our data are too meager to enable us to outline the area of high intensity with confidence, but the following facts seem to be fairly well established: The shock or shocks were produced by movement on the fault line that passes through the Tejon Pass and follows thence east-southeast along the axes of Leonas Valley and Anaverde Valley and northwestward through Cuddy Canyon and Cuddy Valley. The topographic evidence of the fault in the Tejon Pass is very pronounced, but there is topographic evidence of another fault that branches off from the Tejon Pass fault about a mile and a half northwest of Tejon Pass and runs east-northeast from the northwest corner of Los Angeles county, passing along the north side of Castac Lake. The depression occupied by Castac Lake seems to have been formed by a downthrow on the south side of this fault. It has been supposed that the fault through Tejon Pass was a southward prolongation of the San Andreas fault near San Francisco. The identity of these faults is far from being evident. The topography, the distribution of earthquake shocks, and the method of fracture along the fault zones all suggest a series of overlapping faults rather than one continuous fault. Mr. Hamlin says on this subject: “This fault is not a long continuous fracture, but rather a fault zone with numerous branches. Dropped blocks are not uncommon along this zone, some being a mile or more wide and twice as long.” The forms of the isoseismals of this particular earthquake, however, suggest definite relations to this fault zone.

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The San Salvador Earthquake of October 10, 1986—Seismological Aspects and Other Recent Local Seismicity

Earthquake Spectra ◽

10.1193/1.1585439 ◽

1987 ◽

Vol 3 (3) ◽

pp. 419-434 ◽

Cited By ~ 10

Author(s):

Randall A. White ◽

David H. Harlow ◽

Salvador Alvarez

Keyword(s):

Main Shock ◽

Fault Plane ◽

Vertical Plane ◽

Aftershock Sequence ◽

Central American ◽

Fault Plane Solution ◽

San Salvador ◽

The North ◽

Volcanic Chain ◽

Plane Solution

The San Salvador earthquake of October 10, 1986 originated along the Central American volcanic chain within the upper crust of the Caribbean Plate. Results from a local seismograph network show a tectonic style main shock-aftershock sequence, with a magnitude, Mw, 5.6. The hypocenter was located 7.3 km below the south edge of San Salvador. The main shock ruptured along a nearly vertical plane toward the north-northeast. A main shock fault-plane solution shows a nearly vertical fault plane striking N32\sz\E, with left-lateral sense of motion. This earthquake is the second Central American volcanic chain earthquake documented with left-lateral slip on a fault perpendicular to the volcanic chain. During the 2 1/2 years preceeding the earthquake, minor microseismicity was noted near the epicenter, but we show that this has been common along the volcanic chain since at least 1953. San Salvador was previously damaged by a volcanic chain earthquake on May 3, 1965. The locations of six foreshocks preceding the 1965 shock show a distinctly WNW-trending distribution. This observation, together with the distribution of damage and a fault-plane solution, suggest that right-lateral slip occurred along a fault sub-parallel with Central American volcanic chain. We believe this is the first time such motion has been documented along the volcanic chain. This earthquake was also unusual in that it was preceded by a foreshock sequence more energetic than the aftershock sequence. Earlier this century, on June 08, 1917, an Ms 6.4 earthquake occurred 30 to 40 km west of San Salvador Volcano. Only 30 minutes later, an Ms 6.3 earthquake occurred, centered at the volcano, and about 35 minutes later the volcano erupted. In 1919 an Ms 6 earthquake occurred, centered at about the epicenter of the 1986 earthquake. We conclude that the volcanic chain is seismically very active with variable styles of seismicity.

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Rampart seismic zone of central Alaska

Bulletin of the Seismological Society of America ◽

10.1785/bssa0730030813 ◽

1983 ◽

Vol 73 (3) ◽

pp. 813-829

Author(s):

P. Yi-Fa Huang ◽

N. N. Biswas

Keyword(s):

Main Shock ◽

Fault Plane ◽

B Value ◽

Aftershock Sequence ◽

Strike Slip ◽

Lithospheric Plate ◽

Normal Faults ◽

Seismic Zone ◽

The West ◽

Fault Plane Solutions

abstract This paper describes the characteristics of the Rampart seismic zone by means of the aftershock sequence of the Rampart earthquake (ML = 6.8) which occurred in central Alaska on 29 October 1968. The magnitudes of the aftershocks ranged from about 1.6 to 4.4 which yielded a b value of 0.96 ± 0.09. The locations of the aftershocks outline a NNE-SSW trending aftershock zone about 50 km long which coincides with the offset of the Kaltag fault from the Victoria Creek fault. The rupture zone dips steeply (≈80°) to the west and extends from the surface to a depth of about 10 km. Fault plane solutions for a group of selected aftershocks, which occurred over a period of 22 days after the main shock, show simultaneous occurrences of strike-slip and normal faults. A comparison of the trends in seismicity between the neighboring areas shows that the Rampart seismic zone lies outside the area of underthrusting of the lithospheric plate in southcentral and central Alaska. The seismic zone outlined by the aftershock sequence appears to represent the formation of an intraplate fracture caused by regional northwest compression.

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Volcanic centres between Frigg Fjord and Midtkap, eastern North Greenland

Rapport Grønlands Geologiske Undersøgelse ◽

10.34194/rapggu.v106.7768 ◽

1981 ◽

Vol 106 ◽

pp. 69-75

Author(s):

I Parsons

Keyword(s):

Fault Zone ◽

Fold Belt ◽

The South ◽

The North ◽

Rock Types ◽

South West ◽

North Greenland

A series of smal! volcanic centres cut Ordovician turbidites of Formation A in the southem part of Johannes V. Jensen Land between Midtkap and Frigg Fjord (Map 2). Their general location and main rock types were described by Soper et al. (1980) and their nomenclature is adopted here for fig. 22 with the addition of the small pipe B2. A further small intrusion, south-west of Frigg Fjord, was described by Pedersen (1980). The centres lie 5-10 km south of, and parallel to, the important Harder Fjord fault zone (fig. 22) which traverses the southern part of the North Greenland fold belt and shows substantial downthrow to the south (Higgins et al., this report).

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Surface faulting along the Superstition Hills fault zone and nearby faults associated with the earthquakes of 24 November 1987

Bulletin of the Seismological Society of America ◽

10.1785/bssa0790020252 ◽

1989 ◽

Vol 79 (2) ◽

pp. 252-281

Author(s):

R. V. Sharp ◽

K. E. Budding ◽

J. Boatwright ◽

M. J. Ader ◽

M. G. Bonilla ◽

...

Keyword(s):

Fault Zone ◽

Lateral Displacement ◽

Vertical Displacement ◽

Repeated Measurements ◽

The South ◽

Surface Rupture ◽

Imperial Valley ◽

The North ◽

South Central ◽

The Right

Abstract The M 6.2 Elmore Desert Ranch earthquake of 24 November 1987 was associated spatially and probably temporally with left-lateral surface rupture on many northeast-trending faults in and near the Superstition Hills in western Imperial Valley. Three curving discontinuous principal zones of rupture among these breaks extended northeastward from near the Superstition Hills fault zone as far as 9 km; the maximum observed surface slip, 12.5 cm, was on the northern of the three, the Elmore Ranch fault, at a point near the epicenter. Twelve hours after the Elmore Ranch earthquake, the M 6.6 Superstition Hills earthquake occurred near the northwest end of the right-lateral Superstition Hills fault zone. Surface rupture associated with the second event occurred along three strands of the zone, here named North and South strands of the Superstition Hills fault and the Wienert fault, for 27 km southeastward from the epicenter. In contrast to the left-lateral faulting, which remained unchanged throughout the period of investigation, the right-lateral movement on the Superstition hills fault zone continued to increase with time, a behavior that was similar to other recent historical surface ruptures on northwest-trending faults in the Imperial Valley region. We measured displacements over 339 days at as many as 296 sites along the Superstition Hills fault zone, and repeated measurements at 49 sites provided sufficient data to fit with a simple power law. Data for each of the 49 sites were used to compute longitudinal displacement profiles for 1 day and to estimate the final displacement that measured slips will approach asymptotically several years after the earthquakes. The maximum right-lateral slip at 1 day was about 50 cm near the south-central part of the North strand of Superstition Hills fault, and the predicted maximum final displacement is probably about 112 cm at Imler Road near the center of the South strand of the Superstition Hills fault. The overall distributions of right-lateral displacement at 1 day and the estimated final slip are nearly symmetrical about the midpoint of the surface rupture. The average estimated final right-lateral slip for the Superstition Hills fault zone is about 54 cm. The average left-lateral slip for the conjugate faults trending northeastward is about 23 cm. The southernmost ruptured member of the Superstition Hills fault zone, newly named the Wienert fault, extends the known length of the zone by about 4 km. The southern half of this fault, south of New River, expressed only vertical displacement on a sinuous trace. The maximum vertical slip by the end of the observation period there was about 25 cm, but its growth had not ceased. Photolineaments southeast of the end of new surface rupture suggest continuation of the Superstition Hills fault zone in farmland toward Mexico.

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Temporal variations of the fractal properties of seismicity in the western part of the north Anatolian fault zone: possible artifacts due to improvements in station coverage

Nonlinear Processes in Geophysics ◽

10.5194/npg-2-147-1995 ◽

1995 ◽

Vol 2 (3/4) ◽

pp. 147-157 ◽

Cited By ~ 16

Author(s):

A. O. Öncel ◽

Ö. Alptekin ◽

I. Main

Keyword(s):

Fault Zone ◽

Statistical Significance ◽

Rock Fracture ◽

Aegean Sea ◽

B Value ◽

Temporal Variations ◽

North Anatolian Fault ◽

North Anatolian Fault Zone ◽

The North ◽

Northern Aegean Sea

Abstract. Seismically-active fault zones are complex natural systems exhibiting scale-invariant or fractal correlation between earthquakes in space and time, and a power-law scaling of fault length or earthquake source dimension consistent with the exponent b of the Gutenberg-Richter frequency-magnitude relation. The fractal dimension of seismicity is a measure of the degree of both the heterogeneity of the process (whether fixed or self-generated) and the clustering of seismic activity. Temporal variations of the b-value and the two-point fractal (correlation) dimension Dc have been related to the preparation process for natural earthquakes and rock fracture in the laboratory These statistical scaling properties of seismicity may therefore have the potential at least to be sensitive short- term predictors of major earthquakes. The North Anatolian Fault Zone (NAFZ) is a seismicallyactive dextral strike slip fault zone which forms the northern boundary of the westward moving Anatolian plate. It is splayed into three branches at about 31oE and continues westward toward the northern Aegean sea. In this study, we investigate the temporal variation of Dc and the Gutenberg-Richter b-value for seismicity in the western part of the NAFZ (including the northern Aegean sea) for earthquakes of Ms > 4.5 occurring in the period between 1900 and 1992. b ranges from 0.6-1.6 and Dc from 0.6 to 1.4. The b-value is found to be weakly negatively correlated with Dc (r=-0.56). However the (log of) event rate N is positively correlated with b, with a similar degree of statistical significance (r=0.42), and negatively correlated with Dc (r=-0.48). Since N increases dramatically with improved station coverage since 1970, the observed negative correlation between b and Dc is therefore more likely to be due to this effect than any underlying physical process in this case. We present this as an example of how man-made artefacts of recording can have similar statistical effects to underlying processes.

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Seismotectonics of the Ionian-Akarnania Block (IAB) and Western Greece deduced from a local seismic deployment.

10.5194/egusphere-egu2020-21285 ◽

2020 ◽

Author(s):

Antoine Haddad ◽

Athanassios Ganas ◽

Ioannis Kassaras ◽

Matteo Lupi

Keyword(s):

Fault Zone ◽

Velocity Model ◽

Focal Mechanisms ◽

Seismogenic Zone ◽

The South ◽

S Wave ◽

North West ◽

The North ◽

Short Period ◽

Western Greece

From July 2016 to May 2017, we deployed a local seismic network composed of 15 short-period seismic stations to investigate the ongoing seismotectonic deformation of Western Greece with emphasis on the region between Ambrakikos Gulf (to the north) and Kyparissia (to the south). The network was deployed to investigate the behavior of key crustal blocks in western Greece, such as the Ionian-Akarnania Block (IAB).After applying automatic P- and S- wave phase picking we located 1200 local earthquakes using HypoInverse and constrained five 1D velocity model by applying the error minimization technique. Events were relocated using HypoDD and 76&#160; focal mechanisms were computed for events with magnitudes down to ML 2.3 using first motion polarities.We combined the calculated focal mechanisms and the relocated seismicity to shed light on the IAB block boundaries. Three boundaries highlighted by previous studies were also evidenced :-The north-west margin of the block, the Cephalonia Transform Fault, Europe&#8216;s most active fault. NW-striking dextral strike-slip motion was recognized for this fault near the Gulf of Myrtos and the town of Fiskardo.- The south-east margin is the Movri-Amaliada right-lateral Fault Zone, activated during the Movri Mt. Mw 6.4 earthquake sequence.- The Ambrakikos Gulf (a young E-W rift) and the NW-striking left-lateral Katouna-Stamna Fault zone depict the north and north-eastern margins of the IAB block.Seismicity lineaments and focal mechanisms define theKyllini-Cephalonia left-lateral fault, which is also highlighted by bathymetry data. We interpret this fault as the south-western margin of IAB separating an aseismic area observed between Cephalonia and Akarnania from a seismogenic zone north of Zakynthos Island and bridging NW Peloponnese with Cephalonia.

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The 1991 Sierra Madre earthquake sequence in southern California: Seismological and tectonic analysis

Bulletin of the Seismological Society of America ◽

10.1785/bssa0840041058 ◽

1994 ◽

Vol 84 (4) ◽

pp. 1058-1074 ◽

Cited By ~ 2

Author(s):

Egill Hauksson

Keyword(s):

Los Angeles ◽

Fault Plane ◽

B Value ◽

Tectonic Stress ◽

Aftershock Sequence ◽

Strike Slip ◽

Focal Mechanisms ◽

Stress Axis ◽

The North ◽

Sierra Madre

Abstract The (ML 5.8) Sierra Madre earthquake of 28 June 1991 occurred at a depth of 12 km under the San Gabriel Mountains of the central Transverse Ranges. Since at least 1932 this region had been quiescent for M ≧ 3. The mainshock focal mechanism derived from first-motion polarities exhibited almost pure thrust faulting, with a rake of 82° on a plane striking N62°E and dipping 50° to the north. The event appears to have occurred on the Clamshell-Sawpit fault, a splay of the Sierra Madre fault zone. The aftershock sequence following the mainshock occurred at a depth of 9 to 14 km and was deficient in small earthquakes, having a b value of 0.6. Twenty nine single-event focal mechanisms were determined for aftershocks of M > 1.5. The 4-km-long segment of the Clamshell-Sawpit fault that may have ruptured in the mainshock is outlined by several thrust focal mechanisms with an east-northeast-striking fault plane dipping to the north. To the west, several thrust aftershocks with east-striking nodal planes suggest some complexity in the aftershock faulting, such as a curved rupture surface. In addition, several strike-slip and normal faulting events occurred along the edges of the mainshock fault plane, indicating secondary tear faulting. The tectonic stress field driving the coexisting left-lateral strike-slip and thrust faults in the northern Los Angeles basin is north-south horizontal compression with vertical intermediate or minimum principal stress axis.

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Research progress on the Zhongnan-Liyue Fault Zone in the South China Sea Basin

10.5194/egusphere-egu2020-11862 ◽

2020 ◽

Author(s):

Ziying Xu ◽

Jun Wang ◽

Hongfang Gao ◽

Yongjian Yao

Keyword(s):

Spatial Distribution ◽

South China Sea ◽

Fault Zone ◽

South China ◽

The South China Sea ◽

Magnetic Data ◽

Tectonic Deformation ◽

The South ◽

China Sea ◽

The North

We give a review of the up-to-date research situation about The Zhongnan-Liyue Fault Zone (ZLFZ), than analyze the spatial distribution and tectonic deformation feature of the ZLFZ based on the geophysical data including topographic, seismic, gravity and magnetic data. The results show that the ZLFZ has obvious north-south segmentation characteristics in in the South China Sea Basin. The north section, which is between northwest sub-basin and east sub-basin, is a narrow zone with the width of ~16 km, and is NNW trend from 18&#176;N,115.5&#176;E to 17.5&#176;N,116&#176;E. Meanwhile ,the south section, which is between southwest sub-basin and east sub-basin, is a wide zone with the width of 60-80 km, and is NNW trend from the east of ZhongshaBank to the west of LiyueBank. The main fault of the ZLFZ is NNW trend along the seamounts ridge of Zhongnan. the ZLFZ of transition region is NNE trend from the north section to the south section. According the sub-basin&#8217;s sedimentary thickness and oceanic crust thickness exist obvious difference, on both sides of the ZLFZ, we speculate that the ZLFZ play an important role on geological structure of sub-basin. According to the chang of crustal structure, We speculate that the ZLFZ is at least a crustal fracture zone.Key words: South China Sea Basin; Zhongnan-Liyue Fault Zone; Spatial distribution; Tectonic deformation&#160;Foundation item: National Natural Science Foundation of China (41606080, 41576068); The China Geological Survey Program (GZH201400202, 1212011220117, DD20160138, 1212011220116).

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Real-time discrimination of earthquake foreshocks and aftershocks

10.5194/egusphere-egu2020-7590 ◽

2020 ◽

Author(s):

Laura Gulia ◽

Stefan Wiemer

Keyword(s):

Real Time ◽

Large Earthquake ◽

B Value ◽

Aftershock Sequence ◽

Main Question ◽

Traffic Light ◽

Background Value ◽

The North ◽

The Hours ◽

Average Size

Immediately after a large earthquake, the main question asked by the public and decision-makers is whether it was the mainshock or a foreshock to an even stronger event yet to come. So far, scientists can only offer empirical evidence from statistical compilations of past sequences, arguing that normally the aftershock sequence will decay gradually whereas the occurrence of a forthcoming larger event has a probability of a few per cent.We analyse the average size distribution of aftershocks of the 2016 Amatrice&#8211;Norcia (Italy) and Kumamoto (Japan) earthquake sequences and we suggest that in many cases it may be possible to discriminate whether an ongoing sequence represents a decaying aftershock sequence or foreshocks to an upcoming large event.We propose a simple traffic light classification (FTLS, Foreshock Traffic Light System) to assess in real time the level of concern about a subsequent larger event and test it against 58 sequences, achieving a classification accuracy of 95 per cent.We finally test, in near-real-time, the performance of the FTLS to the 2019 Ridgecrest sequence, California: a Mw6.4 followed, about 2 days later, by a Mw7.1. We find that in the hours after the first Ridgecrest event (Mw 6.4, the b-value drops by 23% on average, when compared to the background value, resulting in a &#8216;red&#8217; foreshock traffic light.Mapping in space the changes in b, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. The second mainshock of magnitude 7.1 then indeed occurred in this location and after this event, the b-value increased by 26 percent over the background value, resulting in a green traffic light state.

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