A fault model for the Nahanni earthquakes from aftershock studies

1990 ◽  
Vol 80 (6A) ◽  
pp. 1553-1570 ◽  
Author(s):  
R. B. Horner ◽  
R. J. Wetmiller ◽  
M. Lamontagne ◽  
M. Plouffe
Keyword(s):  
Stress Drop ◽  
Main Shock ◽  
Fault Zones ◽  
Aftershock Sequence ◽  
Aftershock Activity ◽  
Thrust Faulting ◽  
Rupture Plane ◽  
To Come ◽  
Activity Mechanism ◽  
Secondary Fault

Abstract Relative locations of 323 large aftershocks (M 3.0 or greater) in the period from 5 October 1985 to 25 March 1988 show that the Ms 6.6 event on 5 October 1985 initiated at 62.208°N, 124.217°W, about 2.5 km northeast of the Ms 6.9 main shock on 23 December 1985. The overall aftershock distribution suggests the October rupture was primarily a west-dipping, low-angle thrust. In subsequent aftershock activity, the main rupture plane was marked by a distinct quiescent area of about 200 km2 that persisted until the 23 December event. Most of the stress drop and slip occurred in this area. Following the 23 December rupture, a similar sized quiescent zone was also observed; however, it was only evident during the first 24 hr of the aftershock sequence, and the area was about 50 per cent too small to yield the overall stress drop. The additional area appeared to come from secondary rupture zones that developed coincident with the main shock rupture. Precise locations of 182 small (M 3.0 or less) aftershocks recorded during a third field survey from 12 to 21 September 1986 indicated at least one and probably three high-angle faults. Composite mechanism solutions showed thrust faulting except in a region directly south of the main shock rupture areas where there is a bend in one of the secondary fault zones and a concentration of aftershock activity. Mechanism solutions calculated for five of the largest aftershocks in the same region also indicated a similar variability. Development of secondary fault zones explained the increased complexity of the December event and may also provide an explanation for the vertical peak acceleration exceeding 2 g that was recorded about 10 sec after the December rupture initiated.

Preliminary Results From the November 25, 1988 Saguenay (QUEBEC) Earthquake

1989 ◽  
Vol 60 (3) ◽  
pp. 89-93 ◽  
Author(s):  
Robert G. North ◽  
Robert J. Wetmiller ◽  
John Adams ◽  
Frank M. Anglin ◽  
Henry S. Hasegawa ◽  
...  
Keyword(s):  
High Frequency ◽  
Main Shock ◽  
Eastern North America ◽  
Seismic Zoning ◽  
Aftershock Activity ◽  
Thrust Faulting ◽  
Largest Aftershock ◽  
Felt Area ◽  
Historic Seismicity ◽  
Field Network

Abstract The November 25, 1988 Saguenay, Québec earthquake (mb 5.9) was preceded by a foreshock 62 hours earlier (mb 4.4) and followed by over 50 aftershocks, of which only two have been larger than magnitude 3. The unusually large 29 km depth of the main shock is well determined using a field network deployed after the foreshock. Nearly all of the aftershock activity is shallower than the main shock, suggesting that the rupture propagated upwards. The event produced unexpectedly large amounts of high frequency energy, as evidenced by the value of mbLg (6.5) and by high accelerations observed at distances ranging from 40 to 800 km. Focal mechanisms for the foreshock, main shock and largest aftershock indicate thrust faulting. The felt area exceeds 3.5 million km2 and closely approaches that of the 1925 Charlevoix earthquake. The location of the Saguenay event in a region with little historic seismicity, its depth, and its large high-frequency generation, pose several problems for existing models of seismic zoning and hazard assessment for Eastern North America.

High-frequency spectral scaling of a main shock/ aftershock sequence near the Norwegian coast

1988 ◽  
Vol 78 (2) ◽  
pp. 561-570
Author(s):  
Eric P. Chael ◽  
Richard P. Kromer
Keyword(s):  
Stress Drop ◽  
High Frequency ◽  
Background Noise ◽  
Main Shock ◽  
Source Model ◽  
Aftershock Sequence ◽  
Constant Stress ◽  
P Wave ◽  
Western Coast ◽  
Seismic Signals

Abstract A high-frequency seismic element was recently added to the NORESS regional array in Norway. This system can monitor seismic signals at frequencies up to 50 Hz. In February 1986, the high-frequency seismic element recorded an mbLg 4.7 main shock and several aftershocks which occurred 420 km northwest of NORESS, off Norway's western coast. These events produced high-frequency signals which were well above the background noise at the station. P-wave spectra of these events scale in a manner consistent with the ω-square, constant-stress-drop source model. The data do not require any change in this scaling to magnitudes (mbLg) below 2, in contrast to previous reports that constant-stress-drop scaling breaks down at smaller magnitudes.

scholarly journals Characteristics of seismic activity before and after the 2018 M6.7 Hokkaido Eastern Iburi earthquake

2019 ◽  
Vol 71 (1) ◽  
Author(s):  
Takao Kumazawa ◽  
Yosihiko Ogata ◽  
Hiroshi Tsuruoka
Keyword(s):  
Seismic Activity ◽  
Main Shock ◽  
Aftershock Sequence ◽  
Aftershock Activity ◽  
Background Rate ◽  
Etas Model ◽  
Epidemic Type Aftershock Sequence ◽  
Aftershock Sequences ◽  
Before And After ◽  
Largest Aftershock

AbstractWe applied the epidemic type aftershock sequence (ETAS) model, the two-stage ETAS model and the non-stationary ETAS model to investigate the detailed features of the series of earthquake occurrences before and after the M6.7 Hokkaido Eastern Iburi earthquake on 6 September 2018, based on earthquake data from October 1997. First, after the 2003 M8.0 Tokachi-Oki earthquake, seismic activity in the Eastern Iburi region reduced relative to the ETAS model. During this period, the depth ranges of the seismicity were migrating towards shallow depths, where a swarm cluster, including a M5.1 earthquake, finally occurred in the deepest part of the range. This swarm activity was well described by the non-stationary ETAS model until the M6.7 main shock. The aftershocks of the M6.7 earthquake obeyed the ETAS model until the M5.8 largest aftershock, except for a period of several days when small, swarm-like activity was found at the southern end of the aftershock region. However, when we focus on the medium and larger aftershocks, we observed quiescence relative to the ETAS model from 8.6 days after the main shock until the M5.8 largest aftershock. For micro-earthquakes, we further studied the separated aftershock sequences in the naturally divided aftershock volumes. We found that the temporal changes in the background rate and triggering coefficient (aftershock productivity) in respective sub-volumes were in contrast with each other. In particular, relative quiescence was seen in the northern deep zones that includes the M5.8 largest aftershock. Furthermore, changes in the b-values of the whole aftershock activity showed an increasing trend with respect to the logarithm of elapsed time during the entire aftershock period, which is ultimately explained by the spatially different characteristics of the aftershocks.

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

1987 ◽  
Vol 3 (3) ◽  
pp. 419-434 ◽  
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.

Rampart seismic zone of central Alaska

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.

Aftershock sequence and focal parameters of the February 28, 1969 earthquake of the Azores-Gibraltar fracture zone

1972 ◽  
Vol 62 (3) ◽  
pp. 699-719 ◽  
Author(s):  
A. López Arroyo ◽  
A. Udías
Keyword(s):  
Canary Islands ◽  
Fracture Zone ◽  
Main Shock ◽  
Fault Plane ◽  
Aftershock Sequence ◽  
Strike Slip ◽  
Strait Of Gibraltar ◽  
Wide Region ◽  
Focal Parameters ◽  
Source Mechanism Solution

Abstract The earthquake of February 28, 1969, which occurred about 500 km west of the Strait of Gibraltar, was felt over the entire Iberian Peninsula, in a wide region of Morocco, and south to the Canary Islands. It had a long sequence of aftershocks continuing for at least 10 months, but, nevertheless, most of the energy seems to have been liberated in the main shock of which the mb was 7.4. The source mechanism solution indicates a fault plane striking N 67°W and dipping 68°SW, with motion principally of the strike-slip type. There also is some overthrusting. The horizontal extent of faulting is of the order of 90 km.

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

1980 ◽  
Vol 70 (2) ◽  
pp. 559-570 ◽  
Author(s):  
R. A. Uhrhammer
Keyword(s):  
San Francisco ◽  
Fault Zone ◽  
Strong Earthquake ◽  
Main Shock ◽  
B Value ◽  
Aftershock Sequence ◽  
The South ◽  
The North ◽  
Temporal And Spatial Characteristics ◽  
Largest Aftershock

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.

scholarly journals Statistical physics models for aftershocks and induced seismicity

2018 ◽  
Vol 377 (2136) ◽  
pp. 20170397 ◽  
Author(s):  
Molly Luginbuhl ◽  
John B. Rundle ◽  
Donald L. Turcotte
Keyword(s):  
Induced Seismicity ◽  
Statistical Physics ◽  
Aftershock Sequence ◽  
Induced Earthquakes ◽  
Aftershock Activity ◽  
Gas Field ◽  
Natural Time ◽  
Parkfield Earthquake ◽  
Groningen Gas Field

A standard approach to quantifying the seismic hazard is the relative intensity (RI) method. It is assumed that the rate of seismicity is constant in time and the rate of occurrence of small earthquakes is extrapolated to large earthquakes using Gutenberg–Richter scaling. We introduce nowcasting to extend RI forecasting to time-dependent seismicity, for example, during an aftershock sequence. Nowcasting uses ‘natural time’; in seismicity natural time is the event count of small earthquakes. The event count for small earthquakes is extrapolated to larger earthquakes using Gutenberg–Richter scaling. We first review the concepts of natural time and nowcasting and then illustrate seismic nowcasting with three examples. We first consider the aftershock sequence of the 2004 Parkfield earthquake on the San Andreas fault in California. Some earthquakes have higher rates of aftershock activity than other earthquakes of the same magnitude. Our approach allows the determination of the rate in real time during the aftershock sequence. We also consider two examples of induced earthquakes. Large injections of waste water from petroleum extraction have generated high rates of induced seismicity in Oklahoma. The extraction of natural gas from the Groningen gas field in The Netherlands has also generated very damaging earthquakes. In order to reduce the seismic activity, rates of injection and withdrawal have been reduced in these two cases. We show how nowcasting can be used to assess the success of these efforts. This article is part of the theme issue ‘Statistical physics of fracture and earthquakes’.

scholarly journals Application of the Environmental Seismic Intensity scale (ESI 2007) and the European Macroseismic Scale (EMS-98) to the Kalamata (SW Peloponnese, Greece) earthquake (Ms=6.2, September 13, 1986) and correlation with neotectonic structures and active faults

2014 ◽  
Vol 56 (6) ◽  
Author(s):  
Ioannis G. Fountoulis ◽  
Spyridon D. Mavroulis
Keyword(s):  
Structural Damage ◽  
Active Fault ◽  
Main Shock ◽  
Active Faults ◽  
Seismic Hazard Assessment ◽  
Seismic Intensity ◽  
Fault Zones ◽  
Earthquake Sequence ◽  
Ground Subsidence ◽  
Earthquake Scenario

On September 13, 1986, a shallow earthquake (Ms=6.2) struck the city of Kalamata and the surrounding areas (SW Peloponnese, Greece) resulting in 20 fatalities, over 300 injuries, extensive structural damage and many earthquake environmental effects (EEE). The main shock was followed by several aftershocks, the strongest of which occurred two days later (Ms=5.4). The EEE induced by the 1986 Kalamata earthquake sequence include ground subsidence, seismic faults, seismic fractures, rockfalls and hydrological anomalies. The maximum ESI 2007 intensity for the main shock has been evaluated as IX<sub>ESI 2007</sub>, strongly related to the active fault zones and the reactivated faults observed in the area as well as to the intense morphology of the activated Dimiova-Perivolakia graben, which is a 2nd order neotectonic structure located in the SE margin of the Kalamata-Kyparissia mega-graben and bounded by active fault zones. The major structural damage of the main shock was selective and limited to villages founded on the activated Dimiova-Perivolakia graben (IX<sub>EMS-98</sub>) and to the Kalamata city (IX<sub>EMS-98</sub>) and its eastern suburbs (IX<sub>EMS-98</sub>) located at the crossing of the prolongation of two major active fault zones of the affected area. On the contrary, damage of this size was not observed in the surrounding neotectonic structures, which were not activated during this earthquake sequence. It is concluded that both intensity scales fit in with the neotectonic regime of the area. The ESI 2007 scale complemented the EMS-98 seismic intensities and provided a completed picture of the strength and the effects of the September 13, 1986, Kalamata earthquake on the natural and the manmade environment. Moreover, it contributed to a better picture of the earthquake scenario and represents a useful and reliable tool for seismic hazard assessment.

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