The San Salvador earthquake of May 3, 1965

1966 ◽  
Vol 56 (2) ◽  
pp. 561-575
Author(s):  
Cinna Lomnitz ◽  
Rudolf Schultz
Keyword(s):  
Volcanic Activity ◽  
Main Shock ◽  
Tectonic Setting ◽  
Focal Depth ◽  
Building Codes ◽  
General Area ◽  
Populated Area ◽  
Shallow Subsurface ◽  
San Salvador ◽  
Zoning Regulations

abstract The San Salvador earthquake of May 3, 1965 was preceded by a local seismic swarm of three months duration. The main shock was destructive in a densely populated area of not more than 15 km in radius; the same general area was damaged in the earthquakes of 1576, 1659, 1798, 1839, 1854, 1873, 1880, 1917, and 1919. Over 120 casualties were reported. The epicenter has been located on the south rim of the Median Trough, a post-Pliocene structure which accounts for the high seismic and volcanic activity in the region. The observed intensity is attributable to shallow focal depth and to the presence of thick inhomogeneous beds of fluviatile pumice. The tectonic setting and shallow subsurface factors should be recognized in future building codes and zoning regulations.

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.

scholarly journals The Zagreb (Croatia) M5.5 Earthquake on 22 March 2020

Geosciences ◽  
2020 ◽  
Vol 10 (7) ◽  
pp. 252 ◽  
Author(s):  
Snježana Markušić ◽  
Davor Stanko ◽  
Tvrtko Korbar ◽  
Nikola Belić ◽  
Davorin Penava ◽  
...  
Keyword(s):  
Structural Engineering ◽  
Main Shock ◽  
Residential Buildings ◽  
Tectonic Setting ◽  
Time Lapse ◽  
Earthquake Sequence ◽  
Epicentral Area ◽  
Local Soil ◽  
Seismic Amplification ◽  
Zagreb Area

On 22 March 2020, Zagreb was struck by an M5.5 earthquake that had been expected for more than 100 years and revealed all the failures in the construction of residential buildings in the Croatian capital, especially those built in the first half of the 20th century. Because of that, extensive seismological, geological, geodetic and structural engineering surveys were conducted immediately after the main shock. This study provides descriptions of damage, specifying the building performances and their correlation with the local soil characteristics, i.e., seismic motion amplification. Co-seismic vertical ground displacement was estimated, and the most affected area is identified according to Sentinel-1 interferometric wide-swath data. Finally, preliminary 3D structural modeling of the earthquake sequence was performed, and two major faults were modeled using inverse distance weight (IDW) interpolation of the grouped hypocenters. The first-order assessment of seismic amplification (due to site conditions) in the Zagreb area for the M5.5 earthquake shows that ground motions of approximately 0.16–0.19 g were amplified at least twice. The observed co-seismic deformation (based on Sentinel-1A IW SLC images) implies an approximately 3 cm uplift of the epicentral area that covers approximately 20 km2. Based on the preliminary spatial and temporal analyses of the Zagreb 2020 earthquake sequence, the main shock and the first aftershocks evidently occurred in the subsurface of the Medvednica Mountains along a deep-seated southeast-dipping thrust fault, recognized as the primary (master) fault. The co-seismic rupture propagated along the thrust towards northwest during the first half-hour of the earthquake sequence, which can be clearly seen from the time-lapse visualization. The preliminary results strongly support one of the debated models of the active tectonic setting of the Medvednica Mountains and will contribute to a better assessment of the seismic hazard for the wider Zagreb area.

The Godavari Valley earthquake sequence of April 1969

1970 ◽  
Vol 60 (2) ◽  
pp. 601-615 ◽  
Author(s):  
Harsh K. Gupta ◽  
Indra Mohan ◽  
Hari Narain
Keyword(s):  
Main Shock ◽  
Focal Depth ◽  
Earthquake Sequence ◽  
Total Strain ◽  
Type I ◽  
Aftershock Area ◽  
Macroseismic Effects ◽  
Godavari Valley

abstract The Godavari Valley earthquake sequence of April 1969 has been studied in detail. The (Sg − Pg) and the (Pg − Pn) intervals have been used for estimating the extent of aftershock area and focal depth variations respectively. The main shock of magnitude 5.7 was followed by a number of aftershocks which are related by the function Log N = a + b M. The value of b is found to be −0.51. The main shock of April 13 accounted for 70 per cent of the total strain released. This sequence belongs to Type I of Mogi's classification. The macroseismic effects are also discussed briefly.

scholarly journals Volcanic activity and gas emissions along the South Sandwich Arc

2020 ◽  
Vol 83 (1) ◽  
Author(s):  
Emma J. Liu ◽  
Kieran Wood ◽  
Alessandro Aiuppa ◽  
Gaetano Giudice ◽  
Marcello Bitetto ◽  
...  
Keyword(s):  
Carbon Isotope ◽  
Volcanic Activity ◽  
Emission Rate ◽  
Tectonic Setting ◽  
Hydrothermal Systems ◽  
Molar Ratio ◽  
The South ◽  
Gas Emissions ◽  
Volcanic Arc

AbstractThe South Sandwich Volcanic Arc is one of the most remote and enigmatic arcs on Earth. Sporadic observations from rare cloud-free satellite images—and even rarer in situ reports—provide glimpses into a dynamic arc system characterised by persistent gas emissions and frequent eruptive activity. Our understanding of the state of volcanic activity along this arc is incomplete compared to arcs globally. To fill this gap, we present here detailed geological and volcanological observations made during an expedition to the South Sandwich Islands in January 2020. We report the first in situ measurements of gas chemistry, emission rate and carbon isotope composition from along the arc. We show that Mt. Michael on Saunders Island is a persistent source of gas emissions, releasing 145 ± 59 t day−1 SO2 in a plume characterised by a CO2/SO2 molar ratio of 1.8 ± 0.2. Combining this CO2/SO2 ratio with our independent SO2 emission rate measured near simultaneously, we derive a CO2 flux of 179 ± 76 t day−1. Outgassing from low temperature (90–100 °C) fumaroles is pervasive at the active centres of Candlemas and Bellingshausen, with measured gas compositions indicative of interaction between magmatic fluids and hydrothermal systems. Carbon isotope measurements of dilute plume and fumarole gases from along the arc indicate a magmatic δ13C of − 4.5 ± 2.0‰. Interpreted most simply, this result suggests a carbon source dominated by mantle-derived carbon. However, based on a carbon mass balance from sediment core ODP 701, we show that mixing between depleted upper mantle and a subduction component composed of sediment and altered crust is also permissible. We conclude that, although remote, the South Sandwich Volcanic Arc is an ideal tectonic setting in which to explore geochemical processes in a young, developing arc.

scholarly journals Clusty, the waveform-based network similarity clustering toolbox: concept and application to image complex faulting offshore Zakynthos (Greece)

2020 ◽  
Vol 224 (3) ◽  
pp. 2044-2059
Author(s):  
G M Petersen ◽  
P Niemz ◽  
S Cesca ◽  
V Mouslopoulou ◽  
G M Bocchini
Keyword(s):  
Main Shock ◽  
Moment Tensor ◽  
Tectonic Setting ◽  
Active Faults ◽  
Aftershock Sequence ◽  
Strike Slip ◽  
Large Magnitude ◽  
Data Set ◽  
Network Similarity ◽  
Waveform Similarity

SUMMARY Clusty is a new open source toolbox dedicated to earthquake clustering based on waveforms recorded across a network of seismic stations. Its main application is the study of active faults and the detection and characterization of faults and fault networks. By using a density-based clustering approach, earthquakes pertaining to a common fault can be recognized even over long fault segments, and the first-order geometry and extent of active faults can be inferred. Clusty implements multiple techniques to compute a waveform based network similarity from maximum cross-correlation coefficients at multiple stations. The clustering procedure is designed to be transparent and parameters can be easily tuned. It is supported by a number of analysis visualization tools which help to assess the homogeneity within each cluster and the differences among distinct clusters. The toolbox returns graphical representations of the results. A list of representative events and stacked waveforms facilitate further analyses like moment tensor inversion. Results obtained in various frequency bands can be combined to account for large magnitude ranges. Thanks to the simple configuration, the toolbox is easily adaptable to new data sets and to large magnitude ranges. To show the potential of our new toolbox, we apply Clusty to the aftershock sequence of the Mw 6.9 25 October 2018 Zakynthos (Greece) Earthquake. Thanks to the complex tectonic setting at the western termination of the Hellenic Subduction System where multiple faults and faulting styles operate simultaneously, the Zakynthos data set provides an ideal case-study for our clustering analysis toolbox. Our results support the activation of several faults and provide insight into the geometry of faults or fault segments. We identify two large thrust faulting clusters in the vicinity of the main shock and multiple strike-slip clusters to the east, west and south of these clusters. Despite its location within the largest thrust cluster, the main shock does not show a high waveform similarity to any of the clusters. This is consistent with the results of other studies suggesting a complex failure mechanism for the main shock. We propose the existence of conjugated strike-slip faults in the south of the study area. Our waveform similarity based clustering toolbox is able to reveal distinct event clusters which cannot be discriminated based on locations and/or timing only. Additionally, the clustering results allows distinction between fault and auxiliary planes of focal mechanisms and to associate them to known active faults.

A Subsurface Geologic Feature Inferred from Relocation of Local Earthquakes in Al-Refaei District, Southern Iraq

2020 ◽  
Vol 54 (1A) ◽  
pp. 35-43
Author(s):  
Ali Ramthan
Keyword(s):  
High Frequency ◽  
Tectonic Setting ◽  
Arabian Plate ◽  
Stress Regime ◽  
Shallow Subsurface ◽  
Linear Feature ◽  
Recording Station ◽  
Communications System ◽  
System Power ◽  
Southern Iraq

For the last two decades, the District of Al-Refaei, southern Iraq, experienced several small to moderate size earthquakes that seem to be clustered in a relatively small area. The initial locations of these earthquakes from the available local and international bulletins show scattered events without any clear structural patterns. A seven-elements seismic array was installed for the period 2014 to 2018 to monitor this activity. Each element of the array consists of three-component Geospace GS11d high-frequency (4.5 Hz) geophone, a solar system power source, and telemetry communications system to transmit data to a central recording station. During the period of monitoring, the array recorded more than 56 earthquakes having a range of magnitude between 1.5 ML to 4.7 mb. Large number of the low magnitude earthquakes were not detected by other monitoring agencies; however, they were recorded by the array. Fifty-six of the most clearly recorded earthquakes were screened for relocation and analysis. Out of the 56 screened earthquakes, 35 were detected by the array alone. The majority of the selected earthquakes having their initial locations within the array. This improves the relocation process and increase accuracy. The relocated earthquakes express a clear pattern of a linear feature which strikes in the northwest-southeast direction. The direction of the newly inferred pattern coincides with the general tectonic setting of Iraq as it is parallel to Zagros suture zone and the general stress regime of the Arabian Plate. The depth of the relocated earthquakes ranges from about 3.9km to 8.9km. This indicates that these earthquakes occur along a shallow subsurface fault that was not mapped before.

Surface effects and tectonic setting of the 13 December 1982 North Yemen earthquake

1987 ◽  
Vol 77 (6) ◽  
pp. 2018-2037
Author(s):  
George Plafker ◽  
Robert Agar ◽  
A. H. Asker ◽  
M. Hanif
Keyword(s):  
Main Shock ◽  
Arabian Peninsula ◽  
Tectonic Setting ◽  
Tectonic Activity ◽  
Active Volcanism ◽  
The North ◽  
Surface Displacements ◽  
Southwestern Margin ◽  
Surface Manifestation ◽  
Epicentral Region

Abstract The North Yemen earthquake (Mb = 6.0) of 13 December 1982 is the first earthquake in the southern Arabian Peninsula known to be accompanied by surface displacements. The extensive destruction and loss of lives resulted entirely from widespread collapse of unreinforced masonry and mud brick structures; maximum Modified Mercalli intensity was probably VII to VIII. The only surface manifestation of tectonic activity was the occurrence of earthquake-related extensional ground cracks in the epicentral region. The cracks occur mainly in four relatively continuous north- to northwest-trending linear zones that range from a few hundred meters to 15 km in length and in irregular areas of polygonal extension cracks. The area within which the cracks occur is 22.5 km long by about 10 km wide. Continued dilation across some cracks was measured almost 1 month after the main shock. Seismicity and active volcanism in this region are inferred to reflect slow extension of the southwestern margin of the Arabia plate perpendicular to the Red Sea spreading axis.

Lacustrine record of last millennia eruptions in Northern Chilean Patagonia (45–47°S)

The Holocene ◽  
2017 ◽  
Vol 27 (8) ◽  
pp. 1227-1251 ◽  
Author(s):  
Nathalie Fagel ◽  
Denisse Alvarez ◽  
Olivier Namur ◽  
Jean-Luc Devidal ◽  
Laurence Nuttin ◽  
...  
Keyword(s):  
Volcanic Activity ◽  
Tectonic Setting ◽  
Temporal Distribution ◽  
Visual Observation ◽  
Tephra Layer ◽  
Radiocarbon Dates ◽  
The Holocene ◽  
Chilean Patagonia ◽  
Major Chemical ◽  
Glass Shards

Due to its tectonic setting, the Andean Southern Volcanism Zone (SSVZ) is characterized by frequent volcanic activity. Chilean Patagonia lake sediments represent powerful archives of historical and past eruptions since the deglaciation. The lacustrine tephra record is investigated in 10 Holocene sedimentary cores collected in five lakes located along a 45–47° transect through Northern Chilean Patagonia. All the tephras identified by visual observation and strong magnetic susceptibility signal have been characterized for the major chemical composition of their glass shards by microprobe analyses, bulk mineralogical content by x-ray diffraction analyses and grain-size distribution by laser diffraction. Special care has been given to the chronostratigraphical framework in order to determine the age interval for each tephra layer and further to correlate the lacustrine records. The sedimentary age models are based on 210Pb data and calibrated radiocarbon dates measured on macroremains or reservoir effect-corrected bulk sediment. To present a more complete tephrochronological record, 28 microtephras have been confirmed by their mineralogical signature. Our lacustrine tephra record is compared with the Holocene eruptions registered in both surface deposits and continental, lacustrine and peat bog, environments. The different lacustrine eruption records are discussed according to their origin, age and location (distance from volcanoes, wind direction and dispersion of eruption produced). Our data confirm that Chilean Ande SSVZ tephras are mainly derived from historical and past Hudson eruptions. However, the peculiar low-K2O signature of the glass shards observed in one tephra layer from the Northernmost lake, Lake Thompson, confirms an influence from some other SSVZ volcanoes with low-abundance magma type, such as Maca and Cay. Our tephrochronological data compliment the database for volcanic activity in Chile bringing new information essential for the running discussion on the temporal distribution of eruptions over the Holocene.

The St-Donat, Quebec, earthquake sequence of February 18–23,1978

1979 ◽  
Vol 16 (9) ◽  
pp. 1892-1898 ◽  
Author(s):  
R. B. Horner ◽  
R. J. Wetmiller ◽  
H. S. Hasegawa
Keyword(s):  
New York ◽  
Main Shock ◽  
New York State ◽  
Focal Depth ◽  
Maximum Intensity ◽  
Systematic Change ◽  
Aftershock Activity ◽  
Active Volume ◽  
Significant Area ◽  
Recent Earthquakes

The St-Donat earthquake of February 18, 1978, occurred in a diffuse zone of significant seismicity in western Quebec. A field survey detected two aftershocks ([Formula: see text]) in a 3 day period following the magnitude mb(Lg) 4.1 main shock. These were followed by a magnitude mb(Lg) 3.4 earthquake on February 23, after the field stations were removed. Main shock and aftershock activity occurred within a small active volume located at 46 °19.2′N, 74°06.6′W, depth 7 km, with an estimated uncertainty of 1 km on all three hypocentral parameters. The main shock seismic moment was calculated to be 0.8 × 1022 dyne∙cm (8 × 1022 μN∙cm).The tremor was felt over an area of 70 000 km2 in western Quebec, eastern Ontario, and northern New York State with a maximum radius of perceptibility of 185 km and a maximum intensity of V in the St-Donat area. A comparison of the isoseismals of this and three other recent earthquakes of magnitude near 4 suggests a variation of maximum intensity inversely with focal depth; a significant area of intensity V is only observed when the focal depth is less than 10 km. There is also an indication of a systematic change in focal depths across the western Quebec zone, from shallow (upper crustal) depths north of Montreal to deeper (midcrustal) depths north of Ottawa.The P-nodal solution indicates almost pure thrust motion on a plane striking N20°W and dipping either 40°NE or 50°SW. The deviatoric compression axis is nearly horizontal, in a west-southwest direction. Similar results from the 1975 Maniwaki, Quebec, earthquake suggest that a uniform stress condition exists throughout western Quebec.

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