Source parameters of Montana earthquakes (1925-1964) and tectonic deformation in the northern Intermountain Seismic Belt

1989 ◽  
Vol 79 (1) ◽  
pp. 31-50
Author(s):  
Diane I. Doser
Keyword(s):  
Main Shock ◽  
Source Parameters ◽  
Normal Fault ◽  
Strike Slip ◽  
Tectonic Deformation ◽  
Seismic Belt ◽  
Fault Movement ◽  
Waveform Modeling ◽  
Virginia City ◽  
First Motion

Abstract Waveform modeling and first motion analysis are used to determine the source parameters of six 5.8 ≦ M ≦ 6.8 earthquakes that occurred between 1925 and 1964 within the northern Intermountain Seismic Belt of Montana. Results of this study suggest that the 1925 Clarkston earthquake occurred along an oblique normal fault with a trend similar to the southern end of the Clarkston Valley fault. The two largest earthquakes of the 1935 Helena sequence occurred along right-lateral strike-slip faults with trends similar to the Bald Butte and Helena Valley faults. The 1947 Virginia City earthquake occurred along a northwest-southeast trending segment of the Madison fault. Movement at depth was along a fault with strike similar to that of the 1959 Hebgen Lake main shock. A reanalysis of a M = 6.0 aftershock of the 1959 Hebgen Lake sequence suggests the earthquake occurred at a depth of 8 km along a fault that is not seen at the surface. An M = 5.8 earthquake in 1964, located about 10 km from the 1959 aftershock, may have occurred along steeply dipping fault planes (48° to 80°) at depths of 8 to 14 km. Most events could be modeled as simple ruptures.

The July 2020 Mw 6.3 Nima Earthquake, Central Tibet: A Shallow Normal-Faulting Event Rupturing in a Stepover Zone

2021 ◽  
Author(s):  
Jiuyuan Yang ◽  
Caijun Xu ◽  
Yangmao Wen ◽  
Guangyu Xu
Keyword(s):  
Normal Fault ◽  
Strike Slip ◽  
Deformation Analysis ◽  
Postseismic Deformation ◽  
Integrated Analysis ◽  
Tectonic Deformation ◽  
Normal Faulting ◽  
Coseismic Slip ◽  
Synthetic Aperture Radar Data ◽  
Central Tibet

Abstract On 22 July 2020, an Mw 6.3 earthquake with a predominantly normal-faulting mechanism struck the Yibug Caka fault zone, central Tibet, where the overall tectonic environment is characterized by left-lateral strike-slip motion. This event offers a chance to gain insight into the tectonic deformation and the cause of shallow normal-faulting earthquakes in this little studied region. Here, we use Sentinel-1A/B Interferometric Synthetic Aperture Radar data to investigate the coseismic and postseismic deformation related to this earthquake. The earthquake ruptured a previously mapped West Yibug Caka fault and is dominated by normal slip with a peak value of 1.9 m at depth of 6.9 km. Postseismic deformation analysis indicates that the observed subsidence signals of up to ∼4.7 cm are a consequence of afterslip. Most of the afterslip is confined at depths between 0.8 and 8.4 km, peaking at 0.27 m at depth of 6.1 km. The significant coseismic slip and afterslip involved in the earthquake highlights a complex interaction between the major normal fault and the secondary synthetic fault. By an integrated analysis of the inversions, regional geology geomorphology, fault kinematics, and seismicity background, we propose a tectonic model that attributes the occurrence of this normal-faulting event to the release of extensional stress in a stepover zone controlled by the northeast-striking sinistral strike-slip Riganpei Co fault and Bu Zang Ai fault. Compared with that the structural stepover often acts as a barrier to affect the propagation of earthquake rupture, our study demonstrates that the failure of a stepover may potentially induce the occurrence of earthquake along the bounding strike-slip faults.

Inversion of regional surface-wave spectra for source parameters of aftershocks from the Loma Prieta earthquake

1991 ◽  
Vol 81 (5) ◽  
pp. 1726-1736
Author(s):  
Susan L. Beck ◽  
Howard J. Patton
Keyword(s):  
Surface Wave ◽  
Surface Waves ◽  
Focal Mechanism ◽  
Source Parameters ◽  
Strike Slip ◽  
Focal Mechanisms ◽  
P Wave ◽  
Wave Spectra ◽  
Short Period ◽  
First Motion

Abstract Surface waves recorded at regional distances are used to study the source parameters for three of the larger aftershocks of the 18 October 1989, Loma Prieta, California, earthquake. The short-period P-wave first-motion focal mechanisms indicate a complex aftershock sequence with a wide variety of mechanisms. Many of these events are too small for teleseismic body-wave analysis; therefore, the regional surface-waves provide important long-period information on the source parameters. Intermediate-period Rayleigh- and Love-wave spectra are inverted for the seismic moment tensor elements at a fixed depth and repeated for different depths to find the source depth that gives the best fit to the observed spectra. For the aftershock on 19 October at 10:14:35 (md = 4.2), we find a strike-slip focal mechanism with right lateral motion on a NW-trending vertical fault consistent with the mapped trace of the local faults. For the aftershock on 18 October at 10:22:04 (md = 4.4), the surface waves indicate a pure reverse fault with the nodal planes striking WNW. For the aftershock on 19 October at 09:53:50 (md = 4.4), the surface waves indicate a strike-slip focal mechanism with a NW-trending vertical nodal plane consistent with the local strike of the San Andreas fault. Differences between the surface-wave focal mechanisms and the short-period P-wave first-motion mechanisms are observed for the aftershocks analyzed. This discrepancy may reflect the real variations due to differences in the band width of the two observations. However, the differences may also be due to (1) errors in the first-motion mechanism due to incorrect near-source velocity structure and (2) errors in the surface-wave mechanisms due to inadequate propagation path corrections.

scholarly journals Main shock and aftershocks of the December 13, 1990, Eastern Sicily earthquake

1995 ◽  
Vol 38 (2) ◽  
Author(s):  
A. Amato ◽  
R. Azzara ◽  
A. Basili ◽  
C. Chiarabba ◽  
M. Cocco ◽  
...  
Keyword(s):  
Main Shock ◽  
Fault Plane ◽  
Source Parameters ◽  
Source Region ◽  
Aftershock Sequence ◽  
Strike Slip ◽  
Local Network ◽  
Fault Plane Solution ◽  
Slip Mechanism ◽  
Plane Solution

n this paper we describe the location and the fault plane solution of the December 13, 1990, Eastern Sicily earthquake (ML = 5.4), and of its aftershock sequence. Because the main shock location is not well constrained due to the geometry of the permanent National Seismic Network in this area, we used a "master event" algorithm to locate it in relation to a well located aftershock. The revised location is slightly offshore Eastern Sicily, 4.8 km north of the largest aftershock (ML = 4.6) that occurred on December 16, 1990. The main shock has a strike-slip mechanism, indicating SE-NW compression with either left lateral motion on a NS plane, or right lateral on an EW plane. Two days after the main event we deployed a local network of eight digital stations, that provided accurate locations of the aftershocks, and the estimate of source parameters for the strongest earthquake. We observed an unusual quiescence after the ML = 5.4 event, that lasted until December 16, when a ML = 4.6 earthquake occurred. The fault plane solution of this aftershock shows normal faulting on E-W trending planes. Between December 16 and January 6, 1991, a sequence of at least 300 aftershock" was recorded by the local network. The well located earthquakes define a small source region of approximately 5 x 2 x 5 km3, with hypocentral depths ranging between 15 and 20 km. The paucity of large aftershocks, the time gap between the main shock occurrence and the beginning of the aftershock sequence (3.5 days), their different focal mechanisms (strike-slip vs. normal), and the different stress drop between main shock and after- shock suggest that the ML = 5.4 earthquake is an isolated event. The sequence of aftershocks began with the ML = 4.6 event, which is probably linked to the main shock with a complex mechanism of stress redistribution after the main faulting episode.

scholarly journals The July 27, 1976 Tangshan, China earthquake—A complex sequence of intraplate events

1979 ◽  
Vol 69 (1) ◽  
pp. 207-220 ◽  
Author(s):  
Rhett Butler ◽  
Gordon S. Stewart ◽  
Hiroo Kanamori
Keyword(s):  
Surface Waves ◽  
Seismic Moment ◽  
Main Shock ◽  
Normal Fault ◽  
Strike Slip ◽  
Body Waves ◽  
P Wave ◽  
Tangshan Earthquake ◽  
Total Moment ◽  
Slip Event

abstract The Tangshan earthquake (Ms = 7.7), of July 27, 1976 and its principal aftershock (Ms = 7.2), which occurred 15 hr following the main event, resulted in the loss of life of over 650,000 persons in northeast China. This is the second greatest earthquake disaster in recorded history, following the 1556 Shensi Province, Chinese earthquake in which at least 830,000 persons lost their lives. Detailed analyses of the teleseismic surface waves and body waves are made for the Tangshan event. The major conclusions are: (1) The Tangshan earthquake sequence is a complex one, including strike-slip, thrust, and normal-fault events. (2) The main shock, as determined from surface waves, occurred on a near vertical right-lateral strike-slip fault, striking N40°E. (3) A seismic moment of 1.8 × 1027 dyne-cm is obtained. From the extent of the aftershock zone and relative location of the main shock epicenter, symmetric (1:1) bilateral faulting with a total length of 140 km may be inferred. If a fault width of 15 km is assumed, the average offset is estimated to be 2.7 meters with an average stress drop of about 30 bars. (4) The main shock was initiated by an event with a relatively slow onset and a seismic moment of 4 × 1026 dyne-cm. The preferred fault-plane solution, determined from surface-wave analyses, indicates a strike 220°, dip 80°, and rake −175°. (5) Two thrust events follow the strike-slip event by 11 and 19 sec, respectively. They are located south to southwest of the initial event and have a total moment of 8 × 1025 dyne-cm. This sequence is followed by several more events. (6) The principal aftershock was a normal-fault double event with the fault planes unconstrained by the P-wave first motions. Surface waves provide additional constraints to the mechanism to yield an oblique slip solution with strike N120°E, dip 45°SW, and rake −30°. A total moment of 8 × 1026 dyne-cm is obtained. (7) The triggering of lesser thrust and normal faults by a large strike-slip event in the Tangshan sequence has important consequences in the assessment of earthquake hazard in other complex strike-slip systems like the San Andreas.

Aftershocks of the 1952 Kern County, California, earthquake sequence

1999 ◽  
Vol 89 (4) ◽  
pp. 1094-1108 ◽  
Author(s):  
Douglas Dreger ◽  
Brian Savage
Keyword(s):  
Sierra Nevada ◽  
Dynamic Range ◽  
Moment Tensor ◽  
Source Parameters ◽  
High Dynamic Range ◽  
Earthquake Sequence ◽  
Seismic Moment Tensor ◽  
Kern County ◽  
Waveform Modeling ◽  
First Motion

Abstract We have studied the seismograms recorded at the historic Berkeley (BRK) and Pasadena (PAS) stations for 20 aftershocks of the 21 July 1952 Kern County earthquake sequence. These events, in the magnitude range of MW 4.5 to 5.6, are too small to be studied teleseismically, yet they are important for better understanding the tectonics of the southern Sierra Nevada and the Tehachapi Mountains. On-scale recordings of moderate-sized events from this important earthquake sequence were first scanned, digitized, and then subjected to waveform modeling using a seismic moment tensor inverse procedure. In particular, the long-period, three-component Galitzen instrument at BRK and the 6-sec Wood-Anderson at PAS provided very high quality seismograms that could be analyzed in this manner. These two sites have been continuously operated from 1887 and 1927, respectively, and both are current sites of state-of-the-art broadband, high dynamic range instrumentation. First-motion polarities reported by Bath and Richter (1958) were used as additional constraints in the estimation of source parameters. There is considerable variability in the three-component seismograms of the 1952 aftershocks, which in turn result in a diversity of focal mechanisms. The majority of the solutions are northwest-striking reverse mechanisms that likely occurred on various mapped thrust faults in the hanging block of the mainshock. There are several events with northeast-striking, left-lateral mechanisms that are consistent with the strike of the White Wolf fault, as well as several normal slip events. The results of this study indicate that there are a variety of active fault structures adjacent to the White Wolf, Garlock and San Andreas faults in this region.

scholarly journals Mechanisms of the aftershocks of the Kern County, California, earthquake of 1952

1958 ◽  
Vol 48 (2) ◽  
pp. 133-146
Author(s):  
Markus Båth ◽  
Charles F. Richter
Keyword(s):  
Main Shock ◽  
Strike Slip ◽  
Kern County ◽  
The South ◽  
Longitudinal Waves ◽  
Left Hand ◽  
The North ◽  
Right Hand ◽  
First Motion ◽  
Strain Release

abstract Using directions of first motion of longitudinal waves recorded at near-by stations, the orientation of fault traces and the nature of fault motions have been deduced for fifty-seven earthquakes of the Kern County aftershock series. Unlike the main shock, the aftershocks exhibit considerable strike slip with left-hand strike slip dominating on and to the south of the White Wolf fault and right-hand strike slip and dip slip to the north of it. The two last-mentioned mechanisms represent a secondary strain release, beginning not earlier than thirty-seven hours after the main shock.

scholarly journals Source parameters estimation of the 1980 Bajhang Earthquake, far western Nepal

1997 ◽  
Vol 15 ◽  
Author(s):  
K. N. Khanal
Keyword(s):  
Source Parameters ◽  
Parameters Estimation ◽  
Depth Of Focus ◽  
Source Time Function ◽  
Waveform Modeling ◽  
Motion Data ◽  
Earthquake Source Parameters ◽  
First Motion ◽  
The Himalaya ◽  
Moderate Magnitude

The Bajhang earthquake of 1980 is one of the moderate magnitude earthquakes in the Himalaya that has been investigated by many researchers. The methods used by them for determining source parameters vary from the use of the first motion data to the waveform modeling. The ambiguity in focal mechanisms determined by different processes has been resolved by comparing the synthetic seismograms generated using wavenumber integration technique with those observed at the Global Digital Seismograph Networks (GDSN).The earthquake source parameters determined are found to be as follows: Dip=26°, Strike=290°, Rake=90°, Depth of focus=20±5 km and source time function=7 (2, 3, 2) seconds.

Granitoid complexes and the Archean tectonic record in the southern part of northwestern Ontario

1979 ◽  
Vol 16 (10) ◽  
pp. 1965-1977 ◽  
Author(s):  
W. M. Schwerdtner ◽  
D. Stone ◽  
K. Osadetz ◽  
J. Morgan ◽  
G. M. Stott
Keyword(s):  
Large Scale ◽  
Vertical Motion ◽  
Strike Slip ◽  
Horizontal Motion ◽  
Tectonic Deformation ◽  
Northwestern Ontario ◽  
Greenstone Belts ◽  
Second Period ◽  
Transcurrent Faults ◽  
Regional Compression

Two principal, possibly overlapping, periods of tectonic deformation can be distinguished in the Archean of northwestern Ontario, a period of dominantly vertical-motion tectonics and a period of dominantly horizontal-motion tectonics. Gigantic diapirs of foliated to gneissic tonalite–granodiorite developed during the first period and appear to be responsible for the gross structure of, and the major folds within, the metavolcanic–metasedimentary masses ("greenstone belts"). These diapirs are most likely due to mechanical remobilization of early tabular batholiths which originally intruded the oldest supracrustal rocks presently exposed. Later massive to foliated, dioritic to granitic plutons that vary from concordant, crescentic plutons to partly discordant plutons of various shapes and sizes were emplaced into the diapirs.The second period of tectonic deformation is characterized by large-scale dextral shearing and the development of major transcurrent faults under northwesterly regional compression. The strike-slip motions of this period outlasted the late plutonism, and led to the development of mylonitic zones which cut all Archean granitoid plutons.

Erosion rates of the New Zealand Southern Alps reflect long-term tectonics and transient climate

2021 ◽  
Author(s):  
Duna Roda-Boluda ◽  
Taylor Schildgen ◽  
Hella Wittmann-Oelze ◽  
Stefanie Tofelde ◽  
Aaron Bufe ◽  
...  
Keyword(s):  
New Zealand ◽  
Strike Slip ◽  
Southern Alps ◽  
Fault System ◽  
Tectonic Deformation ◽  
Seismic Cycle ◽  
Erosion Rates ◽  
Alpine Fault ◽  
Oblique Convergence

<p>The Southern Alps of New Zealand are the expression of the oblique convergence between the Pacific and Australian plates, which move at a relative velocity of nearly 40 mm/yr. This convergence is accommodated by the range-bounding Alpine Fault, with a strike-slip component of ~30-40 mm/yr, and a shortening component normal to the fault of ~8-10 mm/yr. While strike-slip rates seem to be fairly constant along the Alpine Fault, throw rates appear to vary considerably, and whether the locus of maximum exhumation is located near the fault, at the main drainage divide, or part-way between, is still debated. These uncertainties stem from very limited data characterizing vertical deformation rates along and across the Southern Alps. Thermochronology has constrained the Southern Alps exhumation history since the Miocene, but Quaternary exhumation is hard to resolve precisely due to the very high exhumation rates. Likewise, GPS surveys estimate a vertical uplift of ~5 mm/yr, but integrate only over ~10 yr timescales and are restricted to one transect across the range.</p><p>To obtain insights into the Quaternary distribution and rates of exhumation of the western Southern Alps, we use new <sup>10</sup>Be catchment-averaged erosion rates from 20 catchments along the western side of the range. Catchment-averaged erosion rates span an order of magnitude, between ~0.8 and >10 mm/yr, but we find that erosion rates of >10 mm/yr, a value often quoted in the literature as representative for the entire range, are very localized. Moreover, erosion rates decrease sharply north of the intersection with the Marlborough Fault System, suggesting substantial slip partitioning. These <sup>10</sup>Be catchment-averaged erosion rates integrate, on average, over the last ~300 yrs. Considering that the last earthquake on the Alpine Fault was in 1717, these rates are representative of inter-seismic erosion. Lake sedimentation rates and coseismic landslide modelling suggest that long-term (~10<sup>3</sup> yrs) erosion rates over a full seismic cycle could be ~40% greater than our inter-seismic erosion rates. If we assume steady state topography, such a scaling of our <sup>10</sup>Be erosion rate estimates can be used to estimate rock uplift rates in the Southern Alps. Finally, we find that erosion, and hence potentially exhumation, does not seem to be localized at a particular distance from the fault, as some tectonic and provenance studies have suggested. Instead, we find that superimposed on the primary tectonic control, there is an elevation/temperature control on erosion rates, which is probably transient and related to frost-cracking and glacial retreat.</p><p>Our results highlight the potential for <sup>10</sup>Be catchment-averaged erosion rates to provide insights into the magnitude and distribution of tectonic deformation rates, and the limitations that arise from transient erosion controls related to the seismic cycle and climate-modulated surface processes.</p><p> </p><p> </p>

Inversion of regional surface-wave spectra for source parameters of aftershocks from the 1992 Petrolia earthquake sequence

1995 ◽  
Vol 85 (3) ◽  
pp. 705-715
Author(s):  
Mark Andrew Tinker ◽  
Susan L. Beck
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Source Parameters ◽  
Strike Slip ◽  
Earthquake Sequence ◽  
Fault System ◽  
Accretionary Wedge ◽  
Wave Spectra ◽  
The North ◽  
Gorda Plate

Abstract Regional distance surface waves are used to study the source parameters for moderate-size aftershocks of the 25 April 1992 Petrolia earthquake sequence. The Cascadia subduction zone had been relatively seismically inactive until the onset of the mainshock (Ms = 7.1). This underthrusting event establishes that the southern end of the North America-Gorda plate boundary is seismogenic. It was followed by two separate and distinct large aftershocks (Ms = 6.6 for both) occurring at 07:41 and 11:41 on 26 April, as well as thousands of other small aftershocks. Many of the aftershocks following the second large aftershock had magnitudes in the range of 4.0 to 5.5. Using intermediate-period surface-wave spectra, we estimate focal mechanisms and depths for one foreshock and six of the larger aftershocks (Md = 4.0 to 5.5). These seven events can be separated into two groups based on temporal, spatial, and principal stress orientation characteristics. Within two days of the mainshock, four aftershocks (Md = 4 to 5) occurred within 4 hr of each other that were located offshore and along the Mendocino fault. These four aftershocks comprise one group. They are shallow, thrust events with northeast-trending P axes. We interpret these aftershocks to represent internal compression within the North American accretionary prism as a result of Gorda plate subduction. The other three events compose the second group. The shallow, strike-slip mechanism determined for the 8 March foreshock (Md = 5.3) may reflect the right-lateral strike-slip motion associated with the interaction between the northern terminus of the San Andreas fault system and the eastern terminus of the Mendocino fault. The 10 May aftershock (Md = 4.1), located on the coast and north of the Mendocino triple junction, has a thrust fault focal mechanism. This event is shallow and probably occurred within the accretionary wedge on an imbricate thrust. A normal fault focal mechanism is obtained for the 5 June aftershock (Md = 4.8), located offshore and just north of the Mendocino fault. This event exhibits a large component of normal motion, representing internal failure within a rebounding accretionary wedge. These two aftershocks and the foreshock have dissimilar locations in space and time, but they do share a north-northwest oriented P axis.

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