Source parameters of aftershocks of the 1991 Costa Rica and 1992 Cape Mendocino, California, earthquakes from inversion of local amplitude ratios and broadband waveforms

1995 ◽  
Vol 85 (6) ◽  
pp. 1560-1575 ◽  
Author(s):  
Susan Y. Schwartz
Keyword(s):  
Costa Rica ◽  
Focal Mechanism ◽  
Crustal Structure ◽  
Source Parameters ◽  
Synthetic Seismograms ◽  
Earthquake Location ◽  
Grid Search ◽  
Amplitude Ratios ◽  
The North ◽  
Gorda Plate

Abstract Source parameters of aftershocks of the 22 April 1991 (MW = 7.7) Costa Rica and the 25 April 1992 (MW = 7.1) Cape Mendocino, California, earthquakes are determined using a grid search inversion of P, SH, and SV amplitude ratios recorded by sparse local networks of three-component broadband and short-period stations. The inversion procedure consists of computing synthetic seismograms for three fundamental fault orientations for all source-receiver pairs over a range of source depths; calculating the complex envelopes of the observed and synthetic seismograms to determine peak amplitudes of P, SH, and SV waves; combining the fundamental fault amplitudes for all possible values of strike, dip, and rake, at 10° increments; and determining the best fault orientation and depth as the one that yields the smallest misfit between observed and synthetic P/SH, P/SV, and SV/SH amplitude ratios. The ambiguity in the sense of motion on the nodal planes, arising due to the use of amplitude ratios, is resolved by examining P-wave polarities. The sensitivity of source parameters to uncertainties in earthquake location and crustal structure is explored. For events with good station coverage, focal mechanism determinations are stable for a wide range of assumed values of crustal structure, earthquake location, and depth. Source parameters for many of the largest events (M > 3.4) are also determined by inversion of broadband displacement waveforms using a similar grid-search technique. Comparable results were obtained using both broadband waveforms and amplitude ratios. Focal mechanism solutions for 20 aftershocks of the Costa Rica earthquake reveal a complicated faulting geometry, indicating active thrust, normal, and strike-slip faults in the back-arc of Costa Rica. The 1992 Cape Mendocino earthquake occurred at the intersection of the North American, Gorda, and Pacific plates. While the mainshock was associated with underthrusting of the Gorda plate beneath the North American plate, fault plane solutions for 70% of the 38 largest aftershocks indicate that these events result from either motion between the Gorda and Pacific plates or from internal deformation within the Gorda plate.

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.

scholarly journals Fault source investigation of the 6 December 2016 MwMw 6.5 Pidie Jaya, Indonesia, earthquake based on GPS and its implications of the geological survey result

2020 ◽  
Vol 14 (4) ◽  
pp. 405-412
Author(s):  
Endra Gunawan ◽  
Takuya Nishimura ◽  
Susilo Susilo ◽  
Sri Widiyantoro ◽  
Nanang T. Puspito ◽  
...  
Keyword(s):  
Surface Deformation ◽  
Source Parameters ◽  
Secondary Effects ◽  
Near Surface ◽  
Morphological Attributes ◽  
Ground Shaking ◽  
The North ◽  
Coulomb Failure ◽  
North And South ◽  
Fault Source

AbstractOn 6 December 2016 at 22:03 UTC, a devastating magnitude 6-class strike-slip earthquake occurred along an unidentified and unmapped fault in Pidie Jaya, northern Sumatra. We analysed the possible fault using continuous Global Positioning System (GPS) observation available in the region. In our investigation, we searched for the fault source parameters of the north- and south-dipping left-lateral faults and the west- and east-dipping right-lateral faults. We identified that the fault responsible for the earthquake was located offshore, with a southwest-northeast direction. We also computed the Coulomb failure stress and compared the result with the distribution of the aftershocks. In this study, we demonstrated that the result of the geological field survey conducted soon after the mainshock was attributed to the secondary effects of ground shaking and near-surface deformation, and not surface faulting. The newly identified offshore fault proposed by this study calls for further investigation of the corresponding submarine morphological attributes in this particular region.

Detailed crustal structure of the North China and its implication for seismicity

2014 ◽  
Vol 81 ◽  
pp. 53-64 ◽  
Author(s):  
Wenliang Jiang ◽  
Xin Wang ◽  
Tian Tian ◽  
Jingfa Zhang ◽  
Donglei Wang
Keyword(s):  
Crustal Structure ◽  
North China ◽  
The North

Upper-plate structural controls on the segmentation of the Kefalonia Fault (Ionian Sea, Greece)

2021 ◽  
Author(s):  
Emmanuel Skourtsos ◽  
Haralambos Kranis ◽  
Spyridon Mavroulis ◽  
Efthimios Lekkas
Keyword(s):  
Eastern Mediterranean ◽  
Source Parameters ◽  
Plate Boundary ◽  
Temporal Distribution ◽  
Western Coast ◽  
African Plate ◽  
Plate Structure ◽  
The North ◽  
Macroseismic Effects ◽  
Spatio Temporal

<p>The NNE-SSW, right-lateral Kefalonia Transform Fault (KTF) marks the western termination of the subducting Hellenic slab, which is a part of the oceanic remnant of the African plate. The inception of the KTF, described as a STEP fault, is placed in the Pliocene. KTF is considered to be the most active earthquake source in the Eastern Mediterranean. During the last two decades, four significant earthquakes (M>6.0) have been associated with the KTF. These events are attributed to the reactivation of different segments of the KTF, which are (from North to South) the North Lefkada, South Lefkada, Fiskardo, Paliki and Zakynthos segments: the North Lefkada segment ruptured in the 2003 earthquake, the 2014 Kefalonia events are associated with the Paliki segment and the 2015 Lefkada earthquake with the South Lefkada (and possibly the Fiskardo) segments.</p><p>The upper plate structure in the islands of Lefkada and Kefalonia is characterized by the Ionian Unit, thrusted over the Paxi (or Pre-Apulian) Unit. The Ionian Thrust, which brings the Ionian over the Paxi Unit, is a main upper-plate NNW-SSE, NE-dipping structure. It runs through the island of Lefkada, to be mapped onshore again at the western coast of Ithaki and at SE Kefalonia. Two other major thrusts are mapped on this island: the Aenos thrust, which has a WNW-ESE strike at the southern part of the island and gradually curves towards NNW-SSE in the west and the Kalo Fault in the northern part. These Pliocene (and still active) structures developed during the late-most stages of thrusting in the Hellenides, strike obliquely to the KTF and appear to abut against it.</p><p>We suggest that these thrusts control not only the deformation within the upper plate, but also the earthquake segmentation of the KTF. This suggestion is corroborated by the spatio-temporal distribution and source parameters of the recent, well-documented earthquake events and by the macroseismic effects of these earthquakes. The abutment of the Ionian thrust against the KTF marks the southern termination of the Lefkada earthquake segment, which ruptured in the 2003 earthquake, while the Aenos, (or the Kalo) thrust mark the southern end of the Fiskardo segment. The spatial distribution of the Earthquake Environmental Effects related to the four significant events in the last 20 years displays a good correlation with our interpretation: most of the 2003 macroseismic effects are located in the northern part of Lefkada, which belongs to the upper block of the Ionian thrust; similarly, the effects of the 2014 earthquakes of Kefalonia are distributed mainly in the Paliki Peninsula and the southern part of the island that belong to the footwall of the Aenos thrust and the 2015 effects are found in SW Lefkada, which is part of the footwall of the Ionian thrust.</p><p>We suggest that correlation between upper-plate structure and plate boundary faulting can provide insights in the understanding of faulting pattern in convergent settings, therefore contributing to earthquake management plans.</p>

scholarly journals Characteristics of local earthquake seismograms of varying dislocation sources in a stratified upper crust and modeling for P and S velocity structure: comparison with observations in the Koyna-Warna region, India

2016 ◽  
Vol 58 (6) ◽  
Author(s):  
V. G. Krishna
Keyword(s):  
Velocity Structure ◽  
Source Parameters ◽  
Velocity Model ◽  
Strike Slip ◽  
Synthetic Seismograms ◽  
Structure Comparison ◽  
Earthquake Sources ◽  
Crustal Velocity ◽  
Source Mechanisms ◽  
Local Earthquake

<p>Vertical component record sections of local earthquake seismograms from a state-of-the-art Koyna-Warna digital seismograph network are assembled in the reduced time versus epicentral distance frame, similar to those obtained in seismic refraction profiling. The record sections obtained for an average source depth display the processed seismograms from nearly equal source depths with similar source mechanisms and recorded in a narrow azimuth range, illuminating the upper crustal P and S velocity structure in the region. Further, the seismogram characteristics of the local earthquake sources are found to vary significantly for different source mechanisms and the amplitude variations exceed those due to velocity model stratification. In the present study a large number of reflectivity synthetic seismograms are obtained in near offset ranges for a stratified upper crustal model having sharp discontinuities with 7%-10% velocity contrasts. The synthetics are obtained for different source regimes (e.g., strike-slip, normal, reverse) and different sets of source parameters (strike, dip, and rake) within each regime. Seismogram sections with dominantly strike-slip mechanism are found to be clearly favorable in revealing the velocity stratification for both P and S waves. In contrast the seismogram sections for earthquakes of other source mechanisms seem to display the upper crustal P phases poorly with low amplitudes even in presence of sharp discontinuities of high velocity contrasts. The observed seismogram sections illustrated here for the earthquake sources with strike-slip and normal mechanisms from the Koyna-Warna seismic region substantiate these findings. Travel times and reflectivity synthetic seismograms are used for 1-D modeling of the observed virtual source local earthquake seismogram sections and inferring the upper crustal velocity structure in the Koyna-Warna region. Significantly, the inferred upper crustal velocity model in the region reproduces the synthetic seismograms comparable to the observed sections for earthquake sources with differing mechanisms in the Koyna and Warna regions.</p>

scholarly journals The use of parallel computing for the high-resolution determination of earthquake source parameters

2019 ◽  
pp. 68-75
Author(s):  
A. S. Fomochkina ◽  
V. G. Bukchin
Keyword(s):  
Parallel Computing ◽  
Focal Mechanism ◽  
Source Parameters ◽  
Nodal Plane ◽  
Earthquake Parameters ◽  
Earthquake Source Parameters ◽  
Detailed Search ◽  
High Degree ◽  
Special Case

Alongside the determination of the focal mechanism and source depth of an earthquake by direct examination of their probable values on a grid in the parameter space, also the resolution of these determinations can be estimated. However, this approach requires considerable time in the case of a detailed search. A special case of a shallow earthquake whose one nodal plane is subhorizontal is an example of the sources that require the use of a detailed grid. For studying these events based on the records of the long-period surface waves, the grids with high degree of detail in the angles of the focal mechanism are required. We discuss the application of the methods of parallel computing for speeding up the calculations of earthquake parameters and present the results of studying the strongest aftershock of the Tohoku, Japan, earthquake by this approach.

Secondary faulting near the terminus of a seismogenic strike-slip fault: Aftershocks of the 1976 Guatemala earthquake

1979 ◽  
Vol 69 (2) ◽  
pp. 427-444
Author(s):  
C. J. Langer ◽  
G. A. Bollinger
Keyword(s):  
Focal Mechanism ◽  
Linear Trend ◽  
Strike Slip ◽  
Strike Slip Fault ◽  
Aftershock Activity ◽  
Focal Mechanism Solutions ◽  
The North ◽  
Composite Focal Mechanism ◽  
Theoretical Pattern ◽  
Main Fault

abstract Aftershocks of the February 4, 1976 Guatemalan earthquake (Ms = 7.5) were monitored by a network of portable seismographs from February 9 to February 27. Although seismic data were obtained all along the 230-km surface rupture of the causal Motagua fault, the field program was designed to concentrate on the aftershock activity at the western terminus of the fault. Data from that locale revealed several linear or near-linear trends of aftershock epicenters that splay to the southwest away from the western end of the main fault. These trends correlate spatially with mapped surface lineaments and, to some degree, with ground breakage patterns near Guatemala City. The observed splay pattern of aftershocks and the normal-faulting mode of the splay earthquakes determined from composite focal mechanism solutions, may be explained by a theoretical pattern of stress trajectories at the terminus of a strike-slip fault. Composite focal mechanism solutions for aftershocks located on or near the surface break of the Motagua fault, to the north and east of the linear trend splay area, agree with the mapped surface movements, i.e., left-lateral strike-slip.

Source parameters of the 1949 magnitude 7.1 south Puget Sound, Washington, earthquake as determined from long-period body waves and strong ground motions

1987 ◽  
Vol 77 (5) ◽  
pp. 1530-1557
Author(s):  
Glenn Eli Baker ◽  
Charles A. Langston
Keyword(s):  
Focal Mechanism ◽  
Source Parameters ◽  
Puget Sound ◽  
Vertical Component ◽  
Test Laboratory ◽  
Body Waves ◽  
Polarization Angle ◽  
S Wave ◽  
Fault Surface ◽  
Strong Ground

Abstract Teleseismic P, SH, and SV first motions and SH to SV amplitude ratios recorded at eight teleseismic receivers from the 1949 magnitude 7.1 Olympia, Washington, earthquake in combination with data from three stations at regional distances were utilized in a grid testing routine to constrain focal mechanism. Identification of the pP phase places the event at 54 km depth. Distinct pulses, assumed to be source effects, are observed in the far-field waveforms. Analysis of these pulses for directivity made possible discrimination between the fault and auxiliary planes. The plane taken to represent the fault surface strikes east-west ± 15°, dips 45° ± 15° to the north, and has nearly pure left-lateral slip. The preferred source model has an eastward propagation of 40 km. Surface reflections of successive source pulses suggest an upward component of propagation of 5 km. Bounds on the earthquake location and rupture of the 13 April event were determined using depth and source mechanism constraints from the teleseismic study and characteristics of local strong ground motion recordings. The 9-sec S-instrument trigger time seen in the Seattle acceleration recordings places the event at least 60 km from Seattle. Strong motion velocity at the Olympia Highway Test Laboratory is characterized by an impulsive and rectilinear S wave. The low amplitude of the vertical component of initial S motion suggests that either the epicenter is within 5 km of the Olympia Highway Test Laboratory for a pure incident SV wave or located along an azimuth of N159° if the wave is SH. The combined constraint of minimum distance from Seattle and the S polarization angle implied by the teleseismic data focal mechanism places the initiation of rupture 5 to 10 km north to north-northwest of the Olympia Highway Test Laboratory at 47.13°N, 122.95°W. This is approximately 20 km west of previously determined epicenters. The T axis, gently dipping to the southeast, supports other evidence that the Juan de Fuca plate dips to the southeast in a zone between segments of the plate north and south of the event's location. The fault plane's slip is taken to indicate that subduction is still active beneath Washington and that motion of the two segments is probably independent.

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