Fault Plane Solutions Of Events Research Articles

Seismicity of the 2020 ML3.1 Haenam earthquake and its sequence in South Korea

Recently an earthquake sequence occurred in Haenam, southwestern part of the Korean Peninsula. Earthquake swarms are not unusual in the western region of Korea, which already experienced comparable series of earthquakes before the Haenam earthquake: in the Boryeong offshore (2013) and Baekryeong Island (2013 and 2019) regions. The ML 3.1 earthquake occurred 21km WNW of Haenam on May 3, 2020 and the earthquake sequence occurred from April 26 to May 9 intensively. For two weeks, a dozen small foreshocks preceded the mainshock and more aftershocks occurred. In this paper, relocation of events with focal mechanisms based on the seismicity of the 2020 Haenam earthquake sequence is proposed. The distribution of earthquake sequence in Haenam area is WNW-ESE direction, which is approximately parallel to the fault plane solutions of events with ML ≥ 2.0 indicating strike-slip faults. The sequential events concentrate within the elliptical area of about 0.18 km2 on the surface and the focal depth distributions show some variations between 19.6 km and 21.6 km for different velocity models but their ranges are within about 500–600 m. The depth variations for different models can be due to the different boundary depths between upper and lower crusts. Based on a comparison of the earthquake swarms in the western region of the Korean Peninsula, this study provides seismological information for understanding the seismotectonic structures in the Korean Peninsula.

RECENT SEISMIC ACTIVITY IN CENTRAL GREECE REVEALING LOCAL SEISMOTECTONIC PROPERTIES

The December 2008, M=5.2 earthquake occurred in the Voiotikos–Kifissos basin near the town of Amfikleia in Central Greece and was followed by an intense sequence with hundreds of earthquakes. Mainshock source characteristics derived from the recordings of the Greek National Seismological Network are consistent with previous known earthquakes as well as with the current nearly N–S extensional regime. The adequate azimuthal coverage and the calculated time residuals at each seismological station ensure high location accuracy, whereas the stations operated close to the seismic excitations constrained 80% of the focal depths between 8 and 12km. Distances from the mainshock epicenter to the 10 closest seismological stations vary from 15 to 75 km. Hypoinverse and HypoDD were used for locations, and FPFIT was used for fault plane solutions of events with an adequate number of clear first arrivals. The hypocenters and focal mechanisms illuminate a ≈10km–long fault zone striking nearly E–W with oblique normal faulting and a small left lateral component. The Voiotikos–Kifissos basin is bordered in the south by two left–stepping en echelon segments known as the Pavliani fault zone and the Parnassos detachment, which strike NW and dip NE. In our preferred interpretation, the Amfikleia mainshock ruptured a previously recognized south–dipping fault antithetic to the basin border faults. This fault may be associated with the left step on the border fault, which would be releasing if that fault had a sinistral component.

Seismic Anisotropy Observations in the Mexicali Valley, Baja California, M�xico

A study of seismic anisotropy was performed using data from earthquakes of the Mexicali Valley. The investigated region encompasses the Cerro Prieto Geothermal Field (CPGF), one of the most important fields in the world. The results showed that at most of our stations the average polarization directions of the fast S waves range from N14°W to N17°E. A N-S polarization direction was obtained for the whole area by averaging the polarization directions from all stations used. In terms of the EDA hypothesis, this average trend agrees with the postulated state of stress for southern California, and with fault plane solutions for events of the Mexicali Valley. Notorious deviations from the N-S global trend were found southeast of the CPGF, with polarization trends between N25°E and N67°E, and in the geothermal field, with polarization directions between N7°W and N14°W. The polarization results for these zones indicated stress conditions that are different from the more regional stress pattern. The delay times that were measured between the fast and slow shear waves reached values of up to 0.6 sec, with a mean value of 0.35 sec. Consistent with our polarization results, the larger delay times (0.2–0.6 sec) were found in the CPGF. Smaller or null values were observed at the periphery of the study area. No temporal trends in the delay times were apparent, as shown by data from the two stations that recorded the larger number of events. Overall, we conclude that the splitting effects of this study result from a shallow anisotropy volume. The splitting results are thus interpreted as caused by the preferred orientation of vertical fluid-filled microcracks aligned in a direction that is parallel to the regional stress field. The stronger splitting effects that were observed in the area of the CPGF were found consistent with the geothermal reservoir that is embedded in the sedimentary cover of the zone, at depths of 1 to 4–5 km from the surface. We thus believe that such marked splitting effects have a direct relation with the reservoir of the CPGF.

Do intermediate‐ and deep‐focus earthquakes occur on preexisting weak zones? An examination of the Tonga subduction zone

It has been proposed that intermediate‐ and deep‐focus earthquakes occur on preexisting planes of weakness that were created at shallow depth. The purpose of this study is to test this hypothesis. We examined fault plane solutions for 360 events that occurred in the Tonga subduction zone from January 1976 to February 1999. For events in the slab, each fault plane solution is rotated by the slab dip angle so that the slab is in its original horizontal position. We then compare fault plane solutions for events in the slab with those for events at the outer rise. We find that an asymmetric fault system, corresponding to that found for outer rise events, persists down to about 450 km depth. This suggests that earthquakes down to this depth are caused by the reactivation of preexisting faults created in the oceanic plate before subduction. A similar pattern, though less well constrained, is found in the Kurile subduction zone down to 200 km depth. For earthquakes deeper than 450 km the fault plane solutions are much more scattered. In some places the fault plane pattern suggests the creation of new fault planes along orientations of maximum shear with zero internal friction. The pattern of fault plane solutions in particular localized volumes cannot be easily explained by the creation of new faults in the direction of maximum shear under a coherent regional tectonic stress field. Thus, either a highly heterogeneous stress field exists or preexisting planes of weakness also account for some deep earthquakes.

Tectonic deformation of the Andes and the configuration of the subducted slab in central Peru; results from a microseismic experiment

A microearthquake survey was conducted in the central Andes of Peru, east of the city of Lima, to study the seismicity and style of tectonic deformation of the Peruvian Andes. Although most of the stations forming the temporary seismographic network were located on the high Andes, the vast majority of the microearthquakes recorded occurred to the east of the mountain belt: on the Huaytapallana fault in the Eastern Cordillera and beneath the western margin of the sub-Andes. Thus the sub-Andes appear to be the physiographic province subject to the most intense seismic deformation. Focal depths of the crustal events in this region range generally from 15 to 35 km and some events beneath the sub-Andes appear to be as deep as 40–50 km. The fault-plane solutions of events in the sub-Andean margin show thrust faulting on steep planes oriented roughly north-south, similar to that observed in teleseismic earthquakes studied using body wave modelling. The Huaytapallana fault in the Cordillera Oriental also shows relatively high seismicity along a NE-SW trend that agrees with the fault scarp and the east-dipping nodal plane of two large earthquakes that occurred on this fault on 1969 July 24 and October 1. Microearthquakes of intermediate depth recorded during the experiment show a flat seismic zone about 25 km thick at a depth of about 100 km. This agrees with recent observations showing that beneath Peru the slab first dips at an angle of about 30° to a depth of 100 km and then flattens following a quasi-horizontal trajectory. Fault-plane solutions of intermediate-depth microearthquakes have horizontal T axes oriented east-west suggesting slab pull is the dominant force in the downgoing slab.

Velocity modeling and earthquake relocation in the northeast Caribbean

Abstract Velocity modeling and earthquake relocation are used to examine the structure and seismicity of an area south of the Puerto Rico trench in the northeast Caribbean, and to explore whether and how the North American plate is currently subducting under the Caribbean plate along the Puerto Rico trench. A velocity model is obtained by inverting data for hypocenters, velocities, and station delays from P -wave arrival times for 160 earthquakes with depths of 0 to 85 km recorded by a local network of 18 stations. In the depth interval 0 to 24 km, three layers can be resolved for the region between 64° and 66°W, and 18°20′ and 19°40′N. An extension of the structural investigation south to 18°N, by including 67 additional events with hypocenters as deep as 150 km, shows four well-resolved layers from 0 km to approximately 75 km depth. The velocity model probably represents an average of the overriding Caribbean plate and the underthrust North American plate. A cross-section of the relocated events shows a shallowly dipping seismic zone near 15 to 25 km depth which smoothly changes to a fairly steeply dipping seismic zone at intermediate depth. When coupled with faultplane solutions of events in this region, these data provide evidence that the North American plate is presently involved in active oblique subduction. Two possible faults in the overthrust Caribbean plate have also been found. The two most southern, consistently reporting stations show high residuals for certain P waves that originate north of 18°55′N. Two more nothern stations give high residuals for phases throughout the region, probably indicating their underlying limestone geology.

Tectonic implications of the microearthquake seismicity and fault plane solutions in southern Peru

Using recordings from 15 portable instruments and six permanent stations operated during two field investigations of microearthquakes in southern Peru, we determined locations of 888 shallow and intermediate depth earthquakes and 56 fault plane solutions. A series of tests evaluating the effects of elevation corrections, erroneous velocity structure, and random errors in arrival times on calculated locations suggest that the locations of most of the events are uncertain by less than 7 km. At shallow depths (<60 km) the seismicity is concentrated in three belts: one offshore, one beneath the coast, and one beneath the Western Cordillera. There is also scattered but infrequent shallow activity near the Altiplano. The belts offshore and beneath the coast are apparently associated with slip along the Nazca‐South American plate boundary. Although the style of deformation is complicated, one can infer that beneath the coast the active boundary between the Nazca and South American plates is within the mantle rather than at the base of the South American crust. The belt beneath the Western Cordillera is about 30 km in width and dips southwest from near the surface to a depth of 60 km. Fault plane solutions suggest that crustal shortening is taking place in a northwest‐southeast direction in this region. The significance of this activity is difficult to explain, but these earthquakes provide no clear evidence of “tectonic erosion” in this area. A composite fault plane solution from the few events near the Altiplano shows normal faulting, which is typical of the active tectonics of that region. The results of the investigation of intermediate depth earthquakes shows that the downgoing slab in southern Peru is warped and not torn in the region where the dip changes from nearly horizontal to a dip of about 30°. They also suggest that the axis of the contortion is not perpendicular to the coast but parallel to the direction of plate motion. Moreover, the volcanic line ends at the contortion, which suggests that a relatively thick wedge of aesthenospheric material is necessary for the formation of subduction related volcanism. Fault plane solutions for events between 60‐and 100‐km depth show a large scatter, but some suggest a detachment of a sliver of the downgoing slab. Solutions for deeper events are more consistent with one another with the T axes for most events being nearly horizontal and generally normal to the projections of the seismic zone.