Waveform Cross-correlation Technique Research Articles

Tectonic setting of the northwestern andes Constrained by a high-resolution earthquake catalog: Block Kinematics

A high-precision earthquake catalog was generated using source-specific station terms and waveform cross-correlation techniques. This detailed catalog serves to interpret the crustal structure and deformation related to the tectonic setting in Northwestern South America. The Panamá-Chocó Block (PCB) is in contact with the Northwestern Andes causing crustal deformation and hence, fault interaction in the brittle regime leading to a high earthquake production concentrated in the Murindó Seismic Cluster (MSC) which is located in the northeastern corner of the PCB. Slip on the Uramita Fault Zone (UFZ) is a response to the accommodation of the PCB against the North Andean block (NAB), splitting the fault into two segments, marked by an abrupt change in its strike. The north segment of the UFZ, a dextral transform fault (strike = S52°E and near vertical plane), which produced the Mutatá earthquake (2016-09-14, Mw = 6.2). The central segment of the UFZ strikes N12°E and exhibits mainly reverse slip with a subvertical fault plane which produced the 1987-03-19 Mw = 5.4 and 1987-11-11 Mw = 5.3 events. The Murindó sinistral strike-slip fault (strike = N9°W), which produced the great Murindó earthquake (1992-10-18, Mw = 7.1, epicentral intensity XI) is characterized by its wide zone of brittle deformation as a response to internal deformation of the PCB under a compressional NW-SE stress. Intermediate depth earthquakes are widely observed in the area, where we find evidence of the Caribbean slab subducting southwards beneath Panamá. The subduction of the Malpelo microplate to the south, shows high productivity within the Cauca Cluster and the presence of at least three finger-shaped mantle zones of seismicity.

P-wave velocity structure in the crust and the uppermost mantle of Chao Lake region of the Tan-Lu Fault inferred from teleseismic arrival time tomography

Chao Lake is a Geoheritage site on the active Tan-Lu Fault between the Yangtze craton, the North China craton, and the Dabie orogenic belt in the southeast. This segment of the fault is not well constrained at depth partly due to the overprinting of the fault zone by intrusive materials and its relatively low seismic activity and sparse seismic station coverage. This study took advantage of a dense seismic array deployed around Chao Lake to delineate the P-wave velocity variations in the crust and uppermost mantle using teleseismic earthquake arrival time tomography. The station-pair double-difference with waveform cross-correlation technique was employed. We used a multiscale resolution 3-D initial model derived from the combination of high-resolution 3-D vS models within the region of interest to account for the lateral heterogeneity in the upper crust. The results revealed that the velocity of the upper crust is segmented with structures trending in the direction of the strike of the fault. Sedimentary basins are delineated on both sides of the fault with slow velocities, while the fault zone is characterized by high velocity in the crust and uppermost mantle. The high-velocity structure in the fault zone shows characteristics of magma intrusion that may be connected to the Mesozoic magmatism in and around the Middle and Lower Yangtze River Metallogenic Belt (MLYMB), implying that the Tan-Lu fault might have formed a channel for magma intrusion. Magmatic material in Chao Lake is likely connected to the partial melting, assimilation, storage, and homogenization of the uppermost mantle and the lower crustal rocks. The intrusions, however, seem to have suffered severe regional extension along the Tan-Lu fault driven by the eastward Paleo-Pacific plate subduction, thereby losing its deep trail due to extensional erosion.

Use of Waveform Cross Correlation to Reconstruct the Aftershock Sequence of the August 14, 2016, Sakhalin Earthquake

The waveform cross correlation technique is used to detect signals from aftershocks of the ML = 6.1, August 14, 2016, Sakhalin earthquake using regional seismic network data. Regular P- and S-wave phase arrivals are associated with seismic sources; the resulting cross correlation bulletin is compared with a standard seismic bulletin for the same aftershock sequence. An advantage of the cross correlation bulletin is that it has more detected seismic events tied to three or more stations and a lower catalog completeness threshold; even the epicenters of the weakest aftershocks are located with greater accuracy. The exact aftershock locations tend to cluster along a narrow zone related to the Central Sakhalin Fault.

THE APRIL 2007 SWARM IN TRICHONIS LAKE USING DATA FROM A MICROSEISMIC NETWORK

A series of strong earthquakes took place at the Trichonis lake area, during April 2007. Three events, with M~5.0 occurred within one day, causing damages to almost all nearby villages and especially at the small town of Thermo. The sequence lasted for more than one month with small to moderate size events. It was recorded by regional networks and results related to its time and space evolution have already been published. Just after the major events a microseismic network was deployed in the area, by the University of Patras, Seismology Laboratory. Eight stations were installed, with short period, three component sensors and portable digital recorders. The network was in operation for a period of one month and a lot of events were recorded by enough stations to provide a reliable location. We present here the analysis of these data using modern methodologies like waveform cross-correlation techniques and further relocation of the events using waveform based differential time. The results partially support the findings of the regional networks, as regards the gross characteristics of the aftershock sequence, but provide an enhanced description of it. The space time evolution of the aftershock sequence reveals the activation of more than one fault in the area and suggests the connection of the sequence with an unmapped fault.

Tomography of the Chukou Fault Zone, Southwest Taiwan: Insights from Microearthquake Data

The vigorous collision between the Eurasian plate and Philippine Sea plate in Taiwan causes a series of imbricate fold and thrust belts to develop at the deformation front. The Chukou Fault (CKF), characterized by a thrust type fault, located in Chiayi County, southwest (SW) Taiwan, is a prominent boundary between the fold-thrust belts and the Western Coastal Plain. Most of the seismicity in SW Taiwan is associated with this fault and its neighboring fault systems. The seismotectonic structures in the CKF zone, especially in the east, are complex due to the interactions among fault systems with distinct slip motions. To gain better insights into the seismogenic characteristics in the CKF zone, we used 1661 microearthquakes recorded by a temporary dense broadband seismic network and the Central Weather Bureau Seismic Network (CWBSN) between 2003 and 2004 to investigate the physical properties of the crust in the CKF zone. A waveform cross-correlation technique was applied to 143086 pairs of waveform data to determine the relative differential travel time between the P- and S-waves. By combining both the absolute and relative differential travel time data, we were able to obtain a new 3-D crustal P-wave velocity structure and Vp/Vs ratios. This study suggests that by using both absolute and relative differential travel time data in tomographic inversion can obtain precise 3-D velocity images and also gain better correlation between seismic events and fault structures, which is crucial for understanding the seismogenic process in our study area.

Earthquake-explosion discrimination using waveform cross-correlation technique for mines in southeast of Tehran

The presence of man-made explosions in a seismic catalogue leads to errors in statistical analyses of seismicity. Recently, the need to monitor man-made explosions used for mining, road excavating, and other constructional applications has been become a demanding challenge for the seismologists. In this way, we gain new insight into the cross-correlation technique and conduct this approach to discriminate explosions from seismic datasets. Following this, improved P-wave arrival times are used for more precise relocation. In this study, the waveform cross-correlation technique provides a reliable means for discriminating explosions which have cross-correlation coefficients (CC) of 0.6 or greater with their own corresponding stacked waveforms. The results illustrate that approximately 80 % of seismicity of southeast of Tehran, recorded by the Iranian Seismological Center (IRSC), includes events which have cross-correlation coefficients of ≥0.6 with their corresponding stacked waveforms. Furthermore, with improved P-wave arrival time, there is a better chance to relocate explosions precisely in the region under study.

Application of array-based waveform cross-correlation techniques to aftershock sequences: the 2003 Lefkada Island, Greece, case

Cross-correlation analysis was applied to events in the 2003 Lefkada Island, Greece, sequence in order to identify clusters of seismicity within the extensive aftershock sequence along a fault zone of approximately 100 km length. Data from the small-aperture TRISAR array, covering the first 2 days of aftershock activity, were used. Array-based waveform correlation has a great advantage over single channel correlation analysis in that the validity of waveform matches with relatively low correlation coefficients can be examined by checking the alignment of correlation traces on the different channels. The length of the fault zone leads inevitably to a great diversity in the waveforms, although a small number of clusters of very similar events emerge from the TRISAR data. Events which the correlation analysis had placed within the same cluster were listed in the ISC Bulletin with separations of up to tens of kilometres. This made it necessary to check the validity of the TRISAR clusters by applying the same procedure independently to the three-component stations of the National Seismographic Network of the National Observatory of Athens, located at local to regional distances from the aftershock area. Results suggest that array-based waveform correlation provides a robust tool both for identifying event clusters within large aftershock areas and for identifying situations in which bulletin event location estimates need re-evaluation.

Depth extent of damage zones around the central Calaveras fault from waveform analysis of repeating earthquakes

SUMMARY We systematically investigate spatial variations of temporal changes and depth extent of damage zones along the Calaveras fault that ruptured during the 1984 Morgan Hill earthquake by the waveform analysis of 333 sets of repeating earthquakes. We use a sliding window waveform cross-correlation technique to measure traveltime changes in waveforms generated by each repeating cluster. We find clear traveltime delays in the S- and early S-coda waves for events immediately after the Morgan Hill main shock. The amplitudes of the time delays decrease logarithmically with time since the main shock, indicating a time-dependent recovery (healing) process following the abrupt coseismic temporal changes. The largest temporal changes are observed at station CCO that is the closest to the rupture zone of the Morgan Hill main shock. The time delays at this station are larger for clusters in the top 6 km, and decrease systematically at larger depth. In comparison, the time delays observed at other five stations are much smaller, and do not show clear relationship with hypocentral depth. We suggest that the temporal changes at these five stations mostly occur in the top few hundred metres of the near-surface layers, while the temporal changes at station CCO are likely associated with the damage zone around the Calaveras fault that is well developed in the top few kilometres of the upper crust. Our results are consistent with the inference of a widespread damage and non-linearity in the near-surface layers associated with strong ground motions of nearby large earthquakes, and localized damages and flower-type structures around active faults based on previous studies of fault zone structures and recent 3-D numerical simulations.

Characteristics of the October 2005 Microearthquake Swarm and Reactivation of Similar Event Seismic Swarms over Decadal Time Periods near Socorro, New Mexico

Seismicity in the Rio Grande rift in central New Mexico, southwestern United States, has been dominated by microearthquakes occurring above the midcrustal Socorro magma body (SMB) for at least the past century. The SMB is a sill-like feature ≥3400 km2 in area, with a top surface at 19-km depth and centered below the inner rift half-graben system. A recent swarm of microearthquakes above the magma body began on 20 October 2005. The activity intensified toward 30 October 2005, with the largest event being a shallow felt local magnitude ( M L) 2.4 earthquake at 02:57:35 (UTC). Seismicity increased in the region following this event, with over 1600 earthquakes detectable at a minimum of one local seismic station during the subsequent month. Seismic waveforms for the earthquakes in this sequence are remarkably similar, implying consistently similar location and faulting geometry. The time–space distribution of earthquakes indicates that this sequence is highly swarmlike, in contrast to tectonic mainshock–aftershock sequences. Given the local tectonic setting and shallow (<8 km) depth of individual events above the SMB, a likely explanation for the Socorro earthquake sequence is fluid flow- or pressure-induced triggering associated with the evolution of the SMB. Swarms such as the 2005 sequence are not uncommon in this area (e.g., prominent previous sequences occurred in May and July of 1983). Here we characterize the 2005 earthquake sequence by presenting composite focal mechanisms, magnitude estimates, and high-precision relative locations based on waveform cross-correlation techniques that capitalize on the similar waveforms. A new waveform scanning technique identifies over 1600 small similar earthquakes during this period from continuous seismic records. Waveforms and locations are also observed to be similar between the 2005 and 1983 sequences, suggesting consistency in source location, geometry, and thus mechanism over decadal time scales.

Similar microearthquakes observed in western Nagano, Japan, and implications for rupture mechanics

We have applied a waveform cross correlation technique to study the similarity and the repeatability of more than 21,000 microearthquakes (0 &lt; M &lt; 4.5) in the aftershock zone of the 1984 western Nagano earthquake in central Japan. We find that the seismicity in this particular intraplate fault essentially consists of no repeating earthquakes that occurred on the same patch of the fault in a quasiperiodic manner in the study period between 1995 and 2001. On the other hand, we identify a total of 278 doublets and 62 multiplets (807 events) that occurred consecutively within seconds to days. On the basis of the relative arrival times of the P and S waves, we have obtained precise relative locations of these consecutive events with an error between several meters to a few tens of meters. There is a clear lower bound on the distances measured between these consecutive events and the lower bound appears to be proportional to the size of the first events. This feature is consistent with what Rubin and Gillard [2000] have observed near the San Juan Bautista section of the San Andreas Fault. Shear stress increases at the edge of an earthquake rupture, and the rupture edge becomes the most likely place where the second events are initiated. The observed minimum distance thus reflects the rupture size of the first events. The minimum distance corresponds to the rupture size calculated from a circular fault model with a stress drop of 10 MPa. We found that using different time windows results in a slight difference in the delay time estimates and the subsequent projection locations, which may reflect the finite size nature of earthquake ruptures.

Temporal Changes of Shallow Seismic Velocity Around the Karadere-Düzce Branch of the North Anatolian Fault and Strong Ground Motion

We analyze temporal variations of seismic velocity along the Karadere-Duzce branch of the north Anatolian fault using seismograms generated by repeating earthquake clusters in the aftershock zones of the 1999 Mw7.4 Izmit and Mw7.1 Duzce earthquakes. The analysis employs 36 sets of highly repeating earthquakes, each containing 4–18 events. The events in each cluster are relocated by detailed multi-step analysis and are likely to rupture approximately the same fault patch at different times. The decay rates of the repeating events in individual clusters are compatible with the Omori's law for the decay rate of regional aftershocks. A sliding window waveform cross-correlation technique is used to measure travel time differences and evolving decorrelation in waveforms generated by each set of the repeating events. We find clear step-like delays in the direct S and early S-coda waves (sharp seismic velocity reduction) immediately after the Duzce main shock, followed by gradual logarithmic-type recoveries. A gradual increase of seismic velocities is also observed before the Duzce main shock, probably reflecting post-seismic recovery from the earlier Izmit main shock. The temporal behavior is similar at each station for clusters at various source locations, indicating that the temporal changes of material properties occur in the top most portion of the crust. The effects are most prominent at stations situated in the immediate vicinity of the recently ruptured fault zones, and generally decrease with normal distance from the fault. A strong correlation between the co-seismic delays and intensities of the strong ground motion generated by the Duzce main shock implies that the radiated seismic waves produced the velocity reductions in the shallow material.

Evidence from a cluster of small earthquakes for a fault at 18 km depth beneath Oak Ridge, southern California

Abstract A swarm of about 50 small earthquakes (M ∼ 1.5) occurred for a month during 1989 beneath Oak Ridge, southern California. Location accuracy using conventional analysis of arrival-time picks is limited for these events by the weak, emergent nature of arrivals on the available seismograms. However, waveform cross-correlation techniques are found to provide precise relative event locations due to the similarity of the waveforms recorded at individual stations. The relocated events form a small cluster about 1 km across at a depth of ∼ 18 km and are aligned along a plane that dips 35° to the northwest. Estimated standard errors for the locations are generally less than 50 m. The time evolution of the sequence shows a gradual migration of activity away from its initiation point. Three additional events occurred several months later; these align along the same plane but are displaced about 500 m to the southeast from the main swarm. Reliable fault-plane solutions are difficult to obtain for these events due to the small number of station records available, the limited range of takeoff angles, and the weak initial arrivals on many of the seismograms. Stacking the records at each station over the different events greatly reduces prearrival noise levels and assists in resolving the average P first motions. Analysis of these first-motion data indicates that the slip planes of probable focal mechanisms are not in agreement with the plane defined by the seismicity. The seismicity alignment may represent the extension of the Simi fault, in which case some shallowing of the fault dip would be required to match the observed 35° dip at 18 km.