Double-difference Relocation Research Articles

Seismogenic zone high permeability in the Central Andes inferred from relocations of micro-earthquakes

Precise locations of aftershocks following a large subduction earthquake offers insights into the nature of the seismogenic zone interface and stress redistribution within the subducting plate and its overburden. In 1995 a large magnitude M w = 8.0 earthquake in Antofagasta, northern Chile marked the beginning of an aftershock sequence recorded for 2 months on a dense temporary network surrounding the rupture region. In this study we use cross-correlation and a double-difference relocation technique to obtain high-precision locations for this aftershock dataset. Relocation of the aftershock sequence reveals topography of the subducting Nazca plate and indicates that the slab is deeper to the south. The majority of the seismicity is located in a thin (∼ 1–1.5 km) region on the seismogenic interface. We also find seismicity located just above the seismogenic interface in the overlying continental crust. Particularly a streak of seismicity that extends ∼ 8 km vertically from the seismogenic interface into the overlying continental crust and is in close proximity to a region of large V P/V S ratio interpreted as the migration of trapped subduction fluid. Analysis of the seismicity streak yields a strong correlation between time and focal depth for events within this streak and indicates a fluid flow of ∼ 105 m/day giving a continental crust permeability of 10 − 14 –10 − 15 m 2. Focal mechanism data shows predominantly dip slip motion for these events, in contrast to thrust motion along the seismogenic interface, accommodating stress redistribution in the overriding plate. One could interpret this streak as the migration of fluid along a previous weak fault zone. Alternatively, the seismicity streak is located close to the bottom of the 1995 rupture and therefore one would expect large stress unloading in this area. The normal faulting in the overlying continental plate could be secondary faulting an effect of stress reorientation after the main thrust event. This relaxation of stress could have re-activated previous weak fault zones not only in the overlying continental crust but also in the subducting slab.

Effect of the Data Recorded at Nearby Stations on Earthquake Relative Location

AbstractBased on the fundamental theory of earthquake location, it is found that the data recorded at nearby stations are applicable in double difference relocation algorithm, but not in other relative location algorithms. Equation derivation and simulated calculation demonstrate that double difference algorithm allows us to get the absolute locations of earthquakes with certain accuracy by using the phase arrivals at nearby station. The errors of relative location become larger when using the nearby station records alone than that using the distant station records. Combining the nearby and distant station records, the best locations are obtained. In the case that hypocenter depth is far less than the epicenter distance without depth phases available, only the relative epicenter distribution could be obtained accurately by double difference method, the depth determined would be not stable. The relocation test of a part of aftershocks in Xinjiang‐Jiashi earthquake sequence of 2003 verify the effect of nearby station records on the double difference algorithm mentioned above.

Accurate relocation of İzmit earthquake (Mw = 7.4, 1999) aftershocks in Çınarcık Basin using double difference method

We relocated part of the aftershock activity in Çınarcık Basin and surrounding areas that are associated with the 1999 İzmit earthquake Mw 7.4. Double difference relocation algorithm is used to relocate the aftershocks. The data set was obtained from a temporary seismic network deployed 10 days after the main shock by cooperation between Boğaziçi University Kandilli Observatory and Earthquake Research Institute, LGIT (Grenoble), and IPGP (Paris). For a better station coverage, additional data set was obtained from a network operated by TUBITAK Marmara Research Center. Differential travel times were calculated using both arrival time readings and waveform cross correlation method. We relocated 1145 of the aftershocks and interpreted the results with emphasis on the Yalova and Tuzla cluster located within the Marmara Sea. The results show better focused seismicity patterns for the Yalova cluster, providing clear evidence for the proposed models. We present a revised location of the 1963 Çınarcık earthquake which took place in area of Yalova cluster. Finally we propose that the activity of Tuzla cluster represents a parallel subsidiary fault of the Main Marmara Fault.

P- andS-velocity images of the lithosphere-asthenosphere system in the Central Andes from local-source tomographic inversion

SUMMARY About 50 000 P and S arrival times and 25 000 values of t* recorded at seismic arrays operated in the Central Andes between 20°S and 25°S in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3-D P, S velocity and Qp models. The study volume in the reference model is subdivided into three domains: slab, continental crust and mantle wedge. A starting velocity distribution in each domain is set from a priori information: in the crust it is based on the controlled sources seismic studies; in slab and mantle wedge it is defined using relations between P and S velocities, temperature and composition given by mineral physics. Each iteration of tomographic inversion consists of the following steps: (1) absolute location of sources in 3-D velocity model using P and S arrival times; (2) double-difference relocation of the sources and (3) simultaneous determination of P and S velocity anomalies, P and S station corrections and source parameters by inverting one matrix. Velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double-sided nodes at the domain boundaries. The next iteration is repeated with the updated velocity model and source parameters obtained at the previous step. Different tests aimed at checking the reliability of the obtained velocity models are presented. In addition, we present the results of inversion for Vp and Vp/Vs parameters, which appear to be practically equivalent to Vp and Vs inversion. A separate inversion for Qp has been performed using the ray paths and source locations in the final velocity model. The resulting Vp, Vs and Qp distributions show complicated, essentially 3-D structure in the lithosphere and asthenosphere. P and S velocities appear to be well correlated, suggesting the important role of variations of composition, temperature, water content and degree of partial melting.

The 1998 Hida Mountain, Central Honshu, Japan, earthquake swarm: Double-difference event relocation, frequency–magnitude distribution and Coulomb stress changes

By using the double-difference relocation technique, we have determined the fine structure of seismicity during the 1998 Hida Mountain earthquake swarm. The distribution of seismic activity defines two main directions (N–S and E–W) that probably correspond to the regional stress pattern. The detailed structure of seismicity reveals intense spatio-temporal clustering and earthquake lineations. Each cluster of events contains a mainshock and subsequent aftershock activity that decays according to the Omori law. The seismicity and the b-value temporal and spatial patterns reflect the evolution of the static stress changes during the earthquake swarm. About 80% of the swarm's best-relocated events occur in regions of increased ΔCFF. The smaller value of b found in the northern part of the swarm region and a larger b-value observed to the south, for the same period of time, could be well explained by the static stress changes caused by the larger events of the sequence. We argue that the state of stress in the crust is the main factor that controls the variation of b-value.

Double-difference relocation of earthquakes in central-western China, 1992–1999

The double-difference earthquake location algorithm was applied to the relocation of 10,057 earthquakes that occurred in central-western China (21∘N to 36∘N, 98∘E to 111∘E) during the period from 1992 to 1999. In total, 79,706 readings for P waves and 72,169 readings for S waves were used in the relocation. The relocated seismicity (6,496 earthquakes) images fault structures at seismogenic depths that are in close correlation with the tectonic structure of major fault systems expressed at the surface. The new focal depths confirm that most earthquakes (91%) in this region occur at depths less than 20 km.

Double-difference relocations of the 2004 off the Kii peninsula earthquakes

Abstract Two large earthquakes (Mw7.3 and Mw7.5) occurred off-shore of the Kii peninsula (Japan) on 5th September 2004. The insufficient station coverage in the area where the large events and their aftershocks occurred led to significant uncertainty regarding their depth location. By using a double-difference approach, we were able to determine more accurate hypocenter locations and better define the main characteristics of the aftershock distribution. The data we used in this study comprise of about 700 earthquakes recorded by the Japanese Meteorological Agency (JMA) from 5 to 12 September 2004. We assume several depth ranges for the initial double difference earthquake locations and select the solution with the smallest rms residual. The resulting hypocenter distribution has two distinctive features: 1) on average the depths are about 20 km shallower than the initial JMA location and 2) the hypocenters are divided into two groups: one shallow (around 12 km depth) and another deep (around 27 km depth). The epicenters of the relocated events define a finer, more clustered structure compared with the epicentral distribution of JMA. The splitting of the earthquake locations into two depth groups is still observed when using a significantly different velocity structure. Our results of the two depth groups for the hypocenters resemble the independently determined hypocentral distribution from the OBS survey.

New constraints on seismicity in the Wellington region of New Zealand from relocated earthquake hypocentres

The Wellington region of New Zealand overlies a strongly coupled portion of the shallow subduction interface between the Pacific and Australian plates. To better understand the active deformation in the area, we perform a double-difference (DD) relocation of 6825 local earthquakes that occurred between 1990 and 2001. Using both cross-correlation (CC) and bispectrum (BS) methods, we calculate high-quality waveform-based differential times (WBDTs) for event pairs. We manage to more than triple the S-wave differential time measurements after reasonably estimating many of the unpicked S arrivals. After relocation, the image of the double seismic zone beneath this region is greatly sharpened. We find several northeast-striking linear seismic features near Lake Wairarapa on the North Island, which may result from slip along normal faults within the subducting Pacific Plate. We further search for repeating events that might define creeping patches along the interplate thrust. Using a 20-s data window for CC, we find 287 event clusters with very high waveform similarities. Most of these clusters contain just a doublet that occurred within a short time period. The failure to find repeating earthquakes along the plate boundary in this 12-yr time period is consistent with the hypothesized strong coupling between the Pacific and Australian plates under the Wellington region.

Intra-plate seismicity in the subducting Philippine Sea Plate, southwest Japan: magnitude–depth correlations

Recent studies of intra-plate seismicity have focused on detailed earthquake locations and potential hypotheses for intermediate depth events. Three main competing hypotheses for intra-plate events are: (1) transformational faulting; (2) shear melt instabilities; and (3) dehydration reactions. Recent work, using precise hypocenters and thermal-petrological models, indicates that the spatial distribution of intra-plate events coincides with dehydration reactions. These studies have mainly focused on the location of events and the proposed reactions with little attention given to the magnitude distribution of events. Presented here is a magnitude–depth correlation associated with intra-plate events for southwest Japan. Earthquake hypocenters were obtained from relocating a data set consisting of Japan Meteorological Agency (JMA) seismicity data and Japan University Network Earthquake Catalog (JUNEC). Data from 1985 to 1996 were combined and relocated using a standard location routine (HypoInverse) that allows regional variations in velocity models and station corrections. HypoInverse results show a well defined plane of seismicity associated with the subducting Philippine Sea Plate. The data were further relocated using a double-difference relocation algorithm (HypoDD). HypoDD results show a magnitude–depth correlation for Kii peninsula and northern Kyushu. Histogram plots indicate that events near the plate interface were generally magnitude 3 or less, whereas most large events were located within the lower crust or possibly the subducting oceanic mantle. This observation also appears to hold true for intra-plate events beneath Tohoku (Japan trench) and Hokkaido (Kuriles), but is not clearly observed beneath the Tokai region. A dehydration reaction model can well explain the magnitude–depth correlation.