Aftershock Locations Research Articles

Analysing the 1811–1812 New Madrid earthquakes with recent instrumentally recorded aftershocks

Although dynamic stress changes associated with the passage of seismic waves are thought to trigger earthquakes at great distances, more than 60 per cent of all aftershocks appear to be triggered by static stress changes within two rupture lengths of a mainshock. The observed distribution of aftershocks may thus be used to infer details of mainshock rupture geometry. Aftershocks following large mid-continental earthquakes, where background stressing rates are low, are known to persist for centuries, and models based on rate-and-state friction laws provide theoretical support for this inference. Most past studies of the New Madrid earthquake sequence have indeed assumed ongoing microseismicity to be a continuing aftershock sequence. Here we use instrumentally recorded aftershock locations and models of elastic stress change to develop a kinematically consistent rupture scenario for three of the four largest earthquakes of the 1811-1812 New Madrid sequence. Our results suggest that these three events occurred on two contiguous faults, producing lobes of increased stress near fault intersections and end points, in areas where present-day microearthquakes have been hitherto interpreted as evidence of primary mainshock rupture. We infer that the remaining New Madrid mainshock may have occurred more than 200 km north of this region in the Wabash Valley of southern Indiana and Illinois--an area that contains abundant modern microseismicity, and where substantial liquefaction was documented by historic accounts. Our results suggest that future large mid-plate earthquake sequences may extend over a much broader region than previously suspected.

Breaking into the plate: A 7.6 Mw fracture‐zone earthquake adjacent to the Central Indian Ridge

On 15 July 2003 an extremely large (7.6 Mw) strike‐slip earthquake initiated on or near the end of an active transform along the northern Central Indian Ridge. The event propagated away from the plate boundary along the typically inactive fracture zone, with a sense of slip that opposes the active transform slip direction. Seismically and hydroacoustically determined aftershock locations delineate a 210 ± 25 km long mainshock rupture. The seismic moment and rupture dimensions imply a stress drop of 4.5–6.5 MPa and a mean slip of 3.0 ± 0.5 m. The largest aftershock (5.6 Mw) occurred on the active portion of a neighboring transform at a distance of ∼160 km, where mainshock‐induced static stress changes are predicted to promote failure. Near‐axis fracture‐zone earthquakes may promote and inhibit ridge‐parallel diking along different spreading segments, perhaps contributing to inter‐segment variability in the rate and asymmetry of spreading.

Seismogenic zone structure of the southern Middle America Trench, Costa Rica

The shallow seismogenic portion of subduction zones generates damaging large and great earthquakes. This study provides structural constraints on the seismogenic zone of the Middle America Trench offshore central Costa Rica and insights into the physical and mechanical characteristics controlling seismogenesis. We have located ∼300 events that occurred following the MW 6.9, 20 August 1999, Quepos, Costa Rica, underthrusting earthquake using a three‐dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers. We use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region. These events define a plane dipping at 19° that marks the interface between the Cocos Plate and the Panama Block. The majority of aftershocks occur below 10 km and above 30 km depth below sea level, corresponding to 30–35 km and 95 km from the trench axis, respectively. Relative event relocation produces a seismicity pattern similar to that obtained using absolute locations, increasing confidence in the geometry of the seismogenic zone. The aftershock locations spatially correlate with the downdip extension of the oceanic Quepos Plateau and reflect the structure of the main shock rupture asperity. This strengthens an earlier argument that the 1999 Quepos earthquake ruptured specific bathymetric highs on the downgoing plate. We believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level.

Aftershock activity of Bhuj earthquake of january 26th, 2001

Following a large-sized Bhuj earthquake (M s = 7.6) of January 26th, 2001, a small aperture 4-station temporary local network was deployed, in the epicentral area, for a period of about three weeks and resulted in the recording of more than 1800 aftershocks (-0.07 ≤M L <5.0). Preliminary locations of epicenters of 297 aftershocks (2.0 ≤M L <5.0) have brought out a dense cluster of aftershock activity, the center of which falls 20 km NW of Bhachau. Epicentral locations of after-shocks encompass a surface area of about 50 × 40 km2 that seems to indicate the surface projection of the rupture area associated with the earthquake. The distribution of aftershock activity above magnitude 3, shows that aftershocks are nonuniformly distributed and are aligned in the north, northwest and northeast directions. The epicenter of the mainshock falls on the southern edge of the delineated zone of aftershock activity and the maximum clustering of activity occurs in close proximity of the mainshock. Well-constrained focal depths of 122 aftershocks show that 89% of the aftershocks occurred at depths ranging between 6 and 25 km and only 7% and 4% aftershocks occur at depths less than 5 and more than 25 km respectively. The Gutenberg-Richter (GR) relationship, logN = 4.52 - 0.89ML, is fitted to the aftershock data (1.0<-M L<5.0) and theb-value of 0.89 has been estimated for the aftershock activity.

The Colima, Mexico, Earthquake (MW 5.3) of 7 March 2000: Seismic Activity Along the Southern Colima Rift

The 7 March 2000 earthquake ( M W 5.3), located within the southern Colima rift in southwestern Mexico, is the largest crustal earthquake ever recorded within the state of Colima, Mexico. The mainshock was located at 8 km depth, and the aftershocks are distributed at depths between 5 and 10 km. Aftershock distribution follows a normal decay as measured by the modified Omori's law, with a p value of 0.96. Both aftershock locations and focal mechanisms (determined for the mainshock and some of the aftershocks) delineate a fault dipping toward the northwest and striking west-southwest. This fault segment is parallel to the southern end of the Tamazula fault, which ends at Manzanillo Bay. Regional waveform modeling for the mainshock situates the earthquake on a normal fault with a significant left-lateral strike-slip component on a plane striking 260° and dipping toward the northwest at 46°. We use a small aftershock with similar focal parameters as the mainshock to deconvolve P and S waves recorded at the permanent station COIG. From the deconvolution we obtained a source duration of 0.75 sec from the P waves and 0.9 sec from the S waves for the main event and a stress drop of 320 bars. The aftershock area is much larger than the area measured from the apparent source time function. From the aftershock area and the seismic moment obtained by Harvard University (Centroid Moment Tensor Catalog), we compute a static stress drop of 20 bars. Fault-plane solutions from the mainshock and some of the aftershocks indicate that the northern, and larger, segment of the Tamazula fault is perpendicular to the extensional axis (directed east-southeast). This study confirms the change in the direction of the extensional axis of the southern Colima rift with respect to the northern Colima rift (extensional axis oriented almost east-west). Here we show that the Tamazula fault zone, at least its eastern segment, is active, and a re-evaluation of the seismic hazard posed by this fault to the state of Colima and the cities of Colima and Manzanillo is urgent. Manuscript received 13 September 2002.

Secondary Aftershocks and Their Importance for Aftershock Forecasting

The potential locations of aftershocks, which can be large and damaging, are often forecast by calculating where the mainshock increased stress. We find, however, that the mainshock-induced stress field is often rapidly altered by aftershock-induced stresses. We find that the percentage of aftershocks that are secondary aftershocks, or aftershocks triggered by previous aftershocks, increases with time after the mainshock. If we only consider aftershock sequences in which all aftershocks are smaller than the mainshock, the percentage of aftershocks that are secondary also increases with mainshock magnitude. Using the California earthquake catalog and Monte Carlo trials we estimate that on average more than 50% of aftershocks produced 8 or more days after M ≥5 mainshocks, and more than 50% of all aftershocks produced by M ≥7 mainshocks that have aftershock sequences lasting at least 15 days, are triggered by previous aftershocks. These results suggest that previous aftershock times and locations may be important predictors for new aftershocks. We find that for four large aftershock sequences in California, an updated forecast method using previous aftershock data (and neglecting mainshock-induced stress changes) can outperform forecasts made by calculating the static Coulomb stress change induced solely by the mainshock. Manuscript received 20 November 2002.

Coulomb stress changes in the South Iceland Seismic Zone due to two large earthquakes in June 2000

The South Iceland Seismic Zone experienced the largest earthquakes for 88 years in June 2000, with a MS = 6.6 event on June 17, followed by another MS = 6.6 earthquake on June 21. These events occurred on two parallel N‐S striking, right‐lateral strike slip faults, separated by about 17 km. We calculate the static Coulomb stress change for the June 17 and 21 earthquakes using a distributed slip model derived from joint inversion of InSAR and GPS data. We find that the static stress change caused by the June 17 event is about 0.1 MPa at the location of the June 21 hypocenter, promoting failure on the second fault. Locations of aftershocks agree well with areas of increased Coulomb failure stress. Our calculations indicate that positive stress changes due to the two earthquakes make the area west of the June 21 rupture the most likely site of the next large earthquake in South Iceland.

The 19 January 1995 Tauramena (Colombia) earthquake: geometry and stress regime

The Tauramena (Colombia) earthquake, M w=6.5, occurred on January 19, 1995, in the Andean Eastern Cordillera foothill region, the so-called Piedemonte Llanero. The Harvard CMT focal mechanism indicates an almost pure reverse fault rupture. There was no surface faulting associated with this earthquake. This event was located at the northern tip of a zone, about 90 km in length, with relatively low microseismic activity along the central segment of the Piedemonte Llanero in Colombia. A field expedition to the epicentral area was organized and a temporary portable network was installed for 1 month to register aftershock activity. More than 800 events were recorded during this period. A subset of the best located aftershocks (319 events) shows epicenters extended over an area of 800 km 2, and suggests two antithetic planes on which most of the activity was concentrated. The main event and a subset of 41 aftershocks occurred before the installation of the portable network but were recorded by the Colombian National Seismic Network and were relocated. They show an epicentral distribution similar to that of the subsequent events. Based on geologic information, aftershock locations, and focal mechanisms, we built a model for the Tauramena earthquake. Our results indicate that the Tauramena earthquake was produced by reverse faulting along a steep-dipping plane (dip ∼50°NW) associated with the Guaicaramo System. The fault plane cuts through basement rocks and folded Mesozoic and Cenozoic sedimentary rocks. Distribution of aftershocks in depth, suggesting the presence of splay reverse faults and a backthrust, is consistent with the hypothesis that the Guaicaramo Fault System was an old Mesozoic normal fault, reactivated during the Andean compression as a reverse fault. Stress tensor inversion of P-wave first motion polarities was performed, and focal mechanisms for the best recorded aftershocks were calculated. We found a well-defined sub-horizontal principal compression axis oriented in the NW–SE direction. The predominant reverse faulting for the Tauramena earthquake and the stability of σ 1 suggest that the tectonics in the central segment of the Eastern Cordillera Frontal Fault System is dominated by a compressive regime orthogonal to the cordillera, the controlling processes being probably the Caribbean subduction and the Chocó Block collision.

Detailed Fault Structure of the 2000 Western Tottori, Japan, Earthquake Sequence

We investigate the faulting process of the aftershock region of the 2000 western Tottori earthquake ( M w 6.6) by combining aftershock hypocenters and moment tensor solutions. Aftershock locations were precisely determined by the double difference method using P - and S -phase arrival data of the Japan Meteorological Agency unified catalog. By combining the relocated hypocenters and moment tensor solutions of aftershocks by broadband waveform inversion of FREESIA (F-net), we successfully resolved very detailed fault structures activated by the mainshock. The estimated fault model resolves 15 individual fault segments that are consistent with both aftershock distribution and focal mechanism solutions. Rupture in the mainshock was principally confined to the three fault elements in the southern half of the zone, which is also where the earliest aftershocks concentrate. With time, the northern part of the zone becomes activated, which is also reflected in the postseismic deformation field. From the stress tensor analysis of aftershock focal mechanisms, we found a rather uniform stress field in the aftershock region, although fault strikes were scattered. The maximum stress direction is N107°E, which is consistent with the tectonic stress field in this region. In the northern part of the fault, where no slip occurred during the mainshock but postseismic slip was observed, the maximum stress direction of N130°E was possible as an alternative solution of stress tensor inversion.

Size and orientation of the fault plane for the 2001 Gujarat, India earthquake (Mw7.7) from aftershock observations: A high stress drop event

We used a small array of portable seismographs to determine aftershock locations of the 2001 Gujarat earthquake. Our aftershock locations show a trend that dips toward the south at about 50° which is interpreted as the fault plane of the mainshock. The depth range of the aftershocks is from 10 to 35 km, which is somewhat deeper than other crustal earthquakes, and indicates that the faulting did not reach the surface. The area of the fault is about 40 × 40 km2, which is small for a Mw7.7 earthquake and results in a high static stress drop of 13 to 25 MPa. There are no mapped faults or obvious topographic features along the surface projection of this fault. These findings show that very large damaging earthquakes can occur without producing surface faulting, which is an important issue for earthquake hazard assessments in continental regions.

Analysis of the 23 June 2001 Mw = 8.4 Peru underthrusting earthquake and its aftershocks

On 23 June 2001, a Mw = 8.4 underthrusting earthquake occurred in the southern Peru subduction zone, followed by several large aftershocks, including 26 June (Mw = 6.7) and 7 July (Mw = 7.5). Broadband analyses of seismic data for the largest of these earthquakes show southeastward rupture of 180 km along the portion of the subduction zone previously ruptured in 1868 (Mw 8.8–9). Moment release distributions determined are consistent with aftershock location patterns. Earthquake rupture mode varies along southern Peru, from larger multi‐segment rupture in 1868 to several smaller segment ruptures in 2001, similar to rupture variation observed in northern Peru and other subduction zones. Based on models of subduction zone segment interaction, the 2001 earthquake sequence may suggest a shorter recurrence time for future earthquakes along this portion of the Peru‐Chile subduction zone.

The Combined Inversion of Seismic and Geodetic Data for the Source Process of the 16 October 1999 Mw 7.1 Hector Mine, California, Earthquake

We investigate the source process of the M w 7.1 Hector Mine earthquake by inverting broadband regional and local seismic displacement waveforms com- bined with Global Positioning System (GPS) and synthetic aperture radar interfer- ometry (InSAR) geodetic measurements. We find that the three data sets individually produce remarkably similar slip distributions over a multisegment fault. A simulta- neous inversion of the three data sets is presented, and the sensitivity of the combined inversion to the weighting of the three independent data sets is examined. The results indicate that the overall length of the fault that slipped is 42 km with a peak slip of 5.50 m and total scalar seismic moment of 6.8 10 19 N m. The majority of slip is located on the western branch of the Lavic Lake fault to the northwest of the hypo- center, although the overall rupture is bilateral with appreciable slip on the Bullion fault to the southeast. Some slip is located on an eastern branch of the Lavic Lake fault, an apparent bifurcation, which is also evident from aftershock locations. The average slip and stress drops are 2.70, 1.62, 1.60 m and 78, 36 and 55 bars for the western Lavic Lake, the eastern Lavic Lake, and Bullion faults, respectively. The results also indicate that the overall rupture process is relatively slow, which is char- acterized by a several-second delay before the onset of significant slip, a 1.8 km/sec rupture velocity of the primary asperity, and long, spatially variable, dislocation rise times.

Fault Slip Distribution of the 1999 Mw 7.1 Hector Mine, California, Earthquake, Estimated from Satellite Radar and GPS Measurements

We use a combination of satellite radar and GPS data to estimate the slip distribution of the 1999 M w 7.1 Hector Mine Earthquake, a right-lateral strikeslip earthquake that occurred on a northwest–southeast striking fault in the southern California Mojave Desert. The data include synthetic aperture radar interferograms (InSAR) from both ascending and descending orbits, radar amplitude image offset fields (SARIO) for both ascending and descending azimuth directions, and campaign GPS observations from 55 stations provided by Agnew et al. (2002). We model the fault with nine segments derived from the field-mapped fault rupture, the SARIO data, and aftershock locations. We first estimate the dip of each fault segment, as well as a single constant strike-slip component across each segment, resulting in an average dip of 83° to the northeast and slip of up to 5.6 m. Then, we fix the optimal fault segment dip, discretize the fault segments into 1.5 km × 1.5 km patches, and solve for the variable slip distribution using a nonnegative least-squares method that includes an appropriate degree of smoothing. Our preferred solution has both right-lateral strike-slip and reverse faulting. The estimated geodetic moment is 5.93 × 10 19 N m ( M w 7.1), similar to seismological estimates, indicating that there are insignificant interseismic and postseismic deformation signals in the data. We find strike-slip displacements of up to 6.0 m and reverse faulting of up to 1.6 m, with the maximum slip located just northwest of the epicenter. Most of the slip is concentrated northwest and south of the epicenter; little slip is found on the northeastern branch of the fault. The SARIO data and our modeling indicate that the amount and extent of surface fault rupture were underestimated in the field.

Study of the 1999 M 7.1 Hector Mine, California, Earthquake Fault Plane by Trapped Waves

We recorded fault-zone trapped waves from aftershocks on portable seismometers in a tight linear array across the Lavic Lake fault, which was one of several faults that ruptured in the M 7.1 Hector Mine, California, earthquake on 16 October 1999. Trapped waves with large amplitudes and long duration at 4 to 7 Hz produced by aftershocks occurring within the rupture zone were recorded at stations close to the fault trace. However, the S waves registered at stations farther from the rupture zone for the same events were much briefer. Trapped waves recorded at the Hector Mine rupture zone are similar to those observed in the Landers rupture zone (Li et al., 1994a,b), but show higher frequencies. Simulations of these trapped waves indicate a 75 to 100-m-wide low-velocity and low-Q waveguide along the Hector Mine rupture zone in which the S velocity is reduced by about 40% to 50% from wall-rock velocities, and Q is 10 to 60 in the depth range from the surface to 10 km. We interpret this low-velocity waveguide as being a remnant of the process zone formed by inelastic deformation around the propagating crack tip during dynamic rupture in the 1999 Hector Mine earthquake. The reductions of velocities and Q within the Hector Mine rupture zone are similar to those within the Landers rupture zone, suggesting that the fault-zone rock was damaged to the same degree in the two earthquakes. The wave-guide width (75-100 m) on the Hector Mine rupture zone (40 km in the total length) is half that (150-250 m) of the Landers rupture zone (80 km in the total length), consistent with the scaling of process zone size to rupture length as predicted in some published dynamic rupture models. Locations of aftershocks for which we observed trapped waves show bifurcation of the northern Hector Mine rupture at depth, although only the west rupture branch broke to the surface.

The 1999 Izmit Earthquake Sequence in Turkey: Seismological and Tectonic Aspects

The Izmit/Duzce earthquake sequence is the largest sequence recorded by modern digital networks in the history of Turkish earthquakes. The ( M w 7.4) Izmit mainshock occurred at 29.967° E and 40.729° N, and 13 km depth at 00 hr 01 min 38.7 sec UTC, on 17 August 1999 and was preceded by at least eight immediate foreshocks of magnitude M L < 3.0. Aftershock locations show that a large region has become seismically very energetic following the mainshock and that most of the aftershock zone developed quickly in the first 3 days. The distribution of the aftershocks illuminates numerous seismotectonic structures associated with the various branches of the North Anatolian Fault (NAF) in the aftershock zone. The seismicity pattern defines a rupture plane that extends about 80 km east and 75 km west of the mainshock epicenter, confirming bilateral rupture during the mainshock. The western aftershock activity is almost entirely located offshore, extending along the axis of the Izmit Bay. We show that the westernmost extent of the mainshock rupture reached the offshore region, just 10 km south of the Princes Islands. The dominance of strike-slip motion is one of the main characteristics of the Izmit sequence. A number of significant aftershock clusters occur off the inferred mainshock rupture plane. The spatial and temporal occurrence of these clusters show that they are triggered by the complex redistribution of stresses and dynamic strains imposed by the dislocation of the mainshock. The focal mechanisms of the events located in the western end of the rupture zone show both strike-slip and normal-faulting mechanisms. We infer that the Yalova cluster was triggered by the mainshock. The dominance of the normal faulting mechanisms supports the notion that dynamic extensional strains played a major role in triggering this cluster. Four events located off the Princes Islands indicate similar source mechanisms characterized by right-lateral strike-slip motion parallel to the strike of the Northern Boundary Fault (NBF) and local trend of aftershock distribution. This observation leads to a first order seismotectonic evidence that the nature of the NBF, which is known to be a transtensional feature at the surface, is dominated by strike-slip motion at depth. The coexistence of strike slip and extensional seismotectonic features in the Sea of Marmara highlights the importance of strain partitioning amongst shallow transtentional structures accommodating north-south extension and deeper structures associated with the NAF accommodating east-west right-lateral shear. Manuscript received 22 September 2000.

Aftershock locations and rupture characteristics of the 1995 Mariana Deep Earthquake

The August 23, 1995 (Mw=7.0, depth 586 km) Mariana event produced one of the strongest deep earthquake aftershock sequences known. We combined teleseismic arrival times with arrival times picked from a local network operating directly above the event in the Mariana Islands, and relocated the main shock and aftershocks. We derived the source characteristics from teleseismic body waves by inverting waveform data of the Global Seismograph Network. The main shock ruptured downward across the entire width of the active slab along a plane dipping southwestward. Most of the 17 well‐located aftershocks occurred along or very near the rupture plane. The different focal mechanisms of the main shock and aftershocks indicate that the stress field varies across the width of the Mariana slab at the area of the August 1995 sequence.

Coseismic interferograms of two MS=6.6 earthquakes in the South Iceland Seismic Zone, June 2000

We present InSAR observations of deformation due to two MS=6.6 earthquakes in the South Iceland Seismic Zone in June 2000. Coseismic deformation predominates in the ERS interferograms. Range change, due to mainly right‐lateral strike‐slip on N‐S striking faults, amounting to more than 15 cm is observed, although displacement is mainly perpendicular to the satellite look direction. Using elastic dislocation models in a trial‐and‐error scheme, we find a best‐fitting model that agrees well with the aftershock locations and moment magnitudes estimated from seismograms. The June 17 model has a fault patch 16 km long, 10 km deep, striking N05°E, dipping 86°, with a slip maximum of 2.40 m. The June 21 model has a vertical patch 15 km long, 9 km deep, striking N01°W, with a slip maximum of 2.15 m.

Considering the third dimension in stress‐triggering of aftershocks: 1993 Klamath Falls, Oregon, Earthquake Sequence

Dip slip on a fault of finite dimension produces a static stress change that varies considerably in 3‐D. The 1993 Klamath Falls (Oregon, USA) earthquake sequence shows a shallow, off‐fault cluster of aftershocks with mixed dip‐slip and strike‐slip focal mechanisms. Modeled static stress change and location of aftershocks show a misleading correspondence when viewed in the customary 2‐D presentation. Full 3‐D analysis shows that shallow right‐lateral slip is consistent with the local stress perturbation following the dip‐slip main shocks.

Rupture processes of the 1999 August 17 Izmit and November 12 Düzce (Turkey) earthquakes

SUMMARY We derive the rupture history of the 1999 August 17 Izmit (Mw=7.4) and 1999 November 12 Duzce (Mw=7.1) earthquakes in Turkey from teleseismic body waves using broad-band data of the Global Seismograph Network, aftershock locations and mapped surface breaks. The centroid solutions indicate strike-slip mechanisms for both events. The Izmit earthquake was characterized by rupture propagating predominantly eastwards. It consisted of a main rupture lasting about 25 s followed within 1 min by two more events of Mw=6.9 and Mw=7.0. With the teleseismic data, we could not resolve the westward extent of rupture into the Marmara Sea. However, an upper bound of the seismic moment release west of the epicentre of the Izmit event is estimated to be 1.9×1019 N m. The Duzce earthquake lasted about 14 s and was characterized by a bilateral mode of rupture, in excellent agreement with mapped surface breaks and aftershock locations.

Seismic remote sensing for the earthquake source process and near‐source strong shaking: A case study of the October 16, 1999 Hector Mine earthquake

A new methodology to automatically determine the earthquake source process and near‐source strong ground motions using regional and local distance data was used to study the October 16, 1999 Hector Mine earthquake (MW7.1). Broadband displacement data were inverted for the seismic moment tensor and to resolve the fault plane ambiguity independent of aftershock location and surface faulting information. The identified NNW striking fault plane was used to invert for the distribution of fault slip over a planar surface. The slip model was used to predict near‐fault peak ground velocity, which was found to agree well with the TriNet ShakeMap, and also with near‐fault observations that were not available at the time of the initial analysis. Our method if automated is capable of providing near‐source strong ground motion information within 30 minutes of the earthquake origin time even in cases where there are few near‐fault recording instruments.