Mainshock Rupture Area Research Articles

The Mw 6.6 Gisborne earthquake of 2007

Gisborne city experienced recorded peak ground accelerations exceeding 0.25g for the third time since 1966 in the magnitude Mw 6.6 earthquake at 075516 UT (8:55 pm local time) on 20 December 2007. The earthquake was at a hypocentral distance of 64 km from Gisborne at a depth of 40 km, well within the mantle of the subducted slab of the Pacific plate as it dips beneath the North Island of New Zealand. At this location the plate interface is about 10-15 km deep. The main event was followed by sparse aftershocks consistent with a rupture of the subducted plate, with the largest aftershock of magnitude 4.6 occurring on December 22nd. The GeoNet website received 3,257 felt reports, with a strongest intensity of MM8 (heavily damaging) assigned to the main shock. The 122 strong motion records of this event show a clear regional directional variation in the wave propagation, as well as a distinct 2 Hz peak widely observed throughout the country. At a local scale, three sites in the Gisborne region recorded accelerations around 0.2g. Recordings in Gisborne city also revealed a predominant displacement direction, parallel to the main street where most of the damage occurred. Source studies from moment tensor solution, aftershock relocations, GPS and strong motion data showed that the earthquake occurred within the subducted plate on a 45 degree eastward dipping fault plane. The mainshock rupture area is about 10 km2 reaching a maximum slip of 2.6 m. The computed high stress drop value of 17 MPa is as expected for an intraslab event and consistent with observations of very energetic seismic waves as well as heavy structural damage. GPS data recorded by continuous GPS instruments have also shown that slow slip occurred for about three weeks after the main shock. The slow slip was triggered on the subduction interface, rather than on the same fault plane as the aftershocks. This is the first clear-cut case worldwide of triggered slow slip, although three non-triggered slow-slip events have occurred in the same region since 2002.

High-resolution aftershock observations in the source region of the 2004 mid-Niigata Prefecture Earthquake

Abstract We deployed an extremely dense temporal seismic network in the source region of the 2004 mid-Niigata Prefecture Earthquake (thrust fault) on October 23, 2004, Japan. The seismic network consisted of 145 temporary seismic stations within a 30 km squared and had been kept within approximately a month after the mainshock. High accurate hypocenters of 708 events were determined by inverting the arrival times using double-difference earthquake location algorithm. The aftershocks along the mainshock (M w = 6.6) and the largest aftershock (M w = 6.3) rupture zones are distributed on two 60° westward-dipping planes, located approximately 5 km apart. Conversely, the Oct. 27 aftershock (M w = 5.8) occurred on an eastward dipping plane with a dip angle of 25° that was conjugate to the mainshock fault plane. Most of aftershocks at both northeastern and southwestern edges occurred at shallow depths with eastward-dipping planes. Epicenters of aftershocks in the southwestern region are aligned along N15°E, and rotate approximately 20° counterclockwise from the strike of the mainshock fault. This rotation of the aftershock alignments coincides with the rotation of anticline axes in the southwestern area of the source region. Furthermore, distributions of station corrections for a one-dimensional velocity model suggest that the seismic velocity at the western side of the Muikamachi-fault is lower than that at the eastern side. It is also inferred that the velocity structures in the hangingwall vary along the fault strike. The average velocity in the mainshock rupture area is higher than the periphery in the hangingwall, especially compared with the southwestern side of the hypocenter.

Expansion of aftershock areas caused by propagating post-seismic sliding

SUMMARY It is known that the aftershock areas of large earthquakes often expand over time. This expansion indicates that the stress-increase associated with a main shock gradually propagates outward from the main shock rupture area. It is likely that this propagating stress-increase is caused by the propagation of post-seismic sliding, which has been detected for many large earthquakes from geodetic observations, mainly global positioning system (GPS). In the present study, I perform a numerical simulation of the expansion of the aftershock area caused by the propagation of post-seismic sliding. The model fault exists within an infinite elastic medium and is loaded at a constant displacement rate. The frictional stress on the fault obeys a laboratory-derived rate- and state-dependent friction law. Non-uniformity of the frictional constitutive parameters is introduced to the model fault plane to represent a large asperity for a large earthquake (main shock) and many possible nucleus sites for aftershocks around the main shock asperity. Negative values of A − B are assigned to areas of seismic slip for the main shock and aftershocks, while positive values of A − B are assigned to regions where aseismic sliding occurs. Here, A − B is defined by dτ ss/d ln V that describes the velocity (V ) dependence of steady-state friction stress τ ss. Seismic slip may be nucleated for negative A − B, and aseismic slip occurs for positive A − B. Slip histories on the model fault plane are simulated through numerical integration of the friction law under the condition of uniform shear loading of a constant velocity. The simulation can reproduce aftershocks triggered by stress-increases related to postseismic sliding and expansion of the aftershock area. The rate of expansion of the aftershock area decreases with increasing distance from the main shock asperity or increasing value of A − B in the velocity-strengthening region. This finding suggests that the A − B value for a plate interface can be estimated from aftershock data. The simulation indicates that the 7-day and 30-day aftershock areas are 14‐22 and 24‐39 per cent larger, respectively, than the 1-day aftershock area. These numbers are approximately consistent with observations from large earthquakes at subduction zones.

P-wave Velocity Structure of the Crust and Its Relationship to the Occurrence of the 1999 Izmit, Turkey, Earthquake and Aftershocks

We determined hypocenters and focal mechanism solutions of aftershocks, and three-dimensional P -wave velocity structure in and around the focal area of the 1999 Izmit earthquake ( M w 7.4). Aftershocks form a 170-km-long narrow zone trending in an east-west direction along the northern branch of the North Anatolian Fault Zone (NAFZ). They are not homogeneously distributed in the whole aftershock area, but consist of several clusters. There are three significant clusters in the aftershock activity: (1) near the hypocenter of the mainshock, (2) near 29.2° E in the Marmara Sea, and (3) east of 30.4° E. Focal mechanism solutions of aftershocks have various types. But they are similar to each other within each cluster. The P -wave velocity structure obtained has a distinct low-velocity area to the west of the mainshock hypocenter. The mainshock rupture area estimated by Yagi and Kikuchi (2000) lies outside this low-velocity area. There exists a high-velocity anomaly to the east of 30.4° E. This high-velocity area lies below the aftershock cluster distributed to the east of 30.4° E. This high-velocity anomaly extends to the shallower depth of the southern branch (Iznik-Mekece fault) of NAFZ. The Anatolian earthquake sequence that had migrated westward for the past 60 yr did not propagate into this southern branch. This suggests the possibility that the Anatolian earthquake sequence progressed to the west exploiting an area that might break more easily.

Frictional constraints on crustal faulting

We consider how variations in fault frictional properties affect the phenomenology of earthquake faulting. In particular, we propose that lateral variations in fault friction produce the marked heterogeneity of slip observed in large earthquakes. We model these variations using a rate‐ and state‐dependent friction law, where we differentiate velocity‐weakening behavior into two fields: the strong seismic field is very velocity weakening and the weak seismic field is slightly velocity weakening. Similarly, we differentiate velocity‐strengthening behavior into two fields: the compliant field is slightly velocity strengthening and the viscous field is very velocity strengthening. The strong seismic field comprises the seismic slip concentrations, or asperities. The two “intermediate” fields, weak seismic and compliant, have frictional velocity dependences that are close to velocity neutral: these fields modulate both the tectonic loading and the dynamic rupture process. During the interseismic period, the weak seismic and compliant regions slip aseismically, while the strong seismic regions remain locked, evolving into stress concentrations that fail only in main shocks. The weak seismic areas exhibit most of the interseismic activity and aftershocks but can also creep seismically. This “mixed” frictional behavior can be obtained from a sufficiently heterogeneous distribution of the critical slip distance. The model also provides a mechanism for rupture arrest: dynamic rupture fronts decelerate as they penetrate into unloaded complaint or weak seismic areas, producing broad areas of accelerated afterslip. Aftershocks occur on both the weak seismic and compliant areas around a fault, but most of the stress is diffused through aseismic slip. Rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area by reloading weak fault areas that slipped in the main shock and then healed. We test this frictional model by comparing the seismicity and the coseismic slip for the 1966 Parkfield, 1979 Coyote Lake, and 1984 Morgan Hill earthquakes. The interevent seismicity and aftershocks appear to occur on fault areas outside the regions of significant slip: these regions are interpreted as either weak seismic or compliant, depending on whether or not they manifest interevent seismicity.

Active thrust front of the Greater Caucasus: The April 29, 1991, Racha earthquake sequence and its tectonic implications

Although fault‐bounded thrust sheets are common in the geological record, seismic evidence for their motion is sparse. The April 29, 1991, Racha earthquake (Ms = 7.0), the largest instrumentally recorded earthquake in the Greater Caucasus, is one of the largest recent earthquakes in continental thrust belts and provides evidence on mechanisms of thrust sheet motion. Using data from a deployment of Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) digital seismographs and various other instruments, we locate 1952 aftershocks occurring between May 7 and June 30, 1991. The aftershocks form a zone ∼70 km long and 10–25 km wide striking E‐W, following the Racha ridge at the southern boundary of the Greater Caucasus thrust system. Teleseismic body waves are inverted for source parameters of the mainshock and the two largest aftershocks. The solutions show thrust faulting with centroid depths of 3–10 km, comparable to depths of locally recorded aftershocks (∼2–12 km). The shallow‐dipping nodal plane, the aftershock distribution, and surface geology demonstrate that the main event was caused by faulting on a thrust system dipping NNE at 20°–31° bounding the southern slope of the Greater Caucasus. This fault system thrusts the Greater Caucasus structures south over the Dzhirula basement massif. The inferred fault geometry suggests that the active fault is either a detachment between sediments and Dzhirula basement or cuts through the basement at shallow depths. The 1500‐m‐high Racha ridge overlies the aftershock zone and is a likely consequence of repeated similar earthquakes. Hence the 1991 earthquake sequence shows that the western Greater Caucasus is accommodating plate convergence at a rate possibly comparable to the eastern Greater Caucasus (a few millimeters per year). Along‐strike geological discontinuities above and below the thrust surface correspond to the eastern end of the mainshock rupture area. No strong evidence for transfer structures could be found along strike, suggesting that differences in collisional style between the western and eastern Greater Caucasus may reflect differences in mechanical properties rather than differences in convergence rate. A June 15, 1991, event and its aftershocks, southeast of the primary aftershock zone along strike, show fault planes and slip vectors rotated ∼41° clockwise from the mainshock. This rotation is consistent with an along‐strike change in direction of the thrust front, near 44°E longitude, and demonstrates strong local structural or topographic control on slip direction. The rotation requires along‐strike shortening within the Greater Caucasus thrust system at a rate comparable to the rate of thrusting.

The effect of lateral variations of friction on crustal faulting

We propose that lateral variations in fault friction control the heterogeneity of slip observed in large earthquakes, We model these variations using a rate and state-dependent friction law, where we differentiate velocity-weakening into strong and weak-seismic fields, and velocity-strengthening into compliant and viscous fields. The strong-seismic field comprises the seismic slip concentrations, or asperities. The two «intermediate» frictional fields, weak-seismic and compliant, modulate both the tectonic loading and the dynamic rupture process. During the interseismic period, the compliant and viscous regions slip aseismically while the strong-seismic regions remain locked, evolving into stress concentrations that fail only in main shocks. The weak-seismic regions contain most of the interseismic activity and aftershocks, but also «creep seismically», that is, most of the weak-seismic area slips aseismically, actuating the seismicity on the remaining area. This «mixed» frictional behavior can be obtained from a sufficiently heterogenous distribution for the critical slip distance. The interseismic slip provides an inherent rupture resistance: dynamic rupture fronts decelerate as they penetrate into these unloaded compliant or creeping weak-seismic areas, diffusing into broad areas of accelerated afterslip. Aftershocks occur in both the weak-seismic and compliant areas around the fault, but most of the stress is diffused through aseismic slip. Rapid afterslip on these peripheral areas can also produce aftershocks within the main shock rupture area, by reloading weak fault areas that slipped in the main shock and then healed. We test this frictional model by comparing the interevent seismicity and aftershocks to the coseismic slip distribution for the 1966 Parkfield, 1979 Coyote Lake, and 1984 Morgan Hill earthquakes.

A deep earthquake aftershock sequence and implications for the rupture mechanism of deep earthquakes

A distinguishing characteristic of deep earthquakes has been the absence of observable aftershock sequences1,2. Here we report the first extensive deep-earthquake aftershock sequence to be observed; it was recorded by an array of eight broadband seismographs following the 9 March 1994 deep Tonga earthquake. The aftershocks show a power-law decay with time following the main shock, as is typical of shallow events. Most of the well located aftershocks are concentrated along a steeply dipping plane consistent with one of the nodal planes of the main-shock mechanism and the mechanisms of three large aftershocks. Assuming these aftershocks denote the main-shock rupture area, they define a 50 × 65 km fault plane extending across the entire width of the active seismic zone and into the surrounding aseismic region. The width of the aftershock zone is wider than the expected width of the metastable olivine wedge, demonstrating that either the width of the metastable olivine material exceeds previous estimates, or the aftershocks are not confined in such a wedge.

Coseismic stress changes induced by the 1989 Loma Prieta, California Earthquake

Earthquake focal mechanisms from before and after the 1989 Loma Prieta, California earthquake are used to infer the coseismic stress change. Before the main shock, most earthquakes correspond to right lateral slip on planes sub‐parallel to the San Andreas fault, and imply a generally N‐S most compressional stress axis and a vertical intermediate stress axis. Aftershocks within the main shock rupture zone, however, display almost every style and orientation of faulting, implying an extremely heterogeneous stress field. This suggests that the main shock relieved most, if not all, of the shear stress acting on its fault plane. Aftershocks that lie on the perimeter of the rupture agree with spatially uniform stress states, but only when considered in three groups: north, south, and above the main shock rupture area. In each of these areas the stress state may reflect stress transfer by the main shock.

The Whittier Narrows, California Earthquake of October 1, 1987—Seismology

The October 1, 1987 Whittier Narrows earthquake ( ML = 5.9) was located at 34° 3.0′N, 118° 4.8′W, at the northwestern end of the Puente Hills. The sequence ruptured a small part, 4 km by 5 km, of a previously unidentified, buried, thrust fault that strikes east-west and dips 25° down to the north. This fault may be part of a large system of thrust faults extending across the entire east-west length of the northern margin of the Los Angeles basin. The focus of the mainshock is deep, at 14 ± 1 km. The largest aftershock ( ML = 5.3) produced mostly strike-slip movement on a steeply dipping, northwest plane, that bounds the mainshock rupture area to the west. Enhancement of the Los Angeles basin seismic network would facilitate investigation of the potential of these faults for moderate-sized or large earthquakes.