Large Moment Release Research Articles

A Source Study of the Mw 7.0 Acapulco, Mexico, Earthquake of 8 September 2021

AbstractThe Acapulco earthquake of 2021 broke a segment of the southeast Guerrero seismic gap along the Mexican subduction thrust. The rupture initiated offshore Acapulco (16.770° N, 99.942° W) and propagated down-dip toward northeast. This source directivity is confirmed from both (1) an analysis of local and regional recordings as a function of azimuth and (2) kinematic inversion of near-source, band-pass filtered (0.025–0.5 Hz) displacement seismograms and Global Positioning System static coseismic displacement vectors. The inversion reveals little slip near the hypocenter (<0.5 m) and significant slip distributed over an area of ∼184 km2, with the large slip patches in the northeast part of the fault. The estimated average slip and static stress drop are 260 cm and 18.6 MPa, respectively. Moment rate function reported by National Earthquake Information Center–U.S. Geological Survey from finite-fault modeling is simple, and it resembles other Mexican subduction earthquakes in the 7.0 ≤ M ≤ 7.5 range. Moment rate spectrum is well fit by the Brune ω−2 source model. Radiated seismic energy from teleseismic P waves is 7.5×1014 J, and ER/M0 is 2.1×10−5. Radiated energy enhancement factor—a measure of source complexity—is small, 5.8, similar to other Mexican subduction thrust earthquakes. Seismograms at DeBilt of the 2021 and the 11 May 1962 Acapulco earthquakes show an extraordinary similarity, seldom seen at M 7.0 level. The 2021 earthquake seems a repeat of the 1962 earthquake. The slip deficit since 1962 corresponding to a plate convergence rate of 6.2 cm/yr and perfect coupling is 366 cm. Thus, the seismic slip of 260 cm during that 2021 earthquake suggests a coupling ratio of 0.7, greater than 0.3 and 0.5 reported from geodetic measurements. Large moment release in the southeast seismic gap appears to have a periodicity of ~60 yr. Because 60 yr have elapsed since the last sequence earthquakes (1957 MS 7.5; 1962 MS 7.0 and 6.8), a renewal of large earthquakes in the region may be expected.

An Integrated Multiscale Method for the Characterisation of Active Faults in Offshore Areas. The Case of Sant’Eufemia Gulf (Offshore Calabria, Italy)

Diagnostic morphological features (e.g., rectilinear seafloor scarps) and lateral offsets of the Upper Quaternary deposits are used to infer active faults in offshore areas. Although they deform a significant seafloor region, the active faults are not necessarily capable of producing large earthquakes as they correspond to shallow structures formed in response to local stresses. We present a multiscale approach to reconstruct the structural pattern in offshore areas and distinguish between shallow, non-seismogenic, active faults, and deep blind faults, potentially associated with large seismic moment release. The approach is based on the interpretation of marine seismic reflection data and quantitative morphometric analysis of multibeam bathymetry, and tested on the Sant’Eufemia Gulf (southeastern Tyrrhenian Sea). Data highlights the occurrence of three major tectonic events since the Late Miocene. The first extensional or transtensional phase occurred during the Late Miocene. Since the Early Pliocene, a right-lateral transpressional tectonic event caused the positive inversion of deep (>3 km) tectonic features, and the formation of NE-SW faults in the central sector of the gulf. Also, NNE-SSW to NE-SW trending anticlines (e.g., Maida Ridge) developed in the eastern part of the area. Since the Early Pleistocene (Calabrian), shallow (<1.5 km) NNE-SSW oriented structures formed in a left-lateral transtensional regime. The new results integrated with previous literature indicates that the Late Miocene to Recent transpressional/transtensional structures developed in an ∼E-W oriented main displacement zone that extends from the Sant’Eufemia Gulf to the Squillace Basin (Ionian offshore), and likely represents the upper plate response to a tear fault of the lower plate. The quantitative morphometric analysis of the study area and the bathymetric analysis of the Angitola Canyon indicate that NNE-SSW to NE-SW trending anticlines were negatively reactivated during the last tectonic phase. We also suggest that the deep structure below the Maida Ridge may correspond to the seismogenic source of the large magnitude earthquake that struck the western Calabrian region in 1905. The multiscale approach contributes to understanding the tectonic imprint of active faults from different hierarchical orders and the geometry of seismogenic faults developed in a lithospheric strike-slip zone orthogonal to the Calabrian Arc.

Characterizing Near‐Field Surface Deformation in the 1990 Rudbar Earthquake (Iran) Using Optical Image Correlation

AbstractThe 1990 Rudbar earthquake (Mw 7.3) broke along three right‐stepping segments of the left‐lateral Rudbar fault, in the western Alborz mountains (Iran), producing ∼80 km of surface rupture. Previously reported horizontal (∼60 cm) field offsets were surprisingly low, and the vertical (∼95 cm) unexpected, given the relatively large moment release, shallow source depth, vertical fault geometry, and left‐lateral rake. We characterize the surface displacement from optical image correlation of satellite and aerial images. A maximum displacement of ∼6.5 m is observed in the eastern part of the rupture consistent with both a sharp pulse in the moment release rate ∼10 s after the rupture initiation, and the macroseismic isoseismal curves. We find ∼2.2 m mean surface slip, larger than field measurements, but consistent with empirical scaling laws based upon the seismological moment release. Estimates of fault zone width based on correlation of 1 m resolution aerial photos (and corrected for smoothing bias) are <200 m; these values increase artificially with lower resolution imagery, which is more affected by smoothing bias. The large mismatch between field and correlation displacements suggests the Rudbar earthquake probably featured significant off‐fault deformation, consistent with its low structural maturity. Finally, we document a surface rupture along a structural continuation of the Kabateh segment, east of the Sefid Rud river. Slip along this segment (1–2 m), is less than the neighboring Zard Geli segment (1.5–6 m). The two faults converge at Sefid Rud, which appears to represent a major structural feature controlling fault segmentation in this region.

The Role of Fluid Pressure‐Induced Aseismic Slip in Earthquake Cycle Modulation

AbstractThe evolving state of fault stress during and after the perturbation of fluid pressure gives rise to an intriguing interplay of seismic and aseismic slip on the fault. A better understanding of the possible role of fluids in the triggering mechanism of seismicity is pivotal to effective seismic hazard mitigation, particularly in the context of induced seismicity. Through numerical modeling, we investigate the effect of pore pressure perturbations on the spatio‐temporal evolution of fault slip and the modulation of earthquake cycles. Pressure perturbations are imposed at different magnitudes and different times during a selected interseismic period. Results show a wide range of aseismic responses which can lead to both time advancement and delay of subsequent earthquakes. Specifically, even pressure perturbation <5% of the average event stress drop can trigger aseismic slip that leads to considerable time delay in the next earthquake even when perturbation occurs late in the interseismic period. We find that earthquakes that are delayed in time are associated with large aseismic moment release. Our study highlights the importance of close monitoring of aseismic fault slip in regions prone to the influence of pore fluids and provides physical insights into identifying critical aseismic responses associated with certain triggering outcomes.

Alaska megathrust 1: Seismicity 43 years after the great 1964 Alaska megathrust earthquake

AbstractThe largest moment release during the 1964 Mw 9.2 Alaska earthquake was on the portion of the megathrust under the eastern Kenai Peninsula and the Prince William Sound. The area is currently locked geodetically and corresponds to where the Yakutat terrane is subducting. In 2006–2009, a seismic array consisting of 34 broadband seismometers was deployed in the region. An automatic algorithm was used to detect 12,563 local earthquakes using 13 months of data from the experiment. Of these, 9427 good quality earthquakes could be relocated in a joint inversion for hypocenters and velocity structure. They were then relocated by double difference to generate a final catalog of 8308 hypocenters. These microearthquakes delineate a deeper steeply dipping Wadati‐Benioff zone contiguous with an 11 km wide seismic zone dipping at 3° between 20 km and 40 km depth along the 1964 earthquake's rupture zone. Focal mechanisms do not show interplate thrust faulting events, but mostly normal faulting events with T axes generally parallel the slab dip direction, indicating them to be intraslab seismicity. The shallow narrow band of seismicity lies within the subducting Yakutat terrane and right below the thrust zone. Possibly, thrust faulting has not yet resumed yet in this early phase of the earthquake cycle. Thick subducted sediments overlying the Yakutat terrane could also form a large strong contact zone on a relatively smooth plate boundary, which does not favor seismic sliding on small patches but ruptures homogenously in great earthquakes.

Detailed stress fields in the focal region of the 2011 off the Pacific coast of Tohoku Earthquake—Implication for the distribution of moment release—

We have examined the focal mechanism distributions in the source region of the 2011 off the Pacific coast of Tohoku Earthquake, before and after the mainshock, using F-net data, and then performed a stress tensor inversion by using the multiple inverse method. It is found that earthquakes were mainly reverse fault type events all over the source region before the main shock, whereas, after the main shock, many normal fault type events occurred in the hanging wall but focal mechanisms did not change in the deepest part in the source region. It is inferred from the stress tensor inversion that the stress state is a reverse fault type in the deepest part before, and after, the mainshock but that, in the central part, the stress state changed from reverse fault type before the mainshock to normal fault type after the mainshock. These results suggest that the absolute shear stress on the plate boundary fault in this region was almost completely released by the mainshock. Furthermore, we examined, in detail, the focal mechanism distributions and stress states in the shallower part near the trench axis off the Miyagi Prefecture and found that strike-slip events with the P-axis in the NS direction occurred in the footwall east of the mainshock hypocenter. This finding can be well explained by a slip model with a large moment release near the hypocenter. Our analysis constrains the spatial distribution of the moment release of the 2011 off the Pacific coast of Tohoku Earthquake around the mainshock hypocenter rather than in the vicinity of the trench axis.

Fault zone properties affecting the rupture evolution of the 2009 (Mw6.1) L'Aquila earthquake (central Italy): Insights from seismic tomography

[1] We have inverted P- and S-wave travel times from seismograms recorded by a dense local network to infer the velocity structure in the crustal volume where the April 6th 2009 main shock nucleated. The goal is to image local variations of P-wave velocity and Poisson ratio along the main shock fault zone for interpreting the complexity of the rupture history. The initial stages of the mainshock rupture are characterized by an emergent phase (EP) followed by an impulsive phase (IP) 0.87 s later. The EP phase is located in a very high VP and relatively low Poisson ratio (ν) region. The IP phase marks the beginning of the large moment release and is located outside the low ν volume. The comparison between the spatial variations of VP and Poisson ratio within the main shock nucleation volume inferred in this study with the rupture history imaged by inverting geophysical data allows us to interpret the delayed along-strike propagation in terms of heterogeneity of lithology and material properties.

Modeling the rupture process of the 2003 September 25 Tokachi‐Oki (Hokkaido) earthquake using 1‐Hz GPS data

High‐rate GPS has the potential to recover both dynamic and static displacements accurately. We analyze 1‐Hz GPS data recorded during the 2003 Tokachi‐Oki earthquake. The 1‐Hz GPS displacement waveforms show good agreement with integrated accelerometer records except for low frequency noise that are inherently present in integrated seismic records. The GPS waveforms were inverted to model the spatio‐temporal evolution of the fault slip during the rupture. The slip is found to propagate downdip in the subduction zone with largest moment release ∼50 km northwest of the hypocenter. The region of largest slip agrees in general with traditional seismic studies, indicating that 1‐Hz GPS can be used for finite fault studies. The 1‐Hz GPS slip model shows clearer contrast with afterslip distributions than those inferred from strong motion data, possibly because 1‐Hz GPS is more sensitive to cumulative slip distribution.

Repeating earthquakes and interplate aseismic slip in the northeastern Japan subduction zone

On the basis of a waveform similarity analysis, we detected 321 earthquake clusters with very similar (cross‐correlation coefficient >0.95) waveforms on the plate boundary in the northeastern Japan subduction zone. Most of them were not found within the subducting Pacific plate with a few exceptions. Moreover, even on the plate boundary, they were not located in the large moment release areas of large interplate earthquakes that occurred recently or in the areas where the plates are inferred to be strongly coupled from GPS data analyses. These observations suggest that these similar earthquakes are caused by repeating slips of small asperities with a dimension of around 0.1 to 1 km surrounded by stable sliding areas on the plate boundary. If the aseismic slip portion in these small asperities is negligible, we can estimate the cumulative amount of aseismic slip in the area surrounding each asperity. In other words, repeating earthquake data potentially can be used to estimate the spatiotemporal aseismic slip distribution on the plate boundary. We estimated the spatial distribution of slip rate on the plate boundary from repeating earthquake data. The scaling relation between seismic moment and seismic slip by Nadeau and Johnson [1998] is used for the estimation of the slip amount by each repeating earthquake cluster. Obtained spatial distribution is consistent with that estimated from GPS data on land.

Static and dynamic displacement near a fault with the discrete wavenumber method

When we represent seismic waves by standard wavenumber decomposition schemes such as the discrete wavenumber method, there is ambiguity in the choice of one parameter, the truncate number of the wavenumber integration. Although there is a conventional truncation number to obtain accurate seismograms, few studies have paid attention to seismograms including static displacement (i.e. ω=0). We estimate this parameter in order to accurately calculate not only body waves like P- and S-wave but also static displacement, by overcoming the poor convergence of integration over horizontal wavenumbers. In order to find a suitable value of the truncation number of wavenumbers, we utilize wavenumber-domain solutions from spatial-domain solutions for a homogeneous half space, using the analytic formulation of Okada [Bull. Seismol. Soc. Am. 75 (1985) 1135–1154] and estimate a region where spectral energy is concentrated. We shall identify the range in which the energy is reduced into 1/100 of the maximum amplitude in k x and k y , and define the larger of k x and k y in this estimation as the truncation number of wavenumbers, k max. For a finite rectangular fault buried in a half space, we find that a suitable value of the truncation number of wavenumber k max is 4 km −1 by comparing the analytical solution of Okada for static deformation. If there is large moment release near the surface (e.g. artificial source such as explosion of dynamites), the convergence over horizontal wavenumbers becomes very poor not only for static but also for dynamic motions. Our results show that 4 km −1 is a reasonable value when a fault extend to several kilometers under the surface even if the moment release at each point on the fault surface is constant. In a realistic case, earthquake source has large moment release at several kilometers under the surface, so we can obtain surface displacement accurately, including static displacement in any realistic cases, with the above value of k max.

Source Characteristics of the 12 November 1996 M w 7.7 Peru Subduction Zone Earthquake

The 12 November 1996 M w 7.7 Peru subduction zone earthquake occurred off the coast of southern Peru, near the intersection of the South American trench and the highest topographical point of the subducting Nazca Ridge. We model the broadband teleseismic P-waveforms from stations in the Global Seismic Network to constrain the source characteristics of this subduction zone earthquake. We have analyzed the vertical component P-waves for this earthquake to constrain the depth, source complexity, seismic moment and rupture characteristics. The seismic moment determined from the nondiffracted P-waves is 3–5 ∞ 1020N·m, corresponding to a moment magnitude M w of 7.6–7.7. The source time function for the 1996 Peru event has three pulses of seismic moment release with a total duration of approximately 45–50 seconds. The largest moment release occurs at approximately 35–40 seconds and is located ~90km southeast of the rupture initiation. Approximately 70% of the seismic moment was released in the third pulse.

A note on the depth recurrence and strain release of large Vrancea earthquakes

The large 1940 Vrancea event with body wave magnitude m B = 7.3 was investigated in terms of the depth of rupture initiation and extent. It was found that the rupture started at 150 km depth and propagated downwards. The ruptured zones of the 1977, 1986 and 1990 Vrancea events were estimated from the distribution of aftershocks located with the joint hypocentre determination method. Their depth ranges are 90–110 km, 130–150 km and 70–90 km, respectively. Because the body wave magnitudes m b were saturated for the last three events and the surface wave magnitudes M S were underestimated (not being corrected for depth) we chose the moment magnitudes to quantify the four events: 7.7 (1940), 7.4 (1977), 7.1 (1986) and 6.9 (1990), respectively. The depth interval 110–130 km is found to be unbroken since at least 1838 or 1802 and therefore is expected to accommodate the next large moment release ( M w = 7.0 to 7.4). The strain in the vertical and in the SE-NW directions is about 0.0003, giving a moderate strain rate of about 10 −13 s −1. A correlation between the depth interval of the large events and the year of occurrence in a century is suggested, offering a physical basis to the observed quasi-cyclicity.

Segmentation and Holocene surface faulting on the Median Tectonic Line, southwest Japan

The Median Tectonic Line (MTL) active fault system is a 300‐km‐long, arc‐parallel, right‐lateral strike‐slip fault related to oblique subduction of the Philippine Sea plate beneath the Eurasian plate at the Nankai trough, southwest Japan. The fault on Shikoku Island has a slip rate of 5–10 mm/yr, one of the highest in onshore Japan. We identified 12 geometric segments along the MTL separated by discontinuities such as en echelon steps, bends, changes in strike, and gaps in the surface trace. A chronology of latest Holocene surface faulting on the MTL was constructed based on previously published and new work including trenching, geomorphic mapping, and acoustic profiling and piston coring of the submarine extension of the fault zone. The MTL probably does not rupture along its entire length in a single earthquake but instead consists of multiple “earthquake segments” that rupture independently of one another. The recurrence interval and surficial offset for surface‐rupturing earthquakes at four individual sites on the MTL in Shikoku Island are 1000–3000 years and 5–8 m, respectively. The relatively long recurrence interval and large surficial offset per earthquake suggest large seismic moment release during surface‐rupturing earthquakes on the MTL, as commonly observed for Japanese moderate to large intraplate earthquakes. Because the predicted rupture lengths based on empirical relations between surface displacement and rupture length are considerably larger than those of individual segments, large earthquakes on the MTL probably rupture multiple geometrically defined segments. The fault was previously thought to be unruptured at least in the past 1000 years based on the absence of historical evidence for destructive earthquakes. However, trench excavations show that part of the MTL ruptured most recently during or after the 16th century A.D. with at least 6.9 ± 0.7 m of slip along the main fault trace. This rupture may be correlated to the 1596 Keicho‐Kinki earthquake, but further geological and historical investigations are needed to confirm this hypothesis.

Minute Locating of Faulting beneath Kobe and the Waveform Inversion of the Source Process during the 1995 Hyogo-ken Nanbu, Japan, Earthquake Using Strong Ground Motion Records.

We estimated the source process of the 1995 Hyogo-ken Nanbu (Kobe), Japan, earthquake based on: 1) locating buried fault planes in the Kobe area by examining particle motion at observation stations in the near-source area and 2) multi-time window linear waveform inversion of strong ground motion seismograms. S-wave particle motion diagrams created theoretically show reverse rotation in the horizontal plane at two stations located on opposite sides of the intersection of the earth's surface and the extension of the buried fault plane. We simulated ground motion at various locations surrounding the buried fault plane and compared their particle motions with observed records to obtain constraints on the location of the fault plane. At least two planes are needed for the rupture area northeast of the epicenter if the causative fault is assumed to be made up of a few perfectly planar structures, and the rupture extended at least 26 km northeast from the epicenter.Using a fault model consisting of three planes, two planes on the Kobe side and on the Awaji side, and assuming a step-over at the Akashi strait based on the aftershock distribution, we performed a waveform inversion. The main rupture extended about 45 km horizontally. Three regions had relatively large moment releases: 1) around the rupture starting point; 2) in the shallow (less than 10 km) part of the Nojima fault, which extends along the northwest shore of Awaji Island: and 3) deep (about 10 km) under Kobe City. Even though the second subevent on the Nojima fault had a large moment release, it did not generate pulsive waves, because its rise time was long. The two remarkable pulses seen in the seismograms in the Kobe area came from the first and third subevents.

Estimation of High-frequency Wave Radiation Areas on the Fault Plane of the 1995 Hyogo-ken Nanbu Earthquake by the Envelope Inversion of Acceleration Seismograms.

We estimated the high-frequency wave radiation process on the fault plane of the 1995 Hyogo-ken Nanbu earthquake (MJMA=7.2), which was a strike slip event along active inland faults, by the envelope inversion of strong-motion acceleration seismograms. The horizontal extent of the region that radiated high-frequency (2-10 Hz) waves is about 45 km, which matches the source region estimated from the inversion of strong-motion displacement waveforms. High-frequency waves were mainly radiated at the periphery of this source region, while a clear gap of radiation was seen in the center. Along the Nojima fault, both high- and low-frequency wave radiation were large. This is explained by the fact that the rupture broke the ground surface here. Highfrequency waves were also radiated at the step-over of faults between Awaji Island and the Kobe area. The step-over is interpreted to have behaved as a geometrical barrier and generated high-frequency waves when ruptured. The radiation of high-frequency waves in the northeastern part from the hypocenter to Kobe was small in the large moment release areas estimated from the displacement waveform inversion, and large at their peripheries. This suggests these high-frequency waves were stopping phases from the subevents corresponding to the large moment release areas. The extent of the source region of high-frequency waves roughly matches that of the heavily damaged areas. We inferred that the strong high-frequency waves that came directly from the source was one of the causes of the heavy damage to low-rise structures.

Rupture history of the Great Bolivian Earthquake: Slab interaction with the 660‐km discontinuity?

Teleseismic body waves of the great Bolivian earthquake of June 9, 1994 are analyzed to determine the focal parameters and rupture history. Broadband seismograms reveal a complex rupture process: A small initial event (Mw 7.2) was followed about 10 s later by a large moment release pulse of about 40 s duration. Focal mechanisms determined for the mainshock indicate normal faulting with one very shallow NE dipping plane. Azimuthal variation in body‐wave displacement pulse widths suggest northward rupture. From master event and body‐wave inversion, the main moment release is located 25–50 km NE of the initiation point at about 650‐km depth with only small depth variations between the initial and main event. This suggests that rupture was N directed on the near‐horizontal plane. Because the slab along other parts of the Andean arc at about 600‐km depth dips steeply, a sub‐horizontal plane may imply shearing perpendicular to slab dip. Downdip compression on a sub‐horizontal plane would then imply that the slab does not penetrate the 660‐km seismic discontinuity, but rather, is being sheared out to the NE. This interpretation is not unique as other scenarios are also possible. Such an event immediately above the 660‐km discontinuity suggests massive deformation above the discontinuity with no smooth slab penetration into the lower mantle.

The earthquake sequence of November 1987 and March 1988 in the Gulf of Alaska: A new insight

New information has been obtained by reanalyzing the rupture process of three large strike‐slip earthquakes that occurred in the Gulf of Alaska on November 17 and 30, 1987 and March 6, 1988. We used spectral analysis of P‐waves complemented with a simple technique for inverting body waves using a fixed source mechanism. Our results show that the Ms=7.0 event of November 17, which can be considered to be a foreshock to the November 30 event, consisted of two pulses of large seismic moment release: the first one with a duration of ∼15 sec coincides with those obtained by other studies; the second, which occurs 20 sec after the first arrival is clearly visible on the records, but was not previously reported. The Ms=7.4 earthquake of November 30 is a complex event that ruptured on a fault plane trending north‐south. The Ms=7.7 shock of March 6, 1988 was a simple event along the same north‐south trend south of the November events. P‐wave far field spectra exhibit an ω−1 intermediate trend between the usual ω0 and ω−2 slopes. This trend is due to rupture along a long, narrow fault, so the corner frequency associated with this intermediate trend is related to the reciprocal of rupture propagation time. We suggest that these events were due to a tear in the Pacific plate.

Ultramafic pseudotachylite from the Balmuccia peridotite, Ivrea-Verbano zone, northern Italy

This paper reports an occurrence of ultramafic pseudotachylite from a spinel Iherzolite mass in the Ivre-Verbano zone, northern Italy. The pseudotachylite occurs as discordant dikes and veins crosscutting primary structures of the peridotite and as thin (∼ 1 cm) layers in the middle of shear zones. The pseudotachylite has an ultramafic composition similar to the host peridotite. In thin section it consists of fine-grained euhedral olivine and clinopyroxene crystals densely packed in a glassy matrix with minor amounts of clastic olivine grains and lithic mylonitic clasts. Microstructural observations on the olivines using TEM yields evidence for deformation in the wall rock and olivine clasts at a differential stress level of ∼ 300 MPa. Petrological analysis of the host rock and pseudotachylite combined with an energy calculation for shear heating suggests that the pseudotachylite originated by a near total melting of the host peridotite followed by rapid crystallization on a time scale of 100 s or less, conditions consistent with an earthquake. The presence of an ultramafic pseudotachylite implies a very significant shear heating due presumably to a high shear stress on the fault plane. Faulting in an ultramafic rock associated with melting will result in a large stress drop and resultant large moment release, because resistance to sliding will almost completely be lost upon melting due to the very low viscosity of ultramafic melts.

From tomographic images to fault heterogeneities

Local Earthquake Tomography (LET) is a useful tool for imaging lateral heterogeneities in the upper crust. The pattern of P- and S-wave velocity anomalies, in relation to the seismicity distribution along active fault zones. can shed light on the existence of discrete seismogenic patches. Recent tomographic studies in well monitored seismic areas have shown that the regions with large seismic moment release generally correspond to high velocity zones (HVZ's). In this paper, we discuss the relationship between the seismogenic behavior of faults and the velocity structure of fault zones as inferred from seismic tomography. First, we review some recent tomographic studies in active strike-slip faults. We show examples from different segments of the San Andreas fault system (Parkfield, Loma Prieta), where detailed studies have been carried out in recent years. We also show two applications of LET to thrust faults (Coalinga, Friuli). Then, we focus on the Irpinia normal fault zone (South-Central Italy), where a Ms = 6.9 earthquake occurred in 1980 and many thousands of attershock travel time data are available. We find that earthquake hypocenters concentrate in HVZ's, whereas low velocity zones (LVZ’ s) appear to be relatively aseismic. The main HVZ's along which the mainshock rupture bas propagated may correspond to velocity weakening fault regions, whereas the LVZ's are probably related to weak materials undergoing stable slip (velocity strengthening). A correlation exists between this HVZ and the area with larger coseismic slip along the fault, according to both surface evidence (a fault scarp as high as 1 m) and strong ground motion waveform modeling. Smaller wave-length, low-velocity anomalies detected along the fault may be the expression of velocity strengthening sections, where aseismic slip occurs. According to our results, the rupture at the nucleation depth (~ 10-12 km) is continuous for the whole fault lenoth (~ 30 km), whereas at shallow depth, different fault segments are activated due to lateral heterogeneities in the sedimentary cover. This finding confirms that the rupture process is controlled by lithologic and structural discontinuities in the upper crust, and emphasizes the contribution that LET can make to the study of fault mechanics.

Rupture process and stress‐drop of the Great 1989 MacQuarie Ridge Earthquake

On May 23, 1989, a great (Mw=8.1) earthquake occurred in the Macquarie Ridge complex, south of New Zealand. The earthquake is one of the largest in the Macquarie Ridge complex this century, and represents the right‐lateral strike‐slip component of the motion between the Pacific plate and the Australian plate. Subduction initiation appears to be presently occurring in the Macquarie Ridge complex, and it has been suggested that plate boundary strike‐slip earthquakes in a transitional tectonic environment have a high stress‐drop. We have investigated the source rupture process of the 1989 earthquake, using teleseismic P and SH waves. The best point source depth is 12 km below the ocean bottom. The deconvolved source time function is dominated by a single pulse with a large moment release (1.8×1021 Nm) and a short duration (20 s). The moment rate increases slowly in the first 10 s of the rapture process, and is suddenly truncated at 20 s. There is no resolvable directivity to this sharp truncation, which given the temporal resolution of the data set, means that the spatial location of the sharp truncation is within approximately 50 km of the epicenter. The stress‐drop and average displacement may be as high as 370 bars and 36 m. These values are unusually high, and strongly support the suggestion that high stress‐drop earthquakes are characteristic for transitional tectonic environments.