Decadal Seismicity before Great Earthquakes—Strike-Slip Faults and Plate Interiors: Major Asperities and Low-Coupling Zones

Decadal forerunning seismic activity is examined for very large, shallow earthquakes along strike-slip and intraplate faults of the world. It includes forerunning shocks of magnitude Mw ≥ 5.0 for 21 mainshocks of Mw 7.5 to 8.6 from 1989 to 2020. Much forerunning activity occurred at what are interpreted to be smaller asperities along the peripheries of the rupture zones of great mainshocks at transform faults and subduction zones. Several great asperities as ascertained from forerunning activity agree with the areas of high seismic slip as determined by others using geodetic, mapping of surface faulting, and finite-source seismic modeling. The zones of high slip in many great earthquakes were nearly quiescent beforehand and are identified as the sites of great asperities. Asperities are strong, well-coupled portions of plate inter-faces. Different patterns of forerunning activity on time scales of up to 45 years are attributed to the sizes and spacing of asperities (or lack of). This permits at least some great asperities along transform faults to be mapped decades before they rupture in great shocks. Rupture zones of many great mainshocks along transform faults are bordered either along strike, at depth or regionally by zones of lower plate coupling including either fault creep forerunning activity, aftershocks and/or slow-slip events. Forerunning activity to transforms in continental areas is more widespread spatially than that adjacent to oceanic transforms. The parts of the San Andreas fault themselves that ruptured in great California earthquakes during 1812, precursory activities for various time scales should be sought on the peripheries of great asperities and not just along the major faults themselves. This paper compliments that on decadal forerunning activity to great and giant earthquakes along subduction zones.


Introduction
Great earthquakes have occurred at shallow depths along active transform faults and subduction zones and within the interiors of lithospheric plates. They are particularly important to understand since many have caused extensive destruction and loss of life. Much new seismic and geodetic information has become available for great earthquakes that occurred globally during the last several decades. Here I study 21 mainshocks of magnitude Mw 7.5 to 8.6 that occurred from 1989 to 2020 along 11 strike-slip faults, 8 plate interiors and two in Tibet that may be classified as occurring in either intraplate regions or along slow-moving block boundaries. Emphasis is given to the spatial and in some cases the temporal patterns of activity of moderate to large forerunning events that preceded them in the previous decades. The rupture zones and forerunners of three older earthquakes in California and Guatemala are also described. It is surprising how little has been written about decadal preceding events, which I call forerunning earthquakes to distinguish them from foreshocks of shorter-time duration. The first aim of this paper is simply to describe the spatial distribution of forerunning events to a large number of mainshocks.
A second aim is to use forerunning shocks to map great asperities that were clearly definable, as well as those that were not, in the years to decades before they subsequently ruptured in large, great and giant shocks. Understanding forerunning seismic activity and where it occurs with respect to both the centroids of slip in mainshocks and their regions of high displacements is emphasized. A third aim is to ascertain the implications of this work for risk reduction and shorter-term prediction of large earthquakes. This paper on strike-slip and intraplate mainshocks is a companion paper to [1] that examined decadal activity prior to mainshocks along subduction zones of the world.
Very large mainshocks and their forerunning events are described in terms of the rupture of asperities of various sizes, i.e., strong, well-coupled portions of plate interfaces. Some parts of plate boundaries consist of great asperities that are well coupled, i.e., largely locked, during the slow process of stress buildup to very large earthquakes. Other parts of plate boundaries, so called low-coupling zones (LCZ), often are identified as the sites of either fault creep, smaller asperities and moderate-size forerunning activity and aftershocks. Much forerunning activity as well as several slow-slip events described in the literature occurred in LCZ on the peripheries of great asperities. I describe forerunning cumulative seismic moment before one great strike-slip earthquake and how it changed with time.
In the past, most shorter-term precursory seismic activity was identified only after the occurrence of large earthquakes. Typically, it has been difficult to distinguish it from other seismic events that happen during the long periods of stress buildup to large earthquakes.
Mogi [2] described rupture in the lab of two quite different substances: homogeneous pine resin and three inhomogeneous rocks. The rupture of pine resin at high stress was not preceded by small forerunning seismic events whereas failures of those rocks were. The presence of inhomogeneities seems to be essential, at least in principle, to the occurrences of forerunning, precursory and aftershock activities not only in the lab but also at larger scales at plate boundaries.
More study of the distribution of strong and weak regions along and near major faults is needed not only to understand the physics of earthquakes but also whether long-term earthquake prediction is either possible or unlikely for specific faults. Strong regions that are more difficult to break are called asperities in the rock mechanics and seismological literatures. I use the terms asperities and plate coupling extensively. Some parts of plate boundaries remain locked, i.e., well-coupled during stress accumulation; others are sites of poorer coupling, moderate-size shocks, slow earthquakes, and fault creep. Some information about the physical and geological factors that govern strong and weak asperities is examined here briefly.
Others have analyzed the detailed distribution of large displacement (high slip) over the main rupture zones for many of the mainshocks examined here. They used seismic observations, data from the Geodetic Positioning System (GPS) and observations of surface faulting. They found that areas of high displacements were typically smaller than the sizes of aftershock zones.
A main contribution here is to map the distributions of forerunning strike-slip and intraplate earthquakes and to compare them with areas of high slip as computed by others. Most previous work has focused on individual large earthquakes and not on what can be ascertained by examining many of them worldwide. Relatively little attention has been paid to either the depths of forerunning decadal activity or if locations change with time.
Most very large earthquakes have occurred where plate coupling is high as reported in the literature. Rupture zones of great earthquakes are often bordered by zones of poorer plate coupling either along strike, downdip, updip or adjacent to them along nearby faults. Some of the clearest examples of forerunning activity to large earthquakes are sought with the hope that better knowledge gained from them will permit less well-defined cases to be interpreted better.
How can great asperities be identified beforehand if they do not rupture until the time of a great or giant shock? I find that earthquakes of moderate to major size in the decades before very large events mostly occurred near the peripheries International Journal of Geosciences of great asperities that later broke in great mainshocks. Shocks of moderate to major size can be used to map great asperities that are mostly quiet seismically before they rupture in mainshocks up to decades later. Precursory seismic and geodetic changes to great events should be sought on the peripheries of great asperities and not just along great faults themselves. Some claims that large earthquakes do not have precursors result from sampling only the faults that ruptured in great events. Many were looking in the wrong places.
This study builds upon previous work on great earthquakes, seismic gaps, forerunning activity and aftershocks by Fedotov [3], Mogi [4] [5], Sykes [6], Kelleher et al. [7] and others. Seismic gaps are segments of active plate boundaries that have not been the sites of large earthquakes for decades to hundreds of years. Those early studies typically used the extent of aftershock zones to map the rupture zones of great earthquakes. They did not have access to either GPS data, very long-period centroid locations of earthquakes, information on slow-slip events or finite-fault slip computations of sub-areas of high slip. Forerunning earthquakes of magnitudes as small as 5.0 are used in this paper. This permits many smaller events to be used to map the approximate rupture zones of many coming individual great mainshocks up to decades ahead of time.
One of the major findings of this paper is that the spatial distribution of forerunning earthquakes differs between oceanic and continental transform faults.
Forerunning activity was found to be very concentrated for oceanic transforms and more widespread near continental transform faults. Little to no forerunning activity on a time scale of a few decades was found for the intraplate earthquakes studied. This is attributed to the need to include much longer periods of analysis for intraplate areas where strain buildup likely is slower than for most active plate boundaries.
The distributions of forerunning activity, great and moderate-size asperities, differences in plate coupling, and velocity strengthening and weakening materials can be considered as the building blocks or the architecture of seismic activity along and near strike-slip faults and subduction zones. Some may be useful for more detailed predictions or forecasts of large earthquakes. Older locations and magnitudes are from the Bulletin of the International Seismological Centre and the 1992 catalog of Pacheco and Sykes [11]. The computer program GeoMapApp [12] was used for bathymetry and topography. where rupture initiated, the epicenter as determined from short-period data, is plotted as well for mainshocks.

Methods
After the start of the GCMT database in 1976, I needed to have many years of forerunning activity for an adequate analysis of a subsequent large shocks. Hence, the mainshocks studied here were limited to the period 1989 to August 2020 (Table 1). Results for three older great earthquakes are included as well.

Asperities, Earthquakes and Low-Coupling Segments
The greatest earthquakes along strike-slip faults, which are taken here to break the largest asperities, typically occur at depths of a few to 15 Figure 1 illustrates some of the patterns of forerunning activity before several large strike-slip earthquakes within continents, in this case numerous prior shocks in a broad area near the coming rupture zone of the great 1906 San Francisco earthquake. Surrounding activity was then low from 1920 until about 1954. Most of the 1906 rupture zone along the San Andreas fault itself was quiet in the decades prior to 1906; forerunning activity occurred along other faults including strike-slip faults in the San Francisco Bay area. The four nearby dots northeast of San Francisco Bay denote some of the largest prior shocks. They occurred on or near mapped thrust faults [16]. Plate motion in the area includes a modest component of convergence in addition to strike-slip faulting. Those two types of faulting are partitioned spatially.  [17] from geodetic data and seismic-wave modeling.

A Model of Slip and Asperities-The San Andreas Fault, California
Rupture in the mainshock initiated near the northeastern end of the zone of largest moment release at the short-period epicenter. Rupture progressed southwesterly [17]. It was followed by a smaller release of moment [17], probably in the smaller zone of slip along the northeastern end of surface rupture.
Most of the forerunning activity and many aftershocks of Mw > 5.0 were situated to the northeast of the two main zones of displacements in the mainshock, i.e., well off the rupture zones. Some of that activity may have occurred on sub-parallel faults to the one that broke in 2013. Most of the rupture zone can be considered as a great asperity along its southeastern end and a smaller asperity along its northeastern end. Each remained nearly quiet seismically for decades ahead of time until they ruptured in the Mw 7.7 event. This pattern is like that prior to the 1906 San Francisco event ( Figure 1) and other transform faults within continental areas as well as along many subduction zones [1].
Barnhart et al. [18] mapped surface rupture of the 2013 mainshock using Landsat 8 satellite data with a spacing of 15 m resolution. They found that slip was not parallel to the direction of relative motion between the Arabian and Eurasian plates and hence, that compressional deformation must occur at other times either along either the Hoshab fault itself or in a broader surrounding zone.
In summary, most forerunning activity and aftershocks occurred at a distance from the two main rupture zones of the 2013 mainshock. The two rupture zones can be regarded as major asperities that accumulated stress in the preceding decades and released it suddenly in 2013. Searches for shorter-term foreshock activity to the 2013 and other strike-slip mainshocks within continents should include broad areas of forerunning activity like that in Figure 2 and not just single zones of surface rupture in past great earthquakes. Concentrating just on the main fault itself may well miss prior activities that may be useful for intermediate and short-term predictions.  The well-defined spreading center, which is about 100 km in length from north International Journal of Geosciences to south, spreads at about 75% of the long-term rate of relative movement of about 21 mm/yr. between the North American and Caribbean plates. The Swann transform fault to the west of the spreading center, the site of the 2018 mainshock, takes up nearly the full plate motion. Each of the two transform faults at the eastern ends of the Cayman spreading center are active. The 2020 mainshock broke the northern most active of the two, the Oriente transform, which passes off the coast of southeastern Cuba.
Rupture in the 2020 mainshock proceeded westward toward the Cayman spreading center from its short-period epicenter. No forerunning activity occurred for about 100 km on either side of the centroid location of the 2020 mainshock and along that part of the Oriente transform fault for which [19] computed slip was greater than 6 m ( Figure 3). That area is also notable for an absence of reported earthquakes as small as magnitude, mb, 4.5 since 1920. Clearly, that absence of forerunning activity defines a great asperity along that transform. Some slip as great as 4 m farther west along the Oriente transform closer to the Cayman spreading center was reported [19]. While it experienced five forerunning shocks in Figure 3, whether the area of 4 m slip coincided with them is uncertain. A reanalysis of the locations of those earthquakes is needed.
Abundant forerunning activity in Figure 3 also took place at distances great than 100 km from the centroid of the 2018 mainshock along the Swann transform. Some forerunning activity to the east, however, occurred close to the centroid. Again, those small forerunners need to be relocated. The 2018 mainshock ruptured westward from that area into the region of no forerunning activity in Figure 3. The latter was the site of no shocks of mb 4.5 or greater since 1920. No estimate of detail slip is known to the author for the 2018 mainshock. Hence, it is difficult to estimate the distribution of large asperities for the relatively small shock of Mw 7.5. In summary, forerunning activity did not occur within 100 km of the centroid of the 2020 mainshock and little within 100 km of the 2018 event. The region of computed slip greater than 6 m in the 2020 main event was not the site of forerunning shocks; it is identified as a great asperity. The seismic activity along the two oceanic transforms occurs along two narrow zones, a very different pattern than that seen in Figure 1 and Figure 2 for continental transform fault zones. Forerunning activity did not occur perpendicular to the zone of high slip in the 2020 mainshock.

Macquarie Ridge Earthquake of 1989, Mw 8.0
The Macquarie Ridge, which extends from southern New Zealand to a plate triple junction north of Antarctica, has been the site of many great strike-slip and thrust earthquakes, including the strike-slip mainshock of Mw 8.0 of 1989. Eighty percent of the slip in that mainshock occurred along the plate boundary [20] close the short-period epicenter (Figure 4), more concentrated than reported for many other great strike-slip events. The centroid of the 1989 mainshock (GCMT) was situated near the regions of computed large slip. Forerunning events and aftershocks largely occurred outside those slip zones. Largest blue symbol in center denotes centroidal location of mainshock. Solid black lines indicate short region in which 80% of its computed displacement occurred after [20].
In summary, the zone of largest computed displacement in the 1989 mainshock was nearly quiescent for forerunning and aftershock activity. Thus, it appears to have been a major asperity that built up stress for many decades before it ruptured in 1989.

Intra-Plate Earthquake West of Macquarie Ridge in 2004, Mw 8.1
The strike-slip mainshock of 2004 of Mw 8.1 occurred about 75 to 125 km to the northwest of the plate boundary along the Macquarie Ridge. Neither of its two fault planes is nearly parallel to the strike of the Macquarie Ridge, indicating it likely was an intraplate earthquake shock. Hayes et al. [21] comment on the presence of activity to the northwest of the plate boundary, as in Figure 4 and Figure 5, and relate that deformation to the slow growth of a new plate or block as the ratio of strike-slip to reverse motion along that part of the plate boundary changed over the past few million years. Similar activity, however, does not extend southeast of main plate boundary.
Nearby forerunning activity to the 2004 mainshock in Figure 3 and Figure 4 took place closer to the plate boundary than to its centroid. It may have had little if any relationship to the coming 2004 mainshock. To my knowledge, no one has published a detailed slip distribution for the 2004 event. I consider the mapped distribution of high-frequency radiation by [22] a relatively poor representation of detailed slip.
In summary, the 2004 mainshock occurred well to the northwest of the plate boundary in an area of little forerunning activity. It likely was an oceanic intraplate event. A detailed mapping of slip is needed.

South Scotia Sea Earthquake of 2013, Mw 7.8
A great earthquake in 2013 of Mw 7.8 occurred along a transform fault that bounds the southern margin of the Scotia Sea just to the north of the South Orkney Islands in the southernmost Atlantic Ocean. Long-term plate motion is slow, about 6.4 mm/a, along that boundary between the Scotia and Antarctic plates [23]. It occurred well to the west of the southern end of the Scotia subduction zone (not shown).
Rupture started at the short-period epicenter ( Figure 6) and proceded easterly. Slip in the mainshock as computed by Ye et al. [23] was larger than 5 m in the two zones enclosed by solid black lines. They also computed slip in the earlier 2003 shock of Mw 7.6 to its east, which broke a small part of the eastern region that ruptured more than 5 m in 2013. Since the centroid of the 2013 mainshock is located west of much of the slip computed by [23], at least one of those estimates is problematic. The centroid calculations for the 2013 and 2003 mainshocks indicate mainly strike-slip motion with a normal-faulting component on east-west striking faults that dip 44˚ and 36˚ south under the region of shallower ocean water in Figure 6.
All of the forerunning strike-slip activity prior to the 2013 mainshock was located well to either the east or west of its centroid and outside the computed zones of high slip. A single earthquake of Mw 5.7 with a thrust mechism occurred in 1991 between the two zones of highest slip. Two of the largest forerunning events, those of Mw 6.9 and 6.0, occurred one and four days before the 2013 mainshock. The north-south extent of seismic activity is somewhat broader than that for the two Cayman transforms in Figure 3.  In summary, forerunning strike-slip activity to the 2013 mainshock occurred outside the regions of high computed slip and within what are interpreted as either one or two major asperities. Seismic moment release increased in the 10 years before the mainshock compared to that from 1976 to 2003. Foreshocks occurred within four days of the 2013 event. The 2013 shock took place along a plate boundary with low long-term slip rate.

Earthquake in Central Alaska of 2002, Mw 7.8
The 2002 mainshock of Mw 7.8 ruptured the surface of the Denali and Totschunda faults in central Alaska for about 340 km mainly with strike-slip displacement. The Denali fault has largest long-term slip rate of faults within continenal Alaska with the exception of the Fairweather fault in southeast Alaska. The 2002 mainshock was preceded ten days earlier by a foreshock of Mw 6.6 (large red X in Figure 7) just to the western end of rupture in the the mainshock [24]. Rupture started at the small black diamond in Figure 7 and proceeded easterly and then southeasterly. It subsequently jumped to the Totschunda fault, where slip was smaller. Rupture did not extend farther southeastward along the continuation of the Denali fault.
Eberhart-Phillips et al. [24] state that the long-term slip rate for that part of the Denali fault that broke in 2002 was about 10 mm/a, about 1/7 of the rate for the relative motion of the Pacific and the North American plates. The long-term rate for the part of the Denali fault to the southeast that did not rupture in 2002 is about 2 to 3 mm/a; that for the Totschunda fault, about 5 mm/yr. Rupture in the mainshock was mainly strike slip but with a small compressional component. Most aftershocks occurred at depths less than 10 km, an indication of shallow slip in 2002 [24]. Except for the foreshock, only one small forerunning earthquake occurred within 150 km of the rupture zone of the mainshock. The same result can be seen for events as small as Ms 4.5 in Figure [25]. International Journal of Geosciences

Strike-Slip Earthquake of 2017 along Komandorsky (Commander)
Islands, Mw 7.8 The great earthquake of 2017 in the westernmost Aleutian Islands involved strike-slip motion between the North American and Pacific plates. Plate motion changes from nearly perpendicular convergence south of mainland Alaska to very oblique convergence, i.e. nearly along-strike motion, in the westernmost Aleutians. Plate motion then changes abruptly from near total strike-slip motion near the 2017 rupture zone to underthrusting farther west beneath Kamchatka.
Using GPS data from the Komandorsky Islands and Kamchatka, Kogan et al. [26] concluded that plate motion in the westernmost Aleutians is partitioned into strike-slip motion along a Bering fracture zone (transform fault) to the north of the Komandorsky Islands and along-strike motion on a shallow-dipping interface to their south. They found that model fit the GPS data best with two zones of major long-term plate motion that are separated and partitioned by a narrow Komandorsky block or sliver. About 2/3 of the total plate motion occurs along the Bering fault between the North American plate to its north and the Komandorsky sliver to its south (computed plate motion in the western Aleutians is nearly the same if a distinct Bering plate is analyzed instead of using the North American plate).
Lay et al. [27] computed slip as a function of length along strike for the 2017 mainshock. Slip extended a great distance, nearly 400 km with smaller slip in a zone about 100 km along strike near the centroid of the mainshock ( Figure 10).
Rupture progressed mainly to the southeast from the short-period epicenter but with a minor region of slip to its northwest [27]. The locations of forerunning activity are difficult to correlate with the well-separated patches of computed high slip. Forerunning activity occurred southeast of the region of highest slip in   Forerunning strike-slip activity in Figure 12, an enlarged view, was concentrated well outside the centroid of the mainshock as well as outside the region of the highest measured uplift ( Figure 11). A large region centered near the centroid is inferred to be a great asperity that was largely devoid of forerunning activity.
Four forerunning thrust mechanisms were concentrated south of the centroid just offshore near the region that experienced uplift between 2.5 and 5.0. m in Figure 11. [28] [29] [30] and others deduce that thrusting faulting occurred on and offshore and was a significant contribution to the total moment release and to the generation of a sizable tsunami. [31] attributes the complex distribution of      subducting Mindanao (Philipine) trench to its southeast and the East Luzon trench to its east ( Figure 16 and Figure 17). In that sense, the Phillipine fault system is similar to the Alpine fault system of New Zealand that connects oppositely-dipping subducting zones to its north and south.
Surface displacement was observed along a significant part of the 1990 rupture zone (Figure 16). Silcock and Beavan [34]   The most recent historical earthquake to break the central Luzon section of the Philippine fault occurred in 1645 [36]. From fault trenching, Daligdig [37] finds six or seven past events broke the 1990 zone including that of 1645. He dated a previous event as 1190 to 1390 A.C.E. and estimated an average recurrence interval of 300 to 400 years.
In summary, the rupture zone of the 1990 mainshock was quiet for nearby forerunning earthquakes of Mw > 5.0 from 1976 until its occurrence. It can be regarded as a major asperity. Not much forerunning activity was observed, however.

Patterns of Forerunning Activity-The Tibetan Plateau
Two great earthquakes are described in China-the first in 2008 on the east side of the Tibetan plateau and another within it in 2001. The long-term slip rate for the faults on which they occurred (Table 1) is small compared to that of many plate boundaries but larger than that for many intraplate regions.

Wenchuan China Earthquake of 2008, Mw 7.9
The Wenchuan mainshock of 2008, Mw 7.9, broke about 285 km of the Longmen Shan fault zone in Sichuan Province, China. That rupture zone took place along the boundary between the east side of the Tibetan Plateau and the western side of the Sichuan basin [38]. It caused nearly 90,000 deaths, left millions homeless and caused huge monetary losses [38]. Earthquake mechanisms (Figure 18 and Figure 19) indicate that slip on the fault system during the mainshock and aftershocks consisted of segments with strike-slip and reverse displacements. Similar co-seismic displacements at the surface occurred along various parts of the rupture zone [38]. Another historic earthquake like 2008 has not been identified since 2300 BCE.
Trenching after the 2008 mainshock revealed a paleoseismic event at three sites with an age of 2300 to 3300 years BCE [39]. Wang et al. [40] estimate a long-term slip rate for the Longmen Shan fault zone of about 4 mm/a. Thus, the 2008 rupture zone is not a fast-slipping plate boundary but is better described as the boundary between two major lithospheric blocks or subplates. Rupture in the mainshock started at the short-period epicenter ( Figure 18) near the southwestern end of the rupture zone and broke to the northeast. Nakamura et al. [41] used broadband seismic waves recorded at large distances to deduce the distribution of slip in the mainshock. They concluded that major fault slip of dip about 33˚ occurred along a southwestern segment and about 60˚ along a northeastern zone. They also found two main asperities: one about 40 km and the other about 170 km along strike from the short-period epicenter (near 31˚N). Using data from strong-motion stations, Zhang [42] found that major slip, interpreted as a strong asperity, occurred 20 to 50 km northeast of the short-period epicenter, like the location of the first asperity of [41] and the location of the centroid of the mainshock (Figure 18). None of the forerunners to the mainshock in Figure 18 occurred near its centroid nor the zones of large slip as deduced by [41] [42]. All forerunners took place within the Tibetan plateau at a distance from the faults that ruptured in 2008 and none within the Sichuan basin. Since the number of forerunners of Mw    [47] [48]. It is one of the major fault zones along which Tibet is moving easterly as it is being squeezed by the surrounding Indian and Eurasian plates.

Great Strike-Slip Earthquake in Qinghai
Slip started at the short-period epicenter near 90.5˚E and propagated largely unilaterally to the east. A small amount of slip occurred to its west. Xu et al. [47] report the rupture zone was oriented N100˚ ± 10˚E on average, and slip in 2001 consisted of three main sections-a short westernmost strike-slip section, a short transtensional segment near the start of rupture and a long eastern strike-slip section. Most of the moment release occurred along the eastern segment. They and others subdivide that eastern section into several parts of which several sub-zones slipped more than 5 m. They also report a reassessment of the maximum coseismic horizontal displacement of 7.6 ± 0.4 m at a site just to the east of the centroid, which is consistent with independent measurements derived from interferometric synthetic aperture radar and seismology.  No forerunning shocks in Figure 20 were situated on the surface rupture, much of which can be considered to be the site of one or more major asperities. Rupture in the 1998 mainshock started at the short-period epicenter just to the east of its centroid and propagated 276˚ west-northwesterly but along two separated zones of the same strike [50]. Higher moment release occurred in a first sub-event within 140 km of its initiation. It was followed by a second subevent between 210 and 270 km. Hjörleifsdóttir et al. [51] showed, however, that a solution involving slip between and including those zones adequately fit both the long and short-period seismic data. All but two of the aftershocks occurred between 100 to 200 km of the centroid. Thus, a great asperity that broke in the  mainshock likely coincides with at least the first 100 km of slip in the first sub-event. The 1998 intraplate rupture zone likely has a long repeat time. Hence, it is not surprising that the relatively short period since 1976 was not enough to depict forerunning activity. In summary, no forerunning activity occurred in the vicinity of the 1998 intraplate mainshock. Its rupture zone defines a great asperity.

Earthquake within the Indian-Australian Plate in 2000, Mw 7.9
A great strike-slip mainshock occurred within the Australian plate in the Indian Ocean during 2000. Like the previous event within the Antarctic plate, it was typified by a small number of events afterward but only one unlikely forerunning earthquake to it that was located about 250 km from its centroid ( Figure 23).
Robinson et al. [52] found that it consisted of two sub-events that ruptured the two nodal planes of the mechanism solution. Small aftershocks defined one plane striking about 345˚. They report that the centroid of the mainshock occurred below a major seamount.
In summary, the great intraplate mainshock of 2000 was accompanied by only a few moderate-size events in the 20 years after its occurrence and one preceding shock 250 km away.   like that in the 1998 GCMT mechanism [56]. That strike-slip motion pertains, however, to earlier geological deformation, which involved high-grade metamorphism. To my knowledge no one has computed a detailed distribution of slip for the 1998 mainshock.

Earthquake off the Coast of Kodiak
In summary, the 1998 mainshock occurred along the Banda trough on the south side of the island of Ceram. It does not appear to be a plate boundary today and is located on the forearc side of the Banda arc. It is interpreted as an intraplate earthquake. Forerunning activity in Figure 25 would not have been a good guide to either the location of the coming 1998 mainshock or its magnitude.

Three Great Intraplate Earthquakes off the Coast of Sumatra in 2012 and 2016
Three great mainshocks occurred in the Wharton Basin of the eastern Indian Ocean off the west coast of Sumatra, Indonesia. Two took place about 2 hours apart in 2012 (Mw 8.6 and 8.2) and the other in 2016 (Mw 7.8). The Mw 8.6 earthquake is the largest strike-slip earthquake since the start of instrumental seismology. All three were intraplate strike-slip events whose rupture extended into the upper mantle (GCMT mechanisms, 2012; [57]). Meng et al. [57] attribute the large Mw 8.6 event to high-stress drop, displacements along several faults and rupture penetrating into the uppermost mantle. They occurred in a     In summary, except for the forerunning shocks, whose stresses likely were boosted after by the two giant Sumatran events of 2004 and 2005, the otherwise low forerunning activity can be ascribed to slow stress buildup along intraplate faults. Those closest forerunning events to the 2012 mainshocks need to be relocated to ascertain if they occurred on or off the faults of the coming great events.
The 2016 mainshock had no forerunning events that could be considered related to it and few aftershocks. That intraplate event was located farther from the Sumatran subduction zone than the 2012 mainshocks. Plafker [62] mapped the extent of surface rupture (Figure 31), which was difficult to do along its western end near the volcanic arc and at its eastern end near the Gulf of Honduras. Co-seismic slip of about a meter took place along the segment where Plafker ascertained that clear surface displacement occurred [63].

Older Great and
Smaller slip was computed [63] to its east and west as shown by dashed lines in    in a zone about 80 km wide, which were destressed in the previous great earthquake, and then reloaded to failure in the 25 years before 1906. In contrast, forerunning and aftershock activity along oceanic transform faults, like those along the Cayman trough in Figure 3, occurred along very narrow zones and much simpler plate boundaries. Pre-1906 quiescence of the main San Andreas fault resumed after about 1920 and still exists as of June 2021.

Southern California Earthquake of 1857
The great southern California mainshock of 1857, largely a strike-slip event, ruptured the surface of the San Andreas fault from near Parkfield to Pallett Creek ( Figure 33). It and other fault segments have been sites of many paleoseismic investigations. The 1857 rupture zone has been very quiet for shocks of M > 5.0 since 1920 but is surrounded by activity at a distance, an indication it consists of two or more great asperities that mainly, and perhaps solely, rupture in great earthquakes. Another major segment of the San Andreas fault last broke in a great shock during 1812 between Pallett Creek and the eastern bend of the fault near San Gorgonio Pass in Figure 34 [65] [66]. The southeastern most part of the San Andreas fault also is quiet in Figure 33 and has not ruptured in a great event for hundreds of years.  Several Parkfield earthquakes (P in Figure 33) of about Mw 6 have occurred on the San Andreas fault itself. The rupture zone of the great 1857 Fort Tejon earthquake between Parkfield and Pallett Creek has been particularly quiet for shocks with magnitudes larger than 6.0 since at least 1920. Rupture in the Mw 7.24 Kern County earthquake of 1952 started near the San Andreas but proceeded along the northeast-striking White Wolf fault. It did not break either the San Andreas or Garlock faults. Its slip consisted of left-lateral strike-slip and reverse components [65]. Most, if not all, of the events shown in Figure 34 near the main eastern bend occurred off the main San Andreas fault itself. This indicates that the main fault in much of southern California is locked and consists of one or more great asperities that break in great earthquakes. Those events in Figure 34 are close enough to the San Andreas, however, that their occurrence likely moved it closer to failure. That region should be considered a good place to search for forerunning shocks to great earthquake(s) that will eventually break the San Andreas fault between Pallett Creek and the Salton Sea.
Ben-Zion and Zaliapin [71] describe the localization and coalescence of seismicity before four Mw > 7 earthquakes in southern California, one in the adjacent Baja peninsula of Mexico and a 1999 shock along the North Anatolian fault of Turkey. They find that these mainshocks, which they state were locked ahead of time, were preceded about two years by a reduction in the area where low magnitude forerunning shocks occurred on the peripheries of their rupture zones.
They also report, "the results for regions around the rupture zones of the M > 7 earthquakes show that individual events tend to coalesce rapidly to clusters of growing size ~1 yr before the main shocks". They find that activity before the 2004 Parkfield event of M6, however, indicated decreasing average cluster size.
They show that the Parkfield section of the San Andreas fault is associated with a well-defined linear zone, which is very different from the more diffuse seismicity in most of southern California. The narrow spatial distribution of activity at Parkfield is like that along the oceanic transform faults in Figure 3. The results of Ben-Zion and Zaliapin represent important directions for work on prediction for yearly to decadal time scales, shorter intervals than studied in this paper.
In summary, few, if any, events of magnitude 6 and larger have taken place since 1920 along the southern San Andreas fault itself between Parkfield and its southern end near the Salton Sea. Seismic activity has occurred on the periphery of those quiet zones since 1920. Some of the closest activity has occurred near the eastern bend of the San Andreas, an area that is recommended for intense study to possibly detect premonitory phenomena of yearly to decadal time scales. The Parkfield area, the site of several shocks on the San Andreas and activity before the 1857 great mainshock, is another area worth extensive monitoring for possible precursory events to a great earthquake.  well-coupled and is inferred to consist of two or more great asperities that rupture in great earthquakes [72]. The greater Parkfield area would be a good place to seek forerunning activity and shorter-term precursors to the next great shock just to the south along the San Andreas fault. The Parkfield zone is like low-coupled zones (LCZ) along subduction zones [1]. Many other LCZ have been described as sites of fault creep, smaller earthquakes and slow-slip seismic events. The eastern bend of the San Andreas fault in southern California, the site of many International Journal of Geosciences moderate-size earthquakes since 1920, is also recommended for intense monitoring and study.
Forerunning earthquakes and aftershocks along oceanic transform faults bordering the Cayman trough were distributed differently than those along several continental transforms. Those oceanic events occurred along narrow bands usually just beyond the ends of the rupture zones of two large earthquakes. The forerunning areas of the 1906 earthquake in California and of large mainshocks along other continental transform faults, however, were broad. Moderate-size shocks also occurred and recurred along some narrow segments of the Gofar transform fault at 4˚S on the East Pacific Rise while its other segments moved even without moderate-size earthquakes [73]. Some narrow segments of the Eltanin transform fault system in the southeastern Pacific were sites of repeating moderate-size earthquakes whereas other segments were poorly coupled and did not rupture in shocks of Mw > 5.5 [74].
The Eltanin transform fault system is situated along the fast-spreading East Pacific Rise. Known shocks along it have not exceeded Mw 6.5; most have been smaller than 6.0. The maximum known size of shocks along the Gofar transform, an even faster-moving fault, was even smaller. Moderate size normal-faulting events are very rare along fast-speading ridges themselves. In contrast, transform faults along slow spreading oceanic ridges-including the Cayman trough and the South Scotia Sea-ruptured in larger mainshocks of 7.5 < Mw < 7.8. The largest strike-slip event along the slow-spreading Southwest Indian ridge was Mw 8.0 [11]. The largest known events along transform faults of the Mid-Atlantic ridge-zones of intermediate-rate spreading-are of Mw 7.0 to 7.1. Great earthquakes in 1906 and 1857 along the continental San Andreas fault were of Mw 7.8 and 7.9. Their long-term slip rate is moderate, about 28 to 33 mm/a (Table 1).
Large variations in the sizes of mainshocks among transform faults can be attributed to differences in rock type, the distribution of temperature as a function of depth and geological age (see [15] pp. 307-320). Fast-spreading oceanic transforms like the Eltanin consist of at least three strike-slip segments separated by short spreading ridges [74]. It and other transforms along fast-spreading ridges seem to change their strikes and develop short spreading centers rapidly in response to changes in plate motion. This has prevented the development of very long individual transforms and hence, has restricted the maximum magnitudes of shocks along them. Changing the strike and length of segments of transform faults within continental areas is more difficult. Hence, they rupture in long maximum-size events of Mw 7.8 to 7.9.
The rupture zone of the giant 1906 Ecuador-Colombia earthquake subsequently re-broke in large but not giant shocks in 1942, 1958 and 1979. A similar pattern holds for transform faults, including the rupture zone of the 1906 and 1857 mainshocks in California. Most giant earthquakes-like 1952 Kamchatka, 1960Chile, 1964 Alaska and 2004 Sumatra-Andaman-likely ruptured two or more great asperities [1]. Each probably broke separately in moderate to great shocks at other times, a common pattern for subduction zones and transform International Journal of Geosciences faults.
Cumulative seismic moment accelerated about two years before the New Zealand earthquake of 2016 ( Figure 13). Cumulative moment accelerated on time scales of about a decade before the giant 2011 Tohoku-oki and the great 2014 Iquique earthquakes along subduction zones. Whether those increases in moment were, in fact, casually related to those coming great earthquakes is uncertain and requires more work. This paper examined mainly the distribution of forerunning activity with latitude and longitude but only a few instances of how it varied with depth.
Several other types of possible precursors-increases in the frequency of small repeating earthquakes, slow slip events and changes in b values-are described briefly here and in [1] along with descriptions of forerunning seismicity and great asperities. We need to understand if precursors of various types occur more often just before great earthquakes.
Some of the mainshocks in this paper along transform faults exhibited short-term precursors. [1] reports similar precursors to mainshocks at subduction zones.
[71] describe the localization and coalescence of seismicity on time scales of years before six Mw > 7 earthquakes along continental transform faults. These are the start of next steps toward possible earthquake prediction on shorter time scales than decades. I advocate moving from longer to short-term precursory studies. The recent paper [75] indicates strong new interest in the United States in monitoring and studying earthquake precursors.
Quiet zones and sites of forerunning activity in this paper were identified in hindsight. Is it possible to map similar distributions of forerunning activity ahead of time to better determine both the slip regions of coming large to great strike-slip earthquakes and their magnitudes? This will not be easy since hundreds of quiet zones exist today for the subduction and transform-fault plate boundaries of the world. Many repeat times of great shocks exceed 100 years, i.e., longer than detailed seismic observations. A combination of data and calculations could be used to narrow searches for great events of societal interest including ones that are known to have rupture previously in great or giant events. Future work needs to include many transform faults and subduction zones worldwide instead of concentrating on just a single or a few areas. In addition to operating more local seismic networks, deployment of permanent GPS observations is needed for additional segments of plate boundaries. Double-difference relocations need to be performed for forerunning shocks, including older and smaller earthquakes. Computer processing of seismograph data to possibly identify forerunning and precursory activity could be done on a continuing basis using probabilistic approaches as soon as new data become available.