Decadal Forecasts of Large Earthquakes along the Northern San Andreas Fault System, California: Increased Activity on Regional Creeping Faults Prior to Major and Great Events

The three largest earthquakes in northern California since 1849 were preceded by increased decadal activity for moderate-size shocks along surrounding nearby faults. Increased seismicity, double-difference precise locations of earthquakes since 1968, geodetic data and fault offsets for the 1906 great shock are used to re-examine the timing and locations of possible future large earthquakes. The physical mechanisms of regional faults like the Calaveras, Hayward and Sargent, which exhibit creep, differ from those of the northern San Andreas, which is currently locked and is not creeping. Much decadal forerunning activity occurred on creeping faults. Moderate-size earthquakes along those faults became more frequent as stresses in the region increased in the latter part of the cycle of stress restoration for major and great earthquakes along the San Andreas. They may be useful for decadal forecasts. Yearly to decadal forecasts, however, are based on only a few major to great events. Activity along closer faults like that in the two years prior to the 1989 Loma Prieta shock needs to be examined for possible yearly forerunning changes to large plate boundary earthquakes. Geodetic observations are needed to focus on identifying creeping faults close to the San Andreas. The distribution of moderate-size earthquakes increased significantly since 1990 along the Hayward fault but not adjacent to the San Andreas fault to the south of San Francisco compared to what took place in the decades prior to the three major historic earthquakes in the region. It is now clear from a re-examination of the 1989 mainshock that the increased level of moderate-size shocks in the one to two preceding decades occurred on nearby East Bay faults. Double-difference locations of small earthquakes provide structural information about faults in the region, especially their depths. The northern San Andreas fault is divided into several strongly coupled segments based on differences in seismicity.

Intermediate-Term Seismic Precursors to the Loma Prieta California Earthquake of 1989

We use precise locations of earthquakes to study forerunning seismic activity to the 1989 Loma Prieta earthquake of magnitude 6.9 to the south of San Francisco, California, USA. Relocated shocks of magnitude 4.3 to 5.4 and smaller micro-earthquakes define a distinct zone of nearly the same orientation as the mainshock. That separate zone broke in the 15 months prior to the 1989 mainshock. That feature, which we call the Lake Elsman fault zone, is identified as the site of a prominent intermediate-term (yearly) precursor very close to the coming 1989 mainshock. That zone experienced a relatively large stress decrease during the nearby great earthquake of 1906. From the occurrence of the Lake Elsman shocks, we deduce that stress drop was only restored in the 15 months prior to the 1989 main event. Those stresses are consistent with little forerunning seismic activity in the region after 1906, later increases just before the 1989 mainshock and a decrease in activity thereafter. The southern Santa Cruz mountains segment of the San Andreas Fault zone, the location of the 1989 mainshock, had not been the site of events of magnitude 5 and larger for many decades prior to the occurrence of Lake Elsman earthquakes of magnitude 5.3 and 5.4 in 1988 and 1989. High-preci- sion locations readily available in real-time might be used to monitor similar possible precursory activity very close to the San Andreas and other transform faults.

Decadal Seismicity Prior to Great Earthquakes at Subduction Zones: Roles of Major Asperities and Low-Coupling Zones

Decadal forerunning seismic activity of magnitude Mw ≥ 5.0 is mapped for all 45 mainshocks of Mw 7.7 to 9.1 at subduction zones of the world from 1993 to mid 2020. The zones of high slip in nearly all great earthquakes were nearly quiescent beforehand and are identified as the sites of great asperities and zones of strong seismic coupling. Much forerunning activity occurred at smaller asperities along the peripheries of the rupture zones of many great and giant mainshocks. Those sizes of great asperities as ascertained from forerunning activity generally agree with the areas of high seismic slip as determined by others from geodetic and tide-gauge data and finite-source seismic modeling. Asperities are strong, well-coupled portions of plate interfaces. Different patterns of forerunning activity on time scales of about 5 to 45 years are attributed to either the sizes and spacing of asperities (or lack of). This permits many great asperities to be mapped decades before they rupture in great and giant shocks. Several poorly coupled subduction zones such as Java, Lesser Sunda, Marianas, Tonga and Kermadec are characterized by few great thrust earthquakes and little, in any forerunning activity. Rupture zones of many great and giant earthquakes are bordered either along strike, updip, or downdip by zones of lower plate coupling. Several bordering regions were sites of forerunning activity, aftershocks, and slow-slip events. The detection of forerunning and precursory activities of various kinds should be sought on the peripheries of great asperities as well as within zones of high co-seismic slip.

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 interfaces. 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, 1857 and 1906 have been very quiet since 1920;moderate to large shocks have been concentrated on their peripheries. The intraplate shocks studied, however, exhibited few if any forerunning events, which is attributed to the short period of time studied compared to their repeat times. The detection of forerunning and 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.

Decadal and Yearly Forerunning Earthquakes to Cape Mendocino, Northern California, Mainshock of 1992 Magnitude 6.9

The area near the Cape Mendocino earthquake of 1992, magnitude 6.9, was the site of many moderate to large shocks during the previous decades. It and the Honeydew event of 1991, however, are distinguished from most earthquakes in the region by their thrust-fault mechanisms. The magnitude of the 1991 shock was also unusually large for the preceding decades, Mw 6.1. The mechanisms of most other large events involved strike-slip faulting. The 1992 mainshock occurred in a volume of space characterized by few decadal forerunning earthquakes of moderate to large size. Most of those forerunners took place on the periphery of that volume. The presence of that zone suggests that it broke previously in a large to great earthquake. Precise locations indicate that slip in the 1991 and 1992 earthquakes occurred on faults dipping shallowly to the NE and ENE. They likely took place within the North American plate above the subduction plate boundary. Their implications for earthquake forecasting using sparse precursors are discussed.

Major Asperities that Ruptured in Large California Earthquakes, Surrounding Pre-Shocks, and Some Yearly to Decadal Precursory Changes in Seismic Activity

Double-difference locations of forerunning shocks of seismic magnitude, M, 2 to 6 are examined in the months to decades before 11 mainshocks in California of magnitude 6 and larger. Each of the 11 had large quiet zones beforehand, which are called asperities, that break nearly entirely in large mainshocks. Their surrounding 11 zones were all sites of small to moderate-size shocks that define the approximate magnitudes of the coming large event. The latter, donuts of activity, are places to examine for precursory changes years to decades before larger earthquakes. The quiet asperities and many sites at the earth’s surface are not good places to monitor precursory changes before large mainshocks. Detecting forerunning events to large future earthquakes requires monitoring the right places. A few possible precursory changes are identified and discussed.