Foreshocks and time-dependent earthquake hazard assessment in southern California

The probability that an earthquake in southern California (M ≧ 3.0) will be followed by an earthquake of larger magnitude within 5 days and 10 km (i.e., will be a foreshock) is 6 ± 0.5 per cent (1 S.D.), and is not significantly dependent on the magnitude of the possible foreshock between M = 3 and M = 5. The probability that an earthquake will be followed by an M ≧ 5.0 main shock, however, increases with magnitude of the foreshock from less than 1 per cent at M ≧ 3 to 6.5 ± 2.5 per cent (1 S.D.) at M ≧ 5. The main shock will most likely occur in the first hour after the foreshock, and the probability that a main shock will occur decreases with elapsed time from the occurrence of the possible foreshock by approximately the inverse of time. Thus, the occurrence of an earthquake of M ≧ 3.0 in southern California increases the earthquake hazard within a small space-time window several orders of magnitude above the normal background level

Foreshocks (1966-1980) in the San Andreas system, California

The spatial and temporal distributions of seismicity preceding moderate (ML ≧ 5.0) main shocks in the San Andreas fault system in California have been analyzed to recognize and characterize the patterns of foreshock occurrence. Of 20 main shocks in the San Andreas system, 7, or 35 per cent, have been preceded by immediate foreshock sequences that included events within 1 day and 5 km of the main shocks. A possible correlation of the rate of foreshock occurrence with type of faulting was found such that none of the four main shocks with reverse faulting had foreshocks while 44 per cent of the strike-slip earthquakes had foreshocks. Some enhanced seismic activity was also observed at relatively large distances from the main shock (13 to 30 km) 1 to 5 days before 40 per cent of the main shocks but this activity cannot be clearly distinguished from the background seismicity. Of the seven immediate foreshock sequences, only two had the swarm-like appearance of the class II foreshocks defined by Mogi. The other foreshock sequences appear to be single events (sometimes with their own aftershocks) preceding the respective main shocks. Four of these sequences are spatially correlated with distinct physical discontinuities in their faults between the hypocenters of the foreshock and main shock, and similar discontinuities may also be associated with the other sequences. The durations of the foreshock sequences were found to decrease as the depths of the main shocks increase from 3 to 11 km, which has been interpreted as a dependence on stress. To account for this stress dependence of the duration and the presence of discontinuities, a model for foreshocks occurrence is presented. This model proposes that foreshocks may represent a process of delayed multiple rupture and that the delay between occurrence of foreshock and main shock might represent the time needed for static fatigue to break the stronger rock at the discontinuity in the fault

The 1988 and 1990 Upland Earthquakes: Left-Lateral Faulting Adjacent to the Central Transverse Ranges

Two earthquakes (M_L=4.6 and M_L=5.2) occurred at almost the same location in Upland, southern California, in June 1988 and February 1990 and had similar strike-slip focal mechanisms with left-lateral motion on a northeast striking plane. The focal mechanisms and aftershock locations showed that the causative fault was the San Jose fault, an 18-km-long concealed fault that splays west-southwest from the frontal fault of the central Transverse Ranges. Left-lateral strike-slip faults adjacent to the frontal faults may play an important role in the deformation of the Transverse Ranges and the Los Angeles basin as suggested by these Upland earthquakes, the left-lateral strike-slip 1988 (M_L=4.9) Pasadena earthquake on the Raymond fault, 30 km to the west of Upland, and scattered background seismicity along other active left-lateral faults. These faults may transfer slip away from part of the frontal fault toward the south. Alternatively, these faults could represent secondary faulting related to the termination of the northwest striking right-lateral strike-slip faults to the south of the range front. The 1988 and 1990 Upland earthquakes ruptured abutting or possibly overlapping segments of the San Jose fault. The edges of the overlapping aftershock zones, which are sharply defined, together with background seismicity, outline a 14-km-long aseismic segment of the San Jose fault. The 1988 mainshock originated at 9.5 km depth and caused aftershocks between 5 and 12 km. In contrast, the 1990 mainshock focus occurred at the top of its aftershock zone, at 5 km, and caused aftershocks down to 13 km depth. These deep aftershocks tapered off within 2 weeks. The rate of occurrence of aftershocks in magnitude-time space was the same for both sequences. The state of stress reflected in the focal mechanisms of the aftershocks is identical to that determined from background activity and did not change with time during the aftershock sequence. The constant stress state suggests that the 1988 and 1990 events did not completely release all the stored slip on that segment of the fault. The presence of 14 km of unbroken fault, the abrupt temporal termination of deep aftershocks, and the constant stress state all suggest that a future moderate-sized earthquake (M_L=6.0–6.5) on the San Jose fault is possible with a rupture length of at least 14 km and possibly 18 km

The July 1986 Oceanside (M_L = 5.3) earthquake sequence in the Continental Borderland, Southern California

An earthquake of M_L = 5.3 occurred at 32°58.7′N, 117°51.5′W southwest of Oceanside in San Diego County at 13:47 13 July 1986 (UT). This main shock was followed by an extensive aftershock sequence, with 55 events of M_L ≧ 3.0 during July 1986. The epicenters of the main shock and aftershocks are located at the northern end of the San Diego Trough-Bahia Soledad fault zone (SDT-BS) where it changes strike from northwest to a more westerly direction through a left offset or a bend in the fault. The northwest-striking SDT-BS is one of three strike-slip fault systems that constitute the offshore Agua Blanca fault system. The spatial distribution of the aftershocks indicates a unilateral 7- to 9-km long rupture to the east-southeast away from the epicenter of the main shock. The focal mechanism of the main shock also has an east-southeast striking and south-dipping plane with mostly reverse movement on it. Focal mechanisms of the M_L ≧ 3.0 aftershocks show both reverse and strike-slip movement. The reverse focal mechanisms indicate that this sequence may have occurred on a thrust fault that provides for a left stepping offset or a bend in the San Diego Trough fault as movement is transferred to the west along the Santa Cruz-Catalina Island escarpment. Some of the aftershocks that are located to the southeast of the main shock and have strike-slip focal mechanisms suggest activation of the northwest-trending San Diego Trough fault. A stress inversion of the focal mechanism data shows that the maximum principal stress determined from the focal mechanisms of the main shock and 22 aftershocks that occurred within 36 hours of the main shock has an azimuth of S30°W plunging 18°. The maximum principal stress determined from 30 aftershocks that occurred from 15 July to 2 October 1986 has an azimuth of S20°W, plunging 18°. The φ-values (the measure of the relative sizes of the principal stresses) are approximately 0.07 and 0.1, respectively, indicating that the intermediate and minimum principal stress are of similar magnitude. The results of the stress inversion, and the focal mechanisms that showed reverse faulting, suggest that the Inner Continental Borderland offshore from Oceanside is not currently a pure-strike-slip tectonic regime but rather a strike-slip mixed with reverse faulting regime. When the 52 aftershock focal mechanisms are divided into four groups and the stress inversion is repeated, the change in stress can be described as a progressive counter-clockwise rotation of 14° of the orientation of the maximum principal stress to a more southerly direction, with the greatest change in stress orientation observed shortly after the main shock. The abundance of aftershocks may be related to the large temporal variation in stress orientation that, in turn, may have resulted from the small stress drop of the main shock

The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern California: Seismological and tectonic analysis

The October 1, 1987, Whittier Narrows earthquake (M_L = 5.9) was located at 34°2.96′N, 118°4.86′W, at a depth of 14.6±0.5 km in the northeastern Los Angeles basin. The focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25°±5° and 65°±5°, respectively. The aftershocks define an approximately circular surface that dips gently to the north, centered at the hypocenter of the mainshock with a diameter of 4–6 km. Hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25° north dipping thrust fault striking west and is confined to depths from 10 to 16 km. Although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting, they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock. A cluster of reverse faulting events on north striking planes occurred within hours after the mainshock, 2 km to the west of the mainshock. The largest aftershock (M_L = 5.3) occurred on October 4 and showed mostly right-lateral faulting on the same north-northwest striking plane within the hanging wall. Similarly, several left-lateral focal mechanisms are observed near the eastern edge of the mainshock rupture. The earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays. When relocating the blast using the new models and delays, the absolute hypocentral location bias is less than 0.5 km. The mainshock was followed by nearly 500 locatable aftershocks, which is a small number of aftershocks for this magnitude mainshock. The decay rate of aftershock occurrences with time was fast, while the b value was low (0.67±0.05) for a Los Angeles basin sequence

Reply to 'A Second Opinion on "Operational Earthquake Forecasting: Some Thoughts on Why and How," by Thomas H. Jordan and Lucile M. Jones,' by Stuart Crampin

In folklore, a "silver bullet" is an effective weapon against were-wolves and witches. In earthquake prediction, a silver bullet is a diagnostic precursor—a signal observed before an earthquake that indicates with high probability the location, time, and magnitude of the impending event (Jordan 2006). In his comment, Crampin (2010) claims that shear-wave splitting (SWS) observations provide a silver bullet. He asserts that seismology is thus capable of raising earthquake forecasting out of the low-probability environment to which we assigned it in our recent opinion piece (Jordan and Jones 2010)

Prediction probabilities from foreshocks

When any earthquake occurs, the possibility that it might be a foreshock increases the probability that a larger earthquake will occur nearby within the next few days. Clearly, the probability of a very large earthquake ought to be higher if the candidate foreshock were on or near a fault capable of producing that very large mainshock, especially if the fault is towards the end of its seismic cycle. We derive an expression for the probability of a major earthquake characteristic to a particular fault segment, given the occurrence of a potential foreshock near the fault. To evaluate this expression, we need: (1) the rate of background seismic activity in the area, (2) the long-term probability of a large earthquake on the fault, and (3) the rate at which foreshocks precede large earthquakes, as a function of time, magnitude, and spatial location. For this last function we assume the average properties of foreshocks to moderate earthquakes in California: (1) the rate of mainshock occurrence after foreshocks decays roughly as t^(−1), so that most foreshocks are within three days of their mainshock, (2) foreshocks and mainshocks occur within 10 km of each other, and (3) the fraction of mainshocks with foreshocks increases linearly as the magnitude threshold for foreshocks decreases, with 50% of the mainshocks having foreshocks with magnitudes within three units of the mainshock magnitude (within three days). We apply our results to the San Andreas, Hayward, San Jacinto, and Imperial faults, using the probabilities of large earthquakes from the report of the Working Group on California Earthquake Probabilities (1988). The magnitude of candidate event required to produce a 1% probability of a large earthquake on the San Andreas fault within three days ranges from a high of 5.3 for the segment in San Gorgonio Pass to a low of 3.6 for the Carrizo Plain

Operational Earthquake Forecasting: Some Thoughts on Why and How

The goal of operational earthquake forecasting is to provide the public with authoritative information on the time dependence of regional seismic hazards