古地震学:从岩石中阅读地震史

What Is Paleoseismology?

古地震学通过断层探槽、隆起阶地和海啸沉积物等地质证据研究史前地震的学科，将地震记录延伸至数千年前。 is the branch of earthquake science that studies prehistoric earthquakes using geological evidence preserved in the landscape and in the sedimentary record. Because instrumental and historical earthquake records extend back at most a few hundred years — a tiny fraction of the recurrence intervals of large earthquakes on many faults — paleoseismology is essential for understanding the long-term behavior of fault systems. By identifying, dating, and measuring the physical evidence of past earthquakes preserved in sediments, bedrock, and landforms, paleoseismologists can extend the earthquake record thousands to tens of thousands of years into the past, revealing patterns of rupture that would be invisible in the short instrumental record. This extended record is fundamental to probabilistic seismic hazard analysis (PSHA) and to 地震空区与相邻区段相比长时间未发生地震的活动断层区段，可能预示未来发生地震的概率有所增加。 identification.

The Value of Long Records

The fundamental challenge in earthquake science is that the largest, most dangerous earthquakes occur infrequently. A 断层（地质学）岩石沿其发生位移的破裂面。断层长度从数毫米到数千公里不等。会引发地震的主要断层称为活动断层。 that generates Mw 8.0 earthquakes every 500 years will have produced only one or two events in the entire period covered by modern seismograph networks (roughly 1900 to present). To estimate how often such an earthquake will occur in the future — the foundation of 地震风险评估对特定地区或结构物的地震危险性、建筑物易损性及潜在损失进行评估的过程，综合了危险性图、建筑物清单及损失模型。 — we need many past events, not just one or two. 古地震学通过断层探槽、隆起阶地和海啸沉积物等地质证据研究史前地震的学科，将地震记录延伸至数千年前。 provides this long record, turning the geology of fault zones into a detailed history of earthquake activity stretching across many seismic cycles.

Fault Trench Studies: Reading the Layers

The most widely used technique in 古地震学通过断层探槽、隆起阶地和海啸沉积物等地质证据研究史前地震的学科，将地震记录延伸至数千年前。 is the excavation of trenches across active 断层线断层在地表的痕迹，表现为一条线状或破碎岩石带。地质学家绘制活动断层线图，以评估周边社区的地震危险性。s. A trench typically cuts perpendicular to the fault trace, 1–3 meters wide and 2–5 meters deep, extending far enough on each side to expose undisturbed stratigraphy away from the fault zone. The walls of the trench are carefully logged — every sediment layer, soil horizon, and fault strand is mapped in detail. Earthquakes leave characteristic signatures in the stratigraphy: colluvial wedges (masses of sediment shed from a 断层崖地震期间断层沿垂直方向发生位移而形成的陡崖或陡坡。断层崖可高达数米，是过去地震活动留下的可见证据。 after it is uplifted by an earthquake), buried soil horizons truncated by fault displacement, fissures filled with dike-like injections of sand or mud from below, and angular unconformities where shaking-liquefied sediments were injected upward. Each such event horizon, if it can be confidently identified and dated, represents a past earthquake, providing a time series of rupture events going back as far as the preserved stratigraphy extends.

The Pallett Creek Record

The most extensively studied paleoseismic site in the world is Pallett Creek on the Mojave segment of the San Andreas 断层线断层在地表的痕迹，表现为一条线状或破碎岩石带。地质学家绘制活动断层线图，以评估周边社区的地震危险性。 in southern California. Kerry Sieh's landmark 1978 study documented 12 large earthquakes at this site over the past 2,000 years, including the great 1857 Fort Tejon earthquake. Subsequent work extended the record further and refined the dates. The Pallett Creek record demonstrates that the southern San Andreas does not behave with perfectly periodic repetition — the intervals between events range from roughly 50 to 330 years — but on average produces a major rupture every 130–140 years. The site last ruptured in 1857; this long quiescence, combined with the 闭锁断层因摩擦阻止运动而导致应力持续积累的断层区段。闭锁断层一旦最终破裂，可能引发大地震。 character of the segment, indicates significant accumulated strain.

Dating Past Earthquakes with Radiocarbon

Radiocarbon (¹⁴C) dating is the workhorse technique for determining the ages of organic materials preserved in fault trench stratigraphy. Carbon-14 is produced in the atmosphere by cosmic ray bombardment of nitrogen-14 and incorporated into living organisms during their lifetimes. When an organism dies, it no longer takes in new carbon, and the ¹⁴C in its tissues decays with a half-life of 5,730 years. By measuring the ratio of ¹⁴C to stable ¹²C in charcoal, plant remains, or shell fragments from sediment layers above and below an earthquake horizon, the ages of those layers can be determined, bracketing the time of the earthquake. Radiocarbon dating is most useful for earthquakes younger than about 50,000 years and achieves precision of roughly ±50–200 years with modern accelerator mass spectrometry (AMS) techniques. Other dating methods used in 古地震学通过断层探槽、隆起阶地和海啸沉积物等地质证据研究史前地震的学科，将地震记录延伸至数千年前。 include optically stimulated luminescence (OSL), cosmogenic nuclide exposure dating, and U-series dating of carbonate cements.

Uplifted Terraces and Tsunami Deposits

Not all paleoseismic evidence comes from fault trenches. Coastal geomorphology preserves a rich record of past earthquakes and tsunamis. Uplifted marine terraces — flat benches carved by wave action at sea level during past time periods and now elevated above the ocean — record co-seismic uplift during past subduction earthquakes. The coast of the Huon Peninsula in Papua New Guinea preserves a staircase of uplifted terraces recording repeated earthquakes over hundreds of thousands of years. In Cascadia, ghost forests of drowned trees — standing snags preserved in coastal marshes — record sudden coseismic subsidence during past megathrust earthquakes; radiocarbon dating of these trees has precisely dated the last Cascadia megathrust to January 26, 1700, based on Japanese tsunami records. 海啸由水下地震导致海底突然位移而产生的一系列海浪。海啸可以喷气机般的速度（时速700公里以上）穿越整个大洋盆地。 deposits — anomalous layers of sand and marine microfossils deposited inland by past tsunamis — independently confirm the occurrence and size of past megathrust earthquakes, complementing the fault trench evidence.

Extending the Earthquake Record: Why It Matters for Recurrence Interval

The practical importance of 古地震学通过断层探槽、隆起阶地和海啸沉积物等地质证据研究史前地震的学科，将地震记录延伸至数千年前。 lies in its contribution to earthquake hazard assessment. The 地震重现间隔特定断层上相邻两次大地震之间的平均时间间隔，通过古地震学研究和历史记录估算得出。卡斯凯迪亚俯冲带的重现间隔约为500年。 of a fault — the average time between large earthquakes — cannot be reliably estimated from a record of only one or two events. With a paleoseismic record covering 10 or more events, the mean recurrence interval can be estimated with reasonable confidence, and the variability in that interval can be characterized. This variability is important: the "time-predictable" and "slip-predictable" models assume different relationships between recurrence and slip, and the choice of model affects hazard calculations significantly. 地震空区与相邻区段相比长时间未发生地震的活动断层区段，可能预示未来发生地震的概率有所增加。 analysis — identifying fault segments that have not ruptured in longer than their average interval and are therefore "overdue" — relies directly on paleoseismic data. The Coachella segment of the San Andreas 断层（地质学）岩石沿其发生位移的破裂面。断层长度从数毫米到数千公里不等。会引发地震的主要断层称为活动断层。, which last ruptured around 1680 CE based on paleoseismic evidence, has an estimated recurrence interval of roughly 200 years, making it one of the most worrying seismic gaps in North America.

Paleoseismology and Probabilistic Hazard

古地震学通过断层探槽、隆起阶地和海啸沉积物等地质证据研究史前地震的学科，将地震记录延伸至数千年前。 results feed directly into the fault source models used in 概率地震危险性分析（PSHA）一种量化地震危险性的方法，综合考虑所有可能的地震震源、震级及地震动水平，以超过特定震动水平的概率来表示结果。. The recurrence intervals, slip per event, and fault slip rates断层沿线位移的平均速率，通常以每年若干毫米衡量。滑动速率越高，通常意味着地震发生频率和危险性越高。 derived from trench studies populate the input parameters for fault source characterization models such as those in the United States National Seismic Hazard Model (NSHM). Uncertainties in paleoseismic data — imprecise radiocarbon ages, ambiguous event horizons, incomplete exposure in trenches — are propagated through the hazard model using logic trees that capture the range of plausible interpretations. This formal treatment of uncertainty is essential in 概率地震危险性分析（PSHA）一种量化地震危险性的方法，综合考虑所有可能的地震震源、震级及地震动水平，以超过特定震动水平的概率来表示结果。, ensuring that hazard estimates honestly reflect what is and is not known about the behavior of each fault source. As 古地震学通过断层探槽、隆起阶地和海啸沉积物等地质证据研究史前地震的学科，将地震记录延伸至数千年前。 databases grow and dating techniques improve, the quality and reliability of fault-based seismic hazard assessments continues to improve, directly contributing to safer building codes and more informed land-use planning in earthquake-prone communities.