Weak dynamic seismic stress waves from distant earthquakes can abruptly change the fluid permeability of a hydrologic system, trigger seismicity, or cause liquefaction. Many proposed mechanisms to explain these phenomena require a large fluid pressure gradient to induce fluid flow during the passage of the waves, yet the exact mechanism as to how the fluid pressure can change is unclear. Using full‐waveform numerical modeling, we found that the transient seismic‐wave pressure in a fluid‐filled fracture could increase more than 2 orders of magnitude relative to the incident pressure. We call this transient pressure surge (PS). This pressure increase could be much more pronounced for low‐frequency transient waves than for high frequencies and could develop a large pressure gradient to drive the fluid to flow. This PS phenomenon may be important for understanding many natural phenomena in fluid‐fracture systems.